Department of Inorganic and Physical Chemistry
Research group L3 – Luminescent Lanthanide Lab
Tuning the emission colour of rare-earth
tungstate and vanadate materials towards
white light generation
Thesis submitted to obtain
the degree of Master of Science in Chemistry by
Dorine NDAGSI
Academic year 2013 - 2014
Promoter: prof. dr. Rik Van Deun
Supervisor: Anna Kaczmarek
Department of Inorganic and Physical Chemistry
Research group L3 – Luminescent Lanthanide Lab
Tuning the emission colour of rare-earth
tungstate and vanadate materials towards
white light generation
Thesis submitted to obtain
the degree of Master of Science in Chemistry by
Dorine NDAGSI
Academic year 2013 - 2014
Promoter: prof. dr. Rik Van Deun
Supervisor: Anna Kaczmarek
Acknowledgements
First of all, I would like to thank prof. Van Deun for giving me the opportunity to do my
thesis in his research group. Investigating the luminescence of lanthanide doped materials is
truly an exciting subject. I really learned a great deal this year.
Secondly, I would like to thank my supervisor Anna Kaczmarek. I really liked how she gave
me the space to do research on my own. However, when I had some questions, she was
always available. Thanks to her I learned a great deal about lanthanides and luminescence.
Besides the scientific part, I really liked the humorous atmosphere in the L3 group that she
helped to create. I hope I never forget the laser jokes.
I also want to thank Roel Decadt for answering questions whenever I had some.
Great thanks to Tom Planckaert for performing the XRD measurements and Anna
Kaczmarek for the SEM measurements.
Special thanks to prof. Van Hecke, since without the use of the ICSD database I might not
have been able to characterize the phase of the tungstate material.
I would also like to thank all my fellow students. These past 5 years flew by because of some
amazing people and I wouldn’t want to change a moment of it. A great deal of these
memories I’ve had with one of my best friends and favourite lab partner, Laetitia Vlaminck.
Tineke and Delphine did not follow the same path as we did, but we’re still a foursome.
I’ve enjoyed myself a lot with my fellow thesis students Jason Serck and especially Matthias
Van Zele, whose dark coloured humour will always amuse me.
A warm thanks to Joren Guillaume who not only helped with correcting my thesis, but also
coped with the endless stories (or sometimes nagging) about what happened that day in
the lab.
And most importantly, I want to thank my mom. First of all, for giving me the opportunity to
study for five years. But also for the support. Without her, none of this would be possible.
i
Optimalisatie van de Emissiekleur van Lanthanide-Gedoteerde Wolframaat- en
Vanadaat-Materialen met als doel het verkrijgen van Wit Licht
D. Ndagsia*
, A. M. Kaczmareka, R. Van Deun
a
aVakgroep Anorganische en Fysische Chemie, Universiteit Gent, Krijgslaan 281, gebouw S3,
9000 Gent, België
*E-mail: [email protected]
In dit artikel wordt de synthese van yttrium oxywolframaat (Y2WO6)
en yttrium orthovanadaat (YVO4) beschreven. Beide materialen
werden gesynthetiseerd via een hydrothermale syntheseroute met
behulp van glycerol als ligand. De invloed van verschillende factoren
werd onderzocht. De materialen werden gekarakteriseerd met Diffuse
Reflectantie Infrarood Fourier Transform Spectroscopie (DRIFTS), X-
stralen poeder diffractie (XRD) en Rasterelektronenmicroscopie
(SEM). Na het verkrijgen van een pure fase werden deze materialen
ge(co)doteerd met lanthaniden, met als doel een emissie van zuiver wit
licht te verkrijgen. Excitatie- en emissiespectra werden opgenomen.
Bij excitatie in de ladingsoverdrachtsband vond een efficiënte
ladingsoverdracht plaats van de VO43-
of WO66-
groepen naar de
lanthaniden met als gevolg scherpe 4f-4f transities. Soms was de
ladingsoverdrachtsband nog zichtbaar in het emissiespectrum. De
echte kleur van de materialen werd uitgezet in een CIE-diagram.
Zowel voor als na warmtebehandeling werden materialen gevonden
die wit licht uitzenden.
Kernwoorden: Lanthanide / wolframaten / vanadaten / luminescentie / wit licht
Introductie
Milieuvriendelijkheid is een onderwerp dat
tegenwoordig zeer veel aandacht krijgt (1).
Gloeilampen worden steeds minder gebruikt.
In plaats hiervan schakelt men waar mogelijk
over op compacte fluorescentielampen (beter
gekend als spaarlampen) en TL-buizen (1).
Deze types verlichting gebruiken een mengsel
van fosforen die elk een bepaalde kleur
uitzenden. Deze combinatie van kleuren is de
courante methode om wit licht te verkrijgen.
Met het oog op recyclage is dit geen ideale
situatie, aangezien het chemisch scheiden van
deze verschillende fosforen niet
vanzelfsprekend is (1). Een enkele fosfor die
wit licht uitstraalt is in dit opzicht interessanter.
Het is echter geen sinecure om een materiaal te
verkijgen dat wit licht uitzendt en daarbij ook
een hoog lichtrendement heeft. Deze soort
fosforen kunnen gebruikt worden in licht
emitterende dioden (LEDs) (2). Het principe
van een LED is de recombinatie van
elektronen en gaten onder invloed van een
elektrisch veld. Dit proces veroorzaakt emissie,
ook elektroluminescentie genoemd.
Als mogelijke matrices voor een wit licht
emitterend materiaal worden wolframaat (3-7)
en vanadaat materialen (8-10) naar voor
geschoven. Deze vertonen een brede
ladingsoverdrachtsband van de VO43-
of WO66-
groepen naar de lanthaniden, wat leidt tot
excitatie van de lanthanide-ionen. Deze band
bevindt zich in de blauwe regio van het
spectrum. In combinatie met de scherpe
ii
emissies van de lanthaniden is dit een startpunt
tot het verkrijgen van een materiaal dat wit
licht uitzendt. Tijdens de synthese van deze
fosforen kan een ligand gebruikt worden (1, 3,
11, 12). Liganden worden gebruikt om de
reagentia in oplossing te stabiliseren en hebben
ook een invloed op de morfologie van de
materialen. Het gebruik van een ligand kan
leiden tot verschillende deeltjesgroottes. Elk
ligand beïnvloedt de morfologie op een andere
manier. Een bepaalde morfologie kan zorgen
voor een meer efficiënte ladingsoverdracht en
kan op deze manier een invloed hebben op de
luminescentie-eigenschappen. Het
werkingsmechanisme van het ligand is slechts
empirisch te achterhalen. Vele voorstellen
voor mogelijke vormingsmechanismen zijn op
dit punt al gepubliceerd (6, 14-16). Hoe het
ligand interageert met de bouwblokken
gedurende de synthese kan onderzocht worden
door meerdere syntheses uit te voeren met
verschillende tijdsduren. Wanneer deze
deeltjes onder een elektronenmicroscoop
onderzocht worden, kan men een idee krijgen
van het vormingsmechanisme. Er zijn enkele
liganden die in de literatuur frequent gebruikt
worden zoals: citroenzuur, bis-(2-
ethylhexyl)sulfosuccinaat (AOT),
polyvinlypyrrolidon (PVP), cetyltrimethyl
ammonium bromide (CTAB) (10, 19-23) ...
Glycerol werd hier gekozen als ligand voor de
synthese van lanthanide-gedoteerde
wolframaat- en vanadaat-materialen. Met het
oog op een meer recylagevriendelijke manier
om te verlichten scoort glycerol zeer goed op
de milieuvriendelijkheidsschaal. Dit materiaal
wordt gebruikt in de voedingsindustrie en is
niet giftig (24). Het wordt ook teruggevonden
in vele zepen en shampoos ter bescherming
van de huid. Glycerol werd al gebruikt als
ligand voor de synthese van zeldzame-aarde-
gedoteerde boraten (25), oxides (26-29),
fluorides (30) en fosfaten (31). Er werden geen
artikels gevonden waar men glycerol gebruikte
als ligand voor de synthese van lanthanide-
gedoteerde-vanadaten, maar wel in het geval
van wolframaten (13). Hierbij werden
La2(WO4)3 en NaLa(WO4)2 matrices
gesynthetiseerd met een gemiddelde
deeltjesgrootte van 100 nm. Bij het gebruik
van glycerol als ligand werd een Ostwald
ripening groeiproces gesuggereerd voor de
vorming van de deeltjes. Dit is een proces
waarbij de gevormde kleine kernen oplossen
en afgezet worden op grotere deeltjes. Er werd
geconcludeerd dat glycerol een positieve
invloed had op de deeltjes, aangezien grotere
hoeveelheden glycerol leidden tot kleinere
kernen. Naast het ligand is ook de
hydrothermale syntheseroute milieuvriendelijk,
aangezien water als solvent gebruikt wordt.
Wolframaat- en vanadaat-materialen werden
reeds uitvoerig onderzocht en blijken goede
systemen te zijn voor het doteren van
lanthaniden. Wit-licht-emitterende-lanthanide-
gedoteerde wolframaat- (3-7) en vanadaat-
materialen (8-10) werden al gepubliceerd.
Er zijn geen unieke doteringscondities om wit
licht te bereiken aangezien elke matrix anders
is. Het is belangrijk om te weten dat wit licht
niet exact gedefiniëerd kan worden. Er kan
eerder gesproken worden over een wit licht
regio. Wit licht resulteert typisch uit de
combinatie van een groene, blauwe en rode
emissie. In het ideale geval is het aandeel van
elk van deze kleuren gelijk. Dit wordt
beschreven als perfect wit licht. Wit licht kan
ook resulteren uit combinaties van andere
kleuren, bijvoorbeeld blauw en geel (37). De
‘Commision Internationale de L’Éclairage’
(CIE) heeft een kleurconventie ingevoerd,
waarin elke kleur kan geplot worden in het zgn.
CIE-diagram en overeenkomt met specifieke
coördinaten. Standaard wit licht heeft
coördinaten x = 0,33, y = 0,33. Niet enkel de
exacte kleur, maar ook de temperatuur van de
kleur speelt een belangrijke rol. Wit licht dat
meer naar de blauwe kant van het spectrum
verschoven is, wordt als ‘koud’ geobserveerd,
terwijl een verschuiving naar de rode kant van
het spectrum aangevoeld wordt als een warmer
wit. De kleurtemperatuur (Correlated Colour
iii
Temperature, CCT) wordt uitgedrukt in Kelvin.
Een kleurtemperatuur lager dan 3000 K is
warm. Een kleurtemperatuur hoger dan 5000
K is koud. Wit licht met coördinaten x = 0,33,
y = 0,33 heeft een kleurtemperatuur van
ongeveer 5500 K en is dus koud. De
verschillende schakeringen in wit licht
bemoeilijken het om dit op een
ondubbelzinnige manier te beschrijven.
Verschillende temperaturen wit licht kunnen
nuttig zijn voor verschillende applicaties. Een
blauwer wit licht kan gebruikt worden in
aquaria terwijl wit licht met een roze tint
gebruikt wordt in slagerijen (1).
In dit artikel wordt de synthese besproken van
wolframaat- en vanadaat-materialen via een
hydrothermale route met behulp van glycerol
als ligand. Na het bepalen van de ideale
condities voor de vorming van een pure fase
werden de materialen ge(co)doteerd met
lanthaniden die emissie vertonen in het
zichtbare deel van het spectrum. Na het
opmeten van de emissiespectra werd de ‘echte’
kleur geplot in het CIE-diagram. De invloed
die de syntheseroute heeft op de morfologie
werd ook onderzocht door het uitvoeren van
een sol-gel en microgolfreactie.
Experimenteel
Gebruikte chemicaliën
De gebruikte chemicaliën zijn; yttriumnitraat
(Y(NO3)3∙6H2O, 99,9%, Sigma Aldrich),
europiumnitraat (Eu(NO3)3∙6H2O, 99,9%, Alfa
Aesar), terbiumnitraat (Tb(NO3)3∙5H2O,
99,9% Sigma Aldrich), dysprosiumnitraat
(Dy(NO3)3∙xH2O, 99,9%, Sigma Aldrich),
samariumnitraat (Sm(NO3)3∙6H2O, 99,9%,
Acros Organics, natriumwolframaat
(Na2WO4∙2H2O, 99,9%, Sigma Aldrich),
natriumorthovanadaat (Na3VO4, 99,98%,
Sigma Aldrich), glycerol (bidistilled, 99,5%,
VWR Chemicals), ethanol (EtOH, 96%, Fiers),
salpeterzuur (HNO3, 53%, Roth),
ammoniakoplossing (NH3 (aq), 28-30% NH3
basis, Sigma Aldrich). Alle chemicaliën
werden gebruikt zonder verdere zuivering.
Hydrothermale synthese van lanthanide-
gedoteerde wolframaat- en vanadaat-materialen
In een typische reactieroute werd aan 20 mL
glycerol 10 mL gedestilleerd water toegevoegd.
Hierna werd 1 mmol Y(NO3)3∙6H2O opgelost
in 10 mL gedestilleerd water. Deze oplossing
werd dan toegevoegd aan het glycerol-water
mengsel en hevig geroerd gedurende 15
minuten. Van het Na2WO4∙2H2O of Na3VO4
zout werd 1 mmol opgelost in 10 mL
gedestilleerd water en geroerd gedurende 10
minuten. Hierna werden beide mengsels traag
samengevoegd. De pH werd aangepast met
behulp van HNO3 of NH3(aq). Na 10 minuten
roeren werd de oplossing overgebracht in een
met Teflon beklede autoclaaf die stevig
dichtgeschroefd werd. De autoclaaf werd in
een oven geplaatst en opgewarmd tot 200°C
gedurende 24 uur. Na reactie werd de
autoclaaf traag afgekoeld tot
kamertemperatuur (ongeveer 24 uur). Het
product werd driemaal gewassen met water en
ethanol en gecentrifugeerd gedurende 5
minuten aan 6000-7000 toeren per minuut.
Drogen gebeurde ‘s nachts in een vacuüm
oven op 55°C. Hierna werden de materialen
nabehandeld op hoge temperatuur (900°C voor
vanadaat-materialen en 1100°C voor
wolframaat-materialen) gedurende 3 uur om
alle overtollige producten te verwijderen.
Na de synthese werden alle materialen
gekarakteriseerd via X-straal diffractie,
Diffuse Reflectantie Infrarood Fourier
Transform Spectroscopie en Raster-
elektronenmicroscopie. Verschillende
doteringspercentages werden uitgevoerd (x
varieerde van 0,01 (1% dotering) tot 0,05 (5%
dotering)). De syntheseroute bleef identiek, er
werd nu enkel 1-x mmol Y(NO3)3∙6H2O en x
mmol lanthanidezout gebruikt.
Een microgolf en sol-gel reactie werden ook
verricht. De microgolfreactie werd uitgevoerd
bij 200°C en constante microgolven (200 W)
gedurende 1 uur. De sol-gel reactie werd
uitgevoerd bij 800°C gedurende 5 uur.
iv
Karakterisering
Infrarood metingen werden uitgevoerd met een
Thermo Scientific FT-IR spectrometer (type
Nicolet 6700) uitgerust met een DRIFTS-cel.
Spectra werden opgenomen van 650 cm-1
tot
4000 cm-1
met een resolutie van 4 cm-1
en KBr
als achtergrond. X-stralen diffractogrammen
werden opgenomen met een Thermo Scientific
ARL X’TRA diffractometer, uitgerust met een
CuKα (λ = 1,5405 Å) bron, een goniometer en
een Peltier gekoelde Si(Li) vaste stof detector.
SEM metingen werden uitgevoerd met een
FEI Quanta 200 F SEM en een FEI Nova 600
Nanolab Dual-Beam gefocusseerde ionenstraal
in secundaire elektronmodus. Luminescentie-
metingen werden opgenomen met een
Edinburgh Instruments FLS920 UV-VIS
spectrometer. Steady-state metingen werden
uitgevoerd in het gebied 250 – 1700 nm. Een
450 W xenonlamp werd gebruikt als
excitatiebron. In de regio 200 – 870 nm werd
een Hamamatsu R928P PMT detector gebruikt.
Vervaltijden van 100 µs tot enkele seconden
werden gemeten met een µ920H 60 W
microseconde xenon flashlamp: 0,1-100 Hz,
pulsbreedte ± 2 µs. Vervaltijden korter dan
100 µs werden gemeten met een Continuum
Surelite I laser (450 mJ bij 1064 nm).
Resultaten en Discussie
Eerst worden de gesynthetiseerde wolframaat-
materialen in detail besproken.
Lanthanide gedoteerde wolframaat-materialen
Uitgevoerde reacties Allereerst moeten
verschillende reactieomstandigheden getest
worden met als doel het vinden van de ideale
condities waarbij een pure fase verkregen
wordt. Deze condities zijn zeer specifiek.
Omstandigheden die leiden tot de vorming van
een pure fase leiden niet noodzakelijk ook tot
een pure fase wanneer een ander ligand
gebruikt wordt. Verschillende condities
werden getest en bij een hydrothermale reactie
gedurende 24 uur op een temperatuur van
200°C met gebruik van 20 mL glycerol, pH =
13 en warmtebehandeling op 1100°C werd een
pure fase gevormd.
Diffuse Reflectantie Infrarood Fourier
Transform Spectroscopie Spectra van deze
materialen voor en na warmtebehandeling op
900°C en 1100°C werden opgenomen. In Fig.
1 worden deze weergegeven. De spectra van
de andere wolframaat materialen zijn
vergelijkbaar. De scherpe band op 3559 cm-1
(a) kan toegekend worden aan de O-H
rekvibraties van water en/of glycerol. Na
warmtebehandeling verdwijnt deze piek,
waaruit afgeleid kan worden dat al het water
verwijderd is. Rond 1406 cm-1
(b) is een brede
band zichtbaar die ook toegekend kan worden
aan de O-H rekvibraties van water, al kan deze
band ook veroorzaakt worden door gebruikte
nitraat-zouten tijdens de synthese.
Figuur 1 DRIFTS spectrum van Y2WO6 voor (rood),
na warmtebehandeling op 900°C (blauw) en 1100°C
(groen).
In de regio lager dan 1000 cm-1
zijn er
karakteristieke wolframaat rekvibraties
zichtbaar. Banden (c) en (d) vinden mogelijk
hun oorsprong in de W-O rekvibraties. Band
(e) werd toegeschreven aan de asymmetrische
rekvibraties van W-O-W bruggen (6). Deze
techniek geeft een indicatie dat dit een
wolframaat materiaal is. De pieken van de
spectra na warmtebehandeling op 900°C en
1100°C zijn vergelijkbaar. Er kan
geconcludeerd worden dat na
warmtebehandeling alle water verdwenen is,
maar DRIFTS kan echter niet aantonen of een
pure fase gesynthetiseerd werd en welke fase
dit is.
v
X-stralen poeder diffractie
In Fig. 2. worden de X-straal difractogrammen
weergegeven. Deze werden vergeleken met
een X-straal diffractogram dat berekend werd
op basis van de ICSD kristallografische
databank (40). De meeste wolframaat
materialen vormen al een pure fase na
warmtebehandeling op 900°C of lager (3, 5, 14,
15, 32). Dit was bij de materialen in deze
studie niet het geval. Yttrium oxywolframaat
werd al gepubliceerd als een metastabiel
materiaal (32, 33) dat bij warmtebehandeling
op verschillende temperaturen verscheidene
irreversibele fasen vormt.
Figuur 2 Diffractogram van Y2WO6 voor
warmtebehandeling (A), na warmtebehandeling op
900°C (B), na warmtebehandeling op 1100°C (C) en
het diffractogram berekend op basis van de ICSD
kristallografische databank (D) (40).
Een temperatuur van 1100°C (C) was nodig
om de monokliene Y2WO6 fase te verkrijgen.
Een temperatuur van 900°C (B) leidde tot een
mix van de tetragonale en monokliene fase.
Rasterelektronenmicroscopie (SEM)
Met behulp van rasterelektronenmicroscopie
kon de groote, vorm en de morfologie bepaald
worden. Het is duidelijk uit Fig. 3 dat de
deeltjes niet op een repetitieve manier
geordend zijn. Bij hogere vergroting is
duidelijk dat dit material bolvormige en
staafvormige (smalle en brede) deeltjes bevat.
Waarschijnlijk werden de bolvormige deeltjes
eerst gevormd. De staafvormige deeltjes
hebben een hobbelige vorm, wat vermoedelijk
komt door het samenklitten van de bolvormige
deeltjes. Verdere aggregatie van de deeltjes
leidde tot bredere staafvormige structuren. De
vlakke randen van de structuren werden
waarschijnlijk gevormd tijdens de
warmtebehandeling op hoge temperatuur.
Figuur 3 SEM afbeeldingen van Y2WO6 na
warmtebehandeling op 1100°C bij verschillende
vergrotingen.
Luminescentie-eigenschappen
Verscheidene (co)doteringen werden
uitgevoerd met als doel een materiaal te
verkrijgen dat wit licht uitzendt na
warmtebehandeling, aangezien dit zorgt voor
een meer intense emissie.
De gedoteerde Y2WO6 materialen die leidden
tot witte emissie onder een UV-lamp (254 nm)
voor warmtebehandeling zijn: 2,5% Eu3+
,
2,5% Tb3+
(CCT = 5019 K) / 3% Eu3+
(CCT =
5771 K) / 2% Eu3+
, 3% Tb3+
(CCT = 3952 K) /
2,5% Sm3+
, 2,5% Tb3+
(CCT = 8061 K).
Enkele materialen vertoonden ook witte
emissie na warmtebehandeling: 1% Dy3+
(CCT = 4042 K) en 1% Sm3+
(CCT = 1562 K),
vi
al neigt dit naar een meer roze wit licht. Het
Y2WO6 materiaal dat gedoteerd werd met 1%
Dy3+
en na warmtebehandeling wit licht
uitzendt met een intermediaire temperatuur
wordt hieronder in detail besproken.
Figuur 4 Excitatiespectrum van Y2WO6:Dy
3+(1%) na
warmtebehandeling met λem = 574,0 nm (niet
gecorrigeerd voor de gevoeligheid van de detector).
In het excitatiespectrum is een brede
ladingsoverdrachtsband zichtbaar met een
maximum op 299,6 nm. Deze wordt
toegeschreven aan de ladingsoverdracht van de
2p zuurstoforbitalen naar de 5d wolframaat
orbitalen.
Figuur 5 Emissiespectrum van Y2WO6:Dy3+
(1%) na
warmtebehandeling, geëxciteerd via de
ladingsoverdrachtsband met λex = 299,6 nm
(gecorrigeerd voor de gevoeligheid van de detector).
De ladingsoverdrachtsband is zeer zwak
aanwezig in het emissiespectrum. Dit wijst op
een goede ladingsoverdracht van de WO66-
groepen naar de lanthaniden. Onder de UV
lamp (254 nm) zendt dit materiaal wit licht uit
voor warmtebehandeling maar na
warmtebehandeling is de intensiteit van de
witte emissie veel sterker. Dysprosium toont
transities in de blauwe regio (4F9/2 →
6H15/2),
gele regio (4F9/2 →
6H13/2) en in het rode
gebied (4F9/2 →
6H11/2).
Figuur 6 Vervaltijdcurve van de
4F9/2 →
6H13/2
transitie van Y2WO6:Dy3+
(1%) na
warmtebehandeling met een enkelvoudige
exponentiële fit, geëxciteerd bij 299,6 nm.
De vervaltijd van de 4F9/2 →
6H13/2 transitie
bedraagt 229 µs na warmtebehandeling, wat
een stuk hoger is dan de vervaltijd van 16 µs
voor warmtebehandeling.
Figuur 7 CIE-diagram van Y2WO6:Dy
3+(1%) voor
warmtebehandeling (links) en na warmte-
behandeling op 1100°C (rechts).
In het CIE-diagram is duidelijk te zien hoe de
kleur na warmtebehandeling verschuift van
een koude witte emissie (x = 0,2196, y =
0,2800 en CCT = 26417 K) naar een warme
witte emissie (x = 0,3826, y = 0,3908 en CCT
= 4042 K).
vii
Foto’s van de tungstaat-materialen
Figuur 8 Foto van de gedoteerde materialen onder de
UV lamp (254 nm), van links naar rechts;
Y2WO6:Eu3+
(2,5%), Tb3+
(2,5%) en Y2WO6:Dy3+
(1%)
voor warmtebehandeling, Y2WO6:Dy3+
(1%) na en
Y2WO6:Sm3+
(1%) voor warmtebehandeling.
Figuur 9 Foto van de gedoteerde materialen voor
warmtebehandeling onder de UV lamp (254 nm),
van links naar rechts; Y2WO6:Eu3+
(3%),
Y2WO6:Sm3+
(3%), Y2WO6:Eu3+
(2%), Tb3+
(3%) en
Y2WO6:Sm3+
(2,5%), Tb3+
(2,5%).
Wit-licht-emitterende vanadaat-materialen
werden ook gesynthetiseerd en worden
vervolgens in detail besproken
Lanthanide gedoteerde vanadaat materialen
Uitgevoerde reacties Gelijkaardige
reactieomstandigheden als Wu et al. (36)
publiceerden, maar nu met glycerol als ligand
werden uitgevoerd. De optimale condities die
leidden tot de vorming van een pure fase zijn:
een hydrothermale reactie gedurende 24 uur op
een temperatuur van 200°C met gebruik van
20 mL glycerol, pH = 2 en warmtebehandeling
op 900°C. Deze condities werden gebruikt
voor het doteren met lanthaniden.
Diffuse Reflectantie Infrarood Fourier
Transform Spectroscopie DRIFTS spectra
werden opgenomen voor en na
warmtebehandeling op 900°C, welke
weergegeven worden in Fig. 10.
Figuur 10 DRIFTS spectrum van een vanadaat
materiaal voor (rood) en na warmtebehandeling op
900°C (blauw).
De brede band op 3458 cm-1
(a) wordt
toegeschreven aan de O-H rekvibraties van
water en/of glycerol dat geadsorbeerd is aan
het kristaloppervlak. De banden op 2343 cm-1
(b) en 1625 cm-1
(c) komen overeen met met
de O-H rekvibraties van gecoördineerd water.
Bij golfgetallen kleiner dan 1000 cm-1
zijn de
karakteristieke vibraties van het vanadaat
materiaal zichtbaar. Deze werden
toegeschreven met behulp van de anorganische
infrarood databank van F. Miller et al. (34). De
banden op 977 cm-1
(d) en 706 cm-1
(e) werden
toegewezen aan V-O rekvibraties. Het is
duidelijk dat dit materiaal voor
warmtebehandeling nog zeer veel water bevat
dat erna grotendeels verdwijnt. Na
warmtebehandeling is nog steeds een kleine
hoeveelheid gecoördineerd water aanwezig.
De karakteristieke vanadaat banden suggereren
dat dit poeder een vanadaat-materiaal bevat, al
kan dit enkel bevestigd worden via X-stralen
diffractie.
X-stralen poeder diffractie In Fig. 11
worden de diffractogrammen van de vanadaat
materialen weergegeven en vergeleken met het
diffractogram dat berekend werd op basis van
de kristallografische ICSD databank (39).
viii
Figuur 11 Diffractogram van YVO4 voor
warmtebehandeling (A), na warmtebehandeling op
900°C (B) en het diffractogram berekend op basis
van kristallografische ICSD databank (C) (39).
De pure YVO4 fase werd verkregen voor en na
warmtebehandeling. Voor warmtebehandeling
zijn de reflecties iets breder dan erna, wat
suggereert dat het materiaal kristallijner wordt
na warmtebehandeling op 900°C. Een sol-gel
en microgolfreactie werden ook uitgevoerd.
De X-straaldiffractogrammen van deze
materialen hebben aangetoond dat in beide
gevallen de pure YVO4 fase gevormd was.
Rasterelektronenmicroscopie (SEM)
Met behulp van rasterelektronenmicroscopie
kon de groote, vorm en de morfologie van dit
materiaal bepaald worden.
Figuur 12 SEM afbeeldingen van YVO4
gesynthetiseerd via een hydrothermale syntheseroute
op 180°C in zure omstandigheden (links, pH = 2) en
basische omstandigheden (rechts, pH = 10) en met
behulp van 10 mL glycerol .
Fig. 12 toont de invloed van de zuurtegraad op
de morfologie. Reactie in basische
omstandigheden (pH = 10) leidde tot een
materiaal dat volledig geaggregeerd is. Zure
omstandigheden (pH = 3) daarentegen leidde
tot bolvormige deeltjes die veel minder
geaggregeerd zijn. De deeltjes zijn niet
uniform en ook niet verdeeld op een
regelmatige manier.
Figuur 13 SEM afbeeldingen van YVO4
gesynthetiseerd via een hydrothermale route op
200°C, met behulp van 20 mL glycerol en na
warmtebehandeling op 900°C.
Het gebruik van een grotere hoeveelheid
glycerol en hogere reactietemperatuur heeft
duidelijk een invloed op de deeltjes, welke nu
minder geaggregeerd zijn (Fig. 13). Interessant
genoeg zijn drie verschillende soorten
morfologieën aanwezig. Bolvormige, hoekige
en staafvormige partikels kunnen
onderscheiden worden. Een hogere
hoeveelheid glycerol kan mogelijks leiden tot
deeltjes die uniformer zijn.
Figuur 14 SEM afbeeldingen van YVO4
gesynthetiseerd via een sol-gel route, met behulp van
20 mL glycerol en na warmtebehandeling op 900°C.
ix
De sol-gel route leidde tot deeltjes die volledig
geaggregeerd zijn. Fig. 14 toont hoe eerst
bolvormige deeltjes gevormd werden die dan
na aggregatie leidden tot vlakken met een ruw
oppervlak.
Figuur 15 SEM afbeeldingen van YVO4
gesynthetiseerd via een microgolfroute, met behulp
van 20 mL glycerol en na warmtebehandeling op
900°C.
Bolvormige deeltjes van verschillende groottes
zijn aanwezig in dit YVO4 materiaal (Fig. 15).
Een Ostwald ripening groeiproces kan
gesuggereerd worden aangezien bij hoge
vergroting duidelijk is hoe de grotere sferen
opgebouwd zijn uit kleinere partikels. De
deeltjes zijn vrij uniform verdeeld over het
oppervlak. Dit is een zeer goed resultaat
vergeleken met morfologie bekomen via de
hydrothermale syntheseroute, vooral aangezien
de microgolfsynthese slechts 1 uur duurde, wat
vrij kort is in vergelijking met de 24 uur
durende hydrothermale synthese.
Luminescentie-eigenschappen
Allereerst werd de bekomen YVO4 matrix
enkelvoudig en met verschillende percentages
aan lanthaniden gedoteerd (2%, 3% en 5% aan
Eu3+
, Dy3+
en Sm3+
) . Hieruit kon informatie
verkregen worden over hoe de kleur verschuift
en hoe de intensiteit van de
ladingsoverdrachtsband verandert bij
verschillende doteringspercentages. Later werd
de YVO4 matrix ook co-gedoteerd met als doel
na warmtebehandeling een witte emissie te
verkrijgen. Condities die leidden tot witte
emissie onder een UV lamp (254 nm) voor
warmtebehandeling zijn: 2% Dy3+
(CCT =
4725 K) / 3% Dy3+
(CCT = 4898 K) / 2,5%
Eu3+
, 2,5% Tb3+
(CCT = 2954 K) / 2% Eu3+
,
3% Dy3+
(CCT = 3299 K). Een materiaal dat
gedoteerd werd met 0,5% Eu3+
, 2,5% Dy3+
zendt wit licht uit voor en na
warmtebehandeling op 900°C met CCT =
3474 K (voor) en CCT = 2598 K (na). De
luminescentie eigenschappen van dit materiaal
na warmtebehandeling worden in detail
besproken.
Figuur 16 Excitatiespectrum van YVO4:Eu3+
(0,5%),
Dy3+
(2,5%) na warmtebehandeling, met λem = 574,1
nm (niet gecorrigeerd voor de gevoeligheid van de
detector).
In de regio 250 – 350 nm is een brede band
zichtbaar die piekt op 310,9 nm en te wijten is
aan de ladingsoverdracht van de 2p
zuurstoforbitalen naar de 5d vanadaat orbitalen.
Figuur 17 Emissiespectrum van YVO4:Eu
3+(0,5%),
Dy3+
(2,5%) na warmtebehandeling, geëxciteerd via
de ladingsoverdrachtsband met λex = 310,9 nm
(gecorrigeerd voor de gevoeligheid van de detector).
De transities corresponderend met Eu3+
(rood) en
Dy3+
(blauw) zijn aangeduid in het spectrum.
x
Na warmtebehandeling is de
ladingsoverdrachtsband verdwenen uit het
emissiespectrum. Dit suggereert een goede
ladingsoverdracht van de vanadaat groepen
naar de lanthaniden. De karakteristieke
transities van Eu3+
en Dy3+
zijn duidelijk
zichtbaar. Dysprosium toont een transitie in de
blauwe regio (4F9/2 →
6H15/2), een gele transitie
(4F9/2 →
6H13/2) en een rode transitie (
4F9/2
→6H11/2) die overlapt met de
5D0→
7F3 transitie
van Eu3+
. De intensiteit van de hypersensitieve 5D0→
7F2 elektrische dipooltransitie van Eu
3+
is hoog in vergelijking met de intensiteit van
de magnetische dipool 5D0→
7F1 transitie. Dit
suggereert een lage symmetrie. Voor
warmtebehandeling zijn de dysprosium
transities veel intenser dan de europium
transities, wat niet meer het geval is na
warmtebehandeling. Een ander verschil is dat
voor de warmtebehandeling een
ladingsoverdrachtsband zichtbaar was in het
emissiespectrum van dit materiaal. Deze band
draagt een blauwe kleur bij aan de emissie,
wat zich vertaalt in een hogere CCT waarde.
Figuur 18 Vervaltijdcurve van de
4F9/2 →
6H13/2
transitie (Dy3+
) van YVO4:Eu3+
(0,5%), Dy3+
(2,5%)
na warmtebehandeling met een enkelvoudige
exponentiële fit, geëxciteerd bij 310,9 nm.
De vervaltijd bedraagt 188 µs voor de 4F9/2
→6H13/2 transitie en 484 µs voor de
5D0→
7F2
transitie.
De CIE-coördinaten van dit materiaal voor
warmtebehandeling zijn x = 0,4062, y =
0,3900 met CCT = 3474 K. Na
warmtebehandeling bedragen de coördinaten x
= 0,4769, y = 0,4269 en is de CCT = 2598 K.
Figuur 19 CIE-diagram van YVO4:Eu
3+(0,5%),
Dy3+
(2,5%) voor warmtebehandeling (links), na
warmtebehandeling op 900°C (rechts) en een pure
witte LED (CCT = 2700 K) (midden) (38).
Deze coördinaten zijn meer verschoven naar
de oranje kant van het CIE-diagram. Niettemin
kan dit beschreven worden als wit licht. Ter
vergelijking zijn de coördinaten van een pure
witte LED met CCT = 2700 K ook
weergegeven in het CIE-diagram. De
coördinaten van deze pure witte LED zijn
ontwikkeld op basis van een emissiespectrum
dat gefit werd met drie Gaussiaanse functies
die afkomstig zijn van experimentele data (38).
Het is duidelijk dat dit materiaal voor en na
warmtebehandeling wit licht uitzendt.
Uiteraard toont het materiaal na
warmtebehandeling een veel intensere witte
emissie.
Foto’s van de vanadaat-materialen
Figuur 20 Foto van YVO4:Eu
3+(0,5%), Dy
3+(2,5%)
na warmtebehandeling onder de UV lamp,
geëxciteerd op 254 nm (links) en 302 nm (rechts).
xi
Samenvatting
In dit artikel werden Y2WO6 en YVO4
matrices gesynthetiseerd via een
hydrothermale route met behulp van glycerol
als ligand. De materialen werden
gekarakteriseerd met Diffuse Reflectantie
Infrarood Spectroscopie en X-stralen diffractie.
De matrices werden (co-)gedoteerd met
verschillende lanthaniden die emitteren in het
visuele gebied. Verscheidene materialen die
wit licht uitzenden voor en na
warmtebehandeling werden gesynthetiseerd.
De kleurtemperatuur van de verschillende
materialen varieerde van koud wit (8061 K) tot
warm wit (2598 K).
Dankwoord
Ik zou graag prof. Van Deun bedanken om mij
de kans te geven mijn thesis te maken in zijn
onderzoeksgroep. Ik wil ook mijn begeleidster
Anna Kaczmarek bedanken voor alle hulp
gedurende dit jaar en voor het uitvoeren van de
SEM metingen. Ook wil ik Tom Planckaert
bedanken voor het opnemen van de vele XRD
diffractograms.
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Table of Contents
Acknowledgements ......................................................................................................................
1 Introduction ........................................................................................................................ 1
2 Literature study: rare-earth tungstates ............................................................................. 5
2.1 Tungstate materials ..................................................................................................... 5
2.2 Glycerol as a structure directing agent ........................................................................ 6
2.3 Rare-earth tungstates as a white light source ............................................................. 9
3 Literature study: rare-earth vanadates ........................................................................... 12
3.1 Vanadate materials .................................................................................................... 12
3.2 Rare-earth vanadates as a white light source ........................................................... 16
4 Synthesis and characterization of rare-earth tungstates ................................................ 19
4.1 Synthesis .................................................................................................................... 19
4.2 Performed reactions .................................................................................................. 19
4.3 Diffuse Reflectance Infrared Fourier Transform Spectra .......................................... 20
4.4 X-ray diffractograms .................................................................................................. 21
4.5 Scanning electron microscopy images ....................................................................... 23
4.6 Luminescence properties ........................................................................................... 24
4.6.1 Y2WO6:Eu3+(2.5%), Tb3+(2.5%) before heat treatment – 55a ................................... 26
4.6.2 Y2WO6:Dy3+(1%) after heat treatment – 56c ............................................................ 28
4.6.3 Y2WO6:Eu3+(3%) before heat treatment – 61a ......................................................... 30
4.6.4 Y2WO6:Sm3+(2.5%), Tb3+(2.5%) before heat treatment – 65a .................................. 32
4.6.5 Emission maps .......................................................................................................... 34
5 Synthesis and characterization of rare-earth vanadates ................................................ 37
5.1 Synthesis .................................................................................................................... 37
5.2 Performed reactions .................................................................................................. 37
5.3 Diffuse Reflectance Infrared Fourier Transform Spectra .......................................... 38
5.4 X-ray diffractograms .................................................................................................. 39
5.5 Scanning electron microscopy images ...................................................................... 41
5.6 Luminescence properties .......................................................................................... 44
5.6.1 YVO4:Dy3+(3%) before heat treatment – 43a ..................................................... 46
5.6.2 YVO4:Eu3+(2.5%), Tb3+(2.5%) before heat treatment – 50a ............................... 48
5.6.3 YVO4:Eu3+(2%), Dy3+(3%) before heat treatment – 58a ..................................... 50
5.6.4 YVO4:Eu3+(0.5%), Dy3+(2.5%) after heat treatment – 69c .................................. 52
5.6.5 Emission maps ......................................................................................................... 54
6 Conclusion ........................................................................................................................ 57
7 Bibliography ..................................................................................................................... 59
Appendix A .............................................................................................................................. 63
Experimental ........................................................................................................................ 63
1 Used chemicals .......................................................................................................... 63
2 Hydrothermal synthesis of rare-earth tungstates and vanadates ............................ 63
3 Microwave synthesis of rare-earth tungstates and vanadates ................................. 64
4 Sol-gel synthesis of rare-earth tungstates and vanadates ........................................ 64
5 Overview of all performed syntheses ....................................................................... 65
Appendix B ............................................................................................................................... 67
Equipment and techniques .................................................................................................. 67
1 Scanning electron microscopy (SEM) ........................................................................ 67
2 Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) .................. 67
3 X-ray diffraction (XRD) .............................................................................................. 68
4 Luminescence setup .................................................................................................. 68
5 Commission Internationale de l’Éclairage (CIE) ........................................................ 71
6 Hydrothermal synthesis ............................................................................................. 71
7 Microwave synthesis.................................................................................................. 72
8 Sol-gel synthesis ......................................................................................................... 72
9 Microemulsion synthesis ........................................................................................... 72
10 Molten salt synthesis ................................................................................................. 72
11 Solid state reaction .................................................................................................... 72
12 Czochralski method ................................................................................................... 72
Appendix C ............................................................................................................................... 73
Diffuse Reflectance Infrared Fourier Transform Spectra ..................................................... 73
Appendix D ............................................................................................................................... 78
X-ray diffractograms ............................................................................................................. 78
1 Rare- earth vanadates ................................................................................................ 78
2 Rare-earth tungstates ................................................................................................ 78
Appendix E ............................................................................................................................... 80
Luminescence data ............................................................................................................... 80
1 Correction for detector sensitivity ............................................................................. 80
2 Luminescence spectra ................................................................................................ 81
1
1 Introduction
Green and energy saving technologies are a big issue nowadays. The use of energy efficient
lighting devices is therefore a topic that attracts a lot of interest. Incandescent light bulbs are
being phased out. Instead, compact fluorescent lamps and fluorescent tubes are used. They
contain various phosphors that emit at different wavelengths in the visible range. A lighting
device contains a combination of phosphors that emit different colours, which combined,
generate white light. Synthesizing a single material that emits a natural white light is a
challenge, since both the efficiency and the decay time of this material need to be
maximized. A material that emits natural white light can be used in light emitting diodes
(LEDs). In these devices, electrons and holes are recombined under the influence of an
electric field, which causes emission (electroluminescence). It is likely that in the future the
current trichromatic fluorescent tubes will be phased out in favour of LEDs1.
The lanthanides are the group of 14 elements from atomic number 58 to 71 that contain 4f-
orbitals. The rare-earth term refers to the group of elements containing the lanthanides, as
well as the elements yttrium and scandium, since their properties resemble the f-elements
more than the d-elements. When going over the lanthanide series, the f-orbitals are filled.
As they are filled, these orbitals contract and penetrate the xenon-core appreciably. The 4f-
orbitals are shielded from the environment by the 5s- and 5p-orbitals. Because of this
characteristic, the 4f-orbitals don’t participate in bonding, since they cannot overlap with
ligand orbitals. All lanthanides favour to adopt the 3+ oxidation state, although other states
are possible. An interesting feature of most lanthanide ions is their strong luminescence.
Down-conversion luminescence is the process where after excitation (mostly in the UV
region), the lanthanide emits electromagnetic radiation (in the UV, VIS and/or NIR region)
from an excited electronic state to a lower-lying electronic state. These transitions are
summarized in energy level diagrams (Dieke diagram, Appendix B). Every lanthanide has its
own specific narrow-line emission. According to the Laporte selection rule, transitions
between energy levels of the same parity are forbidden, but the 4f → 4f transitions still
occur. Lanthanides can absorb radiation if a high power excitation source, such as a laser, is
used. Absorption can also occur via ‘the antenna effect’. This is a mechanism where the
ligands harvest the energy and then transfer it to the lanthanide. For this mechanism to
2
occur, the absorption level of the antenna should be slightly higher than the emitting level of
the lanthanide. The energy can be released after the lanthanide has accepted it. Although
this transition is forbidden, emission of radiation will occur when that excited state cannot
return to the ground state radiationlessly. The lanthanides can also be brought in the excited
state by upconversion luminescence11, 63. This is a mechanism where two or more photons of
low energy (long wavelength) are absorbed, after which emission of a high energy (short
wavelength) photon occurs. Usually, this excited state only lives for a very short time (in the
µs-ms range). The decay time of these materials can be determined by measuring the
emission intensity as a function of time. Characteristic decay curves are then obtained.
Quantum yield measurements can be performed to investigate the luminescence efficiency.
More specific information regarding the luminescence can be found in Appendix B.4.
The aim of this thesis is to develop a material that emits white light. For this purpose, rare-
earth tungstates have been shown to be very promising materials4, 5, 7, 15, 20. The tungstate
matrix emits a bluish light itself. When doping these materials with lanthanide ions, which
show a narrow-line emission, it is possible to generate a material that produces white light.
Table 1.1 The most important emissive electronic transitions for the most common luminescent Ln3+ ions in the visible range. The
colours between brackets are less intense colours1.
Ln3+ Transition Wavelength (nm) Wavenumber (cm-1) Transition colour Eu3+ 5D0 → 7F0 578 17300 (Orange)
5D0 → 7F1 590 16950 Orange 5D0 → 7F2 615 16250 Orange 5D0 → 7F3 651 15350 (Red) 5D0 → 7F4 692 14450 Red
Tb3+ 5D4 → 7F6 488 20500 Blue 5D4 → 7F5 545 18350 Green 5D4 → 7F4 588 17000 (Yellow) 5D4 → 7F3 621 16100 (Orange)
Sm3+ 4G5/2 → 6H5/2 560 17850 Yellow 4G5/2 → 6H7/2 599 16700 Yellow 4G5/2 → 6H9/2 645 15500 Red 4G5/2 → 6H11/2 704 14200 Red
Dy3+ 4F9/2 → 6H15/2 478 20900 Blue 4F9/2 → 6H13/2 573 17450 Yellow 4F9/2 → 6H11/2 662 15100 (Red)
Pr3+ 3P0→3H4 483 20700 Blue
3P0→3H5 538 18600 (Green)
3P0→3H6 610 16400 Orange
3P0→3F2 635 15750 Red
3P0→3F3 694 14400 (Red)
3P0→3F4 719 13900 (Red)
Tm3+ 1D2→3F4 449 22250 Blue
3
Various articles regarding the use of tungstate, molybdate or vanadate matrices have
already been published4, 12, 19, 62. In our group, Kaczmareck et al.6 have already synthesized a
white light emitting phosphor. Molybdate and tungstate matrices have already been
compared before4, but no articles where tungstate and vanadate matrices have been
compared are known at this point. Therefore, rare-earth vanadates have also been
synthesized for comparison purposes. Rare-earth vanadates are also promising as white light
emitting materials62, 63, 68.
In order to obtain materials that emit natural white light with a long decay time and a high
efficiency, it is important to find the synthesis conditions for a rare-earth phosphor
compound that forms only one phase instead of a mix of phases. A uniform material of one
phase is essential in the case of obtaining white light, since the operational lifetimes of the
two materials can be different. Over time this may lead to an unwanted shift in the emission
colour. Furthermore, a single phase material facilitates the recycling process. Initially, the
optimal reaction conditions have to be obtained. The influence of the reaction temperature,
reaction time, pH, rare-earth to tungstate/vanadate ratio and the amount of structure
directing agent have been investigated for this purpose. Structure directing agents (SDAs)
are used to tune the size of the nanoparticles and to stabilize the building blocks. Different
SDAs or different amounts of an SDA can lead to a different morphology4. The mechanism by
which these materials are formed during the synthesis is not yet clear, but different
mechanisms have already been suggested by performing time resolved syntheses3, 8, 70.
Various structure directing agents are available on the market, mostly using the same
materials: citric acid, bis-(2-ethylhexyl)sulfosuccinate (AOT), polyvinylpyrrolidone (PVP),
cetyltrimethyl ammonium bromide (CTAB) and others. Particles with different morphologies
have already been synthesized, from flower-like to spindle-like and even dumbbell-like. In
this thesis, glycerol has been used as an SDA. The use of glycerol in this manner has only
been reported once for the synthesis of rare-earth tungstate materials2. There is no record
of using glycerol as an SDA for the synthesis of rare-earth vanadates. Glycerol is an
interesting choice of material, since it is environmentally friendly, especially compared to
other SDAs that are being used. Furthermore glycerol is not toxic, since it is used in the food
industry and it is found in great quantities in soap and detergents to protect the skin. It is
also a material that mixes well with water2, 61. The performed reaction is a hydrothermal
4
synthesis with water and glycerol as the solvents, which is also environmentally friendly.
Glycerol has already proven to be a good structure directing agent for the synthesis of
nanosized lanthanum tungstate materials with absolute quantum yields of up to 69.3%2.
After the synthesis, the material is characterized through powder X-ray diffraction to
investigate if the synthesis resulted in a one phase or a mixed phase material. The obtained
diffractogram can be compared with standard JCPDS data2-5 to investigate which phase of
the material has been formed. Based on the ICSD crystallographic database, a powder XRD
spectrum can also be calculated. This is another way to confirm the phase that was
synthesized. It is not uncommon for certain reaction conditions to lead to the formation of a
mixed phase material49. In many cases, a pure phase material can only form under specific
conditions, which makes finding the right phase difficult.
The morphology is investigated by scanning electron microscopy. The resulting images show
the size, shape and morphology of the particles. When an optimal matrix is successfully
obtained, this material can be doped with various lanthanide ions and the luminescence
properties can be studied. The material can be singly doped, but co-doping with multiple
lanthanides is also possible, especially with the aim of obtaining a material that emits white
light. The observed colour of the materials can be seen under a UV-lamp, although the
wavelength of the UV lamp has an influence on the emitted colour. Because of this, the real
colour of a material cannot be defined unambiguously by the naked eye. To display the real
colour of the materials, the ‘Commission Internationale de l’Éclairage’ (CIE) presented an
absolute way to define colour. The real colour of a material can be plotted in a so-called
chromaticity diagram (see Fig. 1.1). More information
about the CIE-diagram can be found in Appendix B. Fig.
1.1 displays the positions in the CIE-diagram of generic
pure colour LEDs of which the peak shapes are defined by
Gaussian fits. Each peak consists of a fit of three individual
Gaussians that were derived by fitting to experimental
data84.
Figure 1.1 CIE 1931 chromaticity plot. The different markers show the generic pure colour white LEDs with different correlated colour
temperatures. The properties are; a: CCT = 5700 K, x = 0.3275, y = 0.3577; b: CCT = 4000 K, x = 0.3770, y = 0.3677 c: CCT = 2700 K, x =
0.4610, y = 0.4110. The diagram and coordinates are calculated via the ColorCalculator 4.97 program by Sylvania84.
5
2 Literature study: rare-earth tungstates
2.1 Tungstate materials
Rare-earth tungstates have already been proven to be functional materials in various
applications; from catalysis to light emitting diodes, lasers, biological labelling …2-22, 45-60, 70
This type of material is also interesting for lanthanide luminescence. Many articles regarding
the luminescence properties of these materials have already been published4. Combination
of an yttrium- and tungstate source can lead to different phases. The most commonly
occurring phases are the Y2(WO4)37 and NaY(WO4)2
4,70 phases, although less common phases
such as Y2WO613, 57, 58, YW2O6(OH)3
53, Y6WO1259 occur as well. These other phases have also
been reported via similar synthesis routes (hydrothermal, microwave, molten salt, sol-gel,
microemulsion), but at different conditions, type and amount of structure directing agent.
The properties of materials depend on their structure and morphology. The luminescence
properties can also be influenced by the structure, since a certain ordering of the particles
can lead to a better energy transfer to the lanthanide-ions. Being able to fine-tune the
different properties is therefore an interesting topic. The most common method to generate
these materials is the hydrothermal route. Advantages of this method are the use of water
as a solvent, low cost and ability to perform it at large scale. However, disadvantages of this
method include the high reaction temperature and the long reaction times (up to 72 h) after
which the autoclaves have to be cooled down slowly to room temperature. These reactions
are performed in steel vessels that are Teflon-lined. From this point of view, microwave
assisted hydrothermal reactions seem to be a good replacement, since the reactions can be
performed at higher temperatures and shorter reaction times are mostly sufficient. More
information about these synthesis routes can be found in Appendix B.
Excitation spectra of rare-earth tungstates show a broad band in the range of 250-350 nm.
This band is called the charge transfer band (CTB) and is assigned to the charge transfer from
the 2p-oxygen to the 5d-tungsten orbitals. Doping these materials with lanthanides leads to
a combination of this blue CTB with the narrow-line emission of the lanthanides, which is a
promising system to obtain white light. By varying the doping concentrations of the
6
lanthanides, or by co-doping, the emitting colour of the material can be fine-tuned to create
a material that emits white light3.
Structure directing agents are used to regulate the morphology. This can be done by
stabilizing the building blocks, slowing down the nucleation, forming stable complexes with
the metal ions and influencing the growth rate of the surfaces6. Various SDAs which form
complexes in solution for creating nanoparticles can be used. The actual mechanism in which
the particles are formed is unknown, but many suggestions have already been made3, 7, 12, 13.
In literature, various morphologies have been published, going from microstars, to
microflowers12,70, bowknot-like29, urchin-like16 and spindle-like2 structures.
2.2 Glycerol as a structure directing agent
In this thesis, glycerol was chosen as a structure directing agent that also acts as a solvent.
The use of glycerol as the structure directing agent for the synthesis of rare-earth tungstates
has only been reported once before, namely, for the synthesis of europium doped
lanthanum tungstates2. The use of glycerol was effective and led to pure-phase tungstates
with high colour purity after doping with lanthanides. Although glycerol has only once been
published as a structure directing agent for the synthesis of rare-earth tungstates, it has
already been used as an SDA for other rare-earth materials such as borates3, oxides24, 29, 31,
fluorides25 and phosphates27. The use of glycerol for this purpose can be motivated by the
fact that this is a non-toxic material that is even edible. Compared to other SDAs, it is easily
removed by washing with water and/or heat treatment, which prevents it from quenching
the luminescence. This material also mixes well with water, which can be a stable system for
directing the structure of the micromaterials. This makes glycerol an interesting material for
further research regarding structure directed rare-earth tungstates.
Zhenling et al.25 reported the synthesis of europium doped lanthanum fluoride nanoparticles
in a glycerol/water (1:1) reaction medium via a refluxing method. They stated that the
presence of glycerol in the reaction medium can alter the solubility and thus improve the
crystallinity of the material. Because glycerol mixes well with water, it acts as a solvent
during the reaction, but also as a chelating agent since its three OH-groups can form
coordinative bonds with the metals present in the solution. This polar OH-functionality can
be capped on the surface of the material.
7
The SEM image in Fig. 2.1 shows that the material consists of spherical particles of around 30
nm diameter when glycerol was used in the reaction medium.
Yin et al.24 synthesized Eu2O3-doped Y2O3 phosphors via a
solvothermal reaction and investigated the use of water,
ethanol, ethylene glycol and glycerol as reaction media.
The materials with glycerol as a reaction medium showed
an amorphous structure by investigating the XRD
diffractogram. However, after heat treatment the
intensity of the reflections increased, which means that
the crystallinity of the sample increased by heat treating. Moreover, raising the heat
treatment temperature caused the crystallinity to increase even further. Samples prepared
by the solvothermal method with glycerol as a reaction medium led to well-dispersed, near-
spherical particles. The sample showed visible luminescence, although the intensity of the
transitions was much lower when compared to using water as the reaction medium. In this
case, the use of glycerol did not enhance the luminescence intensity, but it is clear that the
use of a high viscosity solvent can effectively alter the morphology of the nanoparticles.
Figure 2.2 Transmission electron microscopy (TEM) images of the materials prepared in a 90% glycerol solution before heat treatment
and after heat treatment at different temperatures24.
The spherical morphology of the materials can be seen in the transmission electron
microscopy (TEM) images. Glycerol is a solvent with a high viscosity. This leads to a low
diffusion rate of the ions in this product and suggests the formation of spherical particles
because crystalline growth was suppressed24.
Eu3+-doped lanthanum tungstates were synthesized by Liu et al.2 via a hydrothermal route.
After the synthesis, spindle-like NaLa(WO4)2 nanoparticles were obtained. However, after
centrifugation and heat treatment at 600°C for 2 h, La2(WO4)3 nanocrystals were obtained.
Fig. 2.3 shows the TEM and SEM images of La2(WO4)3. The average particle size is 100 nm.
The Selected Area Electron Diffraction (SAED) pattern confirms that this is a crystalline
material. Spindle-like NaLa(WO4)2 microcrystals with a length in the range of 2-3 µm are
obtained when not heat treated.
Figure 2.1 SEM image of europium doped
lanthanum fluoride nanoparticles25.
8
To obtain more insight in the growth mechanism of these materials, time-dependent
experiments can be performed. An example of the time dependent syntheses and their
corresponding SEM images is presented in Fig. 2.4.
Figure 2.4 SEM and TEM images of NaLa(WO4)2 synthesized
at 180°C in the presence of 10 mL glycerol for different time
intervals: (a, b) 2 h, (c, d) 5 h, (e, f) 9 h, (g, h), 24 h2.
A short reaction time (2 h) leads to an
amorphous product, which was
determined by X-ray diffraction.
Increasing the reaction time led to the
formation of spindle-like structures and
transformation from an amorphous
phase to a crystalline phase. After 5 h,
the length of the spindles was 1.5 µm to
2.0 µm. Longer reaction times further
increased the size of the spindles to 3
µm. A dissolution nucleation
Ostwald ripening formation mechanism
was suggested. Addition of the tungstate
to the rare-earth elements in glycerol led
to precipitation. Dissolution occurred
during the hydrothermal treatment. The
nuclei were formed during the hydrothermal treatment and then grew to become
nanoparticles through Ostwald ripening (small particles dissolve and are deposited on large
particles, which then become larger).
Figure 2.3 (Left) TEM images of La2(WO4)3 nanocrystals: a) overall view, b) detailed view, c) SAED pattern recorded at b), d) High
resolution TEM image recorded within the black box of b). (Right) SEM and TEM images of spindle-like NaLa(WO4)2 microcrystals: a)
panoramic SEM image, b) TEM image, C) TEM image of an individual spindle, d) HRTEM image recorded within the white box of c)2.
9
Figure 2.5 The possible growth mechanism of a) La2(WO4)3 and b) NaLa(WO4)2 nano-/microcrystals suggested by Liu et al.2.
The influence of the amount of glycerol at different reaction temperatures was investigated
and all conditions led to the same conclusion. Larger amounts of glycerol in the reaction
system led to smaller nuclei since they would be capped more.
Excitation and emission spectra were recorded for the europium doped La2(WO4)3 materials.
A broad W-O charge transfer band was present together with the characteristic narrow
peaks for Eu3+. Liu et al. also measured the absolute quantum yield using an integrating
sphere (excited at 467 nm). They calculated the quantum yield as 69.3%.
2.3 Rare-earth tungstates as a white light source
White light can be created by combining specific emissions of the primary colours (red,
green and blue). Since the human eye can perceive colours in different ways (lighting for
example), a standard has been created by the ‘Commission Internationale de l’Éclairage’
(CIE) to define the actual colour of a material. The CIE coordinates for standard white light
are x = 0.33, y = 0.33. The actual colour can be presented in a chromaticity diagram.
The tungstate matrix emits blue light, which means that only the red and green parts are
necessary in order to obtain white light. Co-doping tungstates with europium (emits red) and
terbium (emits green) is a possible combination for white light generation.
Zeng et al.3 succeeded in creating a white light emitting Gd2(WO4)3 material by co-doping it
with Eu3+ and Tb3+ via a hydrothermal process. The type of lanthanides as well as their
doping concentrations can have a large effect on the true colour of the material, which is
depicted in Fig. 2.6.
10
The colours of the samples vary from green (a) to warm white (d) and red (g). The CIE
coordinates of sample (d) were calculated as x = 0.368, y = 0.370, which is very close to
standard white light. This slight deviation from standard white light leans toward the red
direction and is perceived as warmer white light.
Co-doping with europium and gadolinium can also lead to a white light emitting material, as
has been reported by Lei et al.20. They synthesized NaGdWO4(OH)x and doped it with Eu3+
and Gd3+. A Eu3+/Gd3+ = 0.02 ratio led to a white light emitting material. The gadolinium ions
do not contribute to the emission colour. This is because no 4f-4f transitions occur with
gadolinium since it has a stable half-filled 4f-shell. The Gd3+-ions are used as sensitizers to
effectively transfer energy to the Eu3+-ions.
Co-doping is not always necessary when trying to obtain a white light emitting phosphor.
Huang et al.7 described the synthesis of a europium doped Y2(WO4)3 material via a
hydrothermal method. Doping this material only with europium and altering the doping
concentration led to the following results.
Figure 2.6 Emission spectra of Eu3+/Tb3+ co-doped Gd2(WO4)3 microstars with different co-doping concentrations. Below are the
luminescence photographs of the samples irradiated under a UV lamp (10 W)3.
11
The material with only 1% Eu3+ doping shows a blue colour, resulting from the tungstate that
emits in the blue region. Increasing the doping concentration leads to a white light emitting
tungstate through the combination of blue light (tungstate) and red light (europium).
Increasing the Eu3+ concentration even further leads to a dominant red emission. The CIE
coordinates of sample (B) are x = 0.323, y = 0.280, which is located in the white light zone.
This example clearly shows the advantages of the tungstate matrix that already emits in the
blue region.
In our group Kaczmarek et al.6 synthesized a white light emitting 5% dysprosium doped
Y(WO3)2(OH)3 material. They also showed that heat treatment of the sample can have an
influence on the colour. The CIE coordinates of the material are x = 0.29, y = 0.32 before heat
treatment and x = 0.39, y = 0.42 after heat treatment. The colour changed from near-white
light to yellowish orange light.
Under UV excitation a white light emitting phosphor has also been reported by Lei et al.15.
They report the synthesis of a Eu3+ doped NaY(WO4)3 material with molar ratio Eu3+/Y3+ =
0.02. The CIE coordinates were not reported for this material.
Zheng et al.59 published a europium doped NaY(WO4)2 material that was heat treated at
700°C. From their research it is clear that heat treatment has a large influence on the colour
of the material. CIE coordinates before and after heat treatment are x = 0.3145, y = 0.2663
(near white) and x = 0.6049, y = 0.3234 (red). They also report that the colour of the material
can easily be tuned from blue (1% Eu3+ doping) to white (5% Eu3+ doping) to red (10% Eu3+
doping).
Figure 2.7 (A) Emission spectra and (B) CIE chromaticity diagram of Y2(WO4)3 doped with different Eu3+ concentrations7. The doping
percentages are assigned in the following way; (A) 1% Eu3+, (B) 3% Eu3+, (C) 5% Eu3+, (D) 7% Eu3+, (E) 10% Eu3+.
12
3 Literature study: rare-earth vanadates
3.1 Vanadate materials
Yttrium orthovanadate (YVO4) is a crystalline material with a zirconia structure. This material
can be used as a laser host material (e.g. when doped with Nd3+), a polarizer and in colour
television (when doped with Eu3+). Yttrium orthovanadates are also potential materials for
light emitting diodes (LEDs)49, 62. LEDs might be the future for efficient lighting that emits a
warm white light. Nowadays, white light is created by a combination of a 450-470 nm blue
nitride LED and a yellow-emitting phosphor (usually YAG:Ce3+). However, this combination
leads to a cold white light since the yellow-emitting phosphor also has a strong green
emission. For the creation of warm white light, a material that emits a better yellow light, or
addition of a red emitting phosphor could be the solution. However, for recycling
afterwards, one material that emits white light still is a better option.
The combination of an yttrium and vanadate source can lead to different phases, depending
on the reaction conditions. The most common phase is YVO448, but other phases like YV3O9
or Y2V10O28 have also been reported50.
Many articles regarding the synthesis and luminescent properties of rare-earth yttrium
orthovanadate materials have been published48-53, 62-68. In most cases a hydrothermal route48
is used, but other synthesis methods are employed regularly, such as the solid state
reaction62, the microemulsion route68, a microwave assisted process66 and the Czochralski
method52, with and without the use of structure directing agents. The use of these different
methods leads to various particle sizes. For the hydrothermal and microwave assisted
method, published particle sizes were 25-30 nm and solid state reactions led to particles of
sizes 50-80 nm with the help of an organic template. The use of an organic template clearly
prevents the particles from aggregating and is thus necessary for creating monodispersed
nanoparticles. It has been reported that the microemulsion route leads to particle sizes of
around 20 nm when heat treated at 500°C, 30 nm at 900°C and 80 nm when heat treated at
1100°C. Heat treating the particles clearly has an effect on the particle size. The smallest
sizes are obtained when using a hydrothermal route or a microwave assisted route.
13
As is clear from the SEM and TEM images in Fig. 3.1 and 3.2, the use of a different reaction
method has a large impact on the size and morphology of the particles.
Figure 3.3 Emission spectra of YVO4:Eu3+ phosphors prepared by various methods51.
The reaction method influences the size and shape of the particles. These properties can
have an influence on the luminescence efficiency. From the emission spectrum in Fig. 3.3, it
is clear that in this case the use of a template led to particles with higher luminescence
efficiency. Preparation of these particles via a sintering method showed the lowest
efficiency. The overall quantum yields were calculated to be 10.1% (R1), 8.8% (R2), 3.2% (B)
and 1.9% (C).
Not only the reaction method, but various reaction conditions can influence the size and
properties of these particles. A reaction parameter with a large influence on the formation
of a pure-phase material is the pH. Wu et al.49 researched the influence of the pH and many
Figure 3.2 High resolution TEM images of
YVO4:Eu3+ with a rod-like morphology and size
of 20-30 nm, prepared using a microwave
assisted method66.
Figure 3.1 Low-magnification (a) and high-magnification (b) SEM images of undoped
cereal-like YVO4 with particle size of 31-32 nm prepared by a hydrothermal route48.
14
other reaction conditions with the aim of creating a pure-phase material using a
hydrothermal method.
Scanning the whole pH range led to the conclusion that a pure-phase YVO4 can only be
formed at a certain pH and Y(NO3)3∙6H2O to Na3VO4 ratio. Wu et al. reported that in acidic
conditions the Y:V ratio had to be 1 in order to obtain a pure phase and at basic conditions
the ratio had to be <1. Their results are presented in Table 3.1
Table 3.1 Hydrothermal reaction products from the Y2O3-V2O5 system at 220°C for 4 days via a hydrothermal route 49.
Y:V ratio Acidic medium (pH < 7 ) Basic medium (pH > 7)
>1.0 YVO4 + Y8V2O17 YVO4 + Y(OH)3 1.0 YVO4 YVO4 + Y(OH)3
a <1.0 YVO4 + V2O5 YVO4
aunder much stronger alkaline conditions
By performing reactions in acidic and basic media at different reaction times and following
them via XRD diffractograms, the mechanism for the creation of YVO4 was suggested by Wu
et al. In an acidic medium, the yttrium is present as -cations while the vanadium is
present as anionic oligomers. Via an acid-base reaction between these species, the yttrium
orthovanadate is created ( ) In basic conditions, the
yttrium is present as ( ) and the vanadium is present as anions. This transformation is
believed to follow the next reaction path: ( ( )
).
Figure 3.4 The generation of YVO4 nanoparticles via an acidic and basic route49.
Reactions can be employed with different reaction times. However, a reaction time shorter
than 24 h still contained the starting material V2O5, which means that the reaction is not yet
complete. The reaction temperature also seems to be an important factor since a reaction at
180°C had not finished after 48 h.
15
The presence of organic chelating agents has also been investigated. These molecules can
influence the nucleation and the growth of the particles. The use of a different molecule can
result in a different shaped particle. Nanoflakes4, nanorods4 and nanowires5 are common,
but more complex shapes such as cereal-like48 or rod-like66 also occur. Many different
structure directing agents have been used to control the formation of YVO4 nanoparticles,
although glycerol has not yet been reported as an SDA for the synthesis of YVO4. Particles
that are uniform and ordered in a repetitive way are essential for doping with lanthanides
and obtaining materials with a high efficiency luminescence and a long operational lifetime.
Li et al.48 reported cereal-like YVO4 particles synthesized via a hydrothermal method for high
quantum efficiency photoluminescence. They doped YVO4 in turn with samarium, europium,
terbium and dysprosium.
Figure 3.5 Excitation spectra (left) of cereal-like architectures of YVO4 and YVO4:Ln3+ (Ln = Sm, Eu, Tb, Dy), which were recorded when
monitored using λem = 602 nm for Ln = Sm, 618 nm for Ln = Eu, 542 nm for Ln = Tb , 573 nm for Ln = Dy, and 480 nm for YVO4. Emission
spectra (right) of cereal-like YVO4 and YVO4:Ln3+ (Ln = Sm, Eu, Tb, Dy) excited using a single wavelength light of 320 nm. Top: the eye
visible luminescence photographs of samples excited under irradiation of 254 nm48.
All materials exhibited a V-O charge transfer band (CTB) at a wavelength of 320 nm with
variable intensities. The materials were excited through this CTB to monitor the emission
and see how effective the energy transfer from the vanadate groups to the lanthanides was.
The terbium sample did not show any luminescence when excited at 320 nm. According to Li
et al.48, this originates from an effect that causes the UV excitation energy to migrate
through the VO43- groups to a quenching site and dissipate immediately in a non-radiative
way. They claim that this energy migration is so efficient that it leads to no visible emission.
This migration rate can be reduced by introducing defect centres which will lead to visible
emission.
16
The fact that Li et al. did not see any visible luminescence for the terbium sample might
originate from the low amount of defect centres present in the sample. No other articles
where Tb3+ was doped into an YVO4 matrix were found.
Via calculation methods, the internal quantum efficiencies of the 5D0 level of europium and
dysprosium were estimated to be 14.6% and 11.4%. Although these quantum efficiencies
were calculated and not measured in an absolute way, they are superior to efficiencies that
have been reported for these materials, which is very promising. These particles are known
to have two decay times48, 68. These two lifetimes can be explained by assuming the particle
is spherical, since the particle can then be described as having an inner and outer shell with
the same volume. The ions in the inner shell are known to have longer lifetimes than ions in
the outer shell. The average lifetime of these materials were calculated to be 98 µs (Eu3+)
and 50 µs (Dy3+). Unfortunately, there are few opportunities for comparing the lifetimes,
since most articles do not report them.
3.2 Rare-earth vanadates as a white light source
Yttrium orthovanadate materials have already been proven to be good hosts for lanthanide
luminescense48-53. The good host properties of YVO4 combined with the sharp visible
emission of lanthanides are promising for creating a material that emits white light. Yttrium
orthovanadate materials that emit a yellow and almost-white light have already been
reported62, 63, 68, although these materials can still be improved to obtain a warmer white
light with higher luminescence efficiencies and longer lifetimes.
Multiple ways exist to create one material that emits white light. Zhi-Peng et al.62 created a
yellow emitting phosphor at the edge of white light by co-doping YVO4 with Dy3+ and Bi3+.
The combination of dysprosium and bismuth is promising for white light emission, since
dysprosium has two strong emission bands, one in the blue region (470-500 nm) and one in
the yellow region (560-600 nm). Bismuth was used as a sensitizer, since it is known to
efficiently transfer the energy to the rare-earth. Raising the doping percentage of
dysprosium increased the luminescence intensity, although a maximum is reached at a
doping percentage of 3 mol%. Higher dysprosium doping led to concentration quenching.
The optimal bismuth doping percentage was found to be 10 mol%. The luminescence
intensity was effectively increased by co-doping with bismuth.
17
The chromaticity diagram clearly shows that the
yellow emitting phosphor is situated on the edge
of the white light region. The CIE coordinates of
YVO4: Dy3+(3%) (1) are x = 0.401, y = 0.438.
Doping with bismuth leads to a purer yellow
colour (2), even more pure than the greenish
yellow the YAG:Ce3+ emits. Therefore, it is a
promising material for replacing the YAG:Ce3+ in
white LEDs. However, doping this material (1) with red or blue emitting lanthanides might
lead to a warm white light emitting phosphor.
Another possibility for creating a white light emitting yttrium orthovanadate was proposed
by Luwang et al.68. They combined the sharp yellow and blue emission of dysprosium with
the blue emission of thulium and the sharp red emission of europium. The doped
lanthanides can act as activators, but also as sensitizers by transferring the absorbed light to
the activators. The singly doped europium, dysprosium and thulium yttrium orthovanadates
were synthesized and compared with the emissions of the triply doped yttrium
orthovanadate at different heat treating temperatures. The photoluminescence spectra of
the triply doped yttrium orthovanadate with and without silica shell at heat treating
temperatures of 500°C and 900°C are presented below55. The inset shows that the materials
heat treated at 500°C have much lower luminescence intensity than the materials heat
treated at 900°C. This is probably due to the decrease of the non-radiative transition
probability caused by the surface of the sample. The spectra show the characteristic
emissions of europium, dysprosium and thulium.
Figure 3.6 CIE chromaticity diagram of YVO4:Dy3+ (3%) (1),
Y0.87VO4:Dy3(3%)+, Bi3+(10%) (2) and YAG:Ce3+ (3)62.
Figure 3.7 Photoluminescence spectra of triple lanthanide
ions (Ln3+ = Eu3+, Dy3+ and Tm3+) doped YVO4 and
YVO4@SiO2 heat treated at different temperatures and
monitored at λex = 300 nm (C500 and CS500), λex
=320
(C900) and λex =310 (CS900). The ‘C’ stands for heat treated
samples and the ‘S’ stands for silica coated particles with
the help of TEOS.68.
18
The CIE coordinates for the triply doped yttrium orthovanadate that was heat treated at
900°C are x = 0.42, y = 0.40, which is quite close to white light. However, the corresponding
core-shell sample of this material, heat treated at 500°C, has CIE coordinates x = 0.31, y =
0.32, which is standard white light. The other materials also have high potential as a white
light emitter, possibly by varying their doping percentages or reaction conditions (heat
treating temperature).
Although the core-shell triply doped yttrium orthovanadate material heat treated at 500°C
emits white light, the decay times of these materials are quite low (2.4 µs and 13.1 µs)
compared to the samples that were heat treated at 900°C (87.5 µs and 18.0 µs). Heat
treatment at a higher temperature enhances the decay times significantly. This effect is
explained by the SiO2 that forms a protective layer on the surface of the nanoparticles,
which isolates the lanthanides from the quenching groups that might be present on the
surface.
Doping YVO4 with europium, dysprosium and thulium seems promising for creating a white
light emitting material. However, fine-tuning of the reaction conditions is necessary, since
small differences such as heat treating temperature and doping percentage can have an
influence on the emitted colour.
Figure 3.8 CIE chromaticity diagram: (a) (A)
and B) Eu3+ doped YVO4; (C and D) Dy3+ doped
YVO4; (E and F) Tm3+ doped YVO4; (G and H)
Eu3+, Dy3+ and Tm3+ doped YVO4. Samples A, C,
E and G are heat treated at 500°C, samples B,
D, F and H are heat treated at 900°C. (b) Their
corresponding core-shell samples
(YVO4:Ln3+@SiO2) 55.
19
4 Synthesis and characterization of rare-earth tungstates
4.1 Synthesis
Most materials were synthesized under hydrothermal conditions. For comparison,
microwave and sol-gel reactions were also performed. The reaction pathways and all
performed syntheses are described in Appendix A.
The first challenge in synthesizing rare-earth tungstates is obtaining a pure phase. Some
reaction conditions can lead to a mix of phases, which is not an ideal host material for
luminescence. Different parameters can be varied to try to obtain a pure phase material. The
parameters that have been modified in this thesis are the amount of glycerol, the pH, the
reaction temperature, the rare-earth to ligand ratio and the heat treatment temperature.
The reaction time can also be modified, but for tungstates a 24 h reaction suffices to form a
pure phase4, 6, 7, 13, 14. At shorter reaction times it is not certain that the reaction is already
complete. This can be investigated by performing time resolved syntheses4, 6-8, 70.
Several conditions have been tested to try to obtain a pure phase. At first, the conditions
were chosen at random. When favourable XRD results were obtained, the reaction
conditions were further tuned.
Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) was performed before
and after heat treating the materials to confirm the removal of glycerol and water. X-ray
diffractograms were collected to determine the phase. After phase confirmation, doping
with various lanthanide ions was performed with the goal of obtaining a white light emitting
material.
4.2 Performed reactions
Table 4.1 Performed tungstate reactions with a Y(NO3)3∙6H2O : Na2WO4∙2H2O ratio equal to 1:1 (underlined) and with a Y(NO3)3∙6H2O :
Na2WO4∙2H2O ratio equal to 1:2 (italic). Different reaction temperatures (140-200°C) and amounts of glycerol (0-20 mL) were used.
0 mL 2.5 mL 5 mL 10 mL 15 mL 20 mL
140°C pH = 3, 9 pH = 9
160°C pH = 3 - 10
180°C pH = 6 pH = 6 pH = 6 pH = 3, 6, 6, 9
200°C pH = 11,12,13 pH = 4, 6
20
It was observed that at various reaction conditions no pure phase of a known rare-earth
tungstate material was formed. However, comparison of the XRD diffractograms showed
that higher amounts of glycerol and a higher pH led to sharper reflections. Based on these
findings, a reaction at basic pH (13) and with 20 mL of glycerol was performed. Via a
hydrothermal route, the same reaction conditions were also tested with La(NO3)3∙6H2O and
Gd(NO3)3∙6H2O instead of Y(NO3)3∙6H2O as the rare-earth source, to investigate if this had a
positive influence on the phase formation, as was observed by Liu et al.2.
Table 4.2 Performed tungstate reactions and their synthesis conditions. Different types of syntheses were performed; H: hydrothermal,
S: sol-gel, M: microwave.
# Synthesis T (°C) t (h) pH Glycerol (mL) Y(NO3)3∙6H2O (g) Na2WO4∙2H2O (g)
46 H 200 24 13 20 0.383 0.330 35 S 800 5 13 20 0.383 0.330
67 M 200 1 13 10 0.190 0.165
4.3 Diffuse Reflectance Infrared Fourier Transform Spectra
DRIFTS spectra were recorded before and after heat treatment. Below is an example of an
Y2WO6 material with information about the assigned vibrations. DRIFTS spectra of the other
tungstate materials are similar and can be found in Appendix C.
Table 4.3 Assigned infrared vibrations of undoped Y2WO6 before
heat treatment.
# Wavenumber (cm-1) Vibration
a 3559 O-H
b 1406 O-H / NO3-
c 972 W-O
d 906 W-O
e 815-650 W-O-W
The sharp band at 3559 cm-1 is attributed to the O-H stretch vibrations of water and/or
glycerol. This band disappears after heat treatment. Around 1406 cm-1, a broad band that
can also be attributed to the O-H stretch vibrations of water is visible. However, this band
can also originate from the nitrate salts that have been used as a starting material during the
synthesis14. Below 1000 cm-1 the characteristic tungstate vibrations are visible. Band c and d
are attributed to W-O stretch vibrations and band e was assigned to the asymmetric stretch
vibrations of W-O-W bridges14. The peaks of the material heat treated at 900°C and 1100°C
are similar.
Figure 4.1 DRIFTS spectrum of Y2WO6 before heat treatment
(red), after heat treatment at 900°C (blue) and 1100°C (green).
21
4.4 X-ray diffractograms
After investigating various synthesis conditions, a pure phase Y2WO6 material was obtained.
The materials were first heat treated at
900°C, but no pure phase could be assigned.
It has been reported before that Y2WO6
forms a metastable phase13, 57. Upon heat
treatment at 650°C, Wang et al.13 obtained a
cubic phase. Heat treatment at a higher
temperature (750°C) resulted in a tetragonal
phase and the monoclinic phase was
obtained at 1100°C. This phase
transformation is irreversible, however, the metastable phases are stable at room
temperature. Mostly tungstate materials are heat treated at 900°C or lower, at which a pure
phase already forms2-8. Higher temperatures are not always necessary, but in this case heat
treatment at 900°C led to a mixture of the tetragonal and monoclinic phase. Increasing the
heat treatment temperature to 1100°C led to a pure monoclinic phase Y2WO6 (yttrium
oxytungstate) material.
Reactions were performed with
La(NO3)3∙6H2O and Gd(NO3)3∙6H2O as the
rare-earth source instead of Y(NO3)3∙6H2O
to emphasize how specific these reaction
conditions are. It was observed that the
conditions in which an Y2WO6 material
forms do not necessarily lead to a La2WO6
or Gd2WO6 material when changing the
starting materials. The obtained XRD
Figure 4.2 Powder XRD patterns of sample 46 when heat treated
at 900°C and 1100°C and sample 56 before heat treatment.
These diffractograms are compared with the Y2WO6 powder
pattern calculated from the ICSD crystallographic database87.
Figure 4.3 Powder XRD patterns of a lanthanum (37) and
gadolinium (38) tungstate material after heat treatment at
900°C compared with the powder pattern calculated from the
ICSD crystallographic database88,89.
22
diffractograms are presented in Fig. 4.3. For both materials a dominant phase could be
assigned (that is different from the yttrium oxytungstate phase). However, the spectra are
quite noisy and additional peaks are visible, which means that these materials are no pure
phases. Liu et al.2 synthesized pure phase La2(WO4)3 and NaLa(WO4)2 nanomaterials with
glycerol as a structure directing agent without changing the pH. The diffractogram of sample
37 clearly shows that a La2(WO4)3 phase is present. This is a clear example of how much
influence a difference in pH has on the formation of a pure phase.
The sol-gel type synthesis (35) led to a
mixed phase material. One phase might be
the monoclinic Y2WO6 material, but this
cannot be said with full certainty.
Microwave synthesis led to a material that
consists of very hard granules (all other
materials were powders). The reflections
are quite broad. No phase could be
assigned to this material and the peak
widening is probably due to the low
crystallinity of the material.
The remaining XRD spectra that were recorded can be found in Appendix D.
The optimal reaction conditions that led to a pure monoclinic phase Y2WO6 material were
chosen for doping with various lanthanides to obtain white light. To summarize, these
conditions are a hydrothermal reaction at 200°C for 24 h, at pH = 13, with the use of 20 mL
glycerol and a Y(NO3)3∙6H2O : Na2WO4∙2H2O ratio equal to 1:1.
Figure 4.4 Powder XRD spectra of a sol-gel (35) and microwave (67)
synthesized yttrium tungstate, heat treated at 1100°C.
23
4.5 Scanning electron microscopy images
The morphology of the Y2WO6 phase was investigated via scanning electron microscopy.
Pictures at different magnifications were taken to get insight in the size, shape and
morphology of this material.
Figure 4.5 SEM images of undoped Y2WO6 after heat treatment at 1100°C at different magnifications. (A) 2500x, (B) 5000x, (C) 10000x
and (D, E, F) 20000x.
At a magnification of 2500x (A) it is clear that the particles in this material are not ordered in
a repetitive way. The particles are not uniform since spherical and rod-like particles can be
distinguished. Upon higher magnification (B), it becomes clear that differently sized rods are
present. Some are quite narrow and others are more bulky. At a magnification of 10000x (C),
three sorts of shapes can be distinguished. The smallest visible particles are the spheres. The
narrowest rods are quite bumpy, as if they were created from spheres. The more bulky rods
do not have this bumpy behaviour, but are quite smooth. Intermediate forms can also be
differentiated. During the synthesis, spherical particles were probably formed initially. The
long reaction time presumably led to particles being deposited on top of each other and
forming narrow rod-like structures. In newly formed rods, the individual particles are still
visible, hence the bumpy morphology. As more particles melt onto the rods, the rods
become broader. The edges where the particles were deposited on the rods are smooth.
This probably occurred during heat treatment at 1100°C.
24
4.6 Luminescence properties
White light emitting rare-earth tungstates have already been reported before3, 7, 15, 20, 59, in
our group too by Kaczmarek et al.6.
Based on an extensive literature study of these materials, various doped and co-doped
materials were synthesized in this thesis with the aim of obtaining a white light emitting
material. An overview of all the rare-earth tungstates that were synthesized is presented in
Table 4.4.
Table 4.4 Overview of the synthesized tungstate materials and their doping percentages. All materials were synthesized hydrothermally,
except for sample 67, which was synthesized via a microwave route. The ‘#’ labels the materials before heat treatment with ‘a’ and after
heat treatment with ‘c’. Experiments highlighted in grey are the white light emitting materials that will be discussed in detail.
# Doping Observed colour (UV lamp)
CIE x CIE y CCT CIE
(%) 365 nm 254 nm (K) colour
55a 2.5% Eu3+, 2.5% Tb3+ White White 0.3472 0.3814 5019
55c 2.5% Eu3+, 2.5% Tb3+ Red Orange 0.6300 0.3509 1110
56a 1% Dy3+ White White 0.2196 0.2800 26417
56c 1% Dy3+ White White 0.3826 0.3908 4042
57a 1% Sm3+ White White 0.2282 0.2977 17114
57c 1% Sm3+ White (pink) White (pink) 0.5181 0.3304 1562
61a 3% Eu3+ Brown White 0.3274 0.3124 5771
61c 3% Eu3+ Pinkish (light) White 0.6205 0.3400 1101
62a 3% Sm3+ Brown White (green) 0.2361 0.3069 14220
62c 3% Sm3+ Pink White (green) 0.5508 0.3590 1520
64a 2% Eu3+, 3% Tb3+ White (green) 0.3915 0.4098 3952
64c 2% Eu3+, 3% Tb3+ Red Orange 0.6451 0.3478 1041
65a 2.5% Sm3+, 2.5% Tb3+ White (green) 0.2656 0.3919 8061
65c 2.5% Sm3+, 2.5% Tb3+ Orange (light) Orange 0.6022 0.3781 1347
67a 1% Dy3+ White 0.1881 0.1992 >10000
0
67c 1% Dy3+ White 0.2258 0.1620 >10000
0
Almost all materials presented in Table 4.4 emit a white colour under the UV lamp, however,
the observed colour sometimes differs from the calculated colour. The ‘real’ colour is
presented via the CIE coordinates. This calculation is based on the emission spectrum, where
the material was excited at the most intense point of the charge transfer band. The UV lamp
available in our lab only provides three wavelengths (365 nm, 302 nm and 254 nm). Because
of this, the colour observed under the UV lamp is not always the same as calculated via the
emission spectrum.
25
Exciting the material at different wavelengths obviously has an impact on the emitted
colour. This will be further investigated by recording emission maps (collecting emission
spectra at different excitation wavelengths, see 4.6.5).
Table 4.4 also includes the correlated colour temperature (CCT), which gives more
information about the temperature of the colour (CCT > 5000 K is a cold colour, CCT < 3000 K
is a warm colour).
Luminescence emission, excitation and decay time spectra were recorded for all materials
before and after heat treatment. All transitions were assigned with the help of the Carnall
matrix elements83. Only a few white light emitting materials will be discussed in detail. Four
materials were chosen for further discussion based on their white light emitting properties
and the lanthanides that were doped in this matrix. These doped materials were chosen to
get an extensive overview of the different possibilities that exist for creating a white light
emitting tungstate material. Material 67 was not chosen to be discussed in detail since the
diffractogram (Fig. 4.4) showed that it was not a pure phase. This material has weak
emission and a spectrum that mainly consists of the broad charge transfer band.
Luminescence data of the other materials can be found in Appendix E.2. All spectra are
corrected for detector sensitivity, however, sometimes this overcompensates because of the
relative detector-insensitivity at certain wavelengths. In those cases, the uncorrected
spectrum is presented. An example of this effect can be found in Appendix E.1.
Another quantity that can be introduced is the Luminous Efficiency of Radiation (LER)81. This
parameter is the ratio between the luminous flux and the power (expressed in lumen per
Watt). It describes how well the material produces visible light to an average human
observer. The luminosity has a maximum value of 683 lm/W at monochromatic green light,
since this is the most sensitive wavelength (555 nm) of the human eye. The LER value can be
calculated via the emission spectrum. According to Smet et al.81, white light has a much
lower LER (in the order of 350 lm/W), since white light contains other colour components at
less sensitive wavelengths of the human eye.
In the following paragraphs, a detailed description of the observed luminescence properties
of certain white light emitting tungstate materials is presented.
26
4.6.1 Y2WO6:Eu3+(2.5%), Tb3+(2.5%) before heat treatment – 55a
The charge transfer band is quite narrow and centres around 267.9 nm. The excitation
transitions are assigned for Eu3+ (red) and Tb3+ (green). In the emission spectrum the charge
transfer band is quite weak and spread out over the visible range.
Table 4.5 Overview of the Eu3+ and Tb3+ transitions in Y2WO6:Eu3+(2.5%), Tb3+(2.5%) before heat treatment.
Symbol Wavelength (nm)
Wavenumber (cm-1)
Eu3+ (2.5%) Transitions
Tb3+ (2.5%) Transitions
a 486.4 20559 5D4 → 7F6
b 542.9 18420 5D4 → 7F5
c 579.4 17259 5D0 → 7F0 5D4 → 7F4
d 593.7 16844 5D0 → 7F1
e 612.0 16340 5D0 → 7F2
f 615.0 16260 5D4 → 7F3
g 643.2 15547 5D4 → 7F2
h 651.4 15352 5D0 → 7F3 5D4 → 7F1
i 702.4 14237 5D0 → 7F4
The combination of the red 5D0 → 7F2 europium and green 5D4 → 7F5 terbium transitions in
addition to the blue charge transfer band led to a material which emits white light under the
UV lamp (365 nm), with CIE coordinates x = 0.3472, y = 0.3814 and a CCT of 5019 K. Slightly
altering the doping percentages, as was done with sample 64a, also led to a white light
emitting material. The CIE coordinates of Y2WO6:Eu3+(2%), Tb3+(3%) show a larger divergence
from the standard white light coordinates. However, the coordinates shift to a warmer white
light x = 0.3915, y = 0.4098 and a CCT of 3952 K. This material has an LER value of 313 lm/W.
This brightness is clearly visible in Fig. 4.10.
Figure 4.6 (Left) Excitation spectrum of Y2WO6:Eu3+(2.5%), Tb3+(2.5%) before heat treatment, monitored at 612.2 nm (not corrected for
detector sensitivity). (Right) Emission spectrum of Y2WO6:Eu3+(2.5%), Tb3+(2.5%) before heat treatment, excited through the charge
transfer band at 267.9 nm (corrected for detector sensitivity).
27
Before heat treatment this material emits white light. After heat treatment, the broad
charge transfer band disappears and the intensity of the terbium transitions lower
significantly. This is why the colour shifts towards red. The same effect is visible when the
terbium co-doping percentage is increased. This makes it complicated to obtain a white light
emitting Eu3+, Tb3+ co-doped tungstate after heat treatment.
Table 4.6 Single exponential decay characteristics before heat treatment.
Transition R2 (µs) 5D0 → 7F2 (Eu3+) 0.999 418 5D4 → 7F5 (Tb3+) 0.998 597
The decay curves for the 5D0 → 7F2 transition of Eu3+
and 5D4 → 7F5 transition of Tb3+ were recorded. The
decay time of the Tb3+ transition is longer than that of
the Eu3+ transition.
Fig. 4.10 presents the colours of the
materials observed under the UV lamp
(254 nm). The difference in colour shows
how a minor change in doping percentage
can influence the observed colour.
Figure 4.7 CIE-diagram of Y2WO6:Eu3+(2.5%), Tb3+(2.5%)
before heat treatment (outer left), Y2WO6:Eu3+(2.5%),
Tb3+(2.5%) after heat treatment (outer right),
Y2WO6:Eu3+(2%), Tb3+(3%) before heat treatment
(inner left) and a white LED (2700 K) (inner right).
Figure 4.8 Emission spectrum of Y2WO6:Eu3+(2.5%),Tb3+(2.5%) before heat
treatment, plotted with a rainbow curve (corrected for detector sensitivity).
Figure 4.9 Decay curve of the 5D0 → 7F2 transition
of Y2WO6:Eu3+(2.5%), Tb3+(2.5%) before heat
treatment with a single exponential fit.
Figure 4.10 Pictures of Y2WO6:Eu3+(2.5%), Tb3+(2.5%) before heat
treatment (left), after heat treatment (centre left), Y2WO6:Eu3+(2%),
Tb3+(3%) before heat treatment (centre right) and after heat treatment
(right) under the UV lamp (254 nm).
28
4.6.2 Y2WO6:Dy3+(1%) after heat treatment – 56c
Doping 1% Dy3+ into this matrix led to the formation of a white light emitting rare-earth
tungstate before and after heat treatment. The charge transfer band is intense and has a
maximum around 299.6 nm. In the emission spectrum only a minor CTB is visible, which
suggests good charge transfer from the tungstate groups to the Dy3+ ions.
Table 4.7 Overview of the Dy3+ transitions in Y2WO6:Dy3+(1%) after heat treatment.
Symbol Wavelength (nm) Wavenumber (cm-1) Dy3+ (1%) Transitions
a 479.4 2086 4F9/2 → 6H15/2
b 578.5 1729 4F9/2 → 6H13/2
c 671.1 1490 4F9/2 → 6H11/2
This material emits white light before and after it has been heat treated. Before heat
treatment the white light has a bluish shade. At low doping percentages, the energy transfer
in the material before heat treatment seems to not be very efficient. Because of this, a
charge transfer band is visible in the emission spectrum, which causes the blue shade in the
emitted colour. This translates in CIE coordinates that are shifted to the blue region; x =
0.2196, y = 0.2800 and a very high correlated colour temperature of 26417 K. After heat
treatment the intensity of the CTB drops and the yellow share of the colour increases. Fig.
4.13 presents how the combination of the blue 4F9/2 → 6H15/2, yellow 4F9/2 → 6H13/2 and red
4F9/2 → 6H11/2 transitions led to a warm white colour. The CCT of this material is 4042 K. Heat
treating this material not only increases the intensity of the luminescence, but also shifts the
CCT to a warmer colour. This material has an LER value of 361 lm/W.
Figure 4.11 (Left) Excitation spectrum of Y2WO6:Dy3+(1%) after heat treatment, monitored at 574.0 nm (not corrected for detector
sensitivity). (Right) Emission spectrum of Y2WO6:Dy3+(1%) after heat treatment, excited through the charge transfer band at 299.6 nm
(corrected for detector sensitivity).
29
Under a UV lamp (365/254 nm), this material emits white light before and after heat
treatment. Visually, a difference in colour is observed, but this difference is presented more
clearly in the CIE-diagram. Both materials fall within the white light region, although the
coordinates are located quite far from each other. This illustrates that it is difficult to
unambiguously define white light without the CIE coordinates.
Table 4.8 Single exponential decay characteristics after heat treatment.
Exponential tail fit R2 (µs)
Single 0.9979 229
The decay time of this dysprosium doped tungstate is
significantly higher than the dysprosium doped YVO4
materials (see 5.6.1).
The colour observed under the UV lamp (254 nm) is
consistent with the calculations, which is clear from Fig.
4.15. Before heat treatment the white light has a blue
shade. This shade disappears after heat treatment,
resulting in a white light emission with intermediate
temperature.
Figure 4.12 CIE-diagram of Y2WO6:Dy3+(1%) before
(left) and after heat treatment (right).
Figure 4.13 Emission spectrum of Y2WO6:Dy3+(1%) after heat treatment
plotted with a rainbow curve (corrected for detector sensitivity).
Figure 4.14 Decay curve of Y2WO6:Dy3+(1%) after
heat treatment with a single exponential fit for
the 4F9/2 → 6H13/2 transition.
Figure 4.15 Picture of Y2WO6:Dy3+(1%) before
heat treatment (left) and after heat treatment
(right) under a UV lamp (254 nm).
30
4.6.3 Y2WO6:Eu3+(3%) before heat treatment – 61a
The narrow excitation transitions of the Eu3+ ions are assigned in Fig. 4.16 (left). The charge
transfer band overlaps part of the visible region of the spectrum. Combined with the narrow
line emission of europium, this material emits white light with excellent CIE coordinates. The
intensity of the Eu3+ transitions gives information about the symmetry. The intensity of the
magnetic dipole 5D0 → 7F1 transition is low compared to the intensity of the hypersensitive
electric dipole 5D0 → 7F2 transition. This suggests a low symmetry environment.
Table 4.9 Overview of the Eu3+ transitions in Y2WO6:Eu3+(3%) before heat treatment.
Symbol Wavelength (nm) Wavenumber (cm-1) Eu3+ (3%) Transitions
a 579.4 17259 5D0 → 7F0
b 593.4 16852 5D0 → 7F1 c 612.2 16335 5D0 → 7F2 d 651.0 15361 5D0 → 7F3 e 701.8 14249 5D0 → 7F4
The CIE coordinates of this material (x = 0.3274, y = 0.3214) are close to the standard for
white light (x = 0.33, y = 0.33). Even though this is a not heat treated material, the emission
is quite intense. These results show that co-doping lanthanides or the use of dysprosium
(with blue, yellow and red emission) is not necessary to obtain white light. The temperature
of this colour is cold (CCT = 5771 K), due to the blue share of the CTB. The synthesis of a
white light emitting singly doped tungstate after heat treatment might be possible with low
Eu3+ concentration. At low doping concentrations the charge transfer band might still be
present after heat treatment (as was observed for Dy3+ in 4.6.2). However, the combination
of only blue and red will lead to a cold blue colour, as is the case for this material. This
material has a LER value of 252 lm/W.
Figure 4.16 (Left) Excitation spectrum of Y2WO6:Eu3+(3%) before heat treatment, monitored at 612.2 nm (not corrected for detector
sensitivity). (Right) Emission spectrum of Y2WO6:Eu3+(3%) before heat treatment, excited through the charge transfer band at 265.7 nm.
.(corrected for detector sensitivity).
31
After heat treatment, no charge transfer band is visible anymore. This means that the energy
transfer from the tungstate groups to Eu3+ is better than it was before heat treatment. Due
to this, the colour of this material completely shifts to the red region of the CIE-diagram.
Table 4.10 Single exponential decay characteristics before heat
treatment.
The decay curve was fitted with a single
exponential function. After heat treatment, the
decay time of this material increases up to 732
µs, which quite long compared to lifetimes of
the other doped tungstate materials.
Before heat treatment, the material emits white light that is
observed as cold due to the blue of the charge transfer band.
After heat treatment, the intensity of the emission is much
stronger, since the charge transfer band is not visible
anymore in the emission spectrum (see Appendix E). This
makes the colour shift to the red region.
Exponential tail fit R2 (µs)
Single 0.999 296
Figure 4.17 CIE-diagram of Y2WO6:Eu3+(3%) before
(left) and after heat treatment (right).
Figure 4.18 Emission spectrum of Y2WO6:Eu3+(3%) before heat treatment, plotted
with a rainbow curve (corrected for detector sensitivity).
Figure 4.19 Decay curve of the 5D0 → 7F2 transition of
Y2WO6:Eu3+(3%) before heat treatment with a single
exponential fit.
Figure 4.20 Picture of Y2WO6:Eu3+(3%) before (left) and
after heat treatment (right) under a UV lamp (254 nm).
32
4.6.4 Y2WO6:Sm3+(2.5%), Tb3+(2.5%) before heat treatment – 65a
The CTB is quite narrow and has a maximum around 265.5 nm. In the emission spectrum the
charge transfer band is spread out over the visible region. For creating a white light emitting
tungstate, the combination of the blue emitting CTB with red and green emitting lanthanides
is straightforward. Mostly Eu3+ and Tb3+ are used, however Sm3+ is also a red/orange
emitting lanthanide. Co-doping Sm3+ and Tb3+ could be an interesting system.
Table 4.11 Overview of the Sm3+ and Tb3+ transitions in Y2WO6:Sm3+(2.5%), Tb3+(2.5%) before heat treatment.
Symbol Wavelength (nm)
Wavenumber (cm-1)
Sm3+ (2.5%) Transitions
Tb3+ (2.5%) Transitions
a 485.6 20593 5D4 → 7F6
b 543.4 18403 5D4 → 7F5
c 585.0 17094 4G5/2 → 6H5/2 5D4 → 7F4
d 619.2 16150 4G5/2 → 6H7/2 5D4 → 7F3
e 643.4 15542 4G5/2 → 6H9/2 5D4 → 7F2
f 655.6 15253 5D4 → 7F1
g 682.0 14663 4G5/2 → 6H11/2
For this material, terbium dominates the emission spectrum. One can conclude that the
energy transfer to the Tb3+ ions is more efficient than to the Sm3+ ions. Because of this, the
white light has a blue-green shade. After heat treatment, only samarium peaks are visible in
the emission spectrum (Appendix E). Combining the blue of the charge transfer band and the
strong green 5D4 → 7F5 transition led to a cold colour (CCT = 8061 K). The combination of
terbium and samarium could perhaps work to give a warmer white light if the samarium
transitions were more intense, or in another matrix that does not show this effect.
Figure 4.21 (Left) Excitation spectrum of Y2WO6:Sm3+(2.5%), Tb3+(2.5%) before heat treatment, monitored at 542.0 nm (not corrected for
detector sensitivity). (Right) Emission spectrum of Y2WO6:Sm3+(2.5%), Tb3+(2.5%) before heat treatment, excited through the charge
transfer band at 265.6 nm (corrected for detector sensitivity).
33
After heat treatment, the emission of samarium becomes dominant. This makes the CIE
coordinates shift to the red region. Increasing the doping percentage of samarium might
lead to a warmer white colour and a smaller shift after heat treatment.
Table 4.12 Single exponential decay characteristics before heat
treatment.
Exponential tail fit R2 (µs) 5D4 → 7F5 (Tb3+) 0.998 507
4G5/2 → 6H7/2 (Sm3+) 0.995 475
Contrary to most materials that were synthesized
in this thesis, the decay time of the 5D4 → 7F5
(Tb3+) transition becomes shorter after heat
treatment ( = 461 µs).
Because of the high intensity of the cyan and green
Tb3+ transitions in this material before heat
treatment, the temperature of the colour is cold. This
is also observed under the UV lamp (254 nm). After
heat treatment, the Sm3+ transitions become
dominant (see Appendix E), which results in an orange
colour. This material has an LER value of 319 lm/W.
Figure 4.22 CIE-diagram of Y2WO6:Sm3+(2.5%),
Tb3+(2.5%) before heat treatment (left) and
after heat treatment (right).
Figure 4.23 Emission spectrum of Y2WO6:Sm3+(2.5%), Tb3+(2.5%) before heat
treatment, plotted with a rainbow curve (corrected for detector sensitivity).
Figure 4.24 Decay curve of the 5D4 → 7F5 transition of
Y2WO6:Sm3+(2.5%), Tb3+(2.5%) before heat treatment
with a single exponential fit.
Figure 4.25 Picture of Y2WO6:Sm3+(2.5%), Tb3+(2.5%) before (left)
and after heat treatment (right) under a UV lamp (254 nm).
34
4.6.5 Emission maps
Y2WO6:Eu3+(2.5%), Tb3+(2.5%) before heat treatment – 55a
When this material is excited at different wavelengths in the charge transfer band, the
relative intensity of the transitions changes and the coordinates shift to a warmer white. The
zoom in of the CIE-diagram shows how the different points are almost arranged on a line.
The luminosity of the material clearly changes when exciting at different wavelengths. The
CTB of this material is quite narrow (see Fig. 4.6). One might expect that excitation at 290,
300 and 310 nm will not lead to bright luminescence. However the LER values are
comparable to when the material was excited at the most intense wavelength of the CTB.
Table 4.13 Change in CIE coordinates, CCT and LER values when exciting
Y2WO6:Eu3+(2.5%), Tb3+(2.5%) (not heat treated) at different wavelengths.
Data of the excitation at the most intense wavelength (267.9 nm) are also
presented.
Wavelength (nm)
CIE x CIE y CCT (K)
LER (lm/W)
250 0.3285 0.3481 5668 284 260 0.3245 0.3551 5831 290
267.9 0.3472 0.3814 5019 313
270 0.3297 0.3634 5613 298
280 0.3368 0.3808 5362 312
290 0.3466 0.4051 5095 330
300 0.3567 0.4311 4887 341
310 0.3691 0.4380 4612 318
320 0.3719 0.4208 4476 284
Figure 4.26 Emission maps of
Y2WO6:Eu3+(2.5%), Tb3+(2.5%) before
heat treatment, excited at different
wavelengths of the charge transfer
band (corrected for detector
sensitivity).
Figure 4.27 Zoom in of the CIE-diagram of
Y2WO6:Eu3+(2.5%), Tb3+(2.5%) before heat
treatment at different excitation wavelengths.
35
Y2WO6:Dy3+(1%) after heat treatment – 56c
Contrary to the previous example, the points in the CIE diagram of Y2WO6:Dy3+(1%) do not
show a line behaviour. The points are arranged in a parabolic pathway, first shifting towards
a point (at the maximum of the charge transfer band). When this maximum point is reached,
the coordinates shift back to the position they came from. The same behaviour is observed
with the correlated colour temperatures. All coordinates are situated in the middle of the
white light region. The LER values are comparable when exciting at different wavelengths.
Table 4.14 Change in CIE coordinates, CCT and LER values when
exciting Y2WO6:Dy3+(1%) (after heat treatment) at different
wavelengths. Data of the excitation at the most intense wavelength
(299.6 nm) is also presented.
Wavelength
(nm) CIE x CIE y CCT
(K) LER
(lm/W)
250 0.3471 0.3687 4985 344 260 0.3465 0.3658 4996 341
270 0.3439 0.3612 5075 338
280 0.3411 0.3582 5170 334
290 0.3411 0.3585 5172 334
299.6 0.3826 0.3908 4042 361
300 0.3426 0.3609 5122 336
310 0.3447 0.3634 5052 339
320 0.3478 0.3679 4956 344
330 0.3491 0.3740 4934 344
Figure 4.28 Emission maps of
Y2WO6:Dy3+(1%) after heat
treatment, excited at different
wavelengths of the charge
transfer band (corrected for
detector sensitivity).
Figure 4.29 Zoom in of the CIE-diagram of
Y2WO6:Dy3+(1%) after heat treatment, excited at
different wavelengths.
36
Y2WO6:Eu3+(3%) before heat treatment – 61a
The mappings of the previous tungstate materials only showed minor differences in CIE
coordinates. For Y2WO6:Eu3+(1%) there is a large change in CIE coordinates when deviating
away from the most intense point of the charge transfer band. Excitation at 240-280 nm
resulted in CIE coordinates that group together. The temperature of this emission can be
described as cold. However, excitation at 290 and 300 nm makes the CIE coordinates shift to
the warm white light region. Unfortunately, excitation at the weakest wavelengths of the
charge transfer band results in a weaker emission signal. The LER values steadily increase
when the excitation wavelength is increased, from 252 lm/W (240 nm) to 270 lm/W (300
nm).
Table 4.15 Change in the CIE coordinates and CCT values when exciting Y2WO6:Eu3+(3%) (before heat treatment) at different
wavelengths. Data of the excitation at the most intense wavelength (265.7) is also presented.
Wavelength (nm)
CIE x CIE y CCT (K)
240 0.3236 0.3180 5968 250 0.3265 0.3160 5815
260 0.3267 0.3171 5799
265.7 0.3274 0.3124 5771
270 0.3310 0.3219 5564
280 0.3512 0.3295 4647
290 0.3992 0.3569 3336
300 0.4396 0.3893 2832
Figure 4.31 Zoom in of the CIE-diagram of Y2WO6:Eu3+(3%) before
heat treatment, excited at different wavelengths.
Figure 4.30 Emission maps of
Y2WO6:Eu3+(3%) before heat
treatment, excited at
different wavelengths of the
charge transfer band
(corrected for detector
sensitivity).
37
5 Synthesis and characterization of rare-earth vanadates
5.1 Synthesis
The materials were synthesized under hydrothermal conditions. For comparison, microwave
and sol-gel reactions were also performed. The synthesis routes and all performed reactions
can be found in Appendix A. To synthesize a pure phase rare-earth vanadate material with
glycerol as an SDA, the reaction conditions must be tuned. Reactions similar to what Wu et
al.49 published were first attempted. They reported that a pure-phase yttrium vanadate
material can only be synthesized in acidic conditions if the yttrium to vanadate ratio is 1
(single vanadate), or in basic conditions if the yttrium to vanadate ratio is smaller than 1
(double vanadate) without using a structure directing agent. Based on their findings a few
reactions were performed, but with addition of glycerol as a structure directing agent and
solvent. The optimal reaction conditions to form a pure phase without an SDA are not always
ideal when an SDA is used. It is possible this leads to the same phase, but the formation of a
mixed phase or a different phase is also possible, as was observed with the tungstate
materials (see 4.1). The formation of a specific phase highly depends on the conditions.
Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) was used to confirm
the removal of glycerol and water after heat treatment. Characteristic vibrations of the
molecule were assigned. To confirm whether the materials are pure phases, XRD
diffractograms were recorded and compared with calculated powder XRD diffractograms
from the ICSD crystallographic database. This led to the confirmation of which phase was
obtained. After phase confirmation, this matrix was doped with various lanthanides.
5.2 Performed reactions
Table 5.1 Performed vanadate reactions and their synthesis conditions. Different types of syntheses were performed;
H: hydrothermal, S: sol-gel, M: microwave.
# Synthesis T (°C) t (h) pH Glycerol (mL) Y(NO3)3∙6H2O (g) Na3VO4 (g)
22 H 180 24 2 0 0.383 0.184
23 H 180 24 2 10 0.383 0.184
24 H 180 24 10 10 0.383 0.184
25 H 180 24 10 10 0.383 0.368
34 S 800 5 2 10 0.383 0.184
68 M 200 1 2 10 0.184 0.092
38
Table 5.1 gives an overview of the reactions that were performed. Reaction 22 was a test to
confirm whether these conditions led to a pure phase material, even without the use of
glycerol. Reactions at acid pH with an yttrium to vanadate ratio of 1 (reaction 23) and at
basic pH with an yttrium to vanadate ratio smaller than 1 (reaction 25) were carried out. A
reaction at basic pH with a ratio of 1 was also performed to investigate whether glycerol has
a positive effect on the formation of a pure phase, since these conditions do not lead to a
pure phase without the use of an SDA according to Wu et al.49. A sol gel reaction route51 and
a microwave type reaction76 were performed later on to investigate whether ideal
hydrothermal conditions (pH and amounts of product) were also optimal for other SDA
based techniques.
5.3 Diffuse Reflectance Infrared Fourier Transform Spectra
Diffuse reflectance infrared Fourier transform spectra were recorded before and after heat
treatment of the materials. Table 5.2 and Fig. 5.1 describe an example of an undoped YVO4
material with information about the assigned vibrations. DRIFTS spectra of the other
materials that were synthesized are comparable and can be found in Appendix C.
Table 5.2 Assigned infrared vibrations of undoped YVO4 before
heat treatment.
The broad absorption band at 3458 cm-1
corresponds to the O-H stretch vibrations.
This band is attributed to the water and/or glycerol adsorbed at the surface of the crystals.
The two bands at 2343 cm-1 and 1625 cm-1 are assigned to the O-H stretch vibrations of
coordinated water15. The characteristic vibrations for the vanadate are visible below 1000
cm-1. Characteristic vanadate stretch vibrations were assigned with the help of an inorganic
infrared database by F. Miller et al.54. Bands d and e suggest that this powder contains a
vanadate material. However this cannot be said with complete certainty. Bands a-c show
that the material before heat treatment still contains a lot of water48, 53. The spectrum after
Symbol Wavenumber (cm-1)
Vibration
a 3458 O-H
b 2343 O-H
c 1625 O-H
d 977 V-O
e 706 V-O
Figure 5.1 DRIFTS spectrum of undoped YVO4 before (red) and
after (blue) heat treatment.
39
heat treatment (blue) shows that the broad band at 3459 cm-1 has disappeared. A large
amount of the water has evaporated after heat treatment at 900°C. However, some
coordinated water still is present, as are the characteristic vanadate bands below 1000 cm-1.
For this type of material, DRIFTS is a technique that can confirm the absence or presence of
water in the sample. However, it cannot be said with certainty which material was
synthesized.
5.4 X-ray diffractograms
Figure 5.2 Powder XRD diffractograms of samples 22-25 after heat treatment and sample 32 before heat treatment. These
diffractograms are compared with the YVO4 powder pattern calculated from the ICSD crystallographic database86.
40
Via the ICSD crystallographic database, the powder XRD diffractogram of a material can be
calculated. This is then compared to the obtained diffractograms. Determining the phase is
of paramount importance since it is only useful to dope lanthanide ions in a pure phase
material. Fig. 5.2 shows that an YVO4 material is formed with (23) and without (22) the use
of glycerol under acidic conditions. Using basic conditions and a different yttrium to
vanadate ratio (25) also led to a pure phase material. However, the reflections in the
diffractogram are slightly broadened. According to Wu et al.49 an yttrium to vanadate ratio
of 1 at basic pH values leads to a mix of the YVO4 and Y(OH)3 phase. With glycerol as the
SDA, these reaction conditions (24) lead to the YVO4 phase. However, there are some
additional peaks at the angles of 18.9, 29.9 and 29.6 that were assigned to the Y(OH)3 phase,
as Wu et al.49 reported. The materials before heat treatment (32) are YVO4 pure phases, but
the broader reflections indicate that the material is less crystalline.
The sol-gel synthesis (34) led to an YVO4 phase
with another space group (I41/a)85 than the
materials that were synthesized via a
hydrothermal route (I41/amd)86. The peaks are
broadened which suggests that the material has
a lower crystallinity than the hydrothermally
synthesized materials. The microwave route also
led to the YVO4 phase, but in this case the same
space group as the hydrothermally synthesized
materials was obtained. Peak broadening is also
visible. A hydrothermal reaction obviously gives
the best results. However, the microwave
reaction only lasted for 1 h. Longer reaction
times might give better results. Still, the major
advantage of microwave synthesis is the much
shorter reaction time, compared to
hydrothermal synthesis. The optimal reaction conditions that led to a pure monoclinic phase
YVO4 material were chosen for doping with various lanthanides to obtain white light. To
summarize, these conditions are a hydrothermal reaction at 200°C for 24 h, at pH = 2, with
the use of 20 mL glycerol and a Y(NO3)3∙6H2O : Na3VO4 ratio equal to 1:1.
Figure 5.3 Powder XRD patterns of a sol-gel synthesized
material after heat treatment (34) and of a microwave
synthesized material after heat treatment (68). The
diffractograms are compared with the calculated powder
patterns from the ICSD crystallographic database85,86.
41
5.5 Scanning electron microscopy images
Scanning electron microscopy was used to investigate the size, shape and morphology of the
materials. SEM images of the materials synthesized at different conditions were recorded.
The influence of the pH, reaction temperature, amount of structure directing agent and type
of synthesis are investigated.
The influence of the pH on the morphology of the materials is visible in Fig. 5.4. At basic pH
conditions the particles are completely aggregated, which explains the broader reflections in
the X-ray diffractogram. When a synthesis is performed under acidic conditions, the
materials are much less aggregated. The particles are not distributed in an ordered way and
their size is not uniform. A synthesis at a higher reaction temperature (200°C instead of
180°C) and with a higher amount of glycerol (20 mL instead of 10 mL) was carried out. The
SEM images of the YVO4 material at these conditions are presented in Fig. 5.5.
Figure 5.5 SEM images of an YVO4 material after heat treatment at 900°C at different magnifications. This material was synthesized
hydrothermally at 200°C with the use of 20 mL glycerol and pH = 2. (A) 1000x, (B) 4000x, (C) 8000x, (D and E) 15000x and (F) 30000x.
Figure 5.4 SEM images of an YVO4 material at conditions 23 (A) and 25 (B) (see Table 5.1).
42
At a magnification of 1000x (A), the image looks quite similar to Fig. 5.4 (A). The particles
look spherical and are not ordered. Small clusters of particles are visible, although they are
less aggregated compared to when a lower reaction temperature and lower amount of
glycerol was used. At higher magnification (B) it is clear that the particles are not uniform.
The particles are not really spherical but more angular and spiky structures are also visible.
From the more detailed images (D, E, F) it is clear that three different morphologies can be
distinguished. Spheres of different sizes, narrow rods and angular particles are present. For
the tungstate phase, three different morphologies could also be distinguished (see 4.5).
However, in this case it is plausible that the different shapes are formed separately. When
investigating the image at a higher magnification the angular forms look tetragonal.
However, since the particles are not uniform this cannot be said with complete certainty.
Even though the particles are not uniform, it is interesting how one material consists of
three different morphologies.
The lowest amount of glycerol (10 mL) led to aggregated particles. Increasing the amount of
glycerol to 20 mL resulted in less aggregated particles with three different types of shapes.
Using an even higher amount of glycerol might lead to the particles being capped more and
becoming more unified.
Along with the hydrothermal synthesis, a sol-gel and a microwave synthesis were also
employed with the same amount of glycerol (20 mL) and pH = 2 (see Fig. 5.6).
Figure 5.6 SEM images of undoped YVO4 at different magnifications, prepared via a sol-gel synthesis. (A) 3000x, (B) 10000x and (C)
40000x.
At a magnification of 3000x (A), it is clear that no separate particles are present. All particles
seem to be aggregated, creating a rough surface. Next to this rough surface, a smoother
plane can also be distinguished at a magnification of 10000x (B, top left of the image). At the
bottom left of this image is a clear example of how this plane is created by small particles of
43
different shapes that accumulate. Most of the particles are near-spherical, although more
angular ones can also be distinguished at a higher magnification. Comparison of the SEM
images of the hydrothermally synthesized materials and the sol-gel type synthesized
materials clearly shows how significant the influence of the synthesis route actually is. Even
though the same YVO4 phase is formed, the morphology is entirely different.
The particles that were synthesized via a microwave route are differently shaped than the
particles that were synthesized via other techniques (see Fig. 5.7). At a magnification of
5000x it is obvious that the particles are spread out over the whole surface. Only a small
amount of aggregation is visible.
Figure 5.7 SEM images of YVO4:Eu3+(1%), Tb3+(3%) at different magnifications, prepared via a microwave synthesis. (A) 5000x, (B) 20000x
and (C) 40000x.
Spheres of different sizes can be discerned at higher magnification (C). The surface of the
spheres is quite rough and larger spheres look as if they were built out of smaller spheres.
Based on these findings, an Ostwald ripening process can be suggested. The size of the
particles is rather small. This is probably because of the short reaction time for the
microwave synthesis (1 h) compared to the hydrothermal synthesis (24 h). Longer reaction
times will probably lead to bigger particles. The microwave reaction led to very nice results
compared to the hydrothermal reaction. Only one microwave reaction was performed for
this vanadate material, since the aim was to see how the reaction route influenced the
morphology when all other parameters were kept identical. Performing more research to
further tune reaction parameters for the microwave reaction may lead to even more
uniform and ordered particles.
44
5.6 Luminescence properties
Various singly doped and co-doped vanadate materials have been synthesized. Their colour
properties are presented in Table 5.3.
Table 5.3 Overview of the synthesized vanadate materials and their doping percentages. All materials were synthesized hydrothermally,
except for sample 68, which was synthesized via a microwave route. The ‘#’ labels the materials before heat treatment with ‘a’ and after
heat treatment with ‘c’. Experiments highlighted in grey are the white light emitting materials that will be discussed in detail.
# Doping Observed colour (UV lamp) CIE x CIE y CCT CIE
(%) 365 nm 254 nm (K) colour
31a 5% Eu3+ Red (light) 0.6121 0.3175 1065
31c 5% Eu3+ Red Red (light) 0.6671 0.3303 <1000
32a 5% Dy3+ 0.4116 0.4576 3828
32c 5% Dy3+ Yellow (dark) Yellow (dark) 0.4370 0.4857 3557
33a 5% Sm3+ 0.4024 0.2495 2016
33c 5% Sm3+ Orange Orange (light) 0.6266 0.3689 1200
39a 2% Eu3+ 0.6089 0.3409 1180
39c 2% Eu3+ Red Red (light) 0.6623 0.3306 <1000
40a 2% Dy3+ 0.3612 0.4158 4725
40c 2% Dy3+ Yellow (white) Yellow (white) 0.4351 0.4810 3545
41a 2% Sm3+ 0.5223 0.3705 1771
41c 2% Sm3+ Orange Orange 0.6174 0.3749 1265
42a 3% Eu2+ 0.3415 0.2092 3216
42c 3% Eu3+ Red Red 0.6640 0.3326 <1000
43a 3% Dy3+ 0.3475 0.3597 4898
43c 3% Dy3+ Yellow Yellow (brown) 0.4340 0.4809 3540
44a 3% Sm3+ 0.4877 0.3568 1967
44c 3% Sm3+ Orange Orange 0.6269 0.3693 1200
50a 2.5% Eu3+, 2.5% Tb3+ 0.3795 0.2826 2954
50c 2.5% Eu3+, 2.5% Tb3+ Orange Orange 0.5862 0.3680 1376
51a 2% Eu3+, 2% Tb3+ 0.5467 0.3504 1498
51c 2% Eu3+, 2% Tb3+ Red Red (light) 0.6435 0.3263 <1000
52a 2% Eu3+, 2% Dy3+ 0.4973 0.3849 2152
52c 2% Eu3+, 2% Dy3+ Orange Brown (light) 0.5563 0.3922 1691
58a 2% Eu3+, 3% Dy3+ 0.4121 0.3842 3299
58c 2% Eu3+, 3% Dy3+ Brown (light) Brown (light) 0.5185 0.4006 2037
59a 2% Eu3+, 3% Tb3+ 0.3893 0.2999 2921
59c 2% Eu3+, 3% Tb3+ Red Red 0.6639 0.3307 <1000
68a 1% Eu3+, 3% Tb3+ 0.5152 0.3478 1685
68c 1% Eu3+, 3% Tb3+ Red Red 0.6642 0.3310 <1000
69a 0.5% Eu3+, 2.5% Dy3+ 0.4062 0.3900 3474
69c 0.5% Eu3+, 2.5% Dy3+ White White 0.4769 0.4269 2598
45
Singly doped and co-doped materials were prepared. In Table 5.3 the colour, as observed
under a UV lamp (365 nm and 254 nm), is presented, as well as the ‘real’ colour when
calculated via the CIE coordinates. Table 5.3 clearly shows that excitation at the most intense
point of the charge transfer band results in a different emission colour than when excitation
occurs at the standard wavelengths the UV lamp provides. It appears that the provided
excitation wavelengths of the UV lamp are not efficient for the vanadate materials that were
not heat treated. This explains why no colour is observed. This is also why only pictures of
the materials after heat treatment are presented in the following sections. The correlated
colour temperature (CCT) is also added and gives more information about the temperature
of the colour (CCT > 5000 K is a cold colour, CCT < 3000 K is a warm colour).
Different lanthanide ions were doped in this matrix. At first, different percentages of only
one lanthanide are doped to get an idea of the colours and how they shift after heat
treatment. Based on this knowledge, various co-doped reactions were performed with the
goal of obtaining white light emission.
All materials have been investigated via luminescence spectroscopy. The emission, excitation
and decay curves have been recorded. All transitions were assigned with the help of the
Carnall matrix elements83. Since the aim of this thesis is white light emission, only a few of
the samples with CIE coordinates in the white light region will be discussed in detail. These
materials were chosen based on the different combinations of lanthanides that were doped
in this matrix, to get and extensive overview. With similar doping percentages, the material
with the best white light emitting properties was selected. Luminescence data of the other
samples can be found in Appendix E.
The highest emission efficiency is obtained when excitation occurs at the maximum of the
charge transfer band. However, one can also excite at other wavelengths in the CTB. This will
have an influence on the emission spectrum and thus the CIE coordinates. Emission maps of
the materials with the best white light coordinates were recorded and are presented in
section 5.6.5.
In the following paragraphs, a detailed description of the observed luminescence properties
of certain white light emitting vanadate materials is presented.
46
5.6.1 YVO4:Dy3+(3%) before heat treatment – 43a
A broad charge transfer band in the region of 250 – 350 nm is visible in the excitation
spectrum. Upon excitation at 282.2 nm, the emission spectrum is recorded. The charge
transfer band is visible in the emission spectrum and is centred around 425 nm. This
emission behaviour is different than observed with YVO4:Dy3+(2%). In that case the CTB
overlaps part of the visible region of the spectrum (see Appendix E).
Table 5.4 Overview of the Dy3+ transitions in YVO4:Dy3+(3%) before heat treatment.
Symbol Wavelength (nm) Wavenumber (cm-1) Dy3+ (3%) Transitions
a 483.3 20691 4F9/2 → 6H15/2
b 573.6 17434 4F9/2 → 6H13/2
c 661.9 15108 4F9/2 → 6H11/2
Besides the small red 4F9/2 → 6H11/2 transition, the more intense yellow 4F9/2 → 6H13/2
transition and the blue 4F9/2 → 6H15/2 transition, this material has a larger blue share due to
the CTB centring around 425 nm. This makes the CIE coordinates shift towards the blue
region. The CIE coordinates of this 3% dysprosium doped material are x = 0.3475, y = 0.3597,
which is quite close to standard white light (x = 0.33, y = 0.33). The 2% dysprosium doped
material before heat treatment also emits white light, with CIE coordinates x = 0.3612, y =
0.4158. Increasing the doping percentage by only 1% clearly moves the CIE coordinates more
towards the standard values of white light. The LER value of this material is 336 lm/W.
Figure 5.8 (Left) Excitation spectrum of YVO4:Dy3+(3%) before heat treatment, monitored at 574.0 nm (not corrected for detector
sensitivity). (Right) Emission spectrum of YVO4:Dy3+(3%) before heat treatment, excited through the charge transfer band at 282.2 nm
(corrected for detector sensitivity).
47
The CIE-diagram shows that the colour of this material, when excited at 282.2 nm, is quite
close to the colour of a standard pure colour white LED. Although the 3% Dy3+ doped
material emits white light with better coordinates than 2% Dy3+ doping, this higher doping
percentage also leads to a stronger yellow colour after heat treatment. This makes it difficult
to create a singly Dy3+ doped YVO4 material that emits white light after heat treatment.
Table 5.5 Exponential decay characteristics before heat treatment.
Exponential tail fit R2 (µs)
Single 0.979 55
The CIE coordinates of the 3% Dy3+ doped
material are shifted more towards ideal white
light than the 2% Dy3+ doping but the decay
time is lower with higher Dy3+ doping (τ = 64 µs
for YVO4:Dy3+(2%)).
Fig. 5.12 shows the difference between 2% and 3% Dy3+
doping in the YVO4 matrix. The colour observed under the UV
lamp (254 nm) is yellow. However, when a picture is taken, a
green colour is perceived.
Figure 5.9 CIE-diagram of YVO4:Dy3+(3%) before
heat treatment (bottom left) and after heat
treatment (upper right).
Figure 5.10 Emission spectrum of YVO4:Dy3+(3%) before heat treatment
plotted, with a rainbow curve (corrected for detector sensitivity).
Figure 5.11 Decay curve of the 4F9/2 → 6H13/2 transition of
YVO4:Dy3+(3%) before heat treatment with a single
exponential fit.
Figure 5.12 Picture of YVO4:Dy3+(2%) after heat treatment (left) and
YVO4:Dy3+(3%) after heat treatment (right) under a UV lamp (254 nm).
48
5.6.2 YVO4:Eu3+(2.5%), Tb3+(2.5%) before heat treatment – 50a
Figure 5.13 (Left) Excitation spectrum of YVO4:Eu3+(2.5%), Tb3+(2.5%) before heat treatment, monitored at 618.4 nm (not corrected for
detector sensitivity). (Right) Emission spectrum of YVO4:Eu3+(2.5%), Tb3+(2.5%) before heat treatment, excited through the charge
transfer band at 297.7 nm (corrected for detector sensitivity).
The excitation spectrum shows a CTB in the range 250 – 350 nm. Two excitation transitions
of the Eu3+ ion, 5L6 ← 7F0 (a) and 5D2 ← 7F0 (b) are also visible. The emission spectrum shows
a broad CTB that centres around 450 nm, together with the sharp emissive transitions of
Eu3+ and Tb3+. All transitions were assigned and are presented in Table 5.6.
Table 5.6 Overview of the Eu3+ and Tb3+ transitions in YVO4:Eu3+(2.5%), Tb3+(2.5%) before heat treatment.
Symbol Wavelength (nm) Wavenumber (cm-1) Eu3+ (2.5%) Transitions
Tb3+ (2.5%) Transitions
a 536.5 18639 5D4 → 7F5
b 585.9 17068 5D0 → 7F0 5D4 → 7F4
c 593.0 16863 5D0 → 7F1
d 618.7 16163 5D0 → 7F2 5D4 → 7F3
e 651.4 15352 5D0 → 7F3 5D4 → 7F2
f 697.0 14347 5D0 → 7F4
The 5D4 → 7F5 and 5D4 → 7F4 transitions of Tb3+
are not so intense, but still visible in the
spectrum. However, for a Eu3+(2.5%), Tb3+(2.5%) doped material one would expect more
intense transitions of terbium. Li et al.48 reported that the Tb3+ luminescence gets quenched
in the YVO4 matrix due to an energy migration process. This was investigated before by
DeLosh et al.80. This might be a possible explanation for the low intensity of the Tb3+
transitions. Nevertheless, it is also possible that Tb3+ transfers part of its energy to Eu3+-ion.
The LER value of this material is 214 lm/W.
49
The colour of this material is situated in the white zone, although the CIE coordinates are
shifted towards the pink region due to the intense 5D0 → 7F2 and 5D0 → 7F4 transitions of
europium. Because of this red portion, the observed colour is warmer, which results in a
warm CCT of 2954 K. After heat treatment the colour shifts to orange/red. The intensity of
the hypersensitive 5D0 → 7F2 electric dipole transition is high compared to the intensity of the
5D0→7F1 magnetic dipole transition. This suggests a low symmetry environment1, 70.
Table 5.7 Exponential decay characteristics before heat treatment.
Exponential tail fit R2 (µs) 5D0 → 7F2 (Eu3+) 0.986 304 5D4 → 7F5 (Tb3+) 0.997 10
The lifetime of the 5D0 → 7F2 transition of Eu3+ is
significantly longer than the lifetime of the Tb3+
transition. This makes sense, since this transition is
only weakly visible in the emission spectrum.
Fig. 5.17 presents an example of the shift in colour when
exciting at a different wavelength. Excitation at 302 nm
results in a more intense orange emission than excitation at
254 nm, since this wavelength is closer to the maximum of
the charge transfer band (297.7 nm). This is further
investigated by recording emission maps (see 5.6.5).
Figure 5.14 CIE-diagram of YVO4:Eu3+(2.5%),
Tb3+(2.5%) before heat treatment (left) and
after heat treatment (right).
Figure 5.15 Emission spectrum of YVO4:Eu3+(2.5%), Tb3+(2.5%) before heat
treatment, plotted with a rainbow curve (corrected for detector sensitivity).
Figure 5.16 Decay curve of the 5D0 → 7F2 transition
of YVO4:Eu3+(2.5%), Tb3+(2.5%) before heat
treatment with a single exponential fit.
Figure 5.17 Picture of YVO4:Eu3+(2.5%),
Tb3+(2.5%) after heat treatment under the UV
lamp at a wavelength of 254 nm (left) and
302 nm (right).
50
5.6.3 YVO4:Eu3+(2%), Dy3+(3%) before heat treatment – 58a
Figure 5.18 (Left) Excitation spectrum of YVO4:Eu3+(2%), Dy3+(3%) before heat treatment, monitored at 573.5 nm (not corrected for
detector sensitivity). (Right) Emission spectrum of YVO4:Eu3+(2%), Dy3+(3%) before heat treatment, excited through the charge transfer
band at 303.9 nm (not corrected for detector sensitivity).
A charge transfer band with a maximum at 303.9 nm is visible in the excitation spectrum.
The emission spectrum shows the typical transitions of Eu3+ and Dy3+. Contrary to the
previous examples, the charge transfer band is only slightly visible in the emission spectrum.
This suggests a good energy transfer from the VO43- groups to the lanthanides.
Table 5.8 Overview of the Eu3+ and Dy3+ transitions in YVO4:Eu3+(2%), Dy3+(3%) before heat treatment.
Symbol Wavelength (nm) Wavenumber (cm-1) Eu3+ (2%) Transitions
Dy3+ (3%) Transitions
a 481.8 20756 4F9/2 → 6H15/2
b 572.5 17467 4F9/2 → 6H13/2
c 586.1 17062 5D0 → 7F0
d 592.4 16880 5D0 → 7F1
e 617.8 16186 5D0 → 7F2
f 653.6 15300 5D0 → 7F3 4F9/2 → 6H11/2
g 696.7 14353 5D0 → 7F4
The intense yellow 4F9/2 → 6H13/2 Dy3+ transition combined with the orange 5D0 → 7F2 and
red 5D0 → 7F4 transitions of Eu3+, mixed with a touch of blue, gives this sample a warm white
colour. Although the CIE coordinates of this material do not come close to the standard, they
are still in the white light region, but shifted to the orange side. Different temperatures of
white light can be applied in various applications. A warmer white light is ideal for
illumination. The CIE coordinates for a warm (2700 K) white pure colour LED are x = 0.4610, y
= 0.411084. The CIE coordinates of the YVO4:Eu3+(2%), Dy3+(3%) material are x = 0.4121, y =
0.3842 with a CCT of 3299 K.
51
Since this material shows almost no charge transfer band in the emission spectrum, the
change in CIE coordinates before and after heat treatment is only minor. For comparison,
the position of a standard white LED with a CCT of 2700 K is also presented in the CIE-
diagram. These coordinates do not differ significantly. Although these are good results, the
decay time of this material is fairly short.
Table 5.9 Single exponential decay characteristics before heat treatment.
Exponential tail fit R2 (µs) 4F9/2 → 6H13/2 (Dy3+) 0.998 13
5D0 → 7F2 (Eu3+) 0.989 28
The decay time of the 4F9/2 → 6H13/2 transition
(Dy3+) after heat treatment is 329 µs, which is
significantly longer. One can also try to excite the
heat treated material at different wavelengths to
improve the coordinates and obtain a warmer
white light emitting material.
Even though the wavelength of the UV lamp (302 nm) is quite close to the
maximum wavelength of the charge transfer band (303.9 nm), the visible
emission is rather weak. The LER value of this material before heat
treatment is 311 lm/W and 336 lm/W after heat treatment.
Figure 5.19 CIE-diagram of a standard white
pure colour LED (2700K) (middle)
YVO4:Eu3+(2%), Dy3+(3%) before heat treatment
(left) and after heat treatment (right).
Figure 5.20 Emission spectrum of YVO4:Eu3+(2%), Dy3+(3%) before heat treatment,
plotted with a rainbow curve (not corrected for detector sensitivity).
Figure 5.21 Decay curve of the 5D0 → 7F2 transition
of YVO4:Eu3+(2%), Dy3+(3%) before heat treatment
with a single exponential fit.
Figure 5.22 Picture of YVO4:Eu3+(2%), Dy3+(3%) after heat treatment under a UV lamp (302 nm).
52
5.6.4 YVO4:Eu3+(0.5%), Dy3+(2.5%) after heat treatment – 69c
Figure 5.23 (Left) Excitation spectrum of YVO4:Eu3+(0.5%), Dy3+(2.5%) after heat treatment, monitored at 574.1 nm (not corrected for
detector sensitivity). (Right) Emission spectrum of YVO4:Eu3+(0.5%), Dy3+(2.5%) after heat treatment, excited through the charge transfer
band at 310.9 nm (corrected for detector sensitivity).
The charge transfer band centres around 310.9 nm. Since the emission spectrum shows no
CTB anymore, it can be concluded that the energy transfer from the VO43- groups to the
lanthanides was efficient.
Table 5.10 Overview of the Eu3+ and Dy3+ transitions in YVO4:Eu3+(0.5%), Dy3+(2.5%) after heat treatment.
Symbol Wavelength (nm)
Wavenumber (cm-1)
Eu3+ (0.5%) Transitions
Dy3+ (2.5%) Transitions
a 483.0 20704 4F9/2 → 6H15/2
b 574.0 17422 4F9/2 → 6H13/2
c 592.8 16869 5D0 → 7F1
d 618.2 16176 5D0 → 7F2
e 661.5 15117 5D0 → 7F3 4F9/2 → 6H11/2
f 697.4 14339 5D0 → 7F4
At this point, various white light emitting lanthanide doped YVO4 materials were
synthesized. However, only before heat treatment the emission colour was white. After heat
treatment, the colour shifted towards the most dominant lanthanide. With this in mind, a
dominant Dy3+ doping (2.5%) was chosen with a small amount of Eu3+ (0.5%) to shift the
colour more to warm white light. This goal was met and a material that emits white light
before and after heat treatment was obtained. The colour of the material after heat
treatment can be described as a warm white colour with CIE coordinates x = 0.4769, y =
0.4269 and a CCT of 2598 K. Before heat treatment the LER of this material is 316 lm/W, but
increases up to 397 lm/W after heat treatment, which is quite high for a white light emitting
material.
53
As is obvious from Fig. 5.24, the CIE coordinates of this material are quite similar to the
standard pure colour white LED (2700 K) coordinates. The advantage of a white light
emitting material after heat treatment is a more intense emission than before heat
treatment.
Table 5.11 Single exponential decay characteristics after heat
treatment.
Exponential tail fit R2 (µs) 4F9/2 → 6H13/2 (Dy3+) 0.999 188
5D0 → 7F2 (Eu3+) 0.997 485
In general, heat treated samples show longer
decay times than not heat treated ones, which
is also the case for this material. The lifetime of
the 5D0 → 7F2 (Eu3+) transition is significantly
longer than the 4F9/2 → 6H13/2 (Dy3+) transition.
When the material is excited at 254 nm under the UV
lamp, the white emission has a yellow tone. Exciting
the material at 302 nm shows a more pure white
colour.
Figure 5.24 CIE-diagram of standard white light
(2700K) (middle circle) YVO4:Eu3+, (0.5%),
Dy3+(2.5%) before heat treatment (left circle) and
after heat treatment (right circle).
Figure 5.25 Emission spectrum of YVO4:Eu3+(0.5%), Dy3+(2.5%) after heat
treatment, plotted with a rainbow curve (corrected for detector sensitivity).
Figure 5.26 Decay curve of the 4F9/2 → 6H13/2 transition
of YVO4:Eu3+(0.5%), Dy3+(2.5%) after heat treatment
with a single exponential fit.
Figure 5.27 Picture of YVO4:Eu3+(0.5%), Dy3+(2.5%) after heat treatment under
the UV lamp, excited at 254 nm (left) and 302 nm (right).
54
5.6.5 Emission maps
YVO4:Dy3(3%) before heat treatment – 43a
When the material is excited at 270 nm instead of 282.2 nm, the CIE coordinates are nearly
the ideal white light coordinates. Since 270 nm is not that far off of the optimal excitation
wavelength, the emission is still quit intense. Especially compared to the emission spectrum
upon excitation at 250 nm. The differences in emission intensity when excitation occurs at
different wavelengths are clearly visible in Fig 5.28. The luminosity increases when exciting
at higher wavelengths. However in the range 280 – 300, the LER values are quite similar,
which is probably because of the broad charge transfer band (see Fig. 5.8).
Table 5.12 Change in CIE coordinates, CCT and LER values when exciting sample YVO4:Dy3+(3%) (before heat treatment) at different
wavelengths. Data of the excitation at the most intense wavelength (282.2 nm) are also presented.
Wavelength (nm) CIE x CIE y CCT (K) LER (lm/W)
250 0.3070 0.3139 6992 290 260 0.3191 0.3269 6174 305
270 0.3320 0.3435 5518 321
280 0.3414 0.3566 5155 335
282.2 0.3475 0.3597 4898 336
290 0.3434 0.3590 5088 336
300 0.3448 0.3614 5044 336
310 0.3419 0.3573 5140 329
Figure 5.28 Emission maps of YVO4:Dy3+(3%) before heat treatment, excited at
different wavelengths of the charge transfer band (corrected for detector
sensitivity).
Figure 5.29 Zoom in of the CIE-diagram of
YVO4:Dy3+(3%) (before heat treatment),
excited at different wavelengths.
55
YVO4:Eu3+(2%), Dy3+(3%) before heat treatment – 58a
Table 5.13 Change in CIE coordinates, CCT and LER values when exciting YVO4:Eu3+ (2%), Dy3+(3%) (before heat treatment) at different
wavelengths. Data of the excitation at the most intense wavelength (303.9 nm) are also presented.
For this material, there is no large
variation in coordinates and
temperatures. The coordinates tend to
centre around a value. This is different
from the behaviour of sample 43a,
where the coordinates were arranged
on a line. The luminous efficiency of
radiation is quite similar in all cases.
Wavelength (nm)
CIE x CIE y CCT (K)
LER (lm/W)
250 0.3993 0.3869 3604 329 260 0.4092 0.3883 3395 340
270 0.4096 0.3894 3397 340
280 0.4079 0.3889 3427 340
290 0.4086 0.3893 3417 340
300 0.4068 0.3901 3461 338
303.9 0.4121 0.3842 3299 323
310 0.3971 0.3864 3651 326
Figure 5.30 Emission maps of
YVO4:Eu3+ (2%), Dy3+(3%) before
heat treatment, excited at
different wavelengths of the
charge transfer band (not
corrected for detector sensitivity).
Figure 5.31 Zoom in of the CIE-diagram of YVO4:Eu3+ (2%), Dy3+(3%) (before heat treatment), excited at different wavelengths.
56
YVO4:Eu3+(0.5%), Dy3+(2.5%) after heat treatment – 69c
Table 5.14 Change in CIE coordinates, CCT and LER values when exciting
YVO4:Eu3+ (0.5%), Dy3+(2.5%) (after heat treatment) at different wavelengths.
Data of the excitation at the most intense wavelength (310.9) is also presented.
The same behaviour as observed with the previous
material (YVO4:Eu3+ (2%), Dy3+(3%) (before heat treatment) is also visible for this material.
Excitation at different wavelengths changes the CIE coordinates, which is presented in Fig.
5.33. However, the exact coordinates in Table 5.14 only show minor differences, as do the
CCT and LER values. These coordinates can be seen as warm white light coordinates.
Apparently, exciting at different points in the charge transfer band does not have a large
effect on the emission properties for this material. The fact that this material is excitable at
different wavelengths without major changes in emission properties might be a good
starting point for matching with pumping LEDs81.
Wavelength (nm)
CIE x CIE y CCT (K)
LER (lm/W
) 250 0.4746 0.4246 2610 390 260 0.4744 0.4265 2626 396
270 0.4758 0.4265 2609 396
280 0.4756 0.4266 2612 396
290 0.4748 0.4269 2624 397
300 0.4745 0.4268 2627 396
310 0.4760 0.4279 2617 397
310.9 0.4769 0.4269 2598 397
320 0.4760 0.4288 2623 399
330 0.4775 0.4283 2601 397
Figure 5.33 Zoom in of the CIE-diagram of
YVO4:Eu3+ (0.5%), Dy3+(2.5%) after heat
treatment, excited at different wavelengths.
Figure 5.32 Emission maps of
YVO4:Eu3+ (0.5%), Dy3+(2.5%)
after heat treatment,
excited at different
wavelengths of the charge
transfer band (corrected for
detector sensitivity).
57
6 Conclusion
Creating a pure phase tungstate material with glycerol as the structure directing agent was
challenging. Strong basic conditions eventually led to the formation of the Y2WO6 phase with
a spherical to rod-like morphology. The blue part of the charge transfer band proved to be
helpful in creating a white light emitting tungstate. The charge transfer band disappeared
after heat treatment, since the energy transfer was more efficient. Only at small doping
percentages (see 4.6.2), a charge transfer band was still visible after heat treatment. The
doping of only 1% Dy3+ in this matrix was sufficient to emit a warm white light after heat
treatment with an LER of 361 lm/W. An overview of all white light emitting Y2WO6 materials
that were synthesized in this thesis are presented in Fig. 6.1 and 6.2.
Figure 6.1 Picture of the doped materials; from left to right: Y2WO6:Eu3+(2.5%), Tb3+(2.5%) before heat treatment, Y2WO6:Dy3+(1%)
before heat treatment, Y2WO6:Dy3+(1%) after heat treatment, Y2WO6:Sm3+(1%) before heat treatment.
Figure 6.2 Picture of the doped materials; from left to right: Y2WO6:Eu3+(3%) before heat treatment, Y2WO6:Sm3+(3%) before heat
treatment, Y2WO6:Eu3+(2%), Tb3+(3%) before heat treatment, Y2WO6:Sm3+(2.5%), Tb3+(2.5%) before heat treatment.
58
The synthesized YVO4 matrix proved to be a good host for doping with various lanthanides.
The hydrothermal route led to a material with three differently shaped particles. A
microwave route led to very nice spherical particles. Contrary to previous reports, terbium
was visible in this matrix. However, its intensity was not as high as one would expect. Doping
percentages lower than 5% led to a charge transfer that was efficient enough to detect the
luminescence of the lanthanides. However, not all the energy was transferred, which
resulted in a blue contribution to the visible light. After heat treatment, the charge transfer
always proved to be more efficient and led to colour shifting towards the most dominant
lanthanide. The combination of this colour shifting and terbium being partially quenched in
this matrix made it a challenge to obtain white light after heat treatment. Multiple white
light emitting materials before heat treatment were obtained. Eventually, a warm white light
emitting material after heat treatment was successfully synthesized by co-doping
dysprosium (2.5%) and europium (0.5%) in this matrix. This material had an LER of 397
lm/W, which is quite high for white light.
Figure 6.3 Picture of YVO4:Eu3+(0.5%), Dy3+(2.5%) after heat treatment under the UV lamp, excited at 254 nm (left) and 302 nm (right).
59
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76. S. Faria and E.J. Mehalchick, J. Electrochem. Soc., 1974, 121, 305-307.
77. M. Shang, C. Li and J. Lin, Chem. Soc. Rev., 2014, 43, 1372-1386.
78. Z. Mu, Y. Hu, L. Chen, X. Wang, G. Ju, Z. Yang and Y. Jin, J. Lmin., 2014, 146, 33-36.
79. Q. Wang, Z. Ci, G. Zhu, S. Xin, W. Zheng, M. Q and Y. Wang, Opt. Mater. Exp., 2014, 4, 142-154.
80. R. G. DeLosh, T. Y. Tien, E. F. Gibbons, P. J. Zachmanidis, H. L. Stadler, J. Chem. Phys., 1969, 53 681-683.
81. P. F. Smet, A. B. Parmentier, D. Poelman, J. Electrochem. Soc., 2011, 158, R37-R54.
82. S. Nishiura, S. Tanabe, K Fujioka, Y. Fujimoto, Opt. Mater., 2011, 33, 688-691.
83. W. Carnell, P. Fields, K. Rajnak, J. Chem. Phys, 1968, 49, 4424-4442, 4450-4455, 4443-4446 .
84. ColorCalculator User Guide, OSRAM Sylvania, Inc.
85. ICSD database #246711 (YVO4 I41/a) , first published by X. Wang, I. Loa, K. Syassen, M. Hanfland and B. Ferrand, Phys.Rev. B, 2004, 70(6), 064109-064115
86. ICSD database #246708 (YVO4 I41/amd) , first published by X. Wang, I. Loa, K. Syassen, M. Hanfland and B. Ferrand, Phys.Rev. B, 2004, 70(6), 064109-064115.
87. ICSD database #20955 (Y2WO6), first published by V.A. Efremov, A.V. Tyulin, V.K. Trunov, O. V. Kudin, V.K. Yanovskii and V.I. Voronkova, Kristallogr., 1984, 29, 904-909.
88. ICSD database #62887 (Gd2(WO4)3) , first published by A.V. Tyulin and V.A. Efremov, Kristallogr., 1987, 32, 371-377.
89. ICSD database #78180 (La2(WO4)3) , first published by M. Gaertner, D. Abeln, A. Pring, M. Wilde and A. Reller, J. Solid State Chem., 1994, 111, 128-133.
63
Appendix A
Experimental
1 Used chemicals
The used chemicals are yttrium nitrate (Y(NO3)3∙6H2O, 99.9%, Sigma Aldrich), europium
nitrate (Eu(NO3)3∙6H2O, 99.9%, Alfa Aesar), terbium nitrate (Tb(NO3)3∙5H2O, 99.9% Sigma
Aldrich), lanthanum nitrate (La(NO3)3∙6H2O, 99.9% Merck Chemicals), gadolinium nitrate
(Gd(NO3)3∙6H2O, 99.9%, Sigma Aldrich), dysprosium nitrate (Dy(NO3)3∙xH2O, 99.9%, Sigma
Aldrich), samarium nitrate (Sm(NO3)3∙6H2O, 99.9%, Acros Organics), thulium nitrate
(Tm(NO3)3∙5H2O, 99.9%, Sigma Aldrich), praseodymium nitrate (Pr(NO3)3∙6H2O, 99.9% Sigma
Aldrich), sodium tungstate (Na2WO4∙2H2O, 99.9%, Sigma Aldrich), sodium orthovanadate
(Na3VO4, 99.98%, Sigma Aldrich), glycerol (bidistilled, 99.5%, VWR Chemicals), ethanol (EtOH,
96%, Fiers), nitric acid (HNO3, 53%, Roth), ammonia solution (NH3(aq), 28-30% NH3 basis,
Sigma Aldrich). All chemicals were used without further purification.
2 Hydrothermal surfactant assisted synthesis of rare-earth tungstates
and vanadates
In a typical reaction route, a certain amount of glycerol (0-20 mL) is added to 10 mL of
distilled water. The Y(NO3)3∙6H2O salt (1 mmol) is dissolved in 10 mL of distilled water and
then added to the glycerol dissolved in 10 mL of distilled water (1). This solution is stirred for
15 minutes. The Na2WO4∙2H2O or Na3VO4 salt (1 or 2 mmol) is dissolved in 10 mL of distilled
water and stirred for 15 minutes (2). Solution 2 is now slowly added to solution 1. The pH of
this solution was adjusted to a certain value by addition of HNO3 or NH3(aq). After
magnetically stirring the solution for 10 minutes, it is transferred into a Teflon lined
autoclave, sealed tightly to prevent the material from escaping and heated to a certain
temperature (140-200°C) for a certain amount of time (1-48 h). After the chosen time, the
autoclave was naturally cooled down to room temperature for 24 h. The product was
centrifuged (for 5 minutes at 6000-7000 rpm) and the powder at the bottom of the
centrifuge tube was washed 3 times with water and 3 times with ethanol. The product was
dried in a vacuum oven at 55°C overnight. All samples were heat-treated at 900-1100°C for 3
h to remove all excess products.
64
All samples were investigated by XRD and DRIFTS measurements to determine which
reaction circumstances (pH, temperature, reaction time, amount of glycerol) led to the
formation of a pure phase material.
The synthesis route that led to a pure phase material was performed again, but doped with
various lanthanides (Eu3+, Tb3+, Dy3+, Sm3+, Pr3+, Tm3+). Different doping percentages were
used (x varied from 0.01 (1% doping) to 0.05 (5% doping)). The synthesis route remained the
same, but instead 1-x mmol of Y(NO3)3∙6H2O and x mmol of lanthanide salt was used.
3 Microwave surfactant assisted synthesis of rare-earth tungstates and
vanadates
In a typical reaction route, a certain amount of glycerol (0-10 mL) is added to 5 mL of
distilled water. The Y(NO3)3∙6H2O salt (0.5 mmol) is dissolved in 5 mL of distilled water and
then added to the glycerol dissolved in 5 mL of distilled water (1). This solution is stirred for
15 minutes. The Na2WO4∙2H2O or Na3VO4 salt (0.5 mmol) is dissolved in 5 mL of distilled
water and stirred for 15 minutes (2). Solution 2 is now slowly added to solution 1. The pH is
of this solution was adjusted to a certain value by addition of HNO3 or NaOH.
The product was transferred into a microwave reactor tube and tightly sealed with a cap. A
reaction of 1 hour under continuous stirring was performed at a pressure of 15 bar. The
reaction was performed under continuous microwaves (power of 200 W and PowerMax
function was on). The product was centrifuged (for 5 minutes at 6000-7000 rpm) and the
powder at the bottom of the centrifuge tube was washed 3 times with water and 3 times
with ethanol. The product was dried in a vacuum oven at 55°C overnight. All samples were
heat treated at 900-1100°C for 3h to remove all excess products.
For the microwave synthesis a CEM Discover SP with autosampler was used. The equipment
has a maximum power of 300 W, maximum pressure of 27 bar and Tmax = 270°C.
4 Sol-gel surfactant assisted synthesis of rare-earth tungstates and
vanadates
The Y(NO3)3∙6H2O (1 mmol) is dissolved in distilled water (10 mL) and glycerol (0-20 mL). This
mixture was stirred for 1.5 h at room temperature. Then the Na2WO4∙2H2O or Na3VO4 salt (1
or 2 mmol) is added to this solution. The mixture was heated to 80°C until a gel formed. The
gel is put into a crubicle and then heated in an oven for 5 h at 800°C. The solids were finely
grinded to powders.
65
5 Overview of all performed syntheses
Table A.1 An overview of all performed syntheses in this thesis. Different synthesis routes were performed; H: hydrothermal, F; flask
reaction, S: sol-gel, M: microwave.
# Route T (°C)
t (h)
pH Glycerol (mL)
RE(NO3)3
∙6H2O (g)a
Na2WO4
∙2H2O (g)
Na3VO4 (g)
Doping
1 H 200 6 4 20 0.383 0.33
2 H 200 6 6 20 0.383 0.33
3 H 180 24 6 0 0.383 0.33
4 H 180 24 6 5 0.383 0.33
5 H 180 24 6 10 0.383 0.33
6 H 200 24 6 20 0.364 0.33 5% Eu3+
7 H 200 24 6 20 0.364 0.33 5% Tb3+
8 H 160 24 3 20 0.383 0.33
9 H 160 24 4 20 0.383 0.33
10 H 160 24 5 20 0.383 0.33
11 H 160 24 6 20 0.383 0.33
12 H 160 24 7 20 0.383 0.33
13 H 160 24 8 20 0.383 0.33
14 H 160 24 9 20 0.383 0.33
15 H 160 24 10 20 0.383 0.33
16 H 200 24 11 10 0.383 0.33
17 H 200 24 12 10 0.383 0.33
18 H 200 24 13 10 0.383 0.33
19 H 180 24 3 10 0.383 0.66
20 H 180 24 6 10 0.383 0.66
21 H 180 24 9 10 0.383 0.66
22 H 180 24 2 0 0.383 0.184
23 H 180 24 2 10 0.383 0.184
24 H 180 24 10 10 0.383 0.184
25 H 180 24 10 10 0.383 0.368
26 H 180 24 6 2.5 0.383 0.33
27 H 180 24 6 5 0.383 0.33
28 H 140 24 3 5 0.383 0.33
29 H 140 24 9 5 0.383 0.66
30 H 140 24 9 15 0.383 0.66
31 H 200 24 2 10 0.364 0.184 5% Eu3+
32 H 200 24 2 10 0.364 0.184 5% Dy3+
33 H 200 24 2 10 0.364 0.184 5% Sm3+
34 S 800 5 2 20 0.364 0.184
35 S 800 5 13 20 0.364 0.33
36 H 200 24 2 20 0.364 0.184 5% Tb3+
37 H 200 24 13 20 0.4331 0.33
38 H 200 24 13 20 0.4512 0.33
39 H 200 48 2 20 0.728 0.368 2% Eu3+
40 H 200 48 2 20 0.728 0.368 2% Dy3+
41 H 200 48 2 20 0.728 0.368 2% Sm3+
66
42 H 200 24 2 20 0.364 0.184 3% Eu3+
43 H 200 24 2 20 0.364 0.184 3% Dy3+
44 H 200 24 2 20 0.364 0.184 3% Sm3+
45 F 100 48 2 20 0.364 0.184 5% Eu3+
46 H 200 24 13 20 0.383 0.3299
47 H 200 24 2 15 0.364 0.184 2% Tb3+
48 H 200 24 2 15 0.364 0.184 5% Tb3+
49 H 200 24 2 20 0.364 0.184 2% Eu3+, 2% Dy3+, 2% Tm3+
50 H 200 24 2 20 0.364 0.184 2.5% Eu3+, 2.5% Tb3+
51 H 200 24 2 20 0.364 0.184 2% Eu3+, 2% Tb3+, 2% Tm3+
52 H 200 24 2 20 0.364 0.184 2% Eu3+, 2% Dy3+, 2% Tm3+
53 H 200 24 2 0 0.364 0.184
54 H 200 24 13 0 0.383 0.3299
55 H 200 24 13 20 0.3639 0.3299 2.5% Eu3+, 2.5% Tb3+
56 H 200 24 13 20 0.3792 0.3299 1% Dy3+
57 H 200 24 13 20 0.3792 0.3299 1% Sm3+
58 H 200 24 2 20 0.3639 0.1839 2% Eu3+, 3% Dy3+
59 H 200 24 2 20 0.3639 0.1839 2% Eu3+, 3% Tb3+
60 H 200 24 2 20 0.3811 0.1839 0.5% Dy3+
61 H 200 24 13 20 0.3715 0.3299 3% Eu3+
62 H 200 24 13 20 0.3715 0.3299 3% Sm3+
63 H 200 24 13 20 0.3715 0.3299 3% Pr3+
64 H 200 24 13 20 0.3639 0.3299 2% Eu3+, 3% Tb3+
65 H 200 24 13 20 0.3639 0.3299 2.5% Sm3+, 2.5% Tb3+
66 H 200 24 2 20 0.3639 0.1839 5% Tb3+
67 M 200 1 13 10 0.1896 0.16495 1% Dy3+
68 M 200 1 2 10 0.18385 0.09195 1% Eu3+, 3% Tb3+
69 H 200 24 2 20 0.3677 0.1839 0.5% Eu3+, 2.5% Dy3+ aThe different rare-earth (RE) salts that were used are La(NO3)3∙6H2O
1, Gd(NO3)3∙6H2O2 and Y(NO3)3∙H2O if not indicated otherwise.
Table A.2 Overview of the different doping percentages and amounts that were used.
Eu(NO3)3∙6H2O (g) Dy(NO3)3∙xH2O (g) Sm(NO3)3∙6H2O (g) Tb(NO3)3∙5H2O (g)
M (g/mol) 446.086 456.624 444.47 435.037
1% 0.0045 0.0046 0.0044 0.0044
2% 0.0089 0.0091 0.0089 0.0087
2.5% 0.0112 0.0114 0.0111 0.0109
3% 0.0134 0.0137 0.0133 0.0131
4% 0.0178 0.0183 0.0178 0.0174
5% 0.0223 0.0228 0.0222 0.0218
67
Appendix B
Equipment and techniques
1 Scanning electron microscopy (SEM)
A SEM is an electron microscope that is used to generate an image of a sample to show its
size and morphology. With this technique, a sample is bombarded with accelerated
electrons that interact with the surface. The surface can be grounded in order to remove the
charges which can complicate imaging. These interactions with the sample lead to different
signals such as secondary electrons, X-rays and backscattered electrons. After detection of
these signals an image of the sample’s topography is generated. The different signals give
the detector different information. The secondary electrons give the image a 3D-like
appearance. Interaction with the sample also results in X-rays, which can be interesting for
chemical analysis. The amount of backscattered electrons can give more information about
the atomic number. SEM measurements were performed with a FEI Quanta 200 F SEM and a
FEI Nova 600 Nanolab Dual-Beam focused ion beam in secondary electron mode.
2 Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS)
Absorption of electromagnetic radiation of the right wavelength can excite atoms in
molecules to higher vibrational levels. This energy is absorbed from the infrared region of
the electromagnetic light. Every group of atoms vibrates at a specific wavelength. A
spectrum is collected in which the absorption in function of the wavelength is monitored71.
When a solid sample is being irradiated, the energy will reflect off the sample. There are two
ways in which the light can be reflected: specular reflectance and diffuse reflectance. The
specular component is the light that reflects directly off the surface of the sample while
diffuse reflectance is reflected after the radiation penetrated into the sample and reflected
back. The solid samples were finely ground and mixed with KBr, which functions as a non-
absorbing matrix. This is done to increase the amount of diffuse reflectance. A KBr
background file was used to correct. With the DRIFTS technique the specular reflectance is
minimized while the diffuse reflectance is maximized and this signal is detected by the
infrared detector.
All samples were measured under vacuum and at room temperature, at which most of the
molecules were in their lowest vibrational state. A Thermo Scientific FT-IR spectrometer
(type Nicolet 6700) equipped with a DRIFTS-cel was used. Spectra were recorded from 4000
cm-1 to 650 cm-1 with a resolution of 4 cm-1.
68
3 X-ray diffraction (XRD)
X-ray diffraction is a technique that creates a diffractogram from crystalline materials72. This
diffractogram is material-specific and can thus be used to confirm which material or which
phase has been synthesized. XRD is a non-destructive technique. A monochromatic X-ray
beam is projected onto a well-grinded powdery sample at angle theta. Diffraction occurs at
the lattice. Only the diffracted X-rays that follow Bragg’s Law and lead to constructive
interference are detected. Bragg’s Law states that (with the order of
diffraction (integer), the wavelength of the incident wave, the distance between the
planes of the lattice and the angle between the incoming wave and the planes).
The sample is mounted on a goniometer which rotates and allows the sample to be scanned
through the whole 2 -range by projecting the X-rays at all possible diffraction directions.
The intensity of the reflected X-rays is plotted in function of the 2 -angle and a
diffractogram is generated.
The diffractograms can be compared with the database of the ‘International Centre for
Diffraction Data’ (ICDD), which contains over 50000 powdery diffractograms. Determining
the phase of the material can also be done based on the ICSD crystallographic database.
Based on the crystal structure of a material, the XRD diffractogram can be calculated.
XRD patterns were recorded by a Thermo Scientific ARL X’TRA diffractometer equipped with
a CuKα (λ = 1.5405 Å) source, a goniometer and a Peltier cooled Si(Li) solid state detector.
4 Luminescence setup
A luminescence setup was used for luminescence measurements. For steady-state
measurements a xenon-lamp was used as the excitation source. Time-resolved
measurements were performed with a pulsed beam at a fixed wavelength. This setup is
equipped with excitation and emission monochromators. Depending on the region in which
the lanthanide emits (VIS, IR, NIR), different detectors can be used. Excitation spectra,
emission spectra and decay curves were measured with this setup.
When recording an excitation spectrum, the emission monochromator is fixed at a certain
wavelength, which is normally the most intense emissive peak. The excitation
monochromator then scans the wavelength-range of interest. From this spectrum the most
efficient excitation wavelength can be found. In the case of an emission spectrum, the
process is similar. The excitation monochromator is then fixed at a certain wavelength while
69
the emission monochromator scans the wavelength-range. A scan yields a spectrum where
the intensity is shown in function of the wavelength. Measuring the decay time of a sample
is also an option with this setup. A short-lived excitation source is used for this purpose. The
sample is irradiated for a short time and the received photons of a certain wavelength are
measured in function of time. The decay was measured for 200 s. The decay curves can be
fitted to single or higher exponential equations to obtain the decay times. For a material that
has two lifetimes, the average lifetime can also be calculated.
Single exponential tail fit
Double exponential tail fit
The average lifetime (
)
Where is the intensity at time , the intensity at , the lifetime of the material.
Since f-f-transitions are Laporte forbidden, lanthanides can also be excited via the antenna-
effect. For rare-earth tungstates, there exists a broad charge transfer band from the 2p-
oxygen to the 5d-tungsten orbitals in the emission spectrum that originates from the
tungstate at a maximum of about 270 nm. Rare-earth vanadates also show a broad charge
transfer band (with a maximum intensity at around 320 nm) which is associated with the
charge transfer through the V-O bond. The materials can be excited trough the charge
transfer band to see how efficient the charge transfer is.
Luminescent measurements were performed on a Double Edinburgh Instruments
FLS920/FSP920 spectrometer. The setup contains 2 systems from which only the UV-VIS-NIR
spectrometer is used. Steady-state measurements can be performed in the range 250-1700
nm. This was done with a 450 W xenon lamp with a Hamamatsu R928P PMT detector for the
200-870 nm range. Time-resolved measurements of 100 µs – several seconds were recorded
with a µF920H 60W microsecond xenon flash-lamp: 0.1 - 100 Hz, pulse width ± 2 µs or using
the Continuum Surelite I laser (450 mJ at 1064 nm), operating at a repetition rate of 10Hz
and using the third harmonic (355 nm) as the excitation source, and the photomultiplier
detector.
Quantum yield measurements are performed with the use of an Integrating Sphere (300-
1700 nm). The sphere has a 120 mm inside diameter spherical cavity, coated with Benflec®
and is surrounded by an aluminum shell.
70
The luminescent quantum yield is a measure for the probability that a sample emits the
photons that are absorbed. The quantum yield can be calculated via the following equation.
Quantum yield
∫
∫
Where equals the quantum yield, ε stands for the photons emitted by the sample and α
stands for the photons absorbed by the sample. is the emission spectrum of the
sample, using the sphere. is the spectrum of the light used for excitation, without any
sample present, using the sphere. is the spectrum of the light used to excite the
sample, using the sphere. Only solid samples were measured. This is a non-destructive
technique. All spectra were recorded with a 0.1 nm step size and 0.2 s dwell time. The slit
sizes varied depending on the intensity of the measured samples. The Dieke diagram gives
an overview of all possible 4f-4f transitions.
Figure B.1 Dieke diagram: The energy levels of rare-earth ions in crystals (Interscience Publishers. New York. 1968).
71
5 Commission Internationale de l’Éclairage (CIE)
The colour that is perceived by the human eye can vary from the actual colour of a material.
Since perception is a tricky subject, an objective specification of colour is necessary. The
‘Commission International de l’Éclairage’ (CIE) created this. It developed the CIE 1931
RGBCIE 1931 XYZ colour spaces, which defines colours in a mathematical way.
In this thesis, the goal is to obtain a white-light emitting material. The CIE coordinates can be
derived based on the emission spectra. The CIE coordinates for standard white light are x =
0.333, y = 0.333. For white light, the temperature of the colour is also an important aspect.
White light can be observed warm or cold. To describe the temperature of a colour, the
correlated colour temperature (CCT) is used. Warm colours have a CCT < 3000 K, cold colours
have a CCT > 5000 K.
The CIE coordinates and CCT values were calculated via a ColorCalculator program provided
by Sylvania.
6 Hydrothermal synthesis
The synthesis of the rare-earth tungstates and vanadates can be performed via a
hydrothermal synthesis route4, 6-8, 48-50, 73. These reactions have water as a medium which has
a double function. The water is used as a solvent, but also as a pressure-transmitting
medium. The products in aqueous medium are dissolved in a Teflon-lined steel autoclave
which is filled up to 80% capacity. The autoclave must be sealed tight in order for the
solvents not to evaporate. The autoclave is heated in an oven, usually at temperatures from
140 to 200°C. These conditions lead to high pressures in the autoclave. Heating the
autoclave creates two temperature zones. In the lower and hotter part of the autoclave, the
reagent dissolves and a saturated aqueous solution is created. This solution in the lower part
goes to the upper part due to convective motions in the solution. The solution in the upper
and cooler part of the autoclave thus descends while a solution ascents. In the upper part of
the autoclave, the solution becomes supersaturated because the temperature is reduced
and crystallization occurs.
72
7 Microwave synthesis
The microwave synthesis method is a more efficient heating method than the hydrothermal
synthesis method. When performing a hydrothermal synthesis, the autoclave is placed into
an oven at a certain temperature. It takes a while before the core of the sample reaches the
target temperature. A microwave assisted synthesis directly heats the core of the sample
without first having to heat the vessel itself4, 76.
8 Sol-gel synthesis
In this synthesis method, the reaction medium changes from a sol (solid particles dispersed
in a liquid) to a gel (solid encapsulating a liquid) by heating. When the gel is formed, it can be
dried by evaporation4, 51, 67.
9 Microemulsion synthesis
A microemulsion is a stable liquid mixture that contains two immiscible liquids (oil and
water) that are stabilized by addition of a surfactant. The reagents that are added to this
microemulsion are captured in the core of the micelles. Due to random collisions the
micelles can form dimers at which point they can exchange materials. After the exchange,
the micelles break apart again4, 5, 55.
10 Molten salt synthesis
In this technique, a large amount of a salt is used as the reaction medium. The mixture of
this salt and the reagents are heated to the melting point of the salt and the salt acts as the
solvent. After the reaction, the mixture is cooled down and the salt can be removed with the
use of an appropriate solvent4. This is a fast and easy way to perform a reaction. However,
the reaction conditions are limited to the salt. No SDA’s can be used with this method4, 58.
11 Solid state reaction
This reaction method makes no use of additional salts, solvents or structure directing agents.
The reagents are finely ground and mixed well together to maximize the surface area. The
particles are heated to the appropriate temperature4, 57, 62.
12 Czochralski method
In this method, the reagents are first finely grinded and then mixed. After this they are
heated until a melt is formed. A seed crystal can be used to induce crystallization, after
which the crystal is slowly pulled out of the melt. The reaction is performed in an inert
atmosphere52.
73
Appendix C
Diffuse Reflectance Infrared Fourier Transform Spectra
Before heat treatment – Red graph. After heat treatment – Blue graph. Sample numbers can be found in Table A.1.
Figure C.1 Sample 1 Figure C.2 Sample 2
Figure C.3 Sample 3 Figure C.4 Sample 4
Figure C.5 Sample 5 Figure C.6 Sample 6
74
Figure C.7 Sample 7 Figure C.8 Sample 8
Figure C.9 Sample 9 Figure C.10 Sample 10
Figure C.11 Sample 11 Figure C.12 Sample 12
75
Figure C.13 Sample 13 Figure C.14 Sample 14
Figure C.15 Sample 16 Figure C.16 Sample 17
Figure C.17 Sample 18 Figure C.18 Sample 19
76
Figure C.19 Sample 20 Figure C.20 Sample 21
Figure C.22 Sample 24 Figure C.21 Sample 23
Figure C.23 Sample 25 Figure C.24 Sample 26
77
Figure C.25 Sample 27 Figure C.26 Sample 41
Figure C.27 Sample 62
78
Appendix D
X-ray diffractograms
1 Rare- earth vanadates
2 Rare-earth tungstates
Figure D.1 XRD diffractograms of samples 31, 32, 33, 34 and 53
after heat treatment at 900°C.
Figure D.2 XRD diffractogram of samples 1-7 after
heat treatment at 900°C.
Figure D.3 XRD diffractograms of samples 8-12. The samples
labelled with ‘c’ are heat treated at 900°C. The samples labelled
with ‘e’ are re-heat treated at 1100°C
79
Figure D.5 XRD diffractograms of samples 19, 20, 21, 26, 27,
28. The samples labelled with ‘c’ are heat treated at 900°C.The
samples labelled with ‘e’ are re-heat treated at 1100°C.
Figure D.4 XRD diffractograms of samples 13-18. The samples
labelled with ‘c’ are heat treated at 900°C. The samples
labelled with ‘e’ are re-heat treated at 1100°C.
Figure D.6 XRD diffractograms of samples 29, 30, 35, 54 and 56. The
samples labelled with ‘a’ are not heat treated. The samples labelled with ‘c’
are heat treated at 900°C
80
Appendix E
Luminescence data
Labels; ‘a’: before heat treatment, ‘c’: after heat treatment.
Decay curves were always recorded for the most intense transition that was visible in the
emission spectrum, unless specified differently.
1 Correction for detector sensitivity
The Hamamatsu R928P PMT detector has a range from 200 to 870 nm. At the edges of this
range, the detector is less sensitive. For this sensitivity, the measured spectra should be
corrected. However, correction often leads to the blowing out of proportion of the spectrum
at the edges of the range of the detector, since the sensitivity is much lower there. Below
are some examples of this effect. When this effect is too dominant, the uncorrected
spectrum is presented.
Figure E.1 Emission spectrum of sample 58a, excited through the charge transfer band at 303.9 nm, corrected for detector sensitivity
(left) and not corrected for detector sensitivity (right).
Figure E.2 Excitation spectrum of sample 58a, monitored at 617.7 nm corrected for detector sensitivity (left) and not corrected for
detector sensitivity (right).
81
2 Luminescence spectra
Sample 31a
Figure E.3 (Left) Excitation spectrum of sample 31a, monitored at 618.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 31a, excited through the charge transfer band at 289.0 nm (corrected for detector sensitivity).
Table E.1 Overview of the Eu3+ transitions in sample 31a.
Table E.2 Single exponential decay characteristics of sample 31a.
Exponential tail fit R2 (µs)
Single 0.9921 448.48
Symbol Wavelength (nm) Wavenumber (cm-1) Eu3+ (5%) Transitions
a 586.1 17062 5D0 → 7F0
b 593.5 16849 5D0 → 7F1
c 618.4 16171 5D0 → 7F2
d 651.0 15361 5D0 → 7F3
e 700.5 14276 5D0 → 7F4
Figure E.4 Decay curve of sample 31a with a single
exponential fit for the 5D0 → 7F2 transition.
82
Sample 31c
Table E.3 Overview of the emissive Eu3+ transitions in sample 31c.
Symbol Wavelength (nm) Wavenumber (cm-1) Eu3+ (5%) Transitions
a 586.9 17039 5D0 → 7F0
b 594.0 16835 5D0 → 7F1 c 616.8 16213 5D0 → 7F2 d 650.5 15373 5D0 → 7F3 e 699.6 14294 5D0 → 7F4
Table E.4 Overview of the excitation transitions of Eu3+ in sample 31c.
Symbol Wavelength (nm) Wavenumber (cm-1) Eu3+ (5%) Transitions
a 362.6 27579 5D4 ← 7F0 b 376.0 26596 5G3 ← 7F0 c 395.1 25310 5L6 ← 7F0 d 416.6 24004 5D3 ← 7F0 e 465.7 21473 5D2 ← 7F0
Table E.5 Single exponential decay characteristics of sample 31c.
Exponential tail fit R2 (µs)
Single 0.9967 490.80
Figure E.5 (Left) Excitation spectrum of sample 31c, monitored at 619.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 31c, excited through the charge transfer band at 302.0 nm (corrected for detector sensitivity).
Figure E.6 Decay curve of the 5D0 → 7F2 transition of sample
31c with a single exponential fit.
83
Sample 32a
Figure E.7 (Left) Excitation spectrum of sample 32a, monitored at 574.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 32a, excited through the charge transfer band at 290.0 nm (corrected for detector sensitivity).
Table E.6 Overview of the Dy3+ transitions in sample 32a.
Table E.7 Exponential decay characteristics of sample 32a.
Symbol Wavelength (nm) Wavenumber (cm-1) Dy3+ (5%) Transitions
a 473.4 21124 4F9/2 → 6H15/2
b 571.7 17492 4F9/2 → 6H13/2
c 662.1 15103 4F9/2 → 6H11/2
Exponential tail fit R2 (µs)
Single 0.9764 36.12
Figure E.8 Decay curve of the 4F9/2 → 6H13/2 transition of sample 32a
with a single exponential fit.
84
Sample 32c
Figure E.9 (Left) Excitation spectrum of sample 32c, monitored at 574.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 32c, excited through the charge transfer band at 314.0 nm (corrected for detector sensitivity).
Table E.8 Overview of the emissive Dy3+ transitions in sample 32c.
Symbol Wavelength (nm) Wavenumber (cm-1) Dy3+ (5%) Transitions
a 473.4 21124 4F9/2 → 6H15/2 b 574.3 17413 4F9/2 → 6H13/2 c 662.1 15103 4F9/2 → 6H11/2
Table E.9 Overview of the excitation transitions of Dy3+ in sample 32c.
Symbol Wavelength (nm) Wavenumber (cm-1) Dy3+ (5%) Transitions
a 352.5 28369 6P7/2 ← 6H15/2 b 366.1 27315 6P5/2 ← 6H15/2 c 389.5 25674 4F7/2 ← 6H15/2 d 449.2 22262 4I15/2 ← 6H15/2
Table E.10 Single exponential decay characteristics of sample 32c.
Exponential tail fit R2 (µs)
Single 0.9901 70.74
Figure E.10 Decay curve of the 4F9/2 → 6H13/2 transition of
sample 32c with a single exponential fit.
85
Sample 33a
Table E.11 Overview of the Sm3+ transitions in sample 33a.
Table E.12 Exponential decay characteristics of sample 33a.
Symbol Wavelength (nm)
Wavenumber (cm-1) Sm3+ (5%) Transitions
a 563.8 17737 4G5/2 → 6H5/2 b 601.9 16614 4G5/2 → 6H7/2 c 645.3 15497 4G5/2 → 6H9/2 d 703.6 14213 4G5/2 → 6H11/2
Exponential tail fit R2 (µs)
Single 0.9949 261.01
Figure E.11 (Left) Excitation spectrum of sample 33a, monitored at 646.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 33a, excited through the charge transfer band at 298.0 nm (corrected for detector sensitivity).
Figure E.12 Decay curve of the 4G5/2 → 6H9/2 transition
of sample 33a with a single exponential fit.
86
Sample 33c
Figure E.13 (Left) Excitation spectrum of sample 33c, monitored at 646.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 33c, excited through the charge transfer band at 301.0 nm (corrected for detector sensitivity).
Table E.13 Overview of the Sm3+ transitions in sample 33c.
Symbol Wavelength (nm) Wavenumber (cm-1) Sm3+ (5%) Transitions
a 564.5 17715 4G5/2 → 6H5/2 b 602.1 16609 4G5/2 → 6H7/2 c 645.7 15487 4G5/2 → 6H9/2 d 703.7 14211 4G5/2 → 6H11/2
Table E.14 Single exponential decay characteristics of sample 33c.
Exponential tail fit R2 (µs)
Single 0.9990 429.38
Figure E.14 Decay curve of the 4G5/2 → 6H9/2
transition of sample 33c with a single
exponential fit.
87
Sample 39a
Figure E.15 (Left) Excitation spectrum of sample 39a, monitored at 618.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 39a, excited through the charge transfer band at 301.2 nm (corrected for detector sensitivity).
Table E.15 Overview of the Eu3+ transitions in sample 39a.
Symbol Wavelength (nm) Wavenumber (cm-1) Eu3+ (2%) Transitions
a 585.6 17077 5D0 → 7F0
b 593.8 16841 5D0 → 7F1 c 618.2 16176 5D0 → 7F2 d 651.8 15342 5D0 → 7F3 e 697.7 14333 5D0 → 7F4
Table E.16 Single exponential decay characteristics of sample 39a.
Exponential tail fit R2 (µs)
Single 0.9946 355.33
Figure E.16 Decay curve of the 5D0 → 7F2
transition of sample 39a with a single
exponential fit.
88
Sample 39c
Figure E.17 (Left) Excitation spectrum of sample 39c, monitored at 618.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 39c, excited through the charge transfer band at 314.0 nm (corrected for detector sensitivity).
Table E. 17 Overview of the emissive Eu3+ transitions in sample 39c.
Symbol Wavelength (nm) Wavenumber (cm-1) Eu3+ (2%) Transitions
a 586.0 17064.8 5D0 → 7F0
b 593.0 16863.4 5D0 → 7F1 c 616.8 16212.7 5D0 → 7F2 d 651.6 15346.8 5D0 → 7F3 e 700.5 14275.5 5D0 → 7F4
Table E.18 Overview of the excitation transitions of Eu3+ in sample 39c.
Symbol Wavelength (nm) Wavenumber (cm-1) Eu3+ (2%) Transitions
a 362.2 27609 5D4 ← 7F0 b 380.7 26267 5G2 ← 7F0 c 394.7 25336 5L6 ← 7F0 d 416.6 24004 5D3 ← 7F0 e 465.7 21473 5D2 ← 7F0
Table E.19 Exponential decay characteristics of sample 39c.
Exponential tail fit R2 (µs)
Single 0.9990 520.69
Figure E.18 Decay curve of the 5D0 → 7F2 transition
of sample 39c with a single exponential fit.
89
Sample 40a
Figure E.19 (Left) Excitation spectrum of sample 40a, monitored at 574.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 40a, excited through the charge transfer band at 287.2 nm (corrected for detector sensitivity).
Table E.20 Overview of the Dy3+ transitions in sample 40a.
Symbol Wavelength (nm) Wavenumber (cm-1) Dy3+ (2%) Transitions
a 483.2 20695 4F9/2 → 6H15/2 b 573.1 17449 4F9/2 → 6H13/2 c 661.7 15113 4F9/2 → 6H11/2
Table E.21 Exponential decay characteristics of sample 40a.
Exponential tail fit R2 (µs)
Single 0.9724 64.18
Figure E.20 Decay curve of the 4F9/2 → 6H13/2 transition of
sample 40a with a single exponential fit.
90
Sample 40c
Figure E.21 (Left) Excitation spectrum of sample 40c, monitored at 574.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 40c, excited through the charge transfer band at 310.0 nm (corrected for detector sensitivity).
Table E.22 Overview of the emissive transitions of Dy3+ in sample 40c.
Symbol Wavelength (nm) Wavenumber (cm-1) Dy3+ (2%) Transitions
a 483.2 20694 4F9/2 → 6H15/2 b 574.2 17416 4F9/2 → 6H13/2 c 662.4 15097 4F9/2 → 6H11/2
Table E.23 Overview of the excitation transitions of Dy3+ in sample 40c.
Symbol Wavelength (nm) Wavenumber (cm-1) Dy3+ (2%) Transitions
a 352.3 28385 6P7/2 ← 6H15/2 b 365.8 27337 6P5/2 ← 6H15/2 c 389.4 25681 4F7/2 ← 6H15/2 d 449.0 22272 4I15/2 ← 6H15/2
Table E.24 Exponential decay characteristics of sample 40c.
Exponential tail fit R2 (µs)
Single 0.9913 97.83
Figure E.22 Decay curve of the 4F9/2 → 6H13/2 transition
of sample 40c with a single exponential fit.
91
Sample 41a
Figure E.23 (Left) Excitation spectrum of sample 41a, monitored at 646.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 41a, excited through the charge transfer band at 290.0 nm (corrected for detector sensitivity).
Table E.25 Overview of the Sm3+ transitions in sample 41a.
Symbol Wavelength (nm) Wavenumber (cm-1) Sm3+ (2%) Transitions
a 564.2 17724 4G5/2 → 6H5/2 b 601.9 16614 4G5/2 → 6H7/2 c 645.6 15489 4G5/2 → 6H9/2 d 703.8 14209 4G5/2 → 6H11/2
Table E.26 Exponential decay characteristics in sample 41a.
Exponential tail fit R2 (µs)
Single 0.9951 325.14
Figure E.24 Decay curve of the 4G5/2 → 6H9/2
transition of sample 41a with a single
exponential fit.
92
Sample 41c
Figure E.25 (Left) Excitation spectrum of sample 41c, monitored at 616.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 41c, excited through the charge transfer band at 309.0 nm (corrected for detector sensitivity).
Table E.27 Overview of the emissive Sm3+ transitions in sample 41c.
Table E.28 Overview of the excitation transitions of Sm3+ in sample 41c.
Table E.29 Single exponential decay characteristics of sample 41c.
Symbol Wavelength (nm) Wavenumber (cm-1) Sm3+ (2%) Transitions
a 564.3 17721 4G5/2 → 6H5/2 b 601.3 16631 4G5/2 → 6H7/2 c 646.0 15480 4G5/2 → 6H9/2 d 703.2 14221 4G5/2 → 6H11/2
Symbol Wavelength (nm) Wavenumber (cm-1) Sm3+ (2%) Transitions
a 365.5 27360 4D3/2 ← 6H5/2 b 379.6 26344 4D1/2 ← 6H5/2 c 406.8 24582 4L13/2 ← 6H5/2 d 419.1 23861 4M19/2 ← 6H5/2 e 466.8 21422 4I13/2 ← 6H5/2 f 479.6 20851 4M15/2 ← 6H5/2
Exponential tail fit R2 (µs)
Single 0.9973 576.35
Figure E.26 Decay curve of the 4G5/2 → 6H9/2 transition
of sample 41c with a single exponential fit.
93
Sample 42a
Figure E.27 (Left) Excitation spectrum of sample 42a, monitored at 618.5 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 42a, excited through the charge transfer band at 269.1 nm (not corrected for detector sensitivity).
Table E.30 Overview of the Eu3+ transitions in sample 42a.
Table E.31 Exponential decay characteristics of sample 42a.
Symbol Wavelength (nm) Wavenumber (cm-1) Eu3+ (3%) Transitions
a 593 16863.4 5D0 → 7F1 b 618.8 16160.3 5D0 → 7F2 c 649.9 15387.0 5D0 → 7F3 d 701.6 14253.1 5D0 → 7F4
Exponential tail fit R2 (µs)
Single 0.9914 309.94
Figure E.28 Decay curve of the 5D0 → 7F3
transition of sample 42a with a single
exponential fit.
94
Sample 42c
Figure E.29 (Left) Excitation spectrum of sample 42c, monitored at 618.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 42c, excited through the charge transfer band at 307.4 nm (corrected for detector sensitivity).
Table E.32 Overview of the emissive Eu3+ transitions in sample 42c.
Symbol Wavelength (nm) Wavenumber (cm-1) Eu3+ (3%) Transitions
a 585.8 17071 5D0 → 7F0
b 593.6 16846 5D0 → 7F1 c 614.4 16276 5D0 → 7F2 d 647.6 15442 5D0 → 7F3 e 697.5 14337 5D0 → 7F4
Table E.33 Overview of the excitation transitions of Eu3+ in sample 42c.
Table E.34 Exponential decay characteristics of sample 42c.
Symbol Wavelength (nm) Wavenumber (cm-1) Eu3+ (3%) Transitions
a 362.2 27609 5D4 ← 7F0 b 380.4 26288 5G2 ← 7F0 c 394.6 25342 5L6 ← 7F0 d 416.5 24010 5D3 ← 7F0 e 465.8 21468 5D2 ← 7F0
Exponential tail fit R2 (µs)
Single 0.9988 492.94
Figure E.30 Decay curve of the 5D0 → 7F2 transition
of sample 42c with a single exponential fit.
95
Sample 43c
Figure E.31 (Left) Excitation spectrum of sample 43c, monitored at 574.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 43c, excited through the charge transfer band at 312.0 nm (corrected for detector sensitivity).
Table E.35 Overview of the emissive Dy3+ transitions of sample 43c.
Symbol Wavelength (nm) Wavenumber (cm-1) Dy3+ (3%) Transitions
a 483.6 20678 4F9/2 → 6H15/2 b 574.5 17406 4F9/2 → 6H13/2 c 661.9 15108 4F9/2 → 6H11/2
Table E.36 Overview of the excitation transitions of Dy3+ in sample 43c.
Symbol Wavelength (nm) Wavenumber (cm-1) Dy3+ (3%) Transitions
a 352.4 28377 6P7/2 ← 6H15/2 b 365.8 27337 6P5/2 ← 6H15/2 c 389.3 25687 4F7/2 ← 6H15/2 d 449.0 22272 4I15/2 ← 6H15/2
Table E.37 Exponential decay characteristics of sample 43c.
Exponential tail fit R2 (µs)
Single 0.9922 79.64
Figure E.32 Decay curve of the 4F9/2 → 6H13/2
transition of sample 43c with a single
exponential fit.
96
Sample 44a
Figure E.33 (Left) Excitation spectrum of sample 44a, monitored at 646.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 44a, excited through the charge transfer band at 283.0 nm (corrected for detector sensitivity).
Table E.38 Overview of the emissive Sm3+ transitions in sample 44a.
Symbol Wavelength (nm) Wavenumber (cm-1) Sm3+ (3%) Transitions
a 565.5 17683 4G5/2 → 6H5/2 b 602.0 16611 4G5/2 → 6H7/2 c 645.4 15494 4G5/2 → 6H9/2 d 704.8 14188 4G5/2 → 6H11/2
Table E.39 Overview of the excitation transitions of sample 44a.
Symbol Wavelength (nm) Wavenumber (cm-1) Sm3+ (3%) Transitions
a 406.8 24582 4L13/2 ← 6H5/2 b 466.6 21432 4I13/2 ← 6H5/2
Table E.40 Exponential decay characteristics of sample 44a.
Exponential tail fit R2 (µs)
Single 0.9906 242.85
Figure E.34 Decay curve of the 4G5/2 → 6H9/2
transition of sample 44a with a single
exponential fit.
97
Sample 44c
Figure E.35 (Left) Excitation spectrum of sample 44c, monitored at 646.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 44c, excited through the charge transfer band at 305.0 nm (corrected for detector sensitivity).
Table E.41 Overview of the emissive Sm3+ transitions in sample 44c.
Symbol Wavelength (nm) Wavenumber (cm-1
) Sm3+
(3%) Transitions
a 563.9 17734 4G5/2 →
6H5/2
b 601.5 16625 4G5/2 →
6H7/2
c 646.0 15480 4G5/2 →
6H9/2
d 703.8 14209 4G5/2 →
6H11/2
Table E.42 Overview of the excitation transitions of Sm3+ in sample 44c.
Symbol Wavelength (nm) Wavenumber (cm-1
) Sm3+
(3%) Transitions
a 365.4 27367 4D3/2 ←
6H5/2
b 379.5 26350 4P7/2 ←
6H5/2
c 406.9 24576 4L13/2 ←
6H5/2
d 418.8 23878 4M19/2 ←
6H5/2
e 466.4 21441 4I13/2 ←
6H5/2
f 479.5 20855 4M15/2 ←
6H5/2
Table E.43 Exponential decay characteristics of sample 44c.
Sample 50a
Figure E. 37 Decay curve of the 4D4→7F5 transition (Tb3+) of sample 50a with a single
exponential fit.
Table E. 44 Single exponential decay characteristics of sample 50a.
Exponential tail fit R2 (µs)
Single 0.99669 9.888
Exponential tail fit R2
(µs)
Single 0.9965 414.43
Figure E.36 Decay curve of the 4G5/2 → 6H9/2 transition
of sample 44c with a single exponential fit.
98
Sample 50c
Figure E.38 (Left) Excitation spectrum of sample 50c, monitored at 618.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 50c, excited through the charge transfer band at 306.4 nm (corrected for detector sensitivity).
Table E.45 Overview of the emissive transitions of Eu3+ and Tb3+ in sample 50c.
Symbol Wavelength (nm) Wavenumber (cm-1)
Eu3+ (2.5%) Transitions
Tb3+ (2.5%) Transitions
a 482.5 20725 5D4 → 7F6
b 536.4 18643 5D4 → 7F5 c 572.1 17479 5D4 → 7F4 d 586.3 17056 5D0 → 7F0 e 592.9 16866 5D0 → 7F1 f 618.6 16166 5D0 → 7F2 g 650.2 15380 5D0 → 7F3 h 697.5 14337 5D0 → 7F4
Table E.46 Overview of the excitation transitions of Eu3+ in sample 50c.
Symbol Wavelength (nm) Wavenumber (cm-1)
Eu3+ (2.5%) Transitions
a 394.4 25355 5L6 ← 7F0 b 465.5 21482 5D2 ← 7F0
Table E.47 Exponential decay characteristics of sample 50c.
Exponential tail fit R2 (µs)
Single 0.9954 324.5
Figure E.39 Decay curve of the 5D0 → 7F2 transition of Eu3+
in sample 50c with a single exponential fit.
99
Sample 51a
Figure E.40 (Left) Excitation spectrum of sample 51a, monitored at 618.4 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 51a, excited through the charge transfer band at 87.8 nm (corrected for detector sensitivity).
Table E.48 Overview of the emissive transitions of Eu3+ and Tb3+ in sample 51a.
Symbol Wavelength (nm)
Wavenumber (cm-1)
Eu3+ (2%) Transitions
Tb3+ (2%) Transitions
a 473.9 21101 5D4 → 7F6
b 536.8 18629 5D4 → 7F5 c 556.4 17973 5D4 → 7F4 d 592.1 16889 5D0 → 7F1 e 618.1 16179 5D0 → 7F2 5D4 → 7F3 f 649.6 15394 5D0 → 7F3 5D4 → 7F2
g 697.5 14337 5D0 → 7F4
Table E.49 Exponential decay characteristics of sample 51a.
Exponential tail fit R2 (µs)
Single 0.9922 265.83
Figure E.41 Decay curve of the 5D0 → 7F2
transition of Eu3+ in sample 51a with a single
exponential fit.
100
Sample 51c
Figure E.42 (Left) Excitation spectrum of sample 51c, monitored at 618.3 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 51c, excited through the charge transfer band at 307.1 nm (corrected for detector sensitivity).
Table E.50 Overview of the emissive Eu3+ and Tb3+ transitions in sample 51c.
Symbol Wavelength (nm)
Wavenumber (cm-1)
Eu3+ (2%) Transition
Tb3+ (2%) Transition
a 474.7 21066 5D4 → 7F6
b 536.3 18646 5D4 → 7F5 c 557.3 17944 5D4 → 7F4 d 585.6 17077 5D0 → 7F0 e 593.1 16861 5D0 → 7F1 f 618.5 16168 5D0 → 7F2 5D4 → 7F3 g 649.6 15394 5D0 → 7F3 5D4 → 7F2 h 697.6 14335 5D0 → 7F4
Table E.51 Overview of the excitation transitions of Eu3+ and Tb3+ in sample 51c.
Table E.52 Exponential decay characteristics of sample 51c.
Symbol Wavelength (nm)
Wavenumber (cm-1)
Eu3+ (2%) Transitions
Tb3+ (2%) Transitions
a 362.0 27624 5D4 ← 7F0 5G5 ← 7F6 b 375.9 26603 5G3 ← 7F0 c 381.7 26199 5G2 ← 7F0 5D3 ← 7F6 d 394.8 25329 5L6 ← 7F0 e 416.4 24015 5D8 ← 7F0 f 465.7 21473 5D2 ← 7F0
Exponential tail fit R2 (µs)
Single 0.9980 311.80
Figure E.43 Decay curve of the 5D0 → 7F2 transition
of Eu3+ in sample 51c with a single exponential fit.
101
Sample 52a
Figure E.44 (Left) Excitation spectrum of sample 52a, monitored at 618.2 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 52, excited through the charge transfer band at 290.4 nm (corrected for detector sensitivity).
Table E.53 Overview of the emissive Eu3+ and Dy3+ transitions in sample 52a.
Symbol Wavelength (nm)
Wavenumber (cm-1)
Eu3+ (2%) Transitions
Dy3+ (2%) Transitions
a 473.8 21106 4F9/2 → 6H15/2
b 572.3 17473 4F9/2 → 6H13/2
c 585.6 17077 5D0 → 7F0 d 592.2 16886 5D0 → 7F1 e 617.7 16189 5D0 → 7F2 f 656.6 15230 5D0 → 7F3 4F9/2 → 6H11/2
g 697.3 14341 5D0 → 7F4
Table E.54 Exponential decay characteristics of sample 52a.
Exponential tail fit R2 (µs)
Single 0.9926 221.79
Figure E.45 Decay curve of the 5D0 → 7F2
transition of Eu3+ in sample 52a with a single
exponential fit.
102
Sample 52c
Figure E.46 (Left) Excitation spectrum of sample 52c, monitored at 618.1 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 52c, excited through the charge transfer band at 308.3 nm (corrected for detector sensitivity).
Table E.55 Overview of the emissive Eu3+ and Dy3+ transitions in sample 52c.
Symbol Wavelength (nm)
Wavenumber (cm
-1)
Eu3+
(2%) Transitions
Dy3+
(2%) Transitions
a 473.5 21119 4
F9/2 → 6H15/2
b 572.9 17455 4
F9/2 → 6H13/2
c 585.7 17074 5D0→
7F0
d 592.6 16875 5D0→
7F1
e 618.2 16176 5D0→
7F2
f 655.5 15256 5D0→
7F3
4F9/2 →
6H11/2
g 696.9 14349 5D0→
7F4
Table E.56 Overview of the excitation transitions of Eu3+ in sample 52c.
Symbol Wavelength (nm) Wavenumber (cm-1
) Eu3+
(2%) Transitions
a 394.6 25368 5L6 ←
7F0
b 465.9 21464 5D2 ←
7F0
Table E.57 Exponential decay characteristics of sample 52c.
Exponential tail fit R2
(µs)
Single 0.9981 257.20
Sample 55a
Figure E.48 Decay curve of the 5D4 → 7F5 transition of Tb3+ in sample 55a with a single
exponential fit.
Table E.58 Single exponential decay characteristics of sample 55a.
Exponential tail fit R2 (µs)
Single 0.9984 596.48
Figure E.47 Decay curve of the 5D0 → 7F2 transition
of Eu3+ in sample 52c with a single exponential fit.
103
Sample 55c
Figure E.49 (Left) Excitation spectrum of sample 55c, monitored at 611.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 55c, excited through the charge transfer band at 302.3 nm (corrected for detector sensitivity).
Table E.59 Overview of the emissive Eu3+ and Dy3+ transitions in sample 55c.
Symbol Wavelength (nm)
Wavenumber (cm-1)
Eu3+ (2.5%) Transitions
Tb3+(2.5%) Transitions
a 538.9 18556 5D4 → 7F5
b 579.8 17247 5D0 → 7F0 5D4 → 7F4
c 593.4 16852 5D0 → 7F1 d 611.3 16359 5D0 → 7F2 e 629.6 15883 5D4 → 7F3 f 656.4 15235 5D0 → 7F3 5D4 → 7F2 g 704.6 14192 5D0 → 7F4
Table E.60 Overview of the excitation transitions of Eu3+ and Tb3+ in sample 55c.
Symbol Wavelength (nm)
Wavenumber (cm-1)
Eu3+ (2.5%) Transitions
Tb3+(2.5%) Transitions
a 361.0 27701 5D4 ← 7F0 5G5 ← 7F6 b 381.3 26226 5G2 ← 7F0 5D3 ← 7F6 c 393.1 25439 5L6 ← 7F0 d 464.7 21519 5D2 ←
7F0
Table E.61 Single exponential decay characteristics of sample 55c.
Exponential tail fit R2 (µs)
Single 0.9983 676.85
Figure E.50 Decay curve of the 5D0 → 7F2 transition
of Eu3+ in sample 55c with a single exponential fit.
104
Sample 56a
Figure E.51 (Left) Excitation spectrum of sample 56a, monitored at 573.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 56a, excited through the charge transfer band at 267.8 nm (corrected for detector sensitivity).
Table E.62 Overview of the emissive Dy3+ transition in sample 56a.
Symbol Wavelength (nm) Wavenumber (cm-1) Dy3+ (1%) Transition
a 573.3 17443 4F9/2 → 6H13/2
Table E.63 Overview of the excitation transitions of Dy3+ in sample 56a.
Symbol Wavelength (nm) Wavenumber (cm-1) Dy3+ (1%) Transitions
a 324.4 30826 4M17/2 ← 6H15/2 b 350.7 28514 6P7/2 ← 6H15/2 c 364.9 27405 6P5/2 ← 6H15/2 d 386.5 25873 4I13/2 ← 6H15/2
Table E.64 Single exponential decay characteristics of sample 56a.
Exponential tail fit R2 (µs)
Single 0.9950 15.81
Figure E.52 Decay curve of the 4F9/2 → 6H13/2
transition of Dy3+ in sample 56a with a single
exponential fit.
105
Sample 57a
Figure E.53 (Left) Excitation spectrum of sample 57a, monitored at 646.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 57a, excited through the charge transfer band at 266.5 nm (corrected for detector sensitivity).
Table E.65 Overview of the emissive Sm3+ transition in sample 57a.
Symbol Wavelength (nm) Wavenumber (cm-1) Sm3+ (1%) Transition
a 646.2 15475 4G5/2 → 6H9/2
Table E.66 Single exponential decay characteristics of sample 57a.
Exponential tail fit R2 (µs)
Single 0.9945 22.05
Figure E.54 Decay curve of the 4G5/2 → 6H9/2 transition
of Sm3+ in sample 57a with a single exponential fit.
106
Sample 57c
Figure E.55 (Left) Excitation spectrum of sample 57c, monitored at 656.1 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 57c, excited through the charge transfer band at 301.4 nm (corrected for detector sensitivity).
Table E.67 Overview of the emissive Sm3+ transitions in sample 57c.
Symbol Wavelength (nm) Wavenumber (cm-1) Sm3+ (1%) Transitions
a 568.1 1760 4G5/2 →6H5/2
b 612.4 1633 4G5/2 → 6H7/2 c 656.6 1523 4G5/2 → 6H9/2 d 707.5 1413 4G5/2 → 6H11/2
Table E.68 Single exponential decay characteristics of sample 57c.
Sample 58a
Figure E.57 Decay curve of the 4F9/2 → 6H13/2 transition of Dy3+ in sample 58a with a
single exponential fit.
Table E.69 Single exponential decay characteristics of sample 58a.
Exponential tail fit R2 (µs)
Single 0.9978 661.57
Exponential tail fit R2 (µs)
Single 0.9979 13.20
Figure E.56 Decay curve of the 4G5/2 → 6H9/2 transition of
Sm3+ in sample 57c with a single exponential fit.
107
Sample 58c
Figure E.58 (Left) Excitation spectrum of sample 58c, monitored at 618.3 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 58c, excited through the charge transfer band at 311.3 nm (corrected for detector sensitivity).
Table E.70 Overview of the Eu3+ and Dy3+ transitions in sample 58c.
Symbol Wavelength (nm)
Wavenumber (cm-1)
Eu3+ (2%) Transitions
Dy3+ (3%) Transitions
a 482.3 20734 4F9/2 → 6H15/2
b 572.2 17476 4F9/2 → 6H13/2
c 585.8 17071 5D0 → 7F1 d 592.2 16886 5D0 → 7F2 e 618.3 16173 5D0 → 7F3 4F9/2 → 6H11/2 f 654.9 15270 5D0 → 7F4
Table E.71 Exponential decay characteristics of sample 58c.
Exponential tail fit R2 (µs)
Single 0.9945 329.29
Figure E.59 Decay curve of the 4F9/2 → 6H13/2
transition of Dy3+ In sample 58c with a single
exponential fit.
108
Sample 59a
Figure E.60 (Left) Excitation spectrum of sample 59a, monitored at 618.1 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 59a, excited through the charge transfer band at 301.0 nm (not corrected for detector sensitivity).
Table E.72 Overview of the Eu3+ and Tb3+ transitions in sample 59a.
Symbol Wavelength (nm)
Wavenumber (cm-1)
Eu3+ (2%) Transitions
Tb3+ (3%) Transitions
a 536.1 18653 5D4 → 7F5
b 559.7 17867 5D4 → 7F4 c 585.3 17085 5D0 → 7F1
d 617.8 16186 5D0 → 7F2 5D4 → 7F3
e 649.3 15401 5D0 → 7F3 5D4 → 7F2 f 697.6 14335 5D0 → 7F4
Table E.73 Exponential decay characteristics of sample 59a.
Exponential tail fit R2 (µs)
Single 0.9762 31.19
Figure E.61 Decay curve of the 5D0 → 7F2
transition of Eu3+ of sample 59a with a single
exponential fit.
109
Sample 59c
Figure E.62 (Left) Excitation spectrum of sample 59c, monitored at 618.2 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 59c, excited through the charge transfer band at 307.3 nm (corrected for detector sensitivity).
Table E.74 Overview of the emissive Eu3+ and Tb3+ transitions in sample 59c.
Symbol Wavelength (nm)
Wavenumber (cm-1)
Eu3+ (2%) Transitions
Tb3+ (3%) Transitions
a 536.6 18636 5D4 → 7F5
b 585.6 17077 5D4 → 7F4 c 592.6 16875 5D0 → 7F1
d 618.1 16179 5D0 → 7F2 5D4 → 7F3
e 648.6 15418 5D0 → 7F3 5D4 → 7F2 f 696.6 14355 5D0 → 7F4
Table E.75 Overview of the excitation transitions of Eu3+ in sample 59c.
Symbol Wavelength (nm)
Wavenumber (cm-1)
Eu3+ (2%) Transitions
a 394.3 25361 5L6 ← 7F0 b 465.1 21501 5D2 ← 7F0
Table E.76 Single exponential decay characteristics of sample 59c.
Exponential tail fit R2 (µs)
Single 0.9936 483.56
Figure E.63 Decay curve of the 5D0 → 7F2 transition
of Eu3+ in sample 59c with a single exponential fit.
110
Sample 61c
Figure E.64 (Left) Excitation spectrum of sample 61c, monitored at 611.6 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 61c, excited through the charge transfer band at 303.2 nm (not corrected for detector sensitivity).
Table E.77 Overview of the emissive Eu3+ transitions in sample 61c.
Symbol Wavelength (nm) Wavenumber (cm-1) Eu3+ (3%) Transitions
a 579.2 17265 5D0 → 7F0
b 593.0 16863 5D0 → 7F1 c 611.3 16359 5D0 → 7F2 d 656.0 15244 5D0 → 7F3 e 704.7 14190 5D0 → 7F4
Table E.78 Overview of the excitation transitions of Eu3+ in sample 61c.
Symbol Wavelength (nm) Wavenumber (cm-1) Eu3+ (3%) Transitions
a 360.9 27709 5D4 ← 7F0 b 392.6 25471 5L6 ← 7F0 c 464.5 21529 5D2 ← 7F0
Table E.79 Exponential decay characteristics of sample 61c.
Exponential tail fit R2 (µs)
Single 0.9991 732.14
Figure E.65 Decay curve of the 5D0 → 7F2
transition of Eu3+ in sample 61c with a single
exponential fit.
111
Sample 62a
Figure E.66 (Left) Excitation spectrum of sample 62a, monitored at 646.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 62a, excited through the charge transfer band at 266.1 nm (not corrected for detector sensitivity).
Table E.80 Overview of the emissive Sm3+ transitions in sample 62a.
Symbol Wavelength (nm)
Wavenumber (cm-1) Sm3+ (3%) Transitions
a 603.1 16581 4G5/2 → 6H7/2 b 647.1 15454 4G5/2 → 6H9/2
Table E.81 Overview of the excitation transitions of Sm3+ in sample 62a.
Symbol Wavelength (nm)
Wavenumber (cm-1) Sm3+ (3%) Transitions
a 401.9 24882 6P3/2 ← 6H5/2 b 418.6 23889 4M19/2 ← 6H5/2 c 449.1 22267 4M17/2 ← 6H5/2 d 466.9 21418 4I13/2 ← 6H5/2 e 479.7 20846 4M15/2 ← 6H5/2 f 490.7 20379 4I9/2 ← 6H5/2
Table E.82 Exponential decay characteristics of sample 62a.
Exponential tail fit R2 (µs)
Single 0.9964 21.32
Figure E.67 Decay curve of the 4G5/2 → 6H9/2 transition of
Sm3+ in sample 62a with a single exponential fit.
112
Sample 62c
Figure E.68 (Left) Excitation spectrum of sample 62c, monitored at 612.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 62c, excited through the charge transfer band at 300.2 nm (corrected for detector sensitivity).
Table E.83 Overview of the emissive Sm3+ transitions in sample 62c.
Symbol Wavelength (nm) Wavenumber (cm-1) Sm3+ (3%) Transitions
a 564.4 17718 4G5/2 → 6H5/2 b 612.3 16332 4G5/2 → 6H7/2 c 656.1 15242 4G5/2 → 6H9/2 d 708.8 14108 4G5/2 → 6H11/2
Table E.84 Overview of the excitation transitions of Sm3+ in sample 62c.
Symbol Wavelength (nm) Wavenumber (cm-1) Sm3+ (3%) Transitions
a 361.5 27663 4D3/2 ← 6H5/2 b 376.7 26546 4P7/2 ← 6H5/2 c 404.0 24752 4F7/2 ← 6H5/2
Table E.85 Single exponential decay characteristics of sample 62c.
Exponential tail fit R2 (µs)
Single 0.9963 469.69
Figure E.69 Decay curve of the 4G5/2 → 6H9/2
transition of Sm3+ in sample 62c with a single
exponential fit.
113
Sample 64a
Figure E.70 (Left) Excitation spectrum of sample 64a, monitored at 611.6 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 64a, excited through the charge transfer band at 266.9 nm (not corrected for detector sensitivity).
Table E.86 Overview of the emissive Eu3+ and Tb3+ transitions in sample 64a.
Symbol Wavelength (nm)
Wavenumber (cm-1)
Eu3+ (2%) Transitions
Tb3+ (3%) Transitions
a 485.9 20580 5D4 → 7F6
b 542.9 18420 5D4 → 7F5
c 581.4 17200 5D0 → 7F0 5D4 → 7F4
d 591.3 16912 5D0 → 7F1
e 611.7 16348 5D0 → 7F2 f 622.4 16067 5D4 → 7F3 g 643.1 15550 5D4 → 7F2 h 655.5 15256 5D0 → 7F3 5D4 → 7F1 i 701.5 14255 5D0 → 7F4
Table E.87 Overview of the excitation transitions of Eu3+ and Tb3+ in sample 64a.
Table E.88 Exponential decay characteristics of sample 64a.
Symbol Wavelength (nm)
Wavenumber (cm-1)
Eu3+ (2%) Transitions
Tb3+ (3%) Transitions
a 298.0 33557 5F1 ← 7F0 5H5 ← 7F6 b 302.9 33014 5F2 ← 7F0 5H6 ← 7F6 c 317.9 31456 5H5 ← 7F0 5H7 ← 7F6 d 361.0 27701 5D4 ← 7F0 5G5 ← 7F6 e 380.5 26281 5D3 ← 7F6 f 392.9 25452 5D3 ← 7F0 g 464.2 21542 5D2 ← 7F0
Exponential tail fit
R2 (µs)
5D0 → 7F2 (Eu3+) 0.9989 354.25 5D4 → 7F5 (Tb3+) 0.99785 509.16
Figure E.71 Decay curve of the 5D0 →
7F2 transition of Eu3+ in sample 64a
with a single exponential fit.
Figure E.72 Decay curve of the 5D4
→ 7F5 transition of Tb3+ in sample
64a with a single exponential fit.
114
Sample 64c
Figure E.73 (Left) Excitation spectrum of sample 64c, monitored at 611.6 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 64c, excited through the charge transfer band at 301.9 nm (corrected for detector sensitivity).
Table E.89 Overview of the Eu3+ and Tb3+ transitions of sample 64c. Table E.90 Overview of the excitation transitions of Eu3+ and
Tb3+ in sample 64c.
Table E.91 Single exponential decay characteristics of sample 64c.
Exponential tail fit R2
(µs)
Single 0.9993 770.50
Sample 65a
Table E.92 Single exponential decay characteristics of sample 65a.
Exponential tail fit R2 (µs)
Single 0.9949 475.15
# Wave-length (nm)
Wave-number (cm
-1)
Eu3+
(2%) Trans.
Tb3+
(3%) Trans.
a 360.8 27716 5D4 ←
7F0
5G5 ←
7F6
b 380.6 26274 5D3 ←
7F6
c 392.8 25458 5D3 ←
7F0
d 464.5 21529 5D2 ←
7F0
# Wave-length (nm)
Wave-number
(cm-1
)
Eu3+
(2%) Trans.
Tb3+
(3%) Trans.
a 537.5 18605 5D4 →
7F5
b 579.3 17262 5D0 →
7F0
5D4 →
7F4
c 592.7 16872 5D0 →
7F1
d 611.3 16359 5D0 →
7F2
e 629.9 15876 5D4 →
7F3
f 655.9 15246 5D0 →
7F3
5D4 →
7F2.
7F1
g 704.7 14190 5D0 →
7F4
Figure E.74 Decay curve of the 5D0 → 7F2 transition
of Eu3+ in sample 64c with a single exponential fit.
Figure E.75 Decay curve of the 4G5/2 → 6H7/2 transition of
Sm3+ in sample 65a with a single exponential fit.
115
Sample 65c
Figure E.76 (Left) Excitation spectrum of sample 65c, monitored at 656.0 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 65c, excited through the charge transfer band at 293.7 nm (corrected for detector sensitivity).
Table E.93 Overview of the emissive Sm3+ and Tb3+ transitions in sample 65c.
Symbol Wavelength (nm)
Wavenumber (cm-1)
Sm3+ (2.5%) Transitions
Tb3+ (2.5%) Transitions
a 564.4 17718 4G5/2 → 6H5/2 5D4 → 7F4
b 607.4 16464 4G5/2 → 6H7/2 5D4 → 7F3
c 656.0 15244 4G5/2 → 6H9/2 5D4 → 7F2, 7F1
d 707.8 14128 4G5/2 → 6H11/2
Table E.94 Overview of the excitation transitions of Sm3+ and Tb3+ in sample 65c.
Table E.95 Single exponential decay characteristics of sample 65c.
Exponential tail fit R2 (µs)
Single 0.9967 461.04
Symbol Wavelength (nm)
Wavenumber (cm-1)
Sm3+ (2.5%) Transitions
Tb3+ (2.5%) Transitions
a 345.2 28969 4H9/2 ← 6H5/2 5G3 ← 7F6 b 361.4 27670 4D3/2 ← 6H5/2 5G5 ← 7F6 c 376.6 26553 6P7/2 ← 6H5/2 d 403.2 24802 6P3/2 ← 6H5/2 e 416.9 23987 (6P,4P)5/2 ← 6H5/2 f 466.7 21427 4I13/2 ← 6H5/2
Figure E.77 Decay curve of the 4G5/2 → 6H9/2 transition of
Sm3+ in sample 65c with a single exponential fit.
116
Sample 67a
Figure E.78 (Left) Excitation spectrum of sample 67a, monitored at 573.5 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 67a, excited through the charge transfer band at 294.4 nm (not corrected for detector sensitivity).
Table E.96 Overview of the Dy3+ transition in sample 67a.
Symbol Wavelength (nm) Wavenumber (cm-1) Dy3+ (1%) Transition
a 573.3 17443 4F9/2 → 6H13/2
Table E.97 Single exponential decay characteristics of sample 67a.
Exponential tail fit R2 (µs)
Single 0.9942 25.90
Figure E.79 Decay curve of the 4F9/2 → 6H13/2 transition of
Dy3+ in sample 67a with a single exponential fit.
117
Sample 67c
Figure E.80 (Left) Excitation spectrum of sample 67c, monitored at 572.9 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 67c, excited through the charge transfer band at 302.3 nm (not corrected for detector sensitivity).
Table E.98 Overview of the emissive Dy3+ transitions in sample 67c.
Symbol Wavelength (nm) Wavenumber (cm-1) Dy3+ (1%) Transitions
a 483.2 20695 4F9/2 → 6H15/2 b 576.5 17346 4F9/2 → 6H13/2
Table E.99 Overview of the excitation transitions of Dy3+ in sample 67c.
Symbol Wavelength (nm) Wavenumber (cm-1) Dy3+ (1%) Transitions
a 385.4 25947 4I13/2 ← 6H15/2 b 424.0 23585 4G11/2 ← 6H15/2 c 448.3 22306 4I15/2 ← 6H15/2 d 466.8 21422 4F9/2 ← 6H15/2
Table E.100 Single exponential decay characteristics of sample 67c.
Exponential tail fit R2 (µs)
Single 0.9959 688.88
Figure E.81 Decay curve of the 4F9/2 → 6H13/2 transition
of Dy3+ in sample 67c with a single exponential fit.
118
Sample 68a
Figure E.82 (Left) Excitation spectrum of sample 68a, monitored at 618.3 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 68a, excited through the charge transfer band at 294.3 nm (not corrected for detector sensitivity).
Table E.101 Overview of the emissive Eu3+ and Tb3+ transitions in sample 68a.
Symbol Wavelength (nm)
Wavenumber (cm-1)
Eu3+ (1%) Transition
Tb3+ (3%) Transition
a 537.4 18608 5D4 → 7F5 b 556.4 17973 5D4 → 7F4 c 585.7 17074 5D0 → 7F0 d 592.9 16866 5D0 → 7F1 e 618.2 16176 5D0 → 7F2 5D4 → 7F3 f 650.7 15368 5D0 → 7F3 5D4 → 7F2 g 698.2 14323 5D0 → 7F4
Table E.102 Single exponential decay characteristics of sample 68a.
Exponential tail fit R2 (µs)
Single 0.9924 64.14
Figure E.83 Decay curve of the 5D0 → 7F2 transition of
Eu3+ of sample 68a with a single exponential fit.
119
Sample 68c
Figure E.84 (Left) Excitation spectrum of sample 68c, monitored at 618.3 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 68c, excited through the charge transfer band at 302.9 nm (corrected for detector sensitivity).
Table E.103 Overview of the emissive Eu3+ and Tb3+ transitions in sample 68c.
Symbol Wavelength (nm)
Wavenumber (cm-1)
Eu3+ (1%) Transition
Tb3+ (3%) Transition
a 537.1 18619 5D4 → 7F5 b 585.9 17068 5D0 → 7F0 c 593.0 16863 5D0 → 7F1 d 618.6 16166 5D0 → 7F2 5D4 → 7F3 e 650.7 15368 5D0 → 7F3 5D4 → 7F2 f 697.6 14335 5D0 → 7F4
Table E.104 Overview of the excitation transitions of Eu3+ and Tb3+ in sample 68c.
Symbol Wavelength (nm)
Wavenumber (cm-1)
Eu3+ (1%) Transitions
Tb3+ (3%) Transitions
a 361.7 27647 5D4 ← 7F0 5G5 ← 7F6 b 379.9 26323 5D3 ← 7F6 c 394.5 25349 5D3 ← 7F0 d 465.7 21473 5D2 ← 7F0
Table E.105 Single exponential decay characteristics of sample 68c.
Exponential tail fit R2 (µs)
Single 0.9975 608.55
Figure E.85 Decay curve of the 5D0 → 7F2 transition
of Eu3+ of sample 68c with a single exponential fit.
120
Sample 69a
Figure E.86 (Left) Excitation spectrum of sample 69a, monitored at 572.9 nm (not corrected for detector sensitivity). (Right) Emission
spectrum of sample 69a, excited through the charge transfer band at 302.3 nm (corrected for detector sensitivity).
Table E.106 Overview of the Eu3+ and Dy3+ transitions in sample 69a.
Symbol Wavelength (nm)
Wavenumber (cm-1)
Eu3+ (0.5%) Transitions
Dy3+ (2.5%) Transitions
a 481.9 20751 4F9/2 → 6H15/2
b 572.8 17458 4F9/2 → 6H13/2
c 585.8 17071 5D0 → 7F0 d 592.7 16872 5D0 → 7F1 e 617.9 16184 5D0 → 7F2 f 661.5 15117 5D0 → 7F3 4F9/2 → 6H11/2 g 697.2 14343 5D0 → 7F4
Table E.107 Single exponential decay characteristics of sample 69a.
Exponential tail fit R2 (µs)
Single 0.9987 13.88
Sample 69c
Table E.108 Single exponential decay characteristics of sample 69c.
Exponential tail fit R2 (µs)
Single 0.99653 484.15
Figure E.87 Decay curve of the 4F9/2 → 6H13/2 transition of
Dy3+ of sample 69a with a single exponential fit.
Figure E.88 Decay curve of the 5D0 → 7F2 transition of
Eu3+ of sample 69c with a single exponential fit.
121
Sample 40a – Emission map
Sample 50a – Emission map
Sample 57c – Emission map
Figure E.89 Emission maps of sample 40a,
excited at different wavelengths of the charge
transfer band (corrected for detector
sensitivity).
Figure E.90 Emission maps of sample 50a,
excited at different wavelengths of the charge
transfer band (corrected for detector
sensitivity).
Figure E.91 Emission maps of sample 57c,
excited at different wavelengths of the charge
transfer band (corrected for detector
sensitivity).
122
Sample 58c – Emission map
Sample 64a – Emission map
Sample 65a – Emission map
Figure E.92 Emission maps of sample 58c,
excited at different wavelengths of the charge
transfer band (corrected for detector
sensitivity).
Figure E.93 Emission maps of sample 64a,
excited at different wavelengths of the charge
transfer band (corrected for detector
sensitivity).
Figure E.94 Emission maps of sample 65a,
excited at different wavelengths of the charge
transfer band (corrected for detector
sensitivity).
