Dietrich R. T. ZahnInstitut für Physik, Technische Universität Chemnitz, Germany
Raman Spectroscopy as a Tool to StudyInterfaces, Thin Films,
and Low Dimensional Structures
SemiconductorSemiconductor PhysicsPhysics ––ActivitiesActivities in Chemnitzin Chemnitz
hω
ee
SemiconductorInterface
hω
Electrical Measurements:Current-Voltage (IV)Capacitance-Voltage (CV)(Deep Level) Transient Spectroscopy
Surface Science:Photoemission Spectroscopy(UPS and XPS)X-ray Absorption Fine Structure(NEXAFS)Auger Electron Spectroscopy(AES)Low Energy Electron Diffraction(LEED)Inverse PhotoemissionKelvin Probe (CPD)
Growth:(Organic) Molecular Beam Depositionin Ultra-High Vacuum(Metal-Organic) Vapour Phase Deposition
Optical Spectroscopy:Raman Spectroscopy (RS) PhotoluminescenceSpectroscopic Ellipsometry (SE) UV-visInfrared Spectroscopy (IR)Reflection Anisotropy Spectroscopy (RAS)
The Chemnitz Semiconductor Physics and Organic Semiconductors Groups
Photons in – photons out
Raman Spectroscopy
Optical Spectroscopy
Raman spectroscopy
• Different principles. Based on scattering of (usually) visible monochromatic light by molecules of a gas, liquid or solid
• Two kinds of scattering encountered:– Rayleigh (1 in every 10,000) same frequency– Raman (1 in every 10,000,000) different frequencies
99.99%MONOCHROMATIC
RADIATION
TRANSPARENT DUST-FREE SOLID, LIQUID or GAS
Information Depth
Dielectric Function
describes light – matter interaction
Light – Matter Interaction
incident
reflected
transmitted or absorbed
( ) ( ) κωεω inn +==~
( )xIxI α−= exp)( 0
cωκα 2
=
( ) ( ) ( )ωεωεωε ir i+=
Refractive index:with n real part of refractive index (refraction !) and κ the so-called extinction coefficient (absorption).
Absorption coefficient:Light intensity as function of distance x travelled in a medium:
GaAs
Energy E / eV
1 eV = 1,602×10-19 J
1 nm = 10-9 m = 10 Å
410 495 620 700
Wavelength λ / nm
560
3,0 2,5 2,2 2,05 1,7
UV IR
Energy units for optical spectroscopy
hω =hck = 2πhcλ
k ≡ wave vector; 1/λ = wavenumber
1/λ ∝ energy
1 eV = 8067.5 cm-1
300 cm-1 = 37.2 meV
Historical Introduction
• Studying scattering of light by transparent media
• Serendipitous discovery in 1921
• Phenomenon named for Raman in 1928
• His illuminating source was sunlight!!
Published in Nature in 1928
Origin of Rayleigh and Raman scattering
44 or 1 ν
λ∝I
Raman spectrum for CCl4 excited by laser radiation of λ0 = 488 nm and ν0 = 20, 492 cm-1. The number above the peaks is the Raman shift in cm-1.
Example:
a) At what wavelengths in nm would the stokes and anti-stokes Raman lines for carbon tetrachloride (Δν = 218, 459 and 769 cm-1 ) appear if the source wasa helium/neon laser (632.8nm)?
(an argon ion laser (488.0 nm)?)
a helium/neon laser (632.8nm):
Δν = 218, 459 and 769 cm-1
Raman signal:
Wavenumber: 15802.8 cm-1
Stokes line: ν =(15802.8 – 218) cm-1 =15584.8 cm-1
λ = 641.65 nm
anti-Stokes line: ν =(15802.8 + 218) cm-1 =16020.8 cm-1
λ = 624.2 nm
Infrared absorption and Raman scattering
-300 -200 -100 0 100 200 300Raman shift (cm-1)
0 50 100 150 200 250 300Frequency (cm -1)
Comparison of Raman and infrared spectra
Vibrational Raman
• Symmetric stretching vibration of CO2
• Polarisability changes– therefore Raman band at 1,340 cm-1
• Dipole moment does not– no absorption at 1,340 cm-1 in IR
Vibrational Raman
• Asymmetric stretching vibration of CO2
• Polarisability does not change during vibration– No Raman band near 2,350 cm-1
• Dipole moment does change– CO2 absorbs at 2,349 cm-1 in the IR
Raman SpectroscopyRaman SpectroscopyR - Rayleigh Scattering
S - Stokes Raman Scattering
ωi- ω(q)AS - Anti-Stokes
Raman Scatteringωi+ ω(q)
ωi
v=0v=1
ω(q)ω(q)
Virtual levels
qkk
qEP
i
i
S
S
rh
rh
rh
rhhh
rrrr
±=
±=
=
)(0
ωωωχε
ωi ωiωi+ ω(q)ωi- ω(q)
Inelastic scatteringInelastic scattering of the light mediated by the polarisabilitypolarisability of the medium.
ω
I
Reflected light
Incident light
Scattered light
Theory of Raman Scattering
• Scattering is based on the fact that incident radiation induces an oscillating dipole moment:
)2cos(0 tEM πναrtr
⋅=
)2cos( 110 tπνααα ttt +=• If the atoms execute a periodic motion:
)})(2cos())(2{cos(21
1101 ttE ννπννπα ++−⋅+rt
)2cos(00 tEM πναrtr
⋅=
Depolarization resulting from Raman scattering
Conservation rules for a Raman process
hωL = hωS + hΩphononhKL = hKS +hqphonon
⎧
⎨ ⎪
⎩ ⎪
BACKSCATTERING
qphonon
KS KL
Raman
Brillouin
4π500 nm
π0.5 nm
Ω (cm-1)
300
qphononω L − ωS[ ] q( ) = ω KL( )− ω q − KL( )= cKL − c q − KL( ) = 2cKL − cq
The ‘phonon’ spectrum
IR & Raman Active
Raman Intensity:
polarizability; intensity of the source; the concentration of the active group;
Raman signal intensity increases with the fourth powerof the frequency of the source;
directly proportional to the concentration of the active species
Principle of Raman Scattering
Raman Instrumentation
three major components:
- laser source - sample illumination system and - a suitable spectrophotometer
1,5 2,0 2,5 3,0 3,51
10
100
1000
laser lines
Info
rmat
ion
dept
h / n
m
Photon energy / eV
Information depth for GaAs= ½ of light penetration depth
Typical geometries for Raman scattering
90o scattering
180o scattering
Two sample excitation systems
(a) Schematic of a system for obtaining Raman spectra with a fiber-optic probe; (b) end view of the probe; (c) end view of the collection fibers at the entrance slit of the monochromator. The blackened circle represents the input fiber, and the hatched circles the collection fibers.
Schematic of Raman Spectrometer
Spectrographs for Raman
Spex 1877 triple monochromator
Spex 1403/4 double monochromator
Single Monochromator
Multichannel dispersive Raman spectrometer with a charge-coupled device (CCD). BP is an interference band-pass filter; BR is a Rayleigh band-rejection filter.
Photo-Detectors
Photodiode array detector
Charge coupled device (CCD)
Have a good spectrograph!
Optical diagram of an FT-Raman spectrometer
Spectra of anthracene. A: Conventional instrument, 514.5 nmexcitation; B: FT instrument, 1.064μm excitation.
Raman Spectroscopy
hωs=hωi+hΩ
200 250 300 350
ZnSe LO
Intensity / ctsmW-1s-1
GaAs LO
Raman Shift / cm-1
Raman Spectroscopy
hωs=hωi+hΩ
200 250 300 350
ZnSe LO
Intensity / ctsmW-1s-1
GaAs LO
Raman Shift / cm-1
1,5 2,0 2,5 3,0 3,51
10
100
1000
laser lines
Info
rmat
ion
dept
h / n
m
Photon energy / eV
RamanRaman SpectroscopySpectroscopy
phis Ω±= hhh ωω
buried layers
surface
small focus
intensity∝ω4
high Eg materials
Inten
sity
/ ctsm
W -1
s-1
Raman Shift / cm-1
Inten
sity
/ ctsm
W -1
s-1
Raman Shift / cm-1
Frequency Position and Lineshape
frequency shift by
temperature ≈-2cm-1/100°Cpressure ≈1cm-1/1kbar
lineshape:
asymmetric broadening and shiftoccurs as a result of latticedisturbance
0 100 200 300 400284
286
288
290
292
+/- 10°C
+/- 0.2 cm-1
Peak
Pos
ition
in /
cm -1
Temperature / °C
Determination of Surface Temperature
Using temperature induced shift of substrate phonon peak:
cm-1/100°CInSb: 2.1InP: 2.0GaAs: 1.8Si: 2.2ZnSe: 2.4
Resonance Raman scattering
0ij0
I ∝0 Light j j phononi i Light 0
hωL −hωphonon − Ej⎛
⎝
⎜ ⎜
⎞
⎠
⎟ ⎟ hωL − Ei
⎛
⎝ ⎜
⎞
⎠ ⎟ ij
∑
2
LightLight Phonon
Resonance Raman excitation profiles
100 150 200 250 300Raman shift (cm-1)
100 150 200 250 300Raman shift (cm-1)
100 150 200 250 300Raman shift (cm-1)
100 150 200 250 300Raman shift (cm-1)
100 150 200 250 300Raman shift (cm-1)
1.65 1.70 1.75 1.80 1.85 1.90 1.95
Inte
nsity
(arb
. uni
ts)
Laser Photon Energy (eV)
hωL
100 200 300 400 500 600 70050
100150
2002500.1
0.2
0.3
0.4 LOZnS LOZnSe+LOZnS
2 LOZnSeLOZnSe
Intensity / counts mW -1s -1
Temperature / °CRaman Shift / cm-1
with increasingtemperaturethe bandgapof ZnS0.05Se0.95approaches thephoton energyof 2.66 eV
typical gain oftwo orders ofmagnitude
Resonance enhancement
Sub-Monolayer Sensitivityvia Resonance Enhancement
Growth Chamberultra-high vacuum: base pressure<1⋅10-10mbar
up to 3 Knudsen cells
rf plasma source foratomic nitrogen
LEED/Auger
0.45 0.50 0.55 0.60 0.650
1
2
3
4
visi
ble
ligh
tred
blue
(620 nm)
(414 nm)
InSb
CdTeInPSiGaAs
ZnSe CdS
ZnS
GaN
Ener
gy b
andg
ap /
eV
Lattice constant / nm
Eg vs Lattice Constant
CdS Growth on InP(100)
substrate: ammonium sulfidepassivated InP
wafers annealed in UHV to 330°C for 10 min; TS=200°C compound source for CdS at 620°C
laser excitation:2.34 eV
CdS Growth on InP(100)
0 50 100 150 2000.0
0.1
0.2 calculation experiment
Inte
nsity
LO
CdS
/ co
unts
s-1
mW
-1
CdS Layer Thickness / nm
Determination of CdS Layer Thickness
Fabry-Perotinterferencescause intensitymodulation of Ramansignals
200 300 400
Δd=4nm
Scat
terin
g In
tens
ity
Raman Shift / cm-1
Initial Phase of CdS Depositionon InP(100) at 200°C
broad shoulderon low frequencyside of CdS LO phonon peakindicates an interfacialreaction leadingto an In-S richlayer
CdTe Growth on InSb
substrate: cleaved n-type InSb(110) surface
CdTe deposition from single Knudsencell kept at 550°C
laser excitation: 2.41 eV
CdTe Deposition at 300°C
no CdTe growth
strong interfacereaction
100 150 200 250
In2Te
3
A1g
(Sb)
D
C
B
A
Experiment Fit
Sca
tterin
g In
tens
ity
100 150 200 250
77K
D
C
B
AIn
2Te
3
Sca
tterin
g In
tens
ity
Raman Shift / cm-1
Interfacial Reaction Products
Reaction of Te with InSb leading to the formation of In2Te3 and liberatedSb confirmed.
CdTe Deposition at RT
no interfacereaction
Fabry-Perotmodulation
change in InSbLO/TO ratio
ZnSSe Growth on GaAs(100)
substrate:As capped MBE grownGaAs layer
compound sources for ZnSe and ZnS
atomic nitrogen provided by rf plasma sourcelaser excitation: 2.54 eV for doping at
TS=260°C2.66 eV for ZnSSe at
TS=250°C
MolecularMolecular BeamBeam EpitaxialEpitaxial Growth of Growth of ZnSeZnSe: : EffectEffect of of NitrogenNitrogen DopingDoping
Modulation due to Fabry-Perotinterference: Determination of growth rate and layer thickness
Identical experimental conditions, except: undoped doped
Incorporation of nitrogen causesbroadening of electronicresonance; plus compressivestrain in substrate
0 50 100 150 200 250 300284.7
285.0
285.3
285.6
285.9
286.2
286.5
286.8
ZnSe:N ZnSe undoped
Ram
an S
hift
/ cm
-1
Thickness / nm
Dependence of GaAsLO Frequency on ZnSe Doping
Nitrogeninducescompressivestrain in GaAs
125 150 175 200 225 250 275 300 325 350 375
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
5.7 cm-1
5.7 cm-1
20.2 cm-1
13.7 cm-1
ZnSe LO
GaAs LO
ZnSe:N
ZnSeundoped
TM =260°C
Eex
= 2.54 eV (488 nm)d = 200 nm
Ram
an In
tens
ity /
coun
ts m
W-1
s-1
Raman Shift / cm-1
ZnSe with and without Nitrogen
broadeningof ZnSe LO phonon mode indicateslatticedisturbancebynitrogenincorporation
Raman Monitoring of ZnSSe Growth
ZnS- and ZnSe-like LO phononscatteringobservableup to up to third order
0.0 0.2 0.4 0.6 0.8 1.040
60
80
100
120
140
Theory after Hayashi et al. measured peakdifference
at nominal x
LOZn
S-LO
ZnSe
/ cm
-1
sulphur content x
Determination of S Content in ZnSxSe1-x
dependence of the relative frequency shiftof ZnS- and ZnSe-like LO modes onsulphur contentK.Hayashi et al. ,Jpn.J.Appl.Phys. 30, 501(1991)
200 220 240 260 280 300 320 340
LO1+LO
2
LO2
Sca
tterin
g In
tens
ity
Raman Shift /cm-1
460 480 500 520 540 560 580
xnom
= 0.05
LO1: ZnSe-like
LO2: ZnS-like
LO2-LO
1
2LO1
LO1
Composition of Ternary Compounds
increasing frequencysplitting of ZnS- and ZnSe-like LO modescan be seen in LO and 2LO features
GaN Growth on GaAs(100)substrate:As capped MBE grown
GaAs layer
atomic nitrogen provided by rf plasma source
Ga from Knudsen cell at 870°C
laser excitation: 3.05 eV
Raman Monitoring of GaN Growth on GaAs(100) at 600°C
resonanceenhancement of scattering in thecubic modification:
Eex=3.05eV≈Eg,cub
at 600°C
200 400 600 800 1000
T=600°C
E2
GaAs LO
GaN
E2
A1+LO
dGaN
=
230nm
30nm
clean GaAs
Sca
tterin
g In
tens
ity
Raman Shift / cm-1
GaN Growth on GaAs(100)
high sensitivityachieved for GaNdetection at elevatedtemperatures
Substrate strain and GaN crystalquality
0 50 100 150 200 250
34
36
38
40
42 A1+LO GaN
FWH
M /
cm-1
GaN layer thickness / nm
281
282
283
284
LO GaAsPosi
tion
/ cm
-1
shift of GaAs LO phonon again revealsthe evolution of compressive strain in the substrate
evolution of FWHM is related to thecompetitive growth of cubic and hexagonal GaN
RamanRaman SpectroscopySpectroscopy::
DesorptionDesorption of a of a Se Se CappingCapping LayerLayerCrystallisation during annealing
Temperature induced shift
Background due to roughness
Simple molecules<1nm
IBM PowerPC 750TM
Microprocessor7.56mm×8.799mm
6.35×106 transistors
semiconductor nanocrystal (CdSe)5nm
10-10 10-510-9 10-7 10-610-8 10-4 10-3 10-2
m
Circuit designCopper wiringwidth 0.2μm
red blood cell~5 μm (SEM)DNA
proteins nm
bacteria1 μm
Nanometer memory element(Lieber)1012 bits/cm2 (1Tbit/cm2)
SOI transistorwidth 0.12μm
control biological machines
diatom30 μm
Nanoparticles: What are they?Nanoparticles are small elemental ensembles (typically on the nanometer scale).
They often have unique properties due to the fact that they are do not exhibit the characteristics of the bulk or individual particles.
solid state molecular
Low-dimensional semiconductor structures
3D: 2D: 1D: OD: Crystals Quantum Quantum Quantum
wells (SLs) wires dots
E
N(E
)
E
N(E
)
E
N(E
)
E
N(E
)
SubstrateSubstrate
WL
QD
÷8
÷8
N = 4096n = 1352
N = 4096n = 3584
N = 4096n = 2368
N = total atoms; n = surface atoms
Surface Area
One intrinsic benefit is the increased surface area available in nanoparticles.
Structural characterisation: HRTEM
Ge quantum dots:
Dot base size: ~15 nm,
Height: ~ 1,5 nm
Period of structure is 10
100Å
Structural characterisation of samples: STM
0 20 40 60 80Distance, nm
2.0
1.0
0.0
Height,
nm
Ge quantum dots:
Dot base size: ~15 nm,
Height: ~ 1,5 nm
Density of the dots:
~ 3.1011cm-2
Dot uniformity: ~ 20%
Structural Characterisation: STM
Ge quantum dots:
Dot base size: 10x10 nm,
Height: ~ 1,5 nm
Experimental Set-up
Basic principles of Raman spectroscopy in crystals
1. Energy conservation:
2. Quasi-momentum (wavevector) conservation:
Ω±= hhh si ωω
qkk ±= si
ki
ks qki
ks q 0≈
kiks
q 2k≈ i
λi ~ 5000 Å, a0 ~ 5 Å ⇒ |q| << π/a0 ⇒ only close to BZ center phonons are seen in the 1st order Raman spectra of bulk crystals
i
nqλπ40 ≤≤⇒
sii kk ≈⇒Ω⟩⟩hhω
Raman tensor. Symmetry selection rules.
Scattering intensity: Is ~ |es·χ(q)·ei|2
χ(q) – dielectric susceptibility tensor, modulated by phonons:χ(q) = χ0 + (∂χ/∂q)0q + ... ⇒ 1st order Raman intensity is
Is ~ |es· (∂χ/∂q)0·q·ei|2
R = (∂χ/∂q)0·q — Raman tensor Is ~ |es·R·ei|2
For zinc-blende-type crystals, (001) surface:
⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛=
0000000
)( L
L
LO dd
zR⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛=
00000
00)(
T
T
TO
d
dxR
⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛=
0000
000)(
T
TTO
ddyR
Reduction ofsymmetry:
Td → D2d
• Optical phonons: confinement
• Acoustic phonons: folding
General Properties of Phonons in Superlattices
m = 1, 2, 3... d = (n1+n2)a0
0
Averagedispersion
GaAs AlAs (GaAs) (AlAs)8 8
00.5 0.51 1
0 0.5 1
100
200
300
400
3
2
1
2
1
3Al Ga
As
q (units /a )π 0
Ω(cm )-1
4 nπ λ/ 4 nπ λ/
q (units /d)π Atomic displacements
01 )( anmδ
π+
GaAs AlAs
Selection Rules forGe/Si Superlattices
LA LO
LO TOy‘(x‘,z)-y‘TO
z(x,y)-z,
z(x‘,y‘)-z,
y‘(x‘,x‘)-y‘
LO
z(x,x)-z,z(x‘,x‘)-z
LA
geometryphonon
Typical Raman Spectrum of Ge Dot Superlattice
200 300 400 500
m=±4
m=±3
m=±2
m=±1
Ge-Ge
Ge-Si
localSi-Si
Si
Ram
an in
tens
ity/ a
.u.
Raman shift/ cm-1
40 60 80 100
z(xx)-z geometry
• Ge-Ge phonons,
• Ge/Si phonons,
• folded acoustic phonons
Influence of Strain, Confinement, Intermixing
280 300 320 400 420
Ge-Ge
Ge-Si
Ram
an in
tens
ity/ a
.u.
Raman shift/ cm-1
Strain-induced shift
ω=304 cm-1, εxx=εyy=0.04,
εzz =2C12/C11.εxx
frequency shift due to atomic intermixing
Confinement-induced shift
117)]([21 −=++=Δ cmqp yyxxzz εεεω
ω
Ge-Ge Optical Phonons: Information about Strain
z(xy)-z geometry
wetting layer (no QDs): strained structure; confinement influence
QDs separated by thin Si layers (≤25Å): a partial strain relaxation
(strain is 2.8%)
QDs separated by thick Si layers (≥45Å): strained QDs ( 4%)
280 300 320 340
11/100
14/125
14/45
14/25
14/15
6/100
Ram
an in
tens
ity, a
.u.
Raman shift/ cm-1
Bulk Ge
Ideal strained
Ge-Si Optical Phonons: Information about Atomic Intermixing
z(xy)-z geometry
wetting layer (no QDs): almost abrupt interface
QDs separated by thin Si layer (≤25Å): atomic intermixing x≈0.09
QDs separated by thick Si layer (≥45Å): atomic intermixing x≈0.04
Model of atomic arrangement:380 400 420 440
11/100
14/125
14/45
14/25
14/15
6/100
Ram
an in
tens
ity, a
.u.
Raman shift/ cm-1
Si Si Ge GeGexSi1-x Ge1-xSix
Abrupt interface
Confinement of Optical Phonons
Ge/Si structure with the Ge thickness of 5ML
Ge1 Si
2
0 π/L π/aWavenumber, cm-1
ω1
3
4
5
2
Ge
qm Lm=π
Folded acoustical phonons
Dispersion of folded acoustic phonons (S.M.Rytov, Akoust. Zh.2, 71 (1956))
0 π/d π/aWave vector, cm-1
ω Si
Ge
)sin()sin(2
1)cos()cos()cos(2
2
1
12
2
2
1
1
υω
υω
υω
υω dd
kkddqd +
−=
k =υ ρ
υ ρ1 1
2 2d=d1+d2; d1 and d2, ρ1 and ρ2, υ1 and υ2 are the thickness, density and sound velocity in Ge and Si layers
qs
Folded Acoustical Phonons in Ge Dot SLs
Nominal and calculatedstructure periods:
214 Å 238 Å211 Å 229 ÅElastic continuum theory is applicable for periodical structures with QDs !!!
1,0
0,8
0,6
0,4
0,2
0,0
wav
evec
tor,
π/d
20 40 60 80 100
x10
wavenumber/cm-1
Ram
an in
tens
ity/a
.u.
20 40 60 80
x10
wavenumber/cm-1
14/200 11/200
Experimental Raman Spectra of Ge QD Superlattices
50 100 300 350 400
x3
1.0
0.8
0.6
0.4
0.2
0.0 w
aven
umbe
r /
π/d
LTLOTO
y'(x'x')-y'
y'(x'z)-y'
z(yx)-z
z(x'x')-z
z(xx)-z
y'(zz)-y'
y'(zx')-y'
Ram
an in
tens
ity/ a
.u.
Raman shift/ cm-1
Observation of „prohibited“ phonons:
Deviation fromselection rulesfor ideal Ge/Si superlattices
Resonance Profile of Ge OpticalPhonons
• maximum of Raman intensitycorresponds to resonance with E1exciton in Ge layers
• decreasingfrequency positionof Ge phononsmanifests size-confinement in Ge quantum dots
1,8 2,0 2,2 2,4 2,6Energy/ eV
Ram
an in
tens
ity/ a
.u.
303
306
309
312
315
Ge
phon
on fr
eque
ncy/
cm
-1
Comparison of Strained and Relaxed Ge QDs
• size distribution
• strain in QDs of particular size
• QD shape
1.8 2.0 2.2 2.4 2.6Energy/ eV
Ram
an in
tens
ity/ a
.u.
275
280
285
290
295
300310
315
Ge
phon
on fr
eque
ncy/
cm
-1
SiSi
Ge SiOx
strained relaxedQDs
Raman Spectroscopy and OMBD
Dilor XY 800 SpectrometerMonochromatic light source: Ar+ Laser (2.54eV), Detector: CCD • resonance condition with the absorption band of the organic material.• resolution: ~ 3.5 cm-1.
1.5 2.0 2.5 3.0 3.5 4.0
0
2
4
6
Abs
orbt
ion
coef
ficie
nt *
105
S0-S2 transition
S0-S1 transition
DiMe-PTCDI
PTCDA
Energy / eV
800 700 600 500 400
0
2
4
Wavelength / nm
Ar+ line
PTCDA DiMe-PTCDI
Symmetry D2hRaman active: 19Ag+18B1g+10B2g+7B3g
IR active: +10B1u+18B2u+18B3u
Silent: + 8Au108 internal vibrations
Molecular Vibrational Properties
CC2424HH88OO66
• DiMe-PTCDI: Cambridge Structural Database.
• PTCDA: α- and β-phases: S. R. Forrest, Chem. Rev. 97 (1997), 1793.
Monoclinic crystallographic system in thin films:
CC2626HH1414OO44NN22
C2h44Ag+22Bg
+23Au+43Bu
+ 8Au132 internal vibrations
200 400 600 1200 1350 1500 1650
Inte
nsity
/ ar
b. u
nits
Raman shift / cm-1
Raman Spectra of a PTCDA Crystal
• assignment of modes and their relative atomic contribution using Gaussian `98 (B3LYP, 3-21G).
x0.1
200 400 600 12
Inte
nsity
/ ar
b. u
nits
Raman sh
Raman Spectra of a Raman Spectra of a PTCDAPTCDA CrystalCrystal
• assignment of modes and their relative atomic contribution using Gaussian `98 (B3LYP:3-21G).
Raman shift /cm-1
and a and a DiMeDiMe--PTCDIPTCDI
DiMe-PTCDI PTCDA
PTCDA DiMe-PTCDI
DiMe-PTCDI
PTCDA experimental
ω m= =0.97ω m
ω 221= =0.95ω 233
⎛ ⎞⎜ ⎟⎝ ⎠
• Molecules remaining at the surface:NPTCDAPTCDA(0.04nm) ~ 1013 cm-2
NddSiSi ~ 1012 cm-2
Strong interaction between PTCDAPTCDA molecules and defectsdefects mainlymainly due to SiSi at the GaAsGaAs surface.
Interaction of Interaction of PTCDAPTCDA with the with the SS--GaAs(100):2x1 GaAs(100):2x1 SurfaceSurface
Annealing of a 14 nm thick film at 623 K for 30 min:
1300 1400 1500 1600
Inte
nsity
/ ct
s m
W-1 s
-1
Raman shift / cm-1
0.00
2
40 nmx 0.01
0.45 nm(x 0.6)
0.18 nm
ann.x 4.4
300 600 9000
10
20
30
1200 1400 16000
500
1000
1500
Inte
nsity
/ A
4 am
u-1
Raman shift / cm-1
Calculated Vibrational Properties:PTCDA
1340 1350
2.7 cm-1
• calculations with Gaussian `98 (B3LYP:3-21G).
Raman Monitoring ofRaman Monitoring of PTCDAPTCDA Growth on Growth on SS--GaAs(100):2x1GaAs(100):2x1
• Thin PTCDAPTCDA film: “first layer” SERS effect: molecules in contact with AgAg
• 15 nm PTCDAPTCDA film: mainly long range SERS:no AgAg diffusion into PTCDAPTCDA
S-GaAs(100)
AgAg//PTCDA:PTCDA: Evidence for Abrupt InterfaceEvidence for Abrupt InterfaceSimilar interface formation for AgAg//DiMeDiMe--PTCDIPTCDI
1350 1500 1650
Inte
nsity
/ ct
s m
W-1s-1
0.03
PTCDA(0.4 nm)
Raman shift / cm-11200 1350 1500
PTCDA(15 nm)
0.001
S-GaAs(100)
Ag:1.6 nm/minAg:5.5 nm/min
2.2 nm Ag
11 nm Ag
/ 30
/ 5
Indium/PTCDA: Evidence for Strong Indiffusion
1200 1350 1500 1650
Inte
nsity
/ ct
s m
W-1s-1 0.03
PTCDA
PTCDA(15 nm)
Raman shift / cm-11350 1500 1650
PTCDA(0.4 nm)
0.001
x 0.017+ Inx0.045
In: 0 →100 nm
In: 1 nm/min
PTCDA~0.4 nm(~1 ML) S-GaAs(100)
~15 nm(~50ML)
PTCDA
S-GaAs(100)
PTCDAPTCDA / / SS--GaAs(100)GaAs(100)
Morphology of Organic Thin FilmsMorphology of Organic Thin Films
15 nm thick films (nominal coverage) preferential orientation of DiMeDiMe--PTCDI PTCDI islands with their
long axis parallel to the [011] GaAs[011] GaAs substrate axis.
200nm200nm200nm200nm
DiMeDiMe--PTCDIPTCDI / / SS--GaAs(100)GaAs(100)
Determination of Molecular Orientation:Determination of Molecular Orientation:DiMeDiMe--PTCDIPTCDI
Azimuthal rotation of a 120 nm thick film; normal incidence.Periodic variation of signal in crossed and parallel polarization.
M. Friedrich, G. Salvan, D. Zahn et al., J. Phys.: Cond. Matter, 15 (2003) S2699 .
γ=0°: x II [011]GaAs
γ=90°:x II [0-11]
γ
phononsphonons phononsphonons
STM tip-enhanced Raman spectroscopyA new approach, tip-enhanced Raman spectroscopy (TERS), is explored that combines Raman spectroscopy at smooth surfaces with a local electromagneticfield enhancement provided by an optically active Ag STM or AFM tip. This optical activity is achieved by exciting local surface plasmon modes by focussing the laser light through a thin metal film onon a glass slide onto the tip apex. The local enhancement of the Raman scattering cross section in the vicinity of the tip opens promising avenues towards single molecule Raman spectroscopy.
Raman Spectroscopy