Making graphene magneticMaking graphene magnetic

Irina Grigorieva

Rahul Nair, Margherita Sepioni, I-Ling Tsai, Andre Geim

in collaboration with Arkady Krasheninnikov &OssiLehtinen (University of Helsinki)

why interest in why interest in graphene’sgraphene’s magnetism?magnetism?

from from basic physicsbasic physics standpoint: standpoint:

no no dd-- or or ff--electrons electrons

nonnon--trivial mechanism of magnetic trivial mechanism of magnetic moment formation (moment formation (ππ--magnetism)magnetism)

from from applications applications standpoint: standpoint:

potential for making graphene a potential for making graphene a spin generator spin generator -- important for important for spintronicsspintronics

how? how? -- by by introduction of defectsintroduction of defects. In principle can be done in a . In principle can be done in a controlled manner (unlike e.g. magnetic ions in dilute magnetic controlled manner (unlike e.g. magnetic ions in dilute magnetic ( g g g( g g gsemiconductors)semiconductors)

Theory: many Theory: many possible reasons for possible reasons for magnetismmagnetism

magnetism magnetism in purein pure--carbon carbon systems:systems: atomicatomic--scale scale defect (defect (adatomsadatoms, vacancies) carry, vacancies) carry BB(( ) y) y BB

Lehtinen Lehtinen et al, et al, PRLPRL (2004(2004))Pereira et al,Pereira et al, PRLPRL 9696, 036801 (2006), 036801 (2006)YazyevYazyev Helm, Helm, PR BPR B 7575, 125408 (2007), 125408 (2007)KumazakiKumazaki & & HirashimaHirashima, , J. Phys. Soc. J. Phys. Soc. JpnJpn.. 7676, 064713 (2007), 064713 (2007)UchoaUchoa et al, et al, PPRLRL 101101, 026805 (2008), 026805 (2008)Palacios et al, Palacios et al, PR BPR B 7777, 195428 (2008), 195428 (2008)S &S & CC ( )( )Singh & Kroll, Singh & Kroll, J. Phys: J. Phys: CondensCondens. Matter . Matter 2121, 196002 (2009), 196002 (2009)Krasheninnikov Krasheninnikov et al, et al, PRLPRL (2009)(2009)W. Li et al, W. Li et al, J. Mater. Chem.J. Mater. Chem. 1919, 9274 (2009), 9274 (2009)V lV l t lt l PPR BR B (2009)(2009)Venezuela Venezuela et al, et al, PPR BR B (2009)(2009)LopezLopez--SanchoSancho et al, et al, PR B PR B (2009) (2009) Faccio Faccio et al, et al, PPR BR B (2008(2008), ....), ....

spinspin--polarised polarised states at states at zigzig--zagzag edgesedgesHarigayaHarigaya, , EnokiEnoki (2001,2002) (2001,2002) F jitF jit t l (1996)t l (1996) K b hi t lK b hi t l (2006)(2006)Fujita Fujita et al (1996); et al (1996); Kobayashi et al Kobayashi et al (2006); (2006); Son Son et al, et al, NatureNature (2006)(2006)

Theory: many Theory: many possible reasons for possible reasons for magnetismmagnetism

specific types of defects within grain specific types of defects within grain boundaries :boundaries : Akhukov Fasolino Gornostayevboundaries :boundaries : Akhukov, Fasolino, Gornostayev, Katsnelson, Phys. Rev. B 85, 115407 (2012)

1D defects: ferromagnetic ground state at 1D defects: ferromagnetic ground state at domain boundariesdomain boundaries: : S.S. Alexandre, A. D. Lucio, A. H. Castro Neto and R. W. Nunes, arXiv:1109.6923

t ti i t ti i bilbil

ferromagnetismferromagnetism duedue toto HH--vacanciesvacancies inin graphanegraphane::BerashevichBerashevich ChakrabortyChakraborty,, NanotechnologyNanotechnology 2121,, 355201355201 ((20102010))

spontaneous magnetism in spontaneous magnetism in bilayerbilayergraphene, graphene, E. V. Castro et al, E. V. Castro et al, PRLPRL (2008)(2008)

origin of magnetic momentsorigin of magnetic moments

bipartite nature of graphene latticebipartite nature of graphene lattice

defects create imbalance between thedefects create imbalance between thedefects create imbalance between the defects create imbalance between the two graphene two graphene sublatticessublattices

‘‘ idid ’ t t l li d d d f t’ t t l li d d d f t‘‘midgapmidgap’ states localised around defect ’ states localised around defect sites, extending over several atoms in the sites, extending over several atoms in the vicinity of the defect vicinity of the defect

e g O Yazyev L Helm PRB 75 125408 (2007)e.g., O. Yazyev, L. Helm, PRB 75, 125408 (2007)

V. Pereira et al, PRL 96, 036801 (2006)

experiment: direct detection of magnetic momentsexperiment: direct detection of magnetic moments

magnetometry

magnetometry requires macroscopic quantities of magnetometry requires macroscopic quantities of graphene to detect magnetic moments directlygraphene to detect magnetic moments directly

limit of detection for best magnetometers is ~10limit of detection for best magnetometers is ~101515 BB

1g of graphene contains 101g of graphene contains 102222 atoms atoms many mmany m22 of of graphene needed even if 10% of C atoms are ‘magnetic’graphene needed even if 10% of C atoms are ‘magnetic’g p gg p g

macroscopic samples of graphene15 i t if ti 15 i t if ti 15 min centrifugation 15 min centrifugation

TEMTEMstable suspensionstable suspensionof nonof non coagulatedcoagulated TEMTEM

4040--50 50 hourshours

of nonof non--coagulatedcoagulatedgraphene crystallitesgraphene crystallites

100100 nmnm

4040 50 50 hourshourssonificationsonificationin organic in organic

solventsolvent

Manchester, Nanolett ’08Manchester, Nanolett ’08Dublin group, Nature Nano ‘08Dublin group, Nature Nano ‘08

solventsolvent(NMP(NMP))

collection of collection of graphene graphene nanocrystalsnanocrystals

24

2 cm

16kes

16

% o

f fla

k

100 nmSEM

8

%

layers of nonlayers of non interacting interacting 1401301008060 70504030

200 nm

layers of nonlayers of non--interacting interacting crystallitescrystallites

~50% ~50% monolayersmonolayerst i l t llit i 30t i l t llit i 30 4040

10 140130120110100908060 70504030

Flake size (nm)

20

typical crystallite size ~30typical crystallite size ~30--40nm40nmsuitable for SQUID magnetometry

magnetisation of graphene nanocrystalsmagnetisation of graphene nanocrystals

starting material: HOPGstarting material: HOPG graphene laminategraphene laminate

H ІІ (ab)

300 K

150 KH ІІ (ab)

mostly diamagnetic similar to graphitemostly diamagnetic similar to graphitemostly diamagnetic, similar to graphitemostly diamagnetic, similar to graphiteweak paramagnetic weak paramagnetic signal emerges below signal emerges below 20K20K

M. Sepioni et al, PRL 105, 207205 (2010)

experiment: controlled introduction of defectsexperiment: controlled introduction of defects

two types of atomictwo types of atomic--scale defects studied:scale defects studied:

fl i fl i d td t fluorine fluorine adatomsadatoms

vacancies produced by irradiation with energetic ionsvacancies produced by irradiation with energetic ionsp y gp y g

THEORY: both THEORY: both adatomsadatoms and vacancies are expected to carryand vacancies are expected to carry BBP. O. Lehtinen et al, P. O. Lehtinen et al, PRLPRL (2004)(2004)A V K h i ik t lA V K h i ik t l PRLPRL (2009)(2009)A. V. Krasheninnikov et al, A. V. Krasheninnikov et al, PRLPRL (2009)(2009)O. V. Yazyev, O. V. Yazyev, PRLPRL (2008) (2008) P. Venezuela et al, P. Venezuela et al, PPR BR B (2009)(2009)M P LopeM P Lope SanchoSancho et alet al PR BPR B (2009)(2009)M. P. LopezM. P. Lopez--SanchoSancho et al, et al, PR B PR B (2009) (2009) R. Faccio et al, R. Faccio et al, PPR BR B (2008)(2008)UchoaUchoa et al, et al, PPRLRL 101101, 026805 (2008) ...., 026805 (2008) ....

fluorinated graphene laminatesfluorinated graphene laminates

X F 200°C+ XeF2, 200°C2h

40h F/C = 0.2640h8h

F/C = 0.68

quantitative determination of fluorine

F/C 1

quantitative determination of fluorine concentration (F/C ratio) by XPS

details of graphene fluorination in R. R. Nair et al, Small 2010, 6, No. 24, 2877

paramagnetismparamagnetism in fluorinated graphenein fluorinated graphene

15-times greater saturation magnetisation compared to pristine

h f 90% fl i tigraphene for 90% fluorination

slight decrease in M for full fluorination but still strongly fluorination but still strongly paramagnetic

R.R. Nair et al, Nature Physics 8, 199 (2012)

much larger magnetisation values than for

diamagnetic background subtracted( )

g gferromagnetism reported in graphite

paramagnetismparamagnetism in fluorinated graphenein fluorinated graphene

for all fluorinations: excellent fits to the Brillouin function for J=S=1/2 i t ti ti t ith ti t non-interacting paramagnetic centres with magnetic moments ≈µB

Jxctnh

JJxJctnh

JJNgJM B 22

12

)122

12 TkBgJx BBwhere

JJJJ 2222 g BBwhere

can extract N, the number of spins (magnetic moments)

unambiguous spinunambiguous spin--half half paramagnetismparamagnetism

TC

TkgJNJ

BM

B

B

3

)1( 22

selfself--consistently, excellent fit to Curie law for paramagnetic susceptibilityconsistently, excellent fit to Curie law for paramagnetic susceptibility

nonnon interacting momentsinteracting moments nonnon--interacting momentsinteracting moments

spin concentrations in fluorinated graphenespin concentrations in fluorinated graphene

important parameter - number of spins (magnetic moments) per defect (F adatom)

only 10-3µB per F atom, not consistent with ‘one adatom, one spin’

graphene fluorination graphene fluorination -- mechanismmechanism

tendency towards clustering due to (i) intrinsic ripples(ii) i d h i l i i d (ii) increased chemical activity due to curvature(iii) low migration barriers for (iii) low migration barriers for fluorine adatoms

Osuna et al, J. Phys. Chem. C114, 3340–3345 (2010)

Kelly et al, Chem. Phys. Lett. 313, 445–450 (1999).

Ewels et al, Phys. Rev. Lett. 96, 21610 (2006).

paramagnetismparamagnetism due to clusters of fluorine atomsdue to clusters of fluorine atoms

t F/C 0 5up to F/C ~ 0.5

clustering of adatoms no sublattice imbalance in the ‘bulk’ of fully formed clusters Yazyev, Rep. Prog. Phys. 73, 056501 (2010)y p g y ( )

Wehling, Katsnelson, Lichtenstein, Chem. Phys. Lett. 476, 125 (2009)Rappoport, Uchoa, Castro Neto, Phys. Rev. B 80, 245408 (2009)

total spin is determined by the atom imbalance between the two total spin is determined by the atom imbalance between the two sublattices :

b d N i li i l t f 2000 t ( 8 i )

BA21 NNS

observed Ns implies one spin per cluster of ~2000 atoms (~8 nm size)

even at F/C 1 (=0.999), still ~0.1% defects (missing F atoms)

irradiated graphene laminates

R.H. Telling, M.I. Heggie, Phil Mag. 87, 4797 (2007)

graphene laminateproton

graphene laminate

Proton, 350 keV

8 10~ 8-10 m

advantage compared to graphite: samples sufficiently thin (3-4m) to ensure uniform defect distribution well defined defect concentrations;

on average one vacancy per proton, homogeneous vacancy distributiono a e age o e aca cy pe p oto , o oge eous aca cy d st but o

no implanted ions, only vacancies

paramagnetismparamagnetism in irradiated graphenein irradiated graphene

R.R. Nair et al, Nature Physics 8, 199 (2012)

vacancies are not mobile and vacancies are not mobile and cannot cluster !cannot cluster !cannot cluster !cannot cluster !

0.1 µ0.1 µBB per vacancy per vacancy –– much much BB p yp ygreater than per F greater than per F adatomadatom

qualitatively similar to qualitatively similar to adatomsadatoms (at first sight) :(at first sight) : qualitatively similar to qualitatively similar to adatomsadatoms (at first sight) :(at first sight) :

paramagnetismparamagnetism with spin ½ with spin ½

li i i l i i i h i i li i i l i i i h i i linear increase in total magnetisation with increasing linear increase in total magnetisation with increasing defect density;defect density;

graphene can be made (graphene can be made (parapara)magnetic)magnetic

magnetic moments in graphene can be introduced magnetic moments in graphene can be introduced reliably by reliably by functionalisationfunctionalisation or irradiationor irradiationreliably by reliably by functionalisationfunctionalisation or irradiationor irradiation

only only paramagnetismparamagnetism –– nonnon--interatinginterating magnetic magnetic yy p gp g gg ggmoments moments –– but important first step towards achieving but important first step towards achieving

((ferroferro)magnetism)magnetism

not as straightforward as expected, especially for not as straightforward as expected, especially for adatomsadatoms but broadly agreement with theory but broadly agreement with theoryadatomsadatoms, but broadly agreement with theory, but broadly agreement with theory

amount of magnetic moments can be tuned by amount of magnetic moments can be tuned by g yg ycontrolling the amount defectscontrolling the amount defects

can magnetic moments in graphene be can magnetic moments in graphene be simply and reversibly controlled? simply and reversibly controlled? simply and reversibly controlled? simply and reversibly controlled?

magnetic moments are related to features in the magnetic moments are related to features in the electronic band structure , so should respond to changes electronic band structure , so should respond to changes in the Fermi level:in the Fermi level:

--EE

EF ~0

EE

neutral graphene neutral graphene -- magneticmagnetic

adatoms vacancies

can magnetic moments in graphene be can magnetic moments in graphene be simply and reversibly controlled? simply and reversibly controlled? simply and reversibly controlled? simply and reversibly controlled?

magnetic moments are related to features in the magnetic moments are related to features in the electronic band structure , so should respond to changes electronic band structure , so should respond to changes in the Fermi level:in the Fermi level:

--EE

EF ~ ±1eV

EE

doped graphene doped graphene –– nonnon--magnetic?magnetic?

adatoms vacancies

doping of graphene laminatesdoping of graphene laminates

XX doping by electric field doping by electric field

chemical/molecular dopingchemical/molecular doping

graphene laminates

several gases/liquids shown to be several gases/liquids shown to be effective effective dopantsdopants for graphenefor graphene

200NH3

(

)

100

0CO

-100H2O

I IVII III Schedin et al (Manchester) Nature Mater 2007

t (s)10000 500

-200 NO2

Schedin et al (Manchester), Nature Mater. 2007T. Welhing et al, NanoLett. 8, 173 (2008) ...

effect of doping on vacancy magnetismeffect of doping on vacancy magnetism

vacancies – truly intrinsic magnetism (no foreign atoms)

n ~51011 cm-2EF=

n ~21013 cm-2

EF~0.5 eV

b dl t ith thb dl t ith th

EF 0.5 eVremains remains SS=1/2=1/2

broadly agreement with theorybroadly agreement with theory

effect of doping on vacancy magnetismeffect of doping on vacancy magnetism

EF~0.5 eVft l f HNO F

E 0

after removal of HNO3

EF~ 0

5.00

fS

NN

0SN

universal behaviour for all studied samples universal behaviour for all studied samples R.R. Nair et al, in preparation

number of spins number of spins saturatessaturates at half the initial valueat half the initial value

effect of doping on vacancy magnetism effect of doping on vacancy magnetism –– universal behaviouruniversal behaviour

fS

0SS NNN

initial number b f iinitial number of spins

number of spinsat saturation

5.00S

NN

SN

universal value for saturation of the number of spins at universal value for saturation of the number of spins at EEFF>0.45eV>0.45eV shifting the Fermi level ‘switches off’ only half of the magnetic momentsshifting the Fermi level ‘switches off’ only half of the magnetic moments

covalently bonded impuritiescovalently bonded impurities NMP

organic groups covalently bound to C atoms midgap states

CH3 C2H5 CH2OH

Wehling et al, Wehling et al, PRLPRL 105105, 056802 (2010), 056802 (2010)

Wehling et al, Wehling et al, Phys. Rev. BPhys. Rev. B 8080, 085428 (2009), 085428 (2009)

from transport measurements – always present in graphene in very small concentrations (~10ppm) and act as resonant scatterers( pp )

Z. H. Ni et al, Z. H. Ni et al, NanoLett.NanoLett. 1010, 3868 (2010), 3868 (2010)

annealing promotes binding of organic groups to C atomsannealing promotes binding of organic groups to C atomse.g. L.Yunge.g. L.Yung--Chang et al, Chang et al, NanoLett NanoLett 1212, 414 (2012), 414 (2012)

magnetism of vacancies magnetism of vacancies vsvs covalent impurities covalent impurities

vacanciesvacancies

covalently bonded covalently bonded impurities

(para)magnetism in graphene can be tuned!

can we reversibly control defect magnetism?can we reversibly control defect magnetism?

EF~ 0

EF~0.5 eV

YES WE CANYES, WE CAN

Rah l NairRahul NairManchester

I-Ling-TsaiManchester

Margherita SepioniManchester

Ossi LehtinenHelsinki

Arkady KrasheninnikovUniv. of Helsinki

Andre Geim Manchester

U o e s