presentation

Making graphene magnetic
Irina Grigorieva
Rahul Nair, Margherita Sepioni, I-Ling Tsai,
Andre Geim
in collaboration with Arkady Krasheninnikov &Ossi
Lehtinen (University of Helsinki)
why interest in graphene’s magnetism?
from basic physics standpoint:
 no d- or f-electrons
 non
non--trivial mechanism of magnetic
moment formation (π
(π-magnetism)
from applications standpoint:
 potential for making graphene a spin generator - important for
spintronics
 how? - by introduction of defects.
defects. In principle can be done in a
controlled manner ((unlike e.g.
g magnetic
g
ions in dilute magnetic
g
semiconductors)
Theory: many possible reasons for magnetism
 magnetism in pure
pure--carbon systems:
 atomic
atomic--scale defect ((adatoms
adatoms,, vacancies)) carryy 

B
Lehtinen et al, PRL (2004
(2004))
Pereira et al, PRL 96
96,, 036801 (2006)
Yazyev  Helm, PR B 75
75,, 125408 (2007)
Kumazaki & Hirashima,
Hirashima, J. Phys. Soc. Jpn.
Jpn. 76
76,, 064713 (2007)
Uchoa et al, PRL 101
101,, 026805 (2008)
Palacios et al, PR B 77
77,, 195428 (2008)
S
Singh
& Kroll, J. Phys: Condens.
C
Condens
. Matter 21
21,, 196002 (2009)
(
)
Krasheninnikov et al, PRL (2009)
W. Li et al, J. Mater. Chem. 19
19,, 9274 (2009)
V
l ett al,
l PR B (2009)
Venezuela
Lopez--Sancho et al, PR B (2009)
Lopez
Faccio et al, PR B (2008
(2008),
), ....
 spin
spin--polarised states at zigzig-zag edges
Harigaya, Enoki (2001,2002)
Harigaya,
F jit ett all (1996);
Fujita
(1996) Kobayashi
K b
hi ett all (2006);
(2006)
Son et al, Nature (2006)
Theory: many possible reasons for magnetism
 specific types of defects within grain
boundaries : Akhukov,
Akhukov Fasolino,
Fasolino Gornostayev,
Gornostayev
Katsnelson, Phys. Rev. B 85, 115407 (2012)
 1D defects: ferromagnetic ground state at
domain boundaries: S.S. Alexandre, A. D.
Lucio, A. H. Castro Neto and R. W. Nunes,
arXiv:1109.6923
 ferromagnetism due to H-vacancies in graphane
graphane::
Berashevich  Chakraborty
Chakraborty,, Nanotechnology 21
21,, 355201 (2010)
2010)
t
ti iin bilayer
bil
spontaneous
magnetism
graphene, E. V. Castro et al, PRL (2008)
origin of magnetic moments
bipartite nature of graphene lattice
defects create imbalance between the
two graphene sublattices
‘midgap
midgap’
id
’ states
t t localised
l
li d around
dd
defect
f t
sites, extending over several atoms in the
vicinity of the defect
eg O
e.g.,
O. Yazyev,
Yazyev L.
L Helm
Helm, PRB 75,
75 125408 (2007)
V. Pereira et al, PRL 96, 036801 (2006)
experiment: direct detection of magnetic moments
 magnetometry
 magnetometry requires macroscopic quantities of
graphene to detect magnetic moments directly
 limit of detection for best magnetometers is ~1015 B
 1g of graphene contains 1022 atoms
many m2 of
ggraphene
p
needed even if 10% of C atoms are ‘magnetic’
g
macroscopic samples of graphene
15 min
i centrifugation
t if
ti
40--50 hours
40
sonification
in organic
solvent
(NMP))
(NMP
stable suspension
of nonnon-coagulated
graphene crystallites
Manchester, Nanolett ’08
Dublin group, Nature Nano ‘08
TEM
100 nm
collection of graphene nanocrystals
2 cm
SEM
% of flak
kes
24
16
100 nm
8
200 nm
 layers of nonnon-interacting
crystallites
 ~50% monolayers
typical
t i l crystallite
t llit size
i ~30~30
30-40nm
40
10 20 30 40 50 60 70 80 90 100 110 120 130 140
Flake size (nm)
suitable for SQUID magnetometry
magnetisation of graphene nanocrystals
starting material: HOPG
graphene laminate
H ІІ (ab)
300 K
H ІІ (ab)
150 K
mostly diamagnetic,
diamagnetic similar to graphite
weak paramagnetic signal emerges below 20K
M. Sepioni et al, PRL 105, 207205 (2010)
experiment: controlled introduction of defects

two types of atomicatomic-scale defects studied:
 fluorine
fl i adatoms
d t
 vacancies p
produced byy irradiation with energetic
g ions
THEORY: both adatoms and vacancies are expected to carry B
P. O. Lehtinen et al, PRL (2004)
A V.
A.
V Krasheninnikov
K h i ik ett al,
l PRL (2009)
O. V. Yazyev, PRL (2008)
P. Venezuela et al, PR B (2009)
M P.
M.
P LopezLope -Sancho et al
Lopez
al, PR B (2009)
R. Faccio et al, PR B (2008)
Uchoa et al, PRL 101
101,, 026805 (2008) ....
fluorinated graphene laminates
+ XeF
X F2, 200°C
40h
2h
F/C = 0.26
8h
F/C = 0.68
 quantitative determination of fluorine
F/C  1
concentration (F/C ratio) by XPS
details of graphene fluorination in
R. R. Nair et al, Small 2010, 6, No. 24, 2877
paramagnetism in fluorinated graphene
 15-times greater saturation
magnetisation compared to pristine
graphene
h
for
f 90% fluorination
fl
i ti
 slight decrease in M for full
fluorination but still strongly
paramagnetic
R.R. Nair et al, Nature Physics 8, 199
((2012))
diamagnetic background subtracted
 much larger
g magnetisation
g
values than for
ferromagnetism reported in graphite
paramagnetism in fluorinated graphene
 for all fluorinations: excellent fits to the Brillouin function for J=S=1/2 
non-interacting
with
i t
ti paramagnetic
ti centres
t
ith magnetic
ti moments
t ≈µB
 2J 1
 2 J  1) x   1
 x 
M  NgJB  
ctnh
  ctnh 
 2J
 2J
 2 J 
 2J
where
can extract N, the number of spins (magnetic moments)
xg
gJ B B k BT
unambiguous spinspin-half paramagnetism
2 2
M NJ ( J  1) g  B C



B
T
3k BT
 self
self--consistently, excellent fit to Curie law for paramagnetic susceptibility
 non
non--interacting moments
spin 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 - mechanism
 tendency
towards clustering due to
(i) intrinsic ripples
(ii) increased
i
d chemical
h i l activity
i i d
due
to curvature
(iii) low migration barriers for
fluorine adatoms
Osuna et al, J. Phys. Chem. C
114, 3340–3345 (2010)
Kelly et al, Chem. Phys. Lett. 313, 445–
450 (1999).
Ewels et al, Phys. Rev. Lett. 96, 21610
(2006).
paramagnetism due to clusters of fluorine atoms
up to
t F/C ~ 00.55
 clustering of adatoms  no sublattice imbalance in the ‘bulk’ of fully
Yazyev,
Rep.
formed clusters
y
p Prog.
g Phys.
y 73, 056501 ((2010))
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
sublattices : S  1 NA  NB
2
 observed
b
d Ns implies
i li one spin
i per cluster
l t off ~2000
2000 atoms
t
((~8
8 nm size)
i )
 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 laminate
proton
Proton, 350 keV
8 10 m
~ 8-10
 advantage compared to graphite: samples sufficiently thin (3-4m) to
ensure uniform defect distribution  well defined defect concentrations;
o
on a
average
per p
proton,
distribution
e age one
o e vacancy
aca cy pe
oto , homogeneous
o oge eous vacancy
aca cy d
st but o
 no implanted ions, only vacancies
paramagnetism in irradiated graphene
R.R. Nair et al, Nature Physics
8, 199 (2012)
vacancies are not mobile and
cannot cluster !
y – much
0.1 µB p
per vacancy
greater than per F adatom
 qualitatively similar to adatoms (at first sight) :
 paramagnetism with spin ½
 linear
li
iincrease iin totall magnetisation
i i with
i h iincreasing
i
defect density;
graphene can be made (para
(para)magnetic
)magnetic
magnetic moments in graphene can be introduced
reliably by functionalisation or irradiation
only
paramagnetism
– non
non--interating
yp
g
g magnetic
g
moments – but important first step towards achieving
(ferro)magnetism
ferro)magnetism
not as straightforward as expected, especially for
adatoms,, but broadly agreement with theory
adatoms
amount of magnetic
g
moments can be tuned by
y
controlling the amount defects
can magnetic moments in graphene be
simply and reversibly controlled?
 magnetic moments are related to features in the
electronic band structure , so should respond to changes
in the Fermi level:
EF ~0
- E
neutral graphene - magnetic
adatoms
vacancies
can magnetic moments in graphene be
simply and reversibly controlled?
 magnetic moments are related to features in the
electronic band structure , so should respond to changes
in the Fermi level:
EF ~ ±1eV
- E
doped graphene – non
non--magnetic?
adatoms
vacancies
doping of graphene laminates
X

doping by electric field
chemical/molecular doping
 several gases/liquids shown to be
effective dopants for graphene
graphene laminates
200
NH3
CO
0
H2O
-100
I
II
-200
III
IV
NO2
0
500
t (s)

 ()
100
1000
Schedin et al (Manchester),
(Manchester) Nature Mater
Mater. 2007
T. Welhing et al, NanoLett. 8, 173 (2008) ...
effect of doping on vacancy magnetism
 vacancies – truly intrinsic magnetism (no foreign atoms)
EF=
n ~51011 cm-2
n ~21013 cm-2
EF~0.5
0.5 eV
 broadly
b
dl agreementt with
ith th
theory
remains S=1/2
effect of doping on vacancy magnetism
EF~0.5 eV
after
ft removall off HNO3
EF~ 0
N Sf
 0 .5
0
NS
R.R. Nair et al, in preparation
 universal behaviour for all studied samples
 number of spins saturates at half the initial value
effect of doping on vacancy magnetism – universal behaviour
 N S  N S0  N Sf
N S
 0.5
0
NS
initial number
of spins
b off spins
i
number
at saturation
 universal value for saturation of the number of spins at EF>0.45eV
 shifting the Fermi level ‘switches off’ only half of the magnetic moments
covalently bonded impurities
 organic groups covalently bound to C atoms
CH3
NMP
midgap states
C2H5
CH2OH
Wehling et al, PRL 105
105,, 056802 (2010)
Wehling et al, Phys. Rev. B 80
80,, 085428 (2009)
 from transport measurements – always present in graphene in very
small concentrations ((~10ppm)
pp ) and act as resonant scatterers
Z. H. Ni et al, NanoLett. 10
10,, 3868 (2010)
 annealing promotes binding of organic groups to C atoms
e.g. L.YungL.Yung-Chang et al, NanoLett 12
12,, 414 (2012)
magnetism of vacancies vs covalent impurities
vacancies
covalently bonded
impurities
 (para)magnetism in graphene can be tuned!
can we reversibly control defect magnetism?
EF~ 0
EF~0.5 eV
YES WE CAN
YES,
Rahull Nair
Rah
Manchester
I-Ling-Tsai
Manchester
Margherita Sepioni
Manchester
Ossi Lehtinen
Helsinki
Arkady Krasheninnikov
Univ. o
of Helsinki
U
es
Andre Geim
Manchester