Quantum Hall Effect in Hydrogenated Graphene

2.2 MB·English

by  J. Guillemette

Quantum Hall Effect in Hydrogenated Graphene
J. Guillemette1,2, S.S. Sabri2, B. Wu1, K. Bennaceur1, P.E. Gaskell2, M. Savard1, P.L. L´evesque3,
F. Mahvash2,4, A. Guermoune2,4, M. Siaj4, R. Martel3, T. Szkopek2†, and G. Gervais1†(cid:63)
1Department of Physics, McGill University, Montr´eal, QC, H3A 2T8, Canada
2 Department of Electrical and Computer Engineering,
McGill University, Montr´eal, QC, H3A 2A7, Canada
3 Department of Chemistry, Universit´e de Montr´eal, Montr´eal, QC H3C 3J7, Canada
4 Department of Chemistry, Universit´e du Qu´ebec `a Montr´eal, Montr´eal, QC, H3C 3P8, Canada
†both authors contributed equally to this work and
(cid:63)corresponding author: [email protected]
(Dated: December 11, 2013)
3
1 ThequantumHalleffectisobservedinatwo-dimensionalelectrongasformedinmillimeter-scale
0 hydrogenated graphene, with a mobility less than 10 cm2/V·s and corresponding Ioffe-Regel dis-
2 order parameter (k λ)−1 (cid:29) 1. In zero magnetic field and low temperatures, the hydrogenated
F
grapheneisinsulatingwithatwo-pointresistanceoforderof250h/e2. Applicationofastrongmag-
n
neticfieldgeneratesanegativecolossalmagnetoresistance,withthetwo-pointresistancesaturating
a
J within 0.5% of h/2e2 at 45T. Our observations are consistent with the opening of an impurity-
induced gap in the density of states of graphene. The interplay between electron localization by
7
defect scattering and magnetic confinement in two-dimensional atomic crystals is discussed.

]
l
l
a Two-dimensional atomic crystals have attracted much netic field induces a transition to a well formed quan-
h attention as surfaces with unique low-dimensional elec- tum Hall state with a two point resistance R =h/2e2
2pt
-
tron transport behaviour, most notably the zero-gap accurate to within 0.5%. The insulator-quantum Hall
s
e semiconductor graphene [1]. The relativistic Dirac dis- transition coincides with the magnetic length ((cid:96) ) ap-
B
m
persion of electrons in graphene leads to an anomalous proaching,andbecominglessthanthemeandefectspac-
. four-fold degenerate quantum Hall (QH) sequence [2–5]. ing inferred from Raman spectroscopy (λ ).
t D
a Common to these studies is low disorder, with a mean
m Disorderedgraphenesampleswerepreparedfrompris-
freepathλlargecomparedtoelectronFermiwavelength
tine, large-area, monolayer graphene samples grown by
- λ = 2π/k , with the cleanest graphene samples ex-
d F F chemical vapour deposition (CVD) on Cu foils [11], with
hibiting the fractional quantum Hall (FQH) effect [6, 7].
n growth details reported elsewhere [1]. Graphene mono-
o We report here our discovery of a quantum Hall effect in
layers were transferred to oxidized silicon wafers and
c graphene at the opposite extreme, whereby hydrogena-
[ electricallycontactedforelectrontransportexperiments.
tion was used to induce a short mean-free path beyond
Disorder was controllably introduced into the graphene
1 the Ioffe-Regel limit kFλ ∼ 1 for the onset of strongly by exposure to a beam of atomic hydrogen produced by
v insulating behaviour. Our work illustrates the impor-
thermally cracking molecular hydrogen with a white-hot
7 tance of the interplay between electron localization by
5 tungsten capillary [2]. The room temperature two-point
pointdefectscatteringandmagneticconfinementintwo-
2 resistance R of graphene samples was monitored ver-
dimensional atomic crystals. 2pt
1 sus atomic hydrogen exposure in situ in an ultra-high
.
1 vacuum chamber, Fig.1. The room temperature resis-
We present experimental results on electronic trans-
0 tance of graphene devices was observed to increase with
3 port in graphene where neutral point defects are intro-
exposure to atomic hydrogen, as has been observed in
1 duced by hydrogenation. Previous electron transport
graphene exposed to hydrogen plasma [8]. Atomic hy-
: studiesofhydrogenatedgraphenewereinterpretedasev-
v drogen exposure produces an exponential growth in re-
i idenceforgraphane[8],atwo-dimensionalpolymerofcar-
X sistance, and a corresponding exponential shortening of
bon and hydrogen. More recently, angle resolved photo-
carrier mean-free path λ. The Ioffe-Regel disorder pa-
r emissionspectroscopy(ARPES)[9]andscanningtunnel-
a rameter (k λ)−1 (cid:39) (M/2)·(e2/h)·R , where M = 4
ing microscopy (STM) [10] measurements have demon- F 2pt
accounts for spin and valley degeneracy, is consequently
strated the opening of a gap in the density of states
significantly increased beyond the limit for the onset of
of hydrogenated graphene. The object of our work
strongly insulating behaviour.
is to probe the magnetotransport properties of hydro-
genated graphene. We focus on large area (millimeter- The disorder introduced by hydrogenation was ob-
scale) graphene that was exposed to an atomic hydro- served by Raman spectroscopy, Fig. 2A. The Raman
gen beam resulting in insulating behaviour (dR/dT < 0 Stokes peaks G and G’ associated with pristine sp2 car-
at zero magnetic field) and a low-temperature resistance bonindicateclean,monolayerCVDgraphenepriortohy-
R∼250×h/e2 farabovetheIoffe-Regellimitfortheon- drogenation (blue spectrum). The emergence of strong
set of insulating behaviour. Application of a 45 T mag- disorder-induced peaks D and D’ upon hydrogenation
2
A A
T > 1800 K e– y D hydrogenated CVD graphene
sit G
H2 H• + H• nten D’ G’ G + D
n i
B Ix ma G CVD graphene G’
V R =V /I a
– x+ 2pt x x R
V D G* G”
graphene 1500 2000 2500 3000
VG SiO2 Stokes shift [ cm–1 ]
–+ +n-Si 1cm
B
60V 2 ]
e100
C2e ] 68 2h/e ]100 143550VVV R [ h/2pt101−20 0 T2=01.74K0 60
[ h/2pt 4 R2pt = 0.06 h/e2 R [ 2pt10 –5V VG [V]
R 2
–30V
0
1
0 150 300 450 600 750 VG
Hydrogenation Time [ s ]
10 100
Temperature [ K ]
FIG. 1. Schematic of graphene hydrogenation. A) An elec-
trically contacted CVD grown graphene sample is exposed
to reactive hydrogen atoms produced by thermal cracking of FIG. 2. Characterisation of the hydrogenated graphene de-
hydrogenatT >1800KinaUHVenvironment. B)Thetwo- vices. A) Raman Stokes spectrum of CVD graphene (lower
pointresistanceR ofthegraphenesheetismeasuredinsitu trace)andhydrogenatedCVDgraphene(highertrace). Peaks
2pt
with hydrogen dosing. A photograph of the device is shown. associated with sp2 carbon are classified G, and peaks asso-
C)Thetypicalevolutionofroomtemperatureresistancever- ciated with disorder are classified D, respectively. The ratio
sus hydrogen exposure time (circles), and an exponential fit ID/IG ofDpeakintensitytoGpeakintensityis2.2and0.15
to resistance versus hydrogen dose (dotted line). forhydrogenatedCVDgrapheneandpristineCVDgraphene,
respectively, with the corresponding mean defect spacing λ
D
estimated to be 4.6±0.5 nm and ∼ 35 nm (see text). B)
The temperature dependence of the resistance is insulating
(redspectrum)confirmstheintroductionofpointdefects
(dR /dT < 0) at all gate voltages. The field effect (inset)
2pt
that break the translational invariance of the crystal [3].
shows hole conduction and a zero field effect mobility regime
Thenatureofthepointdefectsintroducedbyhydrogena- at low hole density.
tion is most likely hydrogen-carbon bonds [8]. By direct
comparison of the ratio of D peak intensity to G peak
intensity, I /I , with that observed in graphene disor- 2B, where the Ioffe-Regel parameter is (k λ)−1 (cid:39) 700.
D G F
dered by controlled ion bombardment [4], we estimate In contrast, the Ioffe-Regel parameter in Si [16] and
the mean hydrogen-hydrogen spacing to be in the range GaAs/AlGaAs [17] where QH and FQH was first ob-
λ (cid:39) 4.6±0.5 nm for a 360s hydrogen exposure, corre- served is (k λ)−1 (cid:39) 0.02. The exponential increase in
D F
sponding to a 0.2−0.3% hydrogen to carbon ratio. resistanceasT →0Kappearsconsistentwithavariable-
Electronic transport measurements were performed range hopping behaviour R exp((T /T)n), however we
0 0
on the hydrogenated graphene with large ohmic con- do not find good agreement to either a Mott n=1/3 or
tacts at the opposing corners of a 2.5 mm square us- Efros-Shklovskii n=1/2 exponent over the temperature
ing a standard low-frequency technique. The hydro- range 1.7 K<T <175 K. Rather, a pronounced satura-
genated graphene shows unambiguous insulating be- tion in resistance is observed at the lowest hole density
haviour, ∂R /∂T < 0, over the entire range of acces- andlowesttemperatures,correspondingtotheemergence
2pt
sible temperatures (from 175K to ∼1.7K) and gate in- of a zero field effect mobility regime of transport.
duced carrier density, Fig.2B. Gate voltage modulation The 2-point resistance was measured at T = 575 ±
of the resistance shows hole transport with a field effect 25 mK versus carrier density and in strong magnetic
mobility µ = ∂(1/R )/∂(CV ) (cid:46) 10 cm2/V·s, where fields, Fig.3A. At high hole densities, a colossal negative
2pt G
the gate capacitance C = 11.5 nF/cm2. A regime of magnetoresistance is observed from B =0 T through to
zerofieldeffectmobilityisalsoobserved,seeinsetofFig. B = 45 T. A similar magnetoresistance was observed
3
in weakly fluorinated graphene [18]. Conversely, in the
A
regime of zero field effect mobility, the resistance is very
weakly dependent on magnetic field below a critical field
that is itself dependent upon gate voltage. Remarkably,
100
asetrvaedgaftreomvoaltnaginesuVlGati=ng2,1zeVro,fiaelsdhaerffpecttramnosbitiiloitnyisstaotbe-, 2e ] 100
twohiachreissiswtaitnhciens0a.5tu%raotfinhg/2aet2.aIvnalauequRan2pttum=H12a,ll96st2atΩe,, [ h/pt10 10
2
R
thelongitudinalfourpointresistanceR →0sothatthe
xx 1
twopointresistanceapproachesthetransverseresistance
1
R → |R |. The behaviour of R is fully consistent
2pt xy 2pt 0
with a Hall component admitting the quantized value 10 40
gRrxayph=enhe/R2ex2y.=Fr±omh/4the2e(Nkn+ow1n/2a)n−o1m[2a,lo3u],stQheHhsoelreiedsopo-f Magnetic 2F0ield3 [0 T ]40 –40 –20Gate 0Voltag2e 0[ V ]
ing of the sample, and an assumption of preserved spin
andvalleydegeneracies,thisQHstatecorrespondstothe B Gate Voltage [ V ]
–100 –50 0 50 100
N = −1 Landau level index and a ν = −2 filling factor.
Nafireoeltdoa.bbslye,rvneodSahsutbhnisikQovH-dsetaHteaiassroesaccihlleadtiaotnshiignhrmesaisgtnaentciec 2e ]100 2 [ h/e ]pt011..55 VG = 21V
45TThe, rFeisgi.s3tBan,cerevveeraslussthgaetechvaorlgteagneeauttrcaolintystapnotinBt a=t [ h/2pt10 R2 035 B4 [0 T ] 45 nd edge nd edge
R a a
b b
VG = 71±2 V between electron and hole conduction. 1 B = 45 T ν = –2
Theholedensityatν =−2requiredbythemagneticflux
density is 2eB/h = 2.17×1012/cm2, while the plateau
12 10 8 6 4 2 0 –2
is observed with 3.6×1012/cm2 holes induced from neu- Hole Density [ 1012/cm2 ]
trality. The mobile hole valence band edge is thus lo-
cated at gate voltage V = 51 V, providing a signa- C Magnetic Length, ℓB [ nm ]
G 8 7 6 5 4
ture in electron transport of the formation of a disorder
induced gap. Assuming electron-hole symmetry about 2 ]e100 λD B
h/
cdheanrsgiteynoefu2t.r9a×lit1y0,1w2/ecemst2i.mGaatepaoploencainligzeadndmtihde-gparpessetnactee R [ 2pt110 ℓB < λD
of mid-gap states have been observed in hydrogenated
10 15 20 25 30 35 40 45
graphene by ARPES [9] and STM [10], with a gap of
Magnetic Field [T]
∼ 400−600 meV observed for a similar 0.1−0.3% hy-
drogen to carbon ratio. Neither the ν = −4 or ν = −6
FIG. 3. Quantum Hall effect in hydrogenated graphene. A)
plateaus are clearly observed, however the weak resis- Thetwo-pointresistanceofthehydrogenatedgraphenesheet
tance minimum at V =−51±2 V hints to an emerging is shown versus the magnetic field and the gate voltage. The
G
ν = −6 QH state. Disorder induced broadening of Lan- resistanceaxisisshownonalogarithmicscale,inunitsofthe
dau levels may be responsible for the absence of other resistancequantumRq =h/e2. Alldataweretakenatatem-
perature of 575±25 mK. B) At 45T, the resistance versus
plateaus and the absence of Shubnikov-de Haas oscilla-
gate voltage (upper x-axis) and hole density (lower x-axis),
tions.
with the red line indicating a Hall plateau at R = h/2e2.
From the perspective of two-dimensional transport in The inset shows the resistance at 21V gate voltage approach
a disordered medium, the emergence of a ν = −2 quan- thequantumHallstateplateauwithincreasingmagneticfield.
tum Hall state at 45 T from a zero-field insulating state C)Resistanceofthehydrogenatedgrapheneat21Vgatevolt-
induced by neutral point defects can be understood by age plotted versus both the magnetic field (lower x-axis) and
magnetic length (cid:96) (upper x-axis). The shaded region indi-
comparisonoflengthscales,Fig.3C.Themagneticlength B
(cid:96) = (cid:112)(cid:126)/eB quantifies the magnetic confinement of cates the estimated point defect spacing extracted from the
B Raman spectra.
charge carriers in 2D, independent of material param-
eters. The insulator-QH transition is here observed with
magneticlengthspanning(cid:96) =4−6nm,consistentwith
B
complementary to the picture of extended states float-
acrossoverfromweaktotightmagneticconfinementwith
ing in energy that has been previously used to explain
respecttothepointdefectspacingλ =4.6±0.5nmin-
D
insulator-QH transitions [19].
ferred from Raman spectroscopy. The perspective of an
insulator-QH transition arising from magnetic confine- The emergence of a quantum Hall state from a zero
ment of charge carriers below the mean defect spacing is field effect mobility insulating state raises the question
4
hydrogen-carbon ratio gives an estimated zero-field gap
of E ∼400−600 meV [9]. In comparison, the bandgap
G
of GaAs, the choice material for high mobility 2DEGs,
is ∼ 1.5 eV which is ∼ 20× larger than the cyclotron
energy (cid:126)eB/m∗ = 78 meV separating Landau levels at
45 T. We propose that the energy scales experimentally
accessible in hydrogenated graphene enable the study of
the competition between disorder and magnetic confine-
ment,leadingtonewunderstandingoftheroleofdisorder
in the QHE.
Acknowledgments
WeacknowledgeusefuldiscussionswithA.Grueneis(IFW
Dresden), and the outstanding technical assistance of D.
Berry,M.Nannini,R.Talbot,J.Smeros,R.Gagnon(McGill)
as well as T.P. Murphy, J. Pucci and G. Jones (NHMFL).
This work was funded by the Natural Sciences and Engi-
neering Research Council of Canada (NSERC), the Cana-
dianInstituteforAdvancedResearch(CIFAR),theFondsde
FIG. 4. ‘Ioffe-Regel’ - ‘Wigner-Seitz’ diagram for the main RechercheduQu´ebec-NatureetTechnologies(FRQNT),and
familiesofmaterialsexhibiting the quantumHalleffect. The
theCanadaResearchChairprogram(CRC).Aportionofthis
figure shows the zero-field value of the Ioffe-Regel disorder
workwasperformedattheNationalHighMagneticFieldLab-
parameter(k λ)−1plottedversustheWigner-Seitzradiusr ,
F s
oratory which is supported by NSF Cooperative Agreement
bothdimensionless,forthemain2DEGfamiliesinwhichthe
integer and fractional quantum Hall effect was observed. A No. DMR-0084173, the State of Florida, and the DOE.
briefdescriptionofthematerialsystem,indexednumerically,
is given in the upper right. The size of the labeled regions is
representative of the parameter range from observed mobili-
tiesanddensities. ThedashedlineshowstheIoffe-Regellimit
where kFλ=1. The Wigner crystal (WC), thought to occur [1] K.S. Novoselov et al., Science 306, 696 (2004).
at large rs and low disorder, is highlighted in green. [2] K.S. Novoselov et al., Nature 438, 197 (2005).
[3] Y. Zhang, Y.-W. Tan, H.L. Sto¨rmer, and P. Kim, Nature
438, 201 (2005).
[4] Y. Zhang et al., Phys. Rev. Lett. 96, 136806 (2005).
of how much disorder can be introduced into a 2D elec- [5] A.F. Young et al., Nat. Physics 8, 550 (2012).
tron gas (2DEG) and still form a quantum Hall state. A [6] X.Du,I.Skachko,F.Duerr,A.Luican,andE.Y.Andrei,
diagramsummarizing2DEGsystemswherethequantum Nature 462, 192 (2009).
Hall effect (QHE) has been observed is plotted in Fig.4 [7] C.R. Dean et al., Nat. Physics 7, 693 (2011).
[8] D.C. Elias et al., Science 323, 610 (2009).
versus the dimensionless Ioffe-Regel disorder parameter
[9] D. Haberer et al., Nano Lett. 10, 3360 (2010).
(k λ)−1, with λ the transport mean free path, and the
F [10] D. Haberer et al., ACS Nano, in print (2012).
dimensionless Wigner-Seitz radius r quantifying inter-
S [11] X.S. Li et al. Science 324, 1312 (2009).
actionstrength. Thepreponderanceof2DEGswherethe [12] A. Guermoune et al., Carbon 49, 4204 (2011).
QHE has been observed are low-disorder samples sat- [13] U. Bischler and E. Bertel, J. Vac. Sci. Tech. A 11, 458
isfying the Ioffe-Regel criterion (in zero magnetic field) (1993).
(k λ)−1 (cid:28) 1. However, a previous study in disordered [14] A.Jorioet al.,Raman Spectroscopy in Graphene Related
F
Systems, Wiley-VCH, Singapore, 2011.
GaAs/AlGaAs heterostructures biased to subthreshold
[15] M.M. Lucchese et al., Carbon 48, 1592 (2010).
conditions exhibited strong disorder (k λ)−1 >1 at low
F [16] K. von Klitzing, G. Dorda, and M. Pepper, Phys. Rev.
carrierdensityandatransitionfromlocalizationtoaQH
Lett. 45, 494 (1980).
state[20]. Our observations with hydrogenated graphene [17] D.C. Tsui, H.L. Sto¨rmer, and A.C. Gossard, Phys. Rev.
pushes the limit of disorder to (kFλ)−1 (cid:38)500 where the Lett. 48, 1559 (1982).
QHEcanstillbeattainedinastrongmagneticfield,sug- [18] X. Hong et al., Phys. Rev. B 83, 085410 (2011).
gestingthattheQHEmightberobusttoarbitrarilylarge [19] S. Das Sarma and A. Pinczuk, Eds, Perspectives in
Quantum Hall Effects: Novel Quantum Liquids in Low-
disorder.
Dimensional Semiconductor Structures, p. 23, Wiley-
Theinsulator-QHtransitioninhydrogenatedgraphene VCH, Singapore, 1996.
opens a new regime in energy scales previously unavail- [20] H.-W. Jiang et al., Phys. Rev. Lett. 71, 1439 (1993).
abletoexperiments. TheLandaulevelenergygapabout
√
the N = 0 Landau level is ∆E = 2(cid:126)v /(cid:96) [3],
0→±1 F B
giving ∆E = 240 meV at 45 T. Previous reports
0→±1
of ARPES of graphene hydrogenated to a 0.1−0.3 %
5
SUPPLEMENTARY
INFORMATION
6
G
/ID 5
DEVICE FABRICATION o, I 4
ati
Graphene monolayers were grown by chemical vapour R 3
k ID
deposition (CVD) on electropolished 25 µm thick cop- ea 2 IG
per foils with a methane precursor. Complete CVD P
1
growth procedures are described in detail in previous
0
work[1],includingthedependenceofmaterialqualityon
0 200 400 600 800
growth parameters. Graphene monolayers were trans-
ferred to oxidized silicon substrates using a thin poly- Hydrogenation Time [ s ]
methylmethacrylate handle layer, sacrificial etching of
the Cu foil in a 0.1M (NH ) S O room temperature
4 2 2 8 FIG. 5. Raman analysis of hydrogenated graphene. The
solution. The substrates were 500 µm thick As doped ratio of Raman D peak intensity I to G peak intensity I
D G
Si with 1−5 mΩ−cm room temperature resistivity. A versushydrogenationtime,obtainedwithapumpwavelength
chlorinated, dry thermal oxide of 300 nm thickness was λ=633 nm. Saturation in ID/IG corresponds to an approx-
usedastheback-gatedielectric. Aftergraphenetransfer, imate defect spacing λD ∼ 4 nm. Electron transport mea-
surements were performed on the sample hydrogenated for
ashadowmaskwasusedtodepositTi/Au(2nm/50nm)
360 s.
Ohmiccontacts3mmapart. Ascribewasusedtomanu-
ally isolate individual graphene squares on the substrate
surface.
new Raman peaks associated with increased disorder [3]
HydrogenationwasperformedinaUHVchamberwith
a base pressure in the low 10−9 Torr range. A custom- emerged: G+D (2942 cm−1) and D’ (1628 cm−1). The
evolution of the ratio of D peak intensity (I ) to G
built capillary atomic hydrogen source [2] was used. A D
peakintensity(I )versushydrogenationtimeofsixCVD
tungsten capillary was heated to > 1800 K by electron G
graphene samples is plotted in Fig.5. A non-monotonic
bombardment, with ∼12 W power at ∼600 V accelera-
behaviour in I /I versus hydrogen dose is found, simi-
tionvoltage. Abeamofthermallycrackedhydrogenwas D G
lar to the evolution of I /I versus Ar+-ion bombarded
produceduponintroductionofmolecularhydrogentothe D G
graphene [4]. Under the assumption of similar point-
capillary through a leak valve at an approximate cham-
ber pressure of 10−5−10−6 Torr. The graphene samples defect induced Raman spectra by hydrogenation as by
Ar+-ionbombardment,theobservedsaturationinI /I
were mounted ∼ 10 cm away from the hydrogen source, D G
corresponds to a mean defect spacing λ ∼4 nm.
with a shutter used to precisely control device exposure. D
All devices were subjected to a 1 hour thermal anneal The hydrogenated sample (#HG30) used for high-
at400Kpriortohydrogenexposuretodegasthesample field (45 T) electron transport measurements reported
surface. Betweenhydrogendoses,insitumeasurementof here was analyzed with greater precision by Raman
electricalresistancewasperformed, allowingtheeffectof spectroscopy within 4 days of electron transport mea-
hydrogenation on electrical resistance to be monitored. surement, to minimize the potential effect of envi-
ronmental exposure between electron transport and
Raman spectroscopy. To estimate the hydrogen in-
duced defect density, we apply the same phenomeno-
RAMAN SPECTROSCOPY
logical model for point-defect induced disorder as has
been successfully applied to understanding the correla-
RamanspectroscopywasperformedonCVDgraphene
tion between defect spacing λ and Raman peak ra-
and hydrogenated CVD graphene with a λ = 532 nm D
tio I /I in Ar+-ion bombarded graphene [4]. The
pump laser of 20 mW incident power and a λ=633 nm D G
phenomenological mapping between I /I and λ is,
p4gg0riuoa×mnpsphoebpnlarjeeoscebotrenivdoe.of.x2iWdA0ilzilmtehdmWetshailiseinuccorλiendm,e=nwetni5ttp3sho2ww4needmrre,istbppioneutrcmhftoprst,mpharteohtdiueagwlRhirtaeha-- CIIDGS(cid:104)1=−CexAprr(cid:16)A2A2−−−λ2πr2rrS2S2S2(cid:17)(cid:20)e(cid:105)xpw(cid:16)he−rλπe2DrS2C(cid:17)A−e=xp4(cid:18)D.2−,πG(CrλA2S2D−r=S2)(cid:19)D0(cid:21).8+7,
D
man peaks observed in CVD graphene (Fig. 1A) were r = 3.0 nm and r = 1.0 nm for the pump wave-
A S
D (1347 cm−1), G (1588 cm−1), G* (2462 cm−1), G’ length λ = 514 nm. Our Raman spectra give the ratio
(2685 cm−1) and G” (3255 cm−1). The D peak was I /I =2.88±0.06atλ=532nm,withtheuncertainty
D G
more intense relative to G in the hydrogenated graphene determined by the standard deviation between different
sample used for high-field transport measurements, and measurement points on the sample. In the absence of a
6
ELECTRON TRANSPORT MEASUREMENT
The hydrogenated graphene devices were mounted on
ceramic carriers, in turn mounted on G10 sample hold-
ers, that were then mounted to either a pumped 4He or
3He refrigerator. The samples were cooled in either the
gas or liquid phase of helium. Preliminary characteriza-
tion was performed with a 9 T solenoid (McGill). All
electrical measurements on hydrogenated graphene were
performedusingstandardlock-intechniquesat13Hzand
at a low current excitation I (cid:46) 10 nA. Gate voltage
exc
was applied with a DC voltage source, and gate leak-
age current was monitored for all measurements with a
DC ammeter. The gate leakage resistance was at mini-
mum2GΩ. High-fieldmeasurementswereperformedon
samples using the both a 33 T resistive magnet and the
hybrid superconducting-resistive magnet of the NHMFL
FIG.6. Colossalnegativemagnetoresistanceofhydrogenated facility in Tallahassee. With the hybrid system, a DC
graphene. Resistance versus magnetic field (up to 33 T) and magneticfieldof45Twasappliednormaltothegraphene
gate voltage of sample #HG18 is among five that exhibited surfacewithasuperconductingoutsertcoilgeneratingup
colossal negative magnetoresistance. These measurements, to 11.5 T, and an additional 34 T field generated by a
performedonadifferentsampleandduringadifferentmagnet
resistive insert magnet. Colossal negative magnetoresis-
run,arefullyconsistentwiththemeasurementspresentedfor
tance was observed in five distinct samples, with the re-
sample#HG30describedinthemainbodyofthemanuscript.
sistanceversusmagneticfieldandgatevoltageofsample
#HG18 given in Fig.6.
pumpwavelengthdependenceofI /I forpointdefects,
D G
we conservatively estimate the error in λ as follows.
D
The known I /I scaling of λ4 for edge defects [3] gives
D G
an estimated correction I /I (514 nm)=(514/532)4 =
D G [1] A. Guermoune et al., Carbon 49, 4204 (2011).
I /I (532nm)=2.51±0.05. Weconservativelytakethe
D G [2] U. Bischler and E. Bertel, J. Vac. Sci. Tech. A 11, 458
widest error admitted by the pump wavelength scaled
(1993).
and unscaled Raman ratios ID/IG = 2.70 ± 0.24, giv- [3] A. Jorio et al., Raman Spectroscopy in Graphene Related
ing an estimated point-defect spacing from Eq. 1 of Systems, Wiley-VCH, Singapore, 2011.
λ =4.6±0.5 nm. [4] M.M. Lucchese et al., Carbon 48, 1592 (2010).
D