Table Of ContentExtreme ultraviolet radiation induced defects in single-layer graphene
A. Gao,1,2,a) E. Zoethout,1 J.M. Sturm,1,2,3 C.J. Lee,1,2 and F. Bijkerk1,2
1)FOM-Dutch Institute for Fundamental Energy Research, Edisonbaan 14,3439 MN Nieuwegein,
the Netherlands.
2)XUV group, MESA+ Institute for Nanotechnology, PO Box 217, University of Twente, 7500 AE, Enschede,
the Netherlands.
3)Materials innovation institute M2i, Mekelweg 2, 2628 CD Delft, the Netherlands.
(Dated: 13 January 2014)
We study extreme ultraviolet (EUV) radiation induced defects in single-layer graphene. Two mechanisms
4 for inducing defects in graphene were separately investigated: photon induced chemical reactions between
1 graphene and background residual gases, and breaking sp2 bonds, due to photon and/or photoelectrons
0
induced bond cleaving. Raman spectroscopy shows that D peak intensities grow after EUV irradiation with
2
increasing water partial pressure in the exposure chamber. Temperature-programmed desorption (TPD)
n experiments prove that EUV radiation results in water dissociation on the graphene surface. The oxidation
a of graphene, caused by water dissociation, is triggered by photon and/or photoelectron induced dissociation
J
ofwater. Ourstudiesshowthatthe EUVphotonscleavethesp2 bonds,formingsp3 bonds,leadingtodefects
3
in graphene.
]
i PACS numbers: 61.48.De
c
s
-
l I. INTRODUCTION duced bond cleaving.
r
t In this work, we investigate both of these possible
m
Graphene, a two-dimensional hexagonal packed sheet EUV-induceddamagemechanismsonCVDgrownsingle
t. ofcarbonatoms,hasattractedalotofattentionfromdif- layer graphene. In previous work12, the oxidative dam-
a
ferentresearchfieldsduetoitsuniquephysicalandchem- age to graphene was tentatively attributed to the pres-
m
ical properties1–8. Graphene can act as a diffusion bar- ence of a reactive low density water plasma. But in the
- rier by providing physical separation between an under- ref12,theamountofwater(whichwasthemainsourceof
d
n lying substrate and reactant gases. Indeed, studies show oxidativedamagetographene)adsorbedtothegraphene
o that graphene is highly impermeable to gases9. Further- surfacewasnotcontrolled,norwasitpossibletoperform
c more, single-layer graphene is also highly transparent10, experiments without a substantial plasma-surface inter-
[ which makes it a promising protection layer for optical action. In this work, the influence of the EUV-induced
1 devices, such as mirrors, lenses and screens. Addition- water plasma is controlled in two different ways. In the
v ally, graphene can be grown on large scales by chemical first set of experiments, the partial pressure of water in
2 vapordeposition(CVD)andtransferredtoarbitrarysub- the vacuum chamber was set by introducing water va-
5 strate11, making the rangeof potential applications very por into the interaction chamber. In the second set of
3 wide. experiments, waterlayerswere deliberately depositedon
2
Graphene devices may be used in harsh environments a cold graphene surface (83 K). In the first set of ex-
.
1 e.g., space applications, soft x-ray systems and extreme periments, the graphene is exposed to an EUV-induced
0 ultraviolet (EUV) lithography systems. Since the light plasma, which contains different concentrations of water
4
source of current EUV lithography systems operates at plasma. In the second set of experiments, the solid wa-
1
a wavelength of 13.5 nm, the optical system is in vac- ter layer can be ionized into much denser plasma under
:
v uum, but with a significant partial pressure of water. EUVirradiation. Inaddition,sincethiswaterlayerisph-
Xi The physical and chemical stability of graphene in such ysisorbedonto the graphene surface, the reaction proba-
anenvironmentisofcriticalimportanceifitistobeused bility is much higher compared with that in the first set
r
a as a part of an optical component. Early experimental of experiments. By comparing the nature and density of
work showed that defects were generated in multilayer defectsinducedinthesetwoexperiments,itispossibleto
graphene after EUV exposure. And, even in a reduc- determine the relative contribution of the EUV-induced
ing environment, a small partial pressure of oxidizing plasma to the damage observed in graphene after expo-
agents (water) may cause oxidation12. The damage to sure.
graphene observed in our previous study was attributed Finally, in a third set of experiments, naturally ac-
totwopossibleeffects: photo-inducedchemistrybetween cumulated hydrocarbon contamination (0.7 nm) on a
graphene and background residual gases, and breaking graphenesurfacewasusedasa barrierlayerbetweenthe
sp2 bonds, due to EUV photon and/orphotoelectronin- residual water and graphene surface. In this way, it was
possible to study the damage to graphene, while min-
imizing the reaction rate between graphene and water
plasma.
a)Electronicmail: a.gao@differ.nl. In our experiments, the graphene was character-
1
ized using Raman spectroscopy, X-Ray photoelec- variouswaterpartialpressures. Oneofthegraphenesam-
tron spectroscopy (XPS), and scanning electron mi- pleswasexposedwithoutaddingwaterintothe chamber
croscopy (SEM), before and after exposure. The ra- (water partial pressure less than 10−9 mbar), while an-
tio of the D and G Raman spectral features is used as otherthreesampleswereexposedwithwaterpartialpres-
a measure of how well-ordered the graphene crystalline sures of 1x10−7 mbar, 1x10−6 mbar, and 1x10−5 mbar,
structure is, while XPS is used to determine the amount respectively. These four samples were kept at 289 K by
of oxidation. SEM was used to qualitatively compare backsidecoolingduringtheexposure. Afterexposure,all
the graphene layer completeness. The adsorbed water the samples were examined by Raman spectroscopy and
was examined using temperature-programmed desorp- XPS. A typical Raman spectrum of graphene has three
tion (TPD) spectroscopy to understand the water layer prominent features i.e., D, G and 2D peaks, located at
morphology, water-water, and water-graphene interac- approximately 1350 cm−1, 1580 cm−1, and 2680 cm−1.
tions The G peak is a first order Raman scattering process,
corresponding to an in plane stretching of sp2 bonds.
The D band is due to the breathing modes of six-atom
II. EXPERIMENTS rings, and requires a defect for activation. The ratio of
I(D)/I(G) is commonly used to quantify the defect den-
For the first and second sets of experiments, single sity13. Fig. 1a shows the Raman spectra of the reference
layer graphene samples, obtained from Graphene Super- graphene sample (unexposed) and the exposedgraphene
market Inc., were grown on copper substrates by CVD samples. ItclearlyshowsthattheDpeakheightgrowsaf-
and transferred to SiO /Si substrates. The substrate terexposureforallsamples,indicatingdefectgeneration.
2
size was 10 mm x 10 mm with a 285 nm thick layer Furthermore, the dependence of the I(D)/I(G) ratio on
of SiO . For the third set of experiments, single layer water partial pressure is clearly indicated in Fig. 1b.
2
graphenesamples onCusubstrateswere purchasedfrom The Raman spectra, however,do not clearly show the
Graphenea, which were stored in ambient condition for source of the increased defect density. Fig. 2a shows
severalmonths,leadingtoalayerofhydrocarbonsabout the curve fit for the XPS spectrum of the C1s peak of
0.7 nm thick on top of the graphene layer. For the the pristine sample. The four components of the C1s
first and third sets of experiments, the graphene sam- spectrum, corresponding to sp2 bonds in graphitic like
ples were exposed to EUV from a Xe plasma discharge carbon (284.3 eV), sp3 hybridization (285.2 eV), hy-
source (Philips EUV Alpha Source 2) with a repetition droxyl (C-O) groups (286.1 eV), and carboxyl (C=O)
rate of 500 Hz. The EUV beam profile has a Gaussian groups (288 eV), are plotted separately for the exposed
distributionwithFWHM=3mm. TheEUVintensityat sample. The appearance of sp3, C-O, and C=O bonds
the sample surface was estimated to be 5 W/cm2 with a can be attributed to photo-induced bond cleaving and
dose of 10 mJ/cm2 per pulse. The base pressure of the photo-induced oxidation of graphene, due to presence of
exposure chamber was 2x10−9 mbar. For the second set water. However,Fig.2balsoshowsthatthetotalcarbon
ofexperiments, the graphenesamples wereexposedwith thickness of the exposed samples increases, due to the
EUV intensity of 0.05 W/cm2, and the base pressure of well-knowneffectofhydrocarbondepositionduringEUV
the exposure chamber was 1x10−9 mbar. exposure14. The contributionof the deposited hydrocar-
All exposed samples were characterized using Raman bonsto the XPSspectramakesasingleinterpretationof
spectroscopy, XPS, and SEM after exposure. Raman the data impossible, since the sp3, C-O, and C=O con-
spectra were collected with a home-built system, based tributions can also come from the hydrocarbon contam-
on a 532 nm solid state laser system and a Solar M266 ination. Note that the EUV transmission will decrease
spectrometer with a resolution of 1 cm−1. The illumi- by less than 1 % due to 0.3 nm carbon contamination15,
nation intensity was set at 200 W/cm2. The collection thus, the difference of EUV dose on graphene samples
optics and pixel size of the detector result in a spatial is negligible, and cannot explain the differences between
resolution of 100× 100 um2. The collection efficiency Raman spectra for the five samples.
of the detector system was calibrated using the HG-1
Mercury Argon Calibration Light Source and AvaLight-
D(H)-SDeuterium-HalogenLightSource. XPSwasmea- B. Photo-induced plasma of the adsorbed water
sured by using a monochromatic Al-K, Thermo Fisher
Theta probe with a beam footprint of 1 mm diameter.
Inordertopreventhydrocarboncontamination,alayer
ofwater wasdeliberately deposited onthe graphenesur-
facebydosingthesurfacewithwaterwhileitwasheldat
III. RESULTS AND DISCUSSION
a temperature of 83 K.Directly before waterdosing, the
graphene sample was heated to 600 K to remove surface
A. Photo-induced plasma of the residual water contamination, after which a calibrated surface coverage
of water was adsorbed onto the sample. The water dose
Four graphene samples (single layer graphene on is expressed in Langmuir, with 1 L = 1.33x10−6 mbar.
SiO /Sisubstrate)wereexposedtoEUVfor30minwith Temperature-programmed desorption (TPD) spec-
2
2
x 107 2
16
14
12 1.5
1e−5 mbar
10
o
nts/s 8 1e−6 mbar G rati 1
ou 6 1e−7 mbar D/
c
4
without adding water 0.5
2
unexposed
0
0
−2 10−10 10−8 10−6 10−4
1500 2000 2500 3000
Wave number cm−1 water partial pressure [mbar]
(a) (b)
FIG.1: (coloronline)(a)TheRamanspectraofthegraphenesamplesonSiO2/SisubstrateexposedtoEUVunderdifferentwaterpartial
pressure. (b)TheI(D)/I(G) ratioasfunctionofthewaterpartialpressure. Thexaxisisinlogscale.
400 0.8
C1s
350
0.7
m]
300 n
sp2 ss [0.6
u.250 ne
Counts a.125000 sp3 C−O arbon thick00..45
100 C=O al c
ot
t0.3 unexposed sample 0.34 nm
50
2083 284 285 286 287 288 289 290 01.20 −10 10−8 10−6 10−4
Binding Energy [eV] water partial pressure [mbar]
(a) (b)
FIG.2: (coloronline)(a)ThecurvefittingresultsoftheXPSspectrumforthegraphenesamplesonSiO2/SisubstrateexposedtoEUVwith
waterpartialpressureof1x10−5 mbar. ThesolidcurveindicatestheC1speakwhilethefourdot-dashcurvesarefittedcurves. (b)Thetotal
carbonthicknessoftheexposedsampleswithvariouswaterpartialpressures. Notethattheunexposedsamplehasslightlyhydrocarbon
contaminationonthesurfaceduetotransportandstorageinatmosphericconditions. Thethicknessiscalculatedbasedontheangleresolved
XPSmeasurementsassumingthecarbondensityof2.1g/cm3. Thexaxisisinlogscale.
troscopywasperformedtoinvestigatehowwateradsorbs dosed with water and exposed to EUV. A second TPD
on to the graphene surface. The graphene sample was spectrumwasobtainedfromtheexposedwater-graphene
heated at a rate of 0.5 K/s and water desorption was sample. Finally,thesamplewasdosedwithwaterathird
detected by a quadruple mass spectrometer. To ensure time and a third TPD was performed on the exposed
that the signal is dominated by sample surface desorp- graphene. This procedure was repeated three times on
tion, the entrance to the mass spectrometer consists of the same sample after EUV exposure of 10 min, 30 min,
a cone that faces the sample surface. The cone has an and 180 min respectively. The EUV intensity in this ex-
entranceapertureof4 mm,locatedapproximately2mm posurewasestimatedtobe100µJ/cm2,about100times
from the graphene surface. The temperature was mea- lower than used in the first set of experiments.
suredwith a thermocouple pressedonto the sample with The TPD results are shownin Fig. 3a. The spectra at
a molybdenum clamp. Since this clamp has a relatively low water dosages and high water dosages are markedly
good thermal contact with the cold finger of the manip- different, indicating that the structure of the adsorbed
ulator,themeasuredtemperaturewillunderestimatethe water is different, or evolves differently as function of
actual sample temperature. both temperature and initial dose. As graphene (with
In our experimental procedure, the graphene sample certain hydrocarbon contamination) has a hydrophobic
was dosed with water, and TPD spectroscopy was per- surface, water films tend to dewet upon heating. At low
formedpriorto EUVexposure. The samplewasthenre- coverage,thewatermoleculesformtwo-dimensional(2D)
3
x 105 x 104
3.5 5
10min exposure
3 5L 4.5 30min exposure
4 3 hr exposure
2.5 before exposure
u u3.5 after exposure
s a.m. 2 3L s a.m. 3
unt1.5 unt2.5
Co 2L Co 2
1
1L 1.5
0.5L
0.5
1
0 0.5
120 130 140 150 160 110 120 130 140 150
Temperature [K] Temperature [K]
(a) (b)
FIG.3: (coloronline)Temperature-programmeddesorptionspectraforH2Odesorptiononthegraphenesurface(a)TPDspectraforvarious
H2Odoses;(b)TPDspectraofthegraphenesampleunderdifferentEUVexposuretimewithwaterdosingtimeof1L.Theheatingrateis
0.5K/s. Thecurvesaremanuallyoffsettothehavethesamebackground.
14x 106 2x 108
with H2O
2681 without H2O
12
1.5
10
1585
s/s 8 s/s
nt nt 1
u u
o 6 o
c c
pristine
4 1350
1595 2691 0.5
2
exposed
0 0
1000 1500 2000 2500 3000 3500 1000 1500 2000 2500 3000 3500
Wave number cm−1 Wave number cm−1
(a) (b)
FIG.4: (coloronline)(a)Ramanspectraofthegraphenesamplebefore(pristine)andafterEUVradiationwithwateradsorbedontothe
sample(exposed).(b)Ramanspectraofthesamplesexposedwithorwithoutwateradsorbedontographenesurface.
clusters. Water molecules at edges of these clusters have following.
low coordination to other water molecules, such that In Fig. 3b, the TPD spectra of water on graphene be-
their desorption energy is low, resulting in the desorp- foreandafterexposureareshownforawaterdoseof1L.
tion peak around 130 K16–18. It is shown in Fig. 3b that the spectra of the graphene
However, as the coverage increases, water molecules sampleafterexposureexhibitslowerpeakintensitiesthan
can form 3D clusters of crystalline-ice like structure, re- that of before exposure. This indicates that the amount
sulting in a second peak at 134 K. The position (tem- of water on the surface decreases due to EUV-induced
perature) of this peak increases with coverage,while the desorption of water. Note that, aside from the decrease
leading edge remains similar. This is characteristics for in the total amount of water on the graphene surface, a
zero-order desorption of water molecules from ice17,18. second TPD peak appears at higher temperature after
The rearrangement of the water suppresses the peak at 30minofexposure. Thissecondpeakislikelyduetothe
130 K for water doses above 3 L. At a water dose of 5 L water layer rearranging itself during EUV exposure.
the130Kpeakdisappears,whichindicatesthatallwater EUV photons with an energy of 92 eV can also cause
molecules form multilayers or 3D ice clusters. The TPD water dissociation by direct photon excitation or by an
data from Fig. 3a also indicates that water does not dis- indirect process involving secondary electrons emitted
sociate on adsorption to the graphene surface, thus, any from the substrate. Many studies have shown that the
waterdissociationandresultingchangestothe graphene indirect process, induced by secondary electrons, domi-
aredrivenby EUV-induced processesasdiscussedin the natesoverthedirectphotonexcitationinthesurfacepho-
4
tochemistry14,19,20. Therefore, direct EUV-dissociation full width half maximum of the spectral components for
canbe neglectedandthe watermolecules canbe dissoci- the pristine sample, and samples exposed to EUV with
ated according to the following reactions14: and without adsorbed water are summarized in Tab. I.
The broadening of the sp2 peak from 0.88 eV to 0.98 eV
e−+H2O →H++OH− (1) is usually evidence of a transition from a highly ordered
graphite-like carbon to a less ordered carbon state, sup-
e−+OH− →H++O− (2) porting the conclusion from the Raman results that the
exposedgraphenehasmore defects. The changesin C-O
and C=O concentrations are not significant, consider-
Upon dissociation,the reactive species can either desorb
ing that the uncertainty in the atomic concentration is
from the surface or react (mainly atomic oxygen) with
±2.5 at.%. The C-O and C=O bonds are most likely
the graphene.
to be due to residual poly (methyl meth acrylate), used
Fig.4ashowstheRamanspectraofthegraphenesam-
during the transfer process, and clearly observed on all
ple before and after EUV exposure with adsorbed water
samples using SEM (see Fig. 6).
layer. Comparing these two spectra in Fig. 4a, there is
a clear D peak intensity increase at around 1350 cm−1,
whichisattributedtothepresenceofdefectsingraphene. From the SEM images in Fig. 6, it is apparent that
Previousworkhasshownthatdefectsingraphenecanbe there is less PMMA on the graphene after exposure.
induced by EUV radiation12, however,the shift of the G CracksandholesarealsoobservedintheSEMimagesfor
peak from 1585 cm−1 to 1595 cm−1 and of the 2D peak the exposed samples, indicating an oxidative etching ef-
2681 cm−1 to 2691 cm−1, are usually interpreted as evi- fect. Wecanseethattheexposedgraphenesamplesshow
dence of oxide doping21. more cracks and holes than the pristine sample. And
Fig. 4b shows the Raman spectra of samples exposed theseholesandcracksarepredominantlyalonggraphene
withandwithoutwateradsorbedontothegraphenesur- grain boundaries, a phenomenon also observed for ther-
face. The two spectra differ in both the I(D)/I(G) ra- mal oxidation of graphene22,23. The formation of C-O
tio and the spectrum fluorescence background. Clearly, or C=O groups as a result of oxidation is due to fully
the I(D)/I(G) ratio of the sample exposed with water dissociated water (e.g., atomic oxygen), or partially dis-
is much higher than that of the sample exposed without sociated water (e.g., OH groups). The small holes in the
water,indicating more defects weregeneratedinthe for- pristine sample may come from the transfer process.
mer sample. The water layer adsorbed to the graphene
surfacewillresultinaverydensewaterplasma,andsuch It is also noted that the total thickness of carbon in
a plasma should have a faster reaction rate than that of the exposedsampledecreases,dueto boththe etchingof
water plasma generated in the background residual gas. graphene and the removal of PMMA. This can be seen
Thelargefluorescencesignalinthesampleexposedwith- in the XPS data in Tab. I, where the concentration of
out water is attributed to hydrocarbon contamination. the sp2 and sp3 components decrease, while both the
C-O and C=O contributions remain almost unchanged.
Cleavedsp2 bondscaneitherformsp3 bonds,orthrough
350 oxidation form both sp3 bonds and C-O or C=O bonds.
C1s The combination of decreasing carbon, but increasing
300
oxidation is indicative that the PMMA is preferentially
250 sp2 removed. However, the unchanged sp3 and oxygen con-
u. tent(despitethedisruptiontothegrapheneandtheover-
a.200 allremovalofcarbon)indicates thattwo competing pro-
s
nt cesses are present. The removal of PMMA is compen-
ou150
C sated for by the oxidation of the graphene.
100 sp3 C−O C=O
Until now, we have shown that from Raman results,
50
I(D)/I(G) ratio grows as the water partial pressure in-
0 creases,indicatingmoredefects areformingingraphene.
283 284 285 286 287 288 289 290
Together with the XPS data, it suggests that oxidation,
Binding Energy [eV]
triggeredby the EUVradiationinthe presenceofwater,
FIG.5: (coloronline)TheXPSspectraofthegraphenesample results in the graphenebeing etched, forming cracksand
exposedtoEUVwithadsorbedwaterontographene. Thesolidcurve
indicatestheC1speakwhilethefourdot-dashcurvesarefitted holes,whichissimilarto the effectobservedduringther-
curves. maloxidationofgraphene. InthecaseofEUVexposure,
the oxidation process originates from the dissociation of
Fig.5showsthecurvefitfortheC1speakofthesample water by EUV. However, to determine the relative con-
exposed to EUV with adsorbed water. The components tributions of EUV-induced oxidation and EUV-induced
of the C1s spectrum: sp2, sp3, C-O, and C=O, are plot- bond cleaving, the EUV-induced plasma must be sepa-
ted. The atomic concentration of each component and rated from the graphene surface.
5
TABLEI:AtomicconcentrationofC1scomponent,thetotalcarbonthickness,andFWHMofthepristinesample,andsamplesexposedtoEUV
withandwithoutadsorbedwater. ThetotalthicknessiscalculatedbasedontheangleresolvedXPSmeasurementsassumingthecarbondensity
of2.1g/cm3.
Sample C sp2 (at.%) C sp3 (at.%) C-O (at.%) C=O (at.%) sp2 FWHM (eV) Total thickness(nm)
Pristine (unexposed) 20.4 4.4 2.4 1.0 0.88 0.34
Exposed with adsorbed H O 19.3 2.7 2.4 1.1 0.98 0.30
2
Exposed without adsorbed H O 31.5 6.4 6.4 3.7 1.03 0.74
2
(a) unexposed sample (b) exposed sample
FIG.6: SEMimagesofthepristinesample,andthesampleexposedtoEUVwithadsorbedwatershowingPMMAresidue,cracksandholes. The
PMMAappearsasthedarkerpatchesinbothimages.
C. Exposing graphene with hydrocarbon contamination layerwasfoundtobeabout0.7nmthickfromXPSmea-
surement (see Tab. II). This hydrocarbonlayer acts as a
barrier layer between the graphene and the background
Samples of singlelayergrapheneon Cusubstrate with
residualgasesintheexposurechamber. Inthisway,reac-
a hydrocarbon contamination layer were prepared. The
tionsbetweentheEUV-inducedplasmaandthegraphene
hydrocarbon layer is used as barrier layer between the
can be excluded.
residual water and graphene surface. In this way, it was
possible to study the damage to graphene, while min- Fig.8a shows the Ramanspectra of the reference (un-
imizing the reaction rate between graphene and water exposed) and exposed graphene samples with naturally
plasma. Four graphene samples were exposed to EUV accumulated hydrocarbon contamination. It is noted
for 30 min with background gas conditions that should that all the exposed samples show a clear D peak with
beeitherreducingoroxidizing. Oneofthegraphenesam- approximatelythe sameintensity. The Gpeak,however,
ples was exposed without modifying the background gas is broadened due to a more disordered carbon network
(10−9 mbar,mostlywater),asecondsamplewasexposed after exposure. Analysis is complicated by the fact that
in a water partial pressure of 1x10−5 mbar, a third was all samples, except the one exposed to oxygen, show an
exposed in a hydrogen partial pressure of 1x10−5 mbar, increasein hydrocarboncarbon(see Tab.II),whichcon-
andthelastwasexposedinanoxygenpartialpressureof tributes to, not only the broadening, but also the in-
1x10−5mbar. Allsampleswerekeptat289Kbybackside creased intensity of the G peak. In Tab. II, it is also
cooling during the exposure. apparentthatthe sp2 atomicconcentrationis greatlyre-
duced after exposure, from 53.3 at.% to 34.0 at.%, indi-
Fig.7showsthe RamanspectrumandXPSresultsfor
catingthatsp2 bondsarebeingbrokenandsp3bondsare
an unexposed sample. From the Raman spectra, we can
being formed. The C-O and C=O peaks are obviously
see that the single layergrapheneis still visible after flu-
more pronounced in the exposed samples shown in the
orescence background subtraction, despite the presence
XPS results in Fig. 8b due to EUV induced oxidation of
of hydrocarbon contamination. The XPS results clearly
the covering hydrocarbon carbon layer.
show that, in addition to the sp2 contribution, there is
a significant amount of sp3, C-O, and C=O, which is These results show that direct contact of the EUV-
attributed to the presence of hydrocarbon. Assuming induced plasma and the graphene is not a necessary
a density of 2.1 g/cm3, the hydrocarbon contamination requirement for damage to graphene. The I(D)/I(G)
6
x 106
12 1800
2D C1s
1600
10
1400
8 sp2
G
u.1200
unts/s 6 nts a.1000
o 4 u
c o
C 800
2
600
sp3
0 400 C−O C=O
−2 200
1500 2000 2500 3000 283 284 285 286 287 288 289 290
wavenumber cm−1 Binding Energy [eV]
(a) (b)
FIG.7: (coloronline)Ramanspectrumafterfluorescencebackgroundsubtraction(a)andXPSresults(b)forthereferencesampleofmonolayer
grapheneonCusubstratewithnaturallyaccumulatedhydrocarboncontamination.
TABLEII:Atomicconcentrationandtotalcarbonthicknessofthereferenceandexposedgraphenesampleswithnaturallyaccumulated
hydrocarboncontamination. Theerrormarginofthedatais±1at.%.
Sample Reference Exposed to background gas Exposed to H O Exposed to H Exposed to O
2 2 2
C sp2 (at.%) 53.3 34.0 36.3 36.2 35.7
C sp3 (at.%) 10.0 13.0 10.6 11.0 10.1
C-O (at.%) 1.9 5.0 5.4 7.0 5.1
C=O (at.%) 2.0 11.2 8.6 6.7 6.4
O 1s (at.%) 13.0 22.8 23.2 22.8 23.9
Cu2p3 (at.%) 19.8 14.0 15.9 16.3 18.9
Total thickness (nm) 0.98 1.24 1.10 1.06 0.92
x 107 reference
background 1600
4 O2
1400 C1s
H2
H2O
3 1200
unts/s2 nts a.u.1000 sp2
o u 800
c o
C
1 600
sp3
C−O
C=O
400
0
200
1000 1200 1400 1600 1800 2000 283 284 285 286 287 288 289 290
wavenumber cm−1 Binding Energy [eV]
(b)
(a)
FIG.8: (a)RamanspectraforthereferenceandexposedgraphenesamplesonCusubstratewithnaturallyaccumulatedhydrocarbon
contamination. (b)XPSresultsforthegraphenesampleexposedtooxygen
ratio (0.2) found in Fig. 8 is much lower compared samples, indicating that the process is largely indepen-
with the I(D)/I(G) ratio (1.9) found in Fig. 1 for the dent of the background gases. Considering these facts,
sample exposed to EUV with partial water pressure of the induced defects are predominantly due to photon
1x10−5 mbar, indicating that the hydrocarbon carbon and/orphotoelectroninducedbondcleaving,ratherthan
layer is an effective barrierlayer. In addition, in Fig. 8a, the EUV-induced plasma. The photoelectrons from the
the I(D) intensities are almost the same for all exposed Cusubstratemayplayanimportantroleindefectgener-
7
ationingraphene,sincetheCusubstratehasarelatively gramme is co-financed by Carl Zeiss SMT and ASML,
high photoelectron yield24. These photoelectrons may andtheAgentschapNLthroughtheEXEPTprogramme.
directly attribute in breaking the carbon sp2 bonds. This research is also carried out under project number
It is important to note that the damage to graphene M61.3.11430 in the framework of the Research Program
caused by plasma depends very strongly on the concen- of the Materials innovation institute M2i.
tration and composition of the background gases, while,
1A.K.GeimandK.S.Novoselov,“Theriseofgraphene,”Nature
on the other hand, the damage to graphene caused by
materials6,183–191(2007).
photon and photoelectronis only dependent on the pho- 2A. K. Geim, “Graphene: status and prospects,” science 324,
ton flux. It is, therefore, impossible to determine the 1530–1534 (2009).
relative dominance of these two mechanisms, without 3K. Novoselov, D. Jiang, F. Schedin, T. Booth, V. Khotkevich,
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