SUPPLEMENTARY INFORMATION
Electrical control of nanoscale chemical modification in graphene
Ik-Su Byun1†, Wondong Kim2†, Danil W. Boukhvalov3†, Inrok Hwang1,4, Jong Wan Son1,
Gwangtaek Oh1, Jin Sik Choi1,5, Duhee Yoon6,7, Hyeonsik Cheong7, Jaeyoon Baik8, HyunJoon Shin8, Hung Wei Shiu9, Chia-Hao Chen9, Young-Woo Son3* and Bae Ho Park1*
1
Division of Quantum Phases & Devices, Department of Physics, Konkuk University, Seoul, 143701 Korea
2
Division of Industrial Metrology, Korea Research Institute of Standards and Science, Daejeon 305340, Korea
3
School of Computational Sciences, Korea Institute for Advanced Study, Seoul 130-722, Korea
4
Electronic Materials Research Center, Korea Institute of Science and Technology, Seoul 136-791,
Korea
5
Creative Research Center for Graphene Electronics, Electronics and Telecommunications Research
Institute, Daejeon, 305-700, Korea
6
Department of Physics, Sogang University, Seoul 121-742, Korea
7
Electrical Engineering, University of Cambridge, Cambridge CB3 0FA, United Kingdom
8
Pohang Accelerator Laboratory, Pohang, Kyungbuk 790-784, Korea
9
National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu 30076, Taiwan
†
These authors contributed equally to this work.
*email: hand@kias.re.kr and baehpark@konkuk.ac.kr
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SPEM image of oxidized graphenes
Figure S1. The 16-channel SPEM images obtained near the boundary between the graphene and
SiO2 substrate. The red dotted line and white dotted areas denote the boundary and oxidized
areas on graphene, respectively.
Figure S1 displays a series of 16 SPEM images obtained near the boundary between graphene
and SiO2 with a BE bandwidth of 1 eV and center BE ranging from 278 eV to 293 eV. The
images were taken at a photon energy of 380 eV with an image size of 200 pixels × 200 pixels,
pixel width of 300 nm, dwelling time of 60 ms, pass energy of 23.5 eV, and total acquisition time
of approximately 30 minutes. These graphenes (light yellow area) and graphene oxides (white
dotted rectangular area) on SiO2 can be identified in 9 or 10 channels, clearly. Because of the
distinguished boundary (red dotted line) image, a center BE of 284 eV or 285 eV was used for
further characterization.
2
Effect of pre-annealing treatment
We can identify the three areas of 1LG oxidized with bias voltages of +6 V, +8 V, and +10 V
using a friction force microscope (FFM) image, as shown in Figure S2(a). Figures S2(b) and
S2(c) show the SPEM images near the boundary between graphene and SiO2, which are obtained
at room temperature before and after pre-annealing treatment at 500 K, respectively. Although
the areas of oxidized graphene are not clearly discriminated from that of SiO2 in Figure S2(b)
because of the residual carbon contamination, they become distinguishable after pre-annealing
treatment at 500 K and 10-9 Torr over 2 hours, as shown in Figure S2(c).
Figure S2. FFM (a) and SPEM (b and c) images of pristine and oxidized graphene on SiO2
substrate before (a and b) and after (c) pre-annealing treatment at 500 K.
By combining spatially resolved elemental and chemical mapping with XPS, we are able to
measure a piece of local information with high lateral and energy resolution instead of average
information, and distinguish bonds other than those from pristine graphene after pre-annealing
treatment at 500 K and 700 K, as shown in Figure S3. Before the quantitative survey using by
micro-XPS system, we should find the exact locations of graphene that those SPEM images
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obtained from oxidized graphene areas after pre-annealing treatment at 500 K, as shown in
Figure S3. After additional annealing at 700 K, graphene oxide appears to be partly reduced from
boundaries between graphene and oxidized graphene samples, compared to the sample preannealed at 500 K. Furthermore, we can observe that these oxidized areas of 3LG are reduced
less than those of 1LG and 2LG, implying that such reduction depends on the number of
graphene layers.
Figure S3. SPEM images of oxidized 1LG (a and b) or 2LG and 3LG (c and d) after preannealing treatment at 500 K (a and c) and 700 K (b and d). White dotted rectangular areas
denote the oxidized graphene, and the red lines delineate the boundaries between 2LG and 3LG.
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XPS data analysis for oxidized graphene
We introduced a fitting procedure in which an XPS peak was decomposed with a Doniac and
Sunjic35 line shape using a Lorentzian linewidth of 165 meV and an asymmetry parameter of
0.0636. The C 1s spectra of pristine 1LG consists of two peaks: the main peak at a binding energy
(BE) of 284.5 eV (FWHM = 1.05 eV) is assigned to sp2 C-C; the other peak at higher BE with
core-level shifts (CLS) of +0.7 eV is assigned to sp3 (C-C or C-H)33,34. The C 1s spectra of the
three oxidized 1LG can be fitted with four peaks: one at 284.5 eV is assigned to sp2 C-C, and the
others with CLS of +0.9, +2.1, and +3.1 eV are assigned typically for hydroxyl (C-OH)37, epoxy
(C-O-C)38, and carbonyl (C=O)39, respectively. For quantitative approaches, the area percentage,
normalized area ratio (area of each bond divided by the area of sp2 C-C)40, and oxygen coverage
(sum of peak areas for the oxygen-containing bonds divided by that for all the carbon bonds) are
also calculated from the fitting results for the corresponding XPS spectra, as shown in Figures S4
and S5.
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Figure S4. The area percentage of sp2 C-C, C-OH, C-O-C, and C=O XPS peaks of oxidized
areas on 1–3LG, which depend on number of graphene layers, applied bias, and annealing
temperature.
The pristine 2LG and 3LG also have two peaks assigned to sp2 C-C and sp3 (C-C or C-H)33,34.
The peaks assigned to sp2 C-C bonds are located at 284.4 eV (2LG) and 284.3 eV (3LG) and
have smaller FWHMs of 0.65 eV (2LG) and 0.60 eV (3LG) than that of pristine 1LG. The peak
shift and narrowing may be due to the reduction of the charging effect from the silicon oxide
substrate and the different electrical conductivity (1LG < 2LG < 3LG). The C 1s spectra of
oxidized 2LG and 3LG were also fitted with four peaks: sp2 C-C peak located at 284.4 eV (2LG)
and 284.3 eV (3LG); C-OH, C-O-C, and C=O bond peaks had CLSs of +0.9, +2.1 and +3.1 eV,
respectively37-39. Similar to the oxidized 1LG, the normalized ratio of each bond enables the
quantitative analysis for the oxidation of 2LG and 3LG, as shown in Figure S4.
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Figure S5. The normalized area ratios of C-OH, C-O, and C=O XPS peaks with respect to the
area of sp2 C-C XPS peak (a and b) and oxygen coverage of oxidized areas on graphene (c),
which depend on the number of graphene layers, applied bias, and annealing temperature.
Figures S5(a) and S5(b) show the normalized area ratio of each XPS peak depending on number
of graphene layers, applied bias, and annealing temperature. Using these results, we can
quantitatively analyze each chemical bond that has been induced on graphene by positive AFM
lithography. We were able to confirm the high normalized ratio of C-O-C peaks that have the
values of 4.8 (1LG), 14.7 (2LG), and 7.6 (3LG) with +10 V. Figure S5(c) shows the oxygen
coverage of oxidized areas on graphene calculated using XPS spectra. After annealing at 700 K,
the oxygen coverage of 1LG significantly decreases; however, the coverages of 2LG and 3LG
are negligibly reduced, probably because of both the formation of irreversibly broken sp2 C-C
bonds and the protection effect of the top-most graphene layer.
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Figure S6. AFM (a), FFM (b, and conductive-AFM image (c) on recovered 1LG through UHV
annealing at 700K.
Figure S6 shows AFM, FFM, and conductive-AFM images (obtained at +2 V dc bias) of
oxidized 1LG after UHV annealing at 700K. Partial change in AFM and conductive-AFM
images of 1LG oxidized with +5 V and +7 V are observed after annealing while FFM images
remain nearly unchanged.
Figure S7. Optical microscope images (a), SPEM images at a BE of 284.5 eV (b), and XPS C 1s
spectra (c) of 1LG oxidized with +10 V before and after irradiation of photons with energy of
380 eV for 1 hour.
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Irradiation of photons with energy of 380 eV for 1 hour can change the chemical bonding
characteristics of 1LG oxidized with +10 V, as shown in Figure S7. Spectral weight transfer
from the C=O peak to C-OH peak is observed, and the sp2 C-C peak is unchanged. Thus, we can
argue that the combination method of thermal40 and photon-irradiation treatments on oxidized
graphene could allow for further reduction and recovery to graphene.
Graphene hydrogenated using negative AFM lithography
Figure S8. Schematic diagram of negative AFM lithography (a). Optical microscope (b),
topographic AFM (c), and FFM images (d) of 1LG including areas hydrogenated with negative
AFM lithography using -6 V, -8 V, and -10 V.
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We have hydrogenated graphene in the nanoscale using negative AFM lithography where a
negative dc bias is applied to the graphene, as shown in Figure S8(a). Figures S8(b), S8(c), and
S8(D) respectively exhibit optical microscope, topographic AFM, and FFM images of three areas
on 1LG, which were hydrogenated using -6 V, -8 V, and -10 V. The hydrogenated areas are
indistinguishable in the optical microscope and topographic AFM images, whereas they are
clearly observable in the FFM image because of the higher friction value of hydrogenated
graphene compared to pristine graphene15.
Figure S9. The Raman spectra of pristine and hydrogenated 1LG. The hydrogenation was
performed by negative AFM lithography using -6 V, -8 V, and -10 V biases applied to graphene.
Figure S9 shows the measured Raman spectra of hydrogenated and pristine 1LG. The Raman
spectrum of pristine 1LG reveals sharp G and 2D peaks, whereas defect-related D, D, and D+D
peaks appear on the hydrogenated 1LG irrespective of the external bias applied during negative
AFM lithography.
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Figure S10. XPS C 1s spectra of the three hydrogenated 1LG after UHV annealing for 2 hours at
500 K and 10-9 Torr.
After annealing at 500 K, XPS C 1s spectra of 1LG hydrogenated with -6 V, -8 V, and -10 V
become very similar to that of pristine 1LG, implying that hydrogen atoms can be completely
dissociated by such thermal treatment, as shown in Figure S10.
Transport properties of oxidized and hydrogenated graphene
We fabricated the 1LG/oxidized-(or hydrogenated-)1LG/1LG junctions. Figure S11(a) shows the
current-voltage curves of junctions with 1LG oxidized by +5 V, +7 V, and +10V, as measured
with a voltage sweeping mode at room temperature (300K). The sweeping voltage was applied to
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the conductive AFM tip on right side of 1LG while the left side of 1LG was grounded. More
insulating behaviors are observed in the junction with 1LG oxidized by higher bias voltage. As
shown in Figure S11(b), more insulating behaviors are observed in the junction with 1LG
hydrogenated by a bias voltage with larger absolute value. These results demonstrate that higher
bias voltage applied during AFM lithography induces chemically modified graphene with more
insulating behaviors.
Figure S11. Two terminal current-voltage characteristics of 1LG/oxidized-1LG/1LG (a) and
1LG/hydrogenated-1LG/1LG (b) junctions, which are plotted on semi-logarithmic scale.
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First-principles modeling
The modeling is carried out by the density functional theory realized in the pseudopotential code
SIESTA41, as was done in our previous works42,43. All calculations are done in the local density
approximation (LDA)41. This approximation provides significantly better results for interlayer
distances and interlayer binding energies in such systems42,43. During the optimization, the
electronic ground state was found self-consistently using norm-conserving pseudo-potentials for
cores, a double-ζ plus polarization basis of localized orbitals for carbon and oxygen atoms, and a
double-ζ basis for hydrogen. Optimization of the forces and total energies was performed with an
accuracy of 0.04 eV/Å and 1 meV, respectively. All calculations were carried out for an energy
mesh cut off of 360 Ry and an 8 × 8 × 2 k-point mesh in the Mokhorst-Park scheme45. For the
modeling of the oxidation of graphene, a 3 × 3 graphene supercell (32 carbon atoms, also see
Figures S12(a-d) was used. For the modeling of 3LG, three identical layers of graphene with a
Bernal (AB) stacking order were employed.
For the modeling of step-by-step oxidation, we have used the algorithm which that was
previously employed for the description of nanographenes hydrogenation32. At each step, we
optimized an atomic structure by selecting the structure with the lowest total energy among all
reasonable structures for chemisorption of the next epoxy group on graphene. The energy
required for oxidation is calculated with the standard formula:
Eform = EG+On+1 – (EG+On + EO2/2),
where EG+On is the total energy of graphene with chemisorbed n epoxy groups, and EO2 is the
total energy of an oxygen molecule in a triplet (ground) state in an empty box.
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For the modeling of vacancy formation on graphene, we removed one carbon atom and one of
the nearest oxygen atoms from oxidized graphene at a distance of approximately 2 Å and added
one more oxygen atom to this pair of atoms (to build a CO2 molecule). We optimized the atomic
structure and calculated the vacancy formation energy by the following formula:
Eform = EG – C + On-1 + CO2 – (EG + On + EO2/2),
where EG-C+On-1+CO2 is the total energy of activated graphene and the CO2 molecule (Figures
S12(b) and 3(d)). To check the graphene activation with the formation of a carbon monoxide
(CO) molecule, we performed similar calculations and found that the energies required for this
process are approximately 3 eV higher than for the case of vacancy and carbon dioxide
formation.
The energy required for transformation of epoxy groups to hydroxyl groups in the presence of
water were calculated by the following formula:
Eform = EG + On-1 + (OH)2 – EG +On +H2O,
where EG + On-1 + (OH)2 is the total energy of graphene with n-1 epoxy groups and two hydroxyl
groups (Figure S12(f)), and EG +On +H2O is the total energy of graphene with n epoxy groups and a
water molecule over one of these groups (Figure S12(e)).
The results of these step-by-step calculations are shown in Figure S12. We can divide the process
of the oxidation of graphene (Figures S12(a) and S12(b)) into several stages. These stages can be
correlated with the different applied voltages. We can see that the number of layers affects the
oxidation level at each oxidation stage. The presence of second and third layers provides changes
in the local distortions30 of the graphene sheet induced by the chemisorption of oxygen. These
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distortions play a crucial role in the energetics of C-O-C to C-OH transformation in the presence
of water (Figures S12(e) and S12(f)) because the distortion of the graphene sheet caused by the
hydroxyl group is stronger than that caused by the epoxy group24.
The second step of our modeling is the activation of a vacancy on graphene with the formation of
a carbon dioxide molecule (Figures S12(c) and S12(d)). For 1LG, the energy required for
activation is always higher than that for oxidation, in contrast to 2LG and 3LG. Another
important feature is that passivation of the edges of vacancies by oxygen (Figures 3(g) and 3(j))
is always energetically favorable with formation energies (-2 – -4 eV).
The calculation of the hydrogenation energy has been performed similarly to the calculation of
the oxidation energy. The desorption energy is defined as the energy difference between
hydrogenated graphene (Figures S13(a) and S13(c)) and the corresponding structure with a
single hydrogen atom removed (Figures S13(b) and S13(d)).
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Figure S12. Atomic structures of oxidized graphene: optimized atomic structures of 1LG with
chemisorbed single oxygen atom (a) and maximal coverage of epoxy groups (c); activation of
graphene with formation of carbon dioxide molecule at these concentrations (b and d); initial (e)
and final (f) steps of the transformation of epoxy groups to hydroxyl groups in the presence of
water. Energetics of oxidation (dashed blue line), activation of vacancies with formation of CO2
molecules (solid red line), and hydroxyl groups formation (dashed green line) as function of the
oxidation of graphene surface for 1LG (g), 2LG (h), and 3LG (i).
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Figure S13. Optimized atomic structures in the case of adsorption of the pairs of hydrogen atoms
(a and c) and desorption of the single hydrogen atom from these structures (b and d) for the
hydrogenation of the carbon surface with 6.25 at% (a and b) and 43.25 at% (c and d).
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