Supplementary Data

for

Fabrication of Reduced Graphene Oxide Micro Patterns by Vacuum-

ultraviolet Irradiation: from Chemical and Structural Evolution to

Improving Patterning Precision by Light Collimation

Yudi Tu1, Hiroshi Nakamoto1, Takashi Ichii1, Toru Utsunomiya1, Om Prakash Khatri2,

Hiroyuki Sugimura1*

1 Department of Materials Science and Engineering, Graduate School of Engineering, Kyoto

University, Kyoto 606-8501, Japan

2 Chem. Sci. Division, CSIR-Indian Institute of Petroleum, Mohkampur, Dehradun-248005,

India

* Corresponding author. E-mail: sugimura.hiroyuki.7m@kyoto-u.ac.jp

Tel: +81-75-753-9131S1

1. Hydrophilic modification of SiO2/Si substrate

Si substrates were cut from a 4-inch wafer [p-type silicon (001) substrate with 90 nm

thermal oxides (Electronics and Materials Co., Ltd.)] by diamond pen. The substrates were

then sonicated in ethanol (99.5 %, Nacalai Tesque Inc.) and ultrapure water (UPW, resistivity

≥ 18.2 MΩ·cm, produced by RFD250NB water distillation apparatus, Advantec) for 20 min

respectively in an ultrasonic cleaner (50 Hz, 100 W, VS-F100, As-one), followed by drying

under N2 blow. After removing the tiny particles by sonication, the substrates were submitted

to vacuum-ultraviolet (VUV) light irradiation (172 nm, 10 mW·cm–2, Ushio Inc.) under

ambient atmosphere for 20 min. The organics adsorbed on the substrates were removed by the

oxidation of active oxygen species, which was generated by irradiating VUV light on the

substrates in ambient atmosphere.

S2

2. The power density of the VUV lamp

The power density of the VUV lamp was measured in N2 environment by the accumulated

UV meter (UIT-150, Ushio Inc.) and was plotted against the irradiation distance (Fig. S1).

Since N2 will not absorb 172 nm VUV light, a well-purged N2 environment can be used for

measuring the power density of the VUV lamp. The relationship between the power densities

against the irradiation distance from the lamp window was plotted in Fig. S1. As can be seen

in Fig. S1, the original lamp power density declines along with the irradiation distance

increase because of the non-collimated property of the excimer lamp. The VUV

photoreduction process was conducted at the 1 cm distance where the power density was 13.8

mW·cm−2.

Fig. S1. The original power density of the 172 nm Xe2 excimer VUV lamp in N2 environment.

S3

3. Peak deconvolution details for XPS elemental analysis

The XPS spectra were fitted by Gaussian-Lorentzian [GL(30) shape] curve fitting and were

deconvoluted into 6 peaks, i.e., sp2 C=C (at 284.4 eV with full width half maximum (fwhm)

of 1.50 eV), sp3 C-C (at 285.0 eV with fwhm of 1.36 eV), C-OH (at 286.2 eV, with fwhm of

1.46 eV), epoxides (at 286.9 eV, with fwhm of 1.24 eV), C=O (at 287.9 eV, with fwhm of

1.57 eV) and C(=O)O (at 289.1 eV, with fwhm of 1.41 eV).[1] For pristine GO, a carbon

vacancy (C–V) peak [2,3] was also fitted at 283.5 eV with fwhm of 1.2 eV.

By using specialized software, CASAXPS, the atomic percentages of each composition

(namely, PC=C, PC-C, PC-OH, PEpoxide, PC=O and PC(=O)O) were calculated based on the peak areas.

For pristine GO, the percentage of C–V peak was counted into PC=C. The atomic percentages

of each composition were further utilized to obtain the oxygen/carbon atomic ratio (RO/C) from

the following Equation S1. [4]

RO/C=PC-OH +1

2PEpoxide+PC=O+2 PC(=O)O

PC=C+PC-C + PC-OH + PEpoxide+PC=O + PC(=O)O

(S1)

Previous researches performed the peak fitting of C1s spectra and assigned the peaks at

different binding energy to unambiguous oxygen-containing functional groups.[5,6] However,

it is notable that a certain binding energy can correlate to various oxygenated carbon

structures, especially in GO which contains uncertain oxygenated carbon compositions.[3]

This makes certain errors in this calculation. However, this equation can still be used as a

semi-quantative analysis of RO/C for GO on SiO2/Si substrate with an acceptable precision.

The time evolution of RO/C of VUV reduced GO is shown in Fig. S2.

S4

Fig. S2. The time evolution of RO/C of VUV reduced GO.

S5

4. The strategy to solve the mismatch in VUV patterning

It was found difficult to fulfil the demand to increase the irradiation dose and to maintain

the precision of the patterns for two critical facts: (1) the VUV light utilized here was not

collimated, and (2) the inevitable gap between the substrate and the photomask. As shown in

the illustration of the patterning instruments (Fig. S3a), the VUV light generated by the Xe2

excimer lamp was non-collimated and due to the reflection at the Al mirror, the light would

irradiate the sample surface at different tilted angles. As can be easily predicted, the substrate

would not perfectly contact with the photomask due to the long-range uneven surface.

Therefore, there should be gaps at the photomask-substrate interface, as shown in Fig. S3a.

Estimated by the interference pattern occurred on the interfaces, the gap would be as large as

hundreds of nanometers. The tilted incident light could penetrate into the masked region and

reduce the precision of the pattern. The patterning process will result in a clear mismatch of

patterns, especially under long-time irradiation as shown in Fig. 7b.

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Fig. S3. (a) Schematic illustration of the normal VUV photopatterning. The blue incident light

has the largest incident angle. The magnified schematic illustration at the photomask-substrate

interface showed how deep the tilted incident light can penetrate. (b) Schematic illustration of

the modified VUV photoreduction system. The VUV light would be reflected by the Al mirror

and be angled to irradiate the sample. By using the hollow column, the blue dashed incident

light would be blocked by the Al foil while the green dashed light would be absorbed by the

sodium glass. The critical incident light with the largest incident angle would be the red light.

The magnified schematic illustration at the photomask-substrate interface showed that the

penetraion of incident light decreased.

To solve this unintended mismatch, a hollow column was utilized, which was a sodium

glass covered by Al foil (Fig. S3b). The Al foil would absorb and reflect the VUV light while

the sodium glass would totally absorb the VUV light. The modified light would be relatively

collimated with the critical angle decreasing as shown in Fig. S3b. At this moment, the

mismatch could not be totally eliminated but fairly improved to give a precise pattern with a

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careful cleaning of the substrates and photomasks. On the other hand, the power density

would decline significantly due the absorption and reflection of the light. We measured the

VUV light power density with the hollow column and plotted against the irradiation distance

as shown in Fig. S4a. The power density would be the same at the distance from 4 cm to 6

cm, indicating that only the light over the hollow could transmit to the sample surface. The

modified power density would be 2.7 mW·cm−2. To maintain the exposure dose at the

saturated point of 32 min when irradiating the sample with 13.8 mW·cm−2, the exposure time

was extended to 163 min to achieve the dose of 26.5 J·cm−2.

Fig. S4. (a) The power density of the VUV lamp at different irradiation distance when using

the hollow column. (b) XPS spectrum of the GO irradiated through the hollow column at 5 cm

with the exposure dose of 26.5 J·cm−2.

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5. XPS characterization of UV reduced GO

Fig. S5. Time evolution of the carbon composition of UV reduced GO.

S9

6. Structural characterization by micro Raman spectroscopy (µRS) and Scanning

tunneling microscopy (STM).

Fig. S6. Raman spectra of the graphene, GO and rGO. All the spectra used the excitation

wavelength of 532 nm. The graphene was characterized as a reference. The positions of D

peak, G peak and 2D peak are illustrated by vertical dashed lines.

The micro Raman spectroscopy (µRS) measurements were conducted with a Horiba

XploRA Raman microscope that uses a 532 nm line laser as the excitation source. The µRS

spectra of GO, the VUV reduced rGO and a spectrum of a graphene exfoliated by the Scotch

tape method are shown in Fig. S6. In the case of the graphene, the spectrum shows three

peaks at 1340 cm−1, 1574 cm−1 and 2670 cm−1 that are called D peak, G peak and 2D peak,

respectively. D peak is attributed to an A1g breathing mode of 6-C-atom rings at the K point

and requires defects in graphene for activation, e.g., the edge of graphene and defects in

graphene planar structure, etc.[7] G peak corresponds to the first order E2g phonon scattering

of the in-plane sp2 bonded carbon. The 2D peak is ascribed to a second order scattering of D

peak, which can be enhanced by resonance in the graphene. Compared with the spectrum of

the graphene, the spectrum of GO showed that D peak obviously broadened and enhanced

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along with G peak broadening and shifting to a higher wavelength at 1606 cm−1. The ID/IG

ratio of GO was measured to be 1.07 and showed no clear change after irradiation. By

calculating from the empirical equation[8,9] as shown in Equation S2, the average

interdefects distance was estimated to be ca. 11.7 nm.

LD2 ( nm2) = 4.3× 103

EL4 [I (D)

I (G) ]-1

(S2)

On contrast to µRS, STM can reveal the local structure of the rGO. STM was conducted on

JSPM-5200 (JEOL Ltd.) with a home-built transimpedance amplifier in constant current

mode with mechanically cut tips from Pt/Ir wires. The set point of the tunneling current was

10 pA with a constant tip bias of + 1.0 V. Because STM requires electrical conductivity for

the samples, an epitaxial Au(111) surface formed on mica was used as the substrate only for

STM measurement. The GO was drop-cast onto the Au(111) surface and reduced by the HV-

VUV process as described above. Measurements were performed under ambient environment.

[10,11] The STM image of the VUV-reduced GO is shown in Fig. S7a. The Au(111) surface

was atomically flat with well-defined terraces and islands. The rGO sheet showed a

heterogeneous structure with randomly distributed domains of size of ca. 10 nm. The height

of the sheet was measured from the line profile as shown in Fig. S7b. The basal plane of the

sheet was ca. 0.6 nm thick while the bright small domains were ca. 1.0 nm thick. The thicker

domains were ascribed to the principal of STM; both the electronic and topographic structure

contributed to the contrast in the STM image. On one hand, the thicker domains can be the

graphitic domains that were relatively conductive with a higher density of electronic states

near the Fermi level so that the tunneling current was relatively high and the feedback

enlarged the tip-sample distance, which resulted in the relatively brighter appearance. On the

other hand, the thicker domains can be the highly oxidized-sp3 domains as well. The highly

oxidized-sp3 domains were distorted and decorated with OFGs, resulting in a thick

topographic structure. Both explanations for the bright domains are reasonable and it is

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difficult to distinguish them. However, the sizes of domains were at similar levels of ca. 10

nm, which was consistent with the µRS estimation above and the previously reported STM

and transmission electron microscopic (TEM) results.[11–13] By both studying the global and

local structure of the rGO sheet, it was found that the size of the domains barely changed after

the VUV reduction.

Fig. S7. (a) STM image of the single-layered rGO sheet. (b) Height line profile corresponding

to the dashed line in panel a.

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