Hydrogenated monolayer graphene with reversible and tunable wide band gap and its field-effect transistor

Graphene is currently at the forefront of cutting-edge science and technology due to exceptional electronic, optical, mechanical, and thermal properties. However, the absence of a sizeable band gap in graphene has been a major obstacle for application. To open and control a band gap in functionalized graphene, several gapping strategies have been developed. In particular, hydrogen plasma treatment has triggered a great scientific interest, because it has been known to be an efficient way to modify the surface of single-layered graphene and to apply for standard wafer-scale fabrication. Here we show a monolayer chemical-vapour-deposited graphene hydrogenated by indirect hydrogen plasma without structural defect and we demonstrate that a band gap can be tuned as wide as 3.9 eV by varying hydrogen coverage. We also show a hydrogenated graphene field-effect transistor, showing that on/off ratio changes over three orders of magnitude at room temperature.


Estimation of hydrogen coverage by XPS.
We found that in the case of a band gap of 3.9 eV, about 25% was the hydrogen coverage  as calculated by quantitative analyses of the C 1s core-level measured by synchrotron radiation Xray photoelectron spectroscopy (XPS), which corresponds to stoichiometric C 4 H 1,2 . In the calculation, we assumed that hydrogen atoms are chemically bonded to carbon atoms on the top surface only since the surface of graphene on a SiO 2 /Si substrate is indirectly exposed to the plasma. The C 1s spectrum contains various bonding types, such as sp 3 , sp 2 and H-C-C, of hydrogenated graphene, which we call ,  and , respectively. The CVD graphene is mainly composed of  and also has oxidic phases (Supplementary Figure 1a). As we increased the exposure time t, the  and  peaks gradually became stronger while the  peak became weaker, which indicates that hydrogen is covalently bonded with carbon in graphene. The relative portion of both  and  peaks started to saturate over an exposure time of 800 s, corresponding to an of 25%, at which the optical bandwidth was also maximally saturated (Supplementary Figure   1b). The saturation implies that hydrogen in excess did not react further with carbon when  reached 25%. The H-Gr should therefore be a stoichiometric form of C 4 H.

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Supplementary Note 2

Estimation of a higher-lying direct bandwidth in H-Gr by optical absorption.
The optical transmission T of graphene and H-Gr on a sapphire substrate was measured in the visible-ultraviolet (VUV) region by grating spectrophotometry (Supplementary Figure 2). With the transmission normalized against the substrate effect, the optical absorption coefficient  was obtained according to the standard formula where d is the thickness of the sample. In general, the absorption coefficient due to an interband transition of photo-excited electrons is described by where B is a material-dependent constant, hv is the incident photon energy, and E g is the threshold energy for interband absorption (often a band gap itself for a direct band-gap semiconductor). Our higher-lying absorption background is best fitted with ≈1/2, i.e., a direct interband transition type. The E g for our H-Gr is determined by the energy-intercept of the line tangent to the high-energy (4-6 eV) absorption tail of  2 E 2 .
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Supplementary Note 3
Temperature dependence of resistivity of pristine and hydrogenated graphene.
We search for gapped-semiconductor behavior from our -T data of three types of graphene films (pristine, H-Gr (=12%), and H-Gr (=25%)) over the temperature range of 1.8-250 K. As shown in Supplementary Figure 3a, the resistivity ( of pristine graphene monotonically decreases with decreasing temperature, revealing its metallic transport characteristics, which include saturation behavior toward zero temperature. In sharp contrast, both H-Gr films exhibit insulator-like transport characteristics as their resistivity increases with decreasing temperature. In order to obtain FET mobility by 2-probe measurement, we applied constant source-drain voltage (V sd ), and FET mobility () was extracted from the differential curve of the electric field effect according to the standard formula where L (W) is length (width) of graphene channel. C i is the gate capacitance and it was 3.0×10 -8 F/cm 2 in our 100-nm-thick SiO 2 . V bg is the back gate bias voltage. I sd is the source-drain current measured by the electric field effect measurement. where B is the external magnetic field,  the conductivity of graphene and e the elementary charge.