Applied Science and Convergence Technology 2019; 28(3): 51-54
Published online May 31, 2019
Copyright © The Korean Vacuum Society.
Department of Physics, Chungnam National University, Daejeon 34134, Republic of Korea
Correspondence to:*E-mail: firstname.lastname@example.org
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After the oxidation process of graphene was shown to lead to zero conductance (~ 0 μS) based on the electrochemistry, the natural reduction in the graphene oxide behavior resulting in a recovery of the conductance (~ 200 μS) was observed under exposure to ambient conditions during a 24 h period. Furthermore, damage to the graphene oxide surface during the repeated atomic force microscopy scanning process owing to the weakening of the van der Waals force between the oxidized graphene and SiO2 substrate was characterized. These results provide significant information regarding the remaining time of electrochemical oxidation and the mechanical properties of graphene.
Keywords: Graphene, Reduction, Oxidation, Graphene oxide, Electrochemistry
An electrochemical redox of carbon networks leads to a tuning conductivity from the disruption of an sp2 carbon network when sp3-conjugated carbon is applied in a nanometer-sized point detector, and the source of oxidation in the carbon network is elucidated as a defect in the carbon bonding [1–14]. The oxidation of graphene, which has a two-dimensional carbon bonding structure, is also a significant subject for various applications such as sensors and dielectric layers [1–14]. Although graphene oxide (GO) can be obtained through various avenues, electrochemistry processes using an electrolyte are promising for the fabrication of GO [1,9]. In particular, when employing a negative to positive electrolyte gating voltage potential, we can realize the oxidation and reduction of graphene [1,8,9]. Although many studies have been conducted on the electrical properties in the redox of graphene through electrochemistry [1–14], the electrical and surface characterization of fully oxidized graphene is still required.
In this study, the natural reduction process of graphene oxidized using an electrolyte was considered. In particular, it was observed that the naturally reduced behavior of graphene oxide leads to a recovery of the conductance under exposure to ambient conditions during a 24 h period. Furthermore, the damage to the graphene oxide surface when applying an atomic force microscopy (AFM) scanning process was investigated.
The graphene field-effect transistor (FET) applied in this study was prepared using mechanically exfoliated graphene on a 280-nm-thick SiO2 substrate [see Fig. 1(a)] [8,9,15,16]. Upon employing e-beam lithography, Cr/Au (5/50 nm thickness) metal electrodes were placed in contact with the graphene/SiO2 [see Fig. 1(b)]. To avoid a chemical reaction between the Cr/Au metal electrodes and sulfuric acid (H2SO4, 1 M) electrolyte during the electrochemistry process, a poly methyl methacrylate (PMMA) passivation layer was utilized on a graphene FET. After coating of the PMMA, a window for viewing only the electrochemistry reaction between graphene and electrolyte in the graphene channel area was fabricated using e-beam lithography [see Figs. 1(c) and 1(d)].
Figure 2(a) shows a schematic diagram of the conductance variation of the PMMA window on a graphene FET under application of either an electrolyte (VEG) or a back-gate voltage (VBG). Here, a measurement of the conductance variation was utilized using standard lock-in techniques. The initially hole-doped condition of the graphene was observed based on the accumulated carrier density
To inspect the surface properties of graphene, AFM, and electric force microscopy (EFM) measurements were employed using a commercial AFM system (XE-100, Park Systems Co.), as described in Figs. 4 and 5. The topography measurement applied a noncontact mode by oscillating a cantilever at ~170 kHz. The EFM measurement was utilized by applying an AC voltage of 0.5 V, an amplitude of 17 kHz, and a DC voltage of 1.0 V to the conductive probe [15,16].
Based on the electrolyte gating, the conductance of the graphene FET was modulated, as shown in Fig. 3. By applying VEG = 0.0 to −1.0 V, the conductance of the graphene FET was tuned from 450 to 0 μS. This behavior has been reported as the formation of an sp3 (C-O or C = O) bond on the graphene surface through a chemical reaction with the H2SO4 electrolyte [1–14]. The oxidation of graphene was exactly started under VEG = −1.0 V (marked by blue dashed line) and during applying the electrolyte gating in Fig. 3, the VEG was also tuned from −1.0 to 0.23 V (marked by blue arrow in Fig. 3) for guaranteeing that the conductance variation of graphene depends on VEG, exactly. As a result, the sp3 bonds block the carrier transport on the graphene channel, leading to zero conductance, and this suppressed conductance (0 μS) of the graphene FET cannot be recovered by applying VEG = +0.75 V (see Fig. 3). This indicates that the graphene channel surface is dominantly oxidized, nearly leading to a degradation of the conductance. Note that, although there was no variation of conductance for applying VEG = + 0.75 V during 15 s, the time enough to release sp3 leads to reduction behavior as shown in Fig. 5.
Figure 4 shows the AFM and EFM images for the opened graphene channel area using the PMMA window before (A) and after (B) oxidation, as indicated in Fig. 3. Before oxidation (A) in Fig. 3, a clean topography of the graphene surface was exhibited, as shown in Fig. 4(a). However, a defective topography of the graphene surface after oxidation was observed, as indicated in Fig. 4(b). This means that applying an electrolyte gate of VEG = −1.0 V leads to the oxidation of most of the graphene surface with the formation of undesired defects. Furthermore, the electrical condition of the graphene surface before and after oxidation is as shown in Figs. 4(c) and 4(d). The EFM images of the graphene surface in (A) and (B) of Fig. 3 show a distribution with a potential of VEFM = ~5 and ~1.2 V, as indicated in Figs. 4(c) and 4(d), respectively. This shows that the dominant sp3 bonds on the graphene surface suppress the conductance.
By contrast, as revealed in Fig. 5(a), a slight recovery in the conductance (black
In this study, the oxidation of graphene was achieved using electrochemistry. Furthermore, evidence of the reduction behavior exhibiting a recovery in conductance from oxidized graphene with zero conductance was observed. Because this result can be investigated only under exposure to ambient conditions, it indicates that the weak sp3 bonds can be reduced again to an sp2 bonding condition. Thus, this result allows us to provide significant information indicating that electrochemical oxidation on the graphene surface is not permanent and that the GO surface can be damaged using an AFM tip.
This research was supported by Chungnam National University (2018–2019).