## 3 Results  
### Film Characterization  
The surface morphologies of the synthesized GO, GO/PS bilayer, and pentacene deposited on the GO/PS bilayer were studied by AFM. Figure 1c shows the synthesized GO pieces obtained from a highly diluted solution; the pieces are in the form of flat sheet-type structures with an average sheet size of about 600 nm (with some size deviation from sheet to sheet). The inset of Figure 1c shows the height profile of a 1.5-2.0 nm GO slice, which reveals that the GO flakes are well separated by a single layer. Figure 2a,b, respectively, show the surface morphologies of the GO film (thickness of 70 nm) and PS-coated GO (GO/PS bilayer, total thickness of 75 nm) film used in this system. As can be seen in the AFM images, coating the PS layer on the GO surface can significantly reduce the surface root-mean-square (\(R_{\text{q}}\)) of the GO film, from 4.91 nm to 2.17 nm, and improve the uniformity. Figure 2c shows the morphology of the pentacene film deposited on the GO/PS film; the average grain size is approximately 250 nm, and \(R_{\text{q}}\) roughness is 7.8 nm.  
Figure 1: **(a)** Schematic illustration of procedure for synthesizing graphene oxide (GO) by modified Hummers’ method. **(b)** Schematic device structure of organic field-effect transistor (OFET) used in this study. **(c)** Height-mode atomic force microscopy (AFM) topographs of GO pieces obtained from highly diluted solution (the inset shows the height profile of a GO slice).  
Figure 2: Height-mode AFM topographs of **(a)** GO film, **(b)** polystyrene (PS)-coated GO (GO/PS) film, and **(c)** pentacene film deposited on GO/PS bilayer dielectric.  
To measure the gate dielectric properties of the GO and GO/PS bilayer films, metal-insulator-metal capacitors (herein, Au-GO-heavily p-doped Si wafer or Au-GO/PS-heavily p-doped Si wafer) were fabricated. Figure 3 shows the measured \(C_{\rm i}\) and electric-field-dependent gate leakage current density. The \(C_{\rm i}\) value of GO decreases from 85 to 62.4 nF cm\({}^{-2}\) (average: 73.3 nF cm\({}^{-2}\)) and that of GO/PS decreases from 40.3 to 34.8 nF cm\({}^{-2}\) (average: 37.8 nF cm\({}^{-2}\)), as the frequency increases from 10 kHz to 1 MHz, which agrees well with the reported \(C_{\rm i}\) value of a GO film with a similar thickness [13]. Although the \(C_{\rm i}\) value of the GO/PS bilayer is smaller than that of the GO film because of the low dielectric constant of PS, the former \(C_{\rm i}\) is high enough for GO/PS to serve as a dielectric layer for low-voltage operation of OFETs. As can be observed from the data in Figure 3a, the \(C_{\rm i}\) values of the GO film and the GO/PS bilayer decrease by about 27% and 14%, respectively, with an increase in the frequency to up to 1 MHz; this indicates that the GO/PS bilayer has fewer mobile impurities on the dielectric surface than does the GO single layer. Figure 3b shows the electric-field-dependent gate leakage current. The GO single layer shows a high leakage current (10\({}^{-4}\)-10\({}^{-7}\) A cm\({}^{-2}\)), whereas the gate leakage current of GO/PS bilayer is one order or more less than that of the GO film; this indicates the excellent electrical strength of the GO/PS bilayer. As mentioned in the introduction part, since the hydrophilic hydroxyl and/or epoxide functional groups on the GO surface can function as trap sites and reduce the device characteristics, it is necessary to modify the GO surface with the hydrophobic layer. The leakage current reduction can be attributed to the surface passivation of the hydrophilic GO with the hydrophobic PS, which could facilitate the use of GO as a dielectric of the OFETs.