Applied Science and Convergence Technology 2019; 28(6): 226-228
Published online November 30, 2019
Copyright © The Korean Vacuum Society.
Department of Physics, Chungnam National University, Daejeon 34134, Republic of Korea
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In this work, the conditions of water-gated carrier density accumulation on graphene were investigated. In particular, either negative or positive electric potential in deionized water could be observed, leading to an increase or decrease in the hole charge carrier density, respectively. Furthermore, it was determined that the critical positive electric potential for water gating of graphene is required to return the accumulated carrier density back to its initial state. Taken together, these results provide valuable insight into the relationship between carrier accumulation and the deionized water gating method.
Keywords: Graphene, Carrier density, Water gating
Graphene has been favored for atomically thin electrodes for nano-scale devices due to its high electrical conductivity [1–3]. Since the high charge carrier doping condition of graphene is one of the critical requirements for high conductivity electrodes, the accumulation of carrier density on graphene has been extensively studied. The use of electrolytes to potentially produce highly doped graphene by various methods has also been investigated for applications in several fields [4–13]. However, when a critical electric potential is applied to graphene via electrolytes such as ionic liquid or gel [7–9], undesired sp3 bonds were observed to form on the graphene [4–13] Thus, in order to maintain distinct sp2 electric potential structures on graphene during the application of an electric gate voltage, a stable electrolyte is required. Water is a promising electrolyte for this use, and water-gated field effect transistors have previously been reported [5,12]. However, more detailed information regarding the carrier density variation of graphene with water-gating is required.
In this work, the behavior of carrier density accumulation on graphene is investigated. In particular, the ability to tune the carrier density when employing deionized (DI) water gating with electric potential is explored. Finally, detailed information regarding the effect of accumulating charge carriers on graphene when either negative or positive electric potential is supplied through DI-water is presented.
In order to identify graphene conductance, graphene was prepared on a 280-nm-thick silicon dioxide (SiO2)/highly doped silicon (Si) substrate by the mechanical exfoliation method. Subsequently, titanium (Ti) electrodes were placed in contact with graphene by a conventional electron-beam lithography process [Figs. 1(a) and 1(b)] [8,9,13]. Owing to the natural formation of a thin oxide layer on Ti when exposed to air, this oxide was employed as a barrier for blocking the interaction between the water-gate and Ti electrodes. As shown in Fig. 1(a), the resistance variation of graphene was inspected by two-terminal measurement during the application of either back-gate voltage (VBG) or water-gated voltage (VWG) through a SiO2/Si substrate or DI-water (resistivity = 8 ~ 10 MΩ)/platinum (Pt) bar, respectively.
Figures 2(a) and 2(b) show the resistance of graphene as a function of VBG and VWG, respectively. As shown in these figures, the initially accumulated carrier density in graphene was
Figure 3 shows the resistance of graphene as a function of time for different values of applied VWG. When the (1) VWG = −0.6 V was applied to graphene, a decreased resistance of approximately 4.4 kΩ was observed. Subsequently, the resistance of graphene was measured as 5.4 kΩ for (2) VWG = 0.6 V and as 4.3 kΩ for (7) VWG = 2.0 V. Furthermore, VWG = (1) −0.6 V, (6) 1.5 V, and (7) 2.0 V were gradually applied to graphene again as shown in the second light-green area within Fig. 3. Similar resistances of graphene were obtained from the first measurement denoted by the blue area in Fig. 3. Although, the variation of resistance as a function of VWG was observed, this result was not enough to clearly understand the effect of VWG on the accumulated carrier density variation on graphene. Thus, VCNP-BG from the resistance curves was represented as a function of VBG [Fig. 4] after each VWG at (1) ~ (7) were applied to graphene, as shown in Fig. 3.
Figure 4(a) presents the resistance curves of graphene as a function of VBG after the application of VWG = (0) 0 V, (1) −0.6 V, and (3) 0.7 V ~ (7) 2.0 V. Compared with the neutral point of VCNP-BG = 8.35 V [marked by the black dashed line of the black transport curve in Fig. 4(a)] under (0) VWG = 0 V, VCNP-BG was increased by approximately 10 V [the red transport curve in Fig. 4(a)] for (1) −0.6 V and VCNP-BG remained approximately 10 V during the application of VWG = (2) 0.7 ~ (5) 1.2 V [denoted by the dashed transport curves in Fig. 4(a)]. When VWG greater than (6) 1.5 V and (7) 2.0 V was applied [the blue and brown transport curves in Fig. 4(a)], VCNP-BG decreased by approximately 3 V.
As an estimate of the variation of the accumulated carrier density (
In this work, carrier density accumulation was observed in response to DI-water gating. When a critical negative electric potential (VWG = −0.6 V) was applied via DI-water, the accumulated hole carrier density was observed. Furthermore, once the applied voltage exceeded a critical positive electric potential (VWG ≥ 1.5 V) through DI-water, this accumulated carrier density could be restored to the initial carrier density condition. This result provides meaningful insight into the relationship between carrier accumulation and the DI-water gating method.
This work was supported by the research fund of Chungnam National University.