Ionic Liquid Concentration Dependent Hysteresis in Suspended Bilayer Graphene Transistor
Authors: Weng Wei Yang , Tsai Ming-Sheu , Lu Yu Xuan , Lin Chih Ting 
Published: May 2020

Traditionally, graphene-based field effect transistors comprise graphene flakes lying on SiO2 substrate. On the other hand, suspended graphene structure can help investigate the intrinsic properties of graphene and even show improved sample quality. There have been several publications relating to the researches about transport hysteresis in graphene devices [1-3], which are important for the development of graphene-based sensor. However, there are seldom paper talking about graphene with one-side liquid structure, which means the effects of the substrate can be eliminated, leaving the structure of pure graphene and liquid. Therefore, we can further explore the intrinsic characteristics of graphene, such as hydrophilic or hydrophobic surface properties.
In this work, we used the device structure as shown in Figure 1 to present an investigation compared with the previous device. The device of Figure 1 contains two gold contacts for drain/source electrodes, and the etched area which includes two larger regions for liquid injection and the main trench with some pillar for suspended graphene. We began by measuring the transport hysteresis in the device without trench, and then do the same measurement but using the device for suspended graphene. The solution gate voltage is swept from -1V to 1V in order to avoid any reaction, and both devices are in ambient environment.
Figure 2 shows the different hysteresis directions observed in this work, which means that there are some differences between the carrier transport mechanism of these two devices. As shown in Figure 3, the magnitude of the hysteresis in gate voltage is quite different in the two devices, which it’s larger in suspended-graphene device than in normal device. Besides, the drain currents in two devices also show a large difference, which may result from the fabrication deviation, such as the contact resistance or the sheet resistance of bilayer graphene. On the other hand, the difference between the Dirac point (VDirac) in the potassium chloride (KCl) solution of various concentration shows an almost linear dependent relationship if we plot the concentrations on the logarithmic scale, as shown in Figure 4.
It is believed that the more charge trapping will cause larger hysteresis; however, in this work we found that the magnitude of hysteresis on suspended graphene device is anomalously larger than on-substrate graphene. We suppose that this result is related to the air/graphene/ion liquid structure associated with the intrinsic properties of graphene surface. The second observation of the reduction hysteresis by increasing the solution concentration is also an interesting phenomenon, which may relates to the process of the competition between charge trapping effect and capacitive gating effect. [1]
In conclusion, we compared the electrical characteristics shown by two different structures of graphene-based field effect transistor in this work, and found some interesting results of the transport hysteresis relating to device structure and ion concentration. This development has potentials for the chemical or biological sensing application. Furthermore, it can contribute to the fundamental researches about the interaction on the interface between graphene and solution.
[1] Wang, H.; Wu, Y.; Cong, C.; Shang, J.; Yu, T., Hysteresis of Electronic Transport in Graphene Transistors. ACS Nano 2010,4(12), 7221-8.
[2] Uysal, A.; Zhou, H.; Feng, G.; Lee, S. S.; Li, S.; Fenter, P.; Cummings, P. T.; Fulvio, P.F.; Dai, S.; McDonough, J. K.; Gogotsi, Y. Structural origins of potential dependent hysteresis at the electrified graphene/ionic liquid interface. J. Phys. Chem. C2013,118, 569-574.
[3] Shih, C.-J., Paulus, G. L. C., Wang, Q. H., Jin, Z., Blankschtein, D., Strano, M. S., Understanding Surfactant/Graphene Interactions Using a Graphene Field Effect Transistor: Relating Molecular Structure to Hysteresis and Carrier Mobility. Langmuir, 28(22)

Figure 1