Dielectric Constant and Dipole Moment Dependent Hysteresis in Suspended Bilayer Graphene Transistor
Authors: Yuxuan LU , Shen Yu Te , Tsai Ming Hsiu , Lin Chih-Ting , Lin Shun Chiu 
Published: July 2021

Manipulation of transport hysteresis on graphene-based field effect transistors and understanding electron transfer between graphene and organic solution are important for the development of graphene-based sensor device. Several papers regarding to the researches about hysteresis in graphene devices have been published, which are significant to graphene-based device[1
-3
]. However, there are seldom papers focus on graphene with one-side organic solution which has different relative permittivity and various dipole moment constant. This means that graphene suspended in liquid which eliminates the effect of substrate and leaves graphene and liquid with pure structure. Therefore, we do further researches on the graphene-based device and explore the relationship between graphene and solution.
In our research, the structure of the device is shown in Figure1, which includes two gold electrodes used for drain and source separately and the etched area which contains two larger rectangular area used for injecting liquid and one main trench with some pillars used for suspending graphene. When measuring each set of data, we did experiments in ambient environment and used the same suspended-graphene device to control the influence of the fabrication deviation, such as the contact resistance or the sheet resistance of bilayer graphene.
Figure2(a) shows the SEM image of suspended graphene on the main trench and the significant differences can be seen between the parts where the graphene exists and does not exist, which proves the presence of suspended graphene. We do the same measurement the transport hysteresis in the device by using several solvent includes isopropanol (IPA), ethonal and dimethylacetamide (DMAC). Figure 2(b) shows the different hysteresis situations of three different solutions observed in this work. In the picture, the solid line illustrates the drain current change with the forward voltage which means voltage swept from -1V to 1V and the dotted line depicts the drain current alter with reverse scanning voltage called backward voltage. The value of hysteresis is defined as the difference between the voltage value corresponding to the lowest value of the forward curve and that of the backward curve，which is equal to the change between Dirac point.
As shown in Figure 3(a), the difference between the hysteresis in the organic solution of various permittivity shows an almost curve correlation. Hysteresis value decreases as the dielectric constant drops, which can be explained by less charge trapping will cause smaller hysteresis. At the same time, the variety between the hysteresis of solvent with various dipole moment displays line with more obvious arc, which is shown in Figure3(b). These observations are interesting phenemonon which may related to the change on electrostatic environment of graphene[1] and the process of the competition between charge trapping effect and capacitive gating effect[2].
In conclusion, we made several experiments to explore the relationship between graphene-based device and solvent with different electrical characteristics and found some interesting results of the difference between Dirac point regarding to permittivity and dipole moment. This characteristic may be used for judging the purity of organic solvents and other chemical or biological sensing application. Furthermore, it contribute to the fundamental development of relationship between graphene-based devices and different solvents.
[1]Philipp Klar and Cinzia Casiraghi. Raman spectroscopy of graphene in different dielectric solvents[J].Phys. Status Solidi C,2010,7:2735-2738.
[2]Wang, H.; Wu, Y.; Cong, C.; Shang, J.; Yu, T., Hysteresis of Electronic Transport in Graphene Transistors. ACS Nano 2010,4(12), 7221-8
[3]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. C,2013,118, 569-574.

Figure 1