Electric Double Layer Doping of WSe2 Field-Effect Transistors Using a Monolayer Electrolyte
Authors: Liang Jierui , Xu Ke , Fullerton Susan 
Published: February 2020

Electric double layer (EDL) doping using a polymer electrolyte or ionic liquid is a common technique to explore transport in two-dimensional (2D) materials. The doping relies on the formation of an EDL due to the accumulation of mobile ions at the interface between an electrolyte and a 2D semiconductor. The induced image charge yields large capacitance densities (1-10 μF/cm2) and sheet carrier densities (~1014 cm-2) in 2D crystal field-effect transistors (FETs).1 Our group has developed a monolayer electrolyte that is a single molecule thick and can electrostatically dope the surface of 2D semiconductors.2 The monolayer electrolyte has several advantages over liquid-phase electrolytes because it is solid state, bistability has been demonstrated,2 and DFT calculations predict a low switching voltages (sub-volt) at fast speeds (sub-ns).3

We have demonstrated EDL doping on graphene2 and MoS2 using the monolayer electrolyte. Bistability, state retention and a sheet carrier density of 3.2×1012 cm-2 have been demonstrated. In this study, WSe2 is selected as the 2D channel because it is ambipolar with a sizable bandgap (1.4 eV), large on/off ratio (> 106) and high carrier mobility (~140 cm2V-1s-1).4

The monolayer electrolyte consists of cobalt crown ether phthalocyanine (CoCrPc) and lithium perchlorate (LiClO4) (Fig.1 (a)). CoCrPc is an atomically thin and electrically insulating molecule (band gap ~1.34 eV) that can lie flat on a 2D surface.5 It includes four crown ethers, and each of them can solvate one Li+.6 An ordered array of CoCrPc can be deposited on a 2D material by drop-casting and annealing.5 Back-gated WSe2 FETs were fabricated by electron beam lithography (EBL), and the monolayer electrolyte was deposited on the surface of the WSe2 FETs by drop-casting and annealing. The WSe2 channel surface was cleaned by contact-mode AFM to remove e-beam resist residue before electrolyte deposition to avoid interference to the packing arrangement of the monolayer electrolyte. The quality of the cleaning is confirmed by AFM topography and surface roughness measurements; the roughness after the cleaning equals the value of freshly exfoliated flakes (Rq ~0.22 nm) (Fig.1 (c) and (d)). The cleaning does not deteriorate the back-gated device performance, as indicated by transfer characteristics after the AFM cleaning (Fig.1 (b), no monolayer electrolyte). The device transfer characteristics before and after doping with the monolayer electrolyte will be presented, along with our work on top-gating the devices using h-BN and graphene to create an all-2D EDL FET.
References

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Xu, Ke, et al. ACS nano (2017).

Wang, Wei-Hua, et al. Solid State Ionics 301 (2017): 176-181.

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ACKNOWLEDGEMENTS: This work was supported by NSF-ECCS/GOALI 1408425 and by the Center for Low Energy Systems Technology (LEAST), a STARnet Semiconductor Research Corporation program sponsored by MARCO and DARPA.

Figure 1