Electrical Transport in Suspended Two-Dimensional Materials
Two-dimensional (2D) materials are ones that are only single to a few atomic layers in thickness, and exhibit novel material properties not found in their three-dimensional counterparts. The research of 2D materials starts to take off after the isolation and transport measurements of graphene in 2004. Graphene has a unique dispersion relation and unprecedented material properties such as extreme mechanical strength, high thermal conductivity, and exceedingly high charge carrier mobility. The first part of this thesis describes the fabrication and transport measurements of suspended twisted bilayer graphene devices.
The second part of this dissertation focuses on 2D molybdenum disulfide (MoS2), which, unlike graphene, has a band gap. However, its low mobility severely restricted the applications of MoS2 and the mechanism for mobility bottleneck is unclear. To investigate the role played by substrates, we fabricate suspended MoS2 field effect transistor devices and develop an effective gas annealing technique that significantly improves device quality and increases conductance by 3-4 orders of magnitude. Mobility of the suspended devices ranges from 0.01 to 46 cm2/Vs before annealing, and from 0.5 to 105 cm2/Vs after annealing. Temperature dependence measurements reveal two transport mechanisms: electron-phonon scattering at high temperatures and thermal activation over a gate-tunable barrier height at low temperatures. Our results suggest that transport in these devices is not limited by the substrates, but likely by defects, charge impurities and/or Schottky barriers at the metal-MoS2 interfaces.
Furthermore, we successfully apply ionic liquid for electrolyte gating on suspended MoS2 transistors and achieve very high coupling efficiency, up to 4.4×1013 cm-2V-1. Electrical characterization reveals contact-dominated electrical transport. From the Schottky emission model, the dielectric constant of ionic liquid DEME-TFSI is estimated to be ~11. Comparison between ionic liquid gating of substrate-supported and suspended devices also demonstrates far higher doping efficiency and better screening of charge impurities as well as Schottky barriers.
Recently, the realization of one-dimensional electrical contacts to hexagonal-boron nitride-encapsulated samples points a direction for transport studies of 2D materials. For instance, such MoS2 devices with graphene contacts have demonstrated unprecedented mobility. Furthermore, heterostructures consisting of various 2D atomic layers may be built to create artificial superlattices, thus enable the exploration of novel phenomena and devices with new functionalities.