Many properties of Two-dimensional (2D) materials are vastly different from those of their 3D counterparts. A large family of 2D materials ranging from gapless graphene to metallic NbSe2 (also superconducting), semiconducting MoS2, and insulating hexagonal boron nitride (h-BN) possessing a broad range of exciting new properties have emerged in recent years. Moreover, 2D materials provide the perfect platform to create laterally and vertically stacked heterostructures with intriguing properties. The physics of 2D materials based heterostructures is extremely interesting and novel 2D-heterostructured devices including tunneling diodes, tunneling transistors, photovoltaic cells, and light-emitting diodes have started to emerge.
In this work, we developed a novel lithography-free technique for the fabrication of 2D material-based electrical devices. We fabricated few-layer and multi-layer WS2 devices using a transmission electron microscope (TEM) grid as a shadow mask, and its transport characteristics were studied by electrical measurements. WS2 samples were synthesized by first depositing WO3 followed by sulfurization and characterized by scanning tunneling microscopy (SEM), atomic force microscopy (AFM), and Raman spectroscopy. Hydrazine adsorption on WS2 was studied by measuring the electrical resistances during adsorption (exposing to hydrazine vapor) and subsequent desorption (by pumping). WS2 sample consisting of two layers showed a decrease of resistance upon exposure to hydrazine vapor and showed complete reversibility upon pumping. WS2 sample with three layers showed a decrease of resistance during exposure but showed only partial recovery during desorption. In contrast, multi-layered (12 layers) WS2 sample showed an initial decrease followed by a continued increase of the resistance upon exposure to hydrazine with little or no reversibility upon pumping. The charge transfer from N2H4 to WS2 is believed to be responsible for the decrease of the resistance. Trapping of N2H4 molecules within the multilayers of WS2 causing charge redistribution and possible chemical reactions is believed to be responsible for the increase in resistance during the adsorption and complete irreversibility of resistance during desorption. The experimental results are explained with the help of computational calculations carried out by employing the density functional theory (DFT) framework, as implemented in the Vienna Ab-initio Simulation Package (VASP).
Next, we extended our lithography-free technique for the fabrication of two-dimensional (2D) material based heterostructures. We fabricated graphene-WS2 heterostructured devices again using a TEM grid as a shadow mask. Graphene was directly deposited on a Si/SiO2 substrate by radio frequency (RF) plasma enhanced chemical vapor deposition (PECVD). WS2 was synthesized as before. The temperature dependence of the resistance and magnetoresistance are measured for graphene, WS2, and graphene-WS2 heterostructure. At low temperatures, the transport was found to follow the variable-range hopping (VRH) process, where logarithmic R exhibits a ?−1/3 temperature dependence, an evidence for the 2D Mott VRH transport. The measured low-field magnetoresistance also exhibits a quadratic magnetic field dependence ~?2, consistent with the 2D Mott VRH transport.
Finally, a lithography-free technique was developed to fabricate Graphene/h-BN/Graphene tunnel junctions. Graphene and h-BN were directly deposited on a Si/SiO2 substrate by RF- PECVD using CH4 and ammonia borane as the precursors respectively. Tunnel diodes with varying barriers were fabricated by tuning the thickness of the h-BN layer thickness. The tunneling current was found to scale exponentially with the tunnel barrier thickness.