Electronic Transport And Device Applications Of Two Dimensional Materials And Their Heterostructures

Since the discovery of graphene, two dimensional materials have attracted tremendous attentions. Two dimensional materials have many interesting properties and potential applications. They are expected to play a crucial role in the coming post-Moore era and have many important applications such as electronic and photonic devices. Two dimensional materials have also shown great potential for other functional devices. In the past few years, the heterostructures constituted by different layers of two dimensional materials are becoming emerging as top candidates for device design.Here we investigated the properties of graphene, MoS2, ReS2 and their heterostructures. We fabricated several devices based on these two dimensional materials, such as field effect transistors, digital inverters and photodetectors.At the beginning, we gave a brief introduction about the basic properties and applications of two dimensional materials and their heterostructures, including graphene, transition metal dichalcogenides, black phosphorus. Then we described the measurement platforms and device fabrication which were used in our experiments. Measurement platforms include electrical measurement at room temperature, transport measurement under large magnetic field at low temperature and photoelectric measurement at room temperature. We also introduced the ionic liquid gating technique and transfer methods for the fabrication of heterostructures.In chapter four, we present atomically thin rhenium disulfide flakes with unique distorted 1T structure, which exhibit in-plane anisotropic properties. We fabricated mono-and few-layer ReS2 field effect transistors, which exhibit large current on/off ratios (-107) and low subthreshold swings (100 mV dec-1). The observed anisotropic ratio along two principle axes reaches 3.1, which is the highest among all known two-dimensional semiconducting materials.We successfully demonstrated an integrated digital inverter with good performance by utilizing two ReS2 anisotropic field effect transistors. And we present high responsivity phototransistors based on few-layer ReS2. Depending on the back gate voltage, source drain bias and incident optical light intensity, the maximum attainable photoresponsivity could reach as high as 88,600 A W-1, which is a record value compared to other two-dimensional materials with similar device structures. Such high photoresponsivity is attributed to the increased light absorption as well as the gain enhancement due to the existence of trap states in the few-layer ReS2 flakes. It further enables the detection of weak signals, as successfully demonstrated with weak light sources including a lighter and limited fluorescent lighting.Graphene is the thinnest conductive and elastic material. The electro-mechanical properties of graphene have been studied widely. In chapter six, we observed the positive piezoconductive effect in suspended bi-and multi-layer graphene. The effect is resulting from the strain-induced competition between interlayer coupling and intralayer hopping, as confirmed by numerical simulation based on the non-equilibrium Green’s function method.We obtained the superconducting transformation in bilayer MoS2 by using the ionic liquid gating. The superconducting current is affected by temperature and magnetic field. Using the back gate, we realized the switch on/off of the supercurrent.Then we proposed a p-graphene-n heterostructure photodetector. The photovoltaic effects and diode-like current rectification behavior were observed. Broadband response detectors were demonstrated from visible to the mid-infrared range. The detectors exhibit a remarkable responsivity which is greater than 103 A W-1 and a specific detectivity greater than 1012 Jones at visible range. These results open a new way to fabricate the low energy consumption broadband photovoltaic detectors.At last we summarized our researches in chapter seven, and listed some plans for furture investigation.