Grain boundaries and quantum transport in monolayer MoS2

The growing research on two-dimensional materials reveals their exceptional physical properties and enormous potential for future applications and investigation of advanced physics phenomena. They represent the ultimate limit in terms of active channel thickness as they consist only of few atomic layers and hold a great promise for future scaling of electronic devices. Further, their unique band structure provides a platform for the exploration of novel physics in the field of spin and valleytronics. The presence of a large band gap allows fully electrostatic control over doping and current flow, as well as the realization of novel optoelectronic devices. On of the most appealing and readily available 2D material is molybdenum disulphide (MoS2). This thesis is concentrated on the electronic properties of monolayers MoS2 ranging from the investigation of line defects in synthetically grown material to advanced study of the confinement of charge carriers at low temperature as function of magnetic field. A chapter is describing the details of each of the principal experiments. In Chapter 4 we study the properties of epitaxially grown MoS2 monolayers by means of electrical measurements and advanced scanning probe techniques. After confirming the high quality of the grown crystallites, we investigate the predominant types of occurring grain boundaries by means of Kelvin probe force microscopy. Due to the highly oriented growth, we find that the electronic properties of the film are homogeneous even across the line defects. This fact is confirmed by the observation of constant mobility even in devices over large area containing multiple grain boundaries. Chapter 5 presents different approaches for the fabrication of high-quality dual-gated MoS2 field effect transistors (FETs). Dielectric layers deposited by means of atomic layer deposition can guarantee good mobility and low contact resistance, but the observation of low-dimensional effects may be hindered due to the presence of charged impurities. On the other hand the use of hexagonal boron nitride as dielectric and few layer graphene as contact material is shown to yield very high mobility and low contact resistance. Using dual gated FETs with such material configuration, an electrostatically controlled quantum point contact (QPC) is realized in monolayer MoS2 and presented in Chapter 6. We find that the device conductance is quantized in multiples of e^2/h revealing the lifting of spin and valley degeneracies. Further, we perform bias spectroscopy at different magnetic fields and extract subband energy spacing from 0.8 (at 0T) to 2 meV (at 9.8T) and g-factor in the conduction band of 2.12±0.13. Finally, in Chapter 7 we propose several methods aiming the fabrication of a quantum dot in MoS2. The electrostatic definition of a small conductive island, weakly couples to two highly doped charge carrier reservoirs, requires similar advanced engineering of the dielectric material environment. Despite the low stability, we find possible regimes, in which controlled tunneling from and to a well defined quantum dot can be detected. The charging energy measured electrically is in very good agreement with the lithographically defined size of the island. In conclusion, this work provides insights about the electron transport in monolayer MoS2 exploring grain boundaries, dielectric environment and electron confinement. The results contribute to better understanding of its electronic properties.