Electronic properties of various two-dimensional materials

Abstract

Measuring the physical properties of 2D materials and correlating the spatial variation of these physical properties to the structure provides a route to better understand these materials. A very powerful tool for probing the surface structure and local electronic properties is scanning tunneling microscopy. Scanning tunneling spectroscopy allows the observation of electronic spectral properties with a resolution down to the atomic scale. In this thesis, we mainly employed scanning tunneling microscopy and scanning tunneling spectroscopy to study the structure and electronic properties of different types of 2D materials. Such fundamental research can help us to understand the 2D materials at the nano-scale, and pave the way towards the application of these materials in future electronic devices. In this thesis, we have studied the structural and electronic properties of various 2D materials by using scanning tunneling microscopy and scanning tunneling spectroscopy techniques. Their intrinsic electronic properties are modified via varied ways, such as stacking, hydrogenation, defects and so on. The main points are as follows. In the twisted graphene with a twisted angle of 2o, we have revealed the Van Hove singularities and spatially mapped the electronic structure with the atomic resolution. With the help of dI/dz spectroscopy, we have proved the growth behavior of silicon on TMDs at room temperature. We propose that the silicon atoms prefer to intercalate in the TMDs at the beginning of the growth due to defects in TMDs. For the first time, we have successfully synthesized germanene on a bandgap material, which paves the way towards determining the intrinsic properties of this novel 2D buckled Dirac material. The existence of charge puddles in germanene on MoS2 substrate is resulted from the charge-donating impurities in MoS2. Hydrogenation of germanene, synthesized on Ge2Pt nanocrystals, results in the opening of a bandgap of about 0.5 eV, which means hydrogenated germanene is a promising material for applications of field-effect devices. The high density of defects in HfSe2 mainly degrade its electrical performance, thus the quality of HfSe2 crystal should be carefully evaluated before it is used to make devices.