Scanning Tunneling Microscope Investigation on the Atomistic and Electronic Structures of Two-Dimensional

Abstract

Two dimensional (2D) materials have been taking much attention from researchers due to their promising characteristic for making devices. The 2D van der Waals (vdW) layered materials can be fabricated by a simple mechanical exfoliation from the bulk because of weak vdW coupling. In the 2D limit where electrons can only move freely in the plane perpendicular to the direction of confinement, the contribution on physical properties of the material surface becomes dominant, leading to demand for understanding the surface structure. For this reason, we use scanning tunneling microscopy (STM) to directly observe the surface structure at the atomic level of some materials such as layered vdW crystals (SnSe, SnSe1-xSx, Fe5-xGeTe2), monolayer transition-metal dichalcogenides (TMDs) (VSe2 and ReSe2) on bilayer graphene (BLG), and ultrathin Cu2O films. In this thesis, a brief introduction to 2D materials and scanning tunneling microscope are shown in chapter 1 and 2, respectively. In chapter 3, we figure out the origin of p-type electrical characteristic in SnSe and the alloying behavior of SnSe1-xSx which are helpful for the optimization of thermoelectric properties of SnSe-based materials. Moreover, we find that the non-centrosymmetricity originated from √3×√3 orderings of the Fe(1) - Ge pair resulting in helical magnetism in Fe5-xGeTe2, which is included in chapter 4. Following that, STM results regarding the charge density wave in monolayer VSe2 on BLG and the trapped charge structure at ReSe2-graphene interface are discussed in chapter 5. Finally, chapter 6 is intended for the surface structure and the origin of hole source in ultrathin Cu2O films.