Scanning tunneling microscopic study of transition metal dichalcogenide ultrathin films

Abstract

Monolayer transition metal dichalcogenides (TMDCs) have attracted considerable scientific and technological interests because of their extraordinary electronic and optoelectronic properties. TMDCs exist in different phases such as hexagonal (2H), octahedral (1T) or monoclinic (1T’) structures. Monolayer 2H phase TMDC is commonly a semiconductor with the direct bandgap showing strong photoluminescence, large exciton binding energy as well as novel spin and valley properties. However, the 1T’ phase is semi-metallic and exhibits some interesting properties such as large magnetoresistance, topological insulator and Weyl semimetal states. In this thesis, monolayer and bilayer WSe2 and MoTe2 films grown by molecular-beam epitaxy (MBE) and their different electronic characteristics as revealed by scanning tunneling microscopy and spectroscopy (STM/S) measurements are presented.

Epitaxial growth of WSe2 by MBE reveals layer-by-layer growth mode on highly ordered pyrolytic graphite (HOPG) substrate. By STM/S studies, bandgap narrowing with increasing film thickness is observed. A novel upward band bending effect on both sides of bilayer steps is noted indicating hole accumulation at the boundaries. In monolayer WSe2, quantum quasi-particle interference (QPI) around point defects is reported affirming the presence of spin–valley coupling and large spin splitting at the Q valleys in the conduction band. Comparing with the first-principles calculations, the QPI patterns can be attributed to spin-conserving scattering processes implying long valley and spin lifetime which is promising for applications in spintronic and valleytronic devices.

In contrast, MBE growth of MoTe2 reveals coexistence of 2H and 1T’ phases. By changing the growth conditions or annealing processes, an obvious phase tuning effect is noted. The nucleation and growth of 1T’ phase is favoured at low growth temperature and high Te flux which can be associated with Te adsorbate on the surface. First-principles calculations also support this finding. The convenient phase control of MoTe2 by MBE makes it possible for phase-transition electronic devices based on TMDCs. Furthermore, dense triangular networks of inversion domain boundaries have been observed in 2H-MoTe2 domains by STM/S studies. Quantum dot states confined in the small triangle domains in the 2H phase are revealed. A quantum interference effect in 1T’-MoTe2 is also reported due to intervalley scattering between two electron pockets.

The findings reported in the thesis illustrate the extraordinary and unique electronic properties of two-dimensional TMDCs, which are of interests for both fundamental physics and device applications.