Exploring van der Waals Interactions and Deep Levels in Two-Dimensional Semiconductors via Band-Structure Engineering

Recent advances in two-dimensional materials have enabled the emergence of rich physics at the nanoscale due to the interlayer coupling and strong quantum confinement of phonons, electrons, and other particles. Meanwhile, their ultra-thin thickness makes them appealing for potential channel materials in the field-effect transistors and hence draws tremendous attention in the semiconductor technology. Modulation of conduction and valence bands in semiconductors is among the key parameters to engineer and investigate their electronic, optical and thermal properties. Therefore, in this dissertation, hydrostatic pressure and alloying, two well-known and influential methods of tuning the band structure, were utilized to probe the evolution of physical properties in the van der Waals (vdW) crystals./nThe first part of this dissertation focuses on quantifying van der Waals interactions in layered transition metal dichalcogenides (TMDs) from pressure-enhanced valence band splitting. Despite being relatively weak across the gap between neighboring layers, the vdW forces play a crucial role in the band structure evolution in the layered materials, hence profoundly affecting their physical properties. Therefore, gauging and controlling vdW interactions is essential for understanding the physics and developing applications of these materials. In this dissertation, I experimentally probed the vdW interactions in MoS2 and other TMDs by modulating their band structure and measuring the valence band maximum (VBM) splitting (Δ) at K point as a function of pressure in a diamond anvil cell. As high pressure increases interlayer wavefunction coupling, the VBM splitting is enhanced in 2H-stacked MoS2 multilayers but, due to its specific geometry, not in 3R-stacked multilayers, hence allowing the interlayer contribution to be separated out of the total VBM splitting, as well as predicting a negative pressure (2.4 GPa) where the interlayer contribution vanishes. This negative pressure represents the threshold vdW interaction beyond which neighboring layers are electronically decoupled. This approach is compared to first-principles calculations and found to be widely applicable to other group-VI TMDs./nIn chapters 3 and 4, I described chemical trends of deep levels by alloying the MoS2 and WS2. Properties of semiconductors are largely defined by crystal imperfections including native defects. VdW semiconductors are no exception: defects exist even in the purest materials and strongly affect their electrical, optical, magnetic, catalytic, and sensing properties. However, unlike traditional semiconductors where energy levels of most defects were thoroughly documented, they are experimentally unknow in even the best-studied vdW semiconductors, impeding the understanding and utilization of these materials. Thus, in this dissertation, I directly evaluated deep levels in the bandgap of intrinsic MoS2, WS2, and their alloys by transient spectroscopic study. One of the deep levels is found to follow the conduction band minimum of each host, attributed to the native sulfur vacancy. A switchable, DX center - like deep level has also been identified, whose energy lines up instead on a fixed level across different host materials, which explains the chemical trend of native electron density in the hosts as well as a persistent photoconductivity up to 400K.

Advisor: Wu, Junqiao