Abstract
Atomically thin transition metal dichalcogenides (TMDCs) exhibit rich quantum transport phenomena because their band extrema, spin–valley structure, and electronic phases can be strongly modified by thickness, electrostatic gating, and structural engineering. This thesis investigates two quantum transport problems in atomically thin MoS2: K-valley transport in multilayer 2H-MoS2 and topological edge transport in phase-engineered monolayer MoS2.
For multilayer 2H-MoS2, this thesis demonstrates monolayer-like K-valley electron transport through magnetotransport measurements, contrary to earlier theoretical predictions based on thickness-driven band evolution that the conduction-band minimum of multilayer MoS2 should shift from K to the Q valleys in the first Brillouin zone. This conclusion is consistently supported by the twofold Landau-level degeneracy, the extracted cyclotron effective mass, and the density-dependent even to odd transition of the filling-factor sequences. At higher carrier density, the transport enters a multiband regime, where the first two populated bands behave as spin–orbit-split subbands of the same layer and remain largely insensitive to the external electric field, which contrasts with the behavior observed in thinner layers of MoS2. Qualitative density functional theory analysis indicates that electron doping together with enhanced interlayer coupling can account for this unexpected K-valley transport behavior in multilayer MoS2.
For monolayer MoS2, this thesis develops an on-device phase-engineering strategy to induce metastable 1T′–1H in-plane heterostructures. Raman spectroscopy and cross-sectional scanning transmission electron microscopy confirm the formation of the induced 1T′ phase. Transport measurements reveal quantized resistance plateaus close to h/2e2 at temperatures up to 50 K over channel lengths up to 5 μm. When additional metal electrodes partition the conducting channel, the plateau resistance scales with the number of channel segments, as expected for helical edge transport and supported by theoretical simulations. The quantized transport is further strongly suppressed by magnetic fields, consistent with topological edge transport.
Taken together, these results show that atomically thin MoS2 provides a versatile transport platform in which both valley electrons in multilayers and topological edge states in phase-engineered monolayers can be identified and studied through electrical transport.