Modulation of Electronic Properties in Monolayer–Bilayer Junctions of Transition-Metal Dichalcogenides: Combined First-Principles Calculation and Potential Energy Simulation

Transition-metal dichalcogenides (TMDs) have emerged as promising candidates for advanced nanoelectronic materials owing to their atomically thin nature and tunable electronic properties. However, unavoidable thickness nonuniformity during material growth can introduce in-plane layer discontinuities, which fundamentally modify the local electronic structure. In this work, we demonstrate that in-plane thickness discontinuities in TMDs can be regarded as intrinsic lateral heterojunctions, giving rise to localized interface states and effective transport barriers. By establishing a connection between atomistic electronic structure and device-relevant behavior, we present first-principles density-functional theory (DFT) analysis of in-plane monolayer–bilayer MoS2, MoTe2, WS2, and WSe2, combined with electrostatic potential simulations. The band gap exhibits a pronounced thickness dependence, with a large deviation ranging from 10.7–19.1% at the Heyd–Scuseria–Ernzerhof level when transitioning from monolayers to bilayers, while the change rapidly saturates for thicker layers (e.g., ∼5% from bilayers to trilayers). In contrast to conventional vertical two-dimensional (2D) heterostructures, the in-plane monolayer–bilayer junction introduces hybridized interface states within the forbidden gap. To gain deeper insight into this effect, we performed electrostatic potential simulations coupled with material parameters extracted from DFT calculations to evaluate the effective junction gap to characterize the local transport barrier. The simulation results reveal that the monolayer–bilayer heterojunctions form a type-I band alignment. Specifically, a monolayer–bilayer–monolayer configuration acts as a carrier confinement region, whereas a bilayer–monolayer–bilayer configuration behaves as a quantum well. Such thickness-induced band offsets and interface hybridization can introduce nonuniform electrostatic potentials and undesired carrier localization in device channels. These findings establish in-plane thickness discontinuity as an intrinsic heterojunction in TMDs and provide a quantitative framework linking atomic-scale electronic modulation to device-relevant transport behavior, with important implications for the design and variability of future nanoelectronic devices.