Interlayer exciton diffusion in twist-angle dependent moire potentials of MoS₂-MoSe₂ heterobilayers

Chien-Ju Lee^1*, Bo-Han Lin¹, Fu-Hsien Chu¹, Le-Chih Cho¹, Li-Syuan Lu¹, Wen-Hao Chang¹

¹Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan

* Presenter:Chien-Ju Lee, email:chienju1016@gmail.com

Recent progresss in constructing heterobilayers (HBLs) by stacking monolayer van der Waals semiconductors have emerged as a new approach for engineering exciton properties in two-dimensional materials. HBLs of transition metal dichalcogenides can host interlayer excitons (IXs) with electrons and holes separated in different layers. Such IXs have longer lifetime than intralayer excitons and form a dipolar exciton gas with repulsive Coulomb interactions among the aligned vertical dipoles. In addition, moiré superlattice with a periodic potential landscape can be formed in HBLs of TMDs, which can then modulate the electronic structure and confine excitons. The presence of moiré superlattice imposes an additional length scale that can change the dynamics and transport properties of IXs. Here we study the IX diffusion in MoS₂-MoSe₂ HBLs under the influence of moiré periodicity. We prepared a series of HBLs with small twist angles by stacking monolayer MoSe₂ onto MoS₂ with well-aligned orientations grown by chemical vapor deposition. Twist-angle dependent photoluminescence (PL) energy of interlayer exciton (IX) were observed, signifying the formation of moiré potential. We further investigated the IX diffusion by time-resolved PL and spatially resolved PL imaging. Unlike the pure diffusion of intralayer excitons, the Coulomb repulsion of the vertically aligned dipoles causes a pressure-driven expansion of IXs immediately after the creation of them. A drift-diffusion model is studied and used to describe the observed diffusion. The interplay between moiré potentials and dipole-dipole interactions further leads to exciton-density and twist-angle dependent diffusion of IXs. Our results provide insights in understanding the localization and delocalization of interlayer excitons in the moiré superlattice.

Keywords: Transition metal dichalcogenides, Heterostructures, Interlayer exciton