Persistence of Layer-Tolerant Defect Levels in ReS2

TL;DR

This study investigates defect energetics in ReS2 across thicknesses from monolayer to bulk using density functional theory and a jellium-constrained transfer framework. It reveals layer-tolerant defect levels: both donor and acceptor charge-transition levels remain essentially unchanged with dimensionality for AA and AB stacking, and a neutral two-level defect system with a ~0.40 eV gap persists. The key origin is a combination of exceptionally weak interlayer coupling, minimal quantum confinement, and dielectric screening effects that counterbalance each other, with electronic and structural relaxation terms ($E_{ER}$ and $E_{SR}$) playing critical roles. These findings distinguish ReS2 from other TMDs and position it as a promising platform for thickness-independent optoelectronic and quantum photonic devices, including layer-tolerant single-photon emitters.

Abstract

Defects in two-dimensional (2D) semiconductors play a decisive role in determining their electronic, optical, catalytic and quantum properties. Understanding how defect energy levels respond to variations in layer thickness is essential for achieving reproducible and scalable device performance. We report the persistence of layer-tolerant defect levels in rhenium disulfide (ReS2), where both donor- and acceptor-type charge transition levels remain nearly unchanged from monolayer to bulk in both AA and AB stacking. The associated two-level quantum system also retains its character across thicknesses, enabling ReS2 to serve as a platform for layer-tolerant single-photon emitters. The invariance arises from the interplay between electronic energy minimization and structural relaxation, which together counteract quantum confinement and reduced dielectric screening. Additionally, the intrinsically weak interlayer coupling in ReS2 plays a crucial role. Our findings uncover the microscopic origin of this unique behavior, distinguishing ReS2 from other transitionmetal dichalcogenides and highlighting its potential for thickness-independent optoelectronic and quantum photonic applications.

Figures (4)

• Figure 1: Formation energy of intrinsic point defects in monolayer ReS$_2$ as a function of Fermi level under (a) S-rich and (b) Re-rich conditions. The acceptor and donor levels are represented as solid and hollow circles, respectively. (c) Donor [$\epsilon$(+1/0)] and acceptor [$\epsilon$(0/-1)] charge transition levels for point defects in monolayer ReS$_2$. The valence band and conduction band energy regions are plotted in green and orange, respectively.
• Figure 2: Formation energy as a function of Fermi level of S$_{Re}$ defect in 1L to Bulk ReS$_2$ for (a) AA stacking and (b) AB stacking under S-rich conditions. (c) The cohesive energy of layered ReS$_2$ for AA, AB stacking and layered MoS$_2$ as a function of dimensionality.
• Figure 3: (a) Defect transition-energy levels of S$_{Re}$ as function of number of layers for AA stacking. (b) Schematic of electron occupations for a donor and acceptor for the neutral, negative, and positive charge state. (c) Electronic relaxation energy and (d) structural relaxation energy for AA stacking.
• Figure 4: (a) Static dielectric constant along x, y directions and average as a function of number of layers of AA stacking order. (b) Ionization energy of S$_{Re}$ as a function of average static dielectric constant for AA stacking order. The green and orange bars represent the ionization energy of acceptors and donors, respectively.