Origin of indirect optical transitions in few-layer MoS2, WS2 and WSe2

TL;DR

This study investigates the origin of indirect optical transitions in few-layer MoS2, WS2, and WSe2 by analyzing temperature-dependent photoluminescence shifts, revealing valley competition and the conduction band minimum's nature.

Contribution

It provides new insights into the valley structure and the origin of indirect emissions in few-layer MX2 materials through temperature-dependent PL analysis.

Findings

Identified the origin of indirect emission peaks.

Determined the relative energy of K and Λ valleys.

Showed valley competition in WSe2.

Abstract

It has been well established that single layer MX2 (M=Mo,W and X=S,Se) are direct gap semiconductors with band edges coinciding at the K point in contrast to their indirect gap multilayer counterparts. In few-layer MX2, there are two valleys along the {\Gamma}-K line with similar energy. There is little understanding on which of the two valleys forms the conduction band minimum (CBM) in this thickness regime. We investigate the conduction band valley structure in few-layer MX2 by examining the temperature-dependent shift of indirect exciton PL. Hihgly anisotropic thermal expansion of the lattice and corresponding evolution of the band structure result in distinct peak shift for indirect transitions involving the K and {\Lambda} (midpoint along {\Gamma}-K) valleys. We identify the origin of the indirect emission and concurrently determine the relative energy of these valleys. Our results show that the two valleys compete in energy in few-layer WSe2.