Probing critical point energies of transition metal dichalcogenides: surprising indirect gap of single layer $SL-WSe_2$

TL;DR

This study uses advanced scanning tunneling spectroscopy to reveal that single-layer WSe2 has an indirect quasiparticle gap with the conduction band minimum at the Q point, challenging previous assumptions of a direct gap.

Contribution

It provides detailed quasiparticle electronic structures of TMDs, clarifying the nature of the band gap in single-layer WSe2 and highlighting the importance of experimental techniques.

Findings

Single-layer WSe2 has an indirect quasiparticle gap.

The conduction band minimum is at the Q point, not K.

Rich quasiparticle structures depend on atomic and spin-orbital factors.

Abstract

Understanding quasiparticle band structures of transition metal dichalcogenides (TMDs) is critical for technological advances of these materials for atomic layer electronics and photonics. Although theoretical calculations to date have shown qualitatively similar features, there exist subtle differences which can lead to important consequences in the device characteristics. For example, most calculations have shown that all single layer (SL) TMDs have direct band gaps, while some have shown that $SL-WSe_2$ have an indirect gap. Moreover, there are large variations in the reported quasiparticle gaps, corresponding to large variations in exciton binding energies. By using a comprehensive form of scanning tunneling spectroscopy, we have revealed detailed quasiparticle electronic structures in TMDs, including the quasi-particle gaps, critical point energy locations and their origins in the Brillouin Zones (BZs). We show that $SL-WSe_2$ actually has an indirect quasi-particle gap with the conduction band minimum located at the Q point (instead of K), albeit the two states are nearly degenerate. Its implications on optical properties are discussed. We have further observed rich quasi-particle electronic structures of TMDs as a function of atomic structures and spin-orbital couplings.