Fine structure of negatively charged and neutral excitons in monolayer MoS$_{2}$

TL;DR

This study combines experimental and theoretical approaches to reveal the fine structure of charged excitons in monolayer MoS$_{2}$, identifying novel intra- and inter-valley singlet trions and their implications for excitonic states.

Contribution

It introduces the first detailed identification of intra- and inter-valley singlet trions in monolayer MoS$_{2}$ and links their energy splitting to carrier interactions and dark excitonic ground states.

Findings

Identification of two distinct trion peaks T$_{1}$ and T$_{2}$.

Observation of intra- and inter-valley singlet trions.

Evidence supporting a dark excitonic ground state in MoS$_{2}$.

Abstract

We present experimental and theoretical results on the high-quality single-layer MoS$_{2}$ which reveal the fine structure of charged excitons, i.e., trions. In the emission spectra we resolve and identify two trion peaks, T$_{1}$ and T$_{2}$, resembling the pair of singlet and triplet trion peaks (T$_S$ and T$_{T}$) in tungsten-based materials. However, in polarization-dependent photoluminescence measurements we identify these peaks as novel intra- and inter-valley singlet trions, constituting the trion fine structure distinct from that already known in bright and dark 2D materials with large conduction-band splitting induced by the spin-orbit coupling. We show that the trion energy splitting in MoS$_{2}$ is a sensitive probe of inter- and intra-valley carrier interaction. With additional support from theory we claim that the existence of these singlet trions combined with an anomalous excitonic g-factor and the characteristic temperature dependence of the emission spectra together suggest that monolayer MoS$_{2}$ has a dark excitonic ground state, despite having "bright" single-particle arrangement of spin-polarized conduction bands.