Direct Measurement of Biexcitons in Monolayer WS2

TL;DR

This study directly measures and characterizes biexcitons in monolayer WS2 using advanced spectroscopy, providing clear evidence of their binding energy and valley composition, which clarifies previous ambiguities.

Contribution

The paper demonstrates the first direct measurement of biexcitons in monolayer WS2 using 2Q-MDCS, revealing their binding energy and valley configuration.

Findings

Biexciton binding energy is 26 ± 2 meV.

Biexcitons are composed of two bright excitons in opposite valleys.

2Q-MDCS effectively isolates doubly-excited states in TMDCs.

Abstract

The optical properties of atomically thin transition metal dichalcogenides (TMDCs) are dominated by Coulomb bound quasi-particles, such as excitons, trions, and biexcitons. Due to the number and density of possible states, attributing different spectral peaks to the specific origin can be difficult. In particular, there has been much conjecture around the presence, binding energy and/or nature of biexcitons in these materials. In this work, we remove any ambiguity in identifying and separating the optically excited biexciton in monolayer WS2 using two-quantum multidimensional coherent spectroscopy (2Q-MDCS), a technique that directly and selectively probes doubly-excited states, such as biexcitons. The energy difference between the unbound two-exciton state and the biexciton is the fundamental definition of biexciton binding energy and is measured to be 26 \pm 2 meV. Furthermore, resolving the biexciton peaks in 2Q-MDCS allows us to identify that the biexciton observed here is composed of two bright excitons in opposite valleys.