Room-Temperature Electron–Hole Liquid in Monolayer MoS₂

Abstract

Excitons in semiconductors are usually noninteracting and behave like an ideal gas, but may condense to a strongly correlated liquid-like state, i.e., electron–hole liquid (EHL), at high density and appropriate temperature. An EHL is a macroscopic quantum state with exotic properties and represents the ultimate attainable charge excitation density in steady states. It bears great promise for a variety of fields such as ultra-high-power photonics and quantum science and technology. However, the condensation of gas-like excitons to an EHL has often been restricted to cryogenic temperatures, which significantly limits the prospect of EHLs for use in practical applications. Herein we demonstrate the formation of an EHL at room temperature in monolayer MoS2 by taking advantage of the monolayer’s extraordinarily strong exciton binding energy. This work demonstrates the potential for the liquid-like state of charge excitations to be a useful platform for the studies of macroscopic quantum phenomena and the development of optoelectronic devices.

Near Band‐Edge Optical Excitation Leading to Catastrophic Ionization and Electron–Hole Liquid in Room‐Temperature Monolayer MoS2

“…Compared to the exciton emission at low fluence above the threshold, the spectrum is red-shifted by 70 meV and has a much broader line width (%175 meV). This new PL spectral line shape is identical (width, peak energy, and intensity) to the PL from dense EHP and EHL states created with above-gap excitation, as we have reported elsewhere, [6,8] and also in Figure S1, Supporting Information. We conclude this sharp threshold at elevated photoexcitation originates from a phase transition of excitons into EHL.…”

“…This effect is described as “ionization catastrophe” in the literature and has been observed in several semiconductors . Recent efforts in 2D materials, specifically MoS 2 , WS 2 , and MoTe 2 have revealed such a phase transition under intense photoexcitation . In these observations, ultrathin semiconductors were excited using photon energies higher than the band gap, which created a sufficiently dense population of excitons.…”

“…[10][11][12][13][14] Recent efforts in 2D materials, specifically MoS 2 , WS 2 , and MoTe 2 have revealed such a phase transition under intense photoexcitation. [6,15,16] In these observations, ultrathin semiconductors were excited using photon energies higher than the band gap, which created a sufficiently dense population of excitons. Astonishingly, the below-gap photoexcitation that was utilized in this study triggered ionization of excitons and a phase transition into the EHL phase.…”

“…Hence experiments have to be under cryogenic conditions and laser illumination above the band gap . Recent work on 2D MoS 2 showed that it is possible to create an EHL phase in 2D transition metal dichalcogenides (TMDs) at room temperature . Here we show that it is possible to excite electron–hole liquid (EHL) and dense electron–hole plasma (EHP) phases in 2D MoS 2 with the laser wavelength tuned slightly below the band gap of the material.…”

“…[4,5] Recent work on 2D MoS 2 showed that it is possible to create an EHL phase in 2D transition metal dichalcogenides (TMDs) at room temperature. [6] Here we show that it is possible to excite electron-hole liquid (EHL) and dense electron-hole plasma (EHP) phases in 2D MoS 2 with the laser wavelength tuned slightly below the band gap of the material. While reaching a critical excitation density for the EHL phase transition is not possible with below-gap excitation in conventional semiconductors, such a phase transition is possible in 2D materials because the thermal heating can significantly shift the absorption.…”