Dense Electron–Hole Plasma Formation and Ultralong Charge Lifetime in Monolayer MoS₂ via Material Tuning

Abstract

Many-body interactions in photoexcited semiconductors can bring about strongly interacting electronic states, culminating in the fully ionized matter of electron–hole plasma (EHP) and electron–hole liquid (EHL). These exotic phases exhibit unique electronic properties, such as metallic conductivity and metastable high photoexcitation density, which can be the basis for future transformative applications. However, the cryogenic condition required for its formation has limited the study of dense plasma phases to a purely academic pursuit in a restricted parameter space. This paradigm can potentially change with the recent experimental observation of these phases in atomically thin MoS2 and MoTe2 at room temperature. A fundamental understanding of EHP and EHL dynamics is critical for developing novel applications on this versatile layered platform. In this work, we studied the formation and dissipation of EHP in monolayer MoS2. Unlike previous results in bulk semiconductors, our results reveal that electromechanical material changes in monolayer MoS2 during photoexcitation play a significant role in dense EHP formation. Within the free-standing geometry, photoexcitation is accompanied by an unconstrained thermal expansion, resulting in a direct-to-indirect gap electronic transition at a critical lattice spacing and fluence. This dramatic altering of the material’s energetic landscape extends carrier lifetimes by 2 orders of magnitude and allows the density required for EHP formation. The result is a stable dense plasma state that is sustained with modest optical photoexcitation. Our findings pave the way for novel applications based on dense plasma states in two-dimensional semiconductors.

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.…”

“…At excitation levels below the threshold, the temperature is about 380 K. When the power is increased to 2.4 mW, catastrophic ionization takes place, and the temperature increases to 460 K. The material lattice expands with increasing excitation, which is consistent with a temperature increase from photothermal heating. As reported elsewhere, lattice expansion in MoS 2 leads to a direct‐to‐indirect band transformation, where the conduction band in the K‐valley and valance band in the Γ‐valley form the lowest electron and hole levels, respectively . As a result, the exciton population increase does not lead to a substantial PL increase.…”

“…As reported elsewhere, lattice expansion in MoS 2 leads to a direct-to-indirect band transformation, where the conduction band in the K-valley and valance band in the Γ-valley form the lowest electron and hole levels, respectively. [8] As a result, the exciton population increase does not lead to a substantial PL increase.…”

Exciton Mott transition revisited

“…This behaviour is rather suggestive of a discontinuous transition, unlike what observed in WS 2 [28]. Surprisingly, photoluminescence stops right after the 500 ns pump pulse [35], which is interpreted as the system undergoing a transformation from direct to indirect gap semiconductor, possibly driven by lattice expansion. All this suggests that photoexcited TMD may show rather interesting properties, especially because of the important role played by Coulomb repulsion.…”

“…The dynamics of photoexcited electron-hole pairs in monolayer TMD has been investigated in many experiments, see, e.g., Refs. [28][29][30][31][32][33][34][35]. At low excitation density, there is consensus that both the excitons and the electronic gap red shift [29,31,32], signalling an important contribution of Coulomb repulsion in the semiconducting state of TMD.…”