Many-particle induced band renormalization processes in few- and mono-layer MoS2.

Band renormalization effects play a significant role for two-dimensional (2D) materials in designing a device structure and customizing their optoelectronic performance. However, the intrinsic physical mechanism about the influence of these effects cannot be revealed by general steady-state studies. Here, band renormalization effects in organic superacid treated monolayer MoS2, untreated monolayer MoS2and few-layer MoS2are quantitatively analyzed by using broadband femtosecond transient absorption spectroscopy. In comparison with the untreated monolayer, organic superacid treated monolayer MoS2maintains a direct bandgap structure with two thirds of carriers populated at K valley, even when the initial exciton density is as high as 2.05 × 1014cm-2(under 400 nm excitations). While for untreated monolayer and few-layer MoS2, many-particle induced band renormalizations lead to a stronger imbalance for the carrier population between K and Q valleys inkspace, and the former experiences a direct-to-indirect bandgap transition when the initial exciton density exceeds 5.0 × 1013cm-2(under 400 nm excitations). Those many-particle induced band renormalization processes further suggest a band-structure-controlling method in practical 2D devices.

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

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.

Indirect Bandgap Puddles in Monolayer MoS2 by Substrate-Induced Local Strain.

An unusually large bandgap modulation of 1.23-2.65 eV in monolayer MoS2 on a SiO2 /Si substrate is found due to the inherent local bending strain induced by the surface roughness of the substrate, reaching the direct-to-indirect bandgap transition. Approximately 80% of the surface area reveals an indirect bandgap, which is confirmed further by the degraded photoluminescence compared to that from suspended MoS2 .