Low‐dimensional transition metal dichalcogenide (TMDC) materials are heralding a new era in optoelectronics and valleytronics owing to their unique properties. Photo‐induced dynamics in these systems is mostly studied from the perspective of individual quasi‐particles—excitons, bi‐excitons, or, even, trions—their formation, evolution, and decay. The role of multi‐body and exciton dynamics, the associated collective behavior, condensation, and inter‐excitonic interactions remain intriguing and seek attention, especially in room‐temperature scenarios that are relevant for device applications. In this work, the formation and decay of an unexpected electron–hole quantum liquid phase at room‐temperature on ultrafast timescales in multi‐layer MoS2 nanoparticles is evidenced through femtosecond broadband transient absorption spectroscopy. The studies presented here reveal the complete dynamical picture: the initial electron–hole plasma (EHP) condenses into a quantum electron–hole liquid (EHL) phase that typically lasts as long as 10 ps, revealing its robustness, whereafter the system decays through phonons. The authors employ a successful physical model using a set of coupled nonlinear rate equations governing the individual populations of these constituent phases to extract their contributions to bandgap renormalization (BGR). Beyond the observation of the electron–hole liquid‐like state at room temperature, this work reveals the ultrafast dynamics of photo‐excited low‐dimensional systems arising out of collective many‐particle behavior and correlations.
The need for improved UV emitting luminescent materials underscored by applications in optical communications, sterilization and medical technologies is often addressed by wide bandgap semiconducting oxides. Among these, the Mg-doped ZnO system is of particular interest as it offers the opportunity to tune the UV emission by engineering its bandgap via doping control. However, both the doped system and its pristine congener, ZnO, suffer from being highly prone to parasitic defect level emissions, compromising their efficiency as light emitters in the ultraviolet region. Here, employing the process of femtosecond pulsed laser ablation in a liquid (fs-PLAL), we demonstrate the systematic control of enhanced UV-only emission in Mg-doped ZnO nanoparticles using both photoluminescence and cathodoluminescence spectroscopies. The ratio of luminescence intensities corresponding to near band edge emission to defect level emission was found to be six-times higher in Mg-doped ZnO nanoparticles as compared to pristine ZnO. Insights from UV-visible absorption and Raman analysis also reaffirm this defect suppression. This work provides a simple and effective single-step methodology to achieve UV-emission and mitigation of defect emissions in the Mg-doped ZnO system. This is a significant step forward in its deployment for UV emitting optoelectronic devices.
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