Emergence and Relaxation of an e-h Quantum Liquid Phase in Photoexcited MoS2 Nanoparticles at Room Temperature

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.