Time dynamics of excitons in monolayers of transition metal dichalcogenides

Abstract The review explores the recent studies carried out to understand various dynamical processes, such as the generation, diffusion, recombination, valley depolarization and decoherence of excitons in monolayers of transition metal dichalcogenides. The rate of recombination of excitons in this class of materials has been found to be several orders of magnitude more than in bulk and quantum well structures of conventional semiconductors. This has been attributed to tighter excitonic confinement and stronger Coulomb interaction due to reduced dielectric screening. Trions are found to have orders of magnitude longer life-time than excitons. Moreover, trions show several orders of magnitude longer valley depolarization and valley decoherence times than the neutral excitons. These features give trions an edge over the neutral excitons in terms of controllability of the valley pseudospin for applications in valley based electronics and quantum information processing. However, very little have been done so far to understand the dynamical aspects of trions. Since large area coverage of the monolayer films is necessary for any practical application, it is important to understand the influence of defects and strain on the time dynamics of excitons and their complexes. Unfortunately, only a few such systematic studies are so far been carried out. Various aspects of the dynamical phenomena of excitons are associated with the strong Coulomb interaction, which in principle can be controlled through capping the monolayer with appropriate dielectrics. Comprehensive investigations are needed to understand the effect of the dielectric environment on the timescales of different dynamical processes associated with the excitons and their complexes. Also, the studies of the dynamical aspects of bigger excitonic complexes, such as biexcitons and charged biexcitons, are still lacking. Since dark excitons are difficult to detect optically, their recombination and phase-coherence time-scales are yet to be explored. This knowledge may be helpful in utilizing these entities for future applications.