Nonlinear Photoluminescence in Atomically Thin Layered WSe2 Arising from   Diffusion-Assisted Exciton-Exciton Annihilation
  We studied multi-exciton dynamics in monolayer WSe2 using nonlinear photoluminescence (PL) spectroscopy and Monte Carlo simulations. We observed strong nonlinear saturation behavior of exciton PL with increasing excitation power density, and long-distance exciton diffusion reaching several micrometers. We demonstrated that the diffusion-assisted exciton-exciton annihilation model accounts for the observed nonlinear PL behavior. The long-distance exciton diffusion and subsequent efficient exciton-exciton annihilation process determined the unusual multi-exciton dynamics in atomically thin layered transition metal dichalcogenides. 
Shinichiro Mouri, Yuhei Miyauchi, Minglin Toh, Weijie Zhao, Goki Eda,   and Kazunari Matsuda
cond-mat.mes-hall