  Monolayers of transition-metal dichalcogenides such as WSe2 have become
increasingly attractive due to their potential in electrical and optical
applications. Because the properties of these 2D systems are known to be
affected by their surroundings, we report how the choice of the substrate
material affects the optical properties of monolayer WSe2. To accomplish this
study, pump-density-dependent micro-photoluminescence measurements are
performed with time-integrating and time-resolving acquisition techniques.
Spectral information and power-dependent mode intensities are compared at 290K
and 10K for exfoliated WSe2 on SiO2/Si, sapphire (Al2O3), hBN/Si3N4/Si, and
MgF2, indicating substrate-dependent appearance and strength of exciton, trion,
and biexciton modes. Additionally, one CVD-grown WSe2 monolayer on sapphire is
included in this study for direct comparison with its exfoliated counterpart.
Time-resolved micro-photoluminescence shows how radiative decay times strongly
differ for different substrate materials. Our data indicates exciton-exciton
annihilation as a shortening mechanism at room temperature, and subtle trends
in the decay rates in correlation to the dielectric environment at cryogenic
temperatures. On the measureable time scales, trends are also related to the
extent of the respective 2D-excitonic modes' appearance. This result highlights
the importance of further detailed characterization of exciton features in 2D
materials, particularly with respect to the choice of substrate.
