  As a platform for optoelectronic devices based on exciton dynamics, monolayer
transition metal dichalcogenides (TMDCs) are often placed near metal interfaces
or inside planar cavities. While the radiative properties of point dipoles at
metal interfaces has been studied extensively, those of excitons, which are
delocalized and exhibit a temperature-dependent momentum distribution, lack a
thorough treatment. Here, we analyze the emission properties of excitons in
TMDCs near planar metal interfaces and explore their dependence on exciton
center-of-mass momentum, transition dipole orientation, and temperature.
Defining a characteristic energy scale $k_B T_c = (\hbar k)^2/2m$~($k$ being
the radiative wavevector and $m$ the exciton mass), we find that at
temperatures $T\gg T_c$ and low densities where the momentum distribution can
be characterized by Maxwell-Boltzmann statistics, the modified emission
rates~(normalized to free space) behave similarly to point dipoles at
temperatures $T\gg T_c$. This similarity in behavior arises due to the broad
nature of wavevector components making up the exciton and point dipole
emission. On the other hand, the narrow momentum distribution of excitons for
$T<T_c$ can result in significantly different emission behavior as compared to
point dipoles. These differences can be further amplified by considering
excitons with a Bose Einstein distribution at high phase space densities. We
find suppression or enhancement of emission relative to the point dipole case
by several orders of magnitude. These insights can help optimize the
performance of optoelectronic devices that incorporate 2D semiconductors near
metal electrodes and can inform future studies of exciton radiative dynamics at
low temperatures. Additionally, these studies show that nanoscale optical
cavities are a viable pathway to generating long-lifetime exciton states in
TMDCs.
