Theory and Ab Initio Computation of the Anisotropic Light Emission in Monolayer Transition Metal Dichalcogenides

Abstract

Monolayer transition metal dichalcogenides (TMDCs) are direct gap semiconductors with a unique potential for use in ultrathin light emitters. However, their photoluminescence (PL) is not completely understood. We develop an approach to compute the radiative recombination rate in monolayer TMDCs as a function of photon emission direction and polarization. Using exciton wavefunctions and energies obtained with the ab initio Bethe-Salpeter equation, we obtain polar plots of the PL for different scenarios. Our results can explain the PL anisotropy and polarization dependence measured in recent experiments and predict that light is emitted with a peak intensity normal to the exciton dipole in monolayer TMDCs. We show that excitons emit light anisotropically upon recombination when they are in any quantum superposition state of the K and K' inequivalent valleys. When averaged over the emission angle and exciton momentum, our new treatment recovers the temperature-dependent radiative lifetimes that we previously derived. Our work demonstrates a generally applicable first-principles approach to studying anisotropic light emission in two-dimensional materials.

Saddle‐Point Excitons and Their Extraordinary Light Absorption in 2D β‐Phase Group‐IV Monochalcogenides

“…Given their potential applications in novel optoelectronic and photovoltaic devices, the anisotropic optical properties of several 2D materials have been carefully studied, including phosphorene and chalcogenides . In the following, the anisotropic absorption in the four studied monolayer β‐MXs is investigated taking GeSe as an example.…”

Out-of-plane orientation of luminescent excitons in two-dimensional indium selenide

“…The creation and recombination of excitons, electron–hole pairs bound by Coulomb forces 1 , mediates light–matter interaction in semiconductors. The exciton transition dipole moment can be highly anisotropic, with the dipole’s strength and orientation dictated by the particular electronic properties of the host semiconductor and its dipole selection rules 2,3 . This fundamental relationship implies that different low-dimensional semiconductor structures with varying electronic properties can be found to yield in-plane (IP), out-of-plane (OP) or mixed dipole orientations.…”

Ab initiocalculations of exciton radiative lifetimes in bulk crystals, nanostructures, and molecules

“…Novel 2D semiconductors, such as transition metal dichalcogenides and related layered materials, exhibit unique optical properties and strongly bound excitons that govern their light absorption and emission [35]. We have recently proposed a first-principles approach to compute the radiative lifetime in such 2D materials, as well as its angular and polarization dependence, which gives rise to anisotropic light emission [13,14]. In our approach, exciton recombination is still described using the Fermi Golden rule in Eq.…”

“…Computed radiative lifetimes in a GaAs crystal, shown as a function of temperature up to 50 K. The lifetimes are obtained using the thermal average in Eq (14)…”