Exciton localization in MoSe2 monolayers induced by adsorbed gas molecules

Abstract

Lattice defects and dielectric environment play a crucial role for 2D materials. Gas molecules can get physisorbed easily on the surface through van der Waals forces and can modify dramatically their electronic and optical properties. In this work, we investigate the impact of the physisorbed gas molecules on the optical properties of MoSe2 monolayers by means of low-temperature photoluminescence (PL). More specifically, we focus on the physics of excitons localized by gas molecules. The associated PL peak is observed to show a systematic and large red-shift with temperature and a blue-shift with laser irradiation. Both energy shifts are explained in terms of thermal instability of the localization in combination with hopping effects. Finally, a model is presented, which can reproduce the experimental data with excellent agreement.

Unveiling Valley Lifetimes of Free Charge Carriers in Monolayer WSe2

“…We suspect that this temporal change is due to an degassing of initially adsorbed molecules on top of the devices due to laser illumination. 7 In this context, we note that one cooling cycle usually extends over five days before we have to change the liquid helium container. At this point, the device necessarily has to be warmed up to room temperature and residuals from gas permeation through the elastomer sealing gaskets, micro-leakages or other gas loads my absorb on the devices.…”

“…With band gaps in the optically visible energy range 1-3 and spin-split valleys which allow the creation of valley-and spin-polarized excitons by circularly polarized light, 4,5 monolayer (ML) transition metal dichalcogenides (TMDs) are a very promising family of materials in the field of both spin-and valleytronics. 4,[6][7][8] However, the full potential of TMD-based devices for valleytronic applications critically depends on their valley lifetime of free charge carriers. So far, there is a huge variation in experimentally reported valley lifetimes, which range from the picosecond up to the microsecond timescale.…”

Tailoring of Bound Exciton Photoluminescence Emission in WS2 Monolayers

“…Excitons in the defect band region can be bounded to lattice defects . In addition, Venanzi et al have shown that the defect‐bound exciton band consists of excitons that are localized by physiosorbed gas molecules on the MoSe 2 monolayer surface. This phenomenon appears also in other TMDs like WS 2 .…”

Photoluminescence dynamics in few-layer InSe

“…for disordered semiconductors [35,36]. The idea is to incorporate the disorder, that in our case shows up as inhomogeneous broadening of the PL line, in the effective temperature.…”