Exciton-to-trion conversion as a control mechanism for valley polarization in room-temperature monolayer WS2

Abstract

Transition metal dichalcogenide (TMD) monolayers are two-dimensional semiconductors with two valleys in their band structure that can be selectively addressed using circularly polarized light. Their photoluminescence spectrum is characterized by neutral and charged excitons (trions) that form a chemical equilibrium governed by the net charge density. Here, we use chemical doping to drive the conversion of excitons into trions in $$\text {WS}_{2}$$ WS 2 monolayers at room temperature, and study the resulting valley polarization via photoluminescence measurements under valley-selective optical excitation. We show that the doping causes the emission to become dominated by trions with a strong valley polarization associated with rapid non-radiative recombination. Simultaneously, the doping results in strongly quenched but highly valley-polarized exciton emission due to the enhanced conversion into trions. A rate equation model explains the observed valley polarization in terms of the doping-controlled exciton-trion equilibrium. Our results shed light on the important role of exciton-trion conversion on valley polarization in monolayer TMDs.

Room Temperature Micro-Photoluminescence Studies of Colloidal WS2 Nanosheets

“…The dominating narrow PL peak at ~1.97 eV (X 1 ) might be induced by localized excitons 44 or by trions obtained by an electron surplus due to the sulfur vacancies in colloidal WS 2 nanosheets. [45][46][47][48][49] When comparing the normalized spectra of the colloidal WS 2 monolayer PL at position 3 with the PL of a state-of-the-art hBN-encapsulated exfoliated WS 2 monolayer (sample 3, see Figure 2D), both samples exhibit similar properties in general with colloidal WS 2 nanosheets showing only a fourfold lower PL intensity as the compared exfoliated sample. Surprisingly, the PL features of colloidal WS 2 nanosheets are much stronger pronounced than the exfoliated monolayer.…”

Directional Emission from Tungsten Disulfide Monolayer Coupled to Plasmonic Nanowire‐on‐Mirror Cavity

“…Photoionization and doping are the two main ways to convert exciton into trion. [ 45,46 ] In this case, WS 2 is slightly n ‐doped, so the trions in the system are negatively charged. Exciton to trion conversion is reported by coupling with SPP in metal–insulator–metal (MIM) cavity.…”

“…Photoionization and doping are the two main way to convert exciton into trion [51,52] . In this case, WS2 is slightly n doped, so the trions in the system are negatively charged.…”

Room‐Temperature Low‐Voltage Control of Excitonic Emission in Transition Metal Dichalcogenide Monolayers

“…[11,12] The neutral exciton in TMD monolayers is a bound electron-hole state resulting from the attractive Coulomb interaction, while electron transfer to the conduction band facilitates the formation of negatively charged excitons with tunable emission characteristics. [13][14][15] In contrast to the charge-neutral exciton, the negatively charged exciton preserves the valley polarization and coherence at room temperature, [16][17][18] which makes them promising candidates for valleytronic-based devices for encryption and processing of quantum information.…”