Identifying Defect-Induced Trion in Monolayer WS₂ via Carrier Screening Engineering

Abstract

Unusually high exciton binding energies (BEs), as much as ∼1 eV in monolayer transition-metal dichalcogenides, provide opportunities for exploring exotic and stable excitonic many-body effects. These include many-body neutral excitons, trions, biexcitons, and defect-induced excitons at room temperature, rarely realized in bulk materials. Nevertheless, the defect-induced trions correlated with charge screening have never been observed, and the corresponding BEs remain unknown. Here we report defect-induced A-trions and B-trions in monolayer tungsten disulfide (WS2) via carrier screening engineering with photogenerated carrier modulation, external doping, and substrate scattering. Defect-induced trions strongly couple with inherent SiO2 hole traps under high photocarrier densities and become more prominent in rhenium-doped WS2. The absence of defect-induced trion peaks was confirmed using a trap-free hexagonal boron nitride substrate, regardless of power density. Moreover, many-body excitonic charge states and their BEs were compared via carrier screening engineering at room temperature. The highest BE was observed in the defect-induced A-trion state (∼214 meV), comparably higher than the trion (209 meV) and neutral exciton (174 meV), and further tuned by external photoinduced carrier density control. This investigation allows us to demonstrate defect-induced trion BE localization via spatial BE mapping in the monolayer WS2 midflake regions distinctive from the flake edges.

Room Temperature Micro-Photoluminescence Studies of Colloidal WS2 Nanosheets

“…We assign the nearly constant offset in this energy range to multilayer PL and the transition X 2 to the neutral exciton. The dominating narrow PL peak at ∼1.97 eV ( X 1 ) might be induced by localized excitons or by trions formed by an electron surplus due to sulfur vacancies in colloidal WS 2 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.…”

Engineering Radiative Energy Transfer and Directional Excitonic Emission in van der Waals Heterostructures

“…Some other effects cannot be ignored (Figure S9, Supporting Information) such as the dielectric screening, [33,34] bubble-induced absorption [35] and defect-induced trion emission. [36] In addition, the encapsulated effect [37] and the Förster-type energy transfer enhanced the PL intensity if the thickness of hBN is < 20 nm.…”

Fabrication of high-quality PMMA/SiO$_x$ spaced planar microcavities for strong coupling of light with monolayer WS$_2$ excitons

“…3d) also show much stronger degradation of the exciton oscillator strength for the uncapped monolayer compared to the capped one. Furthermore, the uncapped monolayer shows a pronounced shoulder in the PL spectrum at E ≈ 1.97 eV likely stemming from a trion emission [41,48,49], which indicates a significant defect-induced charge doping effect caused by the deposition process [20,43,50]. These results clearly highlight that the PMMA layer protects the monolayer against the dielectric deposition by PECVD.…”

“…The pronounced PL quenching effect is possibly due to the chemical instability of this material. WS 2 crystals are normally n-type semiconductors due to the presence of sulphur vacancies [40][41][42][43], and therefore, they can be strongly affected by the surrounding molecules [44], such as organic and water molecules, that can act as dopants [45]. As a consequence, the PMMA capping process can modify the doping level of the monolayer effectively causing a reduction of the exciton quantum yield [44].…”