The Role of Defect Geometry in Localized Emission from Monolayer Tungsten Dichalcogenides

Abstract

Understanding the mechanism of single photon emission (SPE) in two-dimensional (2D) material is an unsolved problem important for quantum optical materials and the development of quantum information applications. In 2D transition metal dichalcogenides (TMDs) such as tungsten diselenide (WSe2), quantum emission has been broadly attributed to exciton localization from atomic point defects, yet the precise microscopic origins are not fully understood. This work introduces an empirically grounded computational framework that explains both the origins of facile SPE in WSe2 and its relative scarcity in related TMD, tungsten disulfide. High resolution microscopy identifies native defect geometries existing in monolayer WSe2 lattices providing the ingredients necessary to build a realistic model. The qualitative effects of chalcogen type, defect geometry, and mechanical strain on the electronic structure are then individually assessed using density functional theory, from which a specific divacancy configuration emerges as the candidate for localized single-electron transitions that match observed spectral energies. Spectroscopy and photon correlation measurements further validate this model, establishing a self-consistent link between defect geometry, electronic structure, and quantum emission. By isolating the distinct roles of chalcogen type, defect configuration, and mechanical strain, this work provides a thorough investigation of exciton localization and optical behavior, contributing to a clearer picture of the physical drivers of single photon emission in tungsten-based TMDs.

Figures (5)

• Figure 1: (a) HAADF-STEM image of an approximately 4 nm $\times$ 4 nm area of monolayer $\text{WSe}_{\text{2}}$, with outlined intensity profiles (1,2) quantified in (c). (b) A close-up image of the single and double vacancies outlined by the white box in (a), visually identifying a vertical divacancy (V2) and a single top-plane vacancy (V1).
• Figure 2: Schematic of the pristine crystal and modeled defect configurations: monovacancy (V1), vertical divacancy (V2), and lateral divacancy (V-V). Chalcogen atoms are shown in yellow, while tungsten atoms are shown in grey.
• Figure 3: (a) Band structure of pristine $\text{WSe}_{\text{2}}$ showing the characteristic direct band gap at K. (b) Single vacancy band structure showing two defect midgap states, indicated by the red arrows. (c) Lateral divacancy band structure showing four midgap states (red arrows). (d) Vertical divacancy band structure showing two midgap defect states (red arrows) and two additional defect states hybridizing with the conduction band (green arrows).
• Figure 4: Maps of electron localization for (a) V1, (b) V-V, and(c) V2. For these, localization is determined at the bottom chalcogen plane (d).
• Figure 5: (a) Optical microscope image of $\text{WSe}_{\text{2}}$ monolayer. (b) Atomic force microscopy (AFM) image of the $\text{WSe}_{\text{2}}$ monolayer with areas circled corresponding to smooth (purple), creased (orange), and edge (blue). (c) Low-temperature emission spectra from regions of $\text{WSe}_{\text{2}}$ identified in AFM image. Strained areas exhibit narrow, localized emission features. (d) Map of monolayer emission integrated over the neutral exciton range (700 – 720 nm), showing the uniform intensity of exciton emission across the flake. (e) Map of monolayer emission integrated over wavelengths beyond the neutral exciton ($>720$ nm), showing localized, preferential emission near edges, folds, and cracks as identified with AFM. (f) Localized emission feature from the edge strained (blue) spectrum in (c). Inset shows the characteristic antibunching behavior of a single photon emitter in $g^{(2)}(\tau)$ with $g^{(2)}(0) = 0.17$.