Understanding and Screening Eco-friendly Chemical Passivation Protocols for Monolayer WS2
Abstract
To this end, extensive efforts have been devoted to exploring approaches for improving the semiconducting quality of 2D TMD materials.1-2 Photoluminescence (PL) intensity and charge mobility are quality indicators of 2D TMDs for optoelectronic applications as they are sensitive to traps, structural defects, and charged impurities. The surface chemical strategy, a versatile and non-destructive method, is one of the most effective approaches to tailor the properties of 2D TMD materials for practical device applications. While there have been advances in materials growth in the past years, our understating of defects and how they degrade performance is still unsatisfactory.13–15 Thus, while many defect passivation strategies have been discussed in the literature, most achieve only moderate gains in PL efficiency. Consequently, there is an ongoing search for new chemical treatments that might offer superior passivation and compatibility with device fabrication.
Quantum mechanical computational simulations can be crucial for understanding the mechanisms of passivation in 2D TMD defects, rationalizing the screening process for better passivation agents, and evaluating their effects on the resulting optical properties, such as excitonic effects.
In this context, the computational calculations will be divided into three main parts:
1. Ab initio density functional theory (DFT) calculations will be performed to provide insights into the mechanisms associated with the passivation of sulfur vacancies (SVs) in WS₂ monolayers using the developed chemical treatment. More specifically, using the DFT framework, the mechanism of defect passivation with Li-TFSI in methanol will be clarified, along with the key effects produced on the material’s electronic structure.
2. Computational screening will be conducted to identify other passivation agents (such as salts and solvents) that could further enhance the optical properties of WS₂ layers.
3. To account for excitonic effects or electron-hole interactions, approximations beyond the independent-particle (IP) model must be employed. This can be achieved, for instance, by solving the Bethe-Salpeter equation (BSE). The BSE will be used to obtain the frequency-dependent dielectric function, incorporating excitonic effects. Therefore, using BSE, the optical properties of WS₂ layers with different passivation treatments will be investigated, enabling a comprehensive understanding of the effects of passivation on the optical properties of WS₂ layers.
We would like to emphasize that the DFT/GW/BSE calculations performed in this project are inherently computationally time-consuming and memory-intensive. Modeling WS₂ and passivated WS₂ requires a supercell model, leading to structures with 40–50 atoms, which makes the GW/BSE calculations highly demanding in terms of computational time and resources.
References:
1. Li, Z. et al. Efficient strain modulation of 2D materials via polymer encapsulation. Nature Communications 11, 1–8 (2020).
2. Cadiz, F. et al. Excitonic linewidth approaching the homogeneous limit in MoS2-based van der Waals heterostructures. Physical Review X 7, 1–12 (2017).
3. Cho, K., Pak, J., Chung, S. & Lee, T. Recent Advances in Interface Engineering of Transition-Metal Dichalcogenides with Organic Molecules and Polymers. ACS Nano 13, 9713–9734 (2019).
