First principles study of chalcogen vacancy effect on the optoelectronic and photocatalytic properties of transition metal dichalcogenides monolayers

Technical Summary: First Principles Study of Chalcogen Vacancy Effects on TMDC Monolayers

Problem Statement
Two-dimensional transition metal dichalcogenides (TMDCs) with the formula MX₂ (where M = Mo, W and X = S, Se) are promising candidates for optoelectronic devices and photocatalytic applications, such as water splitting and CO₂ reduction. However, their practical efficiency is often hindered by high carrier recombination rates, low light harvesting capacity, and slow reaction kinetics. Furthermore, during synthesis via methods like chemical vapor deposition (CVD) or mechanical exfoliation, chalcogen vacancies are inevitable. While pristine basal planes in stable TMDCs are often inactive, the specific impact of these unavoidable defects on the structural, optoelectronic, and photocatalytic properties of monolayers requires systematic investigation to determine if they degrade performance or offer functional advantages.

Methodology
The authors employed first-principles quantum mechanical calculations to investigate prototypical MX₂ monolayers (MoS₂, MoSe₂, WS₂, and WSe₂).

• Defect Modeling: Using the Alloy Theoretic Automated Toolkit (ATAT) coupled with the Vienna ab initio Simulation Package (VASP), the authors utilized cluster expansion approximations to identify the most energetically favorable vacancy concentrations by constructing energy convex hulls. The identified concentrations were 6.25% for MoS₂, MoSe₂, and WSe₂, and 1.56% for WS₂.
• Computational Details: Calculations were performed using Density Functional Theory (DFT) with the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation and the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional for more accurate band gap predictions. Van der Waals interactions were corrected using Grimme's DFT-D2 method.
• Stability Analysis: Structural stability was assessed through phonon dispersion calculations (using PHONOPY) and ab initio molecular dynamics (AIMD) simulations at 300 K using a Nosé-Hoover thermostat over an 8 ps trajectory.
• Property Evaluation: The study analyzed electronic band structures, electrostatic potential asymmetry (planar average along the c-axis), work functions, optical absorption coefficients, and the alignment of band edges relative to water redox potentials and CO₂ reduction potentials.

Key Results

• Structural Stability: Structural optimizations revealed that M–X bond lengths around vacancies differ only slightly from pristine structures, indicating minimal local strain. Phonon dispersion and AIMD simulations confirmed that the defective structures are both dynamically and thermally stable at operating temperatures, with no bond breaking observed.
• Electronic Properties: The introduction of chalcogen vacancies creates electronic states within the band gap, leading to a reduction in the band gap width compared to pristine monolayers. HSE06 calculations indicated that the Valence Band Maximum (VBM) and Conduction Band Minimum (CBM) are primarily derived from the transition metal d-orbitals around the vacancy, with minor contributions from chalcogen atoms.
• Electrostatic Asymmetry and Charge Separation: A critical finding is the induction of electrostatic potential asymmetry. The presence of vacancies on one chalcogen layer creates a difference in work function ($\delta\phi$) between the vacancy side and the pristine side. This asymmetry generates a built-in electric field that facilitates charge separation and reduces recombination, a feature absent in pristine, symmetric monolayers. The effect was more pronounced in Se-based structures due to the higher polarizability of Selenium.
• Optical Properties: The introduction of vacancies caused a red shift in the first absorption peak for MoX₂ monolayers, enabling the absorption of lower-energy photons, including infrared radiation, compared to pristine counterparts.
• Photocatalytic Performance:
  • Water Splitting: All defective monolayers possess band gaps greater than 1.23 eV and CBM positions suitable for the Hydrogen Evolution Reaction (HER). However, the Valence Band Maximum (VBM) lies above the potential required for the Oxygen Evolution Reaction (OER), suggesting that while HER is feasible, a co-catalyst is required to assist OER.
  • CO₂ Reduction: The study evaluated the capability to reduce CO₂ into various products (HCOOH, CO, HCHO, CH₃OH, CH₄). WS₂ and WSe₂ defective monolayers were identified as capable of reducing CO₂ into all listed useful chemical products. MoSe₂ was capable of producing HCHO, CH₃OH, and CH₄, while MoS₂ was limited to CH₃OH and CH₄.

Significance and Claims
The paper argues that vacancy-containing TMDC monolayers should not be regarded as flawed products to be discarded. Instead, the results demonstrate that these defects can enhance photocatalytic capabilities compared to pristine counterparts by reducing band gaps, inducing beneficial electrostatic asymmetry for charge separation, and enabling visible light absorption. The authors claim that their findings provide guidelines for the experimental synthesis of vacancy-engineered MX₂ monolayers specifically tailored for optoelectronic devices and photocatalytic applications, particularly highlighting WS₂ and WSe₂ as efficient materials for CO₂ reduction and water splitting (with co-catalyst assistance for OER).