[Paper Review] Substrate screening effects on the quasiparticle band gap and defect charge transition levels in MoS$_2$

Mit H. Naik, Manish Jain|NOT FOUND REPOSITORY (Indian Institute of Science Bangalore)|Oct 26, 2017

2D Materials and Applications4 citations

TL;DR

This study investigates substrate screening effects on the quasiparticle band gap and sulfur vacancy defect charge transition levels (CTLs) in monolayer MoS₂ using the DFT+GW formalism. It reveals that metallic substrates like graphene and graphite reduce the quasiparticle gap by up to 530 meV, while the (0/-1) CTL shifts with the band gap, and the (+1/0) CTL remains pinned within 100 meV of the valence band maximum, enabling substrate-based engineering of defect levels for optoelectronic and catalytic applications.

ABSTRACT

Monolayer MoS$_2$ has emerged as an interesting material for nanoelectronic and optoelectronic devices. The effect of substrate screening and defects on the electronic structure of MoS$_2$ are important considerations in the design of such devices. Here, we present ab initio density functional theory (DFT) and GW calculations to study the effect of substrate screening on the quasiparticle band gap and defect charge transition levels (CTLs) in monolayer MoS$_2$. We find a giant renormalization to the free-standing quasiparticle band gap by 350 meV and 530 meV in the presence of graphene and graphite as substrates, respectively. Our results are corroborated by recent experimental measurements on these systems using scanning tunneling spectroscopy and photoluminescence excitation spectroscopy. Sulfur vacancies are the most abundant native defects found in MoS$_2$. We study the CTLs of these vacancies in MoS$_2$ using the DFT+GW formalism. We find (+1/0) and (0/-1) CTLs appear in the pristine band gap of MoS$_2$. Substrate screening results in renormalization of the (0/-1) level, with respect to the valence band maximum (VBM), by the same amount as the gap. This results in the pinning of the (0/-1) level about $\sim$500 meV below the conduction band minimum for the free-standing case as well as in the presence of substrates. The (+1/0) level, on the other hand, lies less than 100 meV above the VBM for all the cases.

Motivation & Objective

• To understand how substrate dielectric screening affects the quasiparticle band gap in monolayer MoS₂.
• To compute accurate charge transition levels (CTLs) of sulfur vacancy defects in MoS₂ beyond the limitations of standard DFT.
• To investigate whether substrate choice can be used to engineer defect energy levels for improved device performance.
• To provide a quantitative link between experimental observations of gap renormalization and defect behavior and first-principles many-body theory.

Proposed method

• Employed the DFT+GW formalism to compute quasiparticle band gaps and defect formation energies with improved accuracy over standard DFT.
• Used supercell calculations with periodic boundary conditions to model sulfur vacancies in monolayer MoS₂ on various substrates.
• Treated substrate screening effects by embedding the MoS₂ monolayer in a dielectric environment with varying dielectric constants.
• Calculated charge transition levels (CTLs) using the quasiparticle energy correction method, ensuring proper treatment of electron correlation.
• Compared results across substrates including BN, SiO₂, graphene, bilayer graphene, and graphite to assess screening strength effects.
• Assumed identical atomic relaxations in the presence and absence of substrates to isolate the effect of dielectric screening on quasiparticle levels.

Experimental results

Research questions

• RQ1 How does substrate screening affect the quasiparticle band gap of monolayer MoS₂?
• RQ2 What are the accurate charge transition levels (CTLs) of sulfur vacancies in MoS₂ when computed with many-body corrections?
• RQ3 Does the (0/-1) CTL shift proportionally with the band gap under different substrate screening conditions?
• RQ4 Is the (+1/0) CTL pinned near the valence band maximum across different substrates, and why?
• RQ5 Can substrate choice be used to engineer defect levels to optimize hydrogen evolution reaction (HER) activity?

Key findings

• The quasiparticle band gap of free-standing MoS₂ is renormalized by up to 530 meV in the presence of graphite substrates, in agreement with experimental STS and photoluminescence measurements.
• The (0/-1) charge transition level is renormalized by approximately the same amount as the band gap, shifting 500 meV below the conduction band minimum in all substrate configurations.
• The (+1/0) charge transition level remains pinned within 100 meV of the pristine valence band maximum across all substrates, indicating strong screening-induced pinning.
• The (0/-1) CTL remains stable and lies 2.14 eV above the valence band maximum in the quasiparticle gap, consistent with experimental observations of the -1 charge state stability.
• Substrate screening enables tunable defect energy levels, offering a pathway to engineer sulfur vacancy defect states for enhanced hydrogen evolution reaction (HER) catalysis.
• The results demonstrate that substrate choice can be used as a design knob to control defect levels and reduce carrier scattering from charged defects, potentially improving MoS₂ mobility.