Quasiparticle band structures and optical properties of strained monolayer MoS2 and WS2 (1211.5653v2)

Abstract: The quasiparticle (QP) band structures of both strainless and strained monolayer MoS${2}$ are investigated using more accurate many body perturbation \emph{GW} theory and maximally localized Wannier functions (MLWFs) approach. By solving the Bethe-Salpeter equation (BSE) including excitonic effects on top of the partially self-consistent \emph{GW${0}$} (sc\emph{GW${0}$}) calculation, the predicted optical gap magnitude is in a good agreement with available experimental data. With increasing strain, the exciton binding energy is nearly unchanged, while optical gap is reduced significantly. The sc\emph{GW${0}$} and BSE calculations are also performed on monolayer WS${2}$, similar characteristics are predicted and WS${2}$ possesses the lightest effective mass at the same strain among monolayers Mo(S,Se) and W(S,Se). Our results also show that the electron effective mass decreases as the tensile strain increases, resulting in an enhanced carrier mobility. The present calculation results suggest a viable route to tune the electronic properties of monolayer transition-metal dichalcogenides (TMDs) using strain engineering for potential applications in high performance electronic devices.

• The paper shows that accurate scGW₀ methods predict a direct band gap of ~2.80 eV in strainless MoS₂ with a shift to an indirect gap under 1% strain.
• The study finds that while the exciton binding energy remains near 0.63 eV, the optical band gap decreases significantly with applied tensile strain.
• The paper highlights that strained WS₂ has a lower effective electron mass than MoS₂, suggesting enhanced carrier mobility for flexible electronics.

Analysis of Quasiparticle Band Structures and Optical Properties of Strained Monolayer MoS₂ and WS₂

In the research paper titled "Quasiparticle band structures and optical properties of strained monolayer MoS₂ and WS₂," the authors use many-body perturbation GW theory and the maximally localized Wannier functions approach to explore the quasiparticle (QP) band structures of both strained and unstrained monolayer transition-metal dichalcogenides (TMDs), specifically molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂). This paper highlights how modulating strain in these monolayers can efficiently alter their electronic properties, suggesting potential use in high-performance electronic applications.

The authors employ the GW approximation, supplemented with partially self-consistent GW$_0$ (scGW$_0$) calculations and the Bethe-Salpeter equation (BSE), which incorporates excitonic effects. Such methodologies enable more accurate predictions of QP band structures and optical spectra compared to traditional DFT approaches, which often unreliably describe excited states in low-dimensional systems like TMDs. Notably, the paper illustrates that the exciton binding energy remains nearly constant under increasing strain, whereas the optical band gap decreases markedly.

Key Findings and Numerical Insights

1. Quasiparticle Band Gap Calculation: The scGW$_0$ calculations predict that strainless monolayer MoS₂ has a K-K direct band gap of 2.80 eV. This agrees with calculations using a full-potential linearized muffin-tin-orbital method, differing significantly from DFT predictions (1.78 eV). The transition from a direct to indirect band gap was observed when 1% strain was applied, emphasizing the importance of accurate QP modeling beyond DFT for these materials.
2. Optical Properties and Excitonic Effects: The optical gap calculations, obtained by solving the BSE, reveal that the effective exciton binding energy for monolayer MoS₂ is approximately 0.63 eV. The optical band gap of MoS₂, verified through comparison to experimental data, is significantly influenced by strain, showcasing tunability which could be leveraged in electronic device engineering.
3. Impact of Strain: The application of tensile strain results in a decrease of both direct and indirect band gaps for MoS₂ and WS₂. Under strain, WS₂ maintains a lighter effective electron mass relative to MoS₂, proposing WS₂ as a superior channel material due to its higher carrier mobility potential. Such effective mass reductions under strain highlight the capabilities of strain engineering in modulating electronic properties.
4. Comparative TMD Analysis: Alongside MoS₂ and WS₂, supplementary investigations on MoSe₂ and WSe₂ show analogous trends, reinforcing the universality of strain-induced band structure modulations across different monolayer TMDs.

Implications and Future Directions

The implications of this research are substantial for both theoretical understanding and practical applications of TMDs in electronics. Strain engineering, as demonstrated, provides a feasible avenue for tuning electronic properties, enhancing possibilities for the development of flexible electronic devices. The preservation of excitonic binding strength against altered strain suggests reliable operational characteristics of these monolayers in diverse environments.

Future research could extend into dynamic strain applications and devices that necessitate real-time tunability. Additionally, further development in the computational methods, such as increased k-point resolution and consideration of spin-orbital coupling, may yield even more refined predictions. Ultimately, integrating these materials into nanoscale devices will require a detailed understanding of their electron-phonon interactions under strain, offering a rich area for exploration.

In conclusion, the paper presents a comprehensive paper on the strain-dependent electronic properties of MoS₂ and WS₂, proposing practical methods to customize monolayer properties for advanced electronic applications. These findings lay a strong foundation for ongoing research in the field of two-dimensional materials and their technological advancements.