[Paper Review] Excitonic Landscape of Monolayer Transition-Metal Dichalcogenides: Experimental Discrepancies, Theoretical Advances, and Strain Dependence

Cem Sevik, Purushothaman Manivannan+2 more|arXiv (Cornell University)|Jan 13, 2026

2D Materials and Applications0 citations

TL;DR

The paper provides a comprehensive assessment of excitonic properties in monolayer TMDs, reconciling experimental discrepancies with state-of-the-art GW-BSE calculations, and analyzes how biaxial strain modulates direct and indirect excitons.

ABSTRACT

Excitons in monolayer transition-metal dichalcogenides (TMDs) have garnered significant attention because of their large binding energies due to weakly screened Coulomb interaction, and direct bandgap at the K/K$^\prime$ point in the hexagonal Brillouin zone featuring spin-polarised bands due to spin-orbit coupling and lack of inversion symmetry. This makes them prospective for next-generation optoelectronic and quantum devices. However, despite the intense research activity, the reported values for exciton binding energies, quasiparticle gaps, and spectral features exhibit substantial variation across both experimental and theoretical studies. In this article, we present a comprehensive and critical assessment of the current understanding of excitonic properties in single-layer TMDs, integrating results from the angle-resolved photoemission spectroscopy (ARPES), photoluminescence (PL) measurements, and other experimental techniques with first-principles theoretical insights. Special emphasis is placed on the comparison and reconciliation of discrepancies observed across different experimental setups and sample qualities. Furthermore, we highlight our state-of-the-art GW-BSE calculations, which include both equilibrium and laterally strained systems, to systematically analyse the behaviour of direct and indirect excitons. By evaluating the effect of strain as a tunable control variable, we demonstrate its potential to engineer excitonic properties, supported by cross-validation against prior theoretical predictions and experimental findings. In doing so, we clarify the sources of discrepancies in the literature and offer a unified perspective on excited-state engineering strategies in two-dimensional TMDs.

Motivation & Objective

• Assess the current understanding of excitonic properties in single-layer TMDs by integrating experimental results (ARPES, PL, etc.) with first-principles theory.
• Quantify and reconcile discrepancies in quasiparticle gaps and exciton binding energies across samples and setups.
• Evaluate the impact of biaxial strain on direct and indirect excitons and provide strain gauge factors and transition thresholds.

Proposed method

• Perform fully relativistic DFT calculations with SOC using PBE functional and norm-conserving pseudopotentials.
• Apply GW0 corrections with a plasmon-pole model and include a 2D Coulomb truncation for 2D systems.
• Solve the Bethe–Salpeter Equation in the Tamm–Dancoff approximation to obtain exciton energies and wavefunctions.
• Use a tight convergence protocol with specified numbers of bands for W and G, and include spin-orbit coupling throughout.
• Investigate biaxial strain in MoS2, MoSe2, WS2, and WSe2 by repeating calculations for strains from -1.5% to +1.5% with full ionic relaxation.

Experimental results

Research questions

• RQ1 What are the current experimental and theoretical values for exciton binding energies and quasiparticle gaps in monolayer TMDs?
• RQ2 How do discrepancies across different experiments and sample qualities arise and can they be reconciled?
• RQ3 How does biaxial strain influence direct and indirect excitons in representative monolayer TMDs, and what are the gauge factors and thresholds for direct-to-indirect transitions?
• RQ4 Can state-of-the-art GW-BSE calculations reproduce and predict excitonic spectra under strain in MoS2, MoSe2, WS2, and WSe2?

Key findings

• GW-BSE calculations with rigorous convergence and SOC capture the excitonic landscape in monolayer TMDs.
• Strain acts as a tunable control parameter that modulates direct and indirect excitons in the studied materials.
• Discrepancies in the literature are clarified through systematic comparison with experimental techniques and high-level theory.
• Direct-to-indirect exciton transitions under strain can be identified with quantified thresholds and strain gauge factors.
• The analysis provides a unified perspective on exciton engineering strategies in two-dimensional TMDs.