The twist angle at van der Waals interfaces has emerged as a powerful degree of freedom for tailoring excitonic energy landscapes in two-dimensional semiconductors, yet how interlayer coupling and photoinduced dynamic dipole screening cooperatively govern excitonic responses remains elusive. Here, we directly investigate the twist-angle-dependent Coulomb-dominated excitonic effects in bilayer WS₂, including bandgap renormalization (BGR), exciton binding energy, recombination lifetime, and mobility. Femtosecond transient absorption spectroscopy reveals a twist-angle-tunable exciton binding energy from 265 to 336 meV; upon approaching the Mott transition threshold, BGR varies by over 100 meV and is most significant near-30°, where weakened interlayer coupling makes Coulomb interactions dominant. Moreover, the exciton lifetime and mobility are jointly dictated by the twist angle: strong interlayer coupling suppresses recombination while enhancing exciton mobility by threefold. This work integrates photoinduced screening with twist-angle engineering, establishing a unified picture of exciton many-body dynamics and offering guidance for high-performance excitonic devices.

Two-dimensional transition metal dichalcogenides (TMDs) are promising candidates for next-generation optoelectronics, but their performance is often constrained by intrinsic defects. Acid treatment has emerged as a powerful defect engineering strategy, dramatically boosting the photoluminescence (PL) quantum yield of these atomically thin semiconductors. This review systematically examines progress in acid-induced PL enhancement of TMDs over the past decade and distills the underlying mechanisms and ongoing controversies. A central theme is the ubiquitous trade-off: optical properties improve markedly, whereas electrical transport in field-effect transistors frequently degrades after treatment. This counterintuitive behavior is rationalized through a sulfur vacancy-mediated hopping transport model, which shows that the trade-off is an inherent consequence of defect manipulation rather than a side effect. Moving beyond this compromise, synergistic design rules for decoupling optical and electronic responses are critically assessed, with particular emphasis on the emerging use of Lewis acids for targeted property modulation. Finally, future research directions are outlined, including integrating machine learning with atomic-scale defect characterization, testing the generality of defect‑selective passivation across diverse defects, and advancing these strategies toward practical device applications.

Metallic MoO₂ is promising to be one of noble-metal-comparable surface-enhanced Raman spectroscopy (SERS) substrate materials based on the strong electromagnetic mechanism (EM) enhancement. However, the SERS performance is still unable to meet the practical application requirements. Synthesizing large-scale two-dimensional (2D) metallic MoO₂ with controllable thickness, even at atomic level is an effective solution, but it has rarely been reported. The enhancement mechanism based on such kind of metallic metal oxide SERS substrates also lacks a more systematic study. Here, submillimeter-scale (~ 466 μm) atomic-thin (~ 4 nm) metallic MoO₂ was firstly synthesized by chemical vapor deposition (CVD). What’s more, it shows high sensitivity as SERS substrates with a maximum enhancement factor up to 10⁷ and a limit of detection down to 10⁻⁹ M, which is at a high level among most metal oxide-based SERS substrates and even comparable to the values of noble metal substrates with “hot spots”. It was also firstly and systematically found that both the EM (surface plasmon resonance (SPR) effect) and chemical mechanism (CM) (charge transfer process) enhancements exist simultaneously in such MoO₂ substrate. The key lies in the thickness of MoO₂ that determines the dominant enhancement mechanism. MoO₂ flakes not only possess noble-metal-comparable SERS performance, but also show potential in image security and information encryption.

To further improve the quantum efficiency of atomically thin transition metal dichalcogenides (TMDs) is crucial for the realization of high-performance optoelectronic applications. To this regard, a few chemical or physical approaches such as superacid treatment, electrical gating, dielectric screening, and laser irradiation have been developed. In particular, the laser irradiation appears to be a more efficient way with good processability and spatial selectivity. However, the underlying mechanism especially about whether chemisorption or physisorption plays a more important role is still debatable. Here, we unravel the mystery of laser irradiation induced photoluminescence enhancement in monolayer WS₂ by precisely controlling irradiation time and environment. It is found that the synergetic effect of physisorption and chemisorption is responsible for the photoluminescence enhancement, where the physisorption dominates with more than 74% contribution. The comprehensive understanding of the adsorption mechanism in laser-irradiated TMDs may trigger the potential applications for patterned light source, effective photosensor and ultrathin optical memory.