Metallic MoO₂ is promising to be one of noble-metal-comparable 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 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 (SPR effect) and 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.