Submillimeter-scale atomic-thin metallic MoO₂ with noble metal-comparable SERS performance realized by thickness-dependent enhancement

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.