Two-Orders-of-Magnitude Enhancement of SERS Activity via a Simple Surface Engineering of Quasi-Metal Single-Crystal Frameworks.

Beyond noble metals and semiconductors, quasi-metals have recently been shown to be noteworthy substrates for surface enhanced Raman spectroscopy, and their excellent quasi-metal surface-enhanced Raman spectroscopy (SERS) sensing has demonstrated a wider range of application scenarios. However, the underlying mechanism behind the enhanced Raman activity is still unclear. Here, we demonstrate that surface hydroxyls play a crucial role in the enhancement of the SERS activity of quasi-metal nanostructures. As a demonstration material, quasi-metallic MoO₂ single-crystal frameworks rich in surface hydroxyls have been shown to have 100 times higher SERS activity than MoO₂ single-crystal frameworks without hydroxyl functionalization, with a Raman enhancement factor of up to 7.6 × 10⁷. Experimental and first-principles density-functional theory calculation results show that the enhanced Raman activity can be attributed to an effective interfacial charge transfer within the MoO₂/OH/molecule system.