MoN Nanosheets as Sensitive and Reusable Surface-Enhanced Raman Scattering Substrates with Thickness-Dependent Sensitivity

Two-dimensional (2D) materials show advantages as surface-enhanced Raman scattering (SERS) substrates over traditional noble metals for their chemically inert flat surface and uniform SERS signals. However, due to their low density of free electrons, the enhancement mechanism of 2D materials is mainly restricted to chemical enhancement instead of electromagnetic enhancement, resulting in a relatively low enhancement factor. In this work, we report a sensitive 2D SERS substrate based on chemical vapor-deposited MoN nanosheets. The maximum enhancement factor can be 3 × 105 with a limit of detected concentration of 4 × 10–8 M, which is comparable to those noble metal SERS substrates without hot spots. Furthermore, distinct thickness-dependent sensitivity was observed in the MoN SERS substrate; higher sensitivity can be achieved by decreasing the thickness of the nanosheets. The Raman enhancement mechanism is dominated by the surface plasmon resonance of MoN nanosheets, which is evidenced by UV–vis absorption spectroscopy and first-principles calculation. The MoN SERS substrate also shows excellent Raman signal uniformity within the nanosheets, as well as high thermal stability, which can be annealed in air for more than 300 °C and maintain its SERS activity. The high stability of MoN nanosheets allows them to be reused for 20 cycles without obvious signal decay. The overall superior properties in terms of high sensitivity, Raman signal uniformity, stability, and reusability make MoN nanosheets an ideal 2D SERS substrate for practical applications.