Plasmon-Free Surface-Enhanced Raman Sensing Enabled by Phase-Engineered Two-Dimensional Mo₂C-Based Heterostructures

Plasmon-free surface-enhanced Raman scattering (SERS) based on two-dimensional materials is a promising approach for nondestructive analysis. However, the smooth surfaces commonly formed during chemical vapor deposition (CVD) limit the generation of localized high electromagnetic fields and reduce chemical interactions with adsorbed molecules, even in metallic 2D materials. In this study, 2D Mo₂C was synthesized by CVD, and MoOx/Mo₂C, MoS₂/Mo₂C, and MoSe₂/Mo₂C heterostructures were fabricated through thermal oxidation, plasma-assisted sulfurization, and selenization, respectively. The MoS₂ and MoSe₂ top layers were phase-engineered into 1T-rich and 2H-rich structures at 350°C and 550°C, respectively, under the same plasma power of 150 W. The heterostructures exhibit improved SERS activity, mainly due to chemical enhancement (CM) arising from increased surface roughness, defect/edge-rich nanostructures, and heterointerfaces that provide abundant localized chemically active sites for molecular adsorption and interfacial charge transfer. The 1T-rich MoS₂/Mo₂C shows a detection limit of up to 10^-10 M and a Raman enhancement factor of 9.1 × 10⁴, while the 1T-rich MoSe₂/Mo₂C displays a detection limit of up to 10^-9 M and a Raman enhancement factor of 8.9 × 10⁴ for rhodamine B (RhB), attributed to the presence of a high density of states and localized chemically active sites on the 1T-rich MoS₂ and MoSe₂ surfaces.