Noble-metal-free surface-enhanced Raman scattering (SERS) substrates have attracted great attention for their abundant sources, good signal uniformity, superior biocompatibility, and high chemical stability. However, the lack of controllable synthesis and fabrication of noble-metal-free substrates with high SERS activity impedes their practical applications. Herein, we propose a general strategy to fabricate a series of planar transition-metal nitride (TMN) SERS chips via an ambient temperature sputtering deposition route. For the first time, tungsten nitride (WN) and tantalum nitride (TaN) are used as SERS materials. These planar TMN chips show remarkable Raman enhancement factors (EFs) with ~ 10⁵ owing to efficient photoinduced charge transfer process between TMN chips and probe molecules. Further, structural engineering of these TMN chips is used to improve their SERS activity. Benefiting from the synergistic effect of charge transfer process and electric field enhancement by constructing a nanocavity structure, the Raman EF of WN nanocavity chips could be greatly improved to ~ 1.29 × 10⁷, which is an order of magnitude higher than that of planar chips. Moreover, we also design the WN/monolayer MoS₂ heterostructure chips. With the increase of surface electron density on the upper WN and more exciton resonance transitions in the heterostructure, a ~ 1.94 × 10⁷ level EF and a 5 × 10⁻¹⁰ M level detection limit could be achieved. Our results provide important guidance for the structural design of ultrasensitive noble-metal-free SERS chips.

Van der Waals heterojunctions (vdWHs) provide an excellent material system for the research of heterojunction-enhanced Raman scattering (HERS) due to their complexity and diversity. However, the traditional two-dimensional vdWHs are not conducive to the full utilization of near-field light due to the limitation of single dimension. Herein, we fabricate T-shaped mixed-dimensional SnSe₂/ReS₂ vdWHs via chemical vapor deposition and wetting transfer method, and demonstrate that the mixed-dimensional vdWHs can be used as ultrasensitive HERS chips based on the effective photo-induced charge transfer. Besides, the radiative energy transfer effect enhanced by near-field light further magnifies the HERS signals, improving the detection limit of rhodamine 6G (R6G) to femtomolar level. Furthermore, we demonstrate that the ultrasensitive screening of crystal violet in multicomponent solutions adsorbed on SnSe₂/ReS₂ vdWHs can be achieved by adjusting the laser wavelength, which has not been achieved by noble metal materials. This work provides new insights into the mixed-dimensional vdWHs and demonstrates the great application potential of T-shaped heterojunctions.