Improved SERS activity of non-stoichiometric copper sulfide nanostructures related to charge-transfer resonance

The low enhancement factor of semiconductor SERS substrates is a major obstacle for their practical application. Therefore, there is a need to explore the facile synthesis of new SERS substrates and reveal the SERS enhancement mechanism. Here, we develop a simple, facile and low-cost two-step method to synthesize copper sulfide based nanostructures with different Cu_7.2S₄ contents. The as-synthesized sample is composed of nanosheets with the CuS phase structure. With the increase of the annealing temperature to 300 °C, the CuS content gradually decreases and disappears, and the content of Cu_7.2S₄ and CuSO₄ appears and gradually increases. At the annealing temperature of 350 °C, only CuSO₄ exists. Compared with pure CuS or pure CuSO₄, the detection limit of R6G molecules is the lowest for the composite sample with a higher content of Cu_7.2S₄, indicating that the introduction of non-stoichiometric Cu_7.2S₄ can improve the SERS performance and the higher content of Cu_7.2S₄ leads to a higher SERS activity. Furthermore, to investigate the SERS mechanism, the energy band structures and energy-level diagrams of different probe molecules over CuS, Cu_7.2S₄ and Cu_xS are studied by DFT calculations. Theoretical calculations indicate that the excellent SERS behavior depends on charge transfer resonance. Our work provides a general approach for the construction of excellent metal compound semiconductor SERS active substrates.