Improved SERS Activity of non-stoichiometric Copper Sulfide Nanostructures Related to Charge-Transfer Resonance
Low enhancement factor of semiconductor SERS substrate is a major obstacle to its practical application. Therefore, it is requisite to explore the facile synthesis of new SERS substrates and reveal SERS enhancement mechanism. Here, we develop a simple, facile and low-cost two-step methods to synthesis copper sulfide based nanostructures with different Cu7.2S4 contents. The as-synthesized sample is composed of nanosheets with CuS phase structure. With the increase of the annealing temperature to 300 oC, CuS content gradually decreases and disappears, and the content of Cu7.2S4 and CuSO4 appears and gradually increases. At the annealing temperature of 350 oC, only CuSO4 exists. Compared with pure CuS or pure CuSO4, the detection limit of R6G molecule is lowest for the composite sample with higher content of Cu7.2S4, indicating the introduction of non-stoichiometric Cu7.2S4 can imporve the SERS performance and the higher content of Cu7.2S4 brings about the higher SERS activity. Furthermore, to investigate the SERS mechanism, the energy band structures and energy-level diagrams of R6G over CuS, Cu7.2S4 and CuxS are studied by DFT calculation. Theoretical calculations indicates that the excellent SERS behavior depends on charge transfer resonance and molecular resonance. Our work provides a general approach for the construction of excellent metal compound semiconductor SERS active substrates.