Synergistic Defect and Heterojunction Engineering for Significantly Increased Raman Enhancement on Monolayer Metallic Transition Metal Chalcogenide Nanosheets

2D transition metal chalcogenide nanosheets (TMC NSs) have been recognized as ideal platforms for surface-enhanced Raman spectroscopy (SERS). Nevertheless, the unsatisfying Raman enhancement exerts a substantial restriction on their practical applications. Herein, a strategy is demonstrated simultaneously integrating defect and heterojunction engineering within metallic TMC NSs for highly sensitive SERS detection of analytes. Particularly, metallic FeSe-CoSe_x/Cu monolayers with finely controlled Cu ion doping ratios and rich FeSe-CoSe_x/Cu heterojunctions are synthesized. The metallic property endows abundant electronic density of states (DOS) near the fermi level for increasing electron transition probability. The heterojunctions enable the delivery of electrons from FeSe to CoSe_x/Cu, contributing to charge transfer (CT) from CoSe_x/Cu to probe molecules. Moreover, Cu ions doping can well modulate the electronic structure of CoSe_x/Cu, further facilitating the CT between substrates and molecules. Benefiting from the rational engineering of defects and heterojunctions, an ultrasensitive molecular sensing performance is obtained on the FeSe-CoSe_x/Cu30% NSs. The practical application of the FeSe-CoSe_x/Cu30% NSs in trace and quantitative SERS detection of metamitron herbicide in water samples is successfully achieved with the limit of detection (LOD) of 1.3 × 10⁻⁸ M.