Ultrasensitive Plasmon-Free Surface-Enhanced Raman Spectroscopy with Femtomolar Detection Limit from 2D van der Waals Heterostructure

Two-dimensional (2D) materials have been promoted as an ideal platform for surface-enhanced Raman spectroscopy (SERS), as they mitigate the drawbacks of noble metal-based SERS substrates. However, the inferior limit of detection has limited the practical applicability of 2D material-based SERS substrates. Here, we synthesize uniform large-area ReO_xS_y thin films via solution-phase deposition without post-treatments and demonstrate a graphene/ReO_xS_y vertical heterostructure as an ultrasensitive SERS platform. The electronic structure of ReO_xS_y can be modulated by changing the oxygen concentration in the lattice structure, obtaining efficient complementary resonance effects between ReO_xS_y and the probe molecule. In addition, the oxygen atoms in the ReO_xS_y lattice generate a dipole moment on the thin-film surface, which increases the electron transition probability. These synergistic effects outstandingly enhance the Raman effect in the ReO_xS_y thin film. When ReO_xS_y forms a vertical heterostructure on a graphene as the SERS substrate, the enhanced charge-transfer and exciton resonances improve the limit of detection to the femtomolar level, while achieving remarkable flexibility, reproducibility, and operational stability. Our results provide important insights into 2D material-based ultrasensitive SERS based on chemical mechanisms.