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

Abstract

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_<i>x</i>S_<i>y</i> thin films via solution-phase deposition without post-treatments and demonstrate a graphene/ReO_<i>x</i>S_<i>y</i> vertical heterostructure as an ultrasensitive SERS platform. The electronic structure of ReO_<i>x</i>S_<i>y</i> can be modulated by changing the oxygen concentration in the lattice structure, obtaining efficient complementary resonance effects between ReO_<i>x</i>S_<i>y</i> and the probe molecule. In addition, the oxygen atoms in the ReO_<i>x</i>S_<i>y</i> 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_<i>x</i>S_<i>y</i> thin film. When ReO_<i>x</i>S_<i>y</i> 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.