*Article* **Surface-Enhanced Raman Spectroscopy on Hybrid Graphene**/**Gold Substrates near the Percolation Threshold**

**Dmitry E. Tatarkin 1,\*, Dmitry I. Yakubovsky 1, Georgy A. Ermolaev 1,2, Yury V. Stebunov 1, Artem A. Voronov 1, Aleksey V. Arsenin 1, Valentyn S. Volkov 1 and Sergey M. Novikov 1,\***


Received: 19 December 2019; Accepted: 14 January 2020; Published: 17 January 2020

**Abstract:** Graphene is a promising platform for surface-enhanced Raman spectroscopy (SERS)-active substrates, primarily due to the possibility of quenching photoluminescence and fluorescence. Here we study ultrathin gold films near the percolation threshold fabricated by electron-beam deposition on monolayer CVD graphene. The advantages of such hybrid graphene/gold substrates for surface-enhanced Raman spectroscopy are discussed in comparison with conventional substrates without the graphene layer. The percolation threshold is determined by independent measurements of the sheet resistance and e ffective dielectric constant by spectroscopic ellipsometry. The surface morphology of the ultrathin gold films is analyzed by the use of scanning electron microscopy (SEM) and atomic force microscopy (AFM), and the thicknesses of the films in addition to the quartz-crystal mass-thickness sensor are also measured by AFM. We experimentally demonstrate that the maximum SERS signal is observed near and slightly below the percolation threshold. In this case, the region of maximum enhancement of the SERS signal can be determined using the figure of merit (FOM), which is the ratio of the real and imaginary parts of the e ffective dielectric permittivity of the films. SERS measurements on hybrid graphene/gold substrates with the dye Crystal Violet show an enhancement factor of ~10<sup>5</sup> and also demonstrate the ability of graphene to quench photoluminescence by an average of ~60%.

**Keywords:** surface-enhanced Raman scattering; graphene; ultrathin gold films; spectroscopic ellipsometry; percolation threshold
