Raman intensity enhancement of molecules adsorbed onto HfS2 flakes up to 200 layers

Abstract

Newly emerging two-dimensional transition metal dichalcogenide HfS₂ has received considerable attention recently due to its ultrahigh photoresponsivity, well-balanced carrier mobility and an appropriate band gap which offer potential in electronic and optoelectronic devices. In this work, HfS₂ flakes up to 200 layers with varying color contrasts are fabricated and transferred on a SiO₂/Si substrate. The Raman intensities of HfS₂ flakes and Raman intensities of molecules adsorbed on HfS₂ flakes are quantitatively studied both theoretically and experimentally by considering an optical interference effect. The effects of the main experimental factors: thickness of SiO₂ and excitation wavelength on Raman intensities are also theoretically investigated. Due to the low absorption of HfS₂, many strong high-order interference-induced enhancement peaks are observed which are different from high absorption materials like graphene and MoS₂, in which only 2–4 interference-induced enhancement peaks exist. Due to the environmental instability of single layer HfS₂ under ambient conditions, multi-layer HfS₂ is a better choice than single layer HfS₂ as a Raman scattering substrate which has a stronger Raman enhancement and a better environmental stability. The discovery here will expand the application of HfS₂ flakes in molecular detection.