Surface-Enhanced Raman Spectroscopy (SERS) Chemical Enhancement in the Vibronically Coupled Langmuir Layer of Mixed Dichalcogenide 1T-MoSSe with Adsorbed R6G

Abstract

Transition-metal dichalcogenides based on different chalcogen atoms give origins to many new phenomena due to symmetry breaking and exhibit better physicochemical characteristics than pristine dichalcogenides. In this work, the formation of a perfectly tiled uniform monolayer of MoSSe using the Langmuir–Blodgett (LB) technique under ambient conditions is demonstrated. The aligned monolayer of the 1T phase depicts a film thickness of 1.2–2.4 nm corresponding to a single to a bilayer of MoSSe. The LB-prepared substrates are subsequently used for Raman sensing of R6G. The presence of MoSSe quenches the fluorescence of the R6G. Regarding surface-enhanced Raman spectroscopy sensitivity, 1T MoSSe could sense R6G up to picomolar concentrations with an enhancement factor of ∼6 × 106. This is 3 to 4 orders higher than the corresponding sulfide or the selenide analogues (1T MoS2 and 1T MoSe2). This increased sensitivity of 1T MoSSe is attributed to a large number of active sites with intrinsic dipole moment and high electronic conductivity, which leads to strong substrate-analyte vibronic coupling. Absorption and Raman spectroscopy confirms the static coupling between the substrates and R6G molecules. First principle density functional theory calculations reveal high density of states near the Fermi level in the case of distorted 1T MoSSe as compared to the pristine sulfide or selenide with less energy difference between the highest occupied molecular orbital of R6G and the Fermi level of 1T MoSSe. This might result in a facile charge transfer during the photoinduced charge transfer-driven chemical mechanism leading to a strong coupling of the metal–analyte complex, thus resulting in significant Raman enhancement.