1T' Transition Metal Telluride Atomic Layers for Plasmon-Free SERS at Femtomolar Levels

Plasmon-free surface enhanced Raman scattering (SERS) based on the chemical mechanism (CM) is drawing great attention due to its capability for controllable molecular detection. However, in comparison to the conventional noble-metal-based SERS technique driven by plasmonic electromagnetic mechanism (EM), the low sensitivity in the CM-based SERS is the dominant barrier toward its practical applications. Herein, we demonstrate the 1T' transition metal telluride atomic layers (WTe₂ and MoTe₂) as ultrasensitive platforms for CM-based SERS. The SERS sensitivities of analyte dyes on 1T'-W(Mo)Te₂ reach EM-comparable ones and become even greater when it is integrated with a Bragg reflector. In addition, the dye fluorescence signals are efficiently quenched, making the SERS spectra more distinguishable. As a proof of concept, the SERS signals of analyte Rhodamine 6G (R6G) are detectable even with an ultralow concentration of 40 (400) fM on pristine 1T'-W(Mo)Te₂, and the corresponding Raman enhancement factor (EF) reaches 1.8 × 10⁹ (1.6 × 10⁸). The limit concentration of detection and the EF of R6G can be further enhanced into 4 (40) fM and 4.4 × 10¹⁰ (6.2 × 10⁹), respectively, when 1T'-W(Mo)Te₂ is integrated on the Bragg reflector. The strong interaction between the analyte and 1T'-W(Mo)Te₂ and the abundant density of states near the Fermi level of the semimetal 1T'-W(Mo)Te₂ in combination gives rise to the promising SERS effects by promoting the charge transfer resonance in the analyte-telluride complex.