Electrically configurable SERS-FET for the highly sensitive and selective detection of molecules

Abstract

Surface-enhanced Raman scattering (SERS) is a spectroscopic detection technique with extremely high sensitivity and detectivity. However, the performance of the SERS detection in realistic scenarios is always degraded by the interfering molecules in the samples; thus a novel SERS substrate with high sensitivity and selective enhancement capability will be an advantage. Herein, we propose a prototype of SERS field-effect transistor (SERS-FET) with tunable and selective enhancement. In the experiment, a FET with a Li⁺ storage layer (Li_xMoO₃) and a SERS-active layer (WO₃) were deposited on the FTO glass as the gate oxide. Then, the defect density of WO₃ was manipulated by precisely driving the Li⁺ ions from the Li_xMoO₃ layer into the WO₃ layer through applying a pulse voltage on the FTO glass. Using rhodamine 6G (R6G) and methylene blue (MB) as the probe molecules, the results showed that the SERS intensity of R6G and MB was effectively tuned by the applied pulse. Selective enhancement demonstrated that 100-cycle pulses were required for the R6G molecules to achieve two orders of magnitude for the SERS intensity (peak@609 cm⁻¹), while 30-cycle pulses were sufficient for the MB molecules to obtain 4.23 times of the greatest SERS intensity (peak@1628 cm⁻¹). Moreover, first-principles calculation and ultraviolet photoelectron spectroscopy (UPS) revealed that the successful shift of defect levels in the semiconductor to the middle of the bandgap by gradually injecting the Li⁺ ions, and the constant movement of the band alignment to the optimal state activated the resonance charge transfer between the substrate material and specific molecules, and ultimately produced the best Raman enhancement.