A WO/MoO hybrid oxide based SERS FET and investigation on its tunable SERS performance

Active control of the surface-enhanced Raman scattering (SERS) enhancement shows great potential for realizing smart detection of different molecules. However, conventional methods usually involve time-consuming structural design or a sophisticated fabrication process. Herein, we reported an electrically tunable field effect transistor (FET) comprising a WO/MoO hybrid as the SERS active layer. In the experiment, WO/MoO hybrids were first prepared by mixing different molar ratios of WO and MoO oxides. Then, R6G molecules were used as Raman reporters, showing that the intensity of the SERS signal observed on the most optimal hybrids (molar ratio = 1 : 3) could be increased by two times as high as that observed on a single WO or MoO based substrate, which was ascribed to enhanced charge transfer efficiency by the constructed nano-heterojunction between the WO and MoO oxides. Thereafter, a back-gate FET was fabricated on a SiO₂/Si substrate, and the most optimal WO/MoO hybrid was deposited as the gate channel and the SERS active layer. After that, a series of gate biases (from -15 V to 15 V) were implemented to actively tune the SERS performance of the FET. It is evident that the SERS EF can be further tuned from 2.39 × 10⁷ (-15 V) to 6.55 × 10⁷ (+10 V), which is ∼7.4/4.1 times higher than that observed on the pure WO device (8.81 × 10⁶) or pure MoO (1.61 × 10⁷) device, respectively. Finally, the mechanism behind the electrical tuning strategy was investigated. It is revealed that a positive voltage would bend the conduction band down, which increased the electron density near the Fermi level. Consequently, it triggered the resonance charge transfer and significantly improved the SERS performance. In contrast, a negative gate voltage attracted the holes to the Fermi level, which deferred the charge transfer process, and caused the reduction of the SERS enhancement.