Hydrogen-Doping-Induced Metal-Like Ultrahigh Free-Carrier Concentration in Metal-Oxide Material for Giant and Tunable Plasmon Resonance.

Abstract

The practical utilization of plasmon-based technology relies on the ability to find high-performance plasmonic materials other than noble metals. A key scientific challenge is to significantly increase the intrinsically low concentration of free carriers in metal-oxide materials. Here, a novel electron-proton co-doping strategy is developed to achieve uniform hydrogen doping in metal-oxide MoO₃ at mild conditions, which creates a metal-like ultrahigh free-carrier concentration approaching that of noble metals (10²¹ cm^-3 in H_1.68 MoO₃ versus 10²² cm^-3 in Au/Ag). This bestows giant and tunable plasmonic resonances in the visible region to this originally semiconductive material. Using ultrafast spectroscopy characterizations and first-principle simulations, the formation of a quasi-metallic energy band structure that leads to long-lived and strong plasmonic field is revealed. As verified by the surface-enhanced Raman spectra (SERS) of rhodamine 6G molecules on H_x MoO₃ , the SERS enhancement factor reaches as high as 1.1 × 10⁷ with a detection limit at concentration as low as 1 × 10^-9  mol L^-1 , representing the best among the hitherto reported non-metal systems. The findings not only provide a set of metal-like semiconductor materials with merits of low cost, tunable electronic structure, and plasmonic resonance, but also a general strategy to induce tunable ultrahigh free-carrier concentration in non-metal systems.