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

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 MoO 3 at mild conditions, which creates a metal‐like ultrahigh free‐carrier concentration approaching that of noble metals (10 21 cm −3 in H 1.68 MoO 3 versus 10 22 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 MoO 3 , the SERS enhancement factor reaches as high as 1.1 × 10 7 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.