(191c) Engineering Localized Surface Plasmon Resonances in Si Nanowires

Nanoscale semiconductors are rapidly emerging as an alternate materials platform with which to engineer localized surface plasmon resonances (LSPRs). These quantized excitations, which result from the collective oscillation of free carriers at a metallodielectric interface, promise new opportunities for ultrafast information transfer and efficient energy conversion. Here, we show how phosphorus-doped regions positioned along the length of Si nanowires permit user-programmable spectral responses in the mid-infrared. Highly aligned Si nanowire arrays are synthesized via the vapor-liquid-solid (VLS) technique with a combination of Si₂H₆ and PCl₃ precursors. All phosphorus-doped sections are sandwiched between intrinsic base and tip regions to deconvolute substrate and catalyst effects, respectively. In-situ infrared absorption spectroscopy measurements, which mitigate the complications that arise from surface oxidation, reveal intense absorption bands between 800 and 2200 cm^-1 that exhibit dopant concentration and shape-dependent spectral shifts consistent with longitudinal LSPRs. Discrete dipole approximation calculations confirm the observed spectral response results from resonant absorption and that free carrier densities are on the order of 10²⁰ cm^-3. Multimodal spectral responses, achieved by introducing multiple doped regions in each Si nanowire, are also possible. Our findings open the door to surface plasmon engineering in a ubiquitous semiconductor and also highlight the utility of VLS synthesis for this purpose.