Extending semiconductor plasmonics to the UV: Fano resonances and disorder engineering in Ag-In-S nanoparticles

We report widely tunable plasmonic resonance in ternary Ag-In-S and quaternary Ag-In-S-Zn semiconductor nanoparticles (NPs), that exhibit intraband free-carrier localized surface plasmon resonance (LSPR) in the NIR and interband-driven plasmon/plexcitonic Fano mode in the UV spectral regions (300-350 nm), extending semiconductor plasmonics to short wavelengths. These plasmonic properties are precisely tailored by controlling electronic structure, including energy band gaps and Urbach energies in addition to free charge carrier concentration via extrinsic Zn-doping and sonochemically controlled structural disorder. While NIR LSPR bands are symmetric, the UV plasmonic peaks are notably asymmetric and can be modelled with a Fano-like band shape function. The nature of the asymmetry is quantum-mechanical i.e. a Fano-like resonance between a continuous plasmon state and a discrete electronic excitation. By interpolating the experimental interband plasmonic bands in the UV region with a generalized Fano line shape function, relevant parameters such as plasmon damping, resonance energy and asymmetry factor were obtained. In addition, we show that a classical electrodynamics approach based on a driven-damped oscillator also reproduces the asymmetry. The demonstrated plasmon-exciton coupling in Ag-In-S based NPs, which is tunable via both external doping and sonochemical processing, offers a new platform for potential applications requiring precise control of plasmon absorption across a wide spectral region.