(174bx) Colloidal ReO3 Nanocrystals: Extra Rhenium d-Electron Instigating a Plasmonic Response

In this presentation, we will describe a solution based synthesis of nanocrystals (NCs) of rhenium (VI) oxide (ReO₃) which exhibit localized surface plasmon resonance (LSPR) typical of metals and/or degenerately doped semiconductors. Metal oxides are often semiconductors with moderate to wide band gaps and one way of introducing charge carriers is through extensive doping. However, the presence of an extra d-electron in the outer electron shell of the rhenium (VI) ion leads to partial filling of the conduction band and manifests in its characteristic metallic conductivity and optical properties. The colloidal synthesis of these NCs follow an ether based reduction of a Re(VII) precursor to the Re(VI) oxide, different from the usual lysis of the metal alkylcarboxylate precursor by a nucleophile like water, alkylamines or alkyl alcohols. The primary reason for this departure is the availability of a range of oxidation states for Re (+2 to +7) and a careful control of this was necessary to obtain the NCs of interest, coupled with unavailability of suitable Re(VI) precursors. The as-prepared NCs exhibit an LSPR optical response in the visible- near-infrared region (centered at about 650 nm) which imparts an intense blue-green coloration to their solutions. The as- prepared NCs were surface terminated by hydroxyl moieties with additional stabilization in nonpolar solvents offered by L-type coordination by dioctyl ether molecules. The facile removal of the L-type ligands simply by washing with polar solvents enables ease of switching between solvents of choice for transferring these NCs to various substrates. The metallic nature of these NCs was further confirmed by Hall effect measurements on their films. Owing to the visible range LSPR, these metallic ReO3 NC films can serve as excellent surface enhanced Raman scattering (SERS) sensing substrates as demonstrated for rhodamine 6G as the probe molecule. Further, these NC films could undergo reversible phase transformation under electrochemical Li-cycling with concomitant changes in the optical response, as demonstrated by spectroelectrochemical measurements.