Colloidal ReO₃ Nanocrystals: Extra Re d-Electron Instigating a Plasmonic Response

Abstract

Rhenium (+6) oxide (ReO3) is metallic in nature, which means it can sustain localized surface plasmon resonance (LSPR) in its nanocrystalline form. Herein, we describe the colloidal synthesis of nanocrystals (NCs) of this compound, through a hot-injection route entailing the reduction of rhenium (+7) oxide with a long chain ether. This synthetic protocol is fundamentally different from the more widely employed nucleophilic lysing of metal alkylcarboxylates for other metal oxide NCs. Owing to this difference, the NC surfaces are populated by ether molecules through an L-type coordination along with covalently bound (X-type) hydroxyl moieties, which enables easy switching from nonpolar to polar solvents without resorting to cumbersome ligand exchange procedures. These as-synthesized NCs exhibit absorption bands at around 590 nm (∼2.1 eV) and 410 nm (∼3 eV), which were respectively ascribed to their LSPR and interband absorptions by Mie theory simulations and Drude modeling. The LSPR response arises from the oscillation of free electron density created by the extra Re d-electron per ReO3 unit in the NC lattice, which resides in the conduction band. Further, the LSPR contribution facilitates the observation of dynamic optical modulation of the NC films as they undergo progressive electrochemical charging via ion (de)­insertion. Ion (de)­insertion leads to distinct dynamic optical signatures, and these changes are reversible in a wide potential range depending on the choice of the ion (lithium or tetrabutylammonium). Nanostructuring in ReO3 and the description of the associated plasmonic properties of these NCs made this optical modulation feasible, which were hitherto not reported for the bulk material. We envisage that the synthetic protocol described here will facilitate further exploration of such applications and fundamental studies of these plasmonic NCs.

Synthesis and Dual-Mode Electrochromism of Anisotropic Monoclinic Nb12O29Colloidal Nanoplatelets

“…To study the mechanisms underlying an optical response to electrochemical charging, ligand-stripped NPL films were electrochemically reduced using two different cations for charge balance. Small Li + ions can both intercalate into the crystal lattice and be adsorbed on the surface, where they contribute to capacitance, while bulky tetrabutyl­ammonium (TBA + ) ions must remain only at the surface. , When an NPL film is electrochemically reduced with either electrolyte, the transmittance decreases as a result of increased light absorption (Figure a and b). Lower potentials correspond to a larger driving force to reduce the NPL films.…”

Plasmon-induced carrier polarization in semiconductor nanocrystals

“…11,20,[26][27] It is worth mentioning that, some metal oxides, such as VO2 and ReO3, are intrinsically metallic and could possess the LSPR in the NIR and even visible light region. [28][29] Aliovalent doping is a commonly used protocol to tune the electric properties of a semiconductor material in industry. With Sn-doped In2O3 (ITO) being one of the most widely used transparent conducting oxides (TCOs), the optical study of its nanocrystalline form opens the doors for investigation of prototype plasmonic semiconductors NCs.…”

Ultrathin Plasmonic Tungsten Oxide Quantum Wells with Controllable Free Carrier Densities

“…Nevertheless, to the best of our knowledge, this is the highest N e value reported for LSPR-active metal oxide nanostructures including recently reported spherical ReO 3 NCs. 41 Finally, we estimated the direct optical band gap of WO 3-x NPLs from a plot of (αhν) 2 versus hν (Tauc plot). We detected a monotonic increase in the band gap energy with increasing reaction temperature (Figure S8) attributable to the Burstein−Moss shift.…”