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.