Switching
Plasmons: Gold Nanorod–Copper Chalcogenide
Core–Shell Nanoparticle Clusters with Selectable Metal/Semiconductor
NIR Plasmon Resonances
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Abstract
Exerting control over the near-infrared
(NIR) plasmonic response
of nanosized metals and semiconductors can facilitate access to unexplored
phenomena and applications. Here we combine electrostatic self-assembly
and Cd<sup>2+</sup>/Cu<sup>+</sup> cation exchange to obtain an anisotropic
core–shell nanoparticle cluster (NPC) whose optical properties
stem from two dissimilar plasmonic materials: a gold nanorod (AuNR)
core and a copper selenide (Cu<sub>2–<i>x</i></sub>Se, <i>x</i> ≥ 0) supraparticle shell. The spectral
response of the AuNR@Cu<sub>2</sub>Se NPCs is governed by the transverse
and longitudinal plasmon bands (LPB) of the anisotropic metallic core,
since the Cu<sub>2</sub>Se shell is nonplasmonic. Under aerobic conditions
the shell undergoes vacancy doping (<i>x</i> > 0), leading
to the plasmon-rich NIR spectrum of the AuNR@Cu<sub>2–<i>x</i></sub>Se NPCs. For low vacancy doping levels the NIR optical
properties of the dually plasmonic NPCs are determined by the LPBs
of the semiconductor shell (along its major longitudinal axis) and
of the metal core. Conversely, for high vacancy doping levels their
NIR optical response is dominated by the two most intense plasmon
modes from the shell: the transverse (along the shortest transversal
axis) and longitudinal (along the major longitudinal axis) modes.
The optical properties of the NPCs can be reversibly switched back
to a purely metallic plasmonic character upon reversible conversion
of AuNR@Cu<sub>2–<i>x</i></sub>Se into AuNR@Cu<sub>2</sub>Se. Such well-defined nanosized colloidal assemblies feature
the unique ability of holding an all-metallic, a metallic/semiconductor,
or an all-semiconductor plasmonic response in the NIR. Therefore,
they can serve as an ideal platform to evaluate the crosstalk between
plasmonic metals and plasmonic semiconductors at the nanoscale. Furthermore,
their versatility to display plasmon modes in the first, second, or
both NIR windows is particularly advantageous for bioapplications,
especially considering their strong absorbing and near-field enhancing
properties