Near-infrared plasmonics with vacancy doped semiconductor nanocrystals

Plasmonics with heavily doped semiconductor nanocrystals (NCs) is
an emerging field in NC science. However, impurity doping of NCs
remains far from trivial and is, as yet, dominated by a low
chemical control over the incorporated dopant atoms. An appealing
alternative is vacancy doping, where the formation of vacancies in
the structure is responsible for an increased carrier density and
elegantly circumvents the issues related to impurity doping. Due to
high carrier densities of around 10^21cm^(-3) localized surface
plasmon resonances (LSPRs) in the near infrared (NIR) are expected,
and as such highlighted to close the gap between conventionally
doped NCs and noble metal nanoparticles. Copper chalcogenide NCs,
namely copper sulfide (Cu2-xS), copper selenide (Cu2-xSe), and
copper telluride (Cu2-xTe), are an attractive example of vacancy
doped semiconductor NCs, with spectra dominated by intense NIR
resonances. Within this study thorough experimental evidence has
been given to prove the plasmonic nature of those NIR resonances.
By presenting typical plasmonic characteristics, such as refractive
index sensitivity of the LSPR, its intrinsic size dependence,
plasmon dynamics, or interparticle plasmon coupling, the LSPRs in
copper chalcogenide NCs have unambiguously been identified. The
chemical nature of vacancy doping turns out to deliver an
additional, highly attractive means of control over the LSPR in
vacancy doped copper chalcogenide NCs. Through chemical tailoring
of the copper vacancy density via controlled oxidation and
reduction, as shown in this study, a reversible tuning of the LSPR
over a wide range of frequencies in the NIR (1000-2000 nm) becomes
feasible. This highlights copper chalcogenide NCs over conventional
plasmonic materials. Notably, the complete suppression of the LSPR
uncovers the excitonic features present only in the purely
semiconducting, un-doped NCs and reveals the unique option to
selectively address excitons and highly tunable LSPRs in one
material (bandgap Eg~1.2 eV). As such, copper chalcogenide NCs
appear to hold as an attractive material system for the
investigation of exciton plasmon interactions. Indeed, a quenching
of the excitonic transitions in the presence of the developing LSPR
is demonstrated within this work, with a full recovery of the
initial excitonic properties upon its suppression. A theoretical
study on the shape dependent plasmonic properties of Cu2-xTe NCs
reveals a deviation from the usual Drude model and suggests that
the carriers in vacancy doped copper chalcogenide NCs cannot be
treated as fully free. On the other hand, the Lorentz model of
localized oscillators appears to account for the weak shape
dependence, as observed experimentally, indicating an essential
degree of localization of the carriers in vacancy doped copper
chalcogenide NCs. Taken together, this work delivers a huge step
toward the complete optical and structural characterization of
plasmonic copper chalcogenide NCs. The advantages of semiconductor
NC chemistry have been exploited to provide access to novel
plasmonic shapes, such as tetrapods that have not been feasible to
produce so far. A precise size, shape and phase control presents
the basis for this study, and together with a thorough theoretical
investigation delivers important aspects to uncover the tunable
plasmonic properties of vacancy doped copper chalcogenide NCs.