Tuning Intermediate-Band Cu₃VS₄ Nanocrystals from Plasmonic-like to Excitonic via Shell-Coating

Colloidal nanocrystals (NCs) can be generally classified as either excitonic or plasmonic. Excitonic NCs can find applications in photocatalysis and light emitting devices; in contrast, plasmonic NCs can be applied for sensing, nanophotonics, and photothermal conversion, although it is noteworthy that photocatalysis based on plasmon-induced hot carrier transfer is also emerging as an important field. Here we report that Cu₃VS₄ NCs, an intermediate-band material, can be tuned from plasmonic-like to excitonic simply via coating with CdS shells. The plasmonic-like behaviors of pristine Cu₃VS₄ NCs were confirmed by the spectral and dynamic properties of their optical resonances studied using transient absorption (TA) spectroscopy. Upon shell coating, the Cu₃VS₄/CdS core/shell NCs became quasi-type II hetero-NCs, with an emission quantum yield of ∼10% and an exciton lifetime of ∼600 ns. Because of effective electron delocalization throughout Cu₃VS₄ and CdS, we also demonstrated ultrafast and efficient electron extraction from the core/shell NCs. The tunability from plasmonic-like to excitonic behaviors is enabled by state mixing between the core and the shell, which is not limited to Cu₃VS₄ NCs but should be universally applicable to many other intermediate-band materials. Such a general strategy should greatly enrich the toolbox of colloidal NCs for many emerging applications.