Preparation of nanomaterials with controllable size and shape at ambient conditions, without heating or cooling, is extremely attractive from the perspective of cost and energy efficiency. However, highly reactive precursors must be used to obtain NCs at ambient conditions, and this can make the control of particle formation extremely challenging. Degenerately p-doped copper sulfide NCs have attracted much recent interest based on the observation of localized surface plasmon resonance (LSPR) in these materials. These earth-abundant semiconductor NCs have potential applications ranging from photovoltaics to biomedical imaging. Here, we provide the first report of ambient-temperature preparation of covellite nanoplatelets. The lateral dimensions of these are controllable over a wide range while maintaining a constant thickness of 4 nm. The crystalline phase of the NCs is shown here to be controlled by the oxidation state of copper reagent, with a Cu(II) precursor required to prepare phase-pure covellite NCs. The NCs exhibit LSPR absorbance that depends upon their aspect ratio (their lateral dimension, at fixed thickness) and can be tuned over a range of more than 600 nm. Their optical absorbance was modelled quantitatively to extract consistent values of free carrier concentration and background polarizability that apply over a wide range of NC sizes.

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