Direct Quantification of Cu Vacancies and Spatial Localization of Surface Plasmon Resonances in Copper Phosphide Nanocrystals

Copper chalcogenides and pnictogenides often behave as heavily doped p-type semiconductors, because of the presence of a high density of Cu vacancies, with corresponding hole carriers in the valence band. If the free-carrier concentration is high enough, localized surface plasmon resonances can be sustained in nanocrystals of these materials, with frequencies that are typically observed in the infrared region of the spectrum (<1 eV), differently from the typical resonances featured in the visible range by metallic nanoparticles. Here, we demonstrate that Cu vacancies in hexagonal Cu3-xP nanocrystals can be directly quantified by scanning transmission electron microscopy (STEM) analysis. We also report, for the first time, the spatial localization of the plasmon resonances in individual Cu3-xP nanocrystals by means of STEM energy loss spectroscopy (EELS), which is an achievement that, to date, had been possible only on nanoparticles of noble metals. Two plasmon modes can be seen from STEM-EELS, which are in agreement with the resonances calculated from the vacancy concentration obtained from the STEM analysis.