  Copper binary compounds are often intrinsic p-type semiconductors due to the
presence of Cu(I) vacancies, with corresponding hole carriers in the valence
band. If the free carrier concentration is high enough, localized plasmon
resonances can be sustained in nanocrystals, with frequencies in the infra-red
(<1 eV), with respect to the typical resonances seen in the visible range in
the case of metals (Ag, Au, ...). The localization of the resonances can be
demonstrated with scanning transmission electron energy loss spectroscopy
(STEM-EELS) by combining high spatial and high energy resolutions. Here we
demonstrate that Cu(I) vacancies can be directly measured from the STEM images
in Cu(3-x)P hexagonal nanocrystals. Two localized resonances can be seen from
STEM-EELS, which are in agreement with the resonances calculated from the
vacancy concentration obtained from the STEM.
