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Optical absorption spectra of nanocrystalline cupric oxide: possible effects of nanoscopic phase separation

Optical absorption spectra of nanocrystalline cupric oxide CuO samples, obtained using the converging spherical shock wave procedure, reveal significant spectral weight red-shift as compared with spectra of single-crystalline CuO samples. In addition, some of these samples manifest remarkable temperature-dependent resonances near 1.3–1.6 eV. The minimal model suggested to explain both effects implies the CuO nanoceramic to be a metastable system of metallic-like nanoscale droplets with Drude optical response, nucleating in the bare insulating medium under the stimulating influence of the conditions of the specimen treatment. Simple effective medium theory is shown to provide a reasonable description of the experimental spectra in terms of the plasmon features due to the droplet phase.