Size-dependent conductivity-type inversion in ${\mathrm{Cu}}_2\mathrm{O}$ nanoparticles

X-ray photoemission spectroscopy and optical absorption studies have been carried out on ${\mathrm{Cu}}_2\mathrm{O}$ nanoparticles prepared using the activated reactive evaporation technique. A strong shift of the valence band edge (from 0.7 to 1.8 eV) has been observed on reduction of the nanoparticle size from 20 nm to 4 nm. The size-dependent modifications in the energy level diagram and ${\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}-\mathrm{C}\mathrm{u}}_2\mathrm{O}$ nanoparticle Schottky junction characteristics confirm the observation of the inversion of conductivity type from $p$-type bulk conductivity to $n$-type conductivity in ${\mathrm{Cu}}_2\mathrm{O}$ nanoparticles at lower dimensions. This study can initiate a further methodology for tailoring the semiconductor properties of a material by controlling the nanoparticle size.