Modified Semiconductor Band Diagrams Constructed from Optical Characterization of Size-Tunable Cu2O Cubes, Octahedra, and Rhombic Dodecahedra

By making Cu2O nanocubes, octahedra, and rhombic dodecahedra with tunable sizes and recording their light absorption and emission spectra, their absorption and emission bands shift steadily to longer wavelengths with increasing particle sizes from 10 nm to beyond 250 nm. Emission intensities are highest for the smallest nanocubes. Photoluminescence band shifts exceed 130 nm over this size range. For particles having the same volume, rhombic dodecahedra absorb light of shortest wavelength, while cubes show most red-shifted absorption with their band gaps differing by 0.17 eV (or 51.5 nm). They show obviously different colors. The presence of optical size and facet effects in semiconductors means that their emission wavelengths are tunable through facet control and use of nanocrystals much larger than quantum dots. A modified and general band diagram for Cu2O crystals has been constructed incorporating their optical size and facet effects with surface band bending. In addition, a more complete understanding of the different orders of surface band bending for the {100}, {111}, and {110} facets used in explaining the facet-dependent photocatalytic activity, electrical conductivity, and light absorption properties of Cu2O crystals is presented.