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

Photocatalytic Activity Suppression of CdS Nanoparticle-Decorated Cu₂O Octahedra and Rhombic Dodecahedra

Wurtzite CdS nanoparticles have been lightly deposited on Cu₂O cubes, octahedra, and rhombic dodecahedra to examine facet effects on the interfacial charge transfer in a photocatalytic reaction. Instead of an expected photocatalytic activity enhancement on the basis of a favorable band alignment at the heterojunction, CdS-decorated Cu₂O octahedra and rhombic dodecahedra show drastically reduced photocatalytic activities. Further increasing the CdS deposition amount leads to complete suppression of photocatalytic activity. Cu₂O cubes remain inactive even after CdS deposition. Transmission electron microscopy analysis reveals epitaxial growth of the (101) planes of CdS on the (110) planes of a Cu₂O rhombic dodecahedron, whereas the (110) planes of CdS align parallel to the (111) planes of a Cu₂O octahedron. Because facet-dependent photocatalytic activity can be understood from different degrees of band bending at the crystal surfaces, significantly upward bending for the CdS-contacting planes can explain the observed photocatalytic inactivity. This work demonstrates that strong facet effects tuning the band energies of both semiconductors at the heterojunctions make the predictions of an enhanced photocatalytic activity, simply through bulk band energy alignment analysis, highly unreliable

Thermoelectric Properties of Ag-Doped Bi₂(Se,Te)₃ Compounds: Dual Electronic Nature of Ag-Related Lattice Defects

Effects of Ag doping and thermal annealing temperature on thermoelectric transport properties of Bi₂(Se,Te)₃ compounds are investigated. On the basis of the comprehensive analysis of carrier concentration, Hall mobility, and lattice parameter, we identified two Ag-related interstitial (Ag_i) and substitutional (Ag_Bi) defects that modulate in different ways the thermoelectric properties of Ag-doped Bi₂(Se,Te)₃ compounds. When Ag content is less than 0.5 wt %, Ag_i plays an important role in stabilizing crystal structure and suppressing the formation of donor-like Te vacancy (V_Te) defects, leading to the decrease in carrier concentration with increasing Ag content. For the heavily doped Bi₂(Se,Te)₃ compounds (>0.5 wt % Ag), the increasing concentration of Ag_Bi is held responsible for the increase of electron concentration because formation of Ag_Bi defects is accompanied by annihilation of hole carriers. The analysis of Seebeck coefficients and temperature-dependent electrical properties suggests that electrons in Ag-doped Bi₂(Se,Te)₃ compounds are subject to a mixed mode of impurity scattering and lattice scattering. A 10% enhancement of thermoelectric figure-of-merit at room temperature was achieved for 1 wt % Ag-doped Bi₂(Se,Te)₃ as compared to pristine Bi₂(Se,Te)₃