ZnO nanorods were prepared at relatively low temperatures by homogeneous precipitation from zinc nitrate and urea in a water/ethylene glycol mixture. Crystal size and morphology are controlled by growth time and solvent composition. We observed a strong correlation between nanocrystals’ size/morphology and their optoelectronic defect-related properties. Smaller crystallites exhibited intense deep-defect luminescence and enhanced emission related to near-surface excitonic recombination. Longer growth times lead to formation of well-defined nanorods with hexagonal symmetry exhibiting reduced defect emission.
In this work we studied electronic structure of sol–gel grown anatase thin films modified with gold, silver, and zirconia. Surface photovoltage spectroscopy and photoluminescence spectroscopy were employed to obtain band diagrams of TiO2, TiO2/Au, TiO2/Ag, and TiO2/ZrO2 thin films. Our results showed presence of energy levels at ∼2.2, ∼2.3–2.4, and ∼2.7–2.9 eV above the top of the valence band, common in all the films. Most of these states are related to oxygen deficiency. Addition of Ag results in noticeable changes in the electronic structure and, in particular, appearance of an energy level at ∼1.8 eV, apparently associated with Ag doping. Addition of Au did not result in significant changes in electronic states and no Au‐related gap states were detected. Zirconia addition resulted in an increased number of the native defect‐related states whereas no Zr‐related states were observed. These results are discussed in relation to the photocatalytic activity of the studied materials and the role of defect states.
Elucidation of microscopic properties of synthetic diamond films, such as formation and evolution of bulk and surface defects, chemistry of dopants, is necessary for a reliable quality control and reproducibility in applications. Surface photovoltage (SPV) spectroscopy and photoluminescence (PL) spectroscopy were employed to study diamond thin films grown on silicon by microwave plasma-assisted chemical vapor deposition and hot-filament chemical vapor deposition with different levels of boron doping in conjunction with gamma irradiation. SPV experiments showed that while the increase of boron concentration leads to a semiconductor-metal transition, subsequent gamma irradiation reverts quasi-metallic samples back to a semiconducting state by compensating electrical activity of boron possibly via hydrogen. One of the most pronounced common transitions observed at ∼3.1–3.2 eV in the SPV spectra was also present in all of the PL spectra. It is likely that this is a signature of the sp2-hybridized carbon clusters in or in the vicinity of grain boundaries.
In many instances the quality of the surface in ZnO nanoscale systems is a key performance-defining parameter. The surface itself could be a very significant source of lattice defects as well as contaminating impurities, and this influence may extend into the sub-surface vicinity. In our work, key element of the surface analysis is the surface photovoltage (SPV) spectroscopy known for its advantages, such as: identification of conduction vs. valence band nature of the defect-related transitions and the defect level positions within the band gap, ability to measure relatively low densities of surface defects as well as their cross sections. Additional information can be obtained from the SPV transient measurements. In our system, SPV characterization is run in high vacuum, complemented by in situ remote plasma treatment. This combination of surface-sensitive and surface-specific tools is well-suited for studying surface properties with a high degree of reliability since there is no exposure to common air contaminants between processing and characterization cycles. We employed O/He remote plasma treatments of ZnO nanocrystalline surfaces. In situ SPV spectra and transient measurements of the as-received and processed samples revealed, on the one hand, a number of common spectral features in different ZnO nanopowder specimens, and, on the other hand, a noticeable plasma-driven changes in the surface defect properties, as well as in the overall electronic and optical surface characteristics.
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