Yuri M. Strzhemechny

Dominant Effect of Near-Interface Native Point Defects on ZnO Schottky Barriers

The authors used depth-resolved cathodoluminescence spectroscopy and current-voltage measurements to probe metal-ZnO diodes as a function of native defect concentration, oxygen plasma processing, and metallization. The results show that resident native defects in ZnO single crystals and native defects created by the metallization process dominate metal-ZnO Schottky barrier heights and ideality factors. Results for ZnO(0001¯) faces processed with room temperature remote oxygen plasmas to remove surface adsorbates and reduce subsurface native defects demonstrate the pivotal importance of crystal growth quality and metal-ZnO reactivity in forming near-interface states that control Schottky barrier properties.

Enhanced room temperature excitonic luminescence in ZnO/polymethyl methacrylate nanocomposites prepared by bulk polymerization

Homogeneous ZnO/polymethyl methacrylate (PMMA) nanocomposites were prepared by incorporating ZnO nanoparticles synthesized in various diols into a PMMA matrix by the free-radical bulk polymerization. Room temperature photoluminescence spectra of the as-grown and PMMA-embedded ZnO nanoparticles exhibit an excitonic band-gap emission at 3.3 eV, a near band-gap emission at ∼3.1 eV and a broad defect band centered at ∼2.4 eV. Relative intensity of the defect versus band-gap luminescence depends on the parameters of ZnO preparation as well as the average particle size. However, PMMA-embedded particles produce a much stronger excitonic luminescence, whereas the ratio of the 3.1 to 2.4 eV remains approximately constant. There is no indication of random lasing threshold pointing to the ZnO/PMMA interfacial origin of the enhanced band-gap emission.

On microscopic compositional and electrostatic properties of grain boundaries in polycrystalline CuIn1−xGaxSe2

We report on the microscopic characteristics of polycrystalline CuIn1−xGaxSe2 thin films probed with Auger electron spectroscopy, cathodoluminescence spectroscopy, secondary ion mass spectrometry, and work function measurements. Confirming theory, we find a substantial reduction in Cu content from grain interior to boundary and a p-type potential barrier that acts to reduce hole recombination. Such compositional and electrostatic variations between grain boundaries and grain interiors in CuIn1−xGaxSe2 solar cell absorber layers may improve the overall photovoltaic efficiency. The high degree of intergranular inhomogeneity emphasizes the importance of detailed grain-by-grain analysis. These results show that careful specimen preparation and ultrahigh vacuum conditions, coupled with nanoscale instrumental resolution, are pivotal for such analysis.

Low energy electron-excited nanoscale luminescence spectroscopy studies of intrinsic defects in HfO2 and SiO2–HfO2–SiO2–Si stacks

Low energy electron-excited nanoscale (LEEN) luminescence spectroscopy and secondary ion mass spectrometry have been used to probe the defect states and chemical composition in as-deposited relatively thick (∼100nm) HfO2 films and in SiO2∕HfO2∕SiO2∕Si (5nm∕15nm∕5nm) heterojunction stacks grown by plasma enhanced chemical vapor deposition including as well changes in bonding and defects after high temperature (900°C) annealing. LEEN measurements of optical transitions in the thicker HfO2 films are assigned to defect-associated radiative transitions centered at approximately 2.7, 3.4, 4.2 and 5.5eV. These spectra exhibited significant changes in as-deposited films (300°C) and after a 900°C anneal in forming gas (N2∕H2). Qualitative differences in LEEN spectra of stacked films are correlated with (i) formation of Hf silicate during deposition of the HfO2 film onto the SiO2 substrates in the as-deposited films, and (ii) a chemical phase separation of these Hf silicates into a heterogeneous mixture SiO2 and Hf...

Growth of zinc oxide particles in the presence of silicon

In this study we report on the growth behavior of zinc oxide in the presence of different concentrations of silicon. We performed reactions in a continuous tubular reactor in aqueous and ethanolic-aqueous media at different reaction temperatures and for different residence times. It was found that the zinc oxide particles grew from the aqueous medium through the crystallization of amorphous zinc oxide units via different growth stages, i.e., plate-like particles, double-plate-like particles and ellipsoidal particles. The Zn/OH atomic ratio and, consequently, the pH control the particle size and the morphology (i.e., the inclusions on the particle surfaces). The presence of silicon in the reaction solution influences the particle morphology (the formation of planar defects), the particle growth stages (the double-plate-like particles) as well as the particle growth rate. By shifting the UV-VIS absorption maxima of the reaction suspensions we showed that an increased silicon concentration and a decreased reaction temperature significantly retarded the growth rate of the zinc oxide.

MODIFIED EXFOLIATED GRAPHITE AS A MATERIAL FOR SHIELDING AGAINST ELECTROMAGNETIC RADIATION

We report on the structure and morphology of thermoexfoliated graphite (TEG) powders and TEG–metal (Co, Cu, Ni) powders. Electrodynamic parameters of the compacted TEG and TEG–metal specimens have been studied along the compacting axis (c axis) within the range of electromagnetic radiation frequencies between 25.5 and 37.5 GHz. Metal particles attached to the surface of the TEG particles yield enhanced radiation shielding within the entire frequency range. Moreover, it is observed that the absorption coefficient increases with the increase in conductivity of the metal particles and is enhanced due to a high concentration of TEG–metal boundaries, which promote multiple reflections.

Luminescent Properties of Surface Functionalized BaTiO3 Embedded in Poly(methyl methacrylate)

As-received BaTiO3 nanopowders of average grain sizes 50 nm and 100 nm were functionalized by (3-aminopropyl)triethoxysilane (APTES) and mixed with poly(methyl methacrylate)/toluene solution. The nanocomposite solution was spin coated on Si substrates to form thin films. The photoluminescence spectrum of the pure powder was composed of a bandgap emission at 3.0 eV and multiple bands centered about 2.5 eV. Surface functionalization of the BaTiO3 powder via APTES increases overall luminescence at room temperature while only enhancing bandgap emission at low-temperature. Polymer coating of the functionalized nanoparticles significantly enhances bandgap emissions while decreasing emissions associated with near-surface lattice distortions at 2.5 eV.

Magnetometric Studies of Catalyst Refuses in Nanocarbon Materials

It is shown that magnetometry can be employed as an effective tool to control the content of a ferromagnetic constituent in nanocarbon materials. We propose a thermochemical treatment protocol to achieve extensive cleaning of the source nanocarbon materials from ferromagnetic refuses.

Correlation Between Optoelectronic and Positron Lifetime Properties in As‐received and Plasma‐treated ZnO Nanopowders

We employed photoluminescence and positron lifetime measurements on a number of commercially available ZnO nanopowders. The experiments were performed before and after processing of these samples in remote N and O/He plasma. In all the nanopowders, the average lifetime component is substantially longer than in a single‐crystalline sample, consistent with the model of grains with defect‐rich surface and subsurface layers. However, the sample‐to‐sample differences in the quality of the powders, as detected by the photoluminescence spectroscopy, obscure observation of possible size effects. Compression of the powders into pellets yields reductions of the average positron lifetimes. Plasma‐induced modifications are most visible in the low‐temperature photoluminescence spectra of the smallest nanocrystals, indicative of a surface‐specific nature of the chosen treatment procedure.

Correlation of defect-related optoelectronic properties in Zn 5 (OH) 6 (CO 3 ) 2 /ZnO nanostructures with their quasi-fractal dimensionality

Hydrozincite (Zn5(OH)6(CO3)2) is, among others, a popular precursor used to synthesize nanoscale ZnO with complex morphologies. For many existing and potential applications utilizing nanostructures, performance is determined by the surface and subsurface properties. Current understanding of the relationship between the morphology and the defect properties of nanocrystalline ZnO and hydrozincite systems is still incomplete. Specifically, for the latter nanomaterial the structure-property correlations are largely unreported in the literature despite the extensive use of hydrozincite in the synthesis applications. In our work, we addressed this issue by studying precipitated nanostructures of Zn5(OH)6(CO3)2 with varying quasi-fractal dimensionalities containing relatively small amounts of a ZnO phase. Crystal morphology of the samples was accurately controlled by the growth time. We observed a strong correlation between the morphology of the samples and their optoelectronic properties. Our results indicate that a substantial increase of the free surface in the nanocrystal samples generates higher relative concentration of defects, consistent with the model of defect-rich surface and subsurface layers.