Dr. Yuri M. Strzhemechny
| Research - Current Projects |
| Generally, in nanosize crystals, the high surface area-to-volume ratio determines their distinct behavior. It yields a modified density of states and, hence, surface-enhanced phenomena, surface-induced strains, confinement, etc. On the other hand, surface is, in effect, the largest defect of the lattice. Moreover, during growth, then in the process of termination and reconstruction, the surface develops elevated concentration of defects. These defects are not necessarily confined within the immediate vicinity of the surface but may permeate tens of nanometers inward driven by diffusion, stress, cation/anions imbalance, etc. Importantly, in nanoscale objects, the volume ratio of the defective layer vs. relatively defect-free core would become very significant and should generate amplification of the defect-related phenomena scaling with the size of the nanoparticles. Thorough understanding of such phenomena is essential, especially in applications where interface reactivity, surface transport effects or confinement are at play. Nanocrystalline oxides are objects of intense research due to many attractive and beneficial properties. For example, nanoscale ZnO can be synthesized in different geometries, ZnO nanocrystals are chemically robust and stable, etc. It is well known that crystal surface plays a defining role in many applications, and especially on the nanometer scale. On the other hand, this quality of the material (and hence its crystal defect properties) is important for applications as well. In ZnO nanostructures (see above) defective surface layer may extend deep in to the bulk of the material, and thus be comparable to the nanocrystal size. Hence, the contribution of defects should increase with the decreasing size of nanocrystals. Currently an important issue is to correlate the size and morphology of the nanoscale materials with their performance-defining parameters. In our laboratory, we employ a low-temperature inexpensive method for synthesis of metal and metal oxide nanorostructures. By modifying the synthesis parameters we create different nanomorphologies. There is a rich interplay between the defect activity and the surface nanoscale processes. We are performing systematic studies on different nanostructures to elucidate not only the size-dependent effects but also the roles of dimensionality and morphology. Worldwide trend of increasing antibiotic resistance has spun interest in alternative antibacterial agents such as metal oxide particles. Whereas the antibacterial action of many such oxides is well established, the mechanism of this activity is largely unknown. Cytotoxicity could be mediated via such mechanisms as production of reactive oxygen species, release of toxic cations, charged interactions disrupting cell walls and causing osmotic stress. It is proposed that bacteria cannot develop resistance to oxide treatments because antimicrobial activity occurs through multiple mechanisms. Targeted applications of oxide antibacterials are also hindered by a lack of understanding of the role and nature of the local bacterial environment in mediating/hindering antibacterial interactions. In our studies, to address the nature of interactions between oxide surfaces, cellular membranes and bacterial growth media we employ hydrothermally synthesized nano- and microparticles of undoped and Fe-doped ZnO, Ga2O3, as well as GaOOH. Our growth method allows production of particles with tunable morphology and controllable relative abundances of surfaces with desired polarities. The biological assays with Escherichia coli and Staphylococcus aureus are used to examine the antibacterial action and also to run pre- and post-assay comparative studies of the oxide specimens themselves. For the latter we employ a variety of characterization techniques, such as electron microscopy, energy-dispersive X-ray spectroscopy, time and wavelength dependent surface photovoltage, temperature-dependent photoluminescence spectroscopy, etc. Titanium dioxide systems and their properties are studied for decades owing to many applications of this material. For example, this semiconductor is commonly used as a photocatalyst due to its relatively high activity as compared to other materials and its resistance to corrosion. Sensitization of this UV-sensitive system to visible light is a crucial issue. We investigate how doping and irradiation affect formation of nanoscale phases and entailing photocatalytic performance improvements. In particular, we study how addition of other metal oxides, such as zirconia and silica, and noble metal nanoparticles introduces additional levels within the bandgap. We also study how irradiation could modify electronic states available for visible light activation. Our results suggest the possibility of greater photocatalytic efficiencies in the specimens with more complex compositions due to greater number of sites responsive to visible light. Polymer/metal and polymer/metal oxide nanocomposites offer many new promising properties with a wide range of potential applications in optoelectronics, protective coating, sensors, etc. Due to the high nanoparticles surface-to-volume ratio, the surface of the filler is often the dominating factor responsible for the unique properties of a nanocomposite. Surface modification of the grains, by coating or embedding the particles in polymers, may significantly alter the optical and electronic properties of the nanoparticles. In our work, we investigate nanoscale metal particles (e.g. Ag) and nanoscale oxides, such as ZnO, barium titanate, etc. blended with polymers (polymethyl methacrylate, polystyrene, polyvinyl alcohol, etc.). Typically, such embedding leads to dramatic changes in the optical and electronic properties. We investigate mechanisms responsible for such effects. One of the main advantages of the multifunctional high vacuum system built in our lab is a versatile combination of in situ processing and characterization tools. It includes remote plasma treatment simultaneous with resistive annealing as well as such surface-sensitive probes as surface photovoltage (SPV) spectroscopy and Auger electron spectroscopy (AES). |
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