Andrey N. Baranov

Synthesis of rock-salt MeO–ZnO solid solutions (Me=Ni 2+, Co2+, Fe2+, Mn2+) at high pressure and high temperature

A series of metastable Me1−x Zn x O solid solutions (Me = Ni2+, Co2+, Fe2+, Mn2+) with the rock-salt (rs) crystal structure have been synthesized from binary oxides by quenching at 7.7 GPa and 1450–1650 K. Phase composition of the samples, as well as structural properties and stoichiometry of synthesized solid solutions, have been studied by X-ray powder diffraction, both conventional and with synchrotron radiation. The widest (0.3 ≤x≤0.8) composition range of the existence of individual rs solid solution has been established for the NiO–ZnO system. The bulk rs-Co1−x Zn x O, rs-Fe1−x Zn x O and rs-Mn1−x Zn x O solid solutions may be quenched down to ambient conditions with only twice as low ZnO content, i.e. x≤0.5, 0.5 and 0.4, respectively; while formation of rs solid solutions in the CdO–ZnO system has not been observed in the entire concentration range.

Resistive Switching in Al/Graphene Oxide/Al Structure

We report resistive switching behaviors in an Al/graphene oxide/Al planar structure. Graphene oxide was synthesized by a modified Hummers method from graphite rods. The planar structures were fabricated on a Si/SiO2 substrate by spin-coating graphene oxide suspensions and patterning Al electrodes by photolithography. Both diode-like (rectifying) and resistor-like (nonrectifying) behaviors were observed in the device switching characteristics. Electrical characterization of the Al/graphene oxide interface using the induced current identified a potential barrier near the interface and its spatial modulation, caused by local changes of resistance at a bias voltage, which correlated well with the resistive switching of the whole structure. The mechanism of the observed local resistance changes near the electrode and the associated resistive switching of the entire structure is associated with the electrodiffusion of oxygen and the formation of sp2 graphene clusters in an sp3 insulating graphene oxide layer formed near the electrode by a pre-forming process.

Quantum cutting in Li (770 nm) and Yb (1000 nm) co-dopant emission bands by energy transfer from the ZnO nano-crystalline host

Li-Yb co-doped nano-crystalline ZnO has been synthesized by a method of thermal growth from the salt mixtures. X-ray diffraction, transmission electron microscopy, atomic absorption spectroscopy and optical spectroscopy confirm the doping and indicate that the dopants may form Li-Li and Yb(3+)-Li based nanoclusters. When pumped into the conduction and exciton absorption bands of ZnO between 250 to 425 nm, broad emission bands of about 100 nm half-height-width are excited around 770 and 1000 nm, due to Li and Yb dopants, respectively. These emission bands are activated by energy transfer from the ZnO host mostly by quantum cutting processes, which generate pairs of quanta in Li (770 nm) and Yb (1000 nm) emission bands, respectively, out of one quantum absorbed by the ZnO host. These quantum cutting phenomena have great potential for application in the down-conversion layers coupled to the Si solar cells.

Kinetics of the Wurtzite-to-Rock-Salt Phase Transformation in ZnO at High Pressure

Kinetics of the wurtzite-to-rock-salt transformation in ZnO has been studied in the 5-7 GPa pressure range at temperatures below the activation of diffusion processes. The detailed analysis of non-isothermal experimental data using the general evolution equation describing the kinetics of direct phase transformations in solids allowed us to study the kinetic particularities of both nucleation and growth of the rock-salt phase in parent wurtzite ZnO. The main rate-limiting processes are thermally activated nucleation (E(N) = 383 kJ mol(-1) at 6.9 GPa) and thermally nonactivated (most probably quasi-martensitic) growth (k(G) = 0.833 min(-1) at 6.9 GPa). The high impact of thermal deactivation of nucleation places has been evidenced in the case of slow heating, which indirectly indicates that the rs-ZnO nucleation places are mainly produced by pressure-induced stresses in the parent phase.

Preparation, structural and optical characterization of nanocrystalline ZnO doped with luminescent Ag-nanoclusters

Nanocrystalline ZnO doped with Ag-nanoclusters has been synthesized by a salt solid state reaction. Three overlapping broad emission bands due to the Ag nanoclusters have been detected at about 570, 750 and 900 nm. These emission bands are excited by an energy transfer from the exciton state of the ZnO host when pumped in the wavelength range from 250 to 400 nm. The 900 nm emission band shows characteristic orbital splitting into three components pointing out that the anisotropic crystalline wurtzite host of ZnO is responsible for this feature. Heat-treatment and temperature dependence studies confirm the origin of these emission bands. An energy level diagram for the emission process and a model for Ag nanoclusters sites are suggested. The emission of nanocrystalline ZnO doped with Ag nanoclusters may be applied for white light generation, displays driven by UV light, down-convertors for solar cells and luminescent lamps.

Energy-transfer luminescence of a zinc oxide/ytterbium oxide nanocomposite

A newly structured nanocomposite material based on nanocrystalline ZnO/Yb2O3 has been prepared by thermal decomposition of Yb-doped zinc carbonate hydroxide. Transmission electron microscopy has revealed that the prepared nanopowder consists of ZnO nanocrystals of about 50 to 100 nm size decorated by attached smaller Yb2O3 nanocrystals of about 10 to 15 nm size. X-Ray absorption spectroscopy, in particular XANES and EXAFS, indicate the charge of Yb ions equals to +3 and their coordination is oxygen octahedral with the Yb–O and Yb–Yb interatomic distances the same as in bulk Yb2O3. Photoluminescence spectroscopy unambiguously proves an efficient excitation energy transfer from the ZnO nanocrystals to the Yb3+ ions. The energy transfer from the ZnO nanocrystals (absorption range from 250 to 400 nm) to the Yb3+ ions (emission range from 950 to 1100 nm) has been explained by a model, which considers the quantum cutting effect. The prepared nanocomposite is promising for application as a down-conversion layer for enhanced solar cells.

Effect of textured seeds on the morphology and optical properties of solution- and vapor-grown ZnO nanorod arrays

This paper examines the growth of aligned ZnO nanorod arrays through chemical deposition from solution and the vapor phase. The nanorod alignment is ensured primarily by a thin layer of seeds—zinc oxide nanoparticles produced by decomposing zinc acetate directly on the substrate and aligned with their c axes normal to the substrate surface. The acetate route was used to produce nanorod arrays 1 × 1 mm in dimensions on substrates with photoresist.

High-pressure synthesis and luminescent properties of cubic ZnO/MgO nanocomposites

The formation of the nanocrystalline rocksalt ZnO (rs-ZnO) has been in situ studied by x-ray diffraction with synchrotron radiation at high pressure and high temperature. A number of rs-ZnO/MgO nanocomposites with preset grain size were synthesized at 7 GPa and 800 K starting from wurtzite ZnO nanoparticles or nanorods. The use of MgO matrix allowed us to recover metastable rs-ZnO in the nanocrystalline form at ambient pressure. The cathodoluminescence measurements demonstrated the blue shift in the luminescence of rs-ZnO nanocrystals down to 402–408 nm that can be attributed to the enhanced incorporation of point defects with lower activation energy.

Synthesis of ZnO/NiO nanocomposites from ethanol solutions

We propose a process for the synthesis of ZnO/NiO nanocomposites from ethanolic solutions by means of consecutive generation of ZnO and NiO nanoparticles. X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) show that in the range 400–900°C, nanocomposites are two-phase mixtures of particles of hexagonal and cubic phases with ZnO dissolved in NiO; at 1000°C, Ni0.5Zn0.5O single-phase solid solution is generated. The mean particle size determined from TEM data and diffraction peak broadening increases with rising temperature. In the cathodoluminescence spectrum of a sample annealed at 400°C, the luminescence peak shifts to the UV. Specific magnetization versus magnetic field measurements in nanocomposites show hysteresis; the coercive force reaches 200 Oe.

SYNTHESIS OF ZNO NANOTETRAPODS

ZnO tetrapods have been grown on silicon substrates by chemical vapor deposition, and the effect of synthesis conditions on their morphology and size has been studied. The cathodoluminescence spectra of the tetrapods show two emissions characteristic of ZnO, in the UV and green spectral regions. Their relative intensities depend on the vapor composition during synthesis and annealing conditions. A mechanism of tetrapod growth at significant supersaturations is discussed.