Effect of thermochemical reduction on the electrical, optical-absorption, and positron-annihilation characteristics of ZnO crystals

Luminescence Properties of Ho2O3-Doped Y2O3 Stabilized ZrO2 Single Crystals

Single crystals of Ho2O3-doped Y2O3 stabilized ZrO2 (YSZ) with different Y2O3 and Ho2O3 contents were grown by the optical floating zone method. XRD and Raman spectra were measured and showed that crystal samples all had tetragonal structures. Measurements of positron annihilation lifetime spectra indicated that the increase in Y2O3 concentration led to the increases of defects and mean positron lifetime, which enhanced the scattering of light and reduced the luminous intensity and the quantum yield (QY) of the crystal. Under the excitation at 446 nm, photoluminescence (PL) spectra of Ho2O3-doped YSZ crystals showed emission peaks at 540, 551, 670, and 757 nm corresponding to Ho3+ transitions from 5S2, 5F4, 5F5, and 5I4 excited states to the 5I8 ground state, respectively. At low Ho2O3-doped concentrations (0.10–0.50 mol%), the overall emission intensity increased with Ho2O3 contents, reached the maximum value at 0.50 mol%, then decreased with higher Ho2O3 contents, probably as a result of increased non-radiative relaxation caused by increased interactions between Ho3+ ions. Quenching of the PL occurred at Ho2O3 concentrations > 0.5 mol% and due to the electric dipole–dipole interaction. The calculated chromaticity coordinates (CIE) were approximately (0.307, 0.683) and the color purity achieved 99.6%. The results showed that Ho2O3: YSZ crystals were suitable for green light-emitting devices.

Oxygen-Cluster-Modified Anatase with Graphene Leads to Efficient and Recyclable Photo-Catalytic Conversion of CO2 to CH4 Supported by the Positron Annihilation Study

Anatase TiO2 hollow nanoboxes were synthesized and combined with the graphene oxide to get nanocomposite of TiO2/rGO (TG). Graphene oxide was used to modify the Oxygen-Clusters and bulk to surface defects. Anatase and TG composite were characterized with the positron annihilation, XPS, EPR, EIS and photocurrent response analysis. The relative affects of defects on the photocatalytic reduction (CO2 to CH4) were studied. The TG composites showed highest photo-catalytic activity after GO coupling (49 µmol g-1 h-1), 28.6 times higher photocurrent yields much higher quantum efficiency (3.17%@400 nm) when compared to the TiO2 nanoboxes. The mechanism of enhanced photo-catalytic CO2 conversion to CH4 elucidated through electrochemical and photo-catalytic experiments with traceable isotope containing carbon dioxide (13CO2). For the first time we discovered that diminishing the comparative concentration ratio of anatase from the bulk to surface defects could significantly increase the conversion of CO2 to CH4.

ZnO Luminescence and scintillation studied via photoexcitation, X-ray excitation, and gamma-induced positron spectroscopy

The luminescence and scintillation properties of ZnO single crystals were studied by photoluminescence and X-ray-induced luminescence (XRIL) techniques. XRIL allowed a direct comparison to be made between the near-band emission (NBE) and trap emissions providing insight into the carrier recombination efficiency in the ZnO crystals. It also provided bulk luminescence measurements that were not affected by surface states. The origin of a green emission, the dominant trap emission in ZnO, was then investigated by gamma-induced positron spectroscopy (GIPS) - a unique defect spectroscopy method that enables positron lifetime measurements to be made for a sample without contributions from positron annihilation in the source materials. The measurements showed a single positron decay curve with a 175 ps lifetime component that was attributed to Zn vacancies passivated by hydrogen. Both oxygen vacancies and hydrogen-decorated Zn vacancies were suggested to contribute to the green emission. By combining scintillation measurements with XRIL, the fast scintillation in ZnO crystals was found to be strongly correlated with the ratio between the defect luminescence and NBE. This study reports the first application of GIPS to semiconductors, and it reveals the great benefits of the XRIL technique for the study of emission and scintillation properties of materials.

Understanding the effect of surface/bulk defects on the photocatalytic activity of TiO2: anatase versus rutile

The sole effect of surface/bulk defects of TiO2 samples on their photocatalytic activity was investigated. Nano-sized anatase and rutile TiO2 were prepared by hydrothermal method and their surface/bulk defects were adjusted simply by calcination at different temperatures, i.e. 400-700 °C. High temperature calcinations induced the growth of crystalline sizes and a decrease in the surface areas, while the crystalline phase and the exposed facets were kept unchanged during calcination, as indicated by the characterization results from XRD, Raman, nitrogen adsorption-desorption, TEM and UV-Vis spectra. The existence of surface/bulk defects in calcined TiO2 samples was confirmed by photoluminescence and XPS spectra, and the surface/bulk defect ratio was quantitatively analyzed according to positron annihilation results. The photocatalytic activity of calcined TiO2 samples was evaluated in the photocatalytic reforming of methanol and the photocatalytic oxidation of α-phenethyl alcohol. Based on the characterization and catalytic results, a direct correlation between the surface specific photocatalytic activity and the surface/bulk defect density ratio could be drawn for both anatase TiO2 and rutile TiO2. The surface defects of TiO2, i.e. oxygen vacancy clusters, could promote the separation of electron-hole pairs under irradiation, and therefore, enhance the activity during photocatalytic reaction.

Tuning the relative concentration ratio of bulk defects to surface defects in TiO2 nanocrystals leads to high photocatalytic efficiency

TiO(2) nanocrystals with tunable bulk/surface defects were synthesized and characterized with TEM, XRD, BET, positron annihilation, and photocurrent measurements. The effect of defects on photocatalytic activity was studied. It was found for the first time that decreasing the relative concentration ratio of bulk defects to surface defects in TiO(2) nanocrystals could significantly improve the separation efficiency of photogenerated electrons and holes, thus significantly enhancing the photocatalytic efficiency.

Positron annihilation lifetime and photoluminescence studies on single crystalline ZnO

The room temperature positron annihilation lifetime for single crystalline ZnO has been measured as 164 ± 1 ps. The single component lifetime value is very close to but higher than the theoretically predicted value of ~154 ps. Photoluminescence study (at 10 K) indicates the presence of hydrogen and other defects, mainly acceptor related, in the crystal. Defects related to a lower open volume than zinc vacancies, presumably a complex with two hydrogen atoms, are the major trapping sites in the sample. The bulk positron lifetime in ZnO is expected to be a little less than 164 ps.

Microstructural Evolution of Radiation Induced Defects In ZnO During Isochronal Annealing

In this study we discuss the microstructural changes after electron and proton irradiation and the thermal evolution of the radiation induced defects during isochronal annealing. The nominally undoped samples were irradiated either with 3 MeV protons to a fluence of 1.2× 1018 p/cm2 or with 1 MeV electrons to a fluence of 1×1018 e/cm2. The investigation was performed with positron lifetime and Doppler-broadening measurements. The measurements were done at room temperature and in some cases down to 10 K to investigate the thermal dependence of the trapping characteristics of the positrons.

Photoinduced formation of zinc nanoparticles by UV laser irradiation of ZnO

Simple exposure of single-crystal ZnO to 193 nm excimer laser radiation at room temperature results in unexpected coloration. The gray to nearly black colored material, seen principally in the irradiated laser spot, is superficial. We present unambiguous evidence that this coloration is due to high densities of metallic Zn nanoparticles growing on the exposed surface of the crystal. Higher fluence laser exposure generates accumulated surface metal just outside of the irradiated spot. We suggest that the near surface bulk is photodecomposing; thermally driven diffusion leads to surface Zn metal aggregation.

Direct kinetic correlation of carriers and ferromagnetism in Co2+: ZnO

The hypothesis that high-Curie-temperature ferromagnetism in cobalt-doped ZnO (Co2+: ZnO) is mediated by charge carriers was tested by controlled introduction and removal of the shallow donor interstitial zinc. Using oriented epitaxial Co2+: ZnO films grown by chemical vapor deposition, kinetics measurements demonstrate a direct correlation between the oxidative quenching of ferromagnetism and the diffusion and oxidation of interstitial zinc. These results demonstrate controlled systematic variation of a key parameter involved in the ferromagnetism of Co2+: ZnO and, in the process, unambiguously reveal this ferromagnetism to be dependent upon charge carriers. The distinction between defect-bound and free carriers in Co2+: ZnO is discussed.

Evidence of the Zn vacancy acting as the dominant acceptor in n-type ZnO

We have used positron annihilation spectroscopy to determine the nature and the concentrations of the open volume defects in as-grown and electron irradiated (E(el)=2 MeV, fluence 6 x 10(17) cm(-2)) ZnO samples. The Zn vacancies are identified at concentrations of [V(Zn)] approximately 2 x 10(15) cm(-3) in the as-grown material and [V(Zn)] approximately 2 x 10(16) cm(-3) in the irradiated ZnO. These concentrations are in very good agreement with the total acceptor density determined by temperature dependent Hall experiments. Thus, the Zn vacancies are dominant acceptors in both as-grown and irradiated ZnO.