UV-Assisted Room Temperature Gas Sensing with ZnO-Ag Heterostructure Nanocrystals Studied by Photoluminescence

Noble metal-metal oxide nanohybrids play a significant contribution in gas sensing applications at room temperature. Here, Ag-loaded ZnO with different Ag doping concentration are prepared by two-step polymer-network gel method, and NO2 sensing characteristics are tested at room temperature with various concentrations. The nanocrystal sizes are found to be more uniform with increasing with Ag concentration, and photoluminescence spectroscopy further reveals the different defects in ZnO–Ag nanocrystal lattices: pure ZnO has the largest intensity of the conduction band to valence band combination, and ZnO–Ag-1 (1 mol% Ag doping concentration) has the largest oxygen vacancy content, while ZnO–Ag-3 (3 mol% Ag doping concentration) has the largest excess zinc interstitial. It is showed that the gas sensing properties are independent of the size of nanocrystals, and more dependent on the nanocrystal defect structure. In this work, a new sensing mechanism is proposed according to the experimental results.

DC Aging Mechanism of Co2O3-Doped ZnO Varistors

Lowered power loss and asymmetrically electrical parameters are reported in the DC aging of Co2O3-doped ZnO varistors in this paper. Based on the frequency domain dielectric responses of the pristine and degraded samples, the present study explores the roles of point defects in the aging process via dielectric relaxations and their parameters. It is found that breakdown field increased more in the positive direction than the negative direction. Nonlinearity increased in the positive direction, whereas it decreased in the negative direction, and leakage current density increased more in the negative direction than the positive direction. Given the lowest migration energy of Zinc interstitial (Zni, 0.57 eV) and high oxygen ion conductivity in Bi2O3-rich phase, it is speculated that Zni and adsorbed oxygen (Oad) migrate under DC bias, and then change the defect structure and the double Schottky barrier (DSB) at grain boundaries. As a result, the forward-biased barrier height gradually decreases more than the reverse-biased one.

Role of the paramagnetic donor-like defects in the high n-type conductivity of the hydrogenated ZnO microparticles

The magnetic and electronic properties of the hydrogenated highly conductive zinc oxide (ZnO) microparticles were investigated by electron paramagnetic resonance (EPR) and contactless microwave (MW) conductivity techniques in the wide temperature range. The EPR spectra simulation allowed us to resolve four overlapping EPR signals in ZnO microparticles. The Lorentzian EPR line with isotropic g-factor 1.9623(5) was related to the singly ionized oxygen vacancy. Another Lorentzian line with g|| = 1.9581(5), g⊥ = 1.9562(5) was attributed to the zinc interstitial shallow donor center, while EPR signal with g|| = 1.9567(5), g⊥ = 1.9556(5) and Gaussian lineshape was assigned to the hydrogen interstitial shallow effective-mass-like donor. The EPR signal with g|| = 1.9538(5), g⊥ = 1.9556(5) and Lorentzian lineshape was tentatively attributed to the shallow donor center. The charge transport properties in ZnO microparticles have been investigated by the contactless MW conductivity technique at T = 5–296 K. Two conduction mechanisms, including ionization of electrons from the shallow donors to the conduction band and hopping conduction process, have been distinguished. The hopping conduction process follows Mott’s variable-range hopping T−1/4 law at T = 10–100 K. The evaluated values of the average hopping distance (15.86 Å), and hopping energy (1.822 meV at 40 K) enable us to estimate the donor concentration in the investigated ZnO microparticles as ~ 1018 cm−3.

Effects of the thermal annealing on the thermoelectric properties of Ga-doped ZnO thin films

We deposited successfully Ga-doped ZnO (GZO) thin films by using magnetron Dcsputtering technique, followed by annealing. The effects of the thermal annealing on thermoelectric properties of GZO films were investigated. The obtained results showed that due to annealing, the thermoelectric properties of the GZO films were significantly enhanced: (1) power factor increased with an increase of electron mobility due to high film crystallinity; (2) The figure of merit ZT values of the GZO film annealed at 500 oC (ZT = 0.114) was one order higher the asdeposited GZO film (ZT = 0.012). The room temperature photoluminescence (PL) spectra depicted various kinds of point defects which controlled thermoelectric properties and both oxygen vacancies VO and zinc interstitial Zni played an important role.

Morphological Modification and Analysis of ZnO Nanorods and Their Optical Properties and Polarization

We report on the effect of the morphological modification on optical properties and polarization of ZnO nanorods (NR). Here, the morphology and structure of the ZnO NR were modified by introducing different annealing temperatures. The increase of length and diameter and change in density of the ZnO NR were clearly observed by increasing the annealing temperature. We found that the samples show different oxygen vacancy (VO) and zinc interstitial (ZnI) concentrations. We suggest that the different concentrations of VO and ZnI are originated from morphological and structural modification. Our results reveal that optical absorption and polarization of ZnO NR could be enhanced by producing a high concentration of VO and ZnI. The modification of VO and ZnI promotes a decrease in the band gap and coexistence of high optical absorption and polarization in our ZnO NR. Our findings would give a broad insight into the morphological modification and characterization technique on ZnO NR. The high optical and polarization characteristics of ZnO NR are potential for developing the high-performance nanogenerator device for multitype energy harvesting.

Photoluminescence and point defect related emission of ZnO:Mn2+ micro/nanorod fabricated by co-precipitation method

Herein we study point defects and correlation to photoluminescence in ZnO nanorod. ZnO mirco/nanorod structure was successfully fabricated by co-precipitation method with highly homogeneous characteristics. When ion Mn+2 introduced into ZnO structure, the d-spacing distance of ZnO was increased from 0.248 nm to 0.295 nm due to the larger ionic radius of Mn2+ in comparison to Zn2+. The photoluminescence emission evolution of ZnO through doping and annealing processes hinted the relation of point defect transformations. We found that zinc interstitial, zinc vacancy and its related defects were responsible mainly for photoluminescence emission in annealing and/or Mn2+ doped samples.