Search

The effects of surface conditions on photo-induced-carrier (PIC) diffusion are studied by solving one-dimensional continuity equation. Using various forms of ZnO samples with different surface conditions, the PICs lost at sample surface are substantiated to have important contributions to the visible luminescence. For unpassivated ZnO nanorods and single-crystal samples that undergo a high-temperature annealing in air, the visible luminescence originating from the PICs diffused to the surface is estimated more than 99.8 % and 70 % at room temperature, respectively, and more than 99.99 % and 97 % at low temperature. The structureless luminescence caused by PICs, such as the green, yellow, and orange luminescence bands are well fitted under the framework of tail-state luminescence, while the structured green band is suggested to be in association with a specific type of surface states formed during high-temperature annealing in oxidizing atmosphere and can be fitted with Huang-Rhys model by taking into account tail-state luminescence. [ABSTRACT FROM AUTHOR]

The photoluminescence spectra excited by different wavelength light from ZnO samples with and without SiO x passivation demonstrated that the excitation behavior of structured green luminescence (GL-S) band is determined by the surface conditions or the morphological forms. The lowest energy of photons able to excite the GL-S band varied from 3.380 eV down to a value far below the zero phonon line of GL-S band, depending on the defect density in the region near to the sample surface. The imperfections like cracks, scratches, and voids etc. were found decisive for the effectiveness of SiO x passivation on the GL-S band, which is removable only for the samples with good surface conditions. In addition, the phonon energy in GL-S band was determined to be ~3 meV lower than that of longitudinal optical phonons and thus was associated with the surface optical mode. The evidence of a link between the GL-S band and the optical emissions on sample surface revealed that there is no necessity to connect the GL-S band with any specific defects in the inside of samples. This optical emission mechanism is not only helpful to settle the relevant issues of visible luminescence in the research of ZnO, but also important to the study of other phenomena related to photoexcitation. • GL-S band of ZnO is substantiated to originate from the optical emissions on surface. • Excitation behavior of GL-S band depends the density of defects in the near surface region. • Surface imperfections are decisive for the effectiveness of SiO x passivation on GL-S band. • Phonons in the GL-S band is assigned to the surface optical mode. [ABSTRACT FROM AUTHOR]

Highly epitaxial ZnO films deposited on c -plane sapphire substrate by pulsed laser deposition exhibited enhanced extraordinary spectral features in the near-band-edge region, which were studied in comparison with the annealed sample and the bulk ZnO crystal. Based on the characterization in structure and photoluminescence, the near-band-edge emission was assigned to the tail-state luminescence due to the exciton localization, which is different from the bound excitons, instead has the continuous density of states at the deeper levels, probably created by the defects, chemical disorders, and the lattice strains. [ABSTRACT FROM AUTHOR]

The flower-like Ag-ZnO nanorod exhibited a high SERS activity and a long-term optical stability with a shelf life longer than two years. • The flower-like Ag-ZnO NRs exhibited high SERS activity and enable 10−14 M R6G to be detected. • The SERS substrates have a shelf life longer than two years prior to use. • Electron transfer between Ag and ZnO was proved to be important for the SERS stability. The optical stability of silver-coated flower-like ZnO nanorod (NR) arrays were studied in terms of localized surface plasmon resonance and surface-enhanced Raman spectroscopy (SERS). As a highly sensitive SERS substrate, the silver-coated ZnO NR arrays enabled as low as 10−14 M Rhodamine-6G (R6G) molecules to be detected and exhibited a long-term optical stability with a shelf life longer than two years. Using specially prepared samples, the Ag-ZnO nanostructures were studied as a metal–semiconductor junction and the importance of electron transfer between the silver and ZnO NRs for optical stability of silver in the SERS substrates was demonstrated. By addressing the growth behavior of silver in different samples, the good crystallinity of silver was suggested to be another crucial factor of maintaining the SERS activity long-term stable. [ABSTRACT FROM AUTHOR]

ZnO nanorod (NR) arrays in-situ grown on indium tin oxide (ITO) substrates by a solution method are investigated by optimizing the solution concentration and the thickness of ZnO seed layer. Using 5–1000 ppm ethanol as the target gas, the gas sensor prepared with as-grown ZnO NR arrays in 25 mM solution with ∼ 10-nm thick ZnO seed layer is found to have the best gas response at 250 °C and exhibit a pretty good stability. The morphology of ZnO NR arrays is found important for optimizing the gas sensing property and is predominated by coalescence, which varies with solution concentration and the thickness of ZnO seed layer, thus leading to a variation in the defect density and surface state of ZnO NRs and enabling the gas sensing property not to be determined only by the specific surface area of ZnO NRs. Different thick ZnO seed layers will convert into the low-resistance states within a few seconds to minutes after applying an electric voltage and thus the role of ZnO seed layers in determining the gas sensing performance is mainly to control the diameter distribution of ZnO NRs, in which the arrays with a relatively large difference in the diameter of ZnO NRs would exhibit good gas response. [Display omitted] • Gas sensing property of ZnO NR sensors is determined by the morphology. • Coalescence dominates the morphology of ZnO NRs and varies with solution. • A large difference in the diameters of ZnO NRs leads to a good gas response. • Thickness of ZnO seed layer is important for optimizing ZnO NR sensors. • ZnO seed layer on ITO converts into low-resistance state within a short time. [ABSTRACT FROM AUTHOR]

Due to the high sensitivity and lithographic compatibility, gas sensors prepared with ZnO nanorod (NR) arrays in-situ grown on substrates receive much attention. In this work, ZnO NR arrays are grown on ITO glass by a solution method and the thermal stability is studied. It is found that the instability of as-grown sample can be associated with the metastable defects formed during the growth of ZnO NR arrays and the variation of surface states during the measurement. After high-temperature annealing, the thermal stability of ZnO NR arrays are improved, while the best working temperature is shifted to ∼350 °C. By studying the effects of annealing temperature on the thermal stability, the lowest annealing temperature is suggested to be 400 °C for stabilizing ZnO NR arrays. After annealing at 400 °C in air, the ZnO NR arrays exhibit sufficiently good reproducibility with the gas response of ∼45. Based on the dependences of electric currents on working temperature and O 2 partial pressure, chemisorbed O− is suggested to play a predominant role in detecting reducing gases for the ZnO NR arrays and the best working temperature is probably in association with the conversion from irreversible to reversible chemisorption of O−. • Instability of as-grown ZnO NRs is related to instable defects and surface states. • The best working temperature of annealed ZnO NR arrays is shifted to ∼350 °C. • 400 °C is the lowest annealing temperature for stabilizing ZnO NR arrays. • Chemisorbed O− plays a predominant role in detecting reducing gases. • Irreversible to reversible chemisorption decides the best gas response. [ABSTRACT FROM AUTHOR]

The optical emissions of O-annealed ZnO nanorods (ZnO NRs) before and after surface passivation are studied at temperatures of 295 to 17 K in terms of photoluminescence (PL) spectra, PL excitation spectra, and PL quantum efficiency. Because the ZnO NR samples maintain the surface free of mechanical damages during the sample preparation, unambiguous evidence is obtained to substantiate that the structured green luminescence (GL-S) bands come from the optical emissions on the sample surfaces. We suggest that the GL-S bands are excited by the excitons diffused to the sample surface and thus substantially reduced in the PL spectra of ZnO NRs after surface passivation. PL fitting shows that many kinds of surface states may have contributions to the optical emissions in the GL-S bands, leading to the variation of GL-S bands with annealing temperature and ambient environment. [Display omitted] • Flower-like ZnO NRs are successfully passivated by depositing a layer of SiO x films. • PL quantum efficiency of ZnO NRs before and after passivation is determined. • GL-S bands of ZnO NRs are substantiated to come from the sample surface. • GL-S bands is suggested to be excited by the excitons diffused to the surface of ZnO NRs. [ABSTRACT FROM AUTHOR]