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Studies the optical properties of epitaxially grown zinc oxide (ZnO) thin films on sapphire using pulsed laser deposition. Application of zinc oxide in semiconductors; Details on the experiment; Crystal qualities of ZnO.

Abstract: The pressure–volume–temperature relationship and bulk modulus of ZnO with zinc-blende phase are investigated by means of first-principles density functional theory calculations combined with the quasi-harmonic Debye model in which the phononic effects are considered. Particular attention is paid to the predictions of the first and second pressure derivatives of isothermal bulk modulus which play central role in the formulation of approximate equations of state for the first time. The properties of ZnO with zinc-blende phase are summarized in the pressure range of 0–7.45GPa and the temperature up to 2000K. [Copyright &y& Elsevier]

Abstract: Because of the significance of the first and second pressure derivatives of the isothermal bulk modulus, a detailed theoretical study of the high-pressure rocksalt phase of ZnO under high pressure and temperature has been carried out by means of first-principles density functional theory calculations combined with the quasi-harmonic Debye model in which the phononic effects are considered. Particular attention is paid to the predictions of the isothermal bulk modulus and its pressure derivatives which play central role in the formulation of approximate equations of state for the first time. [Copyright &y& Elsevier]

Abstract: n-ZnO nanorod/p-CuAlO2 heterojunction light-emitting diodes have been fabricated on p+-Si substrates. The CuAlO2 thin film was deposited by dc-magnetron sputtering while the ZnO nanorods (NRs) were fabricated using the vapor-phase transport method. The current–voltage characteristics of the devices showed good rectifying behavior with a high forward-to-reverse current ratio of around 120 at ±7V. Strong ultraviolet electro-luminescence centered at ∼390nm and a broad green-band emission were observed from the diode at room-temperature. The p-CuAlO2 layer was found to facilitate hole injection from p+-Si into n-ZnO while confining the electrons at ZnO/CuAlO2 interface, thus effectively enhancing the recombination emission efficiency in ZnO NRs. [Copyright &y& Elsevier]

Abstract: A novel morphology of zinc oxide (ZnO) consisting of resistor-shaped nanowires connected “in-series” has been synthesized by a vapor phase transport method in the presence of water and copper. Scanning electron microscopy showed that the diameter of nanowires changes abruptly from ∼200 to ∼20nm and forms a resistor-shaped nanostructure. Transmission electron microscopy and selected-area electron diffraction revealed a grain boundary at the thicker center (200nm) and the nanowires were composed of a pair of crystallites of wurtzite ZnO growing along ±[0001] directions. A growth mechanism is proposed which takes into account the crystal growth conditions, the catalytic effect of water and copper, as well as the inherent polarization of the wurtzite structure along its c-axis. [Copyright &y& Elsevier]

Tubular ZnO microstructure arrays were fabricated on a large scale by a two-step hydrothermal method. The porous ZnO tubular structures were then used to construct a gas sensor for CO detection. The microtube array gas sensor showed sensitive response to different concentration of CO with an optimum temperature of 250 °C. Because of the large surface to volume ratio, the sensitivity of the microtube arrays was about twice of that of the ZnO rods. Our results indicate that this simple two-step method for fabrication of large-scale tubular microstructure arrays can be potentially used in gas sensor applications with improved performance. [ABSTRACT FROM AUTHOR]

Abstract: The refractive indices of textured indium tin oxide (ITO) and zinc oxide (ZnO) thin films were measured and compared. The ITO thin film grown on glass and ZnO buffered glass substrates by sputtering showed distinct differences; the refractive index of ITO on glass was about 0.05 higher than that of ITO on ZnO buffered glass in the whole visible spectrum. The ZnO thin film grown on glass and ITO buffered glass substrates by filtered vacuum arc also showed distinct differences; the refractive index of ZnO on glass was higher than that of ZnO on ITO buffered glass in the red and green region, but lower in the blue region. The largest refractive index difference of ZnO on glass and ITO buffered glass was about 0.1 in the visible spectrum. The refractive index variation was correlated with the crystal quality, surface morphology and conductivity of the thin films. [Copyright &y& Elsevier]

Abstract: Zinc oxide nanowhiskers with multipod structures have been fabricated on copper-coated Si wafer by sintering the mixture of ZnO and graphite powders in air. The multipod nanowhisker shows tetrapod, tripod and hexapod morphologies with legs of 100–500nm in diameter and several microns in length. The legs of the multipod nanowhisker intercross at a central knot of the whisker. The octahedron twin nucleus is responsible for the tetrapod structure and possibly responsible for the tripod structure as well. The hexapod structure is due to a stacked nucleus of two octahedron nuclei. [Copyright &y& Elsevier]

Zinc oxide (ZnO) crystal fibers were prepared on silicon (1 0 0) substrate via a simple thermal chemical reactions vapor transport deposition method in air with a mixture of ZnO and carbon powders as reactants. The growth process was carried out at 1100°C in a quartz tube with one side opened to the air. There is no other metal catalyst and carrier gas in the process. The scanning electron microscope measurements showed that the ZnO fibers obtained had diameters ranging from 300 nm to 1.5 μm and the maximal lengths up to 1 mm, resulting in an aspect ratio estimated as high as about 600. The ZnO fibers were also characterized by X-ray diffraction, Raman spectroscopy, and room temperature photoluminescence spectroscopy. The growth mechanism is explained by chemical reactions and vapor–liquid–solid mechanism. [Copyright &y& Elsevier]

Nanostructural zinc oxide has been successfully fabricated by heating the mixture of ZnO and graphite powders in air. The growth of these zinc oxide nanostructures with respect to the growing time and temperature has been studied. The morphologies and the crystal structures have been characterized by scanning electronic microscopy and the X-ray diffraction. The results indicated that ZnO nanostructure formed mainly along the crystal orientation [002] on silicon substrate at moderate temperatures. The crystallization was improved by prolonging growth time and the morphologies mainly depended on the distribution of the growth temperature. The growth process was attributed to vapor-liquid-solid mechanism. [ABSTRACT FROM AUTHOR]

Gallium-doped nanostructural zinc oxide fibers have been fabricated by vapor-phase transport method of heating the mixture of zinc oxide, gallium oxide, and graphite powders in air. The zinc oxide fibers grew along [002] direction, forming a vertically aligned array that is predominantly perpendicular to the substrate surface. With a gallium doping concentration of 0.73 at. %, the corresponding carrier concentration and resistivity were 3.77×10[sup 20] cm[sup -3] and 8.9×10[sup -4] Ω cm, respectively. The field emission of these vertically aligned ZnO fiber arrays showed a low field emission threshold (2.4 V/μm at a current density of 0.1 μA/cm[sup 2]), high current density, and high field enhancement factor (2317). The dependence of emission current density on the electric field followed Fowler–Nordheim relationship. The enhanced field emission is attributed to the aligned structure, good crystal quality, and especially, the improved electrical properties (increased conductivity and reduced work function) of the nanofibers due to gallium doping. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: The ZnO micronails were synthesized by the vapor phase transport method. The heads of the micronails show hexagonal disk structure which is suitable for the whispering-gallery mode lasing microcavity. Under the excitation of a nanosecond pulse at 355nm, the single-photon absorption induced lasing was stimulated in the micronail with the head diameter of 3.0μm, the whispering gallery mode and Fabry-Pérot mode lasing were investigated. Under the excitation of femtosecond laser pulses at 804nm, the second harmonic generation and the three-photon absorption induced photoluminescence were observed from a bulk of micronails, then an individual micronail with the diameter of 9.1μm was employed to realize the three-photon absorption induced whispering-gallery mode lasing. [Copyright &y& Elsevier]

Wurtzite ZnO thin films with different epitaxial relationships have been grown on (0 0 1)-, (0 1 1)-, and (1 1 1)LaAlO 3 (LAO) single-crystal substrates by pulsed laser deposition. Nonpolar (1 1 2 ¯ 0)ZnO films with two orthogonal domains were obtained on (0 0 1) LAO, in which the in-plane orientation relationship is demonstrated to be 〈0 0 0 1〉 ZnO //〈1 1 0〉 LAO . For ZnO on (0 1 1)- and (1 1 1)LAO substrates, a single-domain epitaxy with c axial orientation is observed, in which the in-plane relationships were 〈1 1 ¯ 0 0〉 ZnO //〈0 1 1 ¯ 〉 LAO irrespective of the substrate orientations. Based on the in-plane orientation relationship, the lattice mismatch has been obtained for these three oriented ZnO/LAO heterointerfaces. [ABSTRACT FROM AUTHOR]

We have demonstrated rare earths (REs) doped ZnO-based heterojunction light-emitting diodes (LEDs) on p -Si substrates by ultrasonic spray pyrolisis (USP). Room-temperature electroluminescence was clearly observed from reverse-biased n -ZnO:REs/ p -Si diodes, which red and blue color emissions were realized from Eu and Tm ions, respectively. The narrow line-width emissions are attributed to the transitions within the shielded 4f levels of the trivalent REs ions that resulting from direct electron impact excitation during reverse bias. [ABSTRACT FROM AUTHOR]

Abstract: Erbium-doped ZnO (ZnO:Er) thin films with various doping concentrations were deposited on p-Si substrates by ultrasonic spray pyrolysis (USP). The n-ZnO:Er/p-Si heterojunctions were further employed to fabricate light-emitting diodes (LEDs). The devices showed diode-like rectifying current–voltage characteristics with a low reverse breakdown voltage, attributed to the avalanche breakdown. A distinct green electroluminescence peaking at 537nm and 558nm were observed at room temperature under reverse bias. The green electroluminescence originated from the electron impact excitation of Er3+ ions doped in ZnO films. [Copyright &y& Elsevier]

Abstract: An efficient photoelectrode is fabricated by sequentially assembled CdS and CdSe quantum dots (QDs) onto a ZnO-nanowire film. As revealed by UV–vis absorption spectrum and scanning electron microscopy (SEM), CdS and CdSe QDs can be effectively adsorbed on ZnO-nanowire array. Electrochemical impedance spectroscopy (EIS) measured demonstrates that the electron lifetime for ZnO/CdS/CdSe (13.8ms) is calculated longer than that of ZnO/CdS device (6.2ms), which indicates that interface charge recombination rate is reduced by sensitizing CdSe QDs. With broader light absorption range and longer electron lifetime, a power conversion efficiency of 1.42% is achieved for ZnO based CdS/CdSe co-sensitized solar cell under the illumination of one Sun (AM 1.5G, 100mWcm−2). [Copyright &y& Elsevier]

Abstract: We report on a quantum dot sensitized solar cell (QDSSC) based on ZnO nanorod coated vertically aligned carbon nanotubes (VACNTs). Electrochemical impedance spectroscopy shows that the electron lifetime for the device based on VACNT/ZnO/CdSe is longer than that for a device based on ZnO/CdSe, indicating that the charge recombination at the interface is reduced by the presence of the VACNTs. Due to the increased surface area and longer electron lifetime, a power conversion efficiency of 1.46% is achieved for the VACNT/ZnO/CdSe devices under an illumination of one Sun (AM 1.5G, 100mW/cm2). [ABSTRACT FROM AUTHOR]

Abstract: In this study, the effects of the annealing duration of a zinc oxide (ZnO) buffer layer on structural and optical properties of ZnO rods grown by a hydrothermal process are discussed. A ZnO buffer layer was deposited on p-type Si (111) substrates by the metal organic chemical vapor deposition (MOCVD) method. After that, ZnO rods were grown on the ZnO-buffer/Si (111) substrate by a hydrothermal process. In order to determine the optimum annealing duration of the buffer layer for the growth of ZnO rods, durations ranging from 0.5 to 30min were tried. The morphology and crystal structure of the ZnO/ZnO-buffer/Si (111) were measured by field emission scanning electron microscopy (FE-SEM) and x-ray diffraction (XRD). The optical properties were investigated by photoluminescence (PL) measurement. [Copyright &y& Elsevier]

Abstract: Chemical bath method was used to synthesize bilayer ZnO nanostructure on ITO glass in the alkaline solution. As revealed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), the product consists of a layered structure of ZnO nanorods at the bottom and nanoflower atop. The as-prepared sample was assembled in quantum dot sensitized solar cell (QDSSC), which obtained the incident photon to current conversion efficiency (IPCE) of 15% at 400nm and power conversion efficiency (PCE) of 0.45%. Therefore, this novel bilayer ZnO nanostructure has the potential for application in solar cell device as the photoelectrode. [Copyright &y& Elsevier]

Abstract: ZnO nanodisks were fabricated by vapor–vapor transport using zinc powder as source material. The photoluminescence (PL) and Raman scattering spectra were investigated. Based on the mapping technology, the Raman signals combined with PL were used to construct images, which revealed the spatial distribution of defect in ZnO nanodisks. It provides an intuitive method to understand the defect distribution and further helpful to growth control of crystal and design of functional materials and devices. [Copyright &y& Elsevier]

Abstract: In this study, the effects of thermal annealing temperature and duration on ZnO nanorod arrays fabricated by hydrothermal method were investigated. The annealed ZnO/Si(111) substrate was used for ZnO nanorod array growth. The effects of annealing treatment on the structural and optical properties were investigated by scanning electron microscopy, X-ray diffraction, and room-temperature photoluminescence measurements. With the annealing temperature of 750°C and the annealing duration of 10min, both the structural and optical properties of the ZnO nanorod arrays improved significantly, as indicated in the X-ray diffraction and photoluminescence measurement. [Copyright &y& Elsevier]

Abstract: A two-step aqueous thermal decomposition method was used to synthesize a hierarchical ZnO nanostructure on indium–tin-oxide (ITO) glass in a neutral solution. As revealed by scanning electron microscopy and transmission electron microscopy, the product consists of a layered structure of ZnO nanorods at the bottom and nanowire atop. The ZnO nanowires were grown along the [0001] direction with 20nm in diameter and exceeded 10μm in length. The growth mechanism of the hierarchical ZnO nanostructure is discussed. Our results indicate that neutral solution and low Zn concentration are the keys for growing long and thin ZnO nanowires in large scale. Hierarchical ZnO nanostructure has applications in solar cell device as the electrode can largely improve the incident photon to current conversion efficiency (IPCE) and photovoltaic conversion efficiency (PCE) values due to the high aspect ratio. Nanowire ZnO on the top layer can absorb significant amount of quantum dots and the photoexcited electrons can efficiently get transferred into the FTO surface though nanorod array. [Copyright &y& Elsevier]

Abstract: ZnO was deposited on bare Si(100), as-deposited, and annealed ZnO/Si(100) substrates by hydrothermal synthesis. The effects of a ZnO buffer layer and its thermal annealing on the properties of the ZnO deposited by hydrothermal synthesis were studied. The grain size and root mean square (RMS) roughness values of the ZnO buffer layer increased after thermal annealing of the buffer layer. The effect of buffer layer annealing temperature on the structural and optical properties was investigated by photoluminescence, X-ray diffraction, atomic force microscopy, and scanning electron microscopy. Hydrothermal grown ZnO deposited on ZnO/Si(100) annealed at 750°C with the concentration of 0.3M exhibits the best structural and optical properties. [Copyright &y& Elsevier]

Abstract: This paper describes a study on the influence of fabrication temperature on morphologies and field emission properties of zinc oxide (ZnO) nanostructures, which were synthesized by the oxidation of zinc vapor. By adjusting the growth temperature, ZnO multipods, tetrapods, nanoneedles and nanowires were fabricated at 620, 770, 840, and 900°C, respectively. Field emission measurements showed that the turn-on field was 2.1, 2.9, 4.4 and 5.5V/μm, respectively, for ZnO multipods, tetrapods, nanoneedles, and nanowires. And there was much difference in their maximum emission current densities as well. The mechanism of how the synthesis temperature influences the morphologies and field emission abilities of ZnO nanostuctures was discussed in detail. [Copyright &y& Elsevier]

Abstract: The band structures of wurtzite ZnO are calculated using the empirical pseudopotential method (EPM). The 8 parameters of the Zn and O atom pesudopotential form factors with Schluter''s formula are obtained. The effective mass parameters are extracted by using k.p Hamiltonian to fit the EPM results. The calculated band edge energies (E g, E A, E B, and E C) at Γ point are in good agreement with experimental results. The ordering of ZnO at the top of valence band is found to be A(Γ7)–B(Γ9)–C(Γ7) due to a negative spin–orbit (SO) splitting. Based on the band parameters obtained, the valence hole subbands of wurzite ZnO/Mg x Zn1− x O tensile-strained quantum wells (QWs) with different well widths and Mg compositions are calculated using 6-band k.p method. [Copyright &y& Elsevier]

Abstract: Three different ZnO nanostructures include nanoparticles, ripples and regular nanogratings were successfully prepared by femtosecond laser irradiation under different experimental conditions. The in-situ observation of the second harmonic generation (SHG) excited in ZnO crystals before, during, and after the formation of the nanostructures was investigated. The obtained results show that the formed nanostructures contribute to the enhancement of the SHG. We propose that the second harmonics in the sample surface plays an important role in the formation of nanostructures. [Copyright &y& Elsevier]