High-density arrays of low-defect-concentration zinc oxide nanowire grown on transparent conducting oxide glass substrate by chemical vapor deposition

High-density ZnO nanowire arrays with low defect concentrations were directly grown on transparent conducting oxide glass substrates under catalyst-free and low temperature conditions by chemical vapor deposition (CVD). A possible growth mechanism of the nanowires is studied. The experiments indicate that correct levels of supersaturation and evaporation temperature are beneficial to the growth of ZnO nanowires. Photoluminescence exhibits a weak ultraviolet emission at 380 nm and a strong green emission at 495 nm. While using a double-tube growth system, the visible light emission diminishes and the 380 nm emission is the only emission, suggesting that ZnO nanowires with few defects can be prepared using the present CVD technique at low temperature.     

Effects of supply time of Ar gas current on structural properties of Au-catalyzed ZnO nanowires on silicon (1 0 0) grown by vapor–liquid–solid process

ZnO nanowires were grown on Au-coated Si (1 0 0) substrates by the method of vapor–liquid–solid (VLS) growth processing technique. The effects of supply time of Ar gas current on morphology and microstructure of Au-catalyzed ZnO nanowires were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy. The results showed that the morphologies of ZnO nanostructures strongly depended on the time of flowing Ar gas. When the time of flowing Ar gas was 90 s, ZnO showed nanowires with hexagonal structure. Their diameters and lengths were 160 nm and 20 μm, respectively, on average, and the Raman scattering peak located at 438 cm⁻¹ reached maximum intensity. The results also showed that the ZnO growth could be patterned by controlling the initial position of Au-coated area on the Si substrates.     

Growth and photoluminescence of zinc blende ZnS nanowires via metalorganic chemical vapor deposition

We developed a metalorganic chemical vapor deposition to synthesize ZnS nanowires with high purity on Au-coated sapphire substrates at low temperatures. The ZnS nanowires have zinc blende structure, and most of them have raw-like surface on one edge, while is smooth on the other. High-resolution transmission electron microscopic investigations show that the nanowires are well crystalline single crystal grown along [1 1 1] and are free of bulk defects. The growth mechanism is confirmed as a typical vapor-liquid-solid process. The photoluminescence spectrum reveals two prominent blue emissions centered at 452.2 and 468.6 nm, respectively. It is found that sulfur vacancies and surface states should be responsible for the two blue emissions, respectively.     

Synthesis and characterization of well-aligned catalyst-free phosphorus-doped ZnO nanowires

Highly crystalline P-doped ZnO nanowires were grown by physical vapour transport technique without presence of any catalysts. Grown nanowires were well aligned and had good crystallinity with evolution of preferred orientation (0 0 2). Both EDX and Rutherford backscattering spectrometry (RBS) results revealed that phosphorus atoms were incorporated in the ZnO nanowires with the content of less than 1%. The XRD results confirmed an increase in lattice spacing which is attributed to substitution of P on oxygen sites. The photoluminescence spectra of grown nanowires showed a strong emission peak at 3.248 eV with a shoulder at 3.184 eV, corresponding with FA (free electron to acceptor) and DAP (donor-acceptor pair) levels of P-doped ZnO. The lattice spacing from HRTEM agrees with the achieved results from XRD measurement.     

ZnO nanocoral reef grown on porous silicon substrates without catalyst

Porous silicon (PS) technology is utilized to grow coral reef-like ZnO nanostructures on the surface of Si substrates with rough morphology. Flower-like aligned ZnO nanorods are also fabricated directly onto the silicon substrates through zinc powder evaporation using a simple thermal evaporation method without a catalyst for comparison. The characteristics of these nanostructures are investigated using field-emission scanning electron microscopy, grazing-angle X-ray diffraction (XRD), and photoluminescence (PL) measurements of structures grown on both Si and porous Si substrates. The texture coefficient obtained from the XRD spectra indicates that the coral reef-like nanostructures are highly oriented on the porous silicon substrate with decreasing nanorods length and diameter from 800-900 nm to 3.5-5.5 μm and from 217-229 nm to 0.6-0.7 μm, respectively. The PL spectra show that for ZnO nanocoral reefs the UV emission shifts slightly towards lower frequency and the intensity increase with the improvement of ZnO crystallization. This non-catalyst growth technique on the rough surface of substrates may have potential applications in the fabrication of nanoelectronic and nanooptical devices.     

Amorphous SiOx nanowires catalyzed by metallic Ge for optoelectronic applications

Amorphous SiO_x nanowires, with diameters of ∼20 nm and lengths of tens of μm, were grown from self-organized GeSi quantum dots or GeSi alloy epilayers on Si substrates. The morphologies and yield of these amorphous nanowires depend strongly upon the synthesis temperature. Comparative experiments indicate that the present SiO_x nanowires are induced by metallic Ge as catalysts via the solid liquid solid growth mechanism. Two broad peaks centered at 410 nm and 570 nm were observed in photoluminescence spectrum, indicating that such SiO_x nanowires have the potential applications in white light-emitting diodes, full-colour display, full-colour indicator and light sources.     

Effect of growth time on morphology and photovoltaic properties of ZnO nanowire array films

Single-crystalline ZnO nanowire arrays with different aspect ratios and nanowire densities were prepared by the hydrothermal growing method using polyethyleneimine (PEI) as a surfactant. PEI can only hinder the lateral growth of the ZnO nanowires, which is observed by high resolution transmission electron microscopy (HRTEM) analysis. Dye-sensitized solar cells were assembled by the ZnO nanowire arrays with different thicknesses, which can be controlled by the growing time and characterized using photocurrent-voltage measurements. Their photocurrent densities and energy allover conversion efficiencies increased with increasing ZnO nanowire lengths. Short-circuit current den-sity of 4.31 mA-cm-2 and allover energy conversion efficiency of 0.87% were achieved with 12.9-μm-long ZnO nanowire arrays.     

Novel bilayer structure ZnO based photoanode for enhancing conversion efficiency in dye-sensitized solar cells

ZnO film with a novel bilayer structure, which consists of ZnO nanowire (ZnO NW) arrays as underlayer and polydisperse ZnO nanocrystallite aggregates (ZnO NCAs) as overlayer, is fabricated and studied as dye-sensitized solar-cell (DSSC) photoanode. Results indicate that such a configuration of the ZnO nanocrystallite aggregates on the ZnO nanowire arrays (ZnO-(NCAs/NWs)) can significantly improve the efficiency of the DSSC due to its fast electron transport, relatively high surface area and enhanced light-scattering capability. The short-circuit current density (J_sc) and the energy-conversion efficiency (η) of the DSSC based on the ZnO-(NCAs/NWs) photoanode are estimated and the values are 9.19 mA cm⁻² and 3.02%, respectively, which are much better than those of the cells formed only by the ZnO NWs (J_sc = 4.02 mA cm⁻², η = 1.04%) or the ZnO NCAs (J_sc = 7.14 mA cm⁻², η = 2.56%) photoanode. Moreover, the electron transport properties of the DSSC based on the ZnO-(NCAs/NWs) photoanode are also discussed.     

Aging effects on the optical properties of an individual Zn-rich ZnO nanowire

Zn-rich ZnO nanowires are fabricated by a facile thermal evaporation method. The aging and post-annealing effects on an individual nanowire are investigated by cathodoluminescence (CL) spectra. For the nanowire aged in air, the intensity of deep-level (DL) emission is gradually strong with aging time, while are almost unchanged if aged in Ar atmosphere. Whether the individual nanowire was post-annealed in air or Ar atmosphere, the intensity of DL emission was gradually enhanced with temperature. Moreover, blue-shift of the DL emission is detected in the nanowire annealed in air. Nevertheless, for the nanowire annealed in Ar, the DL emission is first blue-shift at temperature of 400 °C, then red-shift at 500 and 600 °C. It is found that different defects can induce different DL emission, and the type of defects varies in the products depending on the experimental conditions.     

Solution-based synthesis of ZnO nanoparticle/CdS nanowire heterostructure

The heterostructure of ZnO nanoparticle (NP)/CdS nanowire (NW) was successfully fabricated by a two-step chemical solution method. The first, CdS nanowires were synthesized by a simple solvothermal route. The second, ZnO nanoparticles were grown on the surface of CdS nanowires in a chemical solution of Zn(CH₃COO)₂·2H₂O and anhydrous ethanol at 200 °C. The heterostructure of synthesized ZnO NP/CdS NW was characterized by transmission electron microscopy (TEM). The effects of reaction conditions, such as different reaction time of CdS nanowires synthesized and deposition reaction time were investigated. Moreover, the formation mechanism of the ZnO NP/CdS NW heterostructure has been phenomenologically discussed.     

Formation of ZnO nanorod arrays on polytetraflouroethylene (PTFE) via a seeded growth low temperature hydrothermal reaction

ZnO nanorod arrays were formed by a low temperature hydrothermal process on seeded polytetraflouroethylene (PTFE) sheets. The seed layer was formed using thermal oxidation of a thin evaporated Zn film on the PTFE sheet at 300 °C in air for 10 min. The formation of ZnO nanorod arrays in the hydrothermal reactive bath consisting of hexamethylamine (HMT) and Zn ions occurred via the reaction of hydroxyl ions released during the thermal degradation of HMT with the Zn ions. The seed layer provided a template for the nucleation of the ZnO and HMT which also acted as a chelating agent that promoted growth of the ZnO along the c-axis, leading to the formation of exclusively (0 0 2) ZnO nanorods. The effect of exposure time of the seeded PTFE to the reactive solution on the formation of the nanorods was investigated. Well aligned, relatively uniform tapered 300 nm long nanorods can be formed after 8 h of exposure. Longer exposure times to 24 h resulted in the formation of more uniform nanorods with base diameter averaged of ∼100 nm and the tip diameter of ∼50 nm. XRD analysis showed that the ZnO nanorod array had a hexagonal wurtzite structure. This result is in agreement with HR-TEM observations and Raman scattering analysis. Photoluminescence study showed that a strong UV emission peak was obtained at 380 nm and a small peak at 560 nm, which is associated with green emission. The optical band gap measured from these plots was at 3.2 eV on average.     

Magnetic properties improvement through off time between pulses and annealing in pulse electrodeposited CoZn nanowires

CoZn alloy nanowire arrays embedded in anodic aluminum oxide (AAO) template were fabricated by alternative current (ac) pulse electrodeposition. Various off times between pulses in an electrolyte with constant concentration of Co⁺² and Zn⁺² and acidity of 4 were employed. The effect of deposition parameters on the alloy contents, microstructures and magnetic properties of Co_xZn_1−x nanowires were studied. It is shown that, Co content increased by increasing the off time between pulses. This phenomenon enables us to fabricate Zn and Co-rich nanowires by adjusting the off time during the deposition procedure. Increasing the off time more than 200 ms increased the coercivity and squareness of CoZn nanowire arrays. A significant increase in the coercivity of CoZn nanowires was observed after annealing which was varied for the samples fabricated with different electrodeposition conditions. A coercivity of 1785 Oe was obtained for the annealed sample (a sample fabricated with 50 ms off time) from initially 240 Oe.     

Zinc oxide (ZnO) grown on flexible substrate using dual-plasma-enhanced metalorganic vapor deposition (DPEMOCVD)

This study investigates the temperature dependence of zinc oxide (ZnO) grown on polyestersulfone (PES) flexible substrates using the dual plasma-enhanced metal–organic chemical vapor deposition (DPEMOCVD) system. The proposed method uses a direct voltage (DC) and radio-frequency (RF) plasma system. The group-VI precursor, oxygen (O₂), can be completely ionized by the DC plasma system. The effect of optimal DC plasma power on ZnO thin films is thoroughly investigated using X-ray diffraction (XRD). The experimental results indicate that the crystalline structure and optical and electrical properties of ZnO thin films grown on PES substrates are dependent on the deposition temperature. The optimum deposition temperature for ZnO thin films deposited on PES substrates is 185 °C, whereas the DC and RF plasma power is 1.8 W and 350 W, respectively. Additionally, the wettability characteristic regarding the UV irradiation time was assessed by measuring the water contact angle. Under the UV irradiation for 60 min, the ZnO film grown at 185 °C represents a low contact angle of 5°, which approaches to a superhydrophilic surface.     

Growth and optical properties of ZnO nanorods prepared through hydrothermal growth followed by chemical vapor deposition

We report on the synthesis of high-quality ZnO nanorods by combining hydrothermal growth (HG) and chemical vapor deposition (CVD) processes. Vertically aligned and closely packed ZnO nanorods were grown by HG on a sputtered ZnO seed layer on a SiO₂/Si (0 0 1) substrate. The top surface of the HG-prepared ZnO nanorods showed very flat surfaces compared with that of the sputtered ZnO seed layer. Therefore, the HG-prepared ZnO nanorods were used as a new alternative seed material for the CVD growth of the ZnO nanorods. Vertical ZnO nanorods were grown by CVD on both the new HG-prepared nanorod seed material and the sputtered ZnO seed layer. The CVD-prepared ZnO nanorods on new HG-prepared nanorod seed material showed better crystalline quality and superior optical properties than the CVD-prepared ZnO nanorods on sputtered seed layer. The former showed negligible deep-level emissions at room temperature photoluminescence measurements. The intensity ratio of near-band-edge emissions to deep-level emissions from the former was about 910, but that from the latter was about 151. This implies that the HG-prepared ZnO nanorods can be used as a promising new seed material for nanostructure synthesis.     

SiC nanowire-toughened MoSi2-SiC coating to protect carbon/carbon composites against oxidation

To protect carbon/carbon (C/C) composites against oxidation, a SiC nanowire-toughened MoSi₂-SiC coating was prepared on them using a two-step technique of chemical vapor deposition and pack cementation. SiC nanowires obtained by chemical vapor deposition were distributed random-orientedly on C/C substrates and MoSi₂-SiC was filled in the holes of SiC nanowire layer to form a dense coating. After introduction of SiC nanowires, the size of the cracks in MoSi₂-SiC coating decreased from 18 ± 2.3 to 6 ± 1.7 μm, and the weight loss of the coated C/C samples decreased from 4.53% to 1.78% after oxidation in air at 1500 °C for 110 h.     

Porous anodic alumina membranes formed by anodization of AA1050 alloy as templates for fabrication of metallic nanowire arrays

The through-hole porous anodic aluminum oxide (AAO) membranes were fabricated by a two-step anodization of the aluminum alloy (AA1050) in 0.3 M oxalic acid at 45 V and 20 °C followed by removal of Al and pore opening/widening procedures. The effect of duration of the second anodizing step on the porous oxide layer thickness and the influence of the pore opening time on structural features of as-prepared membranes were studied in detail. The prepared membranes with a thickness of about 60 μm were used as templates for fabrication of dense arrays of Ag, Au, and Sn nanowires with various aspect ratios by a DC electrochemical deposition process. The successful synthesis of metallic nanowires by simple DC electrodeposition of metals inside the pores of AAO templates fabricated from the AA1050 alloy was reported for the first time. The fabrication costs of nanowire arrays can be reduced by about 500 times when AAO templates, prepared from the AA1050 alloy, are used instead of those usually formed from expensive high purity Al.     

High-quality vertically aligned ZnO nanorods synthesized by microwave-assisted CBD with ZnO-PVA complex seed layer on Si substrates

We successfully synthesized vertically aligned zinc oxide (ZnO) nanorods on seeded silicon substrates using chemical bath deposition assisted by microwave heating. ZnO nanorods were grown on seed layers of ZnO-polyvinyl alcohol (PVA) nanocomposites spin-coated on p-type Si (1 1 1). The nanorod's diameter was found to be dependent on the annealing temperature of the ZnO-PVA seed layer. We produced ZnO nanorods with diameters in the range of 50-300 nm from five groups of seed layers annealed at 250 °C, 350 °C, 380 °C, 450 °C, and 550 °C. The nanorods were examined with X-ray diffraction, transmission electron microscopy, and field emission scanning electron microscopy, which revealed hexagonal wurtzite structures perpendicular to the substrate along the z-axis in the direction of (0 0 2). Photoluminescence measurements revealed high UV emission at a high I_UV/I_vis ratio of 175. We also conducted Raman scattering studies on the ZnO nanorods to estimate the lattice vibration modes.     

Growth of uniform ZnO nanoparticles by a microwave plasma process

ZnO nanoparticle coatings with a controlled size distribution have been grown on quartz substrates by a novel microwave plasma assisted spray (MPAS) technique. This study presents the analysis of structure, photoluminescence, and magnetic properties of particle coatings with two distinctly different mean particle sizes (400 nm and 200 nm). X-ray diffraction patterns show a typical wurtzite structure without any impurity phases for the nanoparticle coatings. SEM and TEM investigations have shown the grown nanoparticles to be spherical and well separated with a narrow size distribution. Nanoparticles are polycrystalline with smaller grain sizes associated with the smaller particle sizes. Photoluminescence (PL) spectra reveal the presence of oxygen vacancy related defects in the 400 nm nanoparticles, which become less pronounced in the 200 nm nanoparticles. The 400 nm nanoparticles are found to exhibit room-temperature ferromagnetism with a clear hysteretic behavior, while the 200 nm nanoparticles are diamagnetic even down to 10 K. These results suggest the oxygen vacancies were the cause for defect-induced ferromagnetism in the 400 nm nanoparticles.     

Spray deposited superhydrophobic ZnO coatings via seed assisted growth

Superhydrophobic zinc oxide (ZnO) coatings were synthesized by a simple and cost-effective spray pyrolysis technique (SPT) via seed assisted growth onto the glass substrates at 723 K from an aqueous zinc acetate precursor solution. Initially, the ZnO seeds were synthesized from an aqueous 0.4 M zinc acetate solution onto the glass substrates at 723 K. For the seed assisted growth of ZnO, the solution concentrations (0.1 M to 0.4 M) were used and its effect on structural, morphological, optical and wettability properties of ZnO thin films was investigated. The synthesized films were found to be polycrystalline, with preferential growth along c-axis. Scanning electron microscopy (SEM) images show the uniform distribution of spherical grains of about 60-80 nm grain size. After seed assisted growth, film surface becomes very rough. The films were specular and transmittance of thin films decreases as the concentration of the precursor solution increases. The optical absorption spectrum shows a sharp absorption band-edge at 381 nm, corresponding to optical gap energy (Eg) of 3.25 eV. All samples are superhydrophobic in nature. The Zn4 sample shows the superhydrophobicity with highest value of the contact angle (CA) i.e. 165°. Such a superhydrophobic coatings can be useful in the anti-snow, anti-fog and self cleaning surfaces.     

Synthesis and characterization ofsingle-crystalline alumina nanowires

Alumina nanowires were synthesized on large-area silicon substrate via simple thermal evaporation method of heating a mixture of aluminum and alumina powders without using any catalyst or template. The phase structure and the surface morphology of the as-grown sample were analyzed by X-ray diffractometry(XRD) and scanning electron microscopy (SEM), respectively. The chemical composition and the microstructure of the as-grown alumina nanowires were characterized using transmission electron microscope(TEM). The nanowires are usually straight and the single crystalline has average diameter of 40 nm and length of 3 - 5μm. The growth direction is along the [002] direction. Well aligned alumina nanowire arrays were observed on the surface of many large particles. The catalyst-free growth of the alumina nanowires was explained under the framework of a vapor-solid(VS) growth mechanism. This as-synthesized alumina nanowires could find potential applications in the fabrication of nanodevices.