ZnO nanowire growth by electric current heating method: A study on the effect of substrate temperature

In this study, we report the growth of ZnO nanowire on quartz glass substrates with Au-catalyst assistance by electric current heating of ZnO ceramic bar. The effect of substrate temperature on the properties of ZnO nanostructures has been investigated systematically. Structural analysis indicates that the grown ZnO crystals belong to hexagonal phase with preferential growth along (0 0 2) orientation. Scanning electron microscopic studies reveal the aligned ZnO nanowires were grown at 800 °C. The typical length and diameter of nanowires are in the uniform ranges of 4–20 μm and 20–100 nm, respectively, showing their high aspect ratio of about 1000. We have made an attempt to discuss about the change in ZnO nanostructures with different substrate temperatures and the possible mechanism for the growth of nanowires. Optical reflectance studies show the infrared reflectivity was controlled through the substrate temperature.     

Structural and morphological properties of ZnO:Ga thin films

Zinc oxide (ZnO) is a promising semiconductor material with a great variety of applications. Compared to undoped ZnO, impurity-doped ZnO has a lower resistivity and better stability. With this aim, Ga has been proposed as a dopant. In this study, the structural characteristics and surface morphology of ZnO films produced by PEMOCVD at a substrate temperature of 250 °C on the c-plane (001) of sapphire were investigated. Doping was realized with 1, 3, 5 and 10 wt.% of Ga₂(AA)₃ in the precursor's mixture. At lower contents, Ga stimulates growth of (002) oriented textured films and the smallest FWHM was obtained as low as 0.17° for ZnO:Ga with 1 wt.%. A change in preferential orientation as well as surface smoothing and roughness decreasing of the films were observed with further increasing Ga content in precursor's mixture. We assume a key role of Ga and note that such a feature would be beneficial for the application of ZnO thin films for formation of abrupt junctions in p-n device structures.     

A rapid and efficient method to prepare aligned SnO2 nanorod arrays for field-emission application

Aligned tin dioxide (SnO₂) nanorods have been synthesized by high-frequency inductive heating. Nanorods were grown on silicon substrates vertically in less than 3 min, using SnO₂ and graphite as the source powder. Scanning electron microscopy and transmission electron microscopy showed nanorod with diameters from 25 to 50 nm. The turn-on field needed to produce a current density of 10 μA/cm² is found to be 1.6 V/μm. This type of SnO₂ nanorods can be applied as field emitters in displays as well as vacuum electric devices.     

Poly(3-hexylthiophene) based field-effect transistors with gate SiO2 dielectric modified by multi-layers of 3-aminopropyltrimethoxysilane

Top-contact organic field-effect transistors based on poly(3-hexylthiophene) (P3HT) active layer were fabricated using gate dielectric (SiO₂) modified with 3-aminopropyltrimethoxysilane (APTMS) multilayers. It has been demonstrated that the treatment of dielectric with APTMS enhances the field-effect mobility as well as the on/off ratio of the devices by nearly two orders of magnitude. This is attributed to conformational changes as well as to an improved uniformity of the spin coat P3HT films on the APTMS-modified substrate as revealed by atomic force microscopy, Fourier transform infrared spectroscopy and UV-Vis measurements.     

Deposition of particulate-free thin films by two synchronised laser sources: effects of ambient gas pressure and laser fluence

The micrometer and sub-micrometer sized particulates present both on the surface and inside of pulsed laser deposited thin films and structures stand for the main drawback of the method in view of technological applications. We applied a two-laser system in order to withdraw the particulates in case of Ta and TaO_x thin films. The Ta targets were irradiated by the first UV laser, while the second IR laser was directed parallel to the target surface, aiming to heat and evaporate the particulates. The morphology of the obtained thin films was studied by scanning electron microscopy. For the TaO_x films, the ambient gas pressure influences, besides the size and density of particulates, their propagation velocity. This in turn results in the variation of the optimum delay time between the ablating UV and the second IR laser pulse. For the Ta films we found that a threshold fluence of the IR laser pulse exists, above which completely particulate-free films were deposited.     

Atomic force microscopy indentation of fluorocarbon thin films fabricated by plasma enhanced chemical deposition at low radio frequency power

Atomic force microscopy (AFM) indentation technique is used for characterization of mechanical properties of fluorocarbon (CF_x) thin films obtained from C₄F₈ gas by plasma enhanced chemical vapour deposition at low r.f. power (5-30 W) and d.c. bias potential (10-80 V). This particular deposition method renders films with good hydrophobic property and high plastic compliance. Commercially available AFM probes with stiff cantilevers (10-20 N/m) and silicon sharpened tips (tip radius < 10 nm) are used for indentations and imaging of the resulted indentation imprints. Force depth curves and imprint characteristics are used for determination of film hardness, elasticity modulus and plasticity index. The measurements show that the decrease of the discharge power results in deposition of films with decreased hardness and stiffness and increased plasticity index. Nanolithography based on AFM indentation is demonstrated on thin films (thickness of 40 nm) with good plastic compliance.     

Structural properties and growth mechanism of flower-like ZnO structures obtained by simple solution method

Flower-shaped zinc oxide (ZnO) structures have been synthesized in the reaction of aqueous solution of zinc nitrate and NaOH at 90 °C. To examine the morphology of ZnO nanostructures, time-dependent experiments were carried out. Detailed structural observation showed that the flower-like structures consist of triangular-shaped leaves, having sharpened tips with wider bases. Photoluminescence spectrum measured at room temperature show a sharp UV emission at 381 nm and a strong and broad green emission at 480-750 nm attributed to structural defects. A possible growth mechanism for the formation of flower-shaped ZnO structures is discussed in detail.     

Influence of dispersant on the morphology of silica-coated cobalt nanoparticles synthesized by a wet based method

Co/SiO₂ nanospheres with nearly perfect core–shell structure were prepared by an improved sol–gel method combined with hydrogen reduction. The products were characterized by X-ray diffraction spectra (XRD), transmission electron microscopy (TEM) and Fourier transform infrared (FT-IR) spectra. The results indicated that polyethylene glycol (PEG 8000) could act as a more efficient dispersive reagent than citric acid monohydrate (CAM). The average size of Co nanoparticles can be well controlled with optimum concentration of PEG 8000 and the average diameter of Co nanoparticles reaches about 30 nm with PEG 8000 concentration of 50 mg ml⁻¹.     

Optical properties of ZnO nanowire arrays electrodeposited on n- and p-type Si(1 1 1): Effects of thermal annealing

Electrodeposition is a low temperature and low cost growth method of high quality nanostructured active materials for optoelectronic devices. We report the electrochemical preparation of ZnO nanorod/nanowire arrays on n-Si(1 1 1) and p-Si(1 1 1). The effects of thermal annealing and type of substrates on the optical properties of ZnO nanowires electroplated on silicon (1 1 1) substrate are reported. We fabricated ZnO nanowires/p-Si structure that exhibits a strong UV photoluminescence emission and a negligible visible emission. This UV photoluminescence emission proves to be strongly influenced by the thermal annealing at 150-800 °C. Photo-detectors have been fabricated based on the ZnO nanowires/p-Si heterojunction.     

Characteristics of plasma hydrogenated ZnO films oriented along the (11-20) plane grown by pulsed laser deposition

Single crystalline phase ZnO layers oriented along the (11-20) plane were successfully grown on (1-102) sapphire substrate by using a modified two-step pulsed laser deposition process. The full width at half maximum of rocking curve for (11-20) ZnO was 430 arc sec. After a hydrogen plasma treatment, the value of surface roughness was decreased from 37.5 to 10.2 nm. And the intensity of ultra-violet emission at 380 nm in cathodoluminescence (CL) spectrum was greatly increased by the hydrogen plasma irradiation. In the time-resolved photoluminescence (TRPL) spectra, the minimum value of decay time was 36 ps after the hydrogen plasma irradiation for 5 min. Especially, CL and TRPL results show the potential of high efficient optoelectronic devices by using the hydrogen plasma treatment.     

Structural and photoluminescence properties of aligned Sb-doped ZnO nanocolumns synthesized by the hydrothermal method

Aligned Sb-doped ZnO nanocolumns were synthesized by a simple hydrothermal method. Based on the analyses of the X-ray diffraction and photoluminescence result, it could be confirmed that the Sb has successfully doped in the ZnO crystal lattices to form an accepter energy level. At 85 K, the recombination of the acceptor-bound exciton was predominant in PL spectrum, which was attributed to the transition of the (Sb_Zn-2V_Zn) complex bound exciton. The acceptor binding energy had been calculated to be 123 meV.     

Growth of high transmittance vertical aligned ZnO nanorod arrays with polyvinyl alcohol by hydrothermal method

Well-aligned ZnO nanorod arrays were synthesized on SnO₂:F (FTO) glass substrates by hydrothermal method with the addition of polyvinyl alcohol. Morphology and optical properties of the well-aligned ZnO nanorod arrays were studied. The transmittance of the as grown samples was dramatically increased by the addition of polyvinyl alcohol. PVA was regarded to play an important role in improving the optical transmittance and its role in hydrothermal growth process is also discussed in this work.     

Fabrication of ZnO thin films by the photochemical deposition method

ZnO thin films were fabricated by the photochemical deposition (PCD) method. The deposition solution contains ZnSO₄, Na₂SO₃, Na₂S₂O₃ and a small amount of NH₄OH for pH adjustment. We blew oxygen or oxygen + ozone (O₃) gas into the solution to increase the dissolved oxygen content and enhance the oxidation reaction. The films were characterized by Auger electron and optical spectroscopy, and a photoelectrochemical (PEC) measurement. On an indium-tin-oxide (ITO) substrate, the films showed high optical transmission in the visible range. In a current-voltage measurement for films on a p-Si substrate, the O₃ bubbling sample showed rectification properties and photovoltaic effects.     

Room temperature deposition of ZnSe thin films by successive ionic layer adsorption and reaction (SILAR) method

The zinc selenide (ZnSe) thin films are deposited onto glass substrate using relatively simple and inexpensive successive ionic layer adsorption and reaction (SILAR) method. The films are deposited using zinc acetate sodium selenosulphate precursors. The concentration, pH, immersion and rinsing times and number of immersion cycles have been optimized to obtain good quality ZnSe thin films. The X-ray diffraction (XRD) study and scanning electron microscopy (SEM) studies reveals nanocrystalline nature alongwith some amorphous phase present in ZnSe thin films. Energy dispersive X-ray (EDAX) analysis shows that the films are Se deficient. From optical absorption data, the optical band gap ‘E_g’ for as-deposited thin film was found to be 2.8 eV and electrical resistivity in the order of 10⁷ Ω cm.     

Reversible CO2 capture by ZnO slurry leading to formation of fine ZnO particles

ZnO powders were synthesized by heating precursors, prepared by the addition of ethylene diamine tetraacetate to an aqueous solution of zinc acetate, at 500?°C in air. A ZnO slurry was prepared by adding distilled water to the synthesized ZnO powder. X-ray diffraction studies confirmed that the ZnO slurry adsorbed CO₂ at 25?°C under a CO₂ gas flow to produce Zn₅(CO₃)₂(OH)₆. The CO₂ adsorption ratio of the ZnO slurry was influenced by the amount of added water. When the weight ratio of water/ZnO was 1, the ZnO slurry had the largest CO₂ adsorption ratio. Scanning electron microscopy and thermal gravimetry showed that the plate-like Zn₅(CO₃)₂(OH)₆ particles thermally decomposed at 300?°C in air leading to a single-phase ZnO powder consisting of nanoparticles with approximately 20?nm in diameter. The specific surface area of the reformed ZnO powder increased to approximately 82?m²/g through the thermal decomposition of Zn₅(CO₃)₂(OH)₆. The ZnO slurry was capable of adsorbing CO₂ under a CO₂ flow at 25?°C and desorbing CO₂ under an air flow at 300?°C. The CO₂ adsorption ratio of the ZnO slurry reached 80%–90% at 25?°C even after the fifth CO₂ adsorption and desorption cycle.     

Photoluminescence and field-emission characteristics of ZnO nanowires synthesized by two-step method

ZnO nanowires with excellent photoluminescence (PL) and field-emission properties were synthesized by a two-step method, and the ZnO nanowires grew along (0 0 2) direction. PL measurements showed that the ZnO nanowires have stronger ultraviolet emission properties at 376 nm and there is 3 nm blue shift after the nanowires were immersed in thiourea (TU) solution compared with those of without immersion. The immersed-ZnO nanowires show a turn-on field of 2.3 V/μm at a current density of 0.1 μA/cm and emission current density up to 1 mA/cm² at an applied field of 6.8 V/μm, which demonstrate that the immersed-ZnO nanowires posses efficient field-emission properties in contrast with those not immersed. The ZnO nanowires may be ideal candidates for making luminescent devices and field-emission displays.     

Fabrication of ZnS/ZnO hierarchical nanostructures by two-step vapor phase method

A simple two-step vapor phase method is presented to fabricate ZnS/ZnO hierarchical nanostructures in bulk quantities. That is ZnS nanobelts were first synthesized and then used as substrate for growth of ZnO nanorod arrays. Investigation results demonstrate that the polar surfaces of ZnS nanobelts could induce a preferred asymmetric growth of ZnO nanorods on the side surfaces. But it is believed that if the local concentration of ZnO was high enough, ZnO nanorods could also grow symmetrically on the top/bottom surface of the ZnS nanobelts. The optical property of the products was also recorded by means of photoluminescence (PL) spectroscopy.     

In situ synthesis of catalytic metal nanoparticle-PDMS membranes by thermal decomposition process

Nanoparticles of various transition elements such as palladium, iron, and nickel were synthesized in situ in the polydimethylsiloxane (PDMS) matrix by thermal decomposition of their corresponding acetylacetonate salts. Various complementary techniques such as XRD, TEM and XPS were used to characterize the nanoparticles formed in the polymer matrix. This synthesis route results in relatively monodisperse nanoparticles with a narrow particle size distribution. In addition, the composite films are pore-free and mechanically stable, making them attractive for a range of applications. Palladium-PDMS membranes can be used as catalytic membrane reactors and show enhanced catalytic activity in ethylene hydrogenation.     

Growth and electrical properties of ZnO thin films deposited by novel ion plating method

The URT(Uramoto-gun with Tanaka magnetic field)-IP(ion plating) method is a technique for depositing a thin film on a substrate. This method offers the advantage of low-ion damage, low deposition temperatures, large area deposition and high growth rates. Ga-doped ZnO thin films were grown using the URT-IP method, and the material properties were evaluated. The quality of ZnO thin films grown by the URT-IP method was found to be sensitive to oxygen supply during growth. It was observed that the saturation point of the growth rate corresponding to the optimum oxygen supply leads to the best electrical properties. The profiles of the dependence of film properties on oxygen supply revealed a part of growth mechanism of the URT-IP method.     

Transmission electron microscopy study of the initial growth stage of GaSb grown on Si (001) substrate by molecular beam epitaxy method

The microstructural properties at the initial growth stage of the GaSb heteroepitaxial growth on a silicon (Si) substrate were investigated using transmission electron microscopy. Well-separated and tall GaSb islands were observed when GaSb was directly grown on a Si substrate (sample A). On the other hand, GaSb was grown to the coalesced and flat islands when a low-temperature AlSb buffer (sample B) was introduced. The different morphologies of the GaSb islands were related to the microstructural properties of the interface between the GaSb and the Si substrate. The GaSb/Si interface was rough, and disordered atomic arrangements were observed at the interface in sample A. On the other hand, the GaSb/Si interface was flat, and well-ordered atomic arrangements appeared at the interface in sample B. Entirely different mechanisms for the relaxation of a misfit strain were demonstrated from a microstructural viewpoint.