Growth and optical properties of aluminum-doped zinc oxide nanostructures on flexible substrates in flexible electronics

Undoped ZnO nanowire arrays and Al-doped ZnO nanostructures with nanowires and nanosheets were successfully synthesized on a polyethylene terephthalate substrate using the rapid hydrothermal synthesis. These undoped ZnO nanowire arrays showed close alignment with highly c-axis oriented and well-defined hexagonal facets (001). The coexistence of the nanowires and nanosheets was observed during the introduction of Al ions. The number of nanosheets increased due to the Al doping concentration and the lack of surface energy. The diameter of the nanosheets and the length of nanowire arrays also increased as a function of the growth time. Room-temperature photoluminescence spectra show that the ZnO:Al nanostructures on the ZnO seeded polyethylene terephthalate substrate yield low level of the defect density compared to the ZnO seeded glass substrate to remove post annealing process.     

Pyramidal nanostructures of zinc oxide

Zinc oxide (ZnO) nanostructures have been prepared by pulsed laser deposition of the oxide onto Si(100) substrate at 600 degrees C. An examination of the morphology using atomic force microscopy and scanning electron microscopy reveals well formed pyramidal structures consistent with the growth habit of ZnO. A domain matched epitaxy across the interface makes the ZnO pyramids orient along the axes of Si(100) surface. The pyramidal nanostructures signify an intermediate state in the growth of hexagonal nanorods of ZnO. The hardness of the nanostructures as well as their response to oxygen gas have been investigated using nanoindentation and conducting probe methods respectively. ZnO nanostructures are much harder than their bulk. The hardness of ZnO pyramids obtained by nanoindentation is 70 +/- 10 GPa which is about one order more that of bulk ZnO. Besides, the nanostructures exhibit high sensitivity towards oxygen. A 70% increase in the resistance of ZnO nanostructures is observed when exposed to oxygen atmosphere.     

A study on electrodeposited zinc oxide nanostructures

Zinc oxide (ZnO) nanostructures prepared by electrochemical deposition method from aqueous zinc nitrate solution at 65 °C onto fluorine doped tin oxide coated glass substrates were investigated. Characterization of ZnO nanostructures was realized using conventional electrochemical techniques, scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. Cyclic voltammetry experiments were performed to elucidate the electrodic processes that occurred when potentials were applied and the optimum potential for electrodeposition were determined. From the Mott-Schottky measurements, the flat-band potential and the donor density for the ZnO nanostructure are determined. From single-step potential experiment in the potential ranges from ?1.1 to ?1.4 V, the formation of ZnO nuclei in the early deposition stages was proceeded according to the three dimensional (3D) instantaneous nucleation followed by diffusion-limited growth rather than a progressive one. SEM images demonstrated that the morphology of ZnO nanostructures depend greatly on the potential depositions. XRD studies revealed that the deposited films were polycrystalline in nature with wurtzite phase.     

Growth of Ge islands on Si substrates

We present a study of Ge islands formation on Si(100) substrates using grazing-incidence small-angle X-ray scattering (GISAXS) and atomic force microscopy (AFM). Samples were prepared by magnetron sputtering of a 5 nm thick Ge layer in a very high vacuum on Si(100) substrate held at different temperatures. The vertical cut (perpendicular to the surface) of the experimental 2D GISAXS pattern has been fitted using a Guinier approximation. The optimum temperature for the islands formation was 650 °C. At this temperature, islands grow in conical shape with very similar dimensions; however, inter-island distances varied significantly.     

Thermal properties of semiconductor zinc oxide nanostructures

A combination of two methods — laser modulation and 3ω — has been used to determine the heat capacity, heat conductivity, and heat diffusivity of zinc oxide nanostructures. A significant difference between the thermal parameters of zinc oxide nanostructures grown by different technological methods has been revealed. It has been shown that the relatively low heat conductivity and heat diffusivity values of oxide zinc nanostructures are due to both the internal defects and the contact resistance between the film and its base — the substrate.     

Nucleation and growth of catalyst-free zinc oxide nanostructures

This paper deals with the investigations on the nucleation and growth of Zinc Oxide (ZnO) nanostructures in a catalyst free synthesis. The ZnO nanostructures have been formed by evaporation of Zn (99.99%) in O2 and Ar atmosphere in single zone furnace under two temperature regions, region A (approximately 1173-1073 K) and region B (approximately 873-773 K). Through application of XRD and TEM techniques, it has been shown that first ZnO is formed which changes to ZnOx through creation of oxygen vacancies. The ZnOx acts as self-catalyst and leads to formation of various nanostructures. Those observed in the present investigation are nanotetrapods (1 D, diameter approximately 70-450 nm, length approximately 2-4.5 approximatelym) nanorods (1 D, diameter approximately 45-95 nm, length approximately 2.5-4.5 microm), nanoflowers(2D, central core diameter approximately 90-185 nm, length of petals/nanorod approximately 1.0-3.5 microm) and nanoparticles (3D, size approximately 0.85-2.5 microm). These nanostructures have been revealed by SEM explorations. Attempts have been made to explain the formation of the various nanostructures in terms of the creation and distribution of the ZnOx, the temperature as well as oxygenation conditions.     

Growth of group III nitride nanostructures on nano-imprinted sapphire substrates

Nanostructures of group III nitrides have been fabricated on nano-imprinted sapphire substrates by nitrogen radical assisted pulsed laser deposition. We have found that the group III nitride nanostructures produced by this method incline toward the incident direction of the nitrogen radical source, probably due to the shadowing effect. The inclinations of the nanostructures are the result of an oblique reactive gas flux and re-evaporation of metal that is shadowed from the gas flux and not nitrided. We have demonstrated that it is possible to control the shape of nanostructure, three dimensionally, by utilizing this inclination phenomenon, which should be quite useful for the fabrication of photonic crystals.     

Growth process from amorphous GaN to polycrystalline GaN on Si (111) substrates

Amorphous GaN (a-GaN) films on Si (111) substrates have been deposited by RF magnetron sputtering with GaN powder target. The growth process from amorphous GaN to polycrystalline GaN is studied by XRD, SEM, PL and Raman. XRD data mean that annealing under flowing ammonia at 850-950 °C for 10 min converts a-GaN into polycrystalline GaN (p-GaN). The growth mechanism can be mostly reaction process through N³⁻ in amorphous GaN replaced by N³⁻ of NH₃. Annealing at 1000 °C, the appearance of GaN nanowires can be understood based on the vapor-liquid-solid (VLS) mechanism. In addition, XRD, PL and Raman measurement results indicate that the quality of GaN films increases with increasing temperature. The tensile stress in the films obtained at 1000 °C is attributable to the expansion mismatch between GaN and Si, with the gallium in the film playing a negligible role.     

Dopant induced bandgap narrowing in Y-doped zinc oxide nanostructures

In this report, hydrothermal synthesis and the absorption properties of the cubic shaped zinc oxide nanostructures doped with different amount of yttrium (Y) metal cation (0 to 15 at.%) are demonstrated. The structural and optical properties of chemically synthesized pure and Y doped ZnO powders are investigated by using powder X-ray diffraction (XRD), field emission scanning electron spectroscopy (FESEM) and transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) absorbance, photoluminescence (PL), and Fourier transform infra-red spectroscopy (FT-IR). It is found that the dopant ions stabilize in wurtzite hexagonal phase of ZnO upto the concentration of less than 6 at.%, which is mainly due to the fact that the ZnO lattice expands and the optical bandgap energy decreases at this level. Increasing the dopant concentration to greater than 6 at.% leads to a contraction of the lattice, which in turn produces a significant structural disorder evidenced by shift in the XRD peaks due to additional interstitial incorporation of Y. The vibrational modes of the metal oxide groups have been identified from the IR transmission spectra. The optical absorption results show that the optical bandgap energy of Y:ZnO nanocrystals is much less as compared to that of the pure bulk ZnO particles. Doping ZnO with trivalent Y produces excess number of electrons in the conduction band and thus, shifts the absorption edge and narrows down to 80 meV approximately. PL spectra are used to study the dependence of doping on the deep-level emission, which show an enhanced blue emission after Y doping. The existence of near band edge (NBE) emission and blue emission, related to zinc interstitials are observed in the luminescence spectra of Zn(1-x)Y(x)O nanostructures.     

Synthesis of zinc oxide nanostructures during zinc oxidation by sub-and supercritical water

Solid zinc (Zn)_S and liquid zinc (Zn)_L are oxidized by water with the formation of zinc oxide (ZnO) nanostructures and the evolution of hydrogen. The maximum rate of this process, called chemical supercondensation by water (CSW), is realized on approaching the melting temperature of zinc from the left and right with increasing density of supercritical water. The CSW process begins with the formation of (ZnO) _n clusters via the reaction (Zn)_S,L + nH₂O = [(Zn)_S,L · (ZnO) _n ] + nH₂, followed by their subsequent growth at n > 7 in the exothermal process of epitaxy on (Zn)_S and coagulation of (ZnO) _n in (Zn)_L. The CSW of (Zn)_S leads predominantly to the formation of nanowires and nanorods, while the CSW of (Zn)_L practically always proceeds with the formation of nanoparticles. The rate of (Zn)_S oxidation increases with the thickness of a layer converted into ZnO. This is related to the self-heating and local melting of (Zn)_S in the course of CSW. The complete CSR of (Zn)_S plates and cylinders results in the formation of highly porous nanostructural ceramics.     

Field-emission stability of hydrothermally synthesized aluminum-doped zinc oxide nanostructures

The Al-doped ZnO (AZO) nanostructures field-emission arrays (FEAs) were hydrothermally synthesized on AZO/glass substrate. The samples with Al-dosage of 3 at.% show the morphology as nanowires vertically grown on the substrates and a structure of c-axis elongated single-crystalline wurtzite. The good field-emission (i.e., the large anode current and low fluctuation of 15.9%) can be found by AZO nanostructure FEAs with well-designed Al-dosage (i.e., 3 at.%) because of the vertical nanowires with the less structural defects and superior crystallinity. Moreover, the Full width at half maximum (FWHM) of near band-edge emission (NBE) decreased as the increase of annealing temperature, representing the compensated structural defects during oxygen ambient annealing. After the oxygen annealing at 500 degrees C, the hydrothermal AZO nanostructure FEAs revealed the excellent electrical characteristics (i.e., the larger anode current and uniform distribution of induced fluorescence) and enhanced field-emission stability (i.e., the lowest current fluctuation of 5.97%).     

Fabrication of zinc oxide nanostructures using solvent-assisted capillary lithography

We present a simple solvent-assisted capillary molding method to fabricate zinc oxide (ZnO) nanostructures using an ultraviolet (UV) curable polyurethane acrylate (PUA) mold. A thin film of the ZnO sol-gel precursor solution in methyl alcohol was prepared by spin coating on a solid substrate and subsequently a nanopatterned PUA mold was brought into conformal contact with the substrate under slight physical pressure (～3.5?bar). After annealing at 230?°C for 4?h, well-defined ZnO nanostructures formed with feature size down to ～50?nm, aided by capillary rise and solvent evaporation. It was found that the height of capillary rise depended highly on the applied pressure. A simple experimental setup was devised to examine the effects of pressure, revealing that the optimum pressure ranged from 3.5?bar to 5?bar. Also, ZnO nanorods could be selectively grown on patterned regions using the seed layer as a pseudocatalyst when the width of the seed layer was larger than ～200?nm.     

Growth mechanism of ZnO nanostructures in wet-oxidation process

Zinc (Zn) thin films were prepared by direct current magnetron sputtering as precursors with different deposition times. Zinc oxide (ZnO) nanostructures such as nanowires, nanobelts and nanoblades were then synthesized from the Zn precursors by wet-oxidation process. The microstructures of the Zn precursor and ZnO nanostructures have been studied by scanning electronic microscopy and X-ray diffractometry. The optoelectronic properties were analyzed by photoluminescence measurement. It was found that the Zn precursor film with a porous top layer consisting of well-crystallized Zn grains is an essential for formation of ZnO nanowires. Along with time dependence study and temperature dependence studies, the ZnO nanostructure growth mechanisms during the wet-oxidation process are proposed: water vapor has a major influence on the initial stage, and the final dimensions of the nanostructure are controlled by the vapor-solid process.     

Growth of one-dimensional zinc sulfide nanostructures through electrophoretic deposition

One-dimensional ZnS nanostructures were grown through EPD of ZnS nanoparticles prepared by a microwave assisted synthesis. The nanostructures were formed without using template membranes after 24 h of deposition on aluminum plates at direct current low voltage. Dimensions of the nanostructures can be varied by deposition time and electric field. AFM results show the polycrystalline character of the nanostructures. Lyosphere distortion and thinning and subsequent dipole–dipole interactions phenomena are proposed as a possible mechanism of the one dimensional nanostructures formation.     

Growth of CNTs on Fe-Si catalyst prepared on Si and Al coated Si substrates

In this paper we report the effect of Al interlayers on the growth characteristics of carbon nanotubes (CNTs) using as-deposited and plasma etched Fe-Si catalyst films as the catalysts. Al interlayers having various thicknesses ranging from 2 to 42?nm were deposited on Si substrates prior to the deposition of Fe-Si catalysts. It was found that the Al interlayer diffuses into the Fe-Si catalyst during the plasma etching prior to the CNT growth, leading to the swelling and amorphization of the catalyst. This allows enhanced carbon diffusion in the catalyst and therefore a faster growth rate of the resulting CNTs. It was also found that use of an Al interlayer having a thickness of ～3 ± 1?nm is most effective. Due to the effectiveness of this, the normally required catalyst etching is no longer needed for the growth of CNTs.     

Synthesis and growth mechanism of zinc oxide nanostructures in arc discharge

Nanodimensional tubes, needles, and tetrapods of zinc oxide (ZnO) with lengths up to ～1 μm and diameters up to several hundred nanometers have been synthesized for the first time using an arc discharge method. A distinctive feature of the proposed method is the broad variety of evaporated and deposited products. The possible mechanism of evaporation, nucleation, and growth is suggested. Initially, the primary sheets are growing; these sheets turn into rolls and tubes, which eventually transform into needles. Tetrapods crystallize and grow in the gas phase and form a white snowlike deposit.     

Changes of tungsten oxide nanostructures in flame vapor deposition process depending on process variables

Tungsten oxide thin film in 1-dimensional (1-D) nanostructures shows high photocatalytic activity and the flame vapor deposition (FVD) process is fast and economical method to prepare 1-D nanostructured tungsten oxide thin films of high purity and crystallinity. We investigated the morphology changes of tungsten oxide thin film prepared by FVD process for various process variables such as total gas flow rate, flame temperature and substrate temperature. For the experimental conditions in this study, we confirmed that the selection of suitable total flow rate is a key factor for 1-D nanostructure growth in fuel-rich condition. As we increase the flame and substrate temperatures, the longer and thinner 1-D nanotubes were obtained, which have the advantages of high surface area and shorter diffusion length of proton for the application to photoelectrochemical water splitting. This study would provide the basic information for the design of FVD process to prepare 1-D nanostructures in future.     

Microwave-assisted synthesis and characterization of flower shaped zinc oxide nanostructures

A microwave-assisted solution-phase approach has been applied for the synthesis of zinc oxide nanostructures. The synthesis procedure was carried out by using two reagents: hydrazine hydrate and ammonia. Flower shaped particles were obtained with hydrazine hydrate whereas mainly spherical agglomerated particles were observed with ammonia. The nanostructures were influenced by microwave irradiation time, reagent concentration and molar ratio of the precursors. High crystalline materials were found without the need of a post-synthesis treatment. The average crystalline size of ZnO nanostructures has been analyzed by X-ray Diffraction (XRD) pattern and estimated to be 18 nm. The presence of flower shaped zinc oxide with nanorods arranged has been confirmed from Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) micrographs. The samples were further analyzed by Fourier Transform InfraRed (FT-IR), Thermogravimetric Analysis (TGA) and photoluminescence spectroscopic techniques.     

Growth condition dependence of structural and electrical properties of Mg2Si layers grown on silicon substrates

Mg₂Si layers were grown on Si substrates by thermal treatment of the substrates in a Mg vapor, and the growth condition dependence of the structural and electrical properties of the layers was investigated. The layers were grown by an interdiffusion process between the deposited Mg atoms and the substrates. The structural and electrical properties of the layers depend on the combinations of the Si substrate and Mg source temperatures during the heat treatment. Any deviation from the isothermal treatment conditions causes degradation of the structural and/or electrical properties of the Mg₂Si layers. It was confirmed that the layers with the optimum structural and electrical properties were obtained when the layers were grown under isothermal growth conditions.