The growth of porous ZnO nanowires by thermal oxidation of ZnS nanowires

The growth of porous ZnO nanowires (NWs) via phase transformation of ZnS NWs at 500-850 degrees C in air was studied. The ZnS NWs were first synthesized by thermal evaporation of ZnS powder at 1100 degrees C in Ar. On subsequent annealing at 500 degrees C in air, discrete ZnO epilayers formed on the surface of ZnS NWs. At 600 degrees C, polycrystalline ZnO and the crack along the (0001) interface between the ZnO epilayer and ZnS NW were observed. At 700-750 degrees C ZnS NWs transformed to ZnO NWs, meanwhile nanopores and interfacial cracks were observed in the ZnO NWs. Two factors, the evaporation of SO2 and SO3 and the stress induced by the incompatible structure at the interface of ZnO epilayer and ZnS NW, can be responsible for the formation of porous ZnO NWs from ZnS NW templates on annealing at 700-750 degrees C in air. Rapid growth of ZnO at 850 degrees C could heal the pores and cracks and thus resulted in the well-crystallized ZnO NWs.     

An advanced fabrication method of highly ordered ZnO nanowire arrays on silicon substrates by atomic layer deposition

In this work, the controlled fabrication of highly ordered ZnO nanowire (NW) arrays on silicon substrates is reported. Si NWs fabricated by a combination of phase shift lithography and etching are used as a template and are subsequently substituted by ZnO NWs with a dry-etching technique and atomic layer deposition. This fabrication technique allows the vertical ZnO NWs to be fabricated on 4 in Si wafers. Room temperature photoluminescence and micro-photoluminescence are used to observe the optical properties of the atomic layer deposition (ALD) based ZnO NWs. The sharp UV luminescence observed from the ALD ZnO NWs is unexpected for the polycrystalline nanostructure. Surprisingly, the defect related luminescence is much decreased compared to an ALD ZnO film deposited at the same time ona plane substrate. Electrical characterization was carried out by using nanomanipulators. With the p-type Si substrate and the n-type ZnO NWs the nanodevices represent p–n NW diodes.The nanowire diodes show a very high breakthrough potential which implies that the ALD ZnO NWs can be used for future electronic applications.     

p-Type ZnO nanowire arrays

Well-aligned ZnO nanowire (NW) arrays with durable and reproducible p-type conductivity were synthesized on alpha-sapphire substrates by using N2O as a dopant source via vapor-liquid-solid growth. The nitrogen-doped ZnO NWs are single-crystalline and grown predominantly along the [110] direction, in contrast to the [001] direction of undoped ZnO NWs. Electrical transport measurements reveal that the nondoped ZnO NWs exhibit n-type conductivity, whereas the nitrogen-doped ZnO NWs show compensated highly resistive n-type and finally p-type conductivity upon increasing N2O ratio in the reaction atmosphere. The electrical properties of p-type ZnO NWs are stable and reproducible with a hole concentration of (1-2) x 10(18) cm(-3) and a field-effect mobility of 10-17 cm2 V(-2) s(-1). Surface adsorptions have a significant effect on the transport properties of NWs. Temperature-dependent PL spectra of N-doped ZnO NWs show acceptor-bound-exciton emission, which corroborates the p-type conductivity. The realization of p-type ZnO NWs with durable and controlled transport properties is important for fabrication of nanoscale electronic and optoelectronic devices.     

Highly efficient visible light photocatalysis of novel CuS/ZnO heterostructure nanowire arrays

Here, a facile approach for the fabrication of CuS nanoparticle (NP)/ZnO nanowire (NW) heterostructures on a mesh substrate through a simple two-step solution method is demonstrated. Successive ionic layer adsorption and reaction (SILAR) was employed to uniformly deposit CuS NPs on the hydrothermally grown ZnO NW array. The synthesized CuS/ZnO heterostructure NWs exhibited superior photocatalytic activity under visible light compared to bare ZnO NWs. This strong photocatalytic activity under visible light is due to the interfacial charge transfer (IFCT) from the valence band of the ZnO NW to the CuS NP, which reduces CuS to Cu(2)S. After repeated cycles of photodecolorization of Acid Orange 7 (AO7), the photocatalytic behavior of CuS/ZnO heterostructure NWs exhibited no significant loss of activity. Furthermore, our CuS/ZnO NWs/mesh photocatalyst floats in solution via partial superhydrophobic modification of the NWs.     

Synthesis and structural-optical properties of Ga-doped ZnO nanowires by hot-walled pulsed laser deposition method

Well-aligned single-crystalline zinc oxide (ZnO) and Ga doped ZnO (GZO) NWs (NWs) were successfully fabricated on Au film catalyzed sapphire substrate using vapor-liquid-solid (VLS) method in hot-walled pulsed laser deposition (HW-PLD). The structural and optical properties of Ga doped ZnO NWs have been investigated depending on various concentration of Ga dopants in ZnO NWs. As increasing Ga concentration, stacking faults were observed by using FE-SEM and an exciton bound to a neutral donor (D(0)X) peak was clearly observed by using PL spectra. From the structural and optical properties, the ZnO NWs by doping could be application to electronic and optoelectronic devices, such as nano-FETs, nano-inverters, nano-logic circuits and nano-sensors.     

Hierarchical Metal/Semiconductor Nanostructure for Efficient Water Splitting

A hierarchically patterned metal/semiconductor (gold nanoparticles/ZnO nanowires) nanostructure with maximized photon trapping effects is fabricated via interference lithography (IL) for plasmon enhanced photo‐electrochemical water splitting in the visible region of light. Compared with unpatterned (plain) gold nanoparticles‐coated ZnO NWs (Au NPs/ZnO NWs), the hierarchically patterned Au NPs/ZnO NWs hybrid structures demonstrate higher and wider absorption bands of light leading to increased surface enhanced Raman scattering due to the light trapping effects achieved by the combination of two different nanostructure dimensions; furthermore, pronounced plasmonic enhancement of water splitting is verified in the hierarchically patterned Au NPs/ZnO NWs structures in the visible region. The excellent performance of the hierarchically patterned Au NPs/ZnO NWs indicates that the combination of pre‐determined two different dimensions has great potential for application in solar energy conversion, light emitting diodes, as well as SERS substrates and photoelectrodes for water splitting.     

A study on rapid growth and piezoelectric effect of ZnO nanowires array

This work presents a rapid and simple synthesis procedure for ZnO nanowires (NWs) array by using the vapor–solid (VS) method. Experimental results indicate that the length and diameter of the grown ZnO NWs are associated with the temperature effect, while the growth density of NWs is strongly related to gas flux during the VS process. Additionally, the synthesized ZnO NWs possess specific crystalline qualities, making them highly promising for piezoelectric device applications. Therefore a piezoelectric type nanogenerator based on the ZnO NWs is also designed in this work, with a high output of piezoelectric current of 0.6 μA cm⁻² obtained as well. Our results further demonstrate the feasibility of applying piezoelectric energy via the rapidly grown ZnO NWs array.     

Optical and field emission characteristics of anodic aluminium oxide/ZnO hybrid nanostructure

Arrays of ZnO nanowires (NWs) were fabricated within the well-distributed pores of anodic aluminium oxide (AAO) template by a simple chemical method. The photoluminescence (PL) and field emission (FE) properties of the AAO/ZnO NWs hybrid structure were investigated in detail. The hybrid nanostructure exhibits interesting PL characteristics. ZnO NWs exhibit UV emission at 378 nm and two prominent blue-green emissions at about 462 and 508 nm. Intense blue emission from the AAO template itself was observed at around 430 nm. Herein, for the first time we report the FE characteristics of the ZnO/AAO hybrid structure to show the influence of the AAO template on the FE property of the hybrid structure. It is found that the turn-on electric field of the vertically grown and aligned ZnO NWs within the pores of AAO template is lower than the entangled unaligned ZnO NWs extracted from the template. Although the AAO template exhibits no FE current but it helps to achieve better FE property of the ZnO NWs through better alignment. The turn-on electric field of aligned NWs was found to be 3 V μm⁻¹ at a current of 0.1 μA. Results indicate that the AAO embedded ZnO NW hybrid structure may find useful applications in luminescent and field emission display devices.     

ZnO nanowires hydrothermally grown on PET polymer substrates and their characteristics

Zinc oxide nanowires (ZnO NWs) were successfully synthesized on the ITO/PET polymer substrates by a hydrothermal method. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy investigations were carried out to characterize the crystallinity, surface morphologies, and orientations of these NWs, respectively. The influence of NW surface morphologies on the optical and electrical properties of ZnO NWs was studied. The hydrothermally grown ZnO NWs with direct band gap of 3.21 eV emitted ultraviolet photoluminescence of 406 nm at room temperature. Field emission measurements revealed that the threshold electric fields (Eth, current density of 1 mA/cm2) of ZnO NWs/ITO/PET and ZnO NWs/ZnO/ITO/PET are 1.6 and 2.2 V/microm with the enhancement factors, beta values, of 3275 and 4502, respectively. Furthermore, the field emission performance of ZnO NWs deposited on the ITO/PET substrate can be enhanced by illumination with Eth of 1.3 V/microm and displays a maximum emission current density of 18 mA/cm2. The ZnO NWs successfully grown on polymer substrate with high transmittance, low threshold electric field, and high emission current density may be applied to a flexible field emission display in the future.     

Direct observation of enhanced cathodoluminescence emissions from ZnO nanocones compared with ZnO nanowire arrays

We report, for the first time, direct observation of enhanced cathodoluminescence (CL) emissions from ZnO nanocones (NCs) compared with ZnO nanowires (NWs). For direct and unambiguous comparison of CL emissions from NWs and nanocones, periodic arrays of ZnO NW were converted to nanocone arrays by our unique HCl [aq] etching technique, enabling us to compare the CL emissions from original NWs and final nanocones at the same location. CL measurements on NW and nanocone arrays reveal that emission intensity of the nanocone at ～ 387 nm is over two times larger than that of NW arrays. The enhancement of CL emission from nanocones has been confirmed by finite-difference time-domain simulation of enhanced light extraction from ZnO nanocones compared to ZnO NWs. The enhanced CL from nanocones is attributed to its sharp morphology, resulting in more chances of photons to be extracted at the interface between ZnO and air.     

Optimization of CVD parameters for long ZnO NWs grown on ITO/glass substrate

The optimization of chemical vapour deposition (CVD) parameters for long and vertically aligned (VA) ZnO nanowires (NWs) were investigated. Typical ZnO NWs as a single crystal grown on indium tin oxide (ITO)-coated glass substrate were successfully synthesized. First, the conducted side of ITO–glass substrate was coated with zinc acetate dihydrate to form seed layer of ZnO nanocrystals. Double zone tube furnace connected to vacuum pump was used for ZnO growth process. Zn metal powder was positioned at the first zone at temperature 900 ^° C. The ITO–glass substrate with pre-coated seed layer was then located in the second zone of tube furnace at growth temperature of 550 ^° C. The growth of ZnO NWs was controlled under constant concentration of seed layer, while other parameters such as argon and oxygen flow rates, substrate position, time and oxygen flow rate were varied. The VA ZnO NWs were finally characterized by scanning electron microscopy, X-ray diffractometer and high-resolution transmission electron microscope equipped with energy-dispersive X-ray spectroscopy. The results show that long and VA ZnO NWs were single crystalline with hexagonal wurtzite structure. The ultimate length and average diameter of ZnO NWs were 10 μm and 50–100 nm, respectively. These were achieved under optimized CVD growth parameters. The mechanism of vertical growth model of ZnO NWs is also discussed.     

Heterojunction solar cells with integrated Si and ZnO nanowires and a chalcopyrite thin film

ZnO nanowires (NWs) have been successfully synthesized using a hydrothermal technique on both glass and silicon substrates initially coated with a sputtered ZnO thin film layer. Varying ZnO seed layer thicknesses were deposited to determine the effect of seed layer thickness on the quality of ZnO NW growth. The effect of growth time on the formation of ZnO NWs was also studied. Experimental results show that these two parameters have an important effect on formation, homogeneity and vertical orientation of ZnO NWs. Silicon nanowires were synthesized by a Ag-assisted electroless etching technique on an n-type Si (100) wafer. SEM observations have revealed the formation of vertically-aligned Si NWs with etching depth of ∼700 nm distributed over the surface of the Si. An electron-beam evaporated chalcopyrite thin film consisting of p-type AgGa_0.5In_0.5Se₂ with ∼800 nm thickness was deposited on the n-type ZnO and Si NWs for the construction of nanowire based heterojunction solar cells. For the Si NW based solar cell, from a partially illuminated area of the solar cell, the open-circuit voltage, short-circuit current density, fill factor and power conversion efficiency were 0.34 V, 25.38 mA cm⁻², 63% and 5.50%, respectively. On the other hand, these respective parameters were 0.26 V, 3.18 mA cm⁻², 35% and 0.37% for the ZnO NW solar cell.     

ZnO nanowire/TiO2 nanoparticle photoanodes prepared by the ultrasonic irradiation assisted dip-coating method

Hybrid ZnO/TiO₂ photoanodes for dye-sensitized solar cells were prepared by combining ZnO nanowire (NW) arrays and TiO₂ nanoparticles (NPs) with the assistance of the ultrasonic irradiation assisted dip-coating method. Results show that the ultrasonic irradiation was an efficient way to promote the gap filling of TiO₂ NPs in the interstices of ZnO NWs. Hybrid ZnO NW/TiO₂ NP electrodes prepared with ultrasonic treatment exhibited better gap filling efficiency and higher visible absorptance. The overall conversion efficiency of the hybrid electrode was 0.79%, representing 35% improvement compared with that of the traditional one (0.58%). The enlarged surface area and improved attachments of TiO₂ NPs onto the walls of ZnO NWs induced by the application of ultrasonic irradiation may be the underlying reason. Electrochemical impedance spectroscopy measurements indicated that hybrid electrodes combined the advantages of improved electron transport along the ZnO NWs and increased surface area provided by infiltrated TiO₂ NPs, both of which are responsible for the improved cell efficiency.     

Enhanced photoluminescence and photoconductivity of ZnO nanowires with sputtered Zn

We have sputtered Zn onto quasi-one-dimensional ZnO nanowires (NWs) in order to investigate the effect of Zn diffusion on the photoluminescence and photoconduction properties of ZnO NWs. Elemental mapping clearly indicates higher Zn concentration in the NWs due to diffusion of Zn. The Zn-sputtered NWs show an enhanced ultraviolet emission with 7 nm red shift. Since the ionization energy of Zni is 51 meV, the enhanced PL emission with a red shift is correlated to the coupling between free exciton and zinc interstitials (Zni) defects. The photocurrent transients show almost 20 times more photocurrent generation in Zn/ZnO NWs compared to the as-grown NWs. In contrast, the thin film shows no significant change in the photoluminescence and photoconductivity. Based on the photoconductivity and photoluminescence results, we predict that Zn diffusion in the NWs occurs easily compared to the films because of the smaller dimensions of the NWs.     

A generic approach for vertical integration of nanowires

We report on the collective integration technology of vertically aligned nanowires (NWs). Si?and ZnO NWs have been used in order to develop a generic technological process. Both mineral and organic planarizations of the as-grown nanowires have been achieved. Chemical vapour deposition (CVD) oxides, spin on glass (SOG), and polymer have been investigated as filling materials. Polishing and/or etching of the composite structures have been set up so as to obtain a suitable morphology for the top and bottom electrical contacts. Electrical and optical characterizations of the integrated NWs have been performed. Contacts ohmicity has been demonstrated and specific contact resistances have been reported. The photoconducting properties of polymer-integrated ZnO NWs have also been investigated in the UV-visible range through collective electrical contacts. A small increase of the resistivity in the ZnO NWs under sub-bandgap illumination has been observed and discussed. A comparison of the photoluminescence (PL) spectra at 300?K of the as-grown and SOG-integrated ZnO nanowires has shown no significant impact of the integration process on the crystal quality of the NWs.     

ZnO nanowire co-growth on SiO2 and C by carbothermal reduction and vapour advection

Vertically aligned ZnO nanowires (NWs) were grown on Au-nanocluster-seeded amorphous SiO(2) films by the advective transport and deposition of Zn vapours obtained from the carbothermal reaction of graphite and ZnO powders. Both the NW volume and visible-to-UV photoluminescence ratio were found to be strong functions of, and hence could be tailored by, the (ZnO+C) source-SiO(2) substrate distance. We observe C flakes on the ZnO NWs/SiO(2) substrates which exhibit short NWs that developed on both sides. The SiO(2) and C substrates/NW interfaces were studied in detail to determine growth mechanisms. NWs on Au-seeded SiO(2) were promoted by a rough ZnO seed layer whose formation was catalysed by the Au clusters. In contrast, NWs grew without any seed on C. A correlation comprising three orders of magnitude between the visible-to-UV photoluminescence intensity ratio and the NW volume is found, which results from a characteristic Zn partial pressure profile that fixes both O deficiency defect concentration and growth rate.     

Rational synthesis of p-type zinc oxide nanowire arrays using simple chemical vapor deposition

We report, for the first time, the synthesis of the high-quality p-type ZnO NWs using a simple chemical vapor deposition method, where phosphorus pentoxide has been used as the dopant source. Single-crystal phosphorus doped ZnO NWs have their growth axis along the 001 direction and form perfect vertical arrays on a-sapphire. P-type doping was confirmed by photoluminescence measurements at various temperatures and by studying the electrical transport in single NWs field-effect transistors. Comparisons of the low-temperature PL of unintentionally doped ZnO (n-type), as-grown phosphorus-doped ZnO, and annealed phosphorus-doped ZnO NWs show clear differences related to the presence of intragap donor and acceptor states. The electrical transport measurements of phosphorus-doped NW FETs indicate a transition from n-type to p-type conduction upon annealing at high temperature, in good agreement with the PL results. The synthesis of p-type ZnO NWs enables novel complementary ZnO NW devices and opens up enormous opportunities for nanoscale electronics, optoelectronics, and medicines.     

A ZnO nanowire vacuum pressure sensor

In this study, we report the growth and characterization of lateral ZnO nanowires (NWs) on ZnO:Ga/glass templates. Using x-ray diffraction and micro-Raman spectroscopy, it was found that crystal quality of the as-grown ZnO NWs is good. It was also found that the average length and average diameter of the laterally grown ZnO NWs were 5?μm and 30?nm, respectively. A vacuum pressure sensor was then fabricated using a single NW bridging across two electrodes. By measuring the current-voltage characteristics of the samples at low pressure, we found that the currents were of 17, 34.28, 57.37 and 96.06?nA for the ZnO NW measured at 1 × 10(-3)?Torr, 1 × 10(-4)?Torr, 3 × 10(-5)?Torr and 5 × 10(-6)?Torr, respectively. These values suggest that the laterally grown ZnO NWs prepared in this study are potentially useful for vacuum pressure sensing.     

Synthesis of high crystallinity ZnO nanowire array on polymer substrate and flexible fiber-based sensor

Well aligned ZnO nanowire (NW) arrays are grown on Kevlar fiber and Kapton film via the chemical vapor deposition (CVD) method. These NWs have better crystallinity than those synthesized through the low-temperature hydrothermal method. The average length and diameter of ZnO NWs grown on Kevlar fiber can be controlled from 0.5 to 2.76 μm and 30 to 300 nm, respectively. A flexible ultraviolet (UV) sensor based on Kevlar fiber/ZnO NWs hybrid structure is made to detect UV illumination quantificationally.     

Growth and characterization of indium phosphide nanowires on transparent conductive ZnO:Al films

The epitaxial growth of indium phosphide nanowires (InP NWs) on transparent conductive aluminum-doped zinc oxide (ZnO:Al) thin films is proposed and demonstrated. ZnO:Al thin films were prepared on quartz substrates by radio frequency magnetron sputtering, then InP NWs were grown on them by plasma enhanced metal organic chemical vapor deposition with gold catalyst. Microstructure and optical properties of InP nanowires on ZnO:Al thin films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectric spectroscopy (XPS), photoluminescence and Raman spectroscopy at room temperature. SEM shows that randomly oriented and intersecting InP nanowires were grown to form a network on ZnO:Al thin films. Both wurtzite (WZ) and zincblende (ZB) structures coexist in the random orientation InP NWs on ZnO:Al thin film had been proved by XRD analysis. XPS result indicates Zn diffusion exists in the InP NWs on ZnO:Al. The photoluminescence spectra of InP nanowires with Zn diffusion present an emission at 915 nm. Zn diffusion also bring effect on Raman spectra of InP NWs, leading to more Raman-shift and larger relative intensity ratio of TO/LO.