Improving the NH(3) gas sensitivity of ZnO nanowire sensors by reducing the carrier concentration

We report a method to improve the sensitivity of a zinc oxide (ZnO) nanowire gas sensor towards ammonia (NH(3)) without the use of catalyst nanoparticles on the nanowire surface. This improvement is achieved by lowering the nominal carrier concentration in the as-grown ZnO nanowires. The carrier concentration in the as-grown ZnO nanowires can be tuned by treating these nanowires to either an oxidizing gas plasma or a reducing gas plasma, as observed from the measured current-voltage (I-V) characteristics response. We demonstrate that a ZnO nanowire sensor device that has been subjected to oxygen plasma treatment, thereby having a reduced carrier concentration, exhibits a sensitivity towards 0.75% NH(3) gas that is improved by approximately four times. The origin of this gas sensitivity improvement is discussed based on x-ray photoelectron spectroscopy analysis results of the plasma-treated ZnO nanowires.     

Effect of chemically reactive species on properties of ZnO nanowires exposed to oxygen and hydrogen plasma

We present a systematic study on the effect of oxygen and hydrogen plasma-generated reactive species on the properties of ZnO nanowires. Upon exposure to oxygen plasma, the electrical conductivity of an individual ZnO nanowire decreased with substantial changes in the surface chemistry, indicating a decrease in the number of donor-like defects and an increase in the number of electron-trapping species. In contrast, an individual ZnO nanowire exposed to hydrogen plasma showed a drastic increase in conductivity up to two orders of magnitude due to the incorporated hydrogen acting as a shallow donor inside the ZnO nanowires without a sputtering process.     

Growth of ZnO nanowires in hollow-type magnetron O2/Ar RF plasma

In this study we investigate how the sputtering of zinc from a zinc target is influenced by the properties of O₂/Ar plasma when the discharge parameters such as gas pressure ratio and target bias voltage are changed. We also investigate plasma conditions for the formation of ZnO nanowires that are created and deposited on the substrate. We found that a plasma condition with strong optical emission from Zn neutrals is an important factor for the deposition of ZnO nanowires. Both the growth of nanowires and the emission intensity from Zn strongly depend on the partial pressure of oxygen.     

Surface modification of an aramid fibre treated in a low-temperature microwave plasma

The surface modification of an aramid fibre treated in a low-temperature microwave (mw) plasma was investigated. Three different plasma gases, oxygen, argon and ammonia, were used to achieve different surface modifications during fibre treatment. The modification of the fibre surface was analysed with electron spectroscopy and electron microscopy. The influence of the surface modification on the fibre-matrix interaction was inspected by measuring the interlamellar shear strength of the composites and the pull-out strength of a fibre bundle in model composites. The process gas and thus the kind of plasma has no significant influence on the fibre modification resulting from plasma treatment. It was shown that a fibre cleaning with subsequent surface ablation is the dominate modification process during mw plasma treatment, independent of the process gas. The degree of surface cleaning and removal of a contamination layer strongly depended on the treatment range. No incorporation of oxygen or nitrogen containing functional groups was observed. This was explained with the composition of the process gas. The improvements of the composite properties demonstrate the advantage of the mw plasma treatment as a fast, environmentprotected, cost-efficient process for fibre modification.     

Cathodoluminescence property of ZnO nano-phosphors fabricated by pulsed Nd:YAG laser ablation in plasma circumstance

Cathodoluminescence (CL) with energy-controlled electron beams was used to study the inner structure of ZnO nano-phosphors fabricated by YAG laser ablation under oxygen gas and oxygen plasma circumstances. Varying accelerating voltages of electron beams from 1.0 to 3.0 kV, it was clearly seen that the CL spectrum in the case of ZnO nano-phosphors fabricated under the oxygen gas background changed from a broad one centered at approximately 500 nm to a narrow one peaked at 380∼400 nm. This result indicates that there is an oxygen vacancy-rich shell in the surface area and a fine crystalline ZnO structure in the inner core region. Under the oxygen plasma circumstance at 500 Pa, however, we found that there is a crystalline structure in the surface layer of ZnO nano-phosphors due to the sufficient supply of oxygen ions and atoms and suppression of the formation of oxygen vacancy.     

The characteristic of the ZnO nanowire morphology grown by the hydrothermal method on various surface-treated seed layers

ZnO nanowires with various density and diameters were obtained by altering the wetting properties of aluminum doped ZnO (AZO) seed layers. AZO seed layers deposited on glass substrates were treated by inductor coupled plasmas (ICP) or atmospheric pressure plasma (APP) systems in order to control wettability before ZnO nanowire growth. Morphology of ZnO nanowires grown by the hydrothermal method was influenced by wettability of AZO seed layers. Our investigation shows that the diameter of ZnO nanowire increases and the length of ZnO nanowire decreases when the contact angle on the AZO seed layer is decreased. As a result, we confirmed that the morphology of ZnO nanowires could be easily controlled by surface treatment of seed layers with the plasma system.     

Enhancement in the photodetection of ZnO nanowires by introducing surface-roughness-induced traps

We investigated the enhanced photoresponse of ZnO nanowire transistors that was introduced with surface-roughness-induced traps by a simple chemical treatment with isopropyl alcohol (IPA). The enhanced photoresponse of IPA-treated ZnO nanowire devices is attributed to an increase in adsorbed oxygen on IPA-induced surface traps. The results of this study revealed that IPA-treated ZnO nanowire devices displayed higher photocurrent gains and faster photoswitching speed than transistors containing unmodified ZnO nanowires. Thus, chemical treatment with IPA can be a useful method for improving the photoresponse of ZnO nanowire devices.     

Growth of ZnO nanowires on a patterned Au substrate

ZnO nanowires were site-selectively deposited on the catalytic gold-patterned substrates using a vapor transport process at low temperature. We observed that Au-Zn phase played an important role in initiating the ZnO nucleation, which was identified from the TEM observation of the interface between the substrate and ZnO nanowires. And then further growth of the nanowires on the ZnO surface was driven by the reaction of the Zn vapor and O₂ gas in vapor-solid growth mode. Finally, it was concluded that the site-specific deposition of ZnO nanowires was ascribed to the vapor-liquid-solid mechanism of Au, Zn vapor, and Au-Zn phases at the initial stage of ZnO nucleation.     

Tailoring the photoluminescence of ZnO nanowires using Au nanoparticles

Rational design of hybrid nanostructures through attaching nanowires with nanoparticles is an effective route to enhance the existing functionalities or to explore new ones. We carry out a systematic investigation on the photoluminescence of ZnO nanowire-Au nanoparticle hybrid nanostructures synthesized by attaching Au nanoparticles onto ZnO nanowires. Citrate-stabilized 40?nm Au nanoparticles effectively quench the green emission and enhance the UV emission of the ZnO nanowires, which is consistent with the wavelength-dependent generation of surface plasmon. The UV/green emission intensity ratio could be reversibly and reproducibly tailored by attaching/detaching Au nanoparticles. This enhancement of UV emission diminishes if the Au nanoparticles are coated with a polymer layer. We also find that the orange-red emission of the ZnO nanowires is related to the excess oxygen on the ZnO surface, and it is also tunable via annealing and surface modifications.     

Influence of oxygen and nitrogen plasma treatment on polyethylene terephthalate (PET) polymers

In this paper we present surface modification of polyethylene terephthalate (PET) polymer, which is commonly used as synthetic vascular graft. Surface modification was made by oxygen and nitrogen plasma at different treatment times. Plasma was created by means of an RF generator at a discharge power of 200 W and gas pressure fixed at 75 Pa. The surface of PET polymer was modified in order to achieve improved attachment of fucoidan, which is a bioactive coating with antithrombogenic properties. In our study we analysed chemical modification of plasma treated surfaces by X-ray photoelectron spectroscopy (XPS), while the changes in morphology and surface roughness were observed with atomic force microscopy (AFM). Our results indicate that attachment of fucoidan is improved by oxygen plasma treatment, especially due to surface roughening.     

Superior Functionality by Design: Selective Ozone Sensing Realized by Rationally Constructed High‐Index ZnO Surfaces

A new technique is reported for the transformation of smooth nonpolar ZnO nanowire surfaces to zigzagged high‐index polar surfaces using polycrystalline ZnO thin films deposited by atomic layer deposition (ALD). The c‐axis‐oriented ZnO nanowires with smooth nonpolar surfaces are fabricated using vapor deposition method and subsequently coated by ALD with a ZnO particulate thin film. The synthesized ZnO–ZnO core–shell nanostructures are annealed at 800 °C to transform the smooth ZnO nanowires to zigzagged nanowires with high‐index polar surfaces. Ozone sensing response is compared for all three types of fabricated nanowire morphologies, namely nanowires with smooth surfaces, ZnO–ZnO core–shell nanowires, and zigzagged ZnO nanowires to determine the role of crystallographic surface planes on gas response. While the smooth and core–shell nanowires are largely non‐responsive to varying O₃ concentrations in the experiments, zigzagged nanowires show a significantly higher sensitivity (ppb level) owing to inherent defect‐rich high‐index polar surfaces.     

An alternative approach to <Emphasis Type="Italic">in situ</Emphasis> synthesize single crystalline ZnO nanowires by oxidizing granular zinc film

ZnO nanowires were synthesized by a relatively simple process—oxidizing granular Zn films at a relatively low temperature (450–600 °C) without catalyst. The zinc film was initially fabricated by sputter deposition in an argon atmosphere at ambient temperature using Zn metal as the sputter target. After subsequent annealing in an air or oxygen atmosphere, ZnO nanowires were found to grow from individual Zn nanograins. The investigation has also showed that the nanowires preferably grow from relatively porous Zn film and a small amount of oxygen flow is beneficial to the growth of nanowires. The resultant single crystal ZnO nanowires obtained from annealing at 600 °C in an oxygen atmosphere had a mean diameter less than 50 nm and had a very good structural quality. This process provides an alternative method to produce ultra-fine ZnO nanowires standing on the substrate.     

Effects of additive gases and plasma post-treatment on electrical properties and optical transmittance of ZnO thin films

A zinc oxide (ZnO) thin film as a channel layer in an oxide thin film transistor (TFT) has been characterized by investigating the effects of additive gases (such as hydrogen and oxygen) during growth and plasma treatment (using argon or hydrogen) after growth on its electrical, optical, and structural properties. By decreasing the additive gas ratio of O₂/H₂ or by increasing the treatment time of hydrogen plasma, the electrical resistivities of ZnO films were significantly reduced, and their transmittances and optical bandgap energies were blue-shifted in wavelength. These results were considered to be closely related to the passivation of oxygen vacancies as well as the formation of shallow donors that were induced by the injection of hydrogen in ZnO via gas addition and plasma treatment. In addition, the injection of hydrogen-including additive gas resulted in a decrease in grain size and crystallinity of ZnO films, whereas the plasma treatment hardly affected their crystalline structures.     

Synthesis of ZnO nanowires by the hydrothermal method,using sol–gel prepared ZnO seed films

Zinc oxide (ZnO) nanowires with various morphologies are synthesized by the hydrothermal method on silicon substrates coated with ZnO thin films. The ZnO films are used as the seed layer and are prepared using the sol–gel technique. Experimental results demonstrate that the synthesis of ZnO nanowires is dependent on the crystalline properties of the ZnO seed-layer films. Sol concentration is the controlled parameter for the preparation of ZnO seed-layer films in this study. The ZnO films are found to have the hexagonal wurtzite structure with highly preferred growth along the c-axis at suitable sol concentrations. The vertically aligned ZnO nanowire arrays on the substrates are believed to be the result of the epitaxial growth of the ZnO seed layer. Scanning electron microscopy shows that nanowires with uniform distribution in length, diameter, and density are obtained. X-ray diffraction patterns clearly reveal that the ZnO nanowires are primarily grown along the c-axis direction. Transmission electron microscopy and selected-area electron diffraction measurements show that the nanowires have good crystalline properties. The well-aligned and high surface areas of the ZnO nanowires make them a potential candidate for applications in solar cells, field emission devices, and ultra-sensitive gas sensors.     

Characterization of infrared chemical sensors modified with ZnO nanowires for the detection of volatile organic compounds

In this paper we describe the application and characterization of zinc oxide (ZnO) nanowires in an infrared (IR) chemical sensing system for the detection of volatile organic compounds (VOCs). Under suitable conditions, we grew ZnO nanowires on the surfaces of IR internal reflection elements (IREs) and obtained successful results for the detection of VOCs. ZnO nanowires offer a large surface area to effectively adsorb the examined species; the sensitivity of these IR sensing systems was increased by 3- to 15-fold after surface treatment with the ZnO nanowires. To explore the performance of this type of sensor, we correlated the morphologies of the ZnO nanowires grown on the surfaces of the IREs with the adsorption behavior observed during the sensing of the VOCs. To characterize the properties of the ZnO nanowires during the detection of VOCs having a range of functionalities, we classified the VOCs and examined their enrichment factors by comparing the IR signals detected in the presence and absence of the ZnO nanowires. Our results indicate that the ZnO nanowires exhibited better performance for the detection of aromatic-type VOCs than they did for non-aromatic compounds. For quantitative analyses, we examined several compounds for their responses toward varying quantities of injected VOCs. Our results indicate that the IREs treated with ZnO nanowires display acceptable linearity in their standard curves; the linear regression coefficients were higher than 0.995 for a range of volatile compounds.     

Local vapor transport synthesis of zinc oxide nanowires for ultraviolet-enhanced gas sensing

A novel local vapor transport technique via induction heating is presented to enable selective, localized synthesis and self-assembly of nanowires, providing a simple and fast method for the direct integration of nanowires into functional devices. The single-crystalline zinc oxide (ZnO) nanowires are grown locally across the silicon-on-insulator microelectrodes within minutes, and the enhancement of gas sensing of ZnO nanowires is demonstrated under ultraviolet (UV) illumination at room temperature. Experiments indicate that when suspended nanowires are exposed to UV light, a twelve-fold increase in conductance and a near five-fold improvement in oxygen response are measured. Furthermore, the UV-enhanced transient responses exhibit a two-level photocurrent decay attributed to carrier recombination and oxygen readsorption. As such, the local vapor transport synthesis and UV-enhanced sensing scheme could provide a promising approach for the construction of miniaturized and highly responsive nanowire-based gas sensors.     

Low temperature epitaxial growth of ZnO layer by plasma-assisted epitaxy

Plasma-assisted epitaxial growth of ZnO layers were achieved on C- and R-plane sapphire substrates in oxygen plasma excited by radio frequency power at 13.56 MHz with evaporation of pure elemental Zn. The ZnO layers were grown at 300–400°C with high growth rate around 1.7 μm/h. Surface cleaning of sapphire substrates using Ar-plasma was crucial for good quality ZnO growth. Photoluminescence spectra at 10 K were dominated by band-edge emission due to bound excitons without deep level emission in green-light region. The intensity of band-edge emission was strongly dependent on applied radio frequency power to excite Ar- and O₂-plasma for sapphire surface cleaning and ZnO growth, respectively, and was about 50 times larger on the layer grown in oxygen plasma than that grown in non-excited oxygen gas. The ZnO layer grown on R-plane sapphire was epitaxially grown above 300°C in oxygen plasma, however, on C-plane sapphire the ZnO layer was easily polycrystallized for thick films even at 400°C. Growth mode and surface morphology of ZnO layers were drastically changed with the substrate orientation.     

Controlled growth of zinc nanowires

Zinc nanowires have been synthesized by heating a mixture of boron and zinc oxide (ZnO) powders at 1050 °C under a nitrogen atmosphere. The influences of the gas flow rate and the substrate character on the nanowire formation were investigated. It was found that higher-flow rate of gas led to the formation of thinner nanowires; while lower-flow rate of gas produced thicker nanowires and even particles due to the higher partial pressure of Zn vapor in this case. Zn nanowires can be produced on alumina and quartz substrates, but not on a stainless-steel substrate under the same or different synthetic conditions. Photoluminescence measurements were conducted on Zn nanowires and particles and weak emission bands at 482 and 493 nm were observed, which may be contributed by the thin ZnO film on the nanowire surface.     

Synthesis and characterization of ZnO nanowires for nanosensor applications

In this paper we report the synthesis of ZnO nanowires via chemical vapor deposition (CVD) at 650 °C. It will be shown that these nanowires are suitable for sensing applications. ZnO nanowires were grown with diameters ranging from 50 to 200 nm depending on the substrate position in a CVD synthesis reactor and the growth regimes. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and Raman spectroscopy (RS) have been used to characterize the ZnO nanowires. To investigate the suitability of the CVD synthesized ZnO nanowires for gas sensing applications, a single ZnO nanowire device (50 nm in diameter) was fabricated using a focused ion beam (FIB). The response to H₂ of a gas nanosensor based on an individual ZnO nanowire is also reported.     

Rapid mass production of ZnO nanowires by a modified carbothermal reduction method

ZnO nanowires are one of the most important nanomaterials for future optoelectronic nanodevices. However, the challenge of producing great quantities of ZnO nanowires has hindered the wide application and commercialization of ZnO nanowires. Here, we employ a modified carbothermal reduction method to synthesize great quantities of ZnO nanowires within 5 min. The synthesis was carried out in a quartz tube furnace at atmospheric pressure without using any catalysts. A nitrogen/air mixed gas was used as a reactive and carrier gas. About 0.85 g ZnO nanowires were obtained using 1 g ZnO and 1 g graphite powder as source materials. The prepared ZnO nanowires exhibited a hexagonal wurtzite crystal structure. The average length and diameter of the prepared ZnO nanowires were about 1.8 μm and 35 nm, respectively. Optical absorption and photoluminescence properties of the prepared ZnO nanowires have also been investigated.