Synthesis of ZnO nanowires on steel alloy substrate by thermal evaporation: Growth mechanism and structural and optical properties

ZnO nanowires having a diameter in the range of 15–40 nm and several tens of micrometers in length were grown on steel alloy substrates by the thermal evaporation technique without the use of any catalyst or additives. A detailed structural analysis revealed that the as-grown ZnO nanowires are single crystalline with wurtzite hexagonal structures and preferentially oriented in the c-axis direction. Origination of a strong and sharp Raman-active E₂ mode at 436.6 cm^-1 indicated that the grown ZnO nanowires have good crystal quality with the hexagonal wurtzite phase. Photoluminescence spectra also exhibited a sharp and strong peak in UV and a suppressed and weak band in the visible region, confirming the good optical properties and less structural defects for the deposited products. Additionally, a systematic growth mechanism is also proposed in detail to acquire a better understanding for the growth of nanowires on steel alloy substrate.     

Synthesis,optical and electrochemical properties of ZnO nanowires/graphene oxide heterostructures

Large-scale vertically aligned ZnO nanowires with high crystal qualities were fabricated on thin graphene oxide films via a low temperature hydrothermal method. Room temperature photoluminescence results show that the ultraviolet emission of nanowires grown on graphene oxide films was greatly enhanced and the defect-related visible emission was suppressed, which can be attributed to the improved crystal quality and possible electron transfer between ZnO and graphene oxide. Electrochemical property measurement results demonstrated that the ZnO nanowires/graphene oxide have large integral area of cyclic voltammetry loop, indicating that such heterostructure is promising for application in supercapacitors.     

Synthesis of ZnO nanowires on Si substrate by thermal evaporation method without catalyst: Structural and optical properties

Synthesis of ZnO nanowires was achieved on Si(100) substrate by the thermal evaporation of high purity metallic zinc powder without the use of any metal catalyst or additives. The diameter and length of the as-grown nanowires were in the range of 20–35 nm and few micrometers, respectively. The shapes and sizes of ZnO nanowires were dependent on the growth time. The high resolution transmission electron microscopy and selected area electron diffraction patterns indicated that the as-grown products are single crystalline with wurtzite hexagonal phase. Room temperature photoluminescence studies exhibited a strong UV emission and a suppressed green emission, confirming the good optical properties for the deposited nanowires.     

Structure and optical properties of ZnO nanowires coated with silica

Abstract

Abstract

It is essential to passivate one-dimensional nanostructures with insulating materials to protect them from contamination and oxidation as well as to avoid cross-talking between the building blocks of complex nanoscale circuits. The ZnO nanowires synthesised by the thermal evaporation of ZnO powders were coated with SiO₂ by the sputtering technique. Transmission electron microscopy and X-ray diffraction analyses revealed that the cores and shells of the ZnO core–SiO₂ shell nanowires were single crystal wurtzite type ZnO and amorphous SiO₂ respectively. Photoluminescence measurements at room temperature showed that the passivation of the ZnO nanowires was successfully achieved with SiO₂ without nearly degrading the near band edge emission from the wires. However, subsequent thermal annealing treatment was found to be undesirable owing to the degradation of the near band edge emission in intensity.     

Effect of Au ion beam on structural,surface, optical and electrical properties of ZnO thin films prepared by RF sputtering

In the present work, ZnO thin films were irradiated with 700?keV Au⁺ ions at different fluence (1?× 10¹³, 1?× 10¹⁴, 2?× 10¹⁴ and 5?× 10¹⁴ ions/cm²). The structural, morphological, optical and electrical properties of pristine and irradiated ZnO thin films were characterized by X-ray diffraction (XRD), Fourier transforms infrared spectroscopy (FTIR), scanning electron microscope (SEM), spectroscopy ellipsometry (SE) and four point probe technique respectively. XRD results showed that the crystallite size decreased from pristine value at the fluence 1?×?10¹³ ions/cm², with further increase of ion fluence the crystallite size also increased due to which the crystallinity of thin films improved. SEM micrographs showed acicular structures appeared on the ZnO thin film surface at high fluence of 5?×?10¹⁴ ions/cm². FTIR showed absorption band splitting due to the growth of ZnO nanostructures. The optical study revealed that the optical band gap of ZnO thin films changed from 3.08?eV (pristine) to 2.94?eV at the high fluence (5?× 10¹⁴ ions/cm²). The electrical resistivity of ZnO thin film decreases with increasing ion fluence. All the results can be attributed to localized heating effect by ions irradiation of thin films and well correlated with each other.     

Effects of rare earth elements doping on gas sensing properties of ZnO nanowires

The effects of rare earth elements (Ce, Eu, and Er) doping on the microstructure and gas sensing properties of ZnO nanowires were investigated. The Ce, Eu, Er-doped ZnO nanowires and pristine ZnO nanowires were synthesized via a solvothermal route. The structure of the prepared samples was studied and compared by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron microscopy. The gas sensors were fabricated by coating the prepared samples on aluminum oxides tube-based Au sensing electrode, with Ni-Cr heating wire to control the operating temperature. The operating temperature of all sensors was determined to be 300 °C with consideration of ethanol response. At this temperature, all sensors showed good ethanol sensing performance, with the 1%Ce-doped ZnO nanowires-based sensor exhibited the highest ethanol response over the other sensors. The mechanism for the microstructure and gas sensing behavior difference of various samples was discussed.     

Microstructural and optical properties of Ta-doped ZnO films prepared by radio frequency magnetron sputtering

Ta-doped ZnO films with different doping levels (0–5.02 at%) were prepared by radio frequency magnetron sputtering. The effects of the doping amount on the microstructure and the optical properties of the films were investigated. The grain size and surface roughness first significantly decrease and then slowly increase with the increase of Ta doping concentration. Both the grain size and the root mean square (RMS) roughness reach their minimum values at the doping content of 3.32 at%. X-ray Diffraction (XRD) patterns confirmed that the prepared Ta-doped ZnO films are polycrystalline with hexagonal wurtzite structure and a preferred orientation along the (002) plane. X-ray photoelectron spectroscopy (XPS) analysis reveals that Ta exists in the ZnO film in the Ta⁵⁺ and Ta⁴⁺ states. The average optical transmission values of the Ta-doped ZnO films are higher than those of the un-doped ZnO film in the visible region. The band gap energy extracted from the absorption edge of transmission spectra becomes large and the near band edge (NBE) emission energy obtained from PL spectra blueshifts to high energy when the Ta doping content grows from 0 at% to 5.02 at%, which can be explained by the Burstein–Moss shift.     

Enhanced UV photosensitivity from rapid thermal annealed vertically aligned ZnO nanowires

We report on the major improvement in UV photosensitivity and faster photoresponse from vertically aligned ZnO nanowires (NWs) by means of rapid thermal annealing (RTA). The ZnO NWs were grown by vapor-liquid-solid method and subsequently RTA treated at 700°C and 800°C for 120 s. The UV photosensitivity (photo-to-dark current ratio) is 4.5 × 10³ for the as-grown NWs and after RTA treatment it is enhanced by a factor of five. The photocurrent (PC) spectra of the as-grown and RTA-treated NWs show a strong peak in the UV region and two other relatively weak peaks in the visible region. The photoresponse measurement shows a bi-exponential growth and bi-exponential decay of the PC from as-grown as well as RTA-treated ZnO NWs. The growth and decay time constants are reduced after the RTA treatment indicating a faster photoresponse. The dark current-voltage characteristics clearly show the presence of surface defects-related trap centers on the as-grown ZnO NWs and after RTA treatment it is significantly reduced. The RTA processing diminishes the surface defect-related trap centers and modifies the surface of the ZnO NWs, resulting in enhanced PC and faster photoresponse. These results demonstrated the effectiveness of RTA processing for achieving improved photosensitivity of ZnO NWs.     

Influence of lead concentration on morphology and optical properties of Pb-doped ZnO nanowires

Undoped and Pb-doped ZnO nanowires with different lead concentrations were grown on Si(111) substrates using a thermal evaporation method. Scanning electron microscopy (SEM) results showed that, the undoped ZnO nanowires were well aligned with uniform diameters and lengths. On the other hand, the Pb-doped ZnO nanowires were tapered and not aligned in a unique direction. X-ray diffraction patterns and Raman measurements clearly indicated hexagonal structures for all of the products. In addition, the Raman results demonstrated that the Pb-doped ZnO nanowires had a lower crystalline quality than the undoped ZnO nanowires. Photoluminescence (PL) studies also confirmed the Raman results and showed a lower optical property for the Pb-doped ZnO nanowires compared to the undoped ZnO nanowires. Moreover, the PL results showed a smaller band-gap for the Pb-doped ZnO nanowires compared to the undoped ZnO.     

Effect of substrate temperature on the structural,electrical, and optical properties of GZO/ZnO films deposited by radio frequency magnetron sputtering

Ga-doped ZnO (GZO)/ZnO bi-layered films were deposited on glass substrates by radio frequency magnetron sputtering at different substrate temperatures of 100, 200 and 300 °C to investigate the effects of substrate temperature on the structural, electrical, and optical properties of the films. Thicknesses of the GZO and ZnO buffer layer were kept constant at 85 and 15 nm by controlling the deposition times.     

Structure and photovoltaic properties of ZnO nanowire for dye-sensitized solar cells

Aligned ZnO nanowires with different lengths (1 to approximately 4 μm) have been deposited on indium titanium oxide-coated glass substrates by using the solution phase deposition method for application as a work electrode in dye-sensitized solar cells (DSSC). From the results, the increases in length of zinc oxide (ZnO) nanowires can increase adsorption of the N3 dye through ZnO nanowires to improve the short-circuit photocurrent (J_sc) and open-circuit voltage (V_oc), respectively. However, the J_sc and V_oc values of DSSC with ZnO nanowires length of 4.0 μm (4.8 mA/cm² and 0.58 V) are smaller than those of DSSC with ZnO nanowires length of 3.0 μm (5.6 mA/cm² and 0.62 V). It could be due to the increased length of ZnO nanowires also resulted in a decrease in the transmittance of ZnO nanowires thus reducing the incident light intensity on the N3 dye. Optimum power conversion efficiency (η) of 1.49% was obtained in a DSSC with the ZnO nanowires length of 3 μm.     

Optical properties of group-I-doped ZnO nanowires

Undoped and group-I elements doped ZnO nanowires were synthesized using a thermal evaporation method. Field emission scanning electron microscopy (FESEM) results showed that, the undoped ZnO nanowires were ultra-long with uniform diameters. On the other hand, the length of the doped ZnO nanowires was in the range of some hundred of nanometers. X-ray diffraction (XRD) patterns clearly indicated hexagonal structures for all of the products. X-ray photoelectron spectroscopy (XPS) studies confirmed the oxidation states of Li, Na, K, in the ZnO lattice. An asymmetric O 1s peak indicated the presence of oxygen in an oxide layer. The effect of doping on the optical band-gap and crystalline quality was also investigated using photoluminescence (PL), UV–vis, and Raman spectrometers. The Raman spectra of the products indicated a strong E₂ (high) peak. The PL spectra exhibited a strong peak in the ultraviolet (UV) region of the electromagnetic spectrum for all of the ZnO nanowires. The UV peak of the doped ZnO nanowires was red-shifted compared to the undoped ZnO nanowires. In addition, the UV–vis spectra of the samples showed similar results compared to the PL results.     

Investigation of Nucleation Mechanism and Tapering Observed in ZnO Nanowire Growth by Carbothermal Reduction Technique

ZnO nanowire nucleation mechanism and initial stages of nanowire growth using the carbothermal reduction technique are studied confirming the involvement of the catalyst at the tip in the growth process. Role of the Au catalyst is further confirmed when the tapering observed in the nanowires can be explained by the change in the shape of the catalyst causing a variation of the contact area at the liquid–solid interface of the nanowires. The rate of decrease in nanowire diameter with length on the average is found to be 0.36 nm/s and this rate is larger near the base. Variation in the ZnO nanowire diameter with length is further explained on the basis of the rate at which Zn atoms are supplied as well as the droplet stability at the high flow rates and temperature. Further, saw-tooth faceting is noticed in tapered nanowires, and the formation is analyzed crystallographically.     

The optical and electrical properties of ZnO:Al thin films deposited at low temperatures by RF magnetron sputtering

Several ZnO:Al thin films have been successfully deposited on glass substrates at different substrate temperatures by RF (radio frequency) magnetron sputtering method. Effects of the substrate temperatures on the optical and electrical properties of these ZnO:Al thin films were investigated. The UV–VIS–NIR spectra of the ZnO:Al thin films revealed that the average optical transmittances in the visible range are very high, up to 88%. X-ray diffraction results showed that crystallization of these films was improved at higher substrate temperature. The band gaps of ZnO:Al thin films deposited at 25 ℃, 150 ℃, 200 ℃, and 250 ℃ are 3.59 eV, 3.55 eV, 3.53 eV, and 3.48 eV, respectively. The Hall-effect measurement demonstrated that the electrical resistivity of the films decreased with the increase of the substrate temperature and the electrical resistivity reached 1.990×10^?3 Ω cm at 250 ℃.     

Synthesis and field emission properties of different ZnO nanostructure arrays

In this article, zinc oxide (ZnO) nanostructures of different shapes were fabricated on silicon substrate. Well-aligned and long ZnO nanowire (NW) arrays, as well as leaf-like ZnO nanostructures (which consist of modulated and single-phase structures), were fabricated by a chemical vapor deposition (CVD) method without the assistance of a catalyst. On the other hand, needle-like ZnO NW arrays were first fabricated with the CVD process followed by chemical etching of the NW arrays. The use of chemical etching provides a low-cost and convenient method of obtaining the needle-like arrays. In addition, the field emission properties of the different ZnO NW arrays were also investigated where some differences in the turn-on field and the field-enhancement factors were observed for the ZnO nanostructures of different lengths and shapes. It was experimentally observed that the leaf-like ZnO nanostructure is most suitable for field emission due to its lowest turn-on and threshold field as well as its high field-enhancement factor among the different synthesized nanostructures.     

Effects of Ni doping on structural,optical and dielectric properties of ZnO

Structural, optical and dielectric properties of Ni doped ZnO samples prepared by the solid state route are presented. X-ray diffraction confirmed the substitution of Ni on Zn sites without changing the hexagonal structure of ZnO. NiO phase appeared for 6% Ni doping. Fourier transform infrared measurements were carried out to study phonon modes in Ni doped ZnO. Significant blueshift with Ni doping was observed in UV–visible studies, strongly supported by photoluminescence spectra that show a high intensity UV emission peak followed by the low intensity green emission band corresponding to oxygen vacancies and defects. The photoluminescence analysis suggest that doping of Ni can affect defects and oxygen vacancies in ZnO and give the possibility of band gap tuning for applications in optoelectronic devices. High values of dielectric constant at low frequency and a strong dielectric anomaly around 320 °C were observed.     

Simultaneous reduction and N-doping of graphene oxides by low-energy N2+ ion sputtering

An easy and catalyst-free method was used to obtain N-doped reduced graphene oxides (N-RGO) through low-energy N₂⁺ ion sputtering of graphene oxides (GO). The simultaneous reduction and N-doping of GO during the sputtering were systematically investigated by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure and Raman spectroscopy. The N-doping and reduction levels, which are determined by the N/C and O/C atomic ratios from the quantitative XPS analysis, respectively, can be easily controlled by varying the N₂⁺ ion sputtering time. In addition, three different N species, namely, nitrile-like N, graphitic N and pyridinic N, can be distinguished in N-RGO.     

沉淀法制备花状ZnO粉体及性能研究

以二水合乙酸锌提供锌源,氨水调节pH值,聚乙二醇-2000为改性剂,采用沉淀法在一定的工艺条件下得到了花状ZnO粉体。并以SEM、TEM及XRD等测试手段对产物的形貌及结构方面进行了表征。利用X射线衍射仪进行结晶结构分析,表明花状ZnO为六方纤锌矿结构;利用场发射电子扫描电镜进行样品形貌分析,表明花状结构ZnO是由纺锤状棒状结构组成,棒的长径比为2.10;利用场发射透射电镜进行结晶结构分析,表明花状ZnO是单晶与多晶的混合体,室温光致发光谱表明花状ZnO在380nm左右存在紫外发射峰。     

Effects of silver impurity on the structural,electrical, and optical properties of ZnO nanowires

1, 3, and 5 wt.% silver-doped ZnO (SZO) nanowires (NWs) are grown by hot-walled pulsed laser deposition. After silver-doping process, SZO NWs show some change behaviors, including structural, electrical, and optical properties. In case of structural property, the primary growth plane of SZO NWs is switched from (002) to (103) plane, and the electrical properties of SZO NWs are variously measured to be about 4.26 × 10⁶, 1.34 × 10⁶, and 3.04 × 10⁵ Ω for 1, 3, and 5 SZO NWs, respectively. In other words, the electrical properties of SZO NWs depend on different Ag ratios resulting in controlling the carrier concentration. Finally, the optical properties of SZO NWs are investigated to confirm p-type semiconductor by observing the exciton bound to a neutral acceptor (A⁰X). Also, Ag presence in ZnO NWs is directly detected by both X-ray photoelectron spectroscopy and energy dispersive spectroscopy. These results imply that Ag doping facilitates the possibility of changing the properties in ZnO NWs by the atomic substitution of Ag with Zn in the lattice.