MBE法生长ZnO纳米线阵列的结构和光学性能

在氧等离子体辅助的MBE系统中, 以1 nm厚的Au薄膜为催化剂, 基于气?液?固(VLS)机制实现了低温ZnO纳米线阵列在Si(111)衬底表面的生长. 通过场发射扫描电子显微镜(FE-SEM)可以观察到, ZnO纳米线阵列垂直生长在衬底上, 直径为20~30 nm. X射线衍射(XRD)和高分辨透射电镜(HRTEM)结果表明: ZnO纳米线为六方纤锌矿结构, 具有沿c轴方向的择优取向. 光致发光(PL)谱显示在380 nm附近有强烈ZnO本征发射峰, 475~650 nm可见光区域有较强的缺陷导致的发射峰.     

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.     

High resolution, two-dimensional image mapping of ZnO nanowires by confocal microphotoluminescence and microraman spectroscopy

High-quality ZnO nanowires were synthesized using both Au catalysts and ZnO seeds by chemical vapor depositionon basal plane sapphire substrates. The nanowires were hexagonal and aligned with their c-axis closely perpendicular to the sapphire substrate surface. The structural characteristics of the nanowiresgrown using the different catalysts/seeds were compared using scanning electron microscopyand X-ray diffraction. Their optical properties were assessed using microphoto-luminescence and confocal microRaman spectroscopy and compared. The nanowires exhibited a strong near band-edge related UV luminescence emission along with a defect related visible emission. The dependence of the luminescence as a function of incident excitation power and depth along the axis of the nanowires was studied. The wurtzite structure of the ZnO was confirmed from the Raman measurements. Two-dimensional mappings of the microphotoluminescence emission at different wavelengths and microRaman scattering from the nanowire samples were carried out using a confocal laser scanning microscope. This enabled the ability to correlate the near band-edge UV and visible emissions over the mapped area.     

Effect of surface morphology of metallic zinc films deposited by ion beam sputter deposition on the formation of ZnO nanowires

Metallic zinc film with various surface roughnesses was deposited on Si (100) substrates by ion beam sputter deposition utilizing beam energies at 8, 12 and 16 keV. The surface roughness of the metallic zinc film increased as ion beam energy increased and was found to act as a crucial factor for the formation of ZnO nanowires by subsequent thermal oxidation. ZnO nanowires with diameters of ∼30 nm and average length of ∼1 μm were obtained from 12 to 16 keV ion beam deposited samples while no ZnO nanowires were found on 8 keV ion beam deposited samples. Photoluminescence study of ZnO nanowires exhibits a strong UV emission at 377.2 nm (3.287 eV) with a full-width at half maximum of 95.0 meV and negligible defect related deep level emission. The ZnO nanowires are grown along the [110] direction and the growth mechanism is likely due to a solid state based-up diffusion process. Field-emission measurement shows a turn-on field of 7.9 MV/m and a field enhancement factor β of 691 is achieved.     

Effect of surface morphology of metallic zinc films deposited by ion beam sputter deposition on the formation of ZnO nanowires

Metallic zinc film with various surface roughnesses was deposited on Si (100) substrates by ion beam sputter deposition utilizing beam energies at 8, 12 and 16 keV. The surface roughness of the metallic zinc film increased as ion beam energy increased and was found to act as a crucial factor for the formation of ZnO nanowires by subsequent thermal oxidation. ZnO nanowires with diameters of ∼30 nm and average length of ∼1 μm were obtained from 12 to 16 keV ion beam deposited samples while no ZnO nanowires were found on 8 keV ion beam deposited samples. Photoluminescence study of ZnO nanowires exhibits a strong UV emission at 377.2 nm (3.287 eV) with a full-width at half maximum of 95.0 meV and negligible defect related deep level emission. The ZnO nanowires are grown along the [110] direction and the growth mechanism is likely due to a solid state based-up diffusion process. Field-emission measurement shows a turn-on field of 7.9 MV/m and a field enhancement factor β of 691 is achieved.     

基于PEG修饰的ZnO量子点光学特性研究

采用溶胶-凝胶法制备了ZnO量子点, 并采用有机高分子试剂PEG(聚乙二醇, M_w=2000)对其表面进行修饰。借助X射线衍射分析、傅立叶红外光谱、光致发光谱和透射显微镜等测试方法, 研究了PEG表面修饰对ZnO量子点结构和光学性能的影响规律。研究表明, 混合加入的PEG聚合物能够成功地包覆在ZnO量子点表面, 但没有改变量子点的晶体结构, 经PEG表面修饰后的ZnO量子点尺寸变小, 稳定性增强, 分散更均匀。同时经PEG修饰的ZnO量子点在400~500 nm波长区域缺陷态发射峰明显减弱, 表明采用PEG来改善ZnO量子点表面缺陷结构具有良好效果。     

A comparative study on the NO2 gas sensing properties of ZnO thin films, nanowires and nanorods

ZnO thin films were fabricated using the spin coating method, ZnO nanowires by cathodically induced sol-gel deposition by the means of an anodic aluminum oxide (AAO) template, and ZnO nanorods with the hydrothermal technique. For thin film preparation, a clear, homogeneous and stable ZnO solution was prepared by the sol-gel method using zinc acetate (ZnAc) precursor which was then coated on a glass substrate with a spin coater. Vertically aligned ZnO nanowires which were approximately 65 nm in diameter and 10 μm in length were grown in an AAO template by applying a cathodic voltage in aqueous zinc nitrate solution at room temperature. For fabrication of the ZnO nanorods, the sol-gel ZnO solution was coated on glass substrate by spin coating as a seed layer. Then ZnO nanorods were grown in zinc nitrate and hexamthylenetetramine aqueous solution. The ZnO nanorods are approximately 30 nm in diameter and 500 nm in length. The ZnO thin film, ZnO nanowires and nanorods were characterized by X-ray diffraction (XRD) analysis and scanning electron microscope (SEM). The NO₂ gas sensing properties of ZnO thin films, nanowires and nanorods were investigated in a dark chamber at 200 °C in the concentration range of 100 ppb-10 ppm. It was found that the response times of both ZnO thin films and ZnO nanorods were approximately 30 s, and the sensor response was depended on shape and size of ZnO nanostructures and electrode configurations.     

Tunable electronic transport characteristics of surface-architecture-controlled ZnO nanowire field effect transistors

Surface-architecture-controlled ZnO nanowires were grown using a vapor transport method on various ZnO buffer film coated c-plane sapphire substrates with or without Au catalysts. The ZnO nanowires that were grown showed two different types of geometric properties: corrugated ZnO nanowires having a relatively smaller diameter and a strong deep-level emission photoluminescence (PL) peak and smooth ZnO nanowires having a relatively larger diameter and a weak deep-level emission PL peak. The surface morphology and size-dependent tunable electronic transport properties of the ZnO nanowires were characterized using a nanowire field effect transistor (FET) device structure. The FETs made from smooth ZnO nanowires with a larger diameter exhibited negative threshold voltages, indicating n-channel depletion-mode behavior, whereas those made from corrugated ZnO nanowires with a smaller diameter had positive threshold voltages, indicating n-channel enhancement-mode behavior.     

Hybrid nanostructures of titanium-decorated ZnO nanowires

Hybrid nanostructures of titanium (Ti)-decorated zinc oxide (ZnO) nanowire were synthesized. Various thick Ti films (6 nm, 10 nm, and 20 nm) were coated to form a titanium oxide (TiO) coating layer around ZnO nanowires. Transmission electron microscope analysis was performed to verify the crystallinity and phases of the TiO layers according to the Ti-coating thickness. Under UV illumination, a bare ZnO nanowire showed a conventional n-type conducting performances. With a Ti coating on a ZnO nanowire, it was converted to a p-type conductor due to the existence of electron-captured oxygen molecules. It discusses the fabrication of Ti-decorated ZnO nanowires including the working mechanisms with respect to UV light.     

Low-temperature fabrication of bunch-shaped ZnO nanowires using a sodium hydroxide aqueous solution

Bunch-shaped ZnO nanowires film was successfully fabricated by the forced-hydrolysis-initiated-nucleation of anhydrous zinc acetate in an aqueous solution of zinc acetate and sodium hydroxide at low temperature. X-ray diffraction and a field emission scanning electron microscope clarified their formation mechanism and morphology development. The morphology was controllable by adjusting the solution temperature and deposition time. ZnO nanowires obtained at 65 degrees C for 6 h have a high aspect ratio of about 106. The smaller diameter with higher aspect ratio of ZnO nanowires, the easier the formation of bunch shapes by the capillary force during the drying process. This fabrication technique indicated that bunched ZnO film was prepared at low cost, and fittable to low heat-resistance substrates such as a polymer substarte.     

Electron field emission characteristics of different surface morphologies of ZnO nanostructures coated on carbon nanotubes

The optimal carbon nanotube (CNT) bundles with a hexagonal arrangement were synthesized using thermal chemical vapor deposition (TCVD). To enhance the electron field emission characteristics of the pristine CNTs, the zinc oxide (ZnO) nanostructures coated on CNT bundles using another TCVD technique. Transmission electron microscopy (TEM) images showed that the ZnO nanostructures were grown onto the CNT surface uniformly, and the surface morphology of ZnO nanostructures varied with the distance between the CNT bundle and the zinc acetate. The results of field emissions showed that the ZnO nanostructures grown onto the CNTs could improve the electron field emission characteristics. The enhancement of field emission characteristics was attributed to the increase of emission sites formed by the nanostructures of ZnO grown onto the CNT surface, and each ZnO nanostructure could be regarded as an individual field emission site. In addition, ZnO-coated CNT bundles exhibited a good emission uniformity and stable current density. These results demonstrated that ZnO-coated CNTs is a promising field emitter material.     

Field emission from zinc oxide nanostructures and its degradation

Arrays of zinc oxide (ZnO) nanowires and nanobelts were synthesized by the thermal evaporation of mixed powders of ZnO and graphite. Neither catalyst nor vacuum environment was involved in the fabrication. For comparison, the ZnO nanowires were grown on a pre-deposited transitional ZnO film on a brass substrate and the ZnO nanobelts were grown directly on a Si substrate. Their field emission properties were systematically measured. Current density of 10 μA/cm² was achieved at the fields of 5.7 and 6.2 V/μm from the nanowires and nanobelts, respectively. Also, the emission sites were found to distribute uniformly on the whole cathode. In the preliminary test on the stability, the ZnO nanobelts, which were sharp at the tip but wide at the root, exhibited better robustness than the ZnO nanowires. The post-test scanning electron microscopy (SEM) observation showed that the degradation of their field emission capability resulted from the breaking of the nanowires, which was tentatively attributed to the resistive heating during the field emission. In contrast, the shedding of the ZnO from the substrate was not so serious as imagined.     

Influence of Sb as a catalyst in the growth of ZnO nano wires and nano sheets using Nanoparticle Assisted Pulsed Laser Deposition (NAPLD)

The methodology on the synthesis of Sb-doped ZnO nanostructures by considering dopant as a catalyst is proposed and demonstrated. The nanostructures were synthesized using intrinsic ZnO as target and Sb-coated Si as substrate, where Sb simultaneously acts as dopant and the catalyst. The catalyst Sb is highly sensitive to temperature conditions resulted in two different nanostructures, the nanowires and the nanosheets. The surface, structural and optical characteristics of the nanowires and the nanosheets are comparatively investigated through SEM, EDX, XRD, Raman spectroscopy and photo luminescence (PL) spectroscopy. The nanowires showed a strong green emission in the PL spectrum and the presence of oxygen vacancies is confirmed thorough Raman peak shift at 556 cm⁻¹. In the case of nanosheets, the defect in oxygen vacancies is completely reduced, and improved UV emission is observed, which confirms the diffusion of Sb in the ZnO lattice.     

Enhanced field electron emission of graphene sheets by CsI coating after electrophoretic deposition

Because of the large quantities of edges, graphene can serve as an efficient edge emitter for field emission (FE). Cesium iodide (CsI) coating was promising to enhance the electron emission and utilized in FE applications. In this work, FE of graphene sheets after electrophoretic deposition (ED) was studied. Electron emission property of GS was obviously improved by coating with CsI. The turn-on field of GS decreased from 4.4 to 2.5 V/ μm; and threshold field decreased from 9 to 5.8 V/μm, respectively. This FE improvement must due to a higher effective density of emission site generated around the GS surface after coating. Scanning electron microscopy (SEM) and computation were taken to reveal the influence after coating. Investigations of CsI coated MWCNTs were also compared in order to better understand the origin of the low turn-on electric field obtained by GS.     

Formation of novel homojunction device using p-type ZnO:Co shell coating on n-type ZnO nanowires

P-type ZnO:Co thin film was spin coated onto n-type ZnO nanowire arrays to form a novel ZnO homojunction device using a fully solution-based process. The optoelectronic and structural properties of the homojunction device were extensively characterized by using scanning electron microscopy, X-ray diffraction, energy dispersive spectroscopy and photoluminescence emission measurement and current voltage measurement. It was found that the applied ZnO:Co coating bundles the nanowires together and suppress surface defects on the nanowire. Dark and illuminated device confirms the pn junction formation and its light sensitivity properties.     

Synthesis,characterization and growth mechanism of ZnO nanowires on NiCl<Subscript>2</Subscript>-coated Si substrates

Well-crystallized ZnO nanowires have been successfully synthesized on NiCl₂-coated Si substrates via a carbon thermal reduction deposition process. The pre-deposited Ni nanoparticles by dipping the substrates into NiCl₂ solution can promote the formation of ZnO nuclei. The as-synthesized nanowires were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectrum. The results demonstrate that the as-fabricated nanowires with about 60 nm in diameter and several tens of micrometers in length are preferentially arranged along [0001] direction with (0002) as the dominate surface. Room temperature PL spectrum illustrates that the ZnO nanowires exist a UV emission peak and a green emission peak, and the peak centers locate at 387 and 510 nm. Finally, the growth mechanism of the nanowires is briefly discussed.     

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.     

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.     

Fullerene C60 coated silicon nanowires as anode materials for lithium secondary batteries

A Fullerene C60 film was introduced as a coating layer for silicon nanowires (Si NWs) by a plasma assisted thermal evaporation technique. The morphology and structural characteristics of the materials were studied by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). SEM observations showed that the shape of the nanowire structure was maintained after the C60 coating and the XPS analysis confirmed the presence of the carbon coating layer. The electrochemical characteristics of C60 coated Si NWs as anode materials were examined by charge-discharge tests and electrochemical impedance measurements. With the C60 film coating, Si NW electrodes exhibited a higher initial coulombic efficiency of 77% and a higher specific capacity of 2020 mA h g(-1) after the 30th cycle at a current density of 100 microA cm(-2) with cut-off voltage between 0-1.5 V. These improved electrochemical characteristics are attributed to the presence of the C60 coating layer which suppresses side reaction with the electrolyte and maintains the structural integrity of the Si NW electrodes during cycle tests.     

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.