ZnO Nanowalls Research Articles

Noise Properties of ZnO Nanowalls Deposited Using Rapid Thermal Evaporation Technology

ZnO nanowalls are rapidly grown on a glass substrate using a low-temperature thermal evaporation method, without the use of a catalyst and the pre-deposition of a ZnO seed layer on the substrate. Most of the ZnO nanowalls are grown vertically and are about 70-200-nm thick and 2-μm long. The room-temperature photoluminescence spectra show a strong intrinsic ultraviolet (UV) emission and a weak defect-related orange emission. The ZnO nanowall UV sensor is highly sensitive to UV light, with an excellent UV-to-visible ratio and good flicker noise characteristics. This shows the strong potential of ZnO nanowalls for use in UV sensors. At an applied bias of 2 V, the noise equivalent power and the normalized detectivity of the ZnO nanowall UV sensor are 1.87 × 10-10 W and 3.38 × 109 cm·Hz0.5·W-1, respectively.

A Facile Method to Fabricate Ultrathin Vertical ZnO Nanowall Arrays

We demonstrate a facile method to synthesize ultrathin vertical ZnO nanowall arrays only through a one-step catalyst fabrication procedure. After being coated with uniformly dispersed gold particles, the substrates were polished gently to create discrete gold nanoparticle alignment as catalyst. The commercial ZnO powder and activated carbon powder with an atom ratio of 1:1 were utilized as source materials, and the whole growth process occurred in a conventional tube furnace chemical vapor deposition (CVD) system. The SEM images confirm that the ZnO nanowall arrays grow vertically along certain tracks made by polishing. The walls are about 17 nm wide, 90 nm high and 1 microm long after 90 min growth. After second growth, the nanowalls become 20 nm wide, 205 nm high, and 2 microm long. AFM images indicate that the alignment of discrete Au particles with certain separations due to polishing plays the key role in realizing vertical ZnO nanowall arrays.

Role of Nickel Catalyst during the Growth of ZnO Nanowalls Investigated by Atom Probe Tomography

The synthesis of ZnO nanowalls using Ni as the catalyst has been studied. ZnO nanowalls were prepared using a (110) α-sapphire substrate coated with a Ni film. ZnO nanowalls were grown using a mixture of ZnO and graphite powders as a source at 950 °C and 15 Torr in a tube furnace. Heating led to the formation of Ni networks. The networks provided nucleation sites for the nucleation and growth of ZnO nanowalls. During the growth phase of the ZnO nanowalls, diffusion of Ni and Al was not expected because the Ni–Sapphire system is known to be nonreactive. However, the results obtained from transmission electron microscopy (TEM) and atom probe tomography (APT) revealed that Al diffused into both the network interface and grain boundary of the ZnO nanowalls. The growth mechanism of the ZnO nanowalls can be inferred from these results, namely, the ZnO nanowall growth was associated with the oxidation of the Ni networks and the interlayers formed between ZnO and the sapphire substrate. APT revealed Al diffusion through the interface via the grain boundaries among the Ni networks, ZnO nanowalls, and Al2O3 substrate.

Influence of the substrate material on the surface morphology of electrochemically deposited ZnO layers

AbstractIn this paper we report results of fabrication of ordered ZnO nanorods (NRs) or nanowalls (NWs) electrochemically deposited on different highly conductive substrates. The following types of conductive substrates have been used: (i) highly doped p‐type multi‐crystalline Si, and (ii) copper thin plates. The influence of a seeding Ag layer deposited on multi‐Si substrates on the growth of ZnO NRs has been studied as well. It is observed that morphology of the deposited ZnO structures depends strongly on the type of the substrate used. It is found that ZnO structures with different shapes, such as NRs, nanotubes or NWs can be grown on top of highly conductive substrates used in this work. It is observed that optical reflection of the deposited layers depends on the substrate used as well as on the time of the electrochemical deposition of ZnO layers. It was found that: (i) for ZnO NWs/Cu structures, diffused reflection exhibit strong enhancement compared to pure Cu substrate; (ii) in case of ZnO NRs/Si structures reflection is decreased compared to that for pure Si substrate. Possible applications of ZnO NRs or NWs‐based structures for the processing of advanced Si‐based solar cells are discussed.

Inverted Organic Solar Cells with ZnO Nanowalls Prepared Using Wet Chemical Etching in a KOH Solution

We report on the photovoltaic (PV) performances of inverted organic solar cells (IOSCs) that were fabricated from PCBM:P3HT polymer with a ZnO thin film and ZnO nanowalls as electron transport and hole block layers. ZnO thin film on ITO/glass substrate was deposited using a simply aqueous solution route. ZnO nanowall structures were obtained via wet chemical etching of ZnO thin films in a KOH solution. The power conversion efficiency (PCE) of the IOSC with ZnO nanowalls was significantly improved by 44% from 1.254% to 1.811% compared to that of the IOSC with ZnO thin film. The short circuit current in IOSCs fabricated with the ZnO nanowalls was increased mainly due to the increase in the charge transport interface area, as a result of enhancement in the PCE. This work suggests a method for fabricating efficient PV devices with a larger charge transport area for future prospects.

Epitaxial growth of self-ordered ZnO nanostructures on sapphire substrates by seed-assisted hydrothermal growth

This article presents the investigation on the epitaxial growth of large scale spatially ordered ZnO nanostructures on (0001) c- and (112¯0) a-plane sapphire substrates by seed-assisted hydrothermal growth without the need for any complicated lithography or pre-patterned catalyst. Prior to hydrothermal growth of ZnO nanostructures, ZnO seeds were epitaxially grown on as-annealed sapphire substrates by metal organic chemical vapor deposition (MOCVD). The periodic ZnO nanorod rows with a fairly constant separation of 100nm were directly grown on the c-plane sapphire substrates. However, the linear periodic array of ZnO nanowalls with the thickness of about 240nm was formed on the a-plane sapphire substrate by coalescence of numerous vertically aligned ZnO nanorods. The average spacing between the ZnO nanowalls is measured to be 450nm. The microstructural characterization of the self-ordered ZnO nanowall array was further performed by cross-sectional transmission electron microscopy. Sapphire surface structure and CVD growth kinetics closely relate to the self-organized epitaxial growth of ZnO nanostructures on c- or a-plane sapphire substrates.

Microstructures of GaN Thin Films Grown on Graphene Layers

Plan-view and cross-sectional transmission electron microscopy images show the microstructural properties of GaN thin films grown on graphene layers, including dislocation types and density, crystalline orientation and grain boundaries. The roles of ZnO nanowalls and GaN intermediate layers in the heteroepitaxial growth of GaN on graphene, revealed by cross-sectional transmission electron microscopy, are also discussed.

Research on the growth of ZnO nanowall by soft-solution route

Three types of two dimensional ZnO nanowall networks were grown uniformly on the surface of ITO glass substrate by two different kinds of soft solutions. Scanning electronic microscopy (SEM) confirmed the formation of nanowalls with three kinds of distinct morphologies, such as (1) interlaced nanowall stood perpendicularly on the substrate, (2) nanowall with triangle cross section with widths ~80–100nm, (3) high-density nanosheets network with about 30nm in thickness. X-ray diffraction (XRD) showed that all of the ZnO nanowall networks were well crystallized in a wurtzite structure and highly oriented direction parallel to [0001]. The growth mechanisms of the ZnO nanowall networks were discussed.

The Growth and Optical Properties of ZnO Nanowalls

Nanowalls are novel nanostructures whose networked morphology holds potential for applications such as solar cells and gas sensors. The realization of such nanowall-based devices depends directly on a comprehensive understanding of the nanowall growth, namely, its competition with nanowire growth and the role of seed particles. We induced a morphological evolution from nanowires to nanowalls by increasing source flux during vapor transport and condensation growth. Nanowall growth kinetics indicates that their morphological dominance was driven by a time-dependent curvature of the nanowall growth facet. Nanowalls have excellent crystalline quality and strong near-band-edge luminescence and were found to grow by a combination of Au- and nonassisted mechanisms, resulting in Au nanoparticles within 300 nm of the substrate whose positions were associated with the origin of green luminescence. These results imply that the growth mechanism causes nanoscale structural variations, which in turn locally affect the ...

Fabrication and electrochemical behavior of flower-like ZnO–CoO–C nanowall arrays as anodes for lithium-ion batteries

This study reported the electrochemical performance of flower-like ZnO–CoO–C nanowall arrays as anodes of lithium-ion batteries. The arrays were fabricated through solution-immersion steps and subsequent calcination at 400 °C. At a rate of 0.5 C, the arrays exhibited a delithiation capacity of 438 mA h g −1 at the 50th cycle. The arrays still delivered a reversible capacity of 224 mA h g −1 at 2.0 C rate, much higher than those of the flower-like ZnO and ZnO–C nanowall arrays. The mechanism for the high capacity of flower-like ZnO–CoO–C nanowall arrays mainly resulted from the catalytic effect of Co phase on the decomposition of Li 2O and the conducting carbon layer formed on ZnO nanowalls. The present finding also provides a kind of nanostructured films that might be applied in solar cells and sensors, etc.

Fabrication of ZnO nanowall-network ultraviolet photodetector on Si substrates

ZnO nanowall networks were prepared by plasma-assisted molecular beam epitaxy without a catalyst on Si (111) substrates. The nanostructures have preferred orientation along the c axis. The nanostructures are about 10 to 20 nm thick and about 50 nm tall. The planar geometry photoconductive type metal-semiconductor-metal photodetector based on the ZnO nanowall networks exhibits a high and wide response spectrum, and no decrease from 250 to 360 nm. With the applied bias below 5 V, the dark current was below 6 μA, and the peak responsivity of 15 A/W was achieved at 360 nm. The UV (360 nm) to visible (450 nm) rejection ratio of around two orders could be extracted from the spectra response.

ZnO Nanomaterials Grown with Fe-Based Catalysts

We have demonstrated that slender, uniform, and vertically aligned ZnO nanorods can be grown with the Fe-based catalysts, while Au and ZnO have been widely accepted as the requisites for growing ZnO nanoarrays. The growth of these ZnO nanorods was investigated on the Fe-based catalyst-covered sapphire substrates by a thermal evaporation–oxidation method at 700 °C for 2 h in the atmospheres of oxygen and nitrogen. In order to develop Fe-related ZnO arrays, catalyst coating techniques of magnetron sputtering and spin-coating were applied. Rate of spin-coating and the reduction procedure of coated substrates were explored. Without a reduction procedure, ZnO nanowalls were obtained. At a medium spin rate, ZnO arrays with a diameter of 80–100 nm were obtained on the Fe/ZnO bilayer-covered sapphire substrates. Growth rates in the diameter and length directions were investigated. The empirical Avrami equation is applied to formulate the linear growth in the axial length with growth duration, by assuming the grow...

1D n–p Heterojunctions of Zinc Oxide on Gallium Nitride: A Structural Characterization

Recently, we showed lateral growth of ZnO nanowires and nanowalls on single crystal GaN and formation of bi- and tridirectional assembly of nanowires and nanowalls using a surface-directed vapor–liquid–solid process (SVLS). Taking advantage of this growth technique, planar arrays of electrically addressable n–p heterojunctions were fabricated. Each n–p heterojunction is formed at the interface of a nanowire or nanowall with the underlying substrate. In the present work, we further investigated the coherency of the crystal structure of the heterojunctions both along their width and length using focused-ion beam microscopy and high-resolution transmission electron microscopy (HRTEM). We explain the anisotropic growth of ZnO in multi directions using a SVLS mechanism, whereas conventional crystal growth techniques only anticipate an isotropic island formation of ZnO on GaN. During the SVLS process, a ZnO nanocrystal forms inside a Au nanodroplet, and elongates via a sequential ZnO island formation during whi...

Scalable network electrical devices using ZnO nanowalls

We report the fabrication and electrical characteristics of scalable nanowall networkdevices and their gas sensor applications. For the network device fabrications,two-dimensional ZnO nanowall networks were grown on AlN/Si substrates with a patternedSiO2 mask layer using selective-area metal-organic vapor-phase epitaxy. The ZnO nanowalls withc-axis orientation were heteroepitaxially grown on AlN/Si substrates, and weresingle-crystalline, as determined by x-ray diffraction and transmission electron microscopy. Theelectrical conductivity of the nanowall networks was measured as a function of nanowalldimensions. The conductance increased linearly with the channel width for widths larger than1 µm, but saturatedat 36 µS forwidths below 1 µm. This conductance scaling behavior is explained by enhanced conduction throughthe regions near the edge of the patterned growth areas, where the density ofthe networks was higher. Gas sensor applications were investigated using thenanowall network devices, and highly sensitive gas detection was demonstrated.

Fabrication and comparative optical characterization of n-ZnO nanostructures (nanowalls, nanorods, nanoflowers and nanotubes)/p-GaN white-light-emitting diodes

White light-emitting diodes (LED) based on ZnO (nanowalls, nanorods, nanoflowers and nanotubes)/p-GaN were fabricated and their electrical, optical and electro-optical characteristics were comparatively characterized. All the LED showed rectifying behavior. The nanowalls and nanorods structures have the highest photoluminescence emission intensity in the visible and UV (at 3.29 eV) regions, respectively. The nanowalls have the highest color rendering index, with a value of 95, and the highest electroluminescence intensity with peaks approximately centered at 420, 450 nm and broad peak covering the visible region.

Optical and Field-Emission Properties of ZnO Nanostructures Deposited Using High-Pressure Pulsed Laser Deposition

ZnO nanostructures were deposited on GaN (0001), Al2O3 (0001), and Si (100) substrates using a high-pressure pulsed laser deposition (PLD) method. Vertically aligned hexagonal-pyramidal ZnO nanorods were obtained on the Al2O3 and Si substrates whereas interlinked ZnO nanowalls were obtained on the GaN substrates. A growth mechanism has been proposed for the formation of ZnO nanowalls based on different growth rates of ZnO polar and nonpolar planes. Both ZnO nanorods and nanowalls exhibit a strong E2H vibration mode in the micro-Raman spectra. The corresponding fluorescence spectra of ZnO nanorods and nanowalls showed near band emission at 3.28 eV. The ZnO nanorods grown on the Si substrates exhibited better crystalline and optical properties compared with the ZnO structures grown on the GaN and Al2O3 substrates. The high aspect ratio, good vertical alignment, and better crystallinity of the ZnO nanorods with tapered tips exhibited promising field emission performance with a low turn-on field of 2 V/μm, a high current density of 7.7 mA/cm2, and a large field enhancement factor.

Formation of Planar Arrays of One-Dimensional p−n Heterojunctions Using Surface-Directed Growth of Nanowires and Nanowalls

We report a surface-directed vapor-liquid-solid process for planar growth of one-dimensional heterojunctions of zinc oxide on single crystal gallium nitride (GaN) that enables their hierarchical assembly to light emitting diodes. An individual heterojunction is about 10 μm in length and 80 nm in width and is formed by planar growth of an n-type ZnO nanowire or nanowall on p-type GaN surface using Au catalyst. Our results show that a ZnO nanocrystal at its nucleation site has six possible growth directions that can be engineered and controlled using an intentional blockade of the nanocrystal growth in certain directions owing to similarities in crystal structures of ZnO and GaN. The ZnO nanowalls are formed when nanowires during their planar growth slowly grow in direction normal to the substrate via a self-catalytic process. The crystal structure of these heterojunctions is examined from two different crystallographic perspectives using high resolution transmission electron microscopy. Results indicate abrupt and epitaxial formation of n-p heterojunctions, which are difficult to achieve in thin film growth of these heterojunctions. The collective light emission of micrometer- to millimeter-size arrays of the heterojunctions is demonstrated via a simple design that is scalable to literally any platform size. This technique allows in situ growth and combinations of II-VI and III-V semiconductors and offers their easier integration to photonic and lab-on-chip platforms with applications in energy generation and light detection.

Parametric Study on Dimensional Control of ZnO Nanowalls and Nanowires by Electrochemical Deposition

A simple electrochemical deposition technique is used to synthesize both two-dimensional (nanowall) and one-dimensional (nanowire) ZnO nanostructures on indium-tin-oxide-coated glass substrates at 70°C. By fine-tuning the deposition conditions, particularly the initial Zn(NO3)2·6H2O electrolyte concentration, the mean ledge thickness of the nanowalls (50–100 nm) and the average diameter of the nanowires (50–120 nm) can be easily varied. The KCl supporting electrolyte used in the electrodeposition also has a pronounced effect on the formation of the nanowalls, due to the adsorption of Cl− ions on the preferred (0001) growth plane of ZnO and thereby redirecting growth on the (100) and (20) planes. Furthermore, evolution from the formation of ZnO nanowalls to formation of nanowires is observed as the KCl concentration is reduced in the electrolyte. The crystalline properties and growth directions of the as-synthesized ZnO nanostructures are studied in details by glancing-incidence X-ray diffraction and transmission electron microscopy.

Soft-solution route to ZnO nanowall array with low threshold power density

ZnO nanowall array (ZNWA) has been directionally grown on the buffer layer of ZnO nanoparticles dip-coated on Si-wafer under a soft solution process. Nanowalls on substrate are in most suitable shape and orientation not only as an optical trap but also as an optical waveguide due to their unique growth habit, V[011¯0]⪢V[0001]≈V[0001¯]. Consequently, the stimulated emission at 384 nm through nanowalls is generated by the threshold power density of only 25 kW/cm2. Such UV lasing properties are superior to those of previously reported ZnO nanorod arrays. Moreover, there is no green (defect) emission due to the mild procedure to synthesize ZNWA.

Growth of ZnO nanostructure on Cu 0.62Zn 0.38 brass foils by thermal oxidation

Cu 0.62Zn 0.38 brass foil was heated at temperatures of 400–700 °C in flowing N 2–5%O 2 at a pressure of 1 atm. for 1–24 h. The oxidized specimens were characterized with a scanning electron microscope, an X-ray diffractometer and a transmission electron microscope. The results show that hexagon ZnO nanowires and nanowalls grew on convoluted oxide scales when brass foils were oxidized at 500 and 600 °C. Thermodynamics of forming ZnO is analyzed based on oxidation theory of an alloy. Pilling–Bedworth ratio of Cu–Zn alloy is calculated based on the volume differences between the formed oxide and the consumed metal. The growth stresses caused by Pilling–Bedworth ratio of the alloy and stress relief of oxide scale at different oxidation temperature are analyzed. The mechanism of forming nanostructural ZnO on the convoluted oxide scales is explained by the change in the growth stresses and stress relief of oxide scale during thermal oxidation.