ZnO Heterostructures Research Articles

Position-controlled vertical arrays of single-crystalline ZnO nanowires on periodically polarity inverted templates

Position controlled single crystalline ZnO nanowires are grown on periodically polarity-inverted (PPI) ZnO heterostructures without catalyst. PPI ZnO templates were fabricated by using selective growth method of polarity with Cr-compound intermediate layers by plasma assisted molecular beam epitaxy. In order to control the position, we used the in-plane two-dimensionally discrete PPI ZnO templates. The lateral polarity inversion in PPI template was confirmed by piezo response microscopy. After syntheses of ZnO nanowires, vertically aligned ZnO nanowires were grown only onto the Zn-polar regions without the O-polar regions. The results clearly show that the position control of ZnO nanowires is possible using the PPI ZnO templates.

Electrochemical Growth of (0001)-n-ZnO Film on (111)-p-Cu2O Film and the Characterization of the Heterojunction Diode

The (111)-p-Cu2O/(0001)-n-ZnO heterostructure was successfully fabricated using a low-temperature electrodeposition method on an Au(111)/Si(100) substrate. The structural, optical and electrical characterizations were carried out by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, transmission electron microscope-energy dispersive X-ray analysis, optical absorption, photoluminescence, and electrical measurements. Isolated hexagonal columnar ZnO grains deposited at a low cathodic current density and the interface between the ZnO and Cu2O film were clearly observed. The (0002)-oriented continuous ZnO film was deposited on the (111)-Cu2O film at higher cathodic current density, but the metallic Cu layer was formed between the ZnO and Cu2O films by reducing Cu2O to Cu0 during the ZnO deposition. The Cu2O/isolated-ZnO heterostructure showed an ohmic feature, and the (111)-Cu2O/Cu/(0002)-ZnO heterostructures showed an excellent rectification with the rectification ratio of ∼105.

Electrochemical Construction of (<i>0001</i>)-ZnO/(<i>111</i>)-Cu<sub>2</sub>O Heterojunction Diode with Excellent Rectification Feature

The (111)-p-Cu2O/(0001)-n-ZnO heterostructure was successfully fabricated using low-temperature electrodeposition method on Au(111)/Si(100) substrate. The ZnO film deposited at positive current density was composed of aggregates of isolated hexagonal columnar grains, and the interface between ZnO and Cu2O layer was clearly observed. (0002)-oriented continuous ZnO layer deposited on the (111)-Cu2O layer at negative current density, but the metallic Cu layer was formed between ZnO and Cu2O layers by reducing Cu2O to Cu during ZnO deposition. The Cu2O/isolated-ZnO heterostructure showed ohmic feature, and (111)-Cu2O/Cu/(0002)-ZnO heterostructures showed an excellent rectification with rectification ratio of 85.

Optical properties of In 2O 3 octahedra nano-beads grown on ZnO nanowires

Octahedra of In 2O 3 have been prepared on the side walls of hexagonal ZnO nanowires using a two-stage vapour–liquid–solid process. Raman studies of In 2O 3 and ZnO heterostructures show distinct phonon peaks corresponding to cubic In 2O 3 and hexagonal ZnO. From the room temperature photoluminescence it is found that the strong UV emission of ZnO nanowires has quenched significantly after In 2O 3 octahedra are grown over them. On the other hand, the emission related to In 2O 3 in the visible and violet regions is also observed. Growing such heterostructures radially is now one step closer to growing them axially in a single nanowire.

Fabrication and characterization of flower-like CuO–ZnO heterostructure nanowire arraysby photochemical deposition

A simple two step solution-based method was applied to fabricate CuO–ZnOheterostructured nanowire (NW) arrays. First, ZnO nanowires were grown ona Si substrate using the ammonia solution hydrothermal reaction. Afterwards,flower-like CuO crystals were photochemically deposited on the tip of the ZnO NWs,using ultraviolet (UV) light (312 nm wavelength) irradiation at room temperature.The morphology of the CuO was controlled by reaction time, density of ZnONWs, and concentration of the solution. Because the deposited CuO is p-type andhas narrow band gap properties, CuO–ZnO heterostructured NWs exhibited astable p–n junction property and good ability to absorb visible light. Throughinvestigation of UV light-triggered reaction phenomena, we found that the production ofOH − from the photocatalytic process on the surface of ZnO NWs plays a critical role in the CuOdeposition mechanism.

Flash synthesis of flower-like ZnO nanostructures by microwave-induced combustion process

Flower-like ZnO particles were fabricated from zinc nitrate and urea via the microwave-induced combustion technique (MICT). The structure and morphology of the products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and selected area electron diffraction (SAED). The effects of growth conditions such as microwave power, radiation time, and molar ratio of urea/Zn 2+ were investigated. Results show that the morphologies and particle sizes of the final products depend on the molar ratio of urea/Zn 2+. The SEM images of ZnO flowers grown in different periods were employed to explain the formation mechanism. This study provides a simple and efficient approach for synthesizing flower-like ZnO heterostructures.

Atomic Layer Deposition of ZnO on Multi-walled Carbon Nanotubes and Its Use for Synthesis of CNT–ZnO Heterostructures

In this article, direct coating of ZnO on PECVD-grown multi-walled carbon nanotubes (MWCNTs) is achieved using atomic layer deposition (ALD). Transmission electron microscopy investigation shows that the deposited ZnO shell is continuous and uniform, in contrast to the previously reported particle morphology. The ZnO layer has a good crystalline quality as indicated by Raman and photoluminescence (PL) measurements. We also show that such ZnO layer can be used as seed layer for subsequent hydrothermal growth of ZnO nanorods, resulting in branched CNT–inorganic hybrid nanostructures. Potentially, this method can also apply to the fabrication of ZnO-based hybrid nanostructures on other carbon nanomaterials.

Synthesis of ZnO Nanotube Arrays and Heterostructures of Cu–ZnO Coaxial Nanotubes by Electrodeposition–Oxidation Method

Semiconductor ZnO nanotube arrays and heterostructures of Cu-ZnO coaxial nanotubes have been synthesized by electrodeposition into porous anodic alumina membranes and subsequent oxidation. Scanning electron microscopy and transmission electron microscopy indicate that the ZnO nanotubular arrays and Cu-ZnO coaxial nanotubular arrays are of large-area and highly ordered. X-ray diffraction patterns show that the nanotubes are polycrystalline. Photoluminescence spectra of the Cu-ZnO nanotubes show that a violet peak, a blue peak and a green peak are centered at 422 nm, 480 nm and 537 nm, respectively. The ordered ZnO nanotube arrays and heterostructures of Cu-ZnO coaxial nanotubes may have promising potential applications in nanodevices.

ZnO-GaN Hybrid Heterostructures as Potential Cost-Efficient LED Technology

Reliable and reproducible p-type doping is the main challenge for fabricating highly efficient ZnO-based light-emitting diodes. During the last few years, the lack of reliable p-type conductivity in ZnO has initiated research concerning the combination of ZnO and other semiconductors, which can then be doped p-type. One of these concepts is the combination of ZnO with GaN heterostructures aiming at the fabrication of hybrid LEDs. We discuss the problems as well as potential benefits from a combination of ZnO and GaN hybrid heterostructures in a single device. We also present our recent results on ZnO-GaN hybrid LEDs using an inverted LED concept. The hybrid LEDs have an external quantum efficiency of more than 35%.

Two‐dimensional polaron mass in ZnO quantum Hall systems

AbstractCyclotron resonance has been observed in ZnO heterostructures with dense two dimensional electron gas at a wide range of the sub‐millimeter wave up to 651 GHz. The typical carrier concentration of the sample is found to be higher than 1012 cm–2 from the Shubnikov de Haas oscillation measurement, and the quantum Hall effect has also been observed at high magnetic fields up to 25 T.The electron effective mass in the ZnO two dimensional system has just been determined as m* = 0.32 ± 0.01 m0 from the magnetic field dependence of the cyclotron resonance experiments. The obtained two dimensional mass is enhanced in comparison with the bare effective mass of three dimensional systems. It is partially due to the large electron‐phonon interaction in ZnO (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Second harmonic generation in periodically polarity-inverted zinc oxide

We report on the second harmonic generation (SHG) in 2D periodically polarity-inverted (PPI) ZnO heterostructures. The grating structures with nanometer-scale periodicity are fabricated on (0001) Al(2)O(3) substrates by using the in situ polarity inversion method. The achievements of SHG with grating in fabricated PPI ZnO structures are demonstrated under consideration of quasi phase matching conditions. In general, grating formation using the this periodical array of differnet polar surface can be extended to the other heteroepitaxial systems with polarity characteristics.

Nucleation and growth mechanisms of ZnO heterostructures controlled by temperature and pressure of CVD

In this work, the ZnO nanostructures with different configurations were grown by using chemical vapour deposition. The morphologies of the above ZnO nanomaterials were characterized with scanning electron microscopy and the corresponding growth mechanisms were also investigated. It was demonstrated that the temperature, pressure, and time of exposure are intrinsic factors that dominate the morphologies of nanostructured ZnO. The temperature dominates different growth rates along different directions of the crystal, determining the shape of the grown ZnO structures. The pressure of furnace triggers the growth of ZnO structures from a nucleation zone, which also explains the variation of the growth direction.

Synthesis and Sensing Properties of ZnO/ZnS Nanocages

Large-scale uniform ZnO dumbbells and ZnO/ZnS hollow nanocages were successfully synthesized via a facile hydrothermal route combined with subsequent etching treatment. The nanocages were formed through preferential dissolution of the twinned (0001) plane of ZnO dumbbells. Due to their special morphology, the hollow nanocages show better sensing properties to ethanol than ZnO dumbbells. The gain in sensitivity is attributed to both the interface between ZnO and ZnS heterostructure and their hollow architecture that promotes analyte diffusion and increases the available active surface area.

Electrical properties of ZnO–BaTiO3–ZnO heterostructures with asymmetric interface charge distribution

We report on capacitance-voltage, current-voltage, Sawyer–Tower, and transient current switching measurements for a ZnO–BaTiO3–ZnO heterostructure deposited on (001) silicon by using pulsed laser deposition. The triple-layer structure reveals asymmetric capacitance- and current-voltage hysteresis and cycling-voltage dependent Sawyer–Tower polarization drift. We explain our findings by coupling of the ferroelectric (BaTiO3) and piezoelectric (ZnO) interface charges and parallel polarization orientation of the ZnO layers causing asymmetric space charge region formation under positive and negative bias. The transient current characteristics suggest use of this structure as nonvolatile memory device.

Facet-Selective Epitaxial Growth of Heterogeneous Nanostructures of Semiconductor and Metal: ZnO Nanorods on Ag Nanocrystals

We demonstrate a new approach for synthesizing Ag-ZnO heterogeneous nanostructures in which single-crystalline ZnO nanorods were selectively grown on {111} rather than {100} facets of single-crystalline Ag truncated nanocubes. We have identified the fine structure of the Ag-ZnO heterostructures and proposed a mechanism indicating that structure match plays a critically important role in this type of facet-selective growth. These heterogeneous nanostructures are of special interest and have potential applications in electrical contacts, functional devices, biological sensors, and catalysis.

Metal organic chemical vapor deposition growth and luminescence of ZnO micro- and nanowires

One dimensional (1D) ZnO heterostructures were deposited on C-plane sapphire using metal organic chemical vapor deposition at atmospheric pressure. Both catalyst-assisted and catalyst-free processes were investigated. In this latter case, growth parameters such as the substrate temperature, the oxygen/zinc ratio, or the carrier gas nature were varied in order to observe the modification of the film morphology. Different 1D shapes were produced, including rods, tubes, and needles, and were observed by scanning electron microscopy. Photoluminescence spectra reveal sharp excitonic transitions, and cathodoluminescence signals recorded along a ZnO conical needle exhibit a blueshift from the base to the top. The vapor-liquid-solid growth of ZnO nanowires using gold droplets was also successful.

Electron transport properties of Zn0.88Mn0.12O∕ZnO modulation-doped heterostructures

The authors have investigated the electron transport properties of Zn1−xMnxO∕ZnO modulation-doped heterostructures grown on ZnO (0001¯) single-crystal substrates. By varying the growth temperature (Tg) of ZnO and Zn1−xMnxO layers, the growth mode changes from three-dimensional grain growth to two-dimensional growth. In this study, Zn0.88Mn0.12O∕ZnO heterostructures with different growth modes of the Zn0.88Mn0.12O barrier layer were fabricated by deposition at different Tg. The effect of the growth mode of the Zn0.88Mn0.12O barrier layer on carrier confinement at the Zn0.88Mn0.12O∕ZnO heterointerface is discussed.

Thin film chemical sensors based on p-CuO/ n-ZnO heterocontacts

Previous work on bulk ceramic heterocontacts ( n-ZnO/ p-CuO) has indicated significant sensitivity to the presence of specific adsorbed chemical species. Here, these results are extended to thin film heterostructures fabricated via chemical solution methods. It is expected that thin film sensor architectures will possess significant advantages over their bulk counterparts. In this study, the desired properties of porosity and crystallinity have been optimized with respect to pyrolysis temperature for each ZnO and CuO sol–gel process . The results of microscopy and X-ray diffraction (XRD) indicated that an optimal balance of these two properties is achieved at a pyrolysis temperature of 250 °C. The CuO films were seen to possess a level of porosity significantly higher than that seen in the ZnO films, making them an ideal candidate for the top layer in a planar thin film heterostructure. Results of current–voltage measurements conducted in 4000 ppm hydrogen have confirmed that the inherent porosity of the CuO films led to an enhanced sensor response in CuO on ZnO heterostructures. Lastly, the fabrication and structural characterization of a mixed solution type heterostructure has been detailed. Atomic force microscopy and XRD data indicated the presence of ZnO pillars dispersed among a matrix of CuO.

Crystal polarity control of ZnO films and nonlinear optical response

AbstractWe report that the nonlinear optical response of polarity controlled ZnO films grown by selective growth technique of Zn-polar and O-polar ZnO layers on sapphire substrate using Cr-compound buffer layers. ZnO layers grown on CrN/sapphire show Zn polar, while those grown on Cr2O3/sapphire result in O-polar ZnO films. In order to verify the origin of nonlinear optical response of ZnO, the polarity-controlled ZnO thin films grown on different buffer layers were investigated as nonlinear optical materials for second harmonic generation (SHG). The effective nonlinear optical coefficient (deff) of ZnO grown on Cr-compound buffer layers showed a higher value than that of ZnO grown on MgO buffer layers. Finally, by combining suggested in-situ polarity control technique with photolithography technique, we have fabricated 1D and 2D periodically-polarity-inverted (PPI) hetro-structures with periodicity ranging from 60 μm to 2 μm. The lateral polarity inversion is confirmed by piezo response microscopy. Such PPI ZnO heterostructures show the enhancement of SHG intensity comparing with the ZnO films.

Ferromagnetism in CuO–ZnO multilayers

We investigated the magnetic properties of CuO–ZnO heterostructures to elucidate the origin of the ferromagnetic signature in Cu doped ZnO. The CuO and ZnO layer thickness were varied from 15 to 150 nm and from 70to350nm, respectively. Rutherford backscattering experiments showed no significant diffusion of either Cu in ZnO or Zn in CuO layers. Magnetic measurements indicate ferromagnetism at 300K, which depends on the CuO particle size, but not on the CuO–ZnO interfacial area. Polarized neutron reflectometry measurements show that the observed magnetization cannot be accounted for solely by spins localized near the CuO–ZnO interface or in the CuO layer.