ZnO Pillars Research Articles

An acoustoelectric-induced tailorable coupled resonator surface acoustic waveguide

Here, we introduce a tailorable coupled resonator surface acoustic waveguide (CRSAW), based on a linear defect of elliptical cylinders inside a phononic crystal of ZnO pillars on a Si substrate. The designed elliptical resonators allow the emergence of a minimally dispersive, monomode shear-guiding band inside the local resonance bandgap, owing to their partially broken structural symmetry in comparison with their previously reported counterparts such as pillars and hollow cylinders. Moreover, by introducing reconfigurable waveguide behavior, we benefit from the acoustoelectric-induced elasticity modulation of ZnO, which is a semiconducting piezoelectric material. Switching the conductivity of ZnO structures between two limiting low and high values (0.01 S m−1 and 100 S m−1), a considerable waveguide modulation at full width at half maximum (FWHM) (ΔFWHM = −21%) and guiding frequency (Δfd = −2.81%) are achieved for the designed optimized elliptical cylinder CRSAW while maintaining an acceptable loss value. Benefiting from the proposed ZnO-based elliptical CRSAW, we have achieved simultaneous monomode and low-bandwidth surface acoustic waveguide behaviors, without involving significant fabrication complications. The designed structures open up new horizons toward the realization of promising building blocks for designing reconfigurable and miniaturized SAW RF-filters, demultiplexers, and Mach–Zehnder devices for wireless communications applications.

Contacting ZnO Individual Crystal Facets by Direct Write Lithography.

Many advanced electronic devices take advantage of properties developed at the surface facets of grown crystals with submicrometer dimensions. Electrical contacts to individual crystal facets can make possible the investigations of facet-dependent properties such as piezoelectricity in ZnO or III-nitride crystals having noncentrosymmetric structure. However, a lithography-based method for developing contacts to individual crystal facets with submicrometer size has not yet been demonstrated. In this report we study the use of electron beam-induced deposition (EBID), a direct write lithography method, for contacting individual facets of ZnO pillars within an electron microscope. Correlating structural and in situ deposition and electrical data, we examine proximity effects during the EBID and evaluate the process against obtaining electrically insulated contact lines on neighboring and diametrically opposite ZnO facets. Parameters such as incident beam energy geometry and size of the facets were investigated with the view of minimizing unwanted proximity broadening effects. Additionally, we show that the EBID direct write method has the required flexibility, resolution, and minimized proximity deposition for creating prototype devices. The devices were used to observe facet-dependent effects induced by mechanical stress on single ZnO pillar structures.

Regular ZnO nanopillar arrays by nanosphere photolithography

Highly regular vertical ZnO nanopillar arrays were hydrothermally grown through a nucleation window pattern generated by nanosphere photolithography. The in-plane intensity modulation of the exposing ultraviolet light in the photoresist was performed by Stöber silica or polystyrene nanospheres in the masking Langmuir–Blodgett monolayer. By comparing six different nanosphere diameters in the 180–700nm range only those with diameter above the exposure wavelength of 405nm generate a pattern in the thin photoresist layer. The pattern quality is improving with increasing diameter, therefore, the masking for nanopillar growth was demonstrated with 700nm polystyrene nanospheres. The results of the nanosphere photolithography were supported by finite-difference time-domain calculations. This growth approach was shown to have the potential for low-cost, low-temperature, large area fabrication of ZnO pillars or nanowires enabling a precise engineering of geometry.

Optoelectronic Properties of Density-Controlled ZnO Nanopillar Arrays

An effective method of controlling the density of ZnO nanopillars (ZnO-NPs) by using the self-assembled silver nanoislands as the growth catalyst is demonstrated. We were able to vary the density of the ZnO-NPs to within the range of ∼10–30 μm–2 by properly manipulating the size distribution of the silver nanoislands partially covering the Si substrates. Continuous silver film can also be used as the catalyst to facilitate the growth of ZnO islands and pillars on Si substrates, albeit with much less control of the resultant density and morphological appearances. The field-emission measurements indicate that the performance of field emission for ZnO-NPs can be improved markedly by reducing the density of the ZnO-NPs. The turn-on field of the low-density ZnO-NPs was as low as 2.39 V/μm with a corresponding current density of 10 μA/cm2 and a field enhancement factor (β value) of ∼3500. The enhancement of field-emission characteristics is attributed to the much reduced field screen effect by properly controll...

Morphology-Controlled Three-Dimensional Nanoarchitectures Produced by Exploiting Vertical and In-Plane Crystallographic Orientations in Hydrothermal ZnO Crystals

Herein we describe the morphology-controlled synthesis of three-dimensional (3D) ZnO nanoarchitectures via a facile hydrothermal route. In this approach, vertical and in-plane crystallographic orientations of ZnO crystals were tuned by appropriate patterning of growth masks and seed layer control, which enabled the formation of three types of ZnO nanoarchitectures. If polycrystalline ZnO layers with poor c-axis orientation were used as a seed, flower-like ZnO nanoarchitectures composed of radially oriented ZnO nanorods were achieved. In the case of c-axis oriented ZnO seeds, polygonal ZnO pillars grew vertically at the center of the growth holes. Even in the latter case, multidomain columnar–joint structures or single domain crystal structures with well-defined hexagonal facets were achieved based on the existence of six-fold in-plane symmetry of the ZnO seed layers. These morphology-controlled ZnO nanoarchitectures exhibited clear differences in light propagation characteristics, which could be ascribed ...

Low temperature growth of ZnO nanorods on flexible polymeric substrates

Vertical ZnO nanorods have been cathodically deposited on conductive polymeric substrates from zinc nitrate electrolyte at low temperatures of 60–80 °C. As-grown ZnO nanorods display a wurzite crystal structure that grows preferentially in the (0 0 2) direction. The interspacing between adjacent lattice fringes is approximately 0.261 nm. The growth rate of ZnO pillars in c-axis is an increasing function of both growth temperature and period. Arrhenius plots determine different stages of apparent activation energy (5.51–8.95 kJ/mol), showing an inhomogeneous surface reactivity on polymeric substrate during the ZnO growth process. This variation of activation energy can be ascribed to the two growth stages of ZnO pillars, the epitaxy and the texture growth.

Electrochemical deposition and superhydrophobic behavior of ZnO nanorod arrays

Vertically aligned ZnO nanorod arrays with different heights are grown on the ZnO seeded indium tin oxide substrate by cathodic electrochemical deposition from zinc nitrate at two temperatures of 60 °C and 80 °C. As-grown ZnO nanorods exhibit wurzite crystal structure and their heights can be well controlled by different deposition times. The fluorination coating tends to induce a superhydrophobicity of ZnO nanorods, i.e., the maximal value of contact angle: 166.9°. The super water repellency can be attributed to the fact that an air layer is confined in the nanorod arrays, and thus leads to water droplets sitting on the ZnO surfaces, referring as Cassie state. Interestingly, their water contact angles are found to vary with the heights of ZnO nanorods, ranged from 99.8 to 746 nm. The superhydrophobicity of ZnO surfaces can be well predicted by a proposed model that is capable of determining the wetted fraction of ZnO pillars. This satisfactory result would shed one light on how the variation of rod height would induce the superhydrophobic behavior of ZnO nanorod arrays.

Cover Picture: Cryst. Res. Technol. 4/2010

AbstractThe cover picture shows schematics of the fabrication process of a two‐dimensional photonic crystal of ZnO pillars on silicon substrate. (see pages 393–397).

Fabrication and optical properties of two‐dimensional photonic crystal of ZnO pillars

AbstractTwo‐dimensional (2D) photonic crystal of ZnO pillars was synthesized on silicon substrate by the combination of template method and vapor‐phase transport method. The microstructure and morphology of the ZnO photonic crystal was evaluated by using scanning electron microscope (SEM) and X‐ray diffraction (XRD). Large‐area specular reflectance measurements showed the presence of photonic stop band. The effect of the photonic band gap and the special structure on the photoluminescence (PL) properties of ZnO photonic crystal has been investigated. Both suppression and enhancement in the PL were observed. Raman scattering analyses demonstrated that the defect of ZnO photonic crystal exists in this experiment. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Growth of zinc oxide nanopillars on an iridium/yttria-stabilized zirconia/silicon substrate

Zinc oxide nanopillars were grown by a self-catalyzed growth process on an epitaxial Ir/yttria-stabilized zirconia/Si(111) multilayer structure in an optically heated tube furnace. The pillars obtained stand upright parallel to each other with their c-axis perpendicular to the sample surface. Problems due to alloying of Zn with Si are completely avoided, and no irregularities of the pillars in the initial growth phase are found. Cathodoluminescence measurements show narrow linewidths below 700μeV due to the excellent crystal quality. Termination of ZnO pillars with a flat metallic iridium layer is an attractive issue towards an optical cavity for laser action.

Large-area growth of vertically aligned ZnO pillars by radio-frequency magnetron sputtering

A new method for fabricating large-area well-aligned ZnO pillars, solely using radio-frequency magnetron sputtering, is reported. A mixture of argon and hydrogen gases was used as the plasma source, acting as a reduction agent. The ZnO pillars grow from a ZnO buffer layer with a ZnMgO nucleation stabilization layer on top, with the sputtered targets sharing the same composition. The entire growth is well controlled and linear. A shadowing effect is also responsible for the growth. The characteristics of the developed method are high uniformity and reproducibility, as commonly known for sputtering, which are vital prerequisites for future developments of nanodevices.

Low-temperature fabrication of single-crystal ZnO nanopillar photonic bandgapstructures

We describe the fabrication of single-crystal nanopillars of ZnO in two-dimensionalperiodic and aperiodic arrays for photonic applications. The structures are definedby a thin polymer template created by e-beam lithography and are grown at90 °C using an electrochemical process. The individual single-crystal ZnO pillars have diameters of∼120 nm and lengthsgreater than 1 µm. Visible light diffraction patterns from the ordered arrays were observed, and the in-planephotonic bandgap was characterized.

Template‐Assisted Large‐Scale Ordered Arrays of ZnO Pillars for Optical and Piezoelectric Applications

Spatially separated ZnO pillars, typically 300 nm in diameter and 2 microm in height, are fabricated via a template-directed approach that leads to long-range hexagonal order. The templates of Au nanodisk arrays are obtained by using metal membranes as a lithography mask. The growth of ZnO pillars is performed in a double-tube system through vapor diffusion-deposition. The growth mechanism of the pillars is studied in detail and is proposed to be a combination of vapor-liquid-solid and vapor-solid models. The piezoelectric and optical properties of single pillars are characterized using piezoresponse force microscopy and micro-photoluminescence spectroscopy, respectively. The pillars show strong excitonic emissions up to room temperature, which indicate a relatively low defect density and good crystalline quality. The obtained piezoelectric coefficient d(33) is (7.5+/-0.6) pm V(-1), which is to our knowledge the first reported value for a single nanopillar.