M.J. Abdullah

Nanotripods of Zinc Oxide

We report the discovery of two-dimensional (2-D) nanotripods of ZnO, a new member of the ZnO nanostructure family. These planar nanotripods are synthesized via a novel approach known as catalyst-free combust-oxidized mesh (CFCOM) process that we developed using a ZnO factory furnace. At about 1200 °C, high velocity zinc vapor is instantly oxidized and captured in a steel mesh for 20s and then air-quenched. From this subminute synthesis process, three types of polycrystalline 2-D tripodal nanostructures are discovered. The ZnO tripods are composed of three planar arms that appear as rectangular nanoplates. We propose two growth routes for these planar tripods, namely base-arm and tripodal-core routes. For the former route, growth begins with the base arm in [[unk] [unk] 20 ] direction. During quenching, the other two arms grow from newly formed tapered facets, ([unk]110) and (1[unk]10). The tripodal-core growth route involves the formation of a hexagonal disc with ±( 0002 ) larger surfaces. From this core, three arms grow simultaneously in [ 11[unk]0 ], [ [unk]10 ] and [ 1[unk]10 ] directions, while the core transforms into a Y-shaped configuration with ±( 10[unk]0), ±( 01[unk]0 ) and ±( [unk]100 ) planes. Morphological analyses are performed using FESEM, EDS, TEM and XRD. Photoluminescence test detects the presence of structural defects associated with green and red peak emissions.

Controlling the shape and gap width of silicon electrodes using local anodic oxidation and anisotropic TMAH wet etching

A simple method for fabricating silicon electrodes with various shapes and gap widths was designed using the special properties of anisotropic tetramethylammonium hydroxide (TMAH) wet etching and local anodic oxidation (LAO). A statistical system was used for the optimization of the parameters of the LAO process to facilitate a better understanding and precise analysis of the process. Analyses of the interaction effects among the significant factors of LAO showed that the relative humidity and applied voltage were interdependent. They had the strongest interaction effect on the dimensions of the oxide mask. TMAH with a concentration of 25% was used as an etchant solution in (1 0 0) silicon with a rectangular oxide mask. The observed undercutting at convex corners, tip shape of emitters and gap widths of electrodes were exactly consistent with theoretical studies. Combination of the LAO method and anisotropic TMAH wet etching was successfully used to fabricate Si nano-gap electrodes. This fabrication method of sharp and round tip emitters was simple, controllable and faster than common techniques. These results indicate that the method can be a new approach for studying the electrical properties of nano-gap electrodes.

Surface and interface phonon polariton characteristics of wurtzite ZnO/GaN heterostructure

Surface and interface phonon polariton modes in wurtzite ZnO/GaN heterostructure on wurtzite 6H–SiC substrate were investigated by a variable angle p-polarized infrared attenuated total reflection spectroscopy. Three dips corresponding to the surface and interface phonon polariton modes were observed; two of the dips that having a lower intensity level of reflectivity were the leaky modes whereas, another one was a real mode. The observations were verified with the surface polariton dispersion curve simulated based on an anisotropy model for a four-layer system. It was shown that the frequencies of leaky modes are predictable by considering the damping of the substrate.Surface and interface phonon polariton modes in wurtzite ZnO/GaN heterostructure on wurtzite 6H–SiC substrate were investigated by a variable angle p-polarized infrared attenuated total reflection spectroscopy. Three dips corresponding to the surface and interface phonon polariton modes were observed; two of the dips that having a lower intensity level of reflectivity were the leaky modes whereas, another one was a real mode. The observations were verified with the surface polariton dispersion curve simulated based on an anisotropy model for a four-layer system. It was shown that the frequencies of leaky modes are predictable by considering the damping of the substrate.

Optical Analysis of Nanocrystalline ZnO Films Coated on Porous Silicon by Radio Frequency (RF) Magnetron Sputtering

Zinc oxide thin films have been deposited onto porous silicon (PSi) substrates at high growth rates by radio frequency (RF) sputtering using a ZnO target. The advantages of the porous Si template are economical and it provides a rigid structural material. Porous silicon is applied as an intermediate layer between silicon and ZnO films and it contributed a large area composed of an array of voids. The nanoporous silicon samples were adapted by photo electrochemical (PEC) etching technique on n-type silicon wafer with (111) and (100) orientation. Micro-Raman and photoluminescence (PL) spectroscopy are powerful and non-destructive optical tools to study vibrational and optical properties of ZnO nanostructures. Both the Raman and PL measurements were also operated at room temperature. Micro-Raman results showed that the A1(LO) of hexagonal ZnO/Si(111) and ZnO/Si(100) have been observed at around 522 and 530 cm–1, re- spectively. PL spectra peaks are distinctly apparent at 366 and 368 cm–1 for ZnO film grown on porous Si(111) and Si(100) substrates, respectively. The peak luminescence energy in nanocrystalline ZnO on porous silicon is blue-shifted with regard to that in bulk ZnO (381 nm). The Raman and PL spectra pointed to oxygen vacancies or Zn interstitials which are responsible for the green emission in the nanocrystalline ZnO.

Increase in Upturn Power Dissipation of Surge Suppressors Due to Highly Defective Nanostructure of Zinc Oxide

Nanodefects are probable root causes for the observed high power dissipation of ZnO‐based surge suppression devices (SSDs). In this work, nanodefects are introduced by overgrinding ZnO for 100 hours via wet milling. Using FESEM, ZnO nanostructures are found to contain fine cracks, chipped‐off surfaces and nanofragments. For the defective ZnO, the zinc relative atomic % (EDS analysis) is observed to be much larger accompanied by higher oxygen vacancy concentration as revealed by PL green emission. Average particle size drops from 0.24 µm to 0.19 µm and specific surface area increases from 4.72 m2/g to 5.67 m2/g. Fabricated SSDs with defective ZnO exhibits higher power dissipation and bigger grain resistivity. A model is proposed to provide a correlation between nanodefects and power dissipation.

Growth Model for Nanoplates and Nanoboxes of Zinc Oxide from a Catalyst‐Free Combust‐Oxidized Process

A novel growth model is proposed for ZnO nanoplates and nanoboxes that are produced via a catalyst‐free combust‐oxidized (CFCO) process. In the CFCO process, molten zinc is vaporized and instantaneously oxidized in normal atmosphere to produce zinc oxide crystals that are transported into a 100–180 m cooling duct. FESEM analyses show clear images of quasi‐rectangular nanoplates and nanoboxes. The plates and boxes can be differentiated by the width‐to‐height ratio (W/H ratio) whereby a W/H ratio of (0.5–1.5) refers to nanoboxes while other ratios classify the nanoplates. These rectangular nanostructures generally have ⟨101¯0⟩ plane growth direction. We propose growth starts from two kinds of nucleation planes parallel to the six‐sided {101¯0} facets of nanorods that provide the nucleation sites for the nucleation planes.

The effects of oxygen-catalysed and heat treatment on the precipitation synthesised ZnO nanoparticles

Nanocrystalline ZnO particles prepared by precipitation method from Zn and I2 reaction with oxygen as catalyst were investigated. The addition of diethanolamine (DEA) as capping agent and fast pyrolysis treatment at 550, 700 and 850 °C were also characterised and elaborated. Compact and small spherical particles were observed for ZnO synthesised with O2 catalysed whilst, uneven surface, fewer dense packing particles and macropore structures were observed for ZnO prepared without excess of O2. It was shown that diffusion of O2 has improved the structural and photoluminescence behaviour of the prepared ZnO nanoparticles. ZnO synthesised with O2-catalysed exhibited better crystalline and leaned towards a pure state as shown by shifting of PL peak to higher energy of pure ZnO while, sample prepared without excess of O2 exhibits poor crystalline and decreasing of the energy band gap with respect to increment of calcination temperatures. Single violet emission was observed in all samples synthesised with excess of O2 whereby the highest intensity was obtained by calcining at 850 °C with photon energy at 2.95 eV. In contrast to sample with excess of O2, ZnO calcined without excess of O2 at 850 °C displays violet and green emission with energy at 2.93 eV and 2.35 eV, respectively.

Fabrication of porous ZnO thin films using wet chemical etching with 0.5% HNO3

Purpose – The purpose of this paper is to synthesize porous zinc oxide (ZnO) by means of strain etching/wet chemical etching method with the use of 0.5% of nitric acid (HNO3) etchant. The structural and surface morphological properties of the samples are accessed by using X‐ray diffraction (XRD) and scanning electron microscopy (SEM) characterization techniques.Design/methodology/approach – ZnO samples used in this work were deposited on the p‐Si (111) substrates by using radio frequency (RF) sputtering technique. Wet chemical etching processes with the use of 0.5% HNO3 etchant was applied on these samples in order to obtain porous structure. The porous ZnO samples are characterized by means of XRD and SEM to access their structural and surface morphological properties.Findings – The XRD and SEM cross‐sectional measurements revealed that the thickness of the etched ZnO thin films is proportional to the etching time. SEM micrographs show that the surface morphology of ZnO changes over etching time. On the ...

Structural and morphological properties of zinc oxide thin films grown on silicon substrates

In this work, the effects of surface orientation of the silicon (Si) substrates on the structural and morphological properties of zinc oxide (ZnO) thin films grown by radio frequency sputtering system were investigated. Silicon substrates with different surface orientations, i.e., Si(100), Si(111), and Si(110) were used. The structural properties of the ZnO thin films were investigated by X-ray diffraction (XRD) technique. The morphological properties of the deposited films were examined by field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The XRD results reveal that all the deposited ZnO thin films exhibit a single phase wurtzite ZnO structure with preferred orientation along (002) direction. The FESEM images indicate a change from leave-like to granular-like structure for Si(100), (110) and (111) substrates, respectively. The AFM root mean square surface roughness for ZnO thin films on Si(100), (110) and (111) reveals a decreasing trend. From the results, it was sugg...

Nanocrystalline ZnO Film Grown on Porous SnO2/Si(111) Substrate

Porous tin oxide was prepared on silicon(111) substrate by the sol–gel route. Then, the samples were dried in air at 600°C for 30 min in an electric furnace. Scanning electron microscope (SEM) images indicated the high density of the pores. Circular microvoids formed by the rigid shaped microarray network of 200–300 nm sizes are clearly seen in the plan view SEM image. The high homogeneity and uniformity of the porous region could also be visualized by this easy method. Nanocrystalline zinc oxide (ZnO) thin films have been deposited onto porous SnO2substrates at high growth rates by radio frequency (RF) sputtering using a ZnO target. The surface morphology of the nanocrystalline ZnO films was characterized by scanning electron microscope (SEM). Photoluminescence (PL) spectroscopy is a powerful, contactless and excellent nondestructive optical tool to study the acceptor binding energy of ZnO nanostructures. The PL measurements were also operated at room temperature. The peak luminescence energy in nanocrystalline ZnO on porous SnO2 is blue-shifted with regard to that in bulk ZnO (381 nm). PL spectra peaks are distinctly apparent at 375 nm for ZnO film grown on porous SnO2/Si(111) substrates.