Kuan Ying Kok

Gold catalysed growth of silicon nanowires and core–shell heterostructures via solid–liquid–solid process and galvanic displacement

Abstract Silicon nanowires (SiNWs) were first synthesised using Au coated p type Si (100) substrate via the solid–liquid–solid (SLS) process. The growth parameters were selectively varied to achieve various stages of growth for studying their effects on the morphology and microstructures of the nanowires (NWs). The SLS growth of SiNWs is discussed in the context of the experimental conditions used. Straight NWs of large aspect ratios, good crystallinity and morphology were generally obtained at a growth temperature of 1000°C along with some worm-like amorphous structures. Te–Si NW core–shell structures were subsequently obtained via post-growth galvanic displacement of the SiNWs in an acidic HF electrolyte containing ions. The core–shell structures obtained were decorated with Te nanoparticles. This increases the NW surface areas and should have great potential in non-reflecting, photovoltaic and thermoelectric applications. Growth study on the SiNWs and Te–Si core–shell structures is presented using various microscopy, diffraction and probe based techniques for structural, morphological and chemical characterisations.

Fabrication of ZnO Nanostructures with Self-Cleaning Functionality

The science of biomimicry has served as a fusion point between nature and technology where one could adopt nature’s best solution for human’s use. Lotus leaf, for example, possesses self-cleaning capability due to the unique physical and chemical properties of its surface structural features. In this work, we aimed to mimic these features on glass surface using ZnO nanostructures to achieve the self-cleaning functionality. A series of ZnO films were electrochemically deposited on indium-doped tin oxide (ITO) conducting glass substrates from different aqueous electrolytes at systematically varied deposition potentials and electrolyte conditions. The surface morphology, density, orientation and aspect ratio of the ZnO micro/nanostructures obtained were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). ZnO ranging from two dimensional plate-like to one-dimensional needle-like micro/nanostructures were observed. Results from these studies show that lower electrolyte concentrations tend to favor one-dimensional growth of ZnO nanostructures that self-assembled into micron-size flower-like clusters at higher deposition temperatures. The ZnO-modified hierarchical dual-structured surface exhibits superhydrophobic property with water contact angle as high as 170o.

Fabrication and Characterization of ZnO Nanofibers by Electrospinning

ZnO nanofibers were successfully prepared by electrospinning a precursor mixture of polyvinylpyrrolidone (PVP)/zinc acetate, followed by calcination treatment of the electrospun composite nanofibers. The effect of applied voltage to the morphology of nanofibers was studied. Both PVP/Zn acetate and ZnO nanofibers were characterized by FESEM and XRD. The results show that the diameter of the nanofibers changed with applied voltage. Results found that the optimum calcined temperature was 500°C to produce continuous ZnO nanofibers.

The Effects of Substrate Orientation on Galvanic Growth of ZnO Structures

Electrochemical route has been a favorite technique for the fabrication of ZnO as it is relatively cheap and capable of producing abundance amount of materials. In this paper, we investigate the morphologies of hydrothermally synthesized ZnO structures assisted by galvanic process on conducting Au-coated silicon substrate. To induce the galvanic process, the substrate was in contact with aluminum so that the difference in the reduction potentials between the two materials provided the driving force for the formation of the ZnO structures. The galvanic process was found to promote compact and anisotropy growth of ZnO nanorods along the (001) plane. It was also found that substrate orientations in the electrolyte solution had an important bearing on the morphologies of ZnO. Well aligned periodic hexagonal arrays of ZnO nanorods of homogeneous diameters were obtained on gold-coated Si substrate with face-down orientation in the electrolyte solution.

Hydrogen sensing enhancement of zinc oxide nanorods via voltage biasing

The capability of zinc oxide (ZnO) as a hydrogen sensing element has been pushed to its limits. Different methods have been explored to extend its sensing capability. In this paper, we report a novel approach which significantly improves the hydrogen sensing capability of zinc oxide by applying a bias voltage to ZnO nanorods as the sensing elements. Zinc oxide in the form of aligned nanorods was first synthesized on an Au-coated Si(111) substrate using a facile method via the galvanic-assisted chemical process. The sensing performance of the zinc oxide nanorods was investigated in response to the applied biasing voltage. It was found that the sensitivity, response time and detection limit of the ZnO sensing elements were dramatically improved with increasing bias voltage. A 100% increment in sensing response was achieved for the detection of 2000 ppm hydrogen gas when the bias voltage was increased from −2 to −6 V with 70% reduction in response and recovery times. This remarkable sensing performance is attributed to the reaction of hydrogen with chemisorbed oxygen ions on the surface of the ZnO nanorods that served as the electron donors to increase the sensor conductance. Higher reverse bias voltages sweep the electrons faster across the electrodes. This shortened the response time and, at the same time, depleted the electrons in the sensor elements and weakens oxygen adsorption. The oxygen ions could then be readily removed by hydrogen, leading to a higher sensitivity of the sensors. This, therefore, envisages a way for high-speed hydrogen gas sensing with high detection sensitivities.

Modification of Optophysical Properties of Poly[(9,9-Di-N-Octylfluorenyl-2,7-Diyl)-Alt-(Benzo[2,1,3]Thiadiazol-4,8-Diyl)] Thin Film via Additions of TiO2 Nanoparticles

The solution of poly [(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo [2,1,3] thiadiazol-4,8-diyl)] (F8BT) F8BT/TiO2 nanocomposites were prepared using solution blending methods. The TiO2 contents were fixed between 0 wt% - 35 wt%. Then, the solutions were spin coated at 1000 rpm for 30 s onto glass substrates to form thin film samples. The optical properties of the nanocomposites were determined using UV-Vis spectroscopy and photoluminescence spectroscopy. The absorption properties of the thin film increased due to existence of intermediate energy band which lead to higher space charge regions for electron insertion. Besides that, the λmax for absorption and emission spectrum were systematically shifted due to incorporation of TiO2 NPs indicating an interaction between nanoparticles and polymer matrix. Furthermore, the intensity of the emission spectrum were enhanced in the presence of TiO2 NPs. This is due to the existence of TiO2 NPs which trapped more electrons at the interface F8BT/TiO2, resulted production of higher number of exciton formation in the nanocomposirte samples.

Controlled growth and alignment of palladium nanowires prepared by template-assisted electrodeposition

Palladium nanowires are good candidates for hydrogen sensing due to the many advantages. In this paper, we discuss the controlled growth of palladium nanowires into the nanopores of anodized alumina oxide (AAO) via template-assisted electrodeposition. This technique provides precise control for the dimensional growth of Pd nanowires. The potential range for electrochemical deposition of palladium nanowires was determined by Linear Sweep Voltammetry (LSV). Crystallinity and microstructural studies of the palladium nanowires were carried out using X-ray diffractometry (XRD) and scanning electron microscopy (SEM) as characterization tools. Systematic study on nanowire alignment across 3 µm-gaps between pairs of microfabricated gold electrodes was carried out using a.c. dielectrophoresis technique. Optimized conditions for good nanowire alignment were reported.

Preparation of Porous Alumina Template for Nanostructure Fabrication

Porous alumina films are widely used as templates for fabricating one-dimensional (1-D) nanostructures such as nanowires or nanotubes. Using a two-step anodisation process, we have successfully optimized the growth conditions for fabricating highly ordered porous alumina films with pore diameters ranging from 20 to 150 nm, to be used as templates for 1-D nanostructure synthesis. The effects of the anodisation conditions on pore structure and the formation rate of the films were systematically studied. It was found that low electrolyte temperatures and agitations decreased the growth rate of the films but favored the process of pore ordering. Removal of oxide layer formed from first anodisation process and removal of barrier oxide at pore ends had an important bearing on pore morphology. Besides the stand-alone porous alumina films, we have also fabricated porous alumina films on rod-shaped Al substrates.

Fabrication of Nanoporous Aluminum Oxide via a Two-Step Anodisation Process

In this study, we report the fabrication of nanoporous aluminum oxide film from high purity aluminium foil via a two-step anodisation process controlled by a constant direct current potential ranging from 40 60 V from a DC power supply. The anodisation process was conducted at 20˚C in an electrochemical cell with the Al foil acting as anode, Pt as cathode and an acidic bath as electrolyte. Porous aluminium oxide films of pore diameters ranging between 30 90 nm were successfully fabricated. The morphologies and phase compositions of the anodized porous alumina films were investigated using scanning electron microscopy (SEM) and x-ray diffraction (XRD) for characterizations.

Anodic Aluminum Oxide Templates for Nickel Nanowires Array Fabrication

This paper reports on the process developed to fabricate anodic aluminum oxide (AAO) templates suitable for the fabrication of nanowires arrays. Anodization process has been used to fabricate the AAO templates with pore diameters ranging from 15 nm to 30 nm. Electrodeposition of parallel arrays of high aspect ratio nickel nanowires were demonstrated using these fabricated AAO templates. The nanowires produced were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that the orientations of the electrodeposited nickel nanowires were governed by the deposition current and electrolyte conditions.