Inn Khuan Ng

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.

Electrochemical Synthesis and Characterisation of BiTe-Based Nanowire Arrays as Thermoelectric Nanogenerators

Thermoelectric (TE) nanogenerators are solid state devices that can directly convert thermal energy into electrical power. The family of bismuth telluride (BiTe)-based semiconductor materials is a good candidate for room-temperature thermoelectric applications. This work aims to synthesize the binary and ternary TE nanowire arrays from BiTe and BiSbTe using template-assisted electrochemical deposition. The compositions and lengths of the nanowires fabricated were precisely tuned by controlling the deposition conditions, while their diameters were confined by the pore sizes of the anodized alumina oxide (AAO) templates used. Chemical compositions of the nanowires were evaluated using Energy Dispersive X-ray (EDX) Spectroscopy and their microstructures were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), conventional transmission electron microscopy (CTEM) and high resolution transmission electron microscopy (HRTEM). TE nanogenerator modules comprising of BiTe and BiSbTe nanowires/AAO composite arrays were fabricated for TE performance evaluation on the basis of their power outputs.

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.

Large-Area Synthesis and Microstructural Investigations of Silicon Nanowires and Te/Bi2Te3-Si Core-Shell Structures

Large-area randomly-oriented silicon nanowires (SiNWs) were synthesized using Au-coated p-type Si (100) substrates via the solid-liquid-solid (SLS) process under different growth conditions. Microstructural studies on the NWs produced showed that straight crystalline nanowires of large aspect ratios were generally obtained at a growth temperature of 1000°C along with some worm-like amorphous structures. Large-area vertically aligned silicon nanowire (SiNW) arrays on p-type (001) Si substrates were also synthesized in an aqueous solution containing AgNO3 and HF by self-selective electroless etching. Diameters of the SiNWs produced from both methods varied from 50 nm to 350 nm and their lengths generally extended from several to approximately a few tens of µm depending on the growth conditions used. Te-Si and Bi2Te3-Si core-shell structures were subsequently obtained via galvanic displacement of SiNWs in acidic HF electrolytes containing HTeO2+ and Bi3+/HTeO2+ ions. The reactions were basically a nanoelectrochemical process due to the difference in redox potentials between the materials. The modified SiNWs of core-shell structures had roughened surface morphologies and, therefore, higher surface-to-bulk ratios compared to unmodified SiNWs. They should have potential applications in sensors, photovoltaic and thermoelectric nanodevices. Microstructural studies on the SiNWs and core-shell structures produced are presented using various microscopy, diffraction and probe-based techniques for characterization.

Aqueous synthesis of silicon nanowire arrays and core-shell structures via electroless nanoelectrochemical process

A solution growth of silicon nanowires (NWs) and core-shell structures is highly desirable due to low growth temperature and large-area synthesis capability. Silicon demonstrates novel electrochemical properties in aqueous solutions containing hydrofluoric acid which have raised significant research interests due to the complex electrochemical etching behavior involved. In the present study, we have demonstrated the synthesis of large-area vertically aligned silicon nanowire (SiNW) arrays (Fig. 1 and 2) in an aqueous solution containing AgNO3 and HF on p-type (001) Si substrate by self-selective electroless etching process [1]. The fabrication process was rather simple and rapid compared to the well-known Vapor-Liquid-Solid (VLS) growth [2] via chemical-vapor deposition (CVD) and other high-vacuum techniques. In this work, the temperature of electrolyte and etching duration were varied in order to achieve different stages of nanowire formation. Diameters of the SiNWs obtained varied from 50 nm to 200 nm and their lengths ranged from several to approximately a few tens of µm, depending on the reaction time and the electrolyte conditions used. Te-Si and Bi2Te3-Si core-shell structures (Fig. 3) were subsequently obtained via galvanic displacement of SiNWs in acidic HF electrolytes containing Bi3+ and Bi3+/HTeO2+ ions respectively. The reactions were basically a nanoelectrochemical process due to the difference in redox potentials between the materials. The modified SiNWs of the core-shell structures exhibit roughened surface morphologies and, therefore, have higher surface-to-bulk ratios compared to the unmodified SiNWs, which should have potential applications in sensor, photovoltaic and thermoelectric nanodevices. Growth study on the SiNWs and core-shell structures produced is presented using various microscopy, diffraction and probe-based techniques for structural, morphological and chemical characterizations.