Fabrication and application of novel highly ordered nano-heterostructure arrays in optoelectronics

Nano-heterostructures are material system formed by the joint of at least two dissimilar semiconductor material components at nanoscale. For nano-hetero-semiconductors, the coupling between them could provide more functional and effective properties than individual components. This is due to the formation of the interface between these semiconductors with unequal band gaps when they are in contact. To date, a great deal of effort has been devoted to the design, fabrication and characterization of various nano-heterostructures in terms of its morphology, dimension, defects, strain, and lattice mismatch. However, the fabrication and manipulation of highly ordered nano-heterostructure arrays are still challenging. This thesis presents the fabrication and characterization of the novel highly ordered ZnO/Si nano-heterostructure arrays, and the subsequent modification to optimize its optoelectronic properties. Focused ion beam system is employed to fabricate the highly ordered Si nanocone arrays, and then the ZnO nanorods are grown on top of each Si nanocone through hydrothermal method. The obtained ZnO/Si nano-heterostructure array is then subject to further modification, including heat treatment, gallium ion irradiation, and metal sulfide decoration to enhance the optoelectronic properties. As an example, attempt has been made to use this highly ordered nano-heterostructure array as the photoelectrode in the photoelectrochemical water splitting system, to demonstrate its application in optoelectronic fields. To fabricate the highly ordered ZnO/Si nano-heterostructure array, ZnO seed layer was first deposited on the surface of Si (p{111}) substrate. The nucleation sites were then defined by patterning the surface with gallium ion beam. After that the hydrothermal process was used to grow ZnO nanorods from the nucleation sites. The whole fabrication process is simple, facile and offers a direct control of the space, length and aspect ratio of the array. The photoluminescence (PL) measurement of the nano-heterostructure arrays was performed. It is found that ZnO/Si nano-heterostructures show an improved interface after heat treatment. The enhanced photoresponse to white light after the heating process is contributed to the recrystallization of ZnO and the developed tensile lattice strain of Si. Photoluminescence measurement of ZnO/Si nano-heterostructure arrays subject to gallium ion irradiation at different ion energies (0.5 keV~16 keV) was performed. The result shows that the intensity of UV emission increases rapidly with the increase of gallium ion energy and reaches the maximum at around 2 keV, approximately 50 times higher than the UV intensity produced from as-grown nanorods. The gentle bombardment of low energy gallium ions removes the defects from ZnO nanorod surfaces. The gallium ions, on the other hand, are implanted into the nanorods, resulting in compressive strain. It is believed that the perfect arrangement of crystal lattice after the removal of surface defects and the introduced compressive strain are two factors that contribute to the significant enhancement of UV emission in the PL spectra. In the present thesis, attempt has been successfully made to decorate ZnO/Si nano-heterostructure arrays with ZnS and Ag2S. ZnS is known to be very stable in aqueous solutions. Meanwhile, Ag2S with a narrow band gap enables more efficient absorption of incident light and faster separation of photogenerated electron-hole pairs. As a result, the Ag2S/ZnS/ZnO/Si nano-heterostructure array has been demonstrated to be a good potential candidate as the photoelectrode in photoelectrochemical water splitting system.Doctor of Philosophy (MSE

A hybrid nanostructure array for gas sensing with ultralow field ionization voltage

We fabricate a unique hybrid nanostructure array for gas sensing based on the polarization mechanism at the nanoscale. It is shown that with platinum nanocrystallites on the top of each nanoneedle, this array can work at ultralow voltages (less than 10 V) as a field ionization gas sensor. We believe that the polarized platinum brings about a local enhanced electrical field, leading to the direct field ionization of gas molecules, which is confirmed by calculations of the charge accumulation and electrical field distribution

Fabrication of hybrid CuO/Pt/Si nanoarray for non-enzymatic glucose sensing

Ordered mushroom-like hybrid CuO/Pt/Si nanoarrays were fabricated using ion etching followed by the electrochemical method. The process started with patterning the surface of a Si (100) substrate coated with a thin layer of Pt, forming a hybrid array of nanoneedles. Each nanoneedle consists of a Pt nanodot sitting on top of a Si nanorod. Cu nanosphere was subsequently grown on the surface of each Pt dot and then oxidized to CuO by the method of continuous cyclic voltammetry in NaOH solution. Meanwhile, the multiple redox cycles produce a rough and porous surface on these CuO nanospheres, which significantly increases the contact area of the nanosphere surfaces with the external environments. This unique nanostructure array shows excellent electrochemical sensitivity to glucose detection.ASTAR (Agency for Sci., Tech. and Research, S’pore

General Approach for MOF-Derived Porous Spinel AFe2O4 Hollow Structures and Their Superior Lithium Storage Properties

A general and simple approach for large-scale synthesis of porous hollow spinel AFe2O4 nanoarchitectures via metal organic framework self-sacrificial template strategy is proposed. By employing this method, we can successfully synthesize uniform NiFe2O4, ZnFe2O4, and CoFe2O4 hollow architectures that are hierarchically assembled by nanoparticles. When these hollow microcubes were tested as anode for lithium ion batteries, good rate capability and long-term cycling stability can be achieved. For example, high specific capacities of 636, 449, and 380 mA h g–1 were depicted by NiFe2O4, ZnFe2O4, and CoFe2O4, respectively, at a high current density of 8.0 A g–1. NiFe2O4 exhibits high specific capacities of 841 and 447 mA h g–1 during the 100th cycle when it was tested at current densities of 1.0 and 5.0 A g–1, respectively. Discharge capacities of 390 and 290 mA h g–1 were delivered by the ZnFe2O4 and CoFe2O4, respectively, during the 100th cycle at 5.0 A g–1.MOE (Min. of Education, S’pore)Published versio

Metal-sulfide-decorated ZnO/Si nano-heterostructure arrays with enhanced photoelectrochemical performance

The present work reports an effective approach to fabricate highly ordered arrays of ZnO/Si heterostructures decorated with metal sulfides (i.e. ZnS and Ag2S). These metal-sulfide-coated ZnO/Si heterostructure arrays are then used as a photoanode in the photoelectrochemical water splitting system. The significant enhancement of ZnO/Si heterostructure arrays in photo-current density and on/off ratio is demonstrated after they are coated with metal sulfides. These metal-sulfide-coated ZnO/Si heterostructure arrays are a promising building block for the fabrication of optoelectronic devices such as photoelectrochemical cells.MOE (Min. of Education, S’pore

Atmospheric microplasma based binary Pt3Co nanoflowers synthesis

The atmospheric microplasma in the gas-liquid phase technique serves as a new potential efficient and green catalyst preparation technique to fabricate nanomaterials. Due to the presence of diverse reactive species, this technique can promote rapid complex reactions in solutions, which are typically sluggish in traditional chemical processes. Here, atmospheric microplasma induced liquid chemistry (AMILC) is applied to fabricate three-dimensional (3D) binary Pt3Co nanoflowers. Nano-architectures of Pt3Co bimetals (2D nanosheets and 3D nanoflowers) can be formed by tuning the initial cobalt molar concentration in the solution. 3D nanoflowers show a 'nano-bouquet' like nanostructure with Co-oxide forming leaves and Pt3Co forming waxberries. 3D nanoflowers show promising electrocatalytic behavior towards ethanol and glucose sensing in alkaline condition. Additionally, AMILC takes less synthesis duration (∼10 min) without hazardous chemicals for Pt3Co bimetal nanostructure preparation compared to conventional chemical approaches (>2 h), indicating that AMILC is a potential candidate with better energy efficiency, lower carbon footprint and green plasma chemistry process for 3D nanostructure material synthesis in catalyst applications.</p

Significant enhancement of UV emission in ZnO nanorods subject to Ga+ ion beam irradiation

Applications of ZnO nanomaterials in optoelectronics are still limited due to their insufficient photoluminescence efficiency. In order to optimize the photoluminescence properties of ZnO nanorods, the UV emission of vertically aligned ZnO nanorods grown on a Si substrate, in correlation with Ga+ ion irradiation at different ion energies (0.5 keV–16 keV), was investigated in the present study. We found that the UV intensity increased rapidly with increasing Ga+ ion energy, up to its maximum around 2 keV, at which point the intensity was approximately 50 times higher than that produced by as-grown ZnO nanorods. The gentle bombardment of low-energy Ga+ ions removes defects from ZnO nanorod surfaces. The Ga+ ions, on the other hand, implant into the nanorods, resulting in compressive strain. It is believed that the perfect arrangement of the crystal lattice upon removal of surface defects and the introduction of compressive strain are two factors that contribute to the significant enhancement of UV light generation.MOE (Min. of Education, S’pore)Accepted versio

Elimination of impurity phase formation in FePt magnetic thin films prepared by pulsed laser deposition

The formation of impurity phases in FePt thin films severely degrades its magnetic properties. The X-ray diffraction patterns of FePt thin films, synthesized using pulsed laser deposition (PLD), showed peaks corresponding to impurity phases, resulting in softer magnetic properties. A systematic investigation was carried to determine the factors that might have led to impurity phase formation. The factors include (i) PLD target composition, (ii) substrate material, (iii) annealing parameters such as temperature, duration and ambience and (iv) PLD deposition parameters such as chamber ambience, laser energy fluence and target–substrate distance. Depositions on the different substrates revealed impurity phase formation only on Si substrates. It was found that the target composition, PLD chamber ambience, and annealing ambience were not the factors that caused the impurity phase formation. The annealing temperature and duration influenced the impurity phases, but are not the cause of their formation. A decrease in the laser energy fluence and increase of the target–substrate distance resulted in elimination of the impurity phases and enhancement in the magnetic and structural properties of FePt thin films. The energy of the ablated plasma species, controlled by the laser energy fluence and the target–substrate distance, is found to be the main factor responsible for the formation of the impurity phases

Hierarchically porous three-dimensional electrodes of CoMoO4 and ZnCo2O4 and their high anode performance for lithium ion batteries

Ternary metal oxides have been receiving wide attention in electrochemical energy storage due to their rich redox reactions and tuneable conductivity. We present a simple solution-based method to prepare a 3D interconnected porous network of ternary metal oxide (CoMoO4 and ZnCo2O4) nanostructures onmacroporous nickel foam. The open-structured networks with different degrees of porosity endow them with high surface areas of electro-active sites. The Li ion storage properties of both anodes are investigated. High rate capability and long term cycling stability are achieved for both systems