Wh Ng

Electrospray synthesis and properties of hierarchically structured PLGA TIPS microspheres for use as controlled release technologies

Microsphere-based controlled release technologies have been utilized for the long-term delivery of proteins, peptides and antibiotics, although their synthesis poses substantial challenges owing to formulation complexities, lack of scalability, and cost. To address these shortcomings, we used the electrospray process as a reproducible, synthesis technique to manufacture highly porous (>94%) microspheres while maintaining control over particle structure and size. Here we report a successful formulation recipe used to generate spherical poly(lactic-co-glycolic) acid (PLGA) microspheres using the electrospray (ES) coupled with a novel thermally induced phase separation (TIPS) process with a tailored Liquid Nitrogen (LN2) collection scheme. We show how size, shape and porosity of resulting microspheres can be controlled by judiciously varying electrospray processing parameters and we demonstrate examples in which the particle size (and porosity) affect release kinetics. The effect of electrospray treatment on the particles and their physicochemical properties are characterized by scanning electron microscopy, confocal Raman microscopy, thermogravimetric analysis and mercury intrusion porosimetry. The microspheres manufactured here have successfully demonstrated long-term delivery (i.e. 1week) of an active agent, enabling sustained release of a dye with minimal physical degradation and have verified the potential of scalable electrospray technologies for an innovative TIPS-based microsphere production protocol.

Intrinsic Resistance Switching in Amorphous Silicon Suboxides: The Role of Columnar Microstructure

We studied intrinsic resistance switching behaviour in sputter-deposited amorphous silicon suboxide (a-SiOx) films with varying degrees of roughness at the oxide-electrode interface. By combining electrical probing measurements, atomic force microscopy (AFM), and scanning transmission electron microscopy (STEM), we observe that devices with rougher oxide-electrode interfaces exhibit lower electroforming voltages and more reliable switching behaviour. We show that rougher interfaces are consistent with enhanced columnar microstructure in the oxide layer. Our results suggest that columnar microstructure in the oxide will be a key factor to consider for the optimization of future SiOx-based resistance random access memory.

Design and Fabrication of Suspended Indium Phosphide Waveguides for MEMS-Actuated Optical Buffering

We present the design and fabrication of suspended optical waveguides on indium phosphide platform for use in an optical buffer device with MEMS actuation, in which the optical delay can be achieved by changing the spacing of the waveguides by electrostatic actuation. The optical and mechanical properties of the waveguides and pillar supports are modeled, and different MEMS actuation schemes are simulated. We also present fabrication and characterization results of the epitaxially grown sample structure and of the suspended waveguide device, exhibiting two parallel waveguides with submicron dimensions separated by a 400-nm air gap, and suspended at 40-μm intervals by S-shaped supports.

MEMS actuation for a continuously tunable optical buffer

We present our work towards development of an optical buffer based on the InP platform that consists of two suspended coupled optical waveguides. A continuously tunable delay in the propagation time can be achieved by varying the spacing between the waveguides using MEMS actuation. Up to 100% variation of the delay time with a driving voltage of 3 V is predicted.

Design and fabrication of InP free-standing optical waveguides for MEMS

We present the design and fabrication of an optical MEMS device on InP platform. The device is based on a suspended parallel waveguide configuration with side pillars supports. Electrodes are integrated on the device layer to provide MEMS actuation functionality to the waveguides. The device is designed to be used as an optical buffer for telecommunication applications.

Controlling and modelling the wetting properties of III-V semiconductor surfaces using re-entrant nanostructures

Inorganic semiconductors such as III-V materials are very important in our everyday life as they are used for manufacturing optoelectronic and microelectronic components with important applications span from energy harvesting to telecommunications. In some applications, these components are required to operate in harsh environments. In these cases, having waterproofing capability is essential. Here we demonstrate design and control of the wettability of indium phosphide based multilayer material (InP/InGaAs/InP) using re-entrant structures fabricated by a fast electron beam lithography technique. This patterning technique enabled us to fabricate highly uniform nanostructure arrays with at least one order of magnitude shorter patterning times compared to conventional electron beam lithography methods. We reduced the surface contact fraction significantly such that the water droplets may be completely removed from our nanostructured surface. We predicted the wettability of our patterned surface by modelling the adhesion energies between the water droplet and both the patterned surface and the dispensing needle. This is very useful for the development of coating-free waterproof optoelectronic and microelectronic components where the coating may hinder the performance of such devices and cause problems with semiconductor fabrication compatibility.

The interplay between structure and function in redox-based resistance switching

We report a study of the relationship between oxide microstructure at the scale of tens of nanometres and resistance switching behaviour in silicon oxide. In the case of sputtered amorphous oxides, the presence of columnar structure enables efficient resistance switching by providing an initial structured distribution of defects that can act as precursors for the formation of chains of conductive oxygen vacancies under the application of appropriate electrical bias. Increasing electrode interface roughness decreases electroforming voltages and reduces the distribution of switching voltages. Any contribution to these effects from field enhancement at rough interfaces is secondary to changes in oxide microstructure templated by interface structure.

Electrospun fabrication of one-dimensional composite nanofibres using colloidal gold/polymer aqueous blends

We successfully demonstrate the facile fabrication of composite polymer/gold nanofibres using the electrospinning technique and show their physical characterization. Scanning Electron Microscopy (SEM) revealed the consistent structural fabrication of uniform nanofibres, and high-resolution Scanning Transmission Electron Microscopy (HR-STEM) images, in both bright field (BF-STEM) and dark field (DF-STEM) modes exposed nanoparticle dispersions in the polymer matrix. There is a critical need for establishing processing methods that are effective on the nanoscale yet are applicable to macroscopic processing; the findings presented here demonstrate that the electrospinning process provides a straightforward technique to fabricate arrays of nanoparticles and potentially functional nanoassemblies for future nanodevices.

Development of Indium Phosphide MEMS for tunable optical buffering

We are developing a MEMS-based device that can produce continuously tunable delays of optical signals. The design is based on coupled dual suspended waveguides fabricated in Indium Phosphide. We report on simulation-based optimisation of optical and mechanical properties of the structure and on current progress in fabrication.

Tunable optical buffer based on III-V MEMS design

We present the design and fabrication of a tunable optical buffer device based on III-V semiconductor platform for telecommunication applications. The device comprises two indium phosphide suspended parallel waveguides with cross sectional dimension of 200 nm by 300 nm, separated by an air gap. The gap between the waveguides was designed to be adjustable by electrostatic force. Our simulation estimated that only 3 V is required to increase the separation distance from 50 nm to 500 nm; this translates to a change in the propagation delay by a factor of 2. The first generation of the suspended waveguide structure for optical buffering was fabricated. The sample was grown on an InP substrate by molecular beam epitaxy. The waveguide pattern is written onto a 300 nm thick InP device layer by electron beam lithography and plasma etching. Electrodes were incorporated into the structure to apply voltages for MEMS actuation.