Passive permeation through biological membranes is an important mechanism for transporting molecules and regulating the cellular content. Studying and understanding passive permeation is also extremely relevant to many industrial applications, including drug design and nanotechnology. In vivo membranes typically consist of mixtures of lamellar and nonlamellar lipids. Lamellar lipids are characterised by their tendency to form lamellar bilayer phases, which are predominant in biology. Nonlamellar lipids, when isolated, instead form non-bilayer structures such as inverse hexagonal phases. While mixed lamellar/nonlamellar lipid membranes tend to adopt the ubiquitous bilayer structure, the presence of nonlamellar lipids is known to have profound effects on key membrane properties, such as internal distributions of stress and elastic properties. This dissertation examines permeation through lamellar and nonlamellar lipid membranes by utilising atomistic molecular dynamics simulations in conjunction with two di erent methods, the z-constraint and the z-restraint, in order to obtain transfer free energy profiles, diffusion profiles and permeation coefficients. An assessment of these methods is performed in search for the optimal, with the goal to create an automated, accurate and robust permeation study framework. Part of the dissertation involves the creation of the corresponding software. Furthermore, this work examines the effect of changing the lamellar vs. nonlamellar lipid composition on the passive permeation mechanism of a series of 13 small molecules and drugs. These nonlamellar lipids are known to affect the lateral pressure distribution inside the membranes. This work investigates the hypothesis that the differences in lateral pressure should increase the resistance to permeation. The results indicate that, upon addition of nonlamellar lipids, permeation is hindered for small molecules but is facilitated for the largest. All results are in agreement with previous experimental and computational studies. This work represents an advancement towards the development of more realistic in silico permeability assays, which may have a substantial future impact in the area of rational drug design.

Hydrophilic POx have very similar behaviour with PEG owing to its peptdomimetic structure, however, POx shows higher chemical stability than PEG and can be further functionalised via substitute R in the side chains or at the end of the chain. In addition, PEtOx has been approved as an indirect food additive by FDA which may indicate the possibility of good immunogenicity of POx-based materials. Synthetic surfaces with reproducibility and biocompatibility for in vitro cell culture offer lots of advantages on adherent cells. A variety of synthetic polymers as well as properties like mechanical and chemical robustness resulting polymer brushes prior to other surface modification methods. Synthetic hydrogels can be further modified and allow a variety of mechanical and biochemical properties that determine the cell phenotype which makes it a good candidate for biomedical applications. Our work has focused on the design of polyoxazolines with controlled end chains for the design of such hydrogels and polymer brushes. In the second chapter, we review the synthesis of defined polyoxazoline and its applications, synthesis of non-fouling surfaces, fabricated hydrogels and characterisations. In the third chapter, we explore the design of poly(2-oxazolines) with controlled end chains and characterise the structure and control of initiation and termination steps. A range of initiators (bromide, iodide as well as tosylates) and termination agents were used to introduce functionalisable or polymerisable end groups on poly(2-oxazolines). Microwave assisted synthesis was used for polyoxazoline synthesis. Polyoxazolines can be simply synthesized in relatively mild conditions using this approach. The structure of the resulting polymers is characterised by NMR, MALDI-ToF and FTIR. In the third chapter, we explore the use of polymerisable polyoxazolines for the design of grafted from polymer brushes. The growth of poly(oxazoline) brushes was studied first and the resulting polymer brushes characterised. We then explored the functionalization of polymer brushes using thiol-ene chemistry and their protein resistance for cell and protein patterning. Hence, we explored the design of polyoxazoline nonfouling coatings. These surfaces allow the control of surface properties such as protein adsorption and bio-functionalization. In the fourth chapter, we designed a series of thiolated poly(oxazolines) to be used for the design of hydrogels crosslinked via thiol-ene chemistry. We fully characterised the thiolated polymers designed and studied the formation of hydrogels using an alkene-functionalised polyoxazoline and a range of thiolated crosslinkers with polyethylene glycol and poly(oxazoline) backbones. Synthetic hydrogels have attracted much attention recently for in vitro cell culture as they allow the control of the properties of soft biomaterials (mechanics, cell adhesion, degradation). Importantly, the gelation conditions used for 3D cell encapsulation are essential as they allow controlling the mechanics and stability of the gel, whilst curing in mild, non-toxic conditions. Properties such as hydrogel chemistry, macroscopic and nanoscale mechanical properties and degradation have indeed been shown to strongly affect cell phenotype and the use of these materials for tissue engineering. To study gelation in situ, photo-rheology was used to characterise the properties and kinetics of the resulting hydrogels. Here, we investigated the formation of hydrogels with different multi-arm PEG thiols. This allowed us to improve the properties of hydrogel even at low weight concentration of materials where gelation is particularly challenging.

<p> UV-C LEDs in the range of 265&ndash;280 nm are needed to develop new disinfection and biotechnology applications. The market share for UV-C LED, versus UV-C lamps (Hg discharge and Xe), increased from 8% in 2008 ($240M) to 25% in 2018 ($810M). However, while low-pressure mercury lamps are ~30% energy efficient, the best commercial UV-C LEDs in the 265&ndash;280 nm range are ~2% energy efficient; InGaN blue LEDs are 80% energy efficient. Research on AlGaN LEDs has made significant progress into AlGaN material quality (including threading dislocation density and n-AlGaN electrical conductivity) but has lagged regarding light extraction efficiency. Light extraction from UV LEDs is limited by p-GaN absorption because of the lack of p-contact to p-AlGaN with AlN fraction (AlN content > 50%). Furthermore, AlGaN emitters at the 265&ndash;280 nm range emit 40&ndash;50% of their emissions as transverse magnetic (TM) waves, which are harder to extract than transverse electric (TE) waves. </p><p> SiC is an absorbing substrate that has been largely overlooked in developing UV-C LEDs, even though it has a small lattice mismatch with AlN (~1%) and a similar Wurtzite crystal structure and is more chemically stable. We demonstrate the first lateral thin-film flip-chip (TFFC) ultraviolet (UV) light-emitting diodes grown on SiC. UV LEDs were made at 310 nm, 298 nm, 278 nm, and 265 nm. </p><p> In this dissertation, we discuss the design, epi development, and fabrication of TFFC AlGaN LEDs with reflective p-contacts. The AlGaN:Mg growth temperature and the Mg doping profile in AlGaN:Mg were found to significantly impact the electroluminescence (EL) efficiency of the AlGaN MQWs. KOH roughening enhanced the light-extraction efficiency (LEE) by 100% and by ~180&ndash;200% for UV LEDs with 10 nm p-GaN and 5 nm p-GaN, respectively, without affecting the devices&rsquo; IV characteristics. The thin-film architecture led to a high LEE of about ~28&ndash;30% without LED encapsulation when used with LEDs with 5 nm p-GaN. The best light extraction efficiency in the literature is ~24% (without LED encapsulation) for a 275 nm flip-chip LED grown on PSS sapphire substrate. KOH roughening of AlN is discussed and is compared to KOH roughening of N-Face GaN. To advance LEE further, we attempted to develop LEDs with transparent current n-AlGaN spreading layers as well as highly doped n⁺-AlGaN tunnel junctions on top of UV-C LEDs. Reflective and ohmic n-contacts with low resistivities were developed for the n-Al_.58Ga_.42N regrown by MBE. Furthermore, a highly reflective MgF₂/Al omnidirectional mirror was developed, which can be used with n-contact microgrid to further enhance the LEE in UV-C LEDs with a transparent tunnel junction. </p><p>

<p> This dissertation explores three related embodiments of light&ndash;matter interactions at the micro&ndash; and nano&ndash;scales, and is focused towards tangible device applications. The first topic provides a fundamentally different transistor or electronic switch mechanism, which is termed a light&ndash;effect transistor (LET). The LET, unlike exotic techniques, provides a practical and viable approach using existing fabrication processes. Electronic devices at the nanoscale operate within the ballistic regime, where the dominate source of energy loss comes from impurity scattering. As a LET does not require extrinsic doping, it circumvents this issue. Electron&ndash;phonon coupling, however, is the second largest source, and it is a pertinent and important parameter affecting electronic conductivity and energy efficiency, such as in LETs. The third topic is laser writing, or the use of a laser to perform post&ndash;growth modifications, to achieve specific optical and electrical characteristics. </p><p> A LET offers electronic&ndash;optical hybridization at the component level, which can continue Moore&rsquo;s law to the quantum region without requiring a FET&rsquo;s fabrication complexity, e.g., physical gate and doping, by employing optical gating and photoconductivity. Multiple independent gates are therefore readily utilized to achieve unique functionalities without increasing chip space. LET device characteristics and novel digital and analog applications, such as optical logic gates and optical amplification, are explored. Prototype cadmium selenide (CdSe) nanowire&ndash;based LETs show output and transfer characteristics resembling advanced FETs, e.g., on/off ratios up to ~1.0x10⁶ with a source-drain voltage of ~1.43 V, gate-power of ~260 nW, and a subthreshold swing of ~0.3 nW/decade (excluding losses). The LET platform offers new electronic&ndash;optical integration strategies and high speed and low energy electronic and optical computing approaches.</p><p> Electron&ndash;phonon coupling is typically studied as an intrinsic property, which is particularly important for electronic transport properties at the nanoscale, where controversy and even contradictory experimental and theoretical findings still persist. Zinc telluride (ZnTe) has important uses in optical or laser refrigeration, and the existing studies do not consider extrinsic effects, such as laser&ndash;forming tellurium&ndash;based species. Nanostructures, with their large surface&ndash;to&ndash;volume ratios, are more susceptible to extrinsic perturbations that ultimately effect coupling. In this dissertation, ZnTe is studied in bulk, thin film, and nanowire forms with primary focus on the latter. Raman spectroscopy under near resonant excitation is used to extract electron&ndash;phonon coupling strengths, which is obtained through the ratio of the first and second order Raman peaks, <i>R</i> = <p style="font-variant: small-caps"> I2LO/I1LO</p> (and is proportional to the Huang&ndash;Rhys factor). Laser&ndash;formation of tellurium&ndash;based species on ZnTe nanowires dynamically altered the ratio R from ~6-7 to 2.4 after laser processing, while tuning the (532 nm) laser power from a few microwatts to 150 microwatts (with constant optical exposure time) did not significantly impact the EPC strength. Other explored effects include size dependence, chemical effects (methanol exposure), and interface effects (e.g., at a gold&ndash;nanowire junction). The findings suggest that the previously reported size dependence in ZnTe was extrinsic in nature. Tunable coupling strengths also suggest the possibility of novel electronic and optoelectronic devices.</p><p> The electrical characteristic of CdSe nanowire M-S-M devices are shown to be tunable with laser illumination. As with any semiconductor material, sufficiently low optical powers produce stable and reproducible electrical properties, while higher optical powers and exposure times can induce laser modifications of the material. Drastic modification of electrical characteristics were observed, such as from converting an ohmic response (linear slope change) to rectified characteristics, and modification of both forward and reverse currents. Results suggest the potential to laser write wavelength&ndash;specific electronic functions that could be used in applications requiring wavelength discrimination, such as with night vision products. Using a combination of laser modification and device fabrication processes provides the ability to offer a menu of electrical behaviors using the same materials and fabrication processes.</p>

<p> Carbon nanotubes are a relatively new area of research which has gained significant attention in published literature. One reason for this interest is their use in multi-phase composites, specifically where they can enhance traditional polymer matrices. Many authors have attempted to adapt conventional micromechanical analyses reserved for microfibers to the nano scale. A review of these works is presented. In depth analysis is provided on one of these two phase (nanotube and matrix) models, the Anumandla-Gibson model, originally published in 2006. A discussion of its strengths and sensitivities is given, with numerical data to support the conclusions. It is extended to three-phase composites through the use of classical laminated plate theory. A literature survey is conducted to gather published two and three-phase experimental results for comparison. Two phase experimental results agree well with the present model, whereas three phase data was limited, but initial comparisons were promising.</p>

<p> III-V self-assembled quantum dots (QDs) and quantum dashes (Q-dashes) grown by epitaxy have numerous applications for optoelectronics and quantum information. Such nanostructures are most commonly formed through strain-driven self-assembly on (001) surfaces. In this process, a thin layer of material deposited under compressive strain reorganizes into three dimensional islands. While compressive self-assembly on the (001) face produces QDs across a wide range of semiconductor materials, few successful reports have addressed QD growth under tensile strain or on other low-index surfaces. Growth of tensile-strained QDs tends to produce dislocations that impair their optical properties. This problem likewise occurs for QD attempts on (111) or (110) surfaces.</p><p> QDs grown under tensile strain or alternative surface orientations would exhibit previously unavailable properties, while providing access to new QD materials for novel optoelectronic devices. Most prominently, tensile strain strongly reduces the bandgaps of nanostructures, allowing them to emit light at much lower energies than they could under compressive strain for long wavelength optoelectronics. Secondly, QDs grown on (111) surfaces are promising candidates for generating polarization-entangled photons. The high electronic symmetry achievable in (111) QDs produces an ideal exciton fine structure for the emission of entangled-photon-pairs. Alternative techniques have been proposed to produce tensile nanostructures and (111) QDs, but these often involve complex processing requirements that lack the simplicity of strain-driven self-assembly.</p><p> To achieve dislocation-free growth of the desired QDs, a growth model is employed that describes the relationship between dislocation nucleation, surface orientation, and strain direction (tensile or compressive). This model shows that both tensile growth on the (001) surface and compressive growth on (111) or (110) surfaces suffers from low dislocation nucleation energy. Instead, dislocation-free QD growth can be achieved by combining the use of tensile strain with a (111) or (110) substrate.</p><p> Using this principle, the present work demonstrates the growth of tensile strained GaAs QDs and Q-dashes, using In_0.52Al_0.48As barriers, grown on IP (110), (111)B, and (111)A substrates by molecular beam epitaxy (MBE). The effects of growth conditions on self-assembly are investigated for each surface orientation, and these trends are utilized to tune the size, shape, and density of the nanostructures. Observations of dislocation-free tensile QDs or Q-dashes on each surface orientation confirm the predictions of the growth model. As a result, strong room temperature luminescence is visible from the nanostructures grown on each surface.</p><p> Due to the high tensile strain, the GaAs nanostructures emit photons as low as 240 meV below the normal bandgap of GaAs. The large bandgap reductions achievable under tension are anticipated to extend QD and Q-dash devices into longer wavelength ranges that are difficult to achieve by other means. Next, by achieving highly symmetric QDs on the (111)A surface, very low exciton tine structure splitting values are observed &ndash; a key requirement for producing entangled photons. Tensile self-assembly thus offers a simple approach for the growth of entangled photon emitters on (111) surfaces. Finally, the results of these QD investigations are anticipated to apply broadly to zinc-blende and diamond-cubic semiconductors, enabling novel devices with a wide range of properties.</p><p> The growth of lattice-matched InAlAs epilayers on InP (110), (111)B, and (111)A substrates is also extensively studied in this work to produce high quality buffer and barrier layers for quantum nanostructure growth. In addition, the development of (110) and (111) semiconductors would allow access to their unique properties, including different alignments of the internal polarization field, compatibility with growth of hexagonal materials, access to different zones of the electronic bandstructure, and long spin lifetimes. Due to these properties, such epilayers are under current investigation to support spintronics, topological insulators, transition metal dichalcogenides, and novel MOSFETs. However, epitaxy on these surface orientations is very challenging due to the formation of hillocks and rough surfaces. Little information is available for growing these semiconductors, which limits the material quality that can be achieved.</p><p> To support emerging (111) and (110) applications, the effects of growth conditions on the morphological, electrical, and optical properties of InAlAs, InGaAs, and InP, grown on InP wafers, are systematically studied for each substrate orientation. Growth parameters are identified that either eliminate or strongly reduce morphological defects on each surface. Conditions for optimizing photoluminescence, carrier mobility, and background doping are also reported. This work therefore offers a comprehensive guide to overcoming material challenges for both epilayers and QDs grown on (110) and (111)-oriented InP substrates. </p>

<p>Hybrid organic/inorganic photovoltaics can overcome many traditional shortcomings of organic photovoltaics, including recombination due to short exciton diffusion length scales, incomplete or tortuous charge transport pathways, and low charge mobility. In this work, aligned pore arrays are electrochemically etched into GaN films, and the semiconducting polymer polyhexylthiophene (P3HT) is intruded into these porous films. This hybrid device uses the polymer as the photoactive phase, electron donor, and hole transport medium, and the GaN as the electron acceptor and electron transport medium. Not only does the nanoporous geometry result in ultrafast charge transfer between the P3HT and the GaN, but a nanoconfined geometry can also drastically enhance charge mobility in the polymer by orienting the polymer alignment such that the fast charge transport direction is oriented vertically.</p><p> Optimal etching parameters are found in various etchants to produce an aligned morphology, and a method to remove the low-porosity overlayer via UV-assisted etching is described. Additionally, the first reported pore formation in GaN using a neutral etching solution is demonstrated, opening up the possibility of safe and environmentally-friendly etching of GaN, in contrast to traditional methods that use extremely toxic hydrofluoric acid.</p><p> Multiple methods to introduce polymer into the pores are described, and it is shown that hot pressing can achieve favorable polymer alignment. Ultrafast charge transport is demonstrated between the confined polymer and the GaN template by time-resolved terahertz spectroscopy. This geometry of an aligned nanoporous template surrounding an organic semiconductor is proposed as a general and beneficial strategy to improve performance of organic solar cells. </p>

Thesis (Ph. D.)--University of Toronto, 2008. / Includes bibliographical references.

Thesis (Ph. D.)--University of Toronto, 2008. / Includes bibliographical references.

<p> As technology becomes more integrated into our daily lives, the need for machines that can safely and comfortably interact with the human body has grown. While the rigidity of traditional robotic materials, such as metals and plastics, can provide mechanical and electrical stability to these devices, it can also reduce safety and comfort when placed in contact with soft human tissue. In recent years, these issues have been addressed by incorporating compliant materials, like liquids or soft polymers, into wearable or biomedical devices. However, these materials, by virtue of their softness, cannot support the high loads required for operations like stabilization or gripping. To address this apparent trade-off between load-bearing stiffness and conformable softness, several groups have constructed stiffness-tuning devices, capable of alternating between a high-stiffness state and a low-stiffness state. Although there exist a wide variety of mechanisms by which we can achieve this switching behavior, thermally activated phase change provides the highest stiffness ratio between the soft and stiff states. In this work, use low-melting point conductive thermoplastics to create electrically activated stiffness-switching devices. When a voltage is applied across this thermoplastic, the resulting electric current causes the polymer to heat and melt. This phase change corresponds to an effective stiffness change. </p><p> In the first study, we introduce a novel stiffness switch layout that employs liquid metal as compliant electrodes oriented across the face of a conductive thermoplastic. This new layout results in an 80% decrease in required voltage, a 60% decrease in activation time, and the ability to switch the stiffness of arbitrary geometries. </p><p> In the second study, we examine the effects of the composition of a conductive thermoplastic composite on its stiffness-switching properties, and use these findings can help guide the design of stiffness-switching composites for a three soft robotic applications.</p><p>