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This thesis concerns the study of amorphous photonic crystal structures: disordered photonic lattices possessing short-range but not long-range order. In the first part of the thesis, I study amorphous structures intended for electrically-pumped random quantum cascade lasers (QCLs). Random lasers are a class of lasers which, unlike traditional lasers, have no well-defined and well-structured cavity; lasing takes place through multiple scattering from a random medium. I design and analyze amorphous lattices of dielectric rods, joined by dielectric veins, which support high quality factor (Q factor) lasing for transverse magnetic (TM) modes. Simulations show that these structures can achieve 2D Q-factors of as large as 10^5, which is not possible using previous designs based on air holes etched in dielectric slabs. In the second part, I perform a numerical study of an amorphous variant of a “photonic topological insulator” (PTI). PTIs are photonic crystal structures with band structures analogous to topological insulator materials, supporting robust electromagnetic edge states that propagate unidirectionally along the edge. Although the existence of these edge states is justified by theoretical arguments that assume long-range order, I show that similarly robust edge states exist in amorphous lattices, which possess only short-range order. This study opens new possibilities for studying disorder in PTIs. In the third and final part, I develop a computational method called "FLAME-slab", which is capable of efficiently solving the wave scattering problem for amorphous slab structures, where scattering elements are distributed within a slab with short-range order. Compared to standard finite element or finite difference methods, the FLAME-slab method allows for much coarser meshes, with as few as three grid layers spanning the thickness of the slab. Solutions are generated based on the Flexible Local Approximation Method (FLAME), using Rigorous Coupled Wave Analysis (RCWA) as a subroutine to generate local basis functions. ​Doctor of Philosophy (SPMS)

The aim of this thesis is to study a new kind of energy storage solution known as “Hybrid Electrochemical capacitor” which allows for the integration of two existing technologies Lithium ion batteries and supercapacitors. The resulting device has the promising properties of both these devices, and can further be fine-tuned to meet the demands of autonomous industry vehicles and Plug-in Hybrid Electric Vehicles. The thesis achieves the above aim by the development and study of electrode materials that can meet the requirements of hybrid electrochemical capacitors. Initial studies are based on the narrowing down of appropriate intercalation materials which can be used as electrodes for a hybrid capacitor. This is achieved in three steps. A benchmark is set using a high Lithium ion conducting type electrode followed by fundamental understanding of the effect of electrode polarity on the energy density of hybrid capacitors. A working prototype cell is constructed using the above rule. Further improvements on the hybrid capacitor are planned by the use of high capacity Lithium rich layered materials. Apart from battery electrode, the thesis looks to develop activated carbon from a biodegradable source with low cost and low environmental impact. The biowaste material chosen in this thesis is human hair. Human hair was chosen because it is cheap to obtain and environmentally benign also the hair follicles are composed of loosely attached flakes which present a possibility of high surface area activated carbon. Dependence of the activation mechanism on the surface area and pore size distribution is studied. The final aim of this section is to develop a supercapacitor material with a high surface and optimum pore size distribution so that the energy stored in the capacitor electrode can be maximised. All the materials studied during the course of this thesis were carefully characterised to study structural morphological and electrochemical properties Doctor of Philosophy (MSE)

Reliability of smart cards depends extensively on the corrosion resistance of the electrical contacts they contain as these contacts are primarily exposed to the environment. Although corrosion as an entirety is a century old problem, the corrosion of thin electrical contacts has been rarely studied in a systematic manner. Cu is typically used as substrate material for electrical contacts and nickel based barrier layer coating have been essential to suppress copper diffusion through the gold topcoat. This thesis aims to bridge knowledge gaps in literature with regards to the corrosion behaviour of Ni and Ni-P barrier coated electrical contacts. Much work has been dedicated to the corrosion resistance of electroless Ni-P deposits but research on electrolytic deposits is limited. The impact of electrodepositing parameters on corrosion resistance has not been well documented. This thesis aids the understanding of the electrodepositing parameters and their implications on corrosion resistance. Moreover, corrosion behaviour due to different arrangement of thin Ni and Ni-P stacks (< 3 μm) is unknown. This thesis also explores different thin Ni and Ni-P stacks and its corrosion resistance to salt spray (SS) and mixed flowing gas (MFG) environments. The corrosion behaviour of these stacks, corrosion mechanism and causes of corrosion disparity have been also discussed in this report. The thesis is bounded by three main considerations viz., method and type of deposition, and corrosion testing methods. Electrodeposition was the choice of deposition technique, Ni and Ni-P electrodeposits were chosen barrier layers and lastly, accelerated tests which simulate atmospheric corrosion were employed. The research starts with a systematic study on the electrodepositing parameters and their influence on the corrosion resistance of electrical contacts. Corrosion products due to the neutral salt spray (NSS) and mixed flowing gas (MFG) tests were characterised through spectroscopic and diffractions studies. Electrical contacts exposed to NSS tests produced green corrosion residues which consist of CuCl (nantokite) and CuCl2(OH)3 (clinoatacamite) and brown residues which consist of Cu2O (cuprite). Electrical contacts exposed to MFG test produces copper sulfides (major) and nickel sulfides (minor). Also increasing the thickness of Ni consistently improved the corrosion resistance in both neutral NSS and MFG corrosion tests. However, the influence of Ni thickness on corrosion was overshadowed by the Ni-P thickness. Increasing the thickness of Ni-P deposits resulted in higher corrosion in the NSS environment and decreased corrosion under the MFG environment. Although initial results pointed towards a disparity that Ni-P may not be suitable for corrosion protection in environments with heavy chlorine concentration but appropriate for sulfur-containing corrosion media, subsequent in-depth studies revealed that this disparity was due to the high edge porosity caused by fast plating. Subsequently, the arrangement of Ni and Ni-P on Cu was hypothesised to affect the eventual corrosion resistance of an electrical contact. The thickness of the overall barrier layer was kept constant and different stacks such as Ni/Au (NA), Ni-P/Ni/Au (PNA), Ni-P/Au (PA), Ni/Ni-P/Au (NPA) and Ni-P/Ni/Ni-P/Au (PNPA) stacks were examined. The discrete and coalesced pits formed due to MFG were explained with a proposed dominant pit concept. Results showed that multi-layer stacks, such as PNA, NPA, PNPA, displayed worse corrosion resistance than single-layered stacks (NA, PA). Removing Ni and Ni-P interfaces was effective in inhibiting corrosion in NSS and MFG environments as Ni/Ni-P interfaces accelerate corrosion due to galvanic coupling. Corrosion pits propagated horizontally and vertically through Ni and Ni-P films, depending on the way Ni and Ni-P were arranged within a stack. Tunnelling corrosion through Ni-P turned distinct with a reduction in Ni-P thickness. This was attributed to possibly higher percentages of discontinuities (i.e. porosity) with decreasing thickness. The reducing thickness coupled with the galvanic coupling between Ni and Ni-P layers further worsened the corrosion in stacks such as Ni-P/Ni/Ni-P/Au. Based on the two aforementioned studies, disparity in corrosion resistance of Ni and Ni-P barrier layers under NSS and MFG corrosion tests was a recurring observation. When corrosion resistance of Ni/Au and Ni-P/Au stacks was assessed using NSS and MFG corrosion tests, each stack emerged superior in only one test. To explain this disparity, factors that could influence corrosion resistance were examined first, namely: internal stress, surface wettability and sulfur co-deposition. None of these factors could explain for the disparity in corrosion performance. Corrosion product analysis was then performed to understand the corrosion product formation due to NSS and MFG. Detailed pits analysis revealed that the disparity was caused by the porosity and corrosion product migration. These concepts were supported experimentally. Based on the understandings generated from the three main studies, some guidelines are proposed to improve the corrosion resistance of electronic contacts under the MFG and NSS corrosion tests. Optimisation of various parameters to generate the current stacking arrangement (i.e. Ni/Ni-P/Au) offer important but limited solution to enhance corrosion protection as individuals layers behave differently when coupled. Different stacking arrangements studied in this thesis show that single-layered stacks such as Ni/Au or Ni-P/Au offer better corrosion protection. The failure of bi-layered and tri-layered stacks does not necessarily stem from the design of such stacks but stem from the industrial practice to layer very thin coatings (such as Ni-P at 500 nm) on Ni under-plates. The increased defects in such thin Ni-P coatings accelerate galvanic corrosion due to difference in corrosion potentials and the small-anode-large-cathode phenomenon. The study on single layered stacks points to the most important aspect of this thesis. Ni-P as a single barrier layer deposited at lower current densities (≤ 1 ASD) and at higher thicknesses (≥ 2 µm) can improve overall good corrosion protection in both NSS and MFG corrosion tests. Doctor of Philosophy (MSE)

The thesis demonstrates for the first time coexistence of write once read many memory (WORM) with resistive random access memory (RRAM). This is the first time such coexistence phenomena are reported in organic materials system. Furthermore, better understanding of working mechanism in resistive switching phenomena in organic materials particularly conjugated polymer has been achieved. Two materials systems are chosen, namely, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT PSS) and Polyvinyl alcohol (PVA) where PEDOT PSS is a conducting polymer and PVA is an insulator polymer. For a device with 5 μm thick PEDOT PSS sandwiched between two Au electrodes, the coexistence of WORM memory and RRAM memory can be achieved (Chapter 4). The WORM result was attributed to dedoping of the conducting PEDOT + PSS- segments to insulating PEDOT 0 segments. The RRAM was explained due to Proton H+ conductive properties of PSS acid domains inside the film after dedoping. This points to a general feature: resistive switching behavior of a whole device is the result of additive/summative resistive switching characteristic of its components. Based on this, we reveal the coexistence of RRAM memory and WORM memory demonstrated in PEDOT PSS blend with PVA (Chapter 5). The WORM memory is again attributed to PEDOT PSS while RRAM memory is attributed to the charge trapping characteristic of PVA. The device performance is improved significantly as compared to the earlier PEFOT PSS system, with more than 1000 cycling stability and much more than 10^4 s retention for RRAM mode. Additional I-V characteristic of this device is further studied in chapter 6 which reveals a new type of WORM memory. This type of WORM memory is driven by metallic migration of Au electrodes. Doctor of Philosophy (MSE)

Transparent glasses are hotly pursued as premium construction materials in transport vehicles and architectural buildings for its aesthetic appeal. Due to widespread usage of transparent glasses, 43% of global world primary energy are spent on heating, ventilation and air-conditioning in buildings. Moreover, individual can also experience uncomfortable situations, such as glaring sunlight, skin and thermal discomfort. Current practice of using multi-layered optical solar films for solar shielding requires costly fabrication process and further results in low visibility and disruption of communication signals. Although solar modulation system driven by external stimuli have been demonstrated, these systems usually involve costly installation, complex device fabrication process and rely on external mechanical or electrical stimuli. Hence, an autonomous system driven by solar heating itself to exhibit broadband solar modulation would be ideal. The closest class of material that could perform in such manner is the thermally responsive hydrogel, a material containing around 90% of water. Unfortunately, their chances of commercialization are largely detracted by their inherent shrinking and aqueous characteristics. In this thesis, a new class of phase transition material that can respond to temperature change was developed. With the increase in temperature, different degree of reversible solar modulation can be achieved and tailored accordingly to a wide range of operating temperatures. The components in this new material consists of ionic liquids which are intrinsically stable and polyurethane elastomer which is not only cost effective, but further endow the material with mechanical robustness. The distinct advantages of broad transition range tunability, cyclic stability and functional versatility in the material system is unattainable by any existing solar modulation solution. This is the first time where ionic liquids were explored as material for thermotropic smart window and the resulting new material super cyclic stability with critical non-volatile and non-freezing properties can be attributed to the superior intrinsic characteristics of ionic liquids. This new material also allows facile and unparalleled tuning range of transition temperature (tuning via multiple parameters). The mechanism of the underlying thermally responsive interactions in this new class of material was extensively elucidated and applied in smart window application for the very first time. In a parallel study, plasmonic particles which exclusively absorb in near infrared region was coupled with thermally responsive hydrogel. Experimental results show that both the response speed and solar modulation of the thermotropic composite could be enhanced. Photothermal characteristics endowed by the plasmonic particle not only enhance the thermal insulation features for tropical climates but further enable optical switching in cold weather climates. The mechanism of plasmonic heating coupled with thermally responsive polymer for smart window was reported for the very first time. Combination of the above novel technologies offers a unique competitive advantage and result in a new class of thermally responsive organic/inorganic hybrid which can be affixed as an interlayer laminate for a fully autonomous smart window to achieve energy conservation. Apart from their application in transport vehicles and architectural buildings, this new material may also find itself in other numerous optical applications, such as over-heating protector for solar panels and other smart devices with memory or sensing functions. It is believed that the novel outcomes from this research thesis will fit neatly into the technological gap concerning green and smart buildings. It will also be propelled by the rapid development and pressing demand of innovative solutions in a highly competitive automotive industry. Doctor of Philosophy (MSE)

Stretchable energy storage devices are prerequisites for the realization of autonomous elastomeric electronics. Microsupercapacitors (MSCs) are promising candidates for this purpose due to their high power and energy densities, potential for miniaturization, and feasibility of embedding in circuits; however, efforts to realize stretchable MSCs have mostly relied on strain-accommodating materials and have suffered from limited stretchability, low conductivity, or complicated patterning processes. Here, we designed and fabricated a stretchable MSC based on reduced-graphene-oxide/Au heterostructures patterned by facile and versatile direct laser patterning. An interconnected and stable 3D network composed of vertically oriented heterostructures was realized by high-repetition-rate femtosecond laser pulses. Upon transferring to polydimethylsiloxane (PDMS), the 3D network achieved a high conductivity of ~105 S m−1, and the conductivity could be maintained at ~104 S m−1 even at 50% strain. A fully laser-patterned stretchable electronics system was integrated with embedded MSCs, which will find applications in soft robotics, wearable electronics, and the Internet of Things. By transforming flat graphene oxide films into vertically oriented sheets, researchers have constructed an energy storage device that may power wearable electronics. Graphene oxide’s ability to quickly store and release charge from its surface makes it attractive for supercapacitors, which recharge significantly faster than conventional batteries. Young-Jin Kim and Pooi See Lee from the Nanyang Technological University in Singapore and colleagues have now used ultrafast lasers to controllably heat graphene oxide into a spiky 3D network that accommodates large amounts of mechanical strain. Following deposition of a gold coating and subsequent transfer to an elastomeric substrate, the team’s supercapacitor retained its high energy output even after being rolled, bent, twisted, and crumpled hundreds of times. The laser-based fabrication enabled production of customized patterns, such as wavy electrode ‘fingers’ that can attach to curved surfaces. An intrinsically stretchable and highly conductive graphene electrode based on vertically oriented graphene/Au bilayer flakes are successfully fabricated by a direct-laser-patterning at a very high-repetition rate and fast scanning speed of laser with a femtosecond pulse, allowing micro-supercapacitors to be integrated with the complete soft electronics systems that can be standalone and be customized by user’s circuit designs.

The creation of crystal phase heterostructures of transition metal chalcogenides, e.g., the 1T/2H heterostructures, has led to the formation of metal/semiconductor junctions with low potential barriers. Very differently, post-transition metal chalcogenides are semiconductors regardless of their phases. Herein, we report, based on experimental and simulation results, that alloying between 1T-SnS2 and 1T-WS2 induces a charge redistribution in Sn and W to realize metallic Sn0.5W0.5S2 nanosheets. These nanosheets are epitaxially deposited on surfaces of semiconducting SnS2 nanoplates to form vertical heterostructures. The ohmic-like contact formed at the Sn0.5W0.5S2/SnS2 heterointerface affords rapid transport of charge carriers, and allows for the fabrication of fast photodetectors. Such facile charge transfer, combined with a high surface affinity for acetone molecules, further enables their use as highly selective 100 ppb level acetone sensors. Our work suggests that combining compositional and structural control in solution-phase epitaxy holds promises for solution-processible thin-film optoelectronics and sensors., Controlling the composition and crystal phase of layered heterostructures is important. Here, the authors report the liquid-phase epitaxial growth of Sn0.5W0.5S2 nanosheets with 83% metallic phase on SnS2 nanoplates, which are used as 100 ppb level chemiresistive gas sensors at room temperature.

The coupling of the localized surface plasma resonance (LSPR) between noble metals of Au, Ag and Cu and semiconductors of Cu2 xE (E ¼ S, Se, Te) opens new regime to design photothermal (PT) agents with enhanced PT conversion efficiency. However, it is rarely explored on fabricating of engineered dual plasmonic hybrid nanosystem for combinatory therapeutic-diagnostic applications. Herein, rattle-type Au@Cu2 xS hollow mesoporous nanoparitcles with advanced PT conversion efficiency are designed for cellular vehicles and chemo-photothermal synergistic therapy platform. The LSPR coupling between the Au core and Cu2 xS shell are investigated experimentally and theoretically to generate a PT conversion efficiency high to 35.2% and enhanced by 11.3% than that of Cu2 xS. By conjugating microRNA (miRNA) gene probe on the surface, it can realize the intracellular oncogenic miRNA detection. After loading of anticancer drug doxorubicin into the cavity of the Au@Cu2 xS, the antitumor therapy efficacy is greatly enhanced in vitro and in vivo due to the NIR photoactivation chemo- and photothermal synergistic therapy. The rattle-type metal-semiconductor hollow mesoporous nanostructure with efficient LSPR coupling and high cargo loading capability will be beneficial to future design of LSPR-based photothermal agents for a broad range of biomedical application. MOE (Min. of Education, S’pore) Accepted version

Vanadium dioxide (VO2) nanoparticles with reversible semiconductor-metal phase transition holds the tremendous potential as a thermochromic material for the energy-saving smart glazing. However, the trade-off between improving the luminous transmittance (Tlum) while sacrificing the solar modulation ability (ΔTsol) hampers its bench-to-market translation. Previous studies of anti-reflection coatings (ARCs) focused primarily on increasing Tlum while neglecting ΔTsol, which is a key energy-saving determinant. The intrinsically low ΔTsol (

The monoclinic scheelite‐type BiVO4 is recognized as one of the promising candidate materials for a photoanode because of its 9.1 % theoretical efficiency for half‐cell solar‐to‐hydrogen conversion. Although significant research efforts have been devoted to improving the performance of the photoelectrochemical cell (PEC) of this material, they have mainly been in small anode areas with only a handful of studies on scaled‐up sizes. Herein, a facile metal–organic decomposition synthesis method was used to produce scaled‐up Mo‐doped BiVO4 photoanodes. Multiple modifications were explored and incorporated to enhance the performance of the photoanode. A large‐area (5 cm×5 cm) photoanode was successfully prepared with all modifications. The resulting photoanode gave rise to an initial photocurrent density of 2.2 mA cm−2 at 1.23 V versus reversible hydrogen electrode, under AM 1.5G illumination in a PEC, which remained at 79 % of this value after 1 h of operation. A deleterious effect of the increased anode surface area on the photocurrent density was observed, which we termed the “areal effect”. Understanding the reasons for the areal effect is indispensable for the development of large‐scale PEC devices for water splitting. NRF (Natl Research Foundation, S’pore) Accepted version

Choroid thinning occurs in age-related macular degeneration (AMD). However, it remains unclear whether the reduction is due to reduction in choroidal vessels or shrinkage of choroidal stroma, or both. The purpose of this study was to evaluate the changes of the choroidal vascular and stromal area in 118 patients with typical AMD (t-AMD) and polypoidal choroidal vasculopathy (PCV) over a 12-month period. We used spectral-domain optical coherence tomography (SD-OCT) with enhanced depth imaging (EDI) mode to measure the subfoveal choroidal thickness (CT), central retinal thickness (CRT) and choroidal vascularity index (CVI - ratio of luminal area to total choroidal area). At baseline, PCV eyes had higher CRT (471.6 µm vs 439.1 µm, p = 0.02), but comparable subfoveal CT and CVI, compared to t-AMD. Eyes with high CVI at baseline showed marked reduction in stromal area compared with eyes with average or low CVI. Over 12 months, CRT and subfoveal CT significantly decreased (p

To meet the growing demand for energy and to reduce the depletion of fossil resources, technology for the harnessing of renewable energy is urgently needed. Among all the renewable energies, solar energy is one of the most clean, abundant and sustainable energy sources that can help to solve the world energy consumption problem. Harvesting and conversion of solar energy to electrical energy required the use of solar cells. Of all the different solar cell technology, copper-indium-gallium sulfide/selenide (CIGS) solar cell is one of the most promising thin film solar cells because of its high optical absorption coefficient, suitable band gap for harvesting a wide range of visible light from the sun as well as some unique electrical and optical properties. Compared to conventional vacuum-based synthesis, there is strong push towards the use of solution-based methods to prepare these CIGS films so that the processing cost can be reduced significantly. In this thesis, the solution-based synthesis of CuIn(S,Se)2 thin films as well as crystals and their application in the solar energy harvesting will be presented. Generally, for the solution-based synthesis routes, selenization/sulfurization process is usually needed to incorporate Se/S, to increase the crystallinity and to encourage grain growth. However, selenization/sulfurization is toxic and requires special processing system to cater for safety. Therefore this thesis sets out to investigate methods to synthesized absorber films without any selenization/sulfurization process and also ways to increase grains size. CuInS2, CuInSe2 and CuInSSe thin films were successfully synthesized without any selenization/sulfurization process. In this work, the thin films were prepared using a vulcanization process and then annealed. The vulcanization process occurred between the molecular precursors, when cross-links are formed between metal acetylacetonates and sulfur/selenium powder even before annealing. Using this method, CuInS2/CuInSe2/CuInSSe thin films could be formed at a relatively low temperature of about 350oC. It was found that crystallinity and morphologies of CuInS2/ CuInSe2/ CuInSSe could be improved by increasing the annealing temperature and annealing time. Raman characterization confirmed that no secondary phase was formed. CuInS2 was selected for the demonstration of solar cell because its film forming ability is better compared to CuInSe2 and CuInSSe This is because the poor solubility of Se in pyridine compared to S. However, since there is no high temperature selenization/sulfurization involved, the grain size of CuInS2 is still relatively small compared to those highly efficient Cu(InGa)(S,Se)2 solar cells. The solar cell efficiency for these CuInS2 is comparable to those Cu(InGa)(S,Se)2 solar cells without selenization/sulfurization and can be further improved by moderating the composition and also increasing the grain size. As mentioned earlier, large CIGS grains will result in films that showed better solar cell performance and one of the reasons is that they have fewer defect sites and recombination centers. As a route to larger grain size, nanoparticles is first synthesized and by encouraging growth in these individual CIGS nanoparticles, micron sized grains can be achieved. CuInSe2 nanoparticles were synthesized by solvothermal reaction at 180oC for 24hrs/48hrs. The effects of precursor concentration and type of solvents on the composition and morphology were investigated. Three different solvents were chosen: ethanol-DMF mixture, methanol-DMF mixture and water-DMF mixture. It was found that CuInSe2 nanoparticles could be obtained from both methanol-DMF mixture and water-DMF mixture but methanol-DMF mixture will result in higher purity CuInSe2 nanoparticles. Size and morphology of the nanoparticles could be tuned by precursor concentration. Raman spectroscopy revealed that although no secondary phases were formed, Cu-Au (CA) phase, which is believed to be harmful for device, existed in the synthesized CuInSe2. The detrimental CA phase can be reduced by increasing the reaction time and the precursor concentration. Photocurrent measurement indicated good photoresponse from the as-synthesized CuInSe2 and CuInSe2 with less CA phase showed improved electrical performance. As mentioned previously, larger grain size is preferred for good solar cells performance. Micron-sized CuIn(S,Se)2 grains were obtained by allowing the CuInSe2 nanoparticles to undergo a second solvothermal reaction step. TEM revealed that micron-sized plates are formed (more than 1μm) and the distribution of different elements is uniform in these plates. The mechanism for the formation of micrograin CuIn(S,Se)2 from CuInSe2 nanoparticles was investigated. In addition, the phases of final products after second-step solvothermal reaction with different reaction time were also studied by XRD and rietveld refinement. Photochemical studies on CuInSe2 and CuInSSe was carried out to investigate the electrical properties and photoresponse. CuInSSe is found to have a bandgap closer to the optimal value and larger carrier concentration. Solar cells fabricated using both CuInS2 thin film and CuInSSe microcrystals showed a better performance compared using just CuInS2 thin film. Our simple, environmental friendly synthesis and fabrication processes suggest a promising potential for the future solar harvesting applications. DOCTOR OF PHILOSOPHY (MSE)

Through millions of years of evolution, living organisms have refined a wide range of biomineralized tissues to meet the key functional requirements that are central to their survival. This includes load-bearing elements and protective armor that must display adequate stiffness and strength or sharp and cutting “biotools” that are key in predation and feeding. These mineralized tissues exhibit multi-scale hierarchical structures to meet their functional requirements, with a precise organization of their building blocks to optimize the combination of their mechanical properties including stiffness, strength, and fracture resistance. A distinctive model system that has gathered recent interest is the dactyl club from stomatopods (mantis shrimps). In comparison to many biomineralized composites that play a passive (defensive) mechanical role, dactyl clubs are dynamically active, hammer-like devices which are used by stomatopods to shatter the hard shells of their prey through repetitive impact loading. The mantis shrimp appendage is one of the most fascinating multifunctional biological material “biotools” in the animal kingdom, which the animal uses for its aggressive predatory strategies. Its hierarchical structure helps to strike and catch its prey 50 times faster than the blink of an eye, while exhibiting exceptional damage tolerance properties. In this research project, the multi-scale hierarchical structure and chemical composition of mantis shrimp appendages was comprehensively probed by various analytical techniques. It is demonstrated that dactyl impact surface consists of a finely-tuned mineral gradient, with fluorapatite substituting amorphous apatite towards the outer surface. Raman spectroscopy measurements show that calcium sulphate, previously not reported in mechanically active biotools, is co-localized with fluorapatite. Ab initio computations suggest that fluorapatite/calcium sulphate interfaces provide binding stability and promote the disordered-to-ordered transition of fluorapatite. Hertzian contact partial loading-unloading indentation measurements revealed that the different layers of the club possess distinctive deformation and energy dissipation mechanisms. High-resolution electron microscopic studies show that sliding and rotation of fluorapatite crystallites leads to a quasi-plastic response in the outer, impact layer of the club. On the other hand, the presence of micro-channels along mineralized chitin fibrils in the inner layer of the club results in a distinct contact mechanics response. Under hydrated conditions, densification of these micro-channels under compressive load leads to a strain hardening behavior in the inner layer of the club. The work demonstrates that the macroscopic size of the club is below the critical size above which Hertizan cone fracture could be formed. Instead, the club’s response remains in the quasi-plastic regime. DOCTOR OF PHILOSOPHY (MSE)

Semiconductor based energy conversion from solar energy to chemical energy is one of the most possible solution for the severe worldwide energy crisis and followed environmental issues. However due to the low efficiency, the commercial application of semiconductor based energy conversion is still limited and further modification for better performance is required. Hence this thesis tries to improve the photocatalytic property of semiconductor by several kinds of modifications including control synthesis of TiO2 with 3D hierarchical structure, loaded Pt nanoclusters with discrete energy levels and combination of TiO2 and Cu2O with different exposed crystal planes. Nano-flower like rutile TiO2 hierarchical structures have been synthesized by a solvent-thermal method. The structure of the nano-flowers was carefully characterized via various techniques, such as XRD, Raman spectra, UV-Vis diffuse reflectance spectra, XPS et al. we found that the building blocks of such nano-flower structures are single-crystalline rutile TiO2 nanorods with their growth along [001] axis and exposed (110) facet on nanarods’ side walls. Owing to this hierarchical nanostructure, this rutile TiO2 showed enhanced photocatalytic activity for the selective oxidation from Benzylamine to N-BIBL. This suggests the great potential of this 3D highly ordered hierarchical structure in many other photocatalytic applications, such as PEC or DSSCs. It is well accepted that Pt is the best cocatalyst for photocatalytic H2 generation from water splitting. However, most consumed Pt is nanoparticles but not Pt nanoclusters with discrete energy levels which show totally different properties due to quantum size effect. In this thesis Pt nanoclusters protected by L-glutathione reduced (GSH) are deposited on the surface of anatase TiO2 with enhanced photocatalytic activity and stability. It is suggested that the synergistic effect of TiO2 and Pt nanoclusters is crucial for the improved photocatalytic performance. And the size of prepared Pt nanoclusters shows great influence on the photocatalytic performance. Pt nanoclusters open a door for better modification of TiO2 by tuning property of co-catalyst atom by atom. Besides the controlled modification to obtain fancy morphology of TiO2 or decorated TiO2 with suitable cocatalysts, facet controlled combination of TiO2 with other semiconductor (Cu2O) is achieved. TiO2 is successfully deposited on the surface of Cu2O with different morphologies which exposed different crystal planes. The combination enhances the photocatalytic activity of the nanocomposites and improves the stability of the TiO2 hybrid Cu2O photocatalysts. In conclusion, we developed different modification strategies for pristine TiO2 to obtain improved photocatalytic activity. These researches on the modification strategies and their positive influence on the photocatalytic performance of the modified TiO2 based photocatalysts suggests the rational design for the nano-structures of these catalysts is very crucial for the acquisition of desired catalytic properties of these catalysts. And this thesis show the promising prospect of the rational designed complex ordered nanostructures and their diverse applications in photocatalysis. DOCTOR OF PHILOSOPHY (MSE)

N-heteroacenes with the CH groups in the backbone of acenes replaced by N atoms are expected to own charming semiconducting properties and potential practical applications in organic electronics. The electron-deficient N-heteroacenes can be used as acceptor moieties in D-A molecules or n-type semiconductors depending on the application requirements. Although the researchers in all over the world are devoted to developing commercially available organic electronics, the study of organic memories, photovoltaics (OPVs) and field-effect transistors (OFETs) can still not stride out laboratory conditions, and the ultimate organic materials, which can satisfy all requirements for practical applications in the three types of devices are in high desire. Inspired by this gap, this dissertation focuses on design and synthesis of novel soluble N-heteroacenes and investigation of their applications in organic memories, OPVs and OFETs. For organic memories, the molecules have been designed based on two assumptions about multiple electron “traps” or multiple electrons intermolecular charge transfer, and accordingly, different electron-deficient N-heteroacene units are combined together to induce multiple electron “traps” or strong electron-deficient N-heteroacenes are utilized to accept multiple electrons. The memory devices have been fabricated and the corresponding switching behaviors, the ON/OFF ratios, the endurance and retention performance have been evaluated. Molecular calculation has been carried out to study the structure-property relationship. In summary, an efficient molecular designing strategy to approach rewritable, multilevel organic memory materials has been developed. For OPVs, the D-π-A structure molecules are designed with large conjugated N-heteroacene as acceptor moiety and linked to widely investigated donor units by π-bridges. The large conjugation of N-heteroacenes is expected to induce broad absorption and facilitate charge carriers’ transport. The typical BHJ architecture OPV devices have been fabricated and the corresponding PCE, Voc, Jsc, and FF are investigated. Morphology analysis of the organic films has been carried out to study the effect of molecular stacking on the device performance. In summary the as-designed molecule has showed PCE up to ~2%, which is expected to own better performance after structure modification. For OFETs, large conjugated N-heteroacenes with deep LUMO level lower than -4.0 eV has been designed. The single-crystal OFET devices have been fabricated and the corresponding transfer and output curves, VTH, ON/OFF ratios and field-effect mobility are investigated. Theoretical calculation has been conducted to study the ideal mobility of molecules and structure-property relationship. In summary a new family of N-heteroacenes have been developed for air-stable, high performance n-type OFETs. DOCTOR OF PHILOSOPHY (MSE)

The creation of thick cardiac tissue for scar replacement therapy following myocardial infarction (MI) has been hindered by the lack of functional and enduring vascular supply. Most cells do not survive more than a few hundred of micrometers diffusion barrier away from the blood vessel in vivo, hence tissue suffering from lack of oxygen and other nutrients experience ischemia and necrosis. Effective vascularization process requires the proper selection of cell types, scaffold containing inherent vasculature infrastructure with suitable mechanical and biochemical properties, and optimized culturing conditions. Since the process of re-vascularization involves the interaction and collaboration of more than one cell type, it is critical to understand the factors affecting the growth dynamics of the co-culture system to direct the process toward a desirable regeneration. Mesenchymal stem cells (MSC) have been used in various engineering applications owing to their regeneration and differentiation capabilities as well as their therapeutic potentials. Efforts have been made by applying both MSC and human umbilical vein endothelial cell (HUVEC) in co-culture system for vasculature development and regeneration, however, no comprehensive studies exist to address and analyze the effects of culturing parameters on the population dynamics. In this thesis, we suggest a modified Lotka-Volterra model to quantitatively describe the population dynamics of co-cultured cells under the influence of different culturing parameters, including cell ratio, medium compositions, and culturing time. This model, commonly referred to describe the prey-and-predator behavior of subpopulations sharing the same closed ecological niche, can now be used to predict the population dynamics in cell co-culture systems under given culturing conditions. The empirical results indicate that under most conditions, endothelial cell (EC) growth was inhibited by their own species but promoted by MSCs, which coincides perfectly with the model prediction. Similar results were also observed when cells were cultured on more complex 3-D acellular extracellular matrix (ECM) scaffolds, derived from porcine left ventricle tissue (pcECM). The decellularized thick pcECM exhibits advantages such as comprehensive 3-D architecture with well-preserved vasculature framework, mechanical and chemical properties that are comparable to native tissue, and ECM attachment proteins that are normally missing in synthetic materials. The use of decellularized animal ECM scaffolds for tissue engineering is not new, but vascularization of cardiac derived ECM remains a critical problem for long term survival of thick and clinically relevant sized tissue constructs. In this study, we demonstrated the supportability of pcECM scaffold surface for the attachment and growth of HUVECs and MSCs, as model cells for angiogenic processes. Two approaches were carried out in parallel to improve HUVEC survival and proliferation on the pcECM scaffold: co-culture of HUVEC with MSC, and protein modification of the pcECM scaffold prior to cell seeding. Studies were carried out first on the surface of small samples (0.5cm2) for screening and optimization purposes, under static culture conditions, i.e. in the tissue culture plate. These studies were followed ex-vivo by dynamic culturing of more complex and clinically relevant thick pcECM patches in a mimicking physiological-like milieu. In the co-culture approach, we demonstrated the supporting function of MSCs in HUVEC attachment and proliferation, which further strengthens the findings of our mathematical modeling performed in standard tissue culture plates. The importance of seeding sequence for co-culture was also revealed. In the second approach, protein treatment of the pcECM was implemented with common attachment proteins, such as gelatin and fibronectin. In both methods, significant improvement of cell growth over time was observed. Finally, we applied the knowledge gained from the latter findings in a 2-D co-culture model and simplified small 3-D ECM scaffolds, on a much more complex thick ECM patch in a dynamic environment mimicking the physiological setting. This dynamic cultivation was provided by a custom built perfusion bioreactor system, which provides efficient medium circulation and shear stress to the cells, seeded within the vasculature. Cells were seeded on the preserved vasculature inherent to the thick acellular pcECM slabs. Confluent monolayers of endothelial cells lining the vasculature lumens were obtained by both sequential co-culturing approach and protein treatment approach following dynamic culturing for up to 21 days. These findings collectively validate the suitability of thick ECM patch for cardiovascular regeneration and replacement therapy. The population dynamics between MSCs and HUVECs were elucidated for the first time using a quantitative model, which could be extended for other co-culture cell models. The successful endothelialization of the thick cardiac tissue patch serves as a proof of concept with a promising potential for cardiac replacement therapy and other clinical applications. DOCTOR OF PHILOSOPHY (MSE)

171 p. Three-dimensional integration, which involves stacking devices vertically, promises several attractive benefits such as high functionality, performance and low system power consumption due to shorter signal delays with the elimination of long wires. One of the common bonding techniques used is metal fhermocompression bonding. The greatest stumbling block for using this technique is the need for high bonding temperature and/or pressure to create strong and reliable bonds. This thesis begins with a study on how temperature and pressure affect bonding characteristics and the mechanical integrity of direct gold joints. One important finding of these experiments is the identification of a critical temperature and pressure below which no bonds can be formed. The criteria for bonding to occur include the removal of surface barrier film between contacting sites through sufficient interfacial mechanical and chemical actions. Freshly exposed surfaces are thus activated thermally, leading to an increase in bond strength. Higher bonding pressure increases bond strength but to a maximum. With further geometrical measurements and fracture analysis, the bonding mechanism is elucidated. This thesis also presents, for the first time, a method to lower the required bonding temperature in gold thermocompression bonded joints with the help of self-assembled monolayers [SAMs]. A comparative study on alkanethiols of various chain lengths in influencing its bondability at several temperatures is evaluated. DOCTOR OF PHILOSOPHY (MSE)

Deformable electronics with the ability to conform to non-planar and curvature surfaces while keeping the functionality is under vigorous development recently to cater for the ever-growing and demanding need of people for mechanically compliant consumer electronics that could achieve human-machine interactions in a deformable way. Prompted by such needs, deformable electronics like wearable bands to capture signals for monitoring human health; stretchable optical projector or camera to realize virtual reality display and foldable smartphones that allows the ultra-portability and durability are under development in the market place as the next generation electronics to realize unprecedented functionalities unreachable for the conventional rigid electronics. As a very important component for the realization of human-machine interaction, a display system is greatly favored to feedback and transport information. Electrochromic display represents a non-emissive display genre with the advantage of high contrast, low power consumption and facile device assembly. The idea of adopting electrochromism for deformable display and information communication is still at its cradle and progressed in a limited fashion. In this study, the research is focused on feasible approaches and techniques for the realization of deformable electrochromic devices, in particular stretchable, wearable and foldable electrochromic devices. To realize these, the research started from the design of deformable electrodes. In order to meet the requirements for low charge transfer resistance with rapid switching and large optical modulation, electrodes need to be endowed with high conductivity. The electrochromic devices subject to deformation are expected to suffer performance degradation as small as possible, which makes requisite the mechanical stability of the electrode against stretching or folding. Ag nanowires with the metallic low resistance and the reticulate network shape of high mechanical compliance are potential with an excellent preservation of the network resistance due to inter-sliding mechanism to accommodate strain. The network structure with open voids that obviates strong absorption could also lead to optical transparency of the percolating network. These beneficial properties well leveraged could be integrated with a stretchable or foldable substrate to function as the deformable electrode. An embedded Ag nanowires network structure in PDMS was engineered to obtain the stretchable electrode. The electrode maintains an excellent conductivity at both the relaxed and 50% strained state. A nanopaper transfer technique was discovered and discussed in the thesis; the versatile transfer technique renders possible the transfer of various functional nanomaterials of different dimensions. Transparent conductive Ag nanowires percolating electrode was demonstrated with a high quality through this technique. The robust electrode was demonstrated as an excellent substrate to practice folding for the electrochromic devices. The display function is realized through the integration of electrochromes on the mentioned deformable electrodes. Unlike rigid conventional electrochromic devices, the morphology and thickness of the electrochrome need to be carefully designed in order to accommodate the deformation. Electrochemical deposition from the precursor offers an excellent solution to manually control the physical property of the electrochromic layer to fulfill the deformation function. Discrete WO3 islands and thin WO3 coatings were deposited for stretchable and foldable electrochromic applications respectively. The performance at relaxed and strained state as well as folded state was analyzed systematically to illuminate approaches for the buildup of durable deformable devices. A fully foldable solid state transparent electrochromic device based on small molecule slime and SWCNT protected Ag nanowire electrode was demonstrated as a potential technique for highly deformable electrochromics. In conclusion, the thesis together with the herein encompassed techniques and approaches is believed to be able to provide some insightful ideas on the emerging topic of deformable electrochromics and meanwhile it serves as a good guidance for the realization of durable deformable electrochromic devices for display applications. DOCTOR OF PHILOSOPHY (MSE)

Two-dimensional (2D) layered materials are emerging candidates for future high-performance electronics and optoelectronics due to their unconventional mechanical, electronic and optoelectronic properties. Furthermore, they can be assembled into van der Waals heterostuctures which display novel properties. The aim of my thesis is to develop a versatile and efficient assembling method to fabricate van der Waals heterostructures with clean interfaces, and to improve the performance of corresponding electronic and optoelectronic devices. To achieve this goal, I have done the following work. First, I developed an interface energy-mediated transfer method to fabricate van der Waals heterostructures. 2D layered materials remained intact after transfer because no harsh reaction or supporting polymer was involved in this method. Few-layer graphene, h-BN, MoS2 and WSe2 were printed on the same PET substrate in a layer-by-layer manner. They acted as gate electrode, dielectric, n-channel and p-channel semiconductors, respectively in the complementary inverter fabricated on flexible substrates. Second, the influence of interfacial properties to the photocurrent generation in MoS2 photo-FETs is studied. By comparing the photocurrent of exposed and h-BN-encapsulated MoS2 photo-FETs, I found that the trapping states from the interfaces between MoS2 and SiO2 is the main reason for prolonged photoresponse. I replaced the common SiO2/Si gating stack with h-BN/FLG and achieved stable and fast photoresponse of MoS2 photo-FETs. The hysteresis of the transistors was eliminated because of the dangling bond-free interfaces. Third, a junction gate field-effect transistor was designed and fabricated on high-performance P-N junction of TMDCs. The quality of P-N junction was improved by doping the intrinsic WSe2 with oxygen plasma. The doping level was high and stable. N-type MoS2 was contacted with FLG electrodes to improve the on-currents. DOCTOR OF PHILOSOPHY (MSE)

The focus on alternative therapeutic strategies to overcome the side effects of the drugs and to act in synergy with drugs to maximize recovery is on the rise. Nutraceuticals are one of the candidates being explored as alternative or adjunctive therapy. Being of plant origin, most of the nutraceuticals are sensitive to degradation and suffer from the disadvantage of loss of bioactivity before they reach the target site. A suitable carrier would help maximize the benefits of these nutraceuticals. Oral route is one of the common methods for the consumption of such nutraceuticals or active ingredients (AI). Despite its numerous advantages, it faces many disadvantages also. Some them being the poor bioavailability of the AI, degradation of the AI during transit of the gastro intestinal tract (GIT), poor absorption and lack of action specificity. In order to overcome this, a food grade carrier system that can encapsulate nutraceuticals of varying solubilities and can also be incorporated into food materials, as a direct food additive is required. In spite of the growing research in this field, a food grade carrier, that can successfully target the delivery of the AI to different parts of the GIT, while preserving its bioactivity, is still lacking commercially. In view of this, a food grade oral delivery system for nutraceuticals was developed using chitosan and starch as the base materials. Chitosan nanoparticles (chnp) were prepared by electrospraying and used for encapsulating the AI. A coating of starch on these particles ensured targeted delivery of the encapsulated AI. The performance of the coating layer was tested in different simulated GIT fluids and was found to be comparable to that of the commercial enteric polymers. The berries of the plant, Withania coagulans, chosen as the nutraceutical candidate, was used to extract the nutraceutical coagulans. The extracts, an aqueous fraction and an organic fraction, were characterized using various analytical techniques and was found to match with that of previously reported extracts from the same plant. Three different bioactive properties of the extract were tested. The aqueous fraction was found to possess wound healing and anti-diabetic effects while the organic fraction was found to have an anti-cancer effect. The small intestine targeting ability of the carrier was proven by encapsulating the aqueous extract into the food grade carrier and demonstrating the anti-diabetic effect both in vitro and in vivo. The large intestine targeting ability of the carrier was proven by encapsulating the organic fraction into the carrier and demonstrating the anti-cancer effect in vitro. Hence the successful working of an oral delivery system, made entirely from food grade materials for targeting nutraceuticals to different parts of the gastro intestinal tract has been showcased as a step towards developing health benefiting food additives. Doctor of Philosophy

Tendon plays an important role in regulating body locomotion and providing additional stability to the body. However, tendon is susceptible to injuries and the healing process could be devastating along with the several issues, namely adhesion formations, slow healing and failure at fixation sites, which have deferred the success of proper tendon healing via tendon tissue engineering. This dissertation thus aims to create a sustained dual-drug therapy to address these issues. For adhesion formation, naproxen sodium (NPS) has been shown to be able to avoid this symptom through inhibiting inflammation process. Therefore, so as to eliminate the side effects of NPS such as drowsiness, headache and nausea, NPS was loaded into an electrospun aligned poly (l-lactic acid)/poly (εcaprolactone) scaffold and its release from the scaffold was optimized to achieve controlled release through adjusting the polymer compositions and water percentage in hexafluoroisopropanol (HFIP). A 2-week sustained release was achieved through modulating the NPS distribution in the fibers of a scaffold which possesses aligned morphology and optimal mechanical properties. Notably, the scaffold exhibited no cytotoxicity and tenocytes could respond to the topographical cues given by the scaffolds, forming a network of cells which mimic the native tendon tissue arrangement. Although NPS is needed to prevent adhesion formations, it has been shown to inhibit the proliferation of tenocytes. Hence the second objective of this dissertation is to release growth factor to promote cell proliferation, and at the same time tackle on the slow healing issues during tendon healing. Insulin-like growth factor 1 (IGF-1) was thus loaded into electrospun poly (lactic-co-glycolic acid)/poly (ε-caprolactone) aligned scaffolds for its ability to promote cellular proliferation and maturation. HFIP was identified as the electrospinning solution for its ability to better maintain IGF-1 stability... Doctor of Philosophy

Renewable energy is fast becoming a mainstay in today’s energy scenario. Some of the main sources of renewable engery are wind, solar in addition to waves,tides,etc. These renewable energy-based production, is however inefficient from a practical as well as financial standpoint. The main reason is being the inability to forecast the exact energy that could be generated. This thesis develops a forecasting approach using interval type-2 fuzzy inferences system to address prediction intervals. The system has been adapted employing a gradient descent learning algorithm and an extended kalman filtering method. Meta-cognition is integrated into the system to improve the learning ability and prevent over-fitting. The proposed systems are used in two real-world renewable energy problems: wind and wave prediction. The wave measurement data were collected from directional waveriders deployed offshore Singapore. The experiments are then conducted on the wave energy characteristics and wind speed forecasting problems. Doctor of Philosophy

In the past half decades, hydrogels have received increasing attention, due to their exceptional wide range of promising applications such as drug delivery, sensing, and tissue engineering, etc. A large number of hydrogel products are sensitive to environmental stimuli, such as temperature, pH, etc. Due to the intrinsic properties, different thermos-responsive hydrogels have been fabricated and developed for energy saving smart windows. Because of the significantly increasing impact from urban heat island (UHI) and global warming, the rising demand for comfortable living and working environments result in serious impact on peak and total electricity demand from building electricity consumption sector. Electrochromic and thermochromic materials are the most promising material applied in smart windows. Compared with conventional thermochromic Vanadium dioxide (VO2), temperature sensitive hydrogel, one of the organic thermochromic materials has the advantages of low critical temperature (τc) (from 32-45 °C), and high luminous transmission (Tlum) below τc and large solar modulation (ΔTsol) which are highly pursued. However, there are several drawbacks of traditional hydrogel based thermochromic materials, such as large shrinkage during heating cross τc, poor water holding capacity, lack of active control and weak mechanical strength. To solve these problems as well as improve the thermochromic properties of smart windows, three approaches have been adopted to address those issues. Firstly, tungsten doped VO2 hybrided with cellulose microgel was synthesized, which can block both visible and infrared (IR) light to give a better thermochromic performance. Secondly, by combining with transparent heater, we reported the first flexible electro-thermochromic device, which responded to both temperature and electricity. Lastly, the first carbon and silica-based hydrogel (PNIPAm-Si-Al gel) were hybrided by dispersing poly-(N-isopropylacrylamide) (PNIPAm) microgels into silica-alumina based gel matrix, which gives negligible shrinkage across τc, high water-holding ability, and printability as it was able to produce complex structure through a 3D printer. Meanwhile, such hybrid hydrogel films enable fabrication of flexible electro-thermochromic devices with up to 75% of ΔTsol, depending on the film thickness and the applied voltage, which is among the highest reported results. A flexible smart thermo-sensitive waveguide switch device was also fabricated by such hybrid PNIPAm-Si-Al gel, which will act as temperature sensitive switch for light signal transfer. Doctor of Philosophy

In recent years, the surge in counterfeit products in the microelectronics industry has escalated to problems such as losses and damages of companies’ reputation and consumers’ safety and confidence. With the aim to protect products and devices from attackers to obtain their circuit design, intellectual property (IP) or even secured data, anti-tampering (security) features have been gaining great interest in companies as well as government bodies. With equipment technology advancement, attackers may utilize them to tamper with the devices to retrieve information in an unorthodox manner. Therefore in this report, the motivation for tamper resistant material for IC packaging is first elaborated, follow by literature review conducted from types of integrated circuits (IC) packaging available on the market to the packaging materials, steps taken to produce counterfeit IC and analysis of the internal structure by removal of packaging materials with different techniques (e.g. laser decapsulation). In addition, the rationale of focusing on laser utilization will be mentioned and lastly, the fundamentals of laser and the laser applications in microelectronics will be elaborated. Experiments are conducted on commercial packages and in-house formulated epoxy resin, Glycidyl-POSS (GP), either with or without carbon black/surface treatment on fillers. The components that formed GP, sample preparation processes, laser parameters and the characterization tools utilized will be described. Different types of GP samples prepared are characterized. Images of these GP samples after laser ablation are captured and step profiler is used to quantify the etch depth for analysis. The resistance to laser ablation is obtained, analyzed and discussed. From the data, it is postulated that due to the strong light absorptivity of carbon black, GP samples which do not contain carbon black but with fillers show better resistance to laser ablation in contrast to samples that contain both carbon black and fillers. In addition, the relationship between adhesion of the matrix and fillers to improve stress transfer and its effect on the extent of laser ablation is also studied. Amino silane treated fillers added into GP matrix were used to fabricate good matrix-filler adhesion samples while Glycidyl silane treated fillers added into GP matrix produced samples with poor adhesion. More details of the results and analysis will be explained in this report. Overall, samples without carbon black showed superior performance, being almost laser resistant. This indicates that the carbon black plays the key role as the light absorber and if the material is designed to not absorb any of the laser light, the damage incurred will be very minimal. In the case where colour pigment such as carbon black cannot be omitted (for blocking light transmission) in the device encapsulating process, it is important to reduce air gaps due to poor adhesion between matrix and fillers or pores/voids as all these defects act as the point of weakness which hinder stress transfer, or to find alternative colour pigments which would absorb the laser light less efficiently. Master of Engineering

Silver/copper-filament-based resistive switching memory relies on the formation and disruption of a metallic conductive filament (CF) with relatively large surface-to-volume ratio. The nanoscale CF can spontaneously break after formation, with a lifetime ranging from few microseconds to several months, or even years. Controlling and predicting the CF lifetime enables device engineering for a wide range of applications, such as non-volatile memory for data storage, tunable short/long term memory for synaptic neuromorphic computing, and fast selection devices for crosspoint arrays. However, conflictive explanations for the CF retention process are being proposed. Here we show that the CF lifetime can be described by a universal surface-limited self-diffusion mechanism of disruption of the metallic CF. The surface diffusion process provides a new perspective of ion transport mechanism at the nanoscale, explaining the broad range of reported lifetimes, and paving the way for material engineering of resistive switching device for memory and computing applications., Resistive random-access memory is operated based on the formation and disruption of nanoscale conductive filaments, but a mechanistic understanding of this process remains unclear. Here, Wang et al. develop a surface-diffusion model to describe lifetime of filaments ranging from microseconds to years.

Metal particles in gap cavities provide an interesting system to achieve hybrid local surface plasmon modes for local field enhancement. Here, we demonstrate a relatively simple method to fabricate Ag nanoparticles positioned on Ag semishells separated by a thin (~5 nm) dielectric layer. The obtained structure can provide strong local electric field enhancement for surface-enhanced Raman scattering (SERS). The fabrication of the ordered array structure was realized by nanosphere self-assembly, atomic layer deposition, and metal thin-film dewetting. Numerical simulation proved that, compared to the conventional metal semishell arrays, the additional Ag particles introduce extra hot spots particularly in the valley regions between adjacent Ag semishells. As a result, the SERS enhancement factor of the metal semishell-based plasmonic structure could be further improved by an order of magnitude. The developed novel plasmonic structure also shows good potential for application in plasmon-enhanced solar water-splitting devices. MOE (Min. of Education, S’pore) Published version

As a promising way to harvest solar energy, photoelectrochemical (PEC) water splitting has attracted intensive interest and attention in recent years. Some issues remain in the field of PEC water splitting and the choice of photoanode is an important problem. Recent decade has witnessed the rapid development of BiVO4 as one of the most successful photoanodes. While it has achieved 90% of its theoretical photocurrent, further improvement is limited by its relatively large bandgap 2.4 eV. Hence, to find alternative photoanode candidates with lower bandgap and comparable charge transport properties is urgently needed. This thesis aims to explore cation substituted metal vanadates which have lower bandgap than BiVO4 and comparable charge transport properties. First, the properties of BiVO4 and the role of catalyst have been explored to understand the limitation of BiVO4 photoanode. Next, Fe was incorporated into BiVO4 system to partially substitute Bi to create a mixture of BiVO4 and FeVO4. All the mixed metal vanadate photoanodes indeed have lower bandgap than BiVO4. PEC measurements show that Bi/Fe in 1:1 ratio has the highest performance and the bandgap has lowered to 2.2 eV. The heterojunction between BiVO4 and FeVO4 is also confirmed to be beneficial to charge separation. FeVO4 has a near-optimal bandgap around 2.07 eV, but its photocurrent achieved has been an order lower than BiVO4. The intrinsic properties of this compound are not available in the literature and hence limit its development as a potential photoanode. In this thesis, a series of characterizations have been carried out for the first time to determine the intrinsic properties and to understand the performance limiting factor. It is found that the performance of FeVO4 is limited by its poor charge carrier separation. With the help of microwave conductivity measurements, the origin of inefficient charge separation is attributed to the low carrier mobility. Doping has been applied to improve the carrier concentration and mobility, but the absolute photocurrent increase is limited and separation efficiency still remains low. Doctor of Philosophy

Implantation of biological corneal inlays, derived from small incision lenticule extraction, may be a feasible method for surgical management of refractive and corneal diseases. However, the refractive outcome is dependent on stromal remodelling of both the inlay and recipient stroma. This study aimed to investigate the refractive changes and tissue responses following implantation of 2.5-mm biological inlays with or without corneal collagen crosslinking (CXL) in a rabbit model. Prior to implantation, rotational rheometry demonstrated an almost two-fold increase in corneal stiffness after CXL. After implantation, haze gradually subsided in the CXL-treated inlays (p = 0.001), whereas the untreated inlays preserved their clarity (p = 0.75). In-vivo confocal microscopy revealed reduced keratocyte cell count at the interface of the CXL inlays at week 8. Following initial steepening, regression was observed in anterior mean curvature from week 1 to 12, being most prominent for the non-CXL subgroups (non-CXL: −12.3 ± 2.6D vs CXL: −2.3 ± 4.4D at 90 μm depth, p = 0.03; non-CXL: −12.4 ± 8.0D vs CXL: −5.0 ± 4.0D at 120 μm depth, p = 0.22). Immunohistochemical analysis revealed comparable tissue responses in CXL and untreated subgroups. Our findings suggest that CXL of biological inlays may reduce the time before refractive stabilization, but longer postoperative steroid treatment is necessary in order to reduce postoperative haze. NRF (Natl Research Foundation, S’pore) NMRC (Natl Medical Research Council, S’pore) MOH (Min. of Health, S’pore) Published version

Corneal neovascularization (CoNV) could be treated by novel anti-angiogenic therapies, though reliable and objective imaging tools to evaluate corneal vasculature and treatment efficacy is still lacking. Optical coherence tomography angiography (OCTA) –currently designed as a retinal vascular imaging system— has been recently adapted for anterior-segment and showed good potential for successful imaging of CoNV. However, further development requires an animal model where parameters can be studied more carefully with histological comparison. Our study evaluated the OCTA in suture-induced CoNV in a rabbit model compared to indocyanine green angiography (ICGA) and slit-lamp photography (SLP). Overall vessel density measurements from OCTA showed good correlation with ICGA (0.957) and SLP (0.992). Vessels density by OCTA was higher than ICGA and SLP (mean = 20.77 ± 9.8%, 15.71 ± 6.28% and 17.55 ± 8.36%, respectively, P

Multiple exciton generation (MEG) or carrier multiplication, a process that spawns two or more electron–hole pairs from an absorbed high-energy photon (larger than two times bandgap energy Eg), is a promising way to augment the photocurrent and overcome the Shockley–Queisser limit. Conventional semiconductor nanocrystals, the forerunners, face severe challenges from fast hot-carrier cooling. Perovskite nanocrystals possess an intrinsic phonon bottleneck that prolongs slow hot-carrier cooling, transcending these limitations. Herein, we demonstrate enhanced MEG with 2.25Eg threshold and 75% slope efficiency in intermediate-confined colloidal formamidinium lead iodide nanocrystals, surpassing those in strongly confined lead sulfide or lead selenide incumbents. Efficient MEG occurs via inverse Auger process within 90 fs, afforded by the slow cooling of energetic hot carriers. These nanocrystals circumvent the conundrum over enhanced Coulombic coupling and reduced density of states in strongly confined nanocrystals. These insights may lead to the realization of next generation of solar cells and efficient optoelectronic devices., The hot carriers in halide perovskite nanocrystals cool much slower than those in conventional semiconductor nanocrystals due to the phonon bottleneck. Here, Li et al. demonstrate enhanced multiple exciton generation with lower threshold in intermediate-confined perovskite nanocrystals based on this effect.

This study evaluated the geospatial distribution of fecal indicator bacteria (FIB) (i.e., Escherichia coli , Enterococcus spp.) and the alternative fecal indicator pepper mild mottle virus (PMMoV) in tropical freshwater environments under different land use patterns. Results show that the occurrence and concentration of microbial fecal indicators were higher for urban than for parkland-dominated areas, consistent with land use weightage. Significant positive correlations with traditional FIB indicate that PMMoV is a suitable indicator of fecal contamination in tropical catchments waters (0.549 ≤ rho ≤ 0.612; P < 0.01). PMMoV exhibited a strong significant correlation with land use weightage (rho = 0.728; P < 0.01) compared to traditional FIB (rho = 0.583; P < 0.01). In addition, chemical tracers were also added to evaluate the potential relationships with microbial fecal indicators. The relationships between diverse variables (e.g., environmental parameters, land use coverage, and chemical tracers) and the occurrence of FIB and PMMoV were evaluated. By using stepwise multiple linear regression (MLR), the empirical experimental models substantiate the impact of land use patterns and anthropogenic activities on microbial water quality, and the output results of the empirical models may be able to predict the sources and transportation of human fecal pollution or sewage contamination. In addition, the high correlation between PMMoV data obtained from quantitative real-time PCR (qPCR) and viral metagenomics data supports the possibility of using viral metagenomics to relatively quantify specific microbial indicators for monitoring microbial water quality (0.588 ≤ rho ≤ 0.879; P < 0.05). IMPORTANCE The results of this study may support the hypothesis of using PMMoV as an alternative indicator of human fecal contamination in tropical surface waters from the perspective of land use patterns. The predictive result of the occurrence of human fecal indicators with high accuracy may reflect the source and transportation of human fecal pollution, which are directly related to the risk to human health, and thereafter, steps can be taken to mitigate these risks.

International audience; Off-axis electron holography allows both the amplitude and the phase shift of an electron wavefield propagating through a specimen in a transmission electron microscope to be recovered. The technique requires the use of an electron biprism to deflect an object wave and a reference wave to form an interference pattern. Here, we introduce an approach based on semiconductor processing technology to fabricate fine electron biprisms with rectangular cross-sections. By performing electrostatic calculations and preliminary experiments, we demonstrate that such biprisms promise improved performance for electron holography experiments.

Engineered nanomaterials (ENMs) raise questions among the scientific community and public health authorities about their potential risks to human health. Studying a prospective cohort of workers exposed to ENMs would be considered the gold standard for identifying potential health effects of nanotechnology and confirming the ‘no effect’ levels derived from cellular and animal models. However, because only small, cross-sectional studies have been conducted in the past 5 years, questions remain about the health risks of ENMs. This essay addresses the scientific, methodological, political and regulatory issues that make epidemiological research in nanotechnology-exposed communities particularly complex. Scientific challenges include the array of physicochemical parameters and ENM production conditions, the lack of universally accepted definitions of ENMs and nanotechnology workers, and the lack of information about modes of action, target organs and likely dose–response functions of ENMs. Standardisation of data collection and harmonisation of research protocols are needed to eliminate misclassification of exposures and health effects. Forming ENM worker cohorts from a combination of smaller cohorts and overcoming selection bias are also challenges. National or international registries for monitoring the exposures and health of ENM workers would be helpful for epidemiological studies, but the creation of such a registry and ENM worker cohorts will require political support and dedicated funding at the national and international levels. Public authorities and health agencies should consider carrying out an ENM awareness campaign to educate and engage all stakeholders and concerned communities in discussion of such a project.

Oral administration of levodopa (LD) is the gold standard in managing Parkinson’s disease (PD). Although LD is the most effective drug in treating PD, chronic administration of LD induces levodopa-induced dyskinesia. A continuous and sustained provision of LD to the brain could, therefore, reduce peak-dose dyskinesia. In commercial oral formulations, LD is co-administrated with an AADC inhibitor (carbidopa) and a COMT inhibitor (entacapone) to enhance its bioavailability. Nevertheless, patients are known to take up to five tablets a day because of poor sustained-releasing capabilities that lead to fluctuations in plasma concentrations. To achieve a prolonged release of LD with the aim of improving its bioavailability, floatable spray-coated microcapsules containing all three PD drugs were developed. This gastro-retentive delivery system showed sustained release of all PD drugs, at similar release kinetics. Pharmacokinetics study was conducted and this newly developed formulation showed a more plateaued delivery of LD that is void of the plasma concentration fluctuations observed for the control (commercial formulation). At the same time, measurements of LD and dopamine of mice administered with this formulation showed enhanced bioavailability of LD. This study highlights a floatable, sustained-releasing delivery system in achieving improved pharmacokinetics data compared to a commercial formulation. MOE (Min. of Education, S’pore) Accepted version

Author(s): Shukla, S; Ager, JW; Xiong, Q; Sritharan, T | Abstract: Iron pyrite (FeS2) holds an enormous potential as a low cost and non-toxic photoelectrochemical and energy-harvesting material owing to its interesting optical, electronic, and chemical properties along with elemental abundance. In this Review, low cost and scalable processing techniques to synthesize phase-pure pyrite thin films and nanocubes are described, and the application of this material in various energy-harvesting devices such as dye-sensitized solar cells, photodiodes, and heterojunction solar cells is discussed. A detailed analysis of the electron transport in single-crystal iron pyrite is presented to shed light on its bulk- and surface-conduction properties, which could be useful in designing better pyrite solar cells and could be exploited for novel device architectures. Finally, future prospects and directions are discussed.

The objective of this work is to design and demonstrate multilevel optical switches by combining different phase change materials. Ge2Sb2Te5 and VO2 nanolayer structures were designed to maximize the optical contrast between four different reflective states. These different optical states arise due to the independent structural phase transitions of VO2 and Ge2Sb2Te5 at different temperatures. The transfer matrix method was used to model Fresnel reflection for each structural phase combination and then to optimize the VO2 and Ge2Sb2Te5 layer thicknesses, which were found to be 70 nm and 50 nm. These multilevel optical switching results provide further possibilities to design composite materials for applications in active and programmable photonics. NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Published version

This thesis deals with colloidal synthesis of hybrid organic-inorganic halide perovskite nanocrystals (NCs) for light emitting diode (LED) applications. The influence of, variations in the precursor ratios, as well as NC size distribution modulation via solvent engineering, during colloidal synthesis of methylammonium lead bromide [CH3NH3PbBr3] perovskite NCs, on the efficiency and stability of the perovskite NC-based LEDs are investigated. A comparison of the CH3NH3PbBr3 and formamidinium lead bromide [CH(NH2)2PbBr3] NC synthesis optimizations for LED applications is also presented. The insights obtained from this research are expected to be useful for the design of perovskite NC synthesis protocols for optoelectronic applications. Doctor of Philosophy

Multiple triggers like exogenous and endogenous factors could affect the equilibrium levels of free radicals and reactive oxygen species in humans. For instance, elevated concentration levels of reactive carbonyl species (carbonyl stress) or reactive oxygen species (oxidative stress) damages a myriad of important biological molecules such as proteins, lipids, carbohydrates and nucleic acids. Damage to these molecules causes cell death and tissue injury, thus contributing to various disorders like type 2 diabetes mellitus (T2D), cancer, and cardiovascular diseases. Therefore, assays for stress related biomarkers could assist in the early diagnosis and could be predictive for disease prognosis. The conventional assay methodologies such as chromatographic and electrophoretic techniques are cumbersome, skill intensive and require expensive instrumentation. Thus, there is sufficient scope to develop a point of care (POC) assay for these biomarkers such that advantages of low cost, fast response time and high sensitivity as well as selectivity can be realized. Carbonyl/Oxidative stress plays a major role in the onset and progression of T2D, and a simple, low cost way of measurement of carbonyl/oxidative stress biomarkers can be beneficial in screening patients for early diagnosis of T2D and its complications thereby assisting current clinical practice. The focus of this thesis was to develop colorimetric assays for monitoring carbonyl/oxidative stress biomarkers associated with T2D. Cationic polythiophene (PT); a water-soluble conjugated polymer was used as a luminescent reporter for the optical detection of these biomarkers. The developed assay consists of two modules; a sample fractioning unit; a filter paper (FUSION 5) for removal of large molecules from clinical samples without requiring external stimuli, and a transduction unit utilizing polyvinylidene fluoride (PVDF) membrane impregnated with (PT) for colorimetric sensing in a flow-through format. The sensing strategy is based on monitoring the changes in optical properties of PT with its associated conformational changes when interacting with a peptide nucleic acid (PNA)/aptamer in the presence and in the absence of target biomarkers. As a proof of concept, the developed assay methodology utilizing PT and PNA/aptamer was validated using different carbonyl/oxidative stress biomarkers such as microRNA (mir21), DNA damage molecule; 8-hydroxy-2′-deoxyguanosine (8-OHdG) and Glyceraldehyde derived Advanced Glycation End Products (Gly-AGEs) without requiring tedious sample pre-treatment and clean up protocols. Colorimetric responses for these biomarkers were obtained at clinically relevant concentrations and could therefore find applications in relation to the management of metabolic diseases such as T2D in a clinical setting as well as in the patient’s home, thus providing attractive and affordable healthcare solutions for field diagnostics. Doctor of Philosophy

PurposeTo evaluate structural changes in the choroid among patients with diabetic macular edema (DME), with varying grades of diabetic retinopathy (DR), using enhance depth imaging spectral domain optical coherence tomography (EDI SD-OCT) scans.MethodsA cross-sectional study was conducted on 82 eyes with DR and DME and 86 healthy control eyes. Eyes with DME were classified according to the severity of DR as per the international DR severity scale. Sub foveal choroidal thickness (SFCT)was obtained using EDI SD-OCT scans. These scans were binarized into luminal and stromal areas, to derive the choroidal vascularity index (CVI). CVI and SFCT were analyzed between the study and control group using paired-T test. Tukey's test was used to correlate the differences in CVI and SFCT between different grades of DR. Further analysis was done to look for the effect of DR severity and type of DME on CVI as well as SFCT using correlation coefficient and linear regression analysis.ResultsSFCT was significantly increased in eyes with DME as compared to the controls (334.47±51.81μm vs 284.53±56.45μm, pConclusionSFCT and CVI are dynamic parameters that are affected by DME. Unlike CVI, SFCT is also affected by ocular and systemic factors like edema and hypertension. CVI may be a more accurate surrogate marker for DME and DR and can potentially be used to monitor the progression of DR.

LaFeO3 nanoparticle-modified TiO2 nanotube arrays were fabricated through facile hydrothermal growth. The absorption edge of LaFeO3 nanoparticle-modified TiO2 nanotube arrays displaying a red shift to ~540 nm was indicated by the results of diffuse reflectance spectroscopy (DRS) when compared to TiO2 nanotube arrays, which means that the sample of LaFeO3 nanoparticle-modified TiO2 nanotube arrays had enhanced visible light response. Photoluminescence (PL) spectra showed that the LaFeO3 nanoparticle-modified TiO2 nanotube arrays efficiently separated the photoinduced electron&ndash, hole pairs and effectively prolonged the endurance of photogenerated carriers. The results of methylene blue (MB) degeneration under simulated visible light illumination showed that the photocatalytic activity of LaFeO3 nanoparticle-modified TiO2 nanotube arrays is obviously increased. LaFeO3 nanoparticle-modified TiO2 nanotube arrays with 12 h hydrothermal reaction time showed the highest degradation rate with a 2-fold enhancement compared with that of pristine TiO2 nanotube arrays.

Organic materials are emerging thermoelectric candidates for flexible power generation and solid-cooling applications. Although the Peltier effect is a fundamental thermoelectric effect that enables site-specific and on-demand cooling applications, the Peltier effect in organic thermoelectric films have not been investigated. Here we experimentally observed and quasi-quantitatively evaluated the Peltier effect in a poly(Ni-ett) film through the fabrication of thermally suspended devices combined with an infrared imaging technique. The experimental and simulation results confirm effective extraction of the Peltier effect and verify the Thomson relations in organic materials. More importantly, the working device based on poly(Ni-ett) film yields maximum temperature differences as large as 41 K at the two contacts and a cooling of 0.2 K even under heat-insulated condition. This exploration of the Peltier effect in organic thermoelectric films predicts that organic materials hold the ultimate potential to enable flexible solid-cooling applications., Observing the Peltier effect, e.g. cooling/heating at material junctions due to current flow, in organic thermoelectric films remains a challenge due the inherent properties of these materials. Here, the authors use IR imaging to experimentally observe the Peltier effect in poly(Ni-ett)-based films.

By in-situ transmission electron microscopy (TEM), we performed a detailed study on the electron-beam radiation damage to nanostructured silicon nitride thin-film process layers in a typical semiconductor NVM device. It was found that high-dose electron-beam radiation at 200 kV led to rapid degradation of silicon nitride process layers, i.e. thin-downing of nanostructured silicon nitride, inter-diffusion of O and N, the formation of bubble-like defects and segregation of N at neighbouring interfaces. Further detailed analysis revealed that radiation-induced modification in the microstructure and chemical composition of silicon nitride layers could be ascribed to the electron radiation induced knock-on damage and ionization damage. The radiation enhanced diffusion (RED) accounted for the continuous thin-down of the nitride process layer and the formation of bubble-like defects in thick nitride spacer process layers. The work well demonstrated the electron-beam sensitivity of nanostructured silicon nitride materials in the semiconductor devices, and thus may give useful information about electron-dose control during TEM failure analysis of the semiconductor devices containing nanostructured silicon nitride process layers. Published version

Destabilizing the polar order unexpectedly boosts the ferroelectric photovoltaic performance in bismuth ferrite., Ferroelectric materials for photovoltaics have sparked great interest because of their switchable photoelectric responses and above-bandgap photovoltages that violate conventional photovoltaic theory. However, their relatively low photocurrent and power conversion efficiency limit their potential application in solar cells. To improve performance, conventional strategies focus mainly on narrowing the bandgap to better match the solar spectrum, leaving the fundamental connection between polar order and photovoltaic effect largely overlooked. We report large photovoltaic enhancement by A-site substitutions in a model ferroelectric photovoltaic material, BiFeO3. As revealed by optical measurements and supported by theoretical calculations, the enhancement is accompanied by the chemically driven rotational instability of the polarization, which, in turn, affects the charge transfer at the band edges and drives a direct-to-indirect bandgap transition, highlighting the strong coupling between polarization, lattice, and orbital order parameters in ferroelectrics. Polar order engineering thus provides an additional degree of freedom to further boost photovoltaic efficiency in ferroelectrics and related materials.

The emergence of novel two-dimensional (2D) materials triggered by graphene opens a new window and provides an exceptional platform in a large variety of potential applications such as flexible electronics, photodetector and clean energy due to their intrinsic novel and fascinating physicochemical properties compared with their bulk counterparts. So far, various 2D materials with versatile functions have been reported and synthesized with varied success. For example, insulating hexagonal boron nitride (h-BN), semiconducting and metallic transition metal dichalcogenides (TMDCs), black phosphors (BP) and even non-layered structure 2D materials pure metal nanosheets have aroused tremendous research interests in recent years, further expanding the 2D family. More importantly, the unique 2D crystal structure and high anisotropy hold great promise for endowing them with tunable electronic and catalytic properties. In this regard, the aim of this PhD thesis is to tune novel 2D materials in a broad regime including defect engineering, interface engineering and stacking engineering and then investigate their possibilities and potential applications for high-performance lithium-ion batteries (LIBs), photocatalytic hydrogen evolution reactions (HER) and electrocatalytic oxygen reduction reactions (ORR). Firstly, highly nitrogen-doped and defect-rich porous nanocarbon has been synthesized by in-situ simple pyrolysis of zeolitic imidazolate framework nanoparticles grown on graphene oxide as negative electrode in lithium ion batteries (LIBs), which demonstrates extraordinary capacities, remarkable cycling performances and rate capability and surpasses carbon-based electrodes in literature and most of metal oxide-based anodes from metal organic frameworks (MOFs). With an impressive initial capacity of 1378 mAh g−1 under current density with 100 mA g−1, it could be operated for 100 and 200 cycles under current densities of 500 mA g−1 and 1000 mA g−1, leading to reversible capacities of 1070 mA g−1 and 948 mAh g−1, separately. If current density was up to 5000 mA g−1, it still reversibly presents a decent capacity of more than 530 mAh g−1 after 400 cycles, exhibiting a high rate of retention of 84.4% in terms of capacity. It is considered that the excellent electrochemical output is due to the perfect bond of an extremely conductive platform (graphene), high-level doping of nitrogen (defect-rich) and hierarchically porous structure. Secondly, via interface engineering, solid edge-on heterostructures between few-layered MoS2 and TiO2 were produced on large scale through a facile two-step, which realized the idea that layered MoS2 can be welded laterally to photocatalysts via edges to enhance the catalytic activity toward water splitting owing to the efficient transport of photoexcited electrons along the highly conductive edges and basal planes of MoS2. The unusual photocatalytic activity is attributed to the improved electronic contact and the optimized electron transport pathways between TiO2 and MoS2, where the recombination of photoexcited electron-hole pairs is effectively suppressed. This work represents a strategy, which marries low-cost layered transition-metal dichalcogenides with backboned photocatalyst as a promising heterostructure for high-efficiency water splitting. Lastly, a facile and large-scale approach has been developed to prepare platinum-based binary alloy nanosheets by stacking engineering target platinum-based alloy foil with aluminum foil, in which asymmetric mechanical rolling technique was employed to repeat the folding and rolling process. With the assistance of sodium hydroxide, aluminum metal layers were selectively etched and removed completely. Combining theoretical calculations, rational material design with optimal compositions of Pt3M (M: Ni, Fe, Co, Ir, Y, Au, Pb) was proposed for electrocatalytic oxygen reduction reactions (ORR). Through a series of characterization techniques, the sheet-like morphology was observed with lateral size of 2-3 μm and the thickness ranges from 5.5 nm to 9.7 nm. The activity and stability of ORR were greatly improved. It is widely believed that altered electronic properties by alloying and changed surface by 2D materials induce a variation for adsorption behavior, which were considered to be the origin of the high ORR activity. Doctor of Philosophy (MSE)

Textiles that are capable of harvesting biomechanical energy via triboelectric effects are of interest for self-powered wearable electronics. Fabrication of conformable and durable textiles with high triboelectric outputs remains challenging. Here we propose a washable skin-touch-actuated textile-based triboelectric nanogenerator for harvesting mechanical energy from both voluntary and involuntary body motions. Black phosphorus encapsulated with hydrophobic cellulose oleoyl ester nanoparticles serves as a synergetic electron-trapping coating, rendering a textile nanogenerator with long-term reliability and high triboelectricity regardless of various extreme deformations, severe washing, and extended environmental exposure. Considerably high output (~250–880 V, ~0.48–1.1 µA cm−2) can be attained upon touching by hand with a small force (~5 N) and low frequency (~4 Hz), which can power light-emitting diodes and a digital watch. This conformable all-textile-nanogenerator is incorporable onto cloths/skin to capture the low output of 60 V from subtle involuntary friction with skin, well suited for users’ motion or daily operations., Incorporation of triboelectric nanogenerators into textiles is attractive for self-powered wearable electronics. Here the authors employ black phosphorus with a hydrophobic coating in a durable, washable, and air permeable textile-based device that converts biomechanical motion into electricity.

Nanotechnology is widely used in the food industry to improve the color, taste, and texture of food products. However, concerns regarding potential undesirable health effects remain. It is expected that interaction of engineered nanomaterials (ENMs) with food ingredients will influence their behavior and the resulting corona. Nonetheless, there are limited systematic studies conducted to clarify this understanding to date. Herein, we investigated the behavior and corona formation of food grade titanium dioxide (TiO2) and silicon dioxide (SiO2) in solutions of model food ingredients including bovine serum albumin (BSA) and sucrose. Measurements using dynamic light scattering (DLS) showed that both TiO2 and SiO2 nanoparticles displayed a decrease in agglomerate sizes in the presence of both food ingredients. Both particles were negatively charged in all the conditions tested. Corona adsorption studies were carried out using multiple complementary methods including Fourier transform infrared (FTIR) spectroscopy, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-ToF-MS), transmission electron microscopy (TEM), micro bicinchoninic acid (BCA) protein assay, and thermogravimetric analysis (TGA). Comparative investigation showed that sucrose could disperse both particles more effectively than BSA and that SiO2 displayed greater adsorption capacity for both BSA and sucrose, compared to TiO2. Taken collectively, this study demonstrated the importance of considering food ingredient effects when mapping the behavior of ENMs in food products. Such understanding could be significant in the evaluation of biological effects, such as toxicity, of ENMs used in food products. Accepted version

Vanadium dioxide (VO2) is one of the most widely studied inorganic phase change material for energy storage and energy conservation applications. Monoclinic VO2 [VO2(M)] changes from semiconducting phase to metallic rutile phase at near room temperature and the resultant abrupt suppressed infrared transmittance at high temperature makes it a potential candidate for thermochromic smart window application to cut the air-condition usage. Meanwhile proper electrical potential, stable structure and good interaction with lithium ions make metastable VO2 [VO2(B)] an attractive material for fabrication of electrodes for batteries and supercapacitors. However, some long-standing issues have plagued its usage. In thermochromic application, high transition temperature (τc), low luminous transmittance (Tlum) and undesirable solar modulation ability (△Tsol) are the key problems, while in energy storage applications, short cycling lifetime and complex three-dimension microstructure are the major challenges. The common methods to produce VO2 polymorph are physical vapour deposition (PVD), chemical vapour deposition (CVD), sol-gel synthesis, and hydrothermal method. CVD is an intensively studied method due to its ability to produce uniform films with precise stoichiometry, phase and morphology control. This paper reviews the various CVD techniques to produce VO2 with controlled phases and the ternary diagram shows the relationship between film stoichiometry and various process conditions. The difference between the various CVD systems are commented and the process window to produce VO2 are tabulated. Some strategies to improve VO2′s performance in both energy conservation and energy storage applications are discussed. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

Corneal transplantation is the only treatment available to restore vision for individuals with blindness due to corneal endothelial dysfunction. However, severe shortage of available donor corneas remains a global challenge. Functional regulatory compliant tissue-engineered corneal endothelial graft substitute can alleviate this reliance on cadaveric corneal graft material. Here, isolated primary human corneal endothelial cells (CEnCs) propagated using a dual media approach refined towards regulatory compliance showed expression of markers indicative of the human corneal endothelium, and can be tissue-engineered onto thin corneal stromal carriers. Both cellular function and clinical adaptability was demonstrated in a pre-clinical rabbit model of bullous keratopathy using a tissue-engineered endothelial keratoplasty (TE-EK) approach, adapted from routine endothelial keratoplasty procedure for corneal transplantation in human patients. Cornea thickness of rabbits receiving TE-EK graft gradually reduced over the first two weeks, and completely recovered to a thickness of approximately 400 µm by the third week of transplantation, whereas corneas of control rabbits remained significantly thicker over 1,000 µm (p

A major challenge for studying authentic liver cell function and cell replacement therapies is that primary human hepatocytes rapidly lose their advanced function in conventional, 2-dimensional culture platforms. Here, we describe the fabrication of 3-dimensional hexagonally arrayed lobular human liver tissues inspired by the liver’s natural architecture. The engineered liver tissues exhibit key features of advanced differentiation, such as human-specific cytochrome P450–mediated drug metabolism and the ability to support efficient infection with patient-derived inoculums of hepatitis C virus. The tissues permit the assessment of antiviral agents and maintain their advanced functions for over 5 months in culture. This extended functionality enabled the prediction of a fatal human-specific hepatotoxicity caused by fialuridine (FIAU), which had escaped detection by preclinical models and short-term clinical studies. The results obtained with the engineered human liver tissue in this study provide proof-of-concept determination of human-specific drug metabolism, demonstrate the ability to support infection with human hepatitis virus derived from an infected patient and subsequent antiviral drug testing against said infection, and facilitate detection of human-specific drug hepatotoxicity associated with late-onset liver failure. Looking forward, the scalability and biocompatibility of the scaffold are also ideal for future cell replacement therapeutic strategies. NMRC (Natl Medical Research Council, S’pore) NRF (Natl Research Foundation, S’pore) Published version

With strongly bound and stable excitons at room temperature, single-layer, two-dimensional organic-inorganic hybrid perovskites are viable semiconductors for light-emitting quantum optoelectronics applications. In such a technological context, it is imperative to comprehensively explore all the factors --- chemical, electronic and structural --- that govern strong multi-exciton correlations. Here, by means of two-dimensional coherent spectroscopy, we examine excitonic many-body effects in pure, single-layer (PEA)$_2$PbI$_4$ (PEA = phenylethylammonium). We determine the binding energy of biexcitons --- correlated two-electron, two-hole quasiparticles --- to be $44 \pm 5$\,meV at room temperature. The extraordinarily high values are similar to those reported in other strongly excitonic two-dimensional materials such as transition-metal dichalchogenides. Importantly, we show that this binding energy increases by $\sim25$\% upon cooling to 5\,K. Our work highlights the importance of multi-exciton correlations in this class of technologically promising, solution-processable materials, in spite of the strong effects of lattice fluctuations and dynamic disorder., Comment: 37 pages including 4 figures and supplemental information