Monolithic Perimeter Gated Single Photon Avalanche Diode Based Optical Detector in Standard CMOS

Since the 1930\u27s photomultiplier tubes (PMTs) have been used in single photon detection. Single photon avalanche diodes (SPADs) are p-n junctions operated in the Geiger mode. Unlike PMTs, CMOS based SPADs are smaller in size, insensitive to magnetic fields, less expensive, less temperature dependent, and have lower bias voltages. Using appropriate readout circuitry, they measure properties of single photons, such as energy, arrival time, and spatial path making them excellent candidates for single photon detection. CMOS SPADs suffer from premature breakdown due to the non-uniform distribution of the electric field. This prevents full volumetric breakdown of the device and reduces the detection effciency by increasing the noise. A novel device known as the perimeter gated SPAD (PGSPAD) is adopted in this dissertation for mitigating the premature perimeter breakdown without compromising the fill-factor of the device. The novel contributions of this work are as follows. A novel simulation model, including SPICE characteristics and the stochastic behavior, has been developed for the perimeter gated SPAD. This model has the ability to simulate the static current-voltage and dynamic response characteristics. It also simulates the noise and spectral response. A perimeter gated silicon photomultiplier, with improved signal to noise ratio, is reported for the first time. The gate voltage reduces the dark current of the silicon photomultiplier by preventing the premature breakdown. A digital SPAD with the tunable dynamic range and sensitivity is demonstrated for the first time. This pixel can be used for weak optical signal application when relatively higher sensitivity and lower input dynamic range is required. By making the sensitivity-dynamic range trade-off the same detector can be used for applications with relatively higher optical power. Finally, an array has been developed using the digital silicon photomultiplier in which the dead time of the pixels have been reduced. This digital photomultiplier features noise variation compensation between the pixels

First-Principles Density Functional Theory Study of Novel Materials for Solar Energy Conversion and Environment Applications

To design an efficient solar energy conversion device, theoretical input is extremely important to provide the basic guideline for experimental scientists, to fabricate the most efficient, cheap, and stable device with less efforts. This desire can be made possible if computational scientist use a proper theoretical protocol, design an energy material, then the experimentalist will only invest weeks or months on the synthetic effort. This thesis highlights my recent efforts in this direction. Monoclinic BiVO4 is has been using as a photocatalyst due to its stability, cheap, easily synthesizable, narrow band gap and ideal VB (-6.80 eV vs vacuum) but inappropriate CB (-4.56 eV vs vacuum) edge position, responsible for its low efficiency. We have carried out a comprehensive experimental and periodic density functional theory (DFT) simulations of the pristine, Oxygen defective (Ov), Se doped monoclinic BiVO4 and heterojunction with Selenium (Se-BiVO4), to improve not only its CB edge position but photocatalytic and charge carrier properties. It is found that Ov (1% Oxygen vacancy) and mild doped BiVO4 (1 to 2% Se) are thermodynamically stable, have ideal band edges ~ -4.30 eV), band gaps (~1.96 eV), and small effective masses of electrons and holes. We have also investigated the contribution of Se to higher performance by effecting morphology, light absorption and charge transfer properties in heterojunction. Finally, it is found that Se makes a direct Z-scheme (band alignments) with BiVO4 where the photoexcited electron of BiVO4 recombine with the VB of Se, consequences electron-hole separation at Se and BiVO4, respectively, as a result, enhanced photocurrent is obtained. Theoretical study of β-TaON in the form of primitive unit cell, supercell and its N, Ta, and O terminated surfaces are carried out with the help of periodic DFT. Optical and electronic properties of all these different species are simulated, which predict TaON as the best candidate for photocatalytic water splitting contrast to their Ta2O5 and Ta3N5 counterparts. The calculated bandgap, valence band, and conduction band edge positions predict that β-TaON should be an efficient photoanodic material. The valence band is made up of N 2p orbitals with a minor contribution from O 2p, while the conduction band is made up of Ta 5d. Turning to thin films, the valence band maximum; VBM (−6.4 eV vs. vacuum) and the conduction band minimum; CBM (−3.3 eV vs. vacuum) of (010)-O terminated surface are respectively well below and above the redox potentials of water as required for photocatalysis. Charge carriers have smaller effective masses than in the (001)-N terminated film (VBM −5.8 and CBM −3.7 eV vs. vacuum). However, due to wide band gap (3.0 eV) of (010)-O terminated surface, it cannot absorb visible wavelengths. On the other hand, the (001)-N terminated TaON thin film has a smaller band gap in the visible region (2.1 eV) but the bands are not aligned to the redox potential of water. Possibly a mixed phase material would produce an efficient photoanode for solar water splitting, where one phase performs the oxidation and the other reduction. Computational study of an optically transparent, near-infrared-absorbing low energy gap conjugated polymer, donor−acceptor−donor (D-A-D) with promising attributes for photovoltaic application is reported herein. The D and A moiety on the polymeric backbone have been found to be responsible for tuning the band gap, optical gap, open circuit (Voc) and short-circuit current density (Jsc) in the polymers solar cells (PSC). Reduction in the band gap, high charge transformation, and enhanced visible light absorption in the D-A-D system is because of strong overlapping of molecular orbitals of D and A. In addition, the enhanced planarity and weak steric hindrance between adjacent units of D-A-D, resulted in red-shifting of its onset of absorption. Finally, PSC properties of the designed D-A-D was modeled in the bulk heterojunction solar cell, which gives theoretical Voc of about 1.02 eV. DFT study has been carried out to design a new All-Solid-State dye-sensitized solar cell (SDSC), by applying a donor-acceptor conjugated polymer instead of liquid electrolyte. The typical redox mediator (I1−/I3−) is replaced with a narrow band gap, hole transporting material (HTM). A unique “upstairs” like band energy diagram is created by packing N3 between HTM and TiO2. Our theoretical simulations prove that the proposed configuration will be highly efficient as the HOMO level of HTM is 1.19 eV above the HOMO of sanitizer (dye); providing an efficient pathway for charge transfer. High short-circuit current density and power conversion efficiency is promised from the strong overlapping of molecular orbitals of HTM and sensitizer. A low reorganization energy of 0.21 eV and exciton binding energy of 0.55 eV, confirm the high efficiency of HTM. Theoretical and experimental studies of a series of four porphyrin-furan dyads were designed and synthesized, having anchoring groups, either at meso-phenyl or pyrrole-β position of a zinc porphyrin based on donor–π–acceptor (D–π–A) approach. The porphyrin macrocycle acts as donor, furan hetero cycle acts as π-spacer and either cyanoacetic acid or malonic acid group acts as acceptor. Optical bandgap, natural bonding, and molecular bonding orbital (HOMO–LUMO) analysis confirm the high efficiency pyrrole-β substituted zinc porphyrins contrast to meso-phenyl dyads. DFT study of polypyrrole-TiO2 composites has been carried out to explore their optical, electronic and charge transfer properties for the development of an efficient photocatalyst. Titanium dioxide (Ti16O32) was interacted with a range of pyrrole (Py) oligomers to predict the optimum composition of nPy-TiO2 composite with suitable band structure for efficient photocatalytic properties. The study has revealed that Py-Ti16O32 composites have narrow band gap and better visible light absorption capability compared to individual constituents. A red-shifting in λmax, narrowing band gap, and strong intermolecular interaction energy (-41 to −72 kcal/mol) of nPy-Ti16O32 composites confirm the existence of strong covalent type interactions. Electron−hole transferring phenomena are simulated with natural bonding orbital analysis where Py oligomers found as donor and Ti16O32 as an acceptor in nPy-Ti16O32 composites. Sensitivity and selectivity of polypyrrole (PPy) towards NH3, CO2 and CO have been studied at DFT. PPy oligomers are used both, in the doped (PPy+) and neutral (PPy) form, for their sensing abilities to realize the best state for gas sensing. Interaction energies and amount of charges (NBO and Mulliken charge analysis) are simulated which reveal the sensing ability of PPy towards these gases. PPy, both in doped and neutral state, is more sensitive to NH3 compared to CO2 and CO. More interestingly, NH3 causes doping of PPy and de-doping of PPy+, providing evidence that PPy/PPy+ is an excellent sensor for NH3 gas. UV-vis and UV-vis-near-IR spectra of nPy, nPy+, and nPy/nPy+-X complexes demonstrate strong interaction of PPy/PPy+ with these atmospheric gases. The applications of graphene (GR) and its derivatives in the field of composite materials for solar energy conversion, energy storage, environment purification and biosensor applications have been reviewed. The vast coverage of advancements in environmental applications of GR-based materials for photocatalytic degradation of organic pollutants, gas sensing and removal of heavy metal ions is presented. Additionally, the presences of graphene composites in the bio-sensing field have been also discussed in this review.College Funded University of Exeter, U

Feasibility Study of Energy Storage Technologies for Remote Microgrid’s Energy Management System

Energy storage systems (ESSs) play a significant role in remote microgrids energy management system (EMS) with the large penetration rate of renewable energy which is intermittent in nature. Energy storage improves system reliability and efficiency in remote microgrids by optimizing the power demand and generation to reduce operational costs. Moreover, it increases the dispatch ability of the energy sources in remote microgrid systems. Lead acid battery (PbA) can be used as an energy storage device in remote microgrids due to its low cost; however, the response rate, short life cycle, and depth of discharge (DoD) lead to high operational costs. Ultracapacitor has a considerably longer life cycle, its energy density is low, and the initial cost is very high. Lithium-ion (Li-ion) and hybrid ion batteries may have comparatively better economical prospects in terms of DoD, life cycle, and operational cost. In this thesis, different energy storage technologies are considered for remote microgrids energy management systems. In addition, the Schiffer weighted Ah throughput model introduces two weight factors to describe that a battery degrades faster in real time operation than the standard test conditions due to different stress factors. These weight factors virtually increase the battery throughput, and accelerate the degradation. To mitigate this problem, different periodical and auto cycling strategies were investigated in this thesis. However, the results demonstrated that frequent full charging prevents the battery from over degradation. Auto cycling strategy was found more cost effective than the periodical cycling. Applying this cycling strategy, the yearly total operational cost of a microgrid system with a 142 kWh PbA battery bank was reduced by 0.62% ($826). Results also showed that the wear cost is an important factor to consider while designing the energy management system. Li-ion and hybrid-ion batteries had lower wear costs and showed great potentiality, although the EMS with a Li-ion battery was found to be 2.55% more cost effective and 1.5% more fuel efficient than hybrid ion batteries. The reduction in operational cost ensures the access to low cost electricity for the people in remote areas. It will accelerate the development of industries, communications, technologies, and the standard of living including the remote health clinics in those areas. Furthermore, the reduction in generators fuel consumption will reduce CO2 emission which will lower the global warming and the greenhouse effect. In this thesis, one of the objectives was to prolong the battery lifetime by preventing the degradation, that may lower the number of yearly battery disposals which are hazardous to the human health and the environment

Using Technologlical Tools to Solve Group Work Problems in Higher Education of Under Developing Countries: A Case Study

Group work activities are one of the main point or task in the high education. Switzer and Shriner (2000) were of the view that students are the most obvious party who benefit from group work among students, faculty members, and the community. According to them there are four overlapping types of benefits for students. These are: 1) immediate educational benefits, 2) immediate social benefits, 3) critical thinking benefits, and 4) long-term career benefits. Lawrance (1992) , and Yates (2001) were of the opinion that face to face communication will not solve the empowerment problems in group work activities. As, through FTF interaction male dominant role can be produced due to identity of speaker, eye contact, nodding, moving the hands , and facial expressions etc. In this situation suitable adoption of technology can be consider as an alternative mode of communication, where there is a chance of discrimination. This case study will be a further step in addition to the previous technological tools & group work related researches. In this researcher will try to explore that how suitable technological tools can play a role to over come the group work problems and to increase the performance of the students in the developing countries like Oman

Influence of Leadership Approaches on Intrinsic Motivation of Career Professionals in Ontario Non-profit Employment Agencies: An Exploratory Case Study

This exploratory case study (Yin, 2009) investigated those leadership approaches that are used by mid-level managers of seven non-profit employment agencies in Ontario, Canada, to support the intrinsic motivation of seven career professionals who work with them. Unlike profit-earning organizations, career professionals of non-profit employment agencies in Ontario do not get any additional financial incentives for exceeding their targets of helping job seekers find sustainable employment. The research used a transformative learning theory lens (Mezirow, 1991), and also an Interpretivist framework (Merriam, 1998) to understand the data. This study also sought to find what motivates career professionals to reach and exceed their pre-set targets without the availability of any additional bonus. Seven mid-level managers and seven career professionals of non-profit employment agencies were interviewed. A semi-structured interview format was used for the one-on-one interviews. Additional data were collected via document perusal, and the researcher’s reflective journals. Data were coded and analyzed thematically using a content analysis method. Triangulation and member-checking were performed for ensuring reliability of data (Yin, 2009). Findings of the study suggest that mid-level managers of Ontario non-profit employment agencies largely use transformational leadership approaches for building and sustaining career professionals’ intrinsic motivation, although, they sometimes use transactional approaches as well. The study also suggests that the career professionals of the seven non-profit employment agencies are by and large, intrinsically motivated, and three of their key motivators are “passion for their jobs”, “empathy for the clients” and “changing other people’s lives”

Anomalous behaviour of molybdenum in steel welds

The addition of molybdenum to steel welds in quite small concentrations leads to a variety of anomalous microstructural and mechanical property effects. In some cases, the effects manifest even when there are no obvious changes in microstructure at the resolution of a transmission electron microscope. There are two particular discrepancies. A quantitative analysis of molybdenum-containing steel welds indicates that there is a degree of strengthening which cannot be explained by the known solid solution or microstructural effects of molybdenum in steels. Secondly, in multirun welds, the addition of molybdenum appears to make the microstructure which evolves during solidification (the primary microstruet1lre) extremely stable. These and other associated phenomena are examined in this thesis. The molybdenum effects mentioned above have been reproduced in detail, using a series of 'high-purity' multirun welds. Having confirmed that molybdenum increases the fraction of primary microstructure in such welds, an attempt was made to see whether the effect is attributable to a change in the austenitisation characteristics with alloying additions. Extensive work using dilatometric techniques backed by microscopy analysis has demonstrated that molybdenum does not lead to any substantial or unexpected changes in the ability to form austenite. The second hypothesis, that the primary microstructure is stabilised as molybdenum increases the tempering resistance, is proven and provides a good explanation of the observations. A series of tempering experiments have established that the anomalously high strength of the molybdenum containing welds cannot be attributed to solid solution strengthening or microstructural effects. Indeed, it appears that there is a submicroscopic secondary hardening type effect which enhances the strength. Even the thermal treatment that occurs as the weld cools from the solidification temperature is shown to be sufficient to induce molybdenum based secondary hardening type effects. Some preliminary atomic resolution effects also lend support to this concept. Titanium as a trace element is important in steel welds, as an element which promotes the intragranular nucleation of acicular ferrite on titanium-rich phases. It is demonstrated that the titanium effect is not intrinsically different for molybdenumcontaining welds. However, the extra hardenability associated with molybdenum certainly helps to suppress the formation of other grain boundary nucleated phases which might swamp events that occur on the inclusions within the grains. An interesting observation is that titanium has a positive effect in limiting the grain boundary nucleated phases, because the intragranularly nucleated acicular ferrite to some extent stifles the formation of other phases

Big data analytics: does organizational factor matters impact technology acceptance?

Ever since the emergence of big data concept, researchers have started applying the concept to various fields and tried to assess the level of acceptance of it with renown models like technology acceptance model (TAM) and it variations. In this regard, this paper tries to look at the factors that associated with the usage of big data analytics, by synchronizing TAM with organizational learning capabilities (OLC) framework. These models are applied on the construct, intended usage of big data and also the mediation effect of the OLC constructs is assessed. The data for the study is collected from the students pertaining to information technology disciplines at University of Liverpool, online programme. Though, invitation to participate e-mails are sent to 1035 students, only 359 members responded back with filled questionnaires. This study uses structural equation modelling and multivariate regression using ordinary least squares estimation to test the proposed hypotheses using the latest statistical software R. It is proved from the analysis that compared to other models, model 4 (which is constructed by using the constructs of OLC and TAM frameworks) is able to explain 44% variation in the usage pattern of big data. In addition to this, the mediation test performed revealed that the interaction between OLC dimensions and TAM dimensions on intended usage of big data has no mediation effect. Thus, this work provided inputs to the research community to look into the relation between the constructs of OLC framework and the selection of big data technology. - 2017, The Author(s).Scopu