Enhancing Charge Carrier Mobility in Colloidal Quantum Dots For Technological Applications

Colloidal quantum dots (QD) are promising semiconducting materials to engineer photovoltaic and optoelectronic devices due to tunable size-dependent absorption and emission properties. These materials are important as they don’t need complicated equipment and huge setup investment for industrial applications. If formulated into a kind of stable nano-ink, these QDs can be incorporated into devices using the most economical processing technologies, spray or roll to roll printing. More importantly, these are compatible with thin-film stacked devices and circuitry that can be formed on heat-sensitive and flexible substrates to make flexible wearable devices and sensors that are difficult to achieve with crystalline-based material in existence. QD processing technology consists of three main steps (I) Synthesis (II) Purification (III) and ligand exchange and device fabrication. With well-established synthetic procedures, great effort has been done on ligand exchange and device fabrication but there has been negligible attention given to purification strategies. Another major hurdle for their industrial applications is very short-lived post-ligand exchange QD solution stability that compromises the QD ink quality even before device fabrication. This thesis is divided into six chapters. First, I will introduce QD chemistry, applications, and post-synthesis purification en route to device fabrication. Second, I’ll introduce Gel Permeation Chromatography (GPC) as a purification technique in parallel to the established precipitation/re-dispersion (PR) method. This section will demonstrate the effectiveness of GPC in removing byproducts and unbound ligands from PbS QDs, and the subsequent applicability of the GPC-purified QDs in optoelectronic devices. In the third and fourth chapters, I will present highly stable 3-mercaptopropionic acid (MPA) capped and halide capped PbS QDs, dispersed in a single non-coordinating organic solvent, to form printable p-type and n-type nano-inks. These inks are stable and suitable for making standalone, heterojunction, and p-n junction solar cell and photodetection devices. These inks should make QDs a viable option for industrial-scale manufacturing of QD devices through spray, or roll-to-roll printing processes. Chapter 5 will introduce AgBiS2 based QDs ink as environment friendly alternative to eliminate toxicity concerns associated with the current state of the art PbS QD system. This ink has been utilized to fabricate flexible photodetectors to show its broad applicability in sensitive areas such as food processing and biomedical applications. Lastly, Chapter 6 of this thesis demonstrates scanning photocurrent microscopy (SPCM) as a diagnostic technique to characterize III-nitride (GaN/AlGaN) based high electron mobility transistor (HEMT) structures for growth defects and current conduction mechanisms via sub-bandgap excitation

Modeling RNA, protein, and synthetic molecules using coarse-grained and all-atom representations

The aim of computational chemistry is to depict and understand the dynamics and interactions of molecular systems. In addition to increased comprehension in the physical and life sciences, this insight yields important applications to therapeutic design and materials science. In computational chemistry, molecules can be modeled in a number of representations depending on the molecular system and phenomena of interest. In this work, both simplified, coarse-grained representations and all-atom representations are used to model the interactions of RNA, cucurbituril host-guest chemistry, and cadmium selenide quantum dot binding to the Src homology 3 domain. For RNA, a coarse-grained model was developed termed RACER (RnA CoarsE-gRained) to accurately predict RNA structure and folding free energy. After optimization to statistical potentials, RACER accurately predicted the structures of 14 RNAs with an average 4.15Å root mean square deviation (RMSD) to the experimental structure. Further, RACER captured the sequence-specific variation in folding free energy for a set of 6 RNA hairpins and 5 RNA duplexes, with a R² correlation of 0.96 to experiment. The binding free energies of a cucurbituril host with 14 guests were computed using a polarizable force field and the free energy techniques of Bennett acceptance ratio and the orthogonal space random walk. The polarizable force field captured binding accurately, yet unexpectedly, the orthogonal space random walk method converged slowly, albeit at still reduced computational expense to the Bennett acceptance ratio. Lastly, the nanotoxicity effects of trioctylphosphine oxide coated cadmium selenide quantum dots are investigated with the model Src homology 3 protein domain in complex with its native proline rich motif ligand. With increasing quantum dot concentration, there is an increasing preference for the quantum dots to bind to the proline rich motif active site, inhibiting Src homology 3 function.Biomedical Engineerin

Synthesis of Nanocrystal Quantum Dots with Enhanced Photoluminescence for Luminescent Optoelectronics

This thesis examines different approaches towards achieving greater luminescence efficiencies in colloidal nanocrystal quantum dots with primary focus towards application within singlet fission photon-multiplier down-conversion systems. Specific challenges to characterising their luminescent properties have been explored in detail, also applicable to other luminescent materials. Additionally, alternative applications for luminescent quantum dots have been investigated, and finally, the gap between small-scale synthesis and industrial production was explored for the material system of most interest. PbS quantum dots were synthesised and their luminescence efficiencies were enhanced through cation exchange with cadmium. It was found that the luminescence improvement was sensitive to a multitude of factors, particularly reaction duration at hitherto unexplored short timescales. Time-resolved optical and structural characterisation was performed indicating a two-step mechanism consisting of cation adsorption followed by subsequent exchange. Halting the reaction to the adsorption stage resulted in the highest luminescence efficiencies. The lead-free quantum dot systems InAs and CuInSe2 were investigated as alternative near-infrared emitters for photon-multiplication. Core-shell approaches with a variety of materials and complex structures were employed to improve their otherwise poor luminescence. The measurement of photoluminescence quantum efficiency is widely used for luminescent material characterisation. As a figure of merit for colloidal quantum dots, the method was examined in great detail. The sources of systematic error were identified and random measurement error quantified for a recognised measurement methodology in order to obtain accurate measurements with meaningful uncertainty bounds, applied for a photon-multiplier demonstrator system. CdSe and InP core-shell quantum dots were synthesised for a novel voltage probe. Their stability under electrolytic environments and strong response to applied electric fields was demonstrated for a biological voltage sensor. PbS quantum dots were synthesised using a commercially-available microfluidic flow reactor. The challenges to product quality and reproducibility upon scale-up were examined. Mitigation strategies against the problems encountered were proposed

Células solares sensibilizadas por novos corantes derivados de cumarinas

Tese de mestrado em Engenharia da Energia e do Ambiente, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2012O trabalho apresentado nesta tese visa a compreensão dos mecanismos que governam o funcionamento de uma célula solar sensibilizada por dois novos corantes derivados de cumarinas, C1-LEN e C2-LEN. O comportamento dos corantes, em filmes de TiO2 mesoporosos, foi comparado com o corante padrão N719. As propriedades das células solares foram analisadas por microscopia eletrónica de varrimento, eficiência quântica, curva de corrente em função da tensão e espectroscopia de impedância eletroquímica. A eficiência, em condições AM1.5, das melhores células sensibilizadas com os corantes N719, C1-LEN e C2,LEN foi de 3,2%, 2,2% e 0,06% respetivamente. A corrente gerada pela célula sensibilizada com o corante C1-LEN é comparável com a da célula sensibilizada com o N719, (6mA/cm2). Contudo, a tensão em circuito aberto é cerca de 200mV inferior, sugerindo que a posição da banda de condução do TiO2 é mais positiva. O desempenho das células sensibilizadas com o corante C2-LEN é inferior devido à densidade de corrente de curto-circuito (0,3mA/cm2) e à tensão em circuito aberto (400mV). Os filmes sensibilizados com os corantes C1-LEN e C2-LEN possuem elevada resistência à recombinação interfacial, sugerindo que maior passivação e grau de cobertura da superfície previnem a recombinação interfacial. Apesar disso, a eficiência das células sensibilizadas com o corante C2-LEN é muito inferior ao das células sensibilizadas com os corantes C1-LEN e N719. O que sugere que a eficiência nestas células não está limitada pelo número reduzido de portadores transportados ou pela elevada recombinação na interface, mas sim pela fraca injeção de eletrões do nível LUMO do corante para a banda de condução do TiO2. Isto também está de acordo com os cálculos TDFDT, uma vez que a densidade eletrónica do nível LUMO do corante C2-LEN está mais afastada do grupo de ancoragem, comparativamente ao corante C1-LEN.The work presented in this thesis aims to understand the underlying mechanisms governing the functioning of dye-sensitized solar cells employing two new recently synthesized types of coumarin derivative dyes, C1-LEN and C2-LEN. The behaviour of the dyes was compared to the standard N719 dye whilst employing mesoporous TiO2 films. The properties of the solar cells were analyzed via scanning electron microscopy, incident photon to conversion efficiency, current voltage curves and electrochemical impedance spectroscopy. The AM1.5 power conversion efficiency of the bests cells was 3.2%, 2.2% and 0.06% for the N719, C1-LEN and C2-LEN respectively. The current produced by the C1-LEN sensitized cell was comparable to that of the N719 sensitized cell, c.a. 6mA/cm2. However, the open-circuit voltage was c.a. 200mV lower with the evidence suggesting that the conduction band edge of the TiO2 is more positive. The C2-LEN sensitized cells performed poorly with the short-circuit current density of c.a 0.3mA/cm2 and open-circuit voltage of c.a. 400mV. It was found that films sensitized with the C1-LEN and C2-LEN demonstrated higher resistance to interfacial recombination, suggesting a higher surface coverage and passivation thus preventing interfacial recombination. However, the power conversion efficiency of the cells sensitized with the C2-LEN dye was significantly lower than those sensitized with the C1-LEN and N719 dye. This suggests that the efficiency is not limited by poor charge transport and high interfacial recombination, but by poor electron injection from the LUMO level of the dye into the conduction band of the TiO2. This is also in accordance with recent TDFDT calculations, where the electron density of the LUMO level of the C2-LEN dye is not as close to the anchoring groups when compared to the C1-LEN dye

Quantum dot sensitized estrogen receptor alpha-recombinant protein electrochemical biosensor for 17-beta estradiol

Philosophiae Doctor - PhDEstrogens play an extraordinary role in the endocrine system regulation through the stimulation and regulation of endocrine pathways. 17β-estradiol is one of the final metabolites in estrogen regulation by hydroxylase enzymes that are well recognized for their metabolic role in hormone fragmentation and dissociation, through hydroxylation reactions that reversibly convert a series of androgens to estrogens (i.e. or one estrogen to the other). However, the 17β-estradiol hormone has been classified as one of the estrogenic endocrine disrupting compounds {i.e. EDC (s)} that show significant adverse effects in the estrogen pathways of male and female animal species. Estrogen receptor alpha (ER-α) is significantly activated by 17β-estradiol, which is a steroid hormone. A biosensor system for the determination of 17β-estradiol was developed based on the highly selective and specific physiological substrate level activation of the ER-α biomolecule by the (17β-estradiol) compound. The chemically-tuned tin selenide quantum dots capped with 3-mercaptopropionic acid were produced at room temperature and employed to capture the ER-α micro-molecule onto the electrode surfaces. These quantum dots possessed average particle size (APS) diameters between 4.6 ± 0.6 nm and an indirect band gap energy (Eg) of 3.14 eV. Surface modification on the quantum dots permitted the formation of efficient amide bonds between the capping molecules of the quantum dots and the estrogen receptor-alpha. The tin selenide quantum dots platform enhanced the surface bio-reactivity of the receptorsensor film. The receptorsensor’s sensitivity towards 17β-estradiol was 5.9 μA/μM associated with a response time (tResponse) of less than 1.2 s. The formal potential, Ep˚ˈ, of the receptorsensor-substrate complex was 149 mV. A detection limit (DL) of 1.9 nM was obtained for the electrochemical biosensing methodology. 17β-estradiol–receptorsensor response kinetics were also evaluated, where a dissociation rate (kd) of 7.6 μM/s, a 50 % inhibition concentration (IC50) value of 3.4 nM and a binding efficiency (Bmax) of 7 nM were obtained. Effective measure of 17β-estradiol concentrations as low as 3.8 nM present in surface waters have been reported to induce feminisation in male aquatic species. The receptorsensor’s dynamic linear range (DLR) nevertheless showed capability of screening a minimum of 0.2 nM to a maximum of 8 nM of the 17β-estradiol concentrations. Furthermore, during the estrogen replacement therapy (ERT), 17β-estradiol concentration levels are monitored at frequent phases, wherein 17β-estradiol concentrations from as low as 0.37 nM are recovered in the serum (i.e. this value was also evaluated to be within the receptorsensor’s-DLR), determining its future capability to be developed for; clinical-diagnosis screening of the 17β-estradiol