In this thesis, cadmium selenide (CdSe) quantum dots (QDs) are synthesized and characterized for patterning applications as well as for photovoltaic devices. The QDs were patterned and embedded into various polymers to form fluorescent composites. Their photophysical properties were investigated in detail. Through template assisted deposition the QDs-polymer composites were patterned into fluorescent nanorods. CdSe QDs were combined with multi-wall carbon nanotubes (CNTs) using a synthesized organic perylene derivative dye (N,N'-di(ethanethiol)-perylene-3,4,9,10-tetracarboxyl diimide) (ETPTCDI) as a link between QDs and CNTs. Upon testing, the QDs-ETPTCDI-CNTs nanocomposite displayed photoactive properties. Photophysical quenching studies of QD-ETPTCDI-CNTs provided better understanding of the electron-hole transfer of each component in the nanocomposite. The nanocomposite material was patterned onto microelectrode devices for photocurrent measurements under an AM1.5 solar simulated light source. These nanocomposites can be used as photovoltaic devices. The preliminary characterization studies of the device show excellent photoresponse under AM1.5 solar simulated light. The band gap alignment of each component of the nanocomposite and the charge transfer kinetics are the key to efficient electron-hole transfer. Optimization of the semiconducting material's interface can potentially make these nanocomposites a system for photovoltaic-based devices.

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Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal.

This dissertation focused on the applications of self-assembled monolayers (SAMs) technique for the investigation of molecule based electronics, plasmon coupling between CdSe quantum dots and metal nanoparticles (MNPs), and copper ion detection using enhanced emission of CdSe quantum dots (QDs). The SAMs technique provides an approach to establish a robust, two-dimensional and densely packed structure which can be formed on metal or semiconductor surfaces. This allows for the design of molecular assemblies that can be used to understand the details of molecular conduction by employing various electrical testbeds. In this work, the strategy of molecular assemblies was used to pattern metal nanoparticles on GaAs surfaces, thereby furnishing a platform to explore the interactions between QDs and MNPs. The enhanced emission of CdSe QDs by MNPs was then used as a probe for ultrasensitive, cheap, and rapid copper(II) detection. The study is divided into three main facets. The first one aimed at controlling electron transport behavior through porphyrins on surfaces with an eye toward optoelectronic and light harvesting applications. The binding of the porphyrin molecules to Au surfaces, pre-covered with a dodecanethiol matrix, was characterized by FTIR, XPS, AFM, STM, of. This study has shown that the perfluoro coupling group between the porphyrin macrocycle and the thiol tether may provide a means of controlling the tunneling behavior. The second area of this study focused on the design of a simple platform to examine the coupling between metal nanostructures and quantum dot assemblies. Here we demonstrate that by using a patterned array of Au or Ag nanoparticles on GaAs, plasmon enhanced photoluminescence (PL) can be directly measured and quantified by direct scaling of regions with and without metal nanostructures. The third field presented a simple manner for using the enhanced PL of CdSe QDs as a probe for ultrasensitive Cu2+ ion detection and quantitative analysis. The PL of QDs was enhanced by two processes: first, photobrightening of the material, and second, plasmonic enhancement by coupling with Ag nanoprisms. This strong PL leads to a high sensitivity of the QDs over a wide dynamic range for Cu2+ detection, as Cu2+ efficiently quenches the QD emission.

Abstract We have extended the spectral range of the current PL-upconversion apparatus to be operated in infrared. Using the IRPL-upconversion¡Awe study the behavior of carrier cooling of InN film and the relationship between the spacer and lifetime in InAs/GaAs stacked QDs . We excited InN film of the band gap of 0.74eV with ultrafast Ti:sapphire laser of the wavelength 404nm. We found the phonon emission time by hot carriers of InN is 14fs. The hot carriers release their excess energy to the lattice of 35K with a timescale of 100ps. We observed in InAs/GaAs QDs that the shorter life time for samples with thin spacer is due to tunneling effect.

No description available.

The motivation of this thesis is to understand the physical properties of semiconductor quantum dots (QDs) and to get insight on the basic physics of charge separation in composites made from QDs with graphene oxide (GO)/organic semiconductors. The flexion phonon interactions is one of fundamental issues in solid state physics, which has a significant effect on both electrical and optical properties of solid state materials. This thesis investigates the physical properties of aqueous grown QDs through exciton-phonon coupling and non-radiative relaxation of excited carriers which have been carried out by temperature dependent photoluminescence spectroscopy. Several e orts have been made in order to understand the basic physics of photo induced charge separation in the hybrid systems made from QDs with graphene oxide and organic semiconductors. Investigations on the photoconductivity of the devices made from these hybrid composites have been carried out keeping the motive of its application in nanotechnology. This thesis work is presented in six chapters inclusive of summary and directions for future work. Chapter 1 discusses the background knowledge and information of the general properties of semiconductor nanostructures, QDs and their hybrid nanocomposites. Chapter 2 deals with the sample preparation and experimental techniques used in this thesis. Chapter 3 elaborates the exciton-phonon scattering and nonradiative relaxations of excited carriers in visible emitting cadmium telluride QDs with help of temperature and size dependent photoluminescence. Chapter 4 presents the investigations on time resolved photoluminescence dynamics and temperature dependent photoluminescence properties of near infrared (NIR) emitting mercury cadmium telluride (CdHgTe). Chapter 5 discusses the importance of NIR emitting silver sulphide (Ag2S) QDs and gives insight of nonradiative recombinations through defect/trap states. Chapter 6 investigates the excited state interactions between CdHgTe QDs and GO. Chapter 7 focuses on the understanding of basic physics of charge separation/transfer between poly (3hexylthiophene) and Ag2S QDs. Chapter 1: Semiconductor nanostructures have attracted significant scientific attention due to their fundamental physical properties and technological interests. Quasi zero dimensional nanocrystals or quantum dots (QDs) have shown unique optical and electrical properties compared to its bulk counterpart. These QDs show discrete energy levels due to the quantum confinement effect hence known as arti cial atoms. Large surface to volume ratio in these QDs is expected to play a crucial role in determing the photo-physical properties. Temperature dependent photoluminescence is a powerful tool for understanding the role of the large surface area on exciton recombination process in QDs. Inorganic QDs combined with different materials like graphene oxide or organic semiconductors forms an exciting class of synthetic materials which integrates the properties of organic and inorganic semiconductors. It is quite important to understand the basic physics of electronic interactions in these composites for its future application in many elds. Chapter 2: Synthesis of the inorganic QDs, graphene oxide, composites and fabrication of devices is an important and integral part of this thesis. Hydrothermal and three necked ask technique is adopted to get highly dispersible colloidal quantum dots in solvents. Synthesis of graphene oxide from graphite through oxidation and ultrasonication has been carried out to obtain homogenous dispersed graphene oxide in water. Structural properties have been studied by techniques like X ray diffraction, Raman spectroscopy, X ray photoelectron spectroscopy and high resolution transmission electron microscopy. Morphological properties are studied by atomic force microscopy and transmission electron microscopy. Optical properties are investigated by absorption spectroscopy, steady state and time resolved photoluminescence spectroscopy. Photoconductivity characteristics are analyzed to understand the basics of enhanced current in the various devices made from QDs composites. Chapter 3:Investigations on exciton phonon coupling and nonradiative relaxations in various sizes of visible light emitting cadmium telluride (CdTe) QDs size have been presented. Due to the large surface area, QDs are prone to have defect/trap states which can affect the exciton relaxation. Hence, understanding the role of such defect/trap states on photoluminescence is very essential for achieving the optimum optical properties. Temperature dependent (15 300 K) photoluminescence has been used to understand nonradiative relaxation of excited carriers. Thermally activated processes and multiple phonons scattering is thoroughly investigated to understand the quenching of photoluminescence with temperature. The strength of exciton-phonon coupling is investigated which determines the variation in energy bandgap of QDs with temperature. Role of exciton phonon scattering is also discussed to understand the basic physics of photoluminescence line width broadening in QDs. Chapter 4 and 5: This part of thesis focuses on the size and temperature pho-toluminescence properties of near infra red emitting ternary alloyed CdHgTe and Ag2S QDs. Near infrared emitting semiconductor quantum dots (QDs) have attracted significant scientific and technological interests due to their potential applications in the fields of photosensor, solar energy harvesting cells, telecommunication and biological tissue imaging etc. Structural and photophysical properties of CdHgTe QDs have been analyzed by high resolution transmission electron microscopy, X rayphotoelectron microscopy, photoluminescence decay kinetics and low temperature photoluminescence. Investigations on the nonradiative recombinations through trap/defects states and exciton phonon coupling are carried out in colloidal Ag 2S QDs which emits in the range of 1065 1260 nm. Particularly, the photoluminescence quenching mechanism with increasing temperature is analyzed in the presence of multiple nonradiative relaxation channels, where the excited carriers are thermally stimulated to the surface defect/trap states of QDs. Chapter 6 and 7: The aim of these chapters is to understand the basic physics of photo induced charge separation in the hybrid systems made from the inorganic QDs with graphene oxide and organic semiconductors. In chapter 6, CdHgTe QDs are decorated on graphene oxide sheets through physisorption. The excited state electronic interactions have been studied by optical and electrical characterizations in these CdHgTe QDs GO hybrid systems. In chapter 7, investigations are carried out for understanding the basic physics of charge separation in the composites of Ag2S QDs and poly (3hexylthiophene 2,5 diyl)(P3HT). These composites of inorganic organic materials are made by simple mixing with help of ultrasonication technique. Steady state and time resolved photoluminescence measurements are used as powerful technique to gain insight of energy/charge transfer process between P3HT and Ag2S QDs. Furthermore, investigations have been carried out on the photoconductivity of the devices made from these hybrid composites keeping the motive of its application in nanotechnology. Chapter 8: The conclusions of the work presented in this thesis are coherently summarized in this chapter. Thoughts and prospective for future directions are also summed up.

Ces dernières années, les énormes progrès réalisés en nanotechnologie ainsi qu’en science des matériaux ont conduit à la préparation de nombreuses nouvelles nanoparticules sans réellement connaître l’ensemble des propriétés associées à leurs dimensions. La première partie de notre travail vise à évaluer les risques et les problèmes associés aux nanomatériaux, en termes de toxicité, en utilisant des nanoparticules de ZnO. Nous avons tout d’abord étudié la capacité de ces nanoparticules à générer des espèces réactives d’oxygènes (EROs) sous irradiation UV en utilisant trois types des quantum dots (QDs) comme modèles, ZnO, ZnO dopé avec des ions Cu2+ et ZnO avec des ions Cu2+ adsorbés à sa surface. Les trois types des QDs ont montré une forte capacité à générer des EROs mais ceux modifiés par les ions Cu2+ en périphérie sont les plus producteurs. Ces QDs inhibent également le plus fortement la croissance de la bactérie E. coli. La toxicité n’est cependant pas dépendante des EROs photo-produits ni du zinc (+2) libéré par les QDs et montre qu’un mécanisme plus complexe doit être considéré. Dans une second partie, nous avons tenté d’améliorer l’activité photocatalytique de nanobâtonnets de ZnO en les associant à de l’oxyde de graphène réduit (rGO). Des nanocomposites ZnO/rGO ont été préparés par voie solvothermale et utilisés pour la phototodégradation du colorant Orange II comme modèle de polluant. Les résultats obtenus montrent que le photocatalyseur ZnO/rGO est très efficace sous irradiation solaire ou visible et qu’il est peu sensible à des variations de pH ou à la présence de perturbateurs dans le milieu. Finalement, le photocatalyseur est très stable et peut être réutilisé plus de dix fois sans perte notable d’activité. / In recent years, tremendous advances in nanotechnology and materials science have led to the synthesis of many new nanoparticles without really knowing all the properties associated with their dimensions. The first part of our work aims to evaluate the risks and problems associated with nanomaterials, in terms of toxicity, using ZnO nanoparticles. We first studied the ability of these nanoparticles to produce reactive oxygen species (ROS) under UV irradiation using three ZnO-based quantum dots (QDs) as models, ZnO, ZnO doped with Cu2+ ions and ZnO with chimisorbed Cu2+ ions at their periphery. The three QDs have a strong capacity of generating ROS but those modified with Cu2+ at their surface were found the be the highest producers. These dots were also found to inhibit more markedly the growth of the E. coli bacteria. The toxicity does neither depend on the amount of photo-generated ROS nor on the amount of Zn(+2) leaked by the QDs, thus indicating that a more complex mechanism should be considered. In a second part, we tried to improve the photocatalytic efficiency of ZnO nanorods by associating these nanomaterials with reduced graphene oxide (rGO). ZnO/rGO composites were prepared by a solvothermal method and applied for the photodegradation of Orange II used as model pollutant. Results obtained demonstrate that the ZnO/rGO photocatalyst is highly efficient under solar and under visible light irradiation and weakly sensitive to pH changes and to the presence of perturbators in the reaction medium. Finally, the photocatalyst is stable and can be reused up to ten times without significant loss of catalytic activity.

Le phototransistor est un nouveau type de photo-détecteur avec une structure MOSFET spéciale qui peut non seulement convertir la lumière absorbée en variation de courant, mais également auto-amplifier ce photo-courant. En particulier, avec des progrès continus dans la synthèse des points Quantum Dots (QDs), les caractères optiques et électriques uniques renforcent le coefficient d'absorption et la génération des trous d'électrons par des processus intégrés faciles. Dans cette thèse, on a synthétisé les PdS infrarouges PbS avec une large absorption infrarouge (IR) (600-1400 nm) et un rendement élevé pour être mélangés avec l'isolateur de porte SU8 des TFT à faible température de poly-silicium (LTPS). Grâce à l'utilisation de cet isolateur de porte photo-sensoriel hybride, ces LTPS TFT peuvent encore obtenir d'excellentes performances électriques telles qu'une mobilité suffisante (3.1 cm2 / Vs), des caractères TFT stables, un rapport marche / arrêt raisonnable (104 ~ 105) et une tension sous-seuil /Déc). De plus, en cas d'exposition à la lumière infrarouge incidente, la sensibilité élevée (1800 A/W) et la sensibilité non négligeable (13 A/W) se trouvent respectivement à 760 nm et 1300 nm. De plus, la photosensibilité atteint également jusqu'à 80 et le temps de réponse est d'environ 30 ms pendant un balayage du signal IR pulsé. Elle prend des mesures concrètes pour l'application générale du phototransistor IR. / Phototransistor is a novel type of photodetector with special MOSFET structure which can not only convert absorbed light into variation of current but also self-amplify this photocurrent. Especially, with continual advances in quantum dots' (QDs) synthesis, the unique optical-electrical characters reinforce absorption coefficient and electron-hole's generation by easy integrated processes. In this thesis, the infrared PbS QDs with wide infrared (IR) absorption (600-1400 nm) and high efficiency were synthesized to be blended with SU8 gate insulator of Low-Temperature-Poly-Silicon (LTPS) TFTs. Through using this hybrid photo-sensing gate insulator, this LTPS TFTs can still obtain excellent electrical performance such as enough mobility (3.1 cm2/Vs), stable TFT's characters, reasonable on/off ratio (104~105) and subthreshold voltage (3.2 V/Dec). Moreover, under incident IR light's exposure, the high responsivity (1800 A/W) and not negligible responsivity (13 A/W) can be found at 760 nm and 1300 nm respectively. In addition, the photosensitivity also reaches up to 80 and the response time is approximately 30 ms during a pulsed IR signal's scanning. It takes concrete steps forward for the broad application of IR phototransistor.

The objective of this thesis is to develop an easy-to-use and computationally economical numerical tool to investigate the flow field in the Pulsed Pressure Chemical Vapour Deposition (PP-CVD) reactor. The PP-CVD process is a novel thin film deposition technique with some advantages over traditional CVD methods. The numerical modelling of the PP-CVD flow field is carried out using the Quiet Direct Simulation (QDS) method, which is a flux-based kinetic-theory approach. Two approaches are considered for the flux reconstruction, which are the true directional manner and the directional splitting method. Both the true directional and the directional decoupled QDS codes are validated against various numerical methods which include EFM, direct simulation, Riemann solver and the Godunov method. Both two dimensional and axisymmetric test problems are considered. Simulations are conducted to investigate the PP-CVD reactor flow field at 1 Pa and 1 kPa reactor base pressures. A droplet flash evaporation model is presented to model the evaporation and transport of the liquid droplets injected. The solution of the droplet flash evaporation model is used as the inlet conditions for the QDS gas phase solver. The droplet model is found to be able to provide pressure rise in the reactor at the predicted rate. A series of parametric studies are conducted for the PP-CVD process. The numerical study confirms the hypothesis that the flow field uniformity is insensitive to the reactor geometry. However, a sufficient distance from the injection inlet is required to allow the injected precursor solution to diffuse uniformly before reaching the substrate. It is also recommended that placement of the substrate at the reactor’s centre axis should be avoided.