Environmental and economic factors demand radical advances in solar cell technologies. Organic and polymer photovoltaics emerged in the 1990's that have led to low cost per unit area, enabled in significant part by the convenient manufacturing of roll-to-roll-processible solution-cast semiconductors. Colloidal quantum dot solar cells dramatically increase the potential for solar conversion efficiency relative to organics by enabling optimal matching of a photovoltaic device's bandgap to the sun's spectrum. Infrared-absorbing colloidal quantum dot solar cells were first reported in 2005. At the outset of this study in 2007, they had been advanced to the point of achieving 1.8% solar power conversion efficiency. These devices degraded completely within a few hours’ air exposure. The origin of the extremely poor device stability was unknown and unstudied. The efficiency of these devices was speculated to be limited by poor carrier transport and passivation within the quantum dot solid, and by the limitations of the Schottky device architecture. This study sought to tackle three principal challenges facing colloidal quantum dot photovoltaics: stability; understanding; and performance. In the first part of this work, we report the first air-stable infrared colloidal quantum dot photovoltaics. Our devices have a solar power conversion efficiency of 2.1%. These devices, unencapsulated and operating in an air atmosphere, retain 90% of their original performance following 3 days’ continuous solar harvesting. The remarkable improvement in device stability originated from two new insights. First, we showed that inserting a thin LiF layer between PbS film and Al electrode blocks detrimental interfacial reactions. Second, we proposed and validated a model that explains why quantum dots having cation-rich surfaces afford dramatically improved air stability within the quantum dot solid. The success of the cation-enrichment strategy led us to a new concept: what if - rather than rely on organic ligands, as all prior quantum dot photovoltaics work had done - one could instead terminate the surface of quantum dots exclusively using inorganic materials? We termed our new materials strategy ionic passivation. The goal of the approach was to bring our nanoparticles into the closest possible contact while still maintaining quantum confinement; and at the same time achieving a maximum of passivation of the nanoparticles' surfaces. We showcase our ionic passivation strategy by building a photovoltaic device that achieves 5.8% solar power conversion efficiency. This is the highest-ever solar power conversion efficiency reported in a colloidal quantum dot device. More generally, our ionic passivation strategy breaks the past tradeoff between transport and passivation in quantum dot solids. The advance is relevant to electroluminescent and photodetection devices as well as to the record-performing photovoltaic devices reported herein.

Solution processed solar cells offer the promise of a low cost solution to global energy concerns. Colloidal quantum dots are one material that can be easily synthesized in and deposited from solution. These nanoparticles also offer the unique ability to select the desired optical and electrical characteristics, all within the same materials system, through small variations in their physical dimensions. These materials, unfortunately, are not without their limitations. To date, films made from colloidal quantum dots exhibit limited mobilities and short minority diffusion lengths. These limitations imply that simple device structures may not be sufficient to make an efficient solar cell. Here we show that through clever manipulation of the geometric and energetic structures, we can utilize the size-tunability of CQDs while masking their poor electrical characteristics. We further outline the physical mechanisms present within these architectures, namely the utilization of a distributed built-in electric field to extract current through drift rather than diffusion. These architectures have consequently exceeded the performance of legacy architectures such as the Schottky cell. Finally, we discuss some of the limiting modes within these architectures and within CQD films in general including the impact of surface traps and polydispersity in CQD populations. Through the development of these novel architectures, the power conversion efficiency of CQD solar cells has increased from ~3.5% to 7.4%; the highest efficiencies yet reported for colloidal quantum dot solar cells.

A novel type of white light emitting semiconductor quantum dot was characterized at the ensemble and single-molecule level. This kind of semiconductor nanocrystal can be made into white light emitting diodes, which have the potential to replace conventional lighting sources. The quantum dots used in this thesis consisted of a cadmium selenide (CdSe) core, capped with ZnS, and have a surface polymer coating of poly(acrylic acid) (PAA). We have characterized the quantum dot size distribution by using dynamic light scattering (DLS), transmission electron microscopy (TEM), atomic force microscopy (AFM) and UV-Vis spectroscopy. Based on these measurements, it is clear that the white quantum dots are polydisperse, with a core size of 2.4 ± 0.5 nm, though the polymer coating swells considerably in aqueous solution. In order to explore the optical properties, the absorption and emission spectra of the ensemble quantum dots solution were measured and compared to “standard” commercial quantum dots. The emission spectrum of the white quantum dots showed two peaks, a strong blue emission peak and a weaker red emission peak. The fluorescence quantum yield of the white quantum dots was found to be less than that of commercial quantum dots. To explore the behavior of individual quantum dots, spatially-resolved single-molecule images were obtained by a dual-view single molecule fluorescence microscopy with a beam splitter which can separate the emission into red and blue components. It was found that individual white CdSe nanocrystals have a broad emission spectrum and the samples did not consist of a mixed population of red emitters and blue emitters. These results suggest that these white light emitting quantum dots can be used for pure white light LEDs and are a good candidate for the replacement for conventional lighting sources.

This thesis presents optical trapping of various single nanoparticles, and the method for integrating the optical trap system into a microfluidic channel to examine the trapping stiffness and to study binding at the single molecule level. Optical trapping is the capability to immobilize, move, and manipulate small objects in a gentle way. Conventional trapping methods are able to trap dielectric particles with size greater than 100 nm. Optical trapping using nanostructures has overcome this limitation so that it has been of interest to trap nanoparticles for bio-analytical studies. In particular, aperture optical trapping allows for trapping at low powers, and easy detection of the trapping events by noting abrupt jumps in the transmission intensity of the trapping beam through the aperture. Improved trapping efficiency has been achieved by changing the aperture shape from a circle; for example, to a rectangle, double nanohole (DNH), or coaxial aperture. The DNH has the advantage of a well-defined trapping region between the two cusps where the nanoholes overlap, which typically allows only single particle trapping due to steric hindrance. Trapping of 21 nm encapsulated quantum dot has been achieved which shows optical trapping can be used in technologies that seek to place a quantum dot at a specific location in a plasmonic or nanophotonic structure. The DNH has been used to trap and unfold a single protein. The high signal-to-noise ratio of 33 in monitoring single protein trapping and unfolding shows a tremendous potential for using the double nanohole as a sensor for protein binding events at a single molecule level. The DNH integrated in a microfluidic chip with flow to show that stable trapping can be achieved under reasonable flow rates of a few µL/min. With such stable trapping under flow, it is possible to envision co-trapping of proteins to study their interactions. Co-trapping is achieved for the case where we flow in a protein (bovine serum albumin – BSA) and co-trap its antibody (anti-BSA). / Graduate / 0544 / 0752 / oskuie@uvic.ca

My Ph.D. thesis is presented in two parts. In the first part, I describe the preparation of organic-inorganic hybrid micelles formed from poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) block copolymers and CdSe quantum dots (QDs). Several distinct morphologies were observed including, spheres, finite-sized wormlike networks and clusters of hollow vesicles. A series of experiments were carried out to explore whether these hybrid colloids were thermodynamically stable or formed under kinetic control. Upon addition of 2-propanol (2-PrOH) to a chloroform solution containing a mixture of PS404-b-P4VP76 plus CdSe QDs (2-PrOH is a good solvent for P4VP block and a precipitant for PS block and QDs), uniform spherical micelles formed almost instantly, with a PS core and a thin P4VP corona to which the QDs were attached. Vigorous stirring of this solution for two days led to the formation of three-dimensional wormlike networks consisted of Y-junctions and cylindrical struts, terminated by bulbous spherical end-caps. Even more profound structural changes occurred when the solution was subjected to prolonged magnetic stirring (e.g. 1 month). ii In contrast, manipulating the chemical composition of the initial block copolymer could trigger a spontaneous structural transition from sphere to network of wormlike micelles over 2 h without the need of stirring. The second part of the thesis begins by describing a modular approach for preparing polyferrocenyldimethylsilane (PFS) block copolymers via a Cu-catalyzed alkyne/azide coupling reaction to covalently combine two homopolymers synthesized separately. This strategy opens the door to a broad library of novel functional PFS block copolymers, for example, poly(ferrocenyldimethylsilane-b-N-isopropyl acrylamide) (PFS-b-PNIPAM). In an attempt to expand our understanding of PFS block copolymer self-assembly in polar solvents, I investigated the self-assembly of a new polymer (PFS26-b-PNIPAM105) in alcohol solvents. When the block polymer was dissolved in methanol, ethanol and 2-propanol, it formed long fiber-like micelles with uniform width. I also showed that micelles of this polymer underwent seeded growth in methanol, leading to cylindrical micelles that were nearly mono- dispersed in length.

My Ph.D. thesis is presented in two parts. In the first part, I describe the preparation of organic-inorganic hybrid micelles formed from poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) block copolymers and CdSe quantum dots (QDs). Several distinct morphologies were observed including, spheres, finite-sized wormlike networks and clusters of hollow vesicles. A series of experiments were carried out to explore whether these hybrid colloids were thermodynamically stable or formed under kinetic control. Upon addition of 2-propanol (2-PrOH) to a chloroform solution containing a mixture of PS404-b-P4VP76 plus CdSe QDs (2-PrOH is a good solvent for P4VP block and a precipitant for PS block and QDs), uniform spherical micelles formed almost instantly, with a PS core and a thin P4VP corona to which the QDs were attached. Vigorous stirring of this solution for two days led to the formation of three-dimensional wormlike networks consisted of Y-junctions and cylindrical struts, terminated by bulbous spherical end-caps. Even more profound structural changes occurred when the solution was subjected to prolonged magnetic stirring (e.g. 1 month). ii In contrast, manipulating the chemical composition of the initial block copolymer could trigger a spontaneous structural transition from sphere to network of wormlike micelles over 2 h without the need of stirring. The second part of the thesis begins by describing a modular approach for preparing polyferrocenyldimethylsilane (PFS) block copolymers via a Cu-catalyzed alkyne/azide coupling reaction to covalently combine two homopolymers synthesized separately. This strategy opens the door to a broad library of novel functional PFS block copolymers, for example, poly(ferrocenyldimethylsilane-b-N-isopropyl acrylamide) (PFS-b-PNIPAM). In an attempt to expand our understanding of PFS block copolymer self-assembly in polar solvents, I investigated the self-assembly of a new polymer (PFS26-b-PNIPAM105) in alcohol solvents. When the block polymer was dissolved in methanol, ethanol and 2-propanol, it formed long fiber-like micelles with uniform width. I also showed that micelles of this polymer underwent seeded growth in methanol, leading to cylindrical micelles that were nearly mono- dispersed in length.

Environmental and economic factors demand radical advances in solar cell technologies. Organic and polymer photovoltaics emerged in the 1990's that have led to low cost per unit area, enabled in significant part by the convenient manufacturing of roll-to-roll-processible solution-cast semiconductors. Colloidal quantum dot solar cells dramatically increase the potential for solar conversion efficiency relative to organics by enabling optimal matching of a photovoltaic device's bandgap to the sun's spectrum. Infrared-absorbing colloidal quantum dot solar cells were first reported in 2005. At the outset of this study in 2007, they had been advanced to the point of achieving 1.8% solar power conversion efficiency. These devices degraded completely within a few hours’ air exposure. The origin of the extremely poor device stability was unknown and unstudied. The efficiency of these devices was speculated to be limited by poor carrier transport and passivation within the quantum dot solid, and by the limitations of the Schottky device architecture. This study sought to tackle three principal challenges facing colloidal quantum dot photovoltaics: stability; understanding; and performance. In the first part of this work, we report the first air-stable infrared colloidal quantum dot photovoltaics. Our devices have a solar power conversion efficiency of 2.1%. These devices, unencapsulated and operating in an air atmosphere, retain 90% of their original performance following 3 days’ continuous solar harvesting. The remarkable improvement in device stability originated from two new insights. First, we showed that inserting a thin LiF layer between PbS film and Al electrode blocks detrimental interfacial reactions. Second, we proposed and validated a model that explains why quantum dots having cation-rich surfaces afford dramatically improved air stability within the quantum dot solid. The success of the cation-enrichment strategy led us to a new concept: what if - rather than rely on organic ligands, as all prior quantum dot photovoltaics work had done - one could instead terminate the surface of quantum dots exclusively using inorganic materials? We termed our new materials strategy ionic passivation. The goal of the approach was to bring our nanoparticles into the closest possible contact while still maintaining quantum confinement; and at the same time achieving a maximum of passivation of the nanoparticles' surfaces. We showcase our ionic passivation strategy by building a photovoltaic device that achieves 5.8% solar power conversion efficiency. This is the highest-ever solar power conversion efficiency reported in a colloidal quantum dot device. More generally, our ionic passivation strategy breaks the past tradeoff between transport and passivation in quantum dot solids. The advance is relevant to electroluminescent and photodetection devices as well as to the record-performing photovoltaic devices reported herein.

Solution processed solar cells offer the promise of a low cost solution to global energy concerns. Colloidal quantum dots are one material that can be easily synthesized in and deposited from solution. These nanoparticles also offer the unique ability to select the desired optical and electrical characteristics, all within the same materials system, through small variations in their physical dimensions. These materials, unfortunately, are not without their limitations. To date, films made from colloidal quantum dots exhibit limited mobilities and short minority diffusion lengths. These limitations imply that simple device structures may not be sufficient to make an efficient solar cell. Here we show that through clever manipulation of the geometric and energetic structures, we can utilize the size-tunability of CQDs while masking their poor electrical characteristics. We further outline the physical mechanisms present within these architectures, namely the utilization of a distributed built-in electric field to extract current through drift rather than diffusion. These architectures have consequently exceeded the performance of legacy architectures such as the Schottky cell. Finally, we discuss some of the limiting modes within these architectures and within CQD films in general including the impact of surface traps and polydispersity in CQD populations. Through the development of these novel architectures, the power conversion efficiency of CQD solar cells has increased from ~3.5% to 7.4%; the highest efficiencies yet reported for colloidal quantum dot solar cells.

Studies various nanostructured materials have gained considerable interest within the past several decades. This novel class of materials has opened up a new realm of possibilities, both for the fundamental comprehension of matter, but also for innovative applications. The size-dependent effect observed for these systems often lies in their interaction with the surrounding environment and understanding such interactions is the pivotal point for the investigations undertaken in this thesis. Three families of nanoparticles are analyzed: semiconductor quantum dots, metallic silver nanoparticles and rare-earth oxide nanomaterials. The radical scavenging ability of cerium oxide nanoparticles (CeO2) is quite controversial since they have been labeled as both oxidizing and antioxidant species for biological systems. Here, both aqueous and organic stabilized nanoparticles are examined in straightforward systems containing only one reactive oxygen species to ensure a controlled release. The apparent absence of their direct radical scavenging ability is demonstrated despite the ease at which CeO2 nanoparticles generate stable surface Ce3+ clusters, which is used to explain the redox activity of these nanomaterials. On the contrary, CeO2 nanoparticles are shown to have an indirect scavenging effect in Fenton reactions by annihilating the reactivity of Fe2+ salts. Cadmium selenide quantum dots (CdSe QD) constitute another highly appealing family of nanocolloids in part due to their tunable, size-dependent luminescence across the visible spectrum. The effect of elemental sulfur treatment is investigated to overcome one of the main drawbacks of CdSe QD: low fluorescence quantum yield. Herein, we report a constant and reproducible quantum yield of 15%. The effect of sulfur surface treatment is also assessed following the growth of a silica shell, as well as the response towards a solution quencher (4-amino-TEMPO). The sulfur treated QD is also tested for interaction with pyronin Y, a xanthene dye that offers potential energy and electron transfer applications with the QD. Interaction with the dye molecule is compared to results obtained with untreated quantum dots, as well as CdSe/ZnS core shell examples. In another chapter of this thesis, the catalytic potential of silver nanoparticles is addressed for the grafting of polyhydrosiloxane polymer chains with various alkoxy groups. A simple one-pot synthesis is presented with silver salts and the polymer. the latter serves as a mild reducing agent and a stabilizing ligand, once silver nanoparticles are formed in-situ. We evaluate the conversion of silane into silyl ethers groups with the addition of several alcohols, whether primary, secondary or tertiary, and report the yields of grafting under the mildest conditions: room temperature, under air and atmospheric pressure.

This thesis focuses on pioneering a scalable route to fabricate quantum information devices based upon single InAs/InP quantum dots emitting in the telecommunications wavelength band around 1550 nm. Using metallic gates in combination with nanotemplate, site-selective epitaxy techniques, arrays of single quantum dots are produced and electrostatically tuned with a high degree of control over the electrical and optical properties of each individual quantum dot. Using metallic gates to apply local electric fields, the number of electrons within each quantum dot can be tuned and the nature of the optical recombination process controlled. Four electrostatic gates mounted along the sides of a square-based, pyramidal nanotemplate in combination with a flat metallic gate on the back of the InP substrate allow the application of electric fields in any direction across a single quantum dot. Using lateral fields provided by the metallic gates on the sidewalls of the pyramid and a vertical electric field able to control the charge state of the quantum dot, the exchange splitting of the exciton, trion and biexciton are measured as a function of gate voltage. A quadrupole electric field configuration is predicted to symmetrize the product of electron and hole wavefunctions within the dot, producing two degenerate exciton states from the two possible optical decay pathways of the biexciton. Building upon these capabilities, the anisotropic exchange splitting between the exciton states within the biexciton cascade is shown to be reversibly tuned through zero for the first time. We show direct control over the electron and hole wavefunction symmetry, thus enabling the entanglement of emitted photon pairs in asymmetric quantum dots. Optical spectroscopy of single InAs/InP quantum dots atop pyramidal nanotemplates in magnetic fields up to 28T is used to examine the dispersion of the s, p and d shell states. The g-factor and diamagnetic shift of the exciton and charged exciton states from over thirty single quantum dots are calculated from the spectra. The g-factor shows a generally linear dependence on dot emission energy, in agreement with previous work on this subject. A positive linear correlation between diamagnetic coefficient and g-factor is observed.