A microfluidic lab-on-a-chip (LOC) device for in-situ synthesis of gold nano-particles was developed. The long term goal is to develop a portable hand-held diagnostic platform for monitoring water quality (e.g., detecting metal ion pollutants). The LOC consists of micro-chambers housing different reagents and samples that feed to a common reaction chamber. The reaction products are delivered to several waste chambers in a pre-defined sequence to enable reagents/ samples to flow into and out of the reaction chamber. Passive flow actuation is obtained by capillary driven flow (wicking) and dissolvable microstructures called ‘salt pillars’. The LOC does not require any external power source for actuation and the passive microvalves enable flow actuation at predefined intervals. The LOC and the dissolvable microstructures are fabricated using a combination of photolithography and soft lithography techniques. Experiments were conducted to demonstrate the variation in the valve actuation time with respect to valve position and geometric parameters. Subsequently, analytical models were developed using one dimensional linear diffusion theory. The analytical models were in good agreement with the experimental data. The microvalves were developed using various salts: polyethylene glycol, sodium chloride and sodium acetate. Synthesized in-situ in our experiments, gold nano-particles exhibit specific colorimetric and optical properties due to the surface plasmon resonance effect. These stabilized mono-disperse gold nano-particles can be coated with bio-molecular recognition motifs on their surfaces. A colorimetric peptide assay was thus developed using the intrinsic property of noble metal nano-particles. The LOC device was further developed on a paper microfluidics platform. This platform was tested successfully for synthesis of gold nano-particles using a peptide assay and using passive salt-bridge microvalves. This study proves the feasibility of a LOC device that utilizes peptide assay for synthesis of gold nano-particles in-situ. It could be highly significant in a simple portable water quality monitoring platform.

Spatial and temporal tracking of passively diffusing functionalized colloids continues to be an improving and auspicious approach to measuring weak specific and non-specific biomolecular interactions. Evidence of this is given by the recent increase in published studies involving the development and implementation of these methods. The primary aim of the work presented in this dissertation was to modify and optimize video microscopy (VM) and total internal reflection microscopy (TIRM) methods to permit the collection of equilibrium binding and sampling data from interaction of surface-immobilized biomolecules. Supported lipid bilayers were utilized as model systems for functionalizing colloid and wall surfaces. Preliminary results measuring calcium-specific protein-protein interactions between surface immobilized cadherin fragments demonstrate the potential utility of this experimental system and these methods. Additionally, quantum dot-modified colloids were synthesized and evanescent wave-excited luminescence from these particles was used to construct potential energy profiles. Results from this work demonstrate that colloids can be used as ultra-sensitive probes of equilibrium interactions between biomolecules, and specialized probes, such as those modified with quantum dots, could be used in a spectral multiplexing mode to simultaneously monitor multiple interactions.

Interparticle interactions determine the microstructure, stability, rheology, and optical properties of concentrated colloidal suspensions involved in paint, paper, cosmetic, and pharmaceutical industries, etc. Frequency domain photon migration (FDPM) involves modeling the photon transport in a multiple scattering medium as a diffusion process in order to simultaneously determine isotropic scattering and absorption coefficients from measured amplitude attenuation and phase shift of the propagating photon density wave. Using FDPM, we investigated the impact of electrostatic interaction upon the optical properties and structure of dense charged suspensions. We demonstrated that electrostatic interactions among charged polystyrene latex may significantly affect the light scattering properties and structure of dense suspensions at low ionic strength (<0.06 mM NaCl equivalent) by actual FDPM measurement. We showed that the structure factor models addressing electrostatic interaction can be used to describe the microstructure of charged suspensions and quenched scattering due to electrostatics, and demonstrated that FDPM has the potential to be a novel structure and surface charge probe for dense suspensions. We also showed that the FDPM measured isotropic scattering coefficients may respond to the change in effective particle surface charge, and displayed the potential of using FDPM for probing particle surface charge in concentrated suspensions. We presented that the interference approximation implies a linear relationship between the absorption coefficient and volume fraction of suspension. We illustrated that FDPM measured absorption coefficient varies linearly with suspension volume fraction and affirmed the interference approximation from a perspective of light absorption. The validation of the interference approximation enables us to develop the methodology for estimating absorption efficiencies and imaginary refractive indices for both particles and suspending fluid simultaneously using FDPM. We further demonstrated a novel application of FDPM measured absorption coefficients in determining pigment absorption spectra, and displayed the potential of using FDPM as a novel analytical tool in pigment and paint industry.

As devices and new technologies continue to shrink, nanocrystalline multi-metal compounds are becoming increasingly important for high efficiency and multifunctionality. However, synthetic methods to make desirable nanocrystalline multi-metallics are not yet matured. In response to this deficiency, we have developed several solution-based methods to synthesize nanocrystalline binary alloy and intermetallic compounds. This dissertation describes the processes we have developed, as well as our investigations into the use of lithographically patterned surfaces for template-directed self-assembly of solution dispersible colloids. We used a modified polyol process to synthesize nanocrystalline intermetallics of late transition and main-group metals in the M-Sn, Pt-M', and Co-Sb systems. These compounds are known to have interesting physical properties and as nanocrystalline materials they may be useful for magnetic, thermoelectric, and catalytic applications. While the polyol method is quite general, it is limited to metals that are somewhat easy to reduce. Accordingly, we focused our synthetic efforts on intermetallics comprised of highly electropositive metals. We find that we can react single-metal nanoparticles with zero-valent organometallic Zinc reagents in hot, coordinating amine solvents via a thermal decomposition process to form several intermetallics in the M''-Zn system. Characterization of the single-metal intermediates and final intermetallic products shows a general retention of morphology throughout the reaction, and changes in optical properties are also observed. Following this principle of conversion chemistry, we can employ the high reactivity of nanocrystals to reversibly convert between intermetallic phases within the Pt-Sn system, where PtSn2 ↔ PtSn ↔ Pt3Sn. Our conversion chemistry occurs in solution at temperatures below 300 °C and within 1 hour, highlighting the high reactivity of our nanocrystalline materials compared to the bulk. Some evidence of the generality for this process is also presented. Our nanocrystalline powders are dispersible in solution, and as such are amenable to solution-based processing techniques developed for colloidal dispersions. Accordingly, we have investigated the use of lithographically patterned surfaces to control the self-assembly of colloidal particles. We find that we can rapidly crystallize 2-dimensional building blocks, as well as use epitaxial templates to direct the formation of interesting superlattice structures comprised of a bidisperse population of particles.

The studies of the dynamics, phase behavior, interparticle interactions, and hydrodynamics of stimuli-responsive pNIPAm-co-AAc microgels were described in this thesis. Due to their responsiveness to external stimuli, these colloidal particles serve as excellent model systems to probe the relationship between colloidal interactions and phase behavior. As a first step, we established our core experimental methodology, by demonstrating that particle tracking video microscopy is an effective technique to quantify various parameters in colloidal systems. Then we used the technique in combination with a microfluidic device that provides in situ control over sample pH to probe the phase behavior of pNIPAm-co-AAc microgel suspensions. In essence, the experimental set-up enables changes in effective particle volume fractions by changing pH, which can be used to construct the phase diagram. In order to explain the unique features of the microgel phase diagram, we measured the underlying pairwise interparticle potential of pNIPAm-co-AAc microgels directly in quasi-2D suspension and proved that the interactions are pH dependent and can range from weakly attractive to soft repulsive. Finally, the hindered Brownian diffusion due of colloidal particles confined by hard walls was investigated systematically and striking differences between hard sphere and soft sphere were found, with soft pNIPAm-co-AAc microgels showing surprising mobility even under strong confinement.

Thin film copper indium gallium selenide (CIGS) solar cells have exhibited single junction power conversion efficiencies above 20% and have been commercialized. The large scale production of CIGS solar cells, however, is hampered by the relatively high cost and poor stoichiometric control of coevaporating tertiary and quaternary semiconductors in high vacuum. To reduce the overall cost of production, CIGS nanocrystals with predetermined stoichiometry and crystal phase were synthesized in solution. Colloidal nanocrystals of CIGS provide a novel route for production of electronic devices. Colloidal nanocrystals combine the well understood device physics of inorganic crystalline semiconductors with the solution processability of amorphous organic semiconductors. This approach reduces the overall cost of CIGS manufacturing and can be used to fabricate solar cells on flexible and light-weight plastic substrates. As deposited CIGS nanocrystal solar cells were fabricated by ambient spray-deposition. Devices with efficiencies of 3.1% under AM1.5 illumination were fabricated. Examining the external and internal quantum efficiency spectrums of the devices reveal that in nanocrystal devices only the space charge region is actively contributing to the extracted photocurrent. The device efficiency of the as-deposited nanocrystal films is presently limited by the small crystalline grains (≈ 15 nm) in the absorber layer and the relatively large interparticle spacing due to the organic capping ligands on the nanocrystal surfaces. Small grains and large interparticle spacing limits high density extraction of electrons and holes from the nanocrystal film. A Mott-Schottky estimation of the space charge region reveals that only 50 nm depth of the nanocrystalline absorber is effectively contributing to the photogenerated current. One strategy to improve charge collection involves increased space charge region for extraction by vertical stacking of diodes. A much longer absorption path for the photons exists in the space charge region with the stacked devices, increasing the probability that the incident radiation is absorbed and then extracted. This method enables an increase in the collected short circuit current. The overall device efficiency, however, suffers with the increased series resistance and shunt conductance of the device. Growth of nanocrystal grains was deemed necessary to achieve power conversion efficiencies comparable to vapor deposited CIGS films. Simple thermal treatment of the nanocrystal layers did not contribute to the growth of the crystalline grain size. At the same time, because of the loss of selenium and increased trap density in the absorber layer, there was a measurable decrease in device efficiency with thermal processing. For increased grain size, the thermal treatment of the absorber layer took place in presence of compensating amounts of selenium vapor. The process of selenization, as it is called, took place at 500°C in a graphite box and led to an increase of the grain size from 15 nm to several microns in diameter. Devices with the increased grain size yielded efficiencies up to 5.1% under AM1.5 radiation. Mott-Schottky analysis of the selenized films revealed a reduction in doping density and a comparable increase in the space-charge region depth with the increased grain size. The increased collection combined with the much higher carrier mobility in the larger grains led to achieved Jsc values greater than 20 mA/cm2. Light beam induced current microscopy (LBIC) maps of the devices with selenized absorber layers revealed significant heterogeneity in photogenerated current. Distribution of current hotspots in the film corresponded with highly selenized regions of the absorber films. In an effort to improve the overall device efficiency, improvements in the selenization process are necessary. It was determined that the selenization procedure is dependent on the selenization temperature and processing environment. Meanwhile, the reactor geometry and nanocrystal inks composition played important roles in determining selenized film morphology and the resulting device efficiency. Further work is necessary to optimize all the parameters to improve device efficiency even further. / text

Soft particle glasses like microgels and compressed emulsions are densely packed, disordered suspensions of deformable particles. Quantitative relationships among the constituent properties and the macroscopic properties of the suspension are determined for their customized design as rheological additives. The microscopic origin of their macroscopic properties is also determined. Advanced characterization techniques like Large Amplitude Oscillatory Shear (LAOS) and microrheology are studied to use them efficiently to characterize these materials. Their microstructure and rheology are investigated through theory, simulations and experiments. Soft particle glasses are used as rheological additives in many applications including coatings, solid inks and textured food and cosmetic products but their formulation is largely empirical. A quantitative connection between their formulation and rheology is critical to enable their rational design. Their microstructure will lead to the microscopic origin of some unique properties in common with other soft crowded materials like intracellular cytoplasm and clays. These are complex fluids and require novel techniques to characterize them. A study of these techniques is essential to efficiently interpret the observations in terms of their macroscopic properties and the microscopic dynamics involved. Particle scale simulations of steady and oscillatory shear flow are developed to predict the nonlinear rheology and microstructure of these glasses. The origin of yielding is determined as escape of particles from their cages giving rise to a shear induced diffusion. Microrheology is studied by developing simulations of a probe particle being pulled at a constant force and the rheological information from microrheology is quantitatively connected to that from bulk rheological measurements. Soft particle glasses develop internal stresses when quenched to a solid state by flow cessation during processing. Experiments are performed to characterize and a priori predict these stresses. Simulations are used to determine the particle scale mechanisms involved in the stress relaxation on flow cessation and the microstructural origin of internal stresses. A pairwise interaction theory is developed for quiescent glasses to quantitatively predict their microstructure and elastic properties. The theory is then extended to sheared glasses to quantitatively predict their nonlinear rheology. The implementation of the pairwise theories is computationally much faster than the full three-dimensional simulations. / text

Controlled patterning of light emitting diodes on semiconductors enables a vast variety of applications such as structured illumination, large-area flexible displays, integrated optoelectronic systems and micro-total analysis systems for real time biomedical screening. We have demonstrated a series of techniques of creating quantum-based (QD) patterned inorganic light emitting devices at room temperature on silicon (Si) substrate. In particular: (I) A combination of QDs self-assembly and microcontact printing techniques were developed to form the light emission monolayer. We expand the self-assembly method with the traditional Langmuir-Schaeffer technique to rapidly deposit monolayers of core: shell quantum dots on flat substrates. A uniform film of QDs self-assembled on water was transferred using hydrophobic polydimethylsiloxane stamps with various nano/micro-scale patterns, and was subsequently stamped. A metal oxide electron transport layer was co-sputtered onto the QDs. The structure was completed by an e-beam evaporating thin metal cathode. Multicolor light emission was observed on application of voltage across the device. (II) We also demonstrate the photolithographic patterning capability of a metal cathode for top emitting QDLEDs on Si substrates. Lithographic patterning technique enables site-controlled patterning and controlled feature size of the electrode with greater accuracy. The stability of inorganic silicon materials and metal oxide based diode structure offers excellent advantages to the device, with no significant damage observed during the patterning and etching steps. Efficient electrical excitation of QDs was demonstrated by both the methods described above. The technique was translated to create localized QD-based light sources for two applications: (1) Three-dimensional scanning probe tip structures for near field imaging. Combined topographic and optical images were acquired using this new class of “self-illuminating” probe in commercial NSOM. The emission wavelength can be tuned through quantum-size effect of QDs. (2) Multispectral excitation sources integrated with microfluidic channels for tumor cell analyses. We were able to detect the variation of sub-cellular features, such as the nucleus-to-cytoplasm ratio, to quantify the absorption at different wavelength upon the near-field illumination of individual tumor cells towards the determination of cancer developmental stage. / text

This dissertation contains four chapters with advances relevant to the fields of nanoparticle synthesis and nanoparticle self-assembly: a review of nanoparticle self-assembly, or “colloidal polymers”; dumbbell heterostructured nanorod synthesis; dipolar matchstick heterostructured nanorod synthesis; and self-assembly of dipolar matchsticks to form colloidal polymers. These chapters are followed by appendices containing supporting data for chapters two through four. The first chapter is a review summarizing current research involving the 1-D assembly of nanocrystals to form “colloidal polymers.” One of the major goals of materials chemistry is to synthesize hierarchical materials with precise controlled particle ordering covering all length scales of interest (termed, the “bottom up” approach). Recent advances in the synthesis of inorganic colloids have enabled the construction of complex morphologies for particles in the range of 1 – 100 nm. The next level of structural order is to control the structure of assemblies formed from these materials. Linear nanoparticle assemblies are particularly challenging to achieve due to the need to impart functionality to colloids such that (typically) only two sites are active per particle. An emerging idea in the literature which addresses this challenge is to consider linear assemblies of inorganic nanoparticles as colloidal analogs to traditional polymers. This conceptual framework has enabled the formation of linear assemblies having controlled composition (to form segmented and statistical copolymers), architecture (linear, branched, cyclic), and degree of polymerization (chain length). However, this emerging field of synthesizing colloidal polymers has not yet been reviewed in terms of methods to control fundamental polymer parameters. Therefore, linear nanoparticle assembly is reviewed in chapter 1 by applying concepts from traditional polymer science to nanoparticle assembly. The emphasis of chapter 1 is on controlling degree of polymerization, architecture, and composition for colloidal polymers, and seminal examples are highlighted which control these parameters. The second chapter is centered on a novel methodology to install ferromagnetic cobalt domains onto core@shell, “CdSe@CdS” nanorods. While the structures synthesized in this work were novel, the key advance from this work was the development of a methodology to separate nanorod activation from deposition of ferromagnetic cobalt domains onto semiconductor nanorods. As synthesized CdSe@CdS nanorods are passivated with strongly binding phosphonic acid ligands, and these ligands prevent direct deposition of many materials (such as cobalt). Synthetic methods must therefore modify nanorod surfaces prior to deposition of additional nanoparticle domains (tips). Previous synthetic methods for the deposition of magnetic domains onto nanorod termini typically combined activation of nanorod termini and metal deposition into a single synthetic step. While these previous reports were successful in achieving tipped nanorods, the coupling of these two reactions required matching the kinetics of nanorod activation and decomposition/reduction of metal precursors in order to achieve the desired heterostructure morphology. However, the presence of ligands used for nanorod activation can also affect the rate of metal precursor decomposition/reduction and the propensity of the metal to form free nanoparticles through homogeneous nucleation. Thus, simultaneous nanorod activation and metal deposition hinders modification of these syntheses to obtain differing heterostructured morphologies. In the work presented in chapter 2, we chemically activate nanorod termini towards cobalt deposition in a separate chemical step from deposition of metallic cobalt nanoparticle domains. First, reductive platinum deposition conditions were utilized to activate nanorod termini towards the deposition of cobalt domains, which were deposited in a subsequent reaction step. Then, the kinetics of nanorod activation during platinum deposition were tracked, and the platinum-tipped nanorod morphologies were correlated with the results of subsequent cobalt deposition reactions. Ultimately, controlled placement of cobalt domains onto one or both nanorod termini was demonstrated based on the degree of activation during platinum deposition. Cobalt nanoparticle tips were then selectively oxidized to form CoₓOy-tipped nanorods, which were a novel class of p-n type nanomaterials achieved over a total of five synthetic steps. Relevant supporting details for the synthesis of these dumbbell tipped nanorods are provided in Appendix A. The third chapter describes the synthesis of CoNP-tipped nanorods with a single, strongly dipolar, ferromagnetic CoNP-tip per nanorod. The key synthetic advance was the ability to activate a single terminus per nanorod without activation of lateral nanorod facets, which was vital in achieving these larger, dipolar, cobalt tips (rather than lateral decoration of cobalt onto nanorod lateral facets). These dipolar “matchstick” CoNP-tipped nanorods then spontaneously formed linear assemblies carrying nanorod side chains as pendant functionality. Activation of CdSe@CdS nanorods was found to occur through the deposition of small (< 2 nm) PtNP-tips which were not readily observable by standard characterization techniques. The finding that small (< 2 nm) PtNP-tips altered nanorod reactivity towards cobalt deposition emphasized the effect of subtle changes to nanorod surface chemistry. Relevant supporting details for the synthesis of these dipolar matchstick tipped nanorods are provided in appendix B. The fourth chapter is centered on the self-assembly of dipolar matchstick cobalt-tipped nanorods to form colloidal (co)polymers reminiscent of traditional bottlebrush polymers, with controlled composition and phase behavior on carbon surfaces. Similar to earlier findings in traditional polymer science, nanorod side chain length was found to significantly impact surface assembly of these colloidal analogs of bottlebrush copolymers, which provided a useful parameter for affecting surface wetting and phase behavior of nanoparticle thin films. This work was also the first demonstration of colloidal copolymers from the dipolar assembly of magnetic nanoparticles, where both segmented and statistical copolymer compositions were achieved. We then demonstrated, for the first time, that a colloidal copolymer with segmented composition can form a mesoscopic phase separated morphology which is similar to that observed for traditional block copolymers. This key advance opens the possibility of controlling structural ordering over still longer length scales by the development of methods to control phase separated morphologies in a manner similar to traditional block copolymers. Relevant supporting details for the synthesis and assembly of these colloidal bottlebrush polymers are provided in appendix C.

This dissertation describes a novel methodology to prepare, functionalize, and assemble polymer-coated ferromagnetic cobalt nanoparticles (PS-CoNPs) and cobalt oxide nanowires. This research demonstrated the ability to use dipolar nanoparticles as `colloidal monomers' to form electroactive 1-D mesostructures via self- and field-induced assembly. The central focus of this dissertation is in developing a novel methodology termed as `Colloidal Polymerization', in the synthesis of well-defined cobalt oxide nanowires as nanostructured electrode materials for potential applications in energy storage and conversion.Ferromagnetic nanoparticles are versatile building blocks due to their inherent spin dipole, which drive 1-D self-assembly of colloids. However, the preparation and utilization of ferromagnetic nanoparticles have not been extensively examined due to the synthetic challenges in preparing well-defined materials that can be easily handled. This dissertation has overcome these challenges through the hybridization of polymeric surfactants with an inorganic colloid to impart functionality, colloidal stability and improved processing characteristics. This modular synthetic approach was further simplified to prepare ferromagnetic nanoparticles in gram scale, which enabled further investigations to develop new chemistry and materials science with these materials. These polymer-coated magnetic nanoparticles self-assembled into extended linear chains due to strong dipolar attractions between colloids. Additionally, novel dipolar assemblies, such as, flux-closure nanorings and lamellae type mesostructures were demonstrated by controlling the interparticle of attractive forces (dipolar versus van der Waals).The research presented herein focused on utilizing polymer-coated ferromagnetic cobalt nanoparticles as `colloidal molecules' to form interconnected 1-D mesostructures via `Colloidal Polymerization'. This process exploited the magnetic organization of dipolar colloids into 1-D mesostructures followed by a facile oxidation reaction to form interconnected electroactive cobalt oxide nanowires. This facile and template free approach enabled the large scale synthesis of semiconductor cobalt oxide nanowires, in which the electronic and electrochemical properties were confirmed for potential applications for energy storage and conversion. This work served as a platform in fabricating a wide range of semiconductor heterostructures, which allowed for structure-property investigation of new nanostructured electrodes.