Characterization and Engineering of the Process of Directed Particle Self-assembly in Thin Films and Sessile Droplets

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Title: Characterization and Engineering of the Process of Directed Particle Self-assembly in Thin Films and Sessile Droplets
Author: Kuncicky, Daniel M.
Advisors: Stefan Franzen, Committee Member
Orlin Velev, Committee Chair
Peter Kilpatrick, Committee Member
John van Zanten, Committee Member
Abstract: Directed self-assembly of colloidal particles confined between a solid and liquid interface has been studied as a versatile tool for organizing 2D and 3D crystalline arrays on solid substrates. The overarching goal of this study was to engineer the process of assembly to achieve simple and cost effective solutions for application needs were self-assembled particle structures work better than microfabricated ones. Two technologically relevant geometrical motifs of assembly were studied in detail. A thin film assembly technique based on controlled withdrawal of a meniscus was employed for modifying solid substrates with arrays of organic and inorganic colloidal particles. The assembled particles were used as a template for sterically directing the meso- and micro structure of metallic films for control over their electro-optical functionality. A sessile droplet templating technique was developed for fabricating arrays of discrete colloidal crystal patches of controlled shape and size. The process of assembly was studied in detail for each motif to optimize the deposition conditions and formulate protocols for controlling the size, composition, internal particle symmetry and overall shape. The methods and the results developed are relevant to different disciplines, including self-assembly, surface chemistry, Atomic Force Microscopy methodology, biological research and spectroscopy. Convective assembly and latex templating of gold nanoparticles was used to fabricate highly efficient nanostructured substrates for surface enhancing Raman scattering (SERS)-based sensors. The structure-dependent performance of these SERS substrates was systematically characterized with cyanide in a flow milli-fluidic chamber to simulate on-line continuous water monitoring. A matrix of experiments was designed to isolate the SERS contributions arising from meso- and microscale porosity, long range ordering of the micropores, and the thickness of the nanoparticle layer. The SERS results were compared to the substrate structure observed by scanning electron microscopy and optical microscopy to correlate substrate structure to SERS performance. A single-step method for rapidly assembling tobacco mosaic virus (TMV) into nanocoatings and macroscopically ordered fibers was developed. Uniform films with long-range alignment or arrays of virus bundles were formed through a combination of shear and dewetting. Discrete, contiguous arrays of the TMV fibers were coated over centimeter length scales using only microliters of TMV suspension. The density and branching of the wire structure were controlled by varying the substrate wettability and meniscus withdrawal speed. The ability to precisely control the wire structure of the bio-scaffold allowed for the fabrication of architectures with advanced chemical and physical functionality. As an example, a procedure was developed where the TMV fibers were conjugated to Au particles followed by Ag enhancement for metal deposition. The procedure developed was used to convert the virus fibers into anisotropically conductive arrays of long wires. A systematic study of a sessile droplet templating process for fabrication of colloidal crystals in small micropatches was undertaken. The methodology was based on drying of a particle suspension on a substrate of controlled contact angle. The process of assembly was correlated to the dynamics of the receding contact line. The kinetics of drying were examined by measurement of droplet profiles and it was found that the rate matched well with diffusion-limited dynamics. The effects of major parameters controlling the process: contact angle, particle concentration, and electrolyte were investigated in detail. A variety of micropatch shapes were observed and categorized within the parameter space. Based on the understanding developed from this cycle of experiments we fabricated arrays of gold SERS substrates in the form of flat, uniformly-shaped micropatches with diameters ranging from microns to millimetres. We also demonstrated that the assemblies can serve as a new class of microidenters that can be used directly for biomechanical characterization and hydraulic permeability studies of whole cells and tissue.
Date: 2007-09-11
Degree: PhD
Discipline: Chemical Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/4129


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