Browsing by Subject "layer-by-layer assembly"
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Item Layer-by-layer Assembly of Nanobrick Wall Ultrathin Transparent Gas Barrier Films(2012-07-16) Priolo, Morgan AlexanderThin layers with high barrier to oxygen and other gases are a key component to many packaging applications, such as flexible electronics, food, and pharmaceuticals. Vapor deposited thin films provide significant gas barrier, but are prone to cracking when flexed, require special, non-ambient processing environments, and can involve complex fabrication when layered with polymers. The addition of clay into polymers can enhance barrier properties relative to the neat polymer; however, these composites are subject to clay aggregation at high loadings, which leads to increased opacity and random platelet alignment that ultimately reduce barrier improvement. Layer-by-layer (LbL) assembly is capable of producing thin films that exhibit super gas barrier properties, while remaining flexible and completely transparent. Montmorillonite (MMT) clay and branched polyethylenimine (PEI) were deposited via LbL assembly to create gas barrier films that can be tailored by altering the pH of the PEI deposition solution or the concentration of the MMT suspension. Films grow linearly as a function of layers deposited, where increasing PEI pH increases spacing between clay layers and increasing MMT concentration increases thin film clay content. An oxygen transmission rate (OTR) below the detection limit of commercial instrumentation (< 0.005 cm3/m2?day?atm) is observed after 70 layers of 0.2 wt % MMT or 24 layers of 2 wt % MMT are deposited with pH 10 PEI onto 179 ?m thick poly(ethylene terephthalate) (PET) film. Three-component films of PEI, poly(acrylic acid) (PAA), and MMT grow exponentially as a function of PEI/PAA/PEI/MMT quadlayers deposited. A transparent, ultrathin film of only four quadlayers deposited onto PET exhibits the lowest oxygen permeability ever reported for any thin film material, at only 51 nm thick. Finally, the first example of LbL assembly using large aspect ratio vermiculite (VMT) clay was performed. PEI/VMT films grow linearly as a function of layers deposited and exhibit 95 % light transmission with 97 wt % VMT. The barrier of these films is due to the highly aligned nanobrick wall structure that creates a tortuous path for permeating molecules. Coupling high flexibility, transparency, and barrier, these coatings are good candidates for a variety of packaging applications.Item Transparent and Conductive Carbon Nanotube Multilayer Thin Films Suitable as an Indium Tin Oxide Replacement(2012-07-16) Park, Yong TaeTransparent electrodes made from metal oxides suffer from poor flexibility and durability. Highly transparent and electrically conductive thin films based on carbon nanotubes (CNTs) were assembled as a potential indium tin oxide (ITO) replacement using layer-by-layer (LbL) assembly. The ultimate objective of this dissertation work is to produce CNT-based assemblies with sheet resistance below 100 Omega/sq and visible light transmission greater than 85 percent. The alternate deposition of positively charged poly(diallyldimethylammonium chloride) [PDDA] and CNTs stabilized with negatively charged deoxycholate (DOC) exhibit linear film growth and thin film properties can be precisely tuned. Ellipsometry, quartz crystal microbalance, and UV-vis were used to measure the growth of these films as a function of PDDA-CNT bilayers deposited, while TEM, SEM, and AFM were used to visualize the nanostructure of these films. Following a literature review describing potential ITO substitutes and LbL technology, the influence of CNT type on optoelectronic performance of LbL assemblies is described. Three different types of nanotubes were investigated: (1) multiwalled carbon nanotubes (MWNTs), (2) few-walled carbon nanotubes (FWNT), and (3) purified single-walled carbon nanotubes (SWNTs). SWNTs produced the most transparent (>85 percent visible light transmittance) and electrically conductive (148 S/cm, 1.62 kOmega/sq) 20-bilayer films with a 41.6 nm thickness, while MWNT-based films are much thicker and more opaque. A 20-bilayer PDDA/(MWNT DOC) film is approximately 103 nm thick, with a conductivity of 36 S/cm and a transmittance of 30 percent. In an effort to improve both transparency and electrical conductivity, heat and acid treatments were studied. Heating films to 300 degree C reduced sheet resistance to 701 Omega/sq (618 S/cm conductivity, 38.4 nm thickness), with no change in transparency, owing to the removal of insulating component in the film. Despite improving conductivity, heating is not compatible with most plastic substrates, so acid doping was investigated as an alternate means to enhance properties. Exposing SWNT-based assemblies to HNO3 vapor reduced sheet resistance of a 10 BL film to 227 Omega/sq. Replacing SWNTs with double walled carbon nanotubes (DWNTs) provided further reduction in sheet resistance due to the greater metallic of DWNT. A 5 BL DWNT film exhibited the lowest 104 Omega/sq sheet resistance (4200 S/cm conductivity, 22.9 nm thickness) with 84 percent transmittance after nitric acid treatment. DWNT-based assemblies maintained their low sheet resistance after repeated bending and also showed electrochemical stability relative to ITO. This work demonstrates the excellent optoelectronic performance, mechanical flexibility, and electrochemical stability of CNT-based assemblies, which are potentially useful as flexible transparent electrodes for a variety of flexible electronics.