Fabrication of Ultrafine Structures and Functional Surfaces Over Large Areas

Micro-/nanofabrication finds wide use in the preparation of patterned surfaces, which are of vital importance in the applications of functional surfaces, electronics, optics, and sensors. This dissertation will present the preparation of ultrafine structures and functional surfaces over large areas utilizing a combination of top-down lithography technique and bottom-up wrinkling method. First, a simple, scalable and cost-effective spacer lithography approach utilizing polydopamine coating technique was developed for the fabrication of well-controlled nanopatterns with feature size in the sub-20 nm scale. Briefly, a thin layer of polydopamine (PDA) was conformally deposited on the sidewalls of pre-patterned poly (methyl methacrylate) (PMMA) features to form a spacer layer. After etching the PDA film on the horizontal surfaces and removing the PMMA resist, only the PDA spacer was left on the substrate, forming a new pattern. The pattern density of the new features was doubled and the feature size was well tuned to a sub-20 nm scale. We also utilized a bottom-up wrinkling technique to spontaneously pattern and functionalize polymer films. Through a reactive silane-infusion induced surface instability, wrinkled patterns with tunable wavelengths were easily produced over large areas and the surface chemical functionality of the wrinkles was well-tuned by the infusion of different functional silanes. Hierarchically wrinkled patterns with micro/nano structure were achieved by combining wrinkling with nanoimprint lithography. The hierarchically wrinkled surfaces exhibited superhydrophobicity with water contact angles higher than 160°and water sliding angle lower than 5°. To scale up these patterned functional surfaces over large areas, roll-to-roll nanoimprint lithography technique was investigated for the continuous fabrication of superhydrophobic surfaces and lubricant infused patterned surfaces based on hierarchically wrinkled surfaces. These functionally patterned surfaces displayed self-cleaning properties to a variety of liquid contaminations and anti-biofouling properties when challenged with Escherichia coli bacteria. This study suggested a potential transformation of artificial biomimetic structures from small, lab scale coupons to low cost, large area platforms

DIRECT PATTERNING OF NATURE-INSPIRED SURFACES FOR BIOINTERFACIAL APPLICATIONS

There are three major challenges for the design of patterned surfaces for biointerfacial applications: (i) durability of antibacterial/antifouling mechanisms, (ii) mechanical durability, and (iii) lifetime of the master mold for mass production of patterned surfaces. In this dissertation, we describe our contribution for the development of each of these challenges. The bioinspired surface, Sharklet AFTM, has been shown to reduce bacterial attachment via a biocide-free structure-property relationship effectively. Unfortunately, the effectiveness of polymer-based sharkskin surfaces is challenged over the long term by both eventual bacteria accumulation and a lack of mechanical durability. To address these common modes of failure, hard, multifunctional, antifouling, and antibacterial shark-skin patterned surfaces were fabricated via a solvent-assisted imprint patterning technique. A UV-crosslinkable adhesive material was loaded with titanium dioxide (TiO2)nanoparticles (NPs) from which shark skin microstructures were imprinted on a polyethylene terephthalate substrate. Furthermore, hard, multifunctional, antifouling, and antibacterial shark skin patterned surfaces were fabricated using inks comprised of zirconium dioxide (ZrO2) NPs and TiO2 NPs. The ZrO2 NPs provide an extremely hard and durable matrix in the final structure, while the TiO2 NPs provide active antibacterial functionality in the presence of UV light via photooxidation. The dynamic water contact angle, mechanical, antibacterial, and antifouling characteristics of the shark skin patterned surfaces were investigated as a function of TiO2 content. We then demonstrated the multifunctional shark skin system’s suitability for use as an antifouling biosensor. Lastly, we described the design of a durable, hard master mold for pattern transfer. The lifetime of many of the current molds is limited by a lack of mechanical durability as well as cost. In this study, ZrO2 NPs were imprinted on a variety of substrates using a solvent-assisted patterning technique and subsequently annealed to increase the mechanical durability of the mold. Polymer replications were demonstrated using the hard ZrO2 mold with thermal and UV nanoimprinting lithography techniques, and injection molding. After up to 115,000 injection molding cycles, there was no delamination or breakage in the ZrO2 mold. The high hardness and durability, as demonstrated through the many replication cycles, suggests that the ZrO2 mold has excellent potential for use in the mass production of patterned polymer replicas. We also explored the nanopatterning of stainless steel using the ZrO2 mold. The solution-processability and simple patterning technique of ZrO2 NPs enable large-area and cost-effective fabrication of the hard molds which can be used for the variety of nano and micro-replication technologies

Inquiry of Graphene Electronic Fabrication

Graphene electronics represent a developing field where many material properties and devices characteristics are still unknown. Researching several possible fabrication processes creates a fabrication process using resources found at Cal Poly a local industry sponsor. The project attempts to produce a graphene network in the shape of a fractal Sierpinski carpet. The fractal geometry proves that PDMS microfluidic channels produce the fine feature dimensions desired during graphene oxide deposit. Thermal reduction then reduces the graphene oxide into a purified state of graphene. Issues arise during thermal reduction because of excessive oxygen content in the furnace. The excess oxygen results in devices burning and additional oxidation of the gate contacts that prevents good electrical contact to the gates. Zero bias testing shows that the graphene oxide resistance decreases after thermal reduction, proving that thermal reduction of the devices occurs. Testing confirms a fabrication process producing graphene electronics; however, revision of processing steps, especially thermal reduction, should greatly improve the yield and functionality of the devices

Micro-electroforming metallic bipolar electrodes for mini-DMFC stacks

This paper describes the development of metallic bipolar plate fabrication using micro-electroforming process for mini-DMFC (direct methanol fuel cell) stacks. Ultraviolet (UV) lithography was used to define micro-fluidic channels using a photomask and exposure process. Micro-fluidic channels mold with 300 micrometers thick and 500 micrometers wide were firstly fabricated in a negative photoresist onto a stainless steel plate. Copper micro-electroforming was used to replicate the micro-fluidic channels mold. Following by sputtering silver (Ag) with 1.2 micrometers thick, the metallic bipolar plates were completed. The silver layer is used for corrosive resistance. The completed mini-DMFC stack is a 2x2 cm2 fuel cell stack including a 1.5x1.5 cm2 MEA (membrane electrode assembly). Several MEAs were assembly into mini-DMFC stacks using the completed metallic bipolar plates. All test results showed the metallic bipolar plates suitable for mini-DMFC stacks. The maximum output power density is 9.3mW/cm2 and current density is 100 mA/cm2 when using 8 vol. % methanol as fuel and operated at temperature 30 degrees C. The output power result is similar to other reports by using conventional graphite bipolar plates. However, conventional graphite bipolar plates have certain difficulty to be machined to such micro-fluidic channels. The proposed micro-electroforming metallic bipolar plates are feasible to miniaturize DMFC stacks for further portable 3C applications.Comment: Submitted on behalf of EDA Publishing Association (http://irevues.inist.fr/handle/2042/16838

Compression molding processed superhydrophobic CB/CeO2/PVDF/CF nanocomposites with highly robustness, reusability and multifunction

Bioinspired superhydrophobic treatment imparts unique features to surfaces such as self-cleaning, water-proofing, anti-icing, anti-fouling, etc. Here we introduce a simple approach to manufacture carbon fiber based superhydrophobic nanocomposite materials. The developed materials had high mechanochemical durability and electrical conductivity which should find promising applications in many engineering fields. The nanocomposites were manufactured via molding process and comprised of carbon fiber (CF), poly(vinylidene fluoride) (PVDF), carbon black (CB) and cerium dioxide (CeO2) nanoparticles, which is typically applied to fabricate carbon fiber reinforced plastics (CFRP) for structural use. The CFRP nanocomposites show a number of excellent functionalities such as superhydrophobicity (water contact angle ∼156° and sliding angle ∼5°), excellent structural properties (tensile strength ∼ 109 MPa and tensile modulus ∼ 10 GPa) and electrical conductivity (∼6.8 S/cm). The nanocomposites maintain excellent superhydrophobicity even after 200 cycles of sand paper abrasion, 24 h of strong base and/or 60 min of strong acid erosion. Additionally, both the superhydrophobicity and mechanical properties can be recovered by re-molding process after the nanocomposites were cut into pieces or ground into powders. This demonstrates good reusability and clear potential for recycling of the developed materials

The effects of the processing parameters on the laser machining of a green china ceramic

The purpose of this study was to investigate the quality of the cut, the thickness of the heat-affected zone, and the amount of dross produced by laser cutting green china ceramics;This study seeks an alternative method to cut greenware, which will reduce the high rate of defect found in conventional punching and trimming. The study was centered on laser cutting and in any machining process the surface properties of the material were considered. This study provides the processing parameters relating to one type of china greenware, but the information gathered will be important in the development of any new laser cutting process for green china ceramic;The independent variables that were manipulated in this experiment were: moisture content of the greenware, feed rate at which samples are cut, and power of the laser beam. Specimens were laser cut at high, medium, and low levels of each of the independent variables. The investigation was concerned with the effect(s) the independent variations may have on the following dependent variables: quality of cut measured in micro inch, thickness of the heat-affected zone measured in thousandths of an inch, and weight of dross measured in thousandths of a gram;Statistical analyses were performed on the sets of data obtained after laser cutting. Greatest attention was given to qualifying and quantifying the effects and any relationship with respect to the quality of cut, the thickness of the heat-affected zone, and the amount of dross produced;The results of this study indicated that the higher the moisture content the better the quality of cut. Low feed rate and/or high power causes the widest heat-affected zone. The higher the power the greater the weight of dross

Development of a process for fabricating high aspect ratio parylene microstructures.

PARYLENE (poly-para-xylylene) is mostly used as a conformal protective polymer pin-hole free coating material to uniformly protect any component configuration on diverse substrates. This thesis describes in detail how the unique properties of parylene can be conveniently combined with MEMS technology to meet biocompatibility requirements of biological and chemical applications and develop unique microstructure shapes. Since etching of parylene is not readily possible, the best way to mold it into any shape would be to etch hollow molds in silicon and deposit parylene in them. It is easy to etch away the silicon mold for releasing these parylene structures. Parylene is nonreactive in wet etchants (like TMAH or KOH) that are used to etch silicon. These microstructures can be helpful in implants and other biomedical applications. This technique allows for the production of unique microstructures, many of which are not realizable by other fabrication technique. Any other material that conforms easily in silicon molds and is non-reactive with silicon and silicon etchants, can be molded in the shape of the fabricated molds. A material that is tested for these properties can be deposited because most of the fabrication processes (like etching, lithography, oxidation and wafer bonding) are performed only on silicon for preparing the molds. Materials deposited by CVD (chemical vapor deposition) or less viscous liquids that solidify on cooling, can be investigated for deposition in molds. Many useful applications can be derived by combining this method with various materials. CAD tools were used to simulate the mask features for designing this microstructure and to layout the photomask pattern. A fabrication procedure is devised from these simulation results and the process is implemented in a Class 100/1000 Cleanroom facility at the Lutz Micro/Nanotechnology Cleanroom core facility, University of Louisville. A complete guide to fabricate this MEMS-based parylene structure is provided in this thesis project. Important observations, complete experimental procedure and results are discussed in detail

Vacuum Die Casting of Silicon Sheet for Photovoltaic Applications

The development of a vacuum die-casting process for producing silicon sheet suitable for photovoltaic cells with a terrestrial efficiency greater than 12 percent and having the potential to be scaled for large quantity production is considered. The initial approach includes: (1) obtaining mechanical design parameters by using boron nitride, which has been shown to non-wetting to silicon; (2) optimizing silicon nitride material composition and coatings by sessile drop experiments; (3) testing effectiveness of fluoride salt interfacial media with a graphite mold; and (4) testing the effect of surface finish using both boron nitride and graphite. When the material and mechanical boundary conditions are established, a finalized version of the prototype assembly will be constructed and the casting variables determined