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

Nanolithography on non-planar surfaces and self-assembly of metal salt-polymer nanomaterials

This thesis is focused on fabrication of high aspect ratio nanostructures on non-planar surfaces using evaporated electron beam resist (Part I), and a novel fabrication methods of high resolutionhigh-resolution surface nanostructures using metal salt: polymer nanocomposites self-assembly (Part II). Various top-down and bottom-up nanopatterning techniques are currently available with the rapid progress in instrumentation and material engineering. However, patterning on non-planar surfaces of various materials still remains an overwhelming challenge because the conventional resist coating method, spin-coating, works well for only planar surfaces such as a flat wafer. On the other hand, the ability to pattern any given surface at the nanoscale, in particular surfaces with high inherent roughness or with pre-patterned micro-scale features, opens new perspectives in various fields from multi-scale biomimetics to optoelectronics. Part I (Chapter 1-4) of the present thesis aims to address this issue using evaporated electron beam resist. Electron beam lithography (EBL) is a versatile technique for creating arbitrary patterns on substrates with sub-10 nm resolution. Contrary to conventional lithography techniques, EBL was previously shown to be able to pattern non-planar surfaces using modified lithography system to adjust the beam position along z-axis, spray coating of the resist, and evaporation of the resist. Among them, evaporation of the resist is more favorable as it can be done on any irregular surfacesurfaces using commonly available thermal evaporation equipment. Yet, previous evaporated resist materials suffer from low resolution and sensitivity, as well as poor dry etching resists for subsequent pattern transfer to the sub-layer. Here, evaporation of polystyrene electron beam resist is studied which was used to pattern on irregular surfaces such as the cantilever of atomic force microscope and side surface of an optical fiber. Furthermore, in order to drastically increase the resist’s dry etching resistance, chromium that is a hard etching mask material was successfully incorporated into the resist by co-evaporating or Cr and polystyrene. This nanocomposite resist enabled the fabrication of very high aspect ratio structures by electron beam lithography followed by dry plasma etching. As this material can be evaporated on any substrate, including non-planar surfaces, it can open new era to spectroscopy and bio-sensing techniques. Part II (Chapter 5-6) presents a low-cost bottom-up fabrication techniques for creatingto create dense surface nanostructures without long-range ordering. Recently, micro- and nano-structured surfaces have become a hot topic in nanotechnology where performance of devices is enhanced due to such surface nanostructuring. Such structures are often called as a “smart” coating on the surfaces where they could provide wetting/de-wetting, adhesion, thermal and/or electrical conductivity, super-hydrophobicity, self-cleaning, anti-icing, anti-reflectivity, etc. Bottom-up techniques, such as self-assembly lithography, areis undoubtedly much more cost-effective than top down lithography techniques for applications that do not need long range ordering. Block co-polymer lithography, colloidal lithography, sol-gel processing, wet/dry etching are some commonly used techniques of bottom-up fabrication. However, fabrication of those structures with low costslow-cost as well as high performance is still challenging. Here a novel fabrication method is introduced, which involves spin-coating of metal salt : polymer composite followed by its phase-separation upon thermal annealing. Both spin-coating and thermal annealing are very low- cost processes. With this method, after pattern transfer to the substrate using the self-formed metal salt islands as mask, dense and high resolutionhigh-resolution nanostructures over large area without long-range ordering is achieved, which offered greatly enhanced super-hydrophobic and anti-reflective properties

Nanoscale Architecture for Site-controlled Epitaxy and Antireflective Coatings

The deployment of novel nanofabrication and -characterization techniques has paved the way for tunable functionalities of materials by engineering the size of the structures at the nanoscale. The key concepts in this thesis are the exploitation of quantum mechanical properties in nanostructures and sub-wavelength optical materials. This thesis covers nanoimprint lithography based fabrication techniques for the realization of quantum dot based semiconductor devices and the formation of sub-wavelength antireflection coatings for high efficiency solar cells. Both types of patterning processes were developed aiming for the integration with molecular beam epitaxy, which was used for the material fabrication. The structural and optical properties of the quantum dots fabricated by this new method indicated excellent suitability of the fabrication process for large scale quantum dot based devices. The increasing demand of renewable energy sources has led to rapid development in the field of photovoltaics. In addition to the significant improvements in the preparation of active materials, the performance of a solar cell is directly increased by carefully designing the antireflection coating on the surface of the cell. The reflectivity of the solar cell surface has to be minimized in order to transfer the maximum amount of solar energy into the cell to be converted into electricity. In this thesis, a novel nanostructured antireflection coating was developed. The antireflective properties of the coating were outstanding within a broad spectral range and a clear increase in the solar cell performance was achieved