Bioinspired large scale fabrication of periodic nanostructures

This dissertation basically contains two different research topics. The first topic is covered in Chapter 1 to Chapter 3, describing how to generate periodic nanostructures in large scale on different substrates by assembling nanoparticles via a spin-coating technique. For the second topic, Chapter 4 and Chapter 5 describe how to make single graphene sheets and then use them as nanofillers for making polymer nanocomposites.;Antireflection coatings (ARCs) have been widely utilized to reduce the unwanted reflective losses of incident light from crystalline silicon surface. More than 30% of incident light is reflected back from the surface of crystalline silicon because of its high refractive index. Quarter-wavelength silicon nitride (SiNx) films deposited by plasma-enhanced chemical vapor deposition (PECVD) are the industrial standard for ARCs on crystalline silicon substrates. However, the PECVD-deposited SiNx films are expensive to fabricate. Additionally, commercial SiNx ARCs are typically designed to suppress reflection efficiently at wavelengths around 600 nm. The reflective loss is rapidly increased for near-infrared and other visible wavelengths, which contain a large portion of the incident solar energy.;In order to resolve the throughput and scalability issue of generating nanometer features, we have recently developed a versatile spin-coating technique that enables wafer-scale assembly of submicrometer-sized particles into non-close-packed structures. This technique provides a scalable templating fabrication platform to produce a majority of nanostructured materials with submicrometer-scale periodicity. My research mainly focuses on developing subwavelength-structured "moth-eye" antireflection coatings on silicon substrates by our spin-coating technique to improve the light collection efficiencies of crystalline silicon solar cells. The templated subwavelength silicon nipple arrays exhibit excellent broadband antireflective and superhydrophobic properties. Besides, "moth-eye" antireflection coatings can also be generated on GaSb substrates by spin-coating technique and on glass substrates via soft lithography successfully. To greatly extend the capability of the spin-coating technique in creating periodic nanostructures with resolution beyond the optical diffraction limit, I also demonstrate the scalable production of periodic sub-100 nm features with more than one order of magnitude higher number density than our previous approaches by assembling 70 nm diameter silica particles.;The second topic in this dissertation covers the fabrication of single graphene sheets and graphene-chitosan nanocomposites - Graphene - a single layer of carbon atoms bonded together in a hexagonal lattice - has stimulated a vast amount of research in recent years. Its remarkable properties including high values of Young's modulus (∼1100 GPa), thermal conductivity(∼5000 Wm-1K-1) and specific surface area (∼2630 m2g-1) make it an excellent carbon-based nanofiller material for polymer-based nanocomposites. Nanocomposites have great potential to exceed the performance of conventional composites due to its nanoscale fillers at low loading. However, maximal mechanical enhancement can only be achieved when the nanofiller is homogeneously dispersed in the polymer matrix and the external tensile load is efficiently transferred via a strong interaction at the interface between the filler and the matrix.;Chitosan, a natural-biopolymer with unique structure features, possesses the primary amine at the C-2 position of the glucosamine residues and is soluble in aqueous acidic media at pH < 6.5. It is commonly used to disperse nanomaterials and immobilize enzymes for constructing biosensors due to its excellent capability for film formation, nontoxicity, biocompatibility, mechanical strength, and good water permeability. My research is to disperse strong, highly stiff graphene sheets in a chitosan biopolymer matrix with self-assembling layered structures by using a simple evaporation method for developing high-performance lightweight composites in different applications. The photograph and the cross-sectional SEM images of the final composite film show that graphene sheets are uniformly dispersed in the polymer matrix and aligned parallel to the surface of the film, indicating external tensile loads can be successfully transmitted to the strong graphene nanofiller across the graphene--chitosan interface via interfacial interactions. Based on my results, a 90% increase in tensile strength can be achieved at a very low filler content (∼4.76 wt %).