Timothy Collins

Large Block Copolymer Self-Assembly for Fabrication of Subwavelength Nanostructures for Applications in Optics

Nanostructured surfaces are common in nature and exhibit properties such as antireflectivity (moth eyes), self-cleaning (lotus leaf), iridescent colors (butterfly wings), and water harvesting (desert beetles). We now understand such properties and can mimic some of these natural structures in the laboratory. However, these synthetic structures are limited since they are not easily mass produced over large areas due to the limited scalability of current technologies such as UV-lithography, the high cost of infrastructure, and the difficulty in nonplanar surfaces. Here, we report a solution process based on block copolymer (BCP) self-assembly to fabricate subwavelength structures on large areas of optical and curved surfaces with feature sizes and spacings designed to efficiently scatter visible light. Si nanopillars (SiNPs) with diameters of ∼115 ± 19 nm, periodicity of 180 ± 18 nm, and aspect ratio of 2-15 show a reduction in reflectivity by a factor of 100, <0.16% between 400 and 900 nm at an angle of incidence of 30°. Significantly, the reflectivity remains below 1.75% up to incident angles of 75°. Modeling the efficiency of a SiNP PV suggests a 24.6% increase in efficiency, representing a 3.52% (absolute) or 16.7% (relative) increase in electrical energy output from the PV system compared to AR-coated device.

Linking Precursor Alterations to Nanoscale Structure and Optical Transparency in Polymer Assisted Fast-Rate Dip-Coating of Vanadium Oxide Thin Films

Solution processed metal oxide thin films are important for modern optoelectronic devices ranging from thin film transistors to photovoltaics and for functional optical coatings. Solution processed techniques such as dip-coating, allow thin films to be rapidly deposited over a large range of surfaces including curved, flexible or plastic substrates without extensive processing of comparative vapour or physical deposition methods. To increase the effectiveness and versatility of dip-coated thin films, alterations to commonly used precursors can be made that facilitate controlled thin film deposition. The effects of polymer assisted deposition and changes in solvent-alkoxide dilution on the morphology, structure, optoelectronic properties and crystallinity of vanadium pentoxide thin films was studied using a dip-coating method using a substrate withdrawal speed within the fast-rate draining regime. The formation of sub-100 nm thin films could be achieved rapidly from dilute alkoxide based precursor solutions with high optical transmission in the visible, linked to the phase and film structure. The effects of the polymer addition was shown to change the crystallized vanadium pentoxide thin films from a granular surface structure to a polycrystalline structure composed of a high density of smaller in-plane grains, resulting in a uniform surface morphology with lower thickness and roughness.

In-depth TEM characterization of block copolymer pattern transfer at germanium surfaces

Dry plasma etching for the pattern transfer of mask features is fundamental to semiconductor processing and the development of device and electrically conducting elements becomes more challenging as features reach the deep nanoscale regime. In this work, high resolution transmission electron microscopy (TEM) coupled with energy dispersive x-ray (EDX) characterization were used to analyze the pattern transfer of graphoepitaxially aligned block copolymer (BCP) features to germanium (Ge) substrates as a function of time. The BCP patterns were converted into metal oxide hardmasks in order to affect good aspect ratios of the transferred features. An unusual interface layer between metal oxide nanowires and the germanium-on-insulator substrate was observed. EDX analysis shows that the origin of this interface layer is a result of the presence of a negative tone e-beam resist material, HSQ (hydrogen silsesquioxane). HSQ was employed as a guiding material to align line-space features of poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) BCP with 16 nm half-pitch topography. Additionally, the existence of a metal oxide layer (from the initial PS-b-P4VP film) is also shown through ex situ TEM and EDX characterization. Three dimensional modeling of features is also provided giving a unique insight into the arrangement and structure of BCP features prior to and after the pattern transfer process. The results presented in this article highlight the accuracy of high resolution electron microscopy and elemental mapping of BCP generated on-chip etch masks to observe and understand through-film features affecting pattern transfer.

Nanopatterned protein-polysaccharide thin films by humidity regulated phase separation

Greater sustainability in mass manufacturing is essential to alleviating anthropogenic climate change. High surface-area, micro- and nano-patterned films have become a fundamental tool in materials science, however these technologies are subject to a dwindling petrochemical supply, increasing costs and disposability concerns. This paper describes the production of patterned biopolymer films utilizing controlled phase separation of biopolymeric thin films into nanopatterns using easily transferable variables and methods. Similar morphologies to those commonly observed with synthetic block-copolymers (BCPs) were achieved across a large range of feature sizes, from 160 nm to >5 μm: Bicontinuous, porous, droplet-matrix, particulated and dimpled. Protein and polysaccharide type, protein to polysaccharide ratio, casting method and ambient humidity were primary conditions found to influence the pore morphology of the films. High protein concentrations (4:1 and 2:1 blends) generally resulted in porous structures whereas high polysaccharide concentrations (1:2 and 1:4 blends) resulted in spherical structures. High humidity conditions (60% + relative humidity) resulted in the growth of large protuberances up to 10 µm in diameter while lower humidity (10-30%) resulted in discrete features smaller than 200 nm.