Jason B. Sorge

Circular birefringence dependence on chiral film porosity

Obliquely deposited thin films with helical microstructures exhibit circular Bragg effects. In this study, the effect of film porosity on the circular birefringence of helical thin films is investigated in TiO(2) films deposited at angles ranging from 30 degrees to 87 degrees in order to determine the various mechanisms responsible for the circular Bragg effects. Specular transmittance and diffuse scattering measurements indicate two film growth regimes of enhanced circular Bragg effects: The first regime is due to a maximum in form birefringence while the second regime is caused by strong anisotropic scattering.

Ion-Beam Assisted Glancing Angle Deposition for Relative Humidity Sensors

Ion-beam assisted glancing-angle deposition is used to fabricate relative humidity sensors. Ion currents of 3,5, and 7 mA were tested. The morphology and capacitance of the sensors are found to be dependent on the ion current density. The capacitance increases with increasing ion current, especially for the 7 mA case which exhibits a capacitance approximately one order of magnitude greater than a standard glancing-angle deposited film over much of the sensor range.

Directed Branch Growth in Aligned Nanowire Arrays

Branch growth is directed along two, three, or four in-plane directions in vertically aligned nanowire arrays using vapor-liquid-solid glancing angle deposition (VLS-GLAD) flux engineering. In this work, a dynamically controlled collimated vapor flux guides branch placement during the self-catalyzed epitaxial growth of branched indium tin oxide nanowire arrays. The flux is positioned to grow branches on select nanowire facets, enabling fabrication of aligned nanotree arrays with L-, T-, or X-branching. In addition, a flux motion algorithm is designed to selectively elongate branches along one in-plane axis. Nanotrees are found to be aligned across large areas by X-ray diffraction pole figure analysis and through branch length and orientation measurements collected over 140 μm(2) from scanning electron microscopy images for each array. The pathway to guided assembly of nanowire architectures with controlled interconnectivity in three-dimensions using VLS-GLAD is discussed.

Control of the principal refractive indices in biaxial metal oxide films

We provide both an extensive experimental characterization and a model for metal oxide, slanted columnar thin films fabricated using glancing angle deposition. The model is applicable to slanted posts of any type, deposited at a constant deposition angle, with variable azimuthal substrate rotation. The model is capable of predicting the column tilt, principal refractive indices, and in-plane birefringence under a single unified framework, given knowledge of common material parameters. This paper also establishes a number of additional important results, including the occurrence of negative in-plane birefringence and the occurrence of uniaxial films with nonzero columnar tilt.

Understanding the Effects of a High Surface Area Nanostructured Indium Tin Oxide Electrode on Organic Solar Cell Performance

Organic solar cells (OSCs) are a complex assembly of disparate materials, each with a precise function within the device. Typically, the electrodes are flat, and the device is fabricated through a layering approach of the interfacial layers and photoactive materials. This work explores the integration of high surface area transparent electrodes to investigate the possible role(s) a three-dimensional electrode could take within an OSC, with a BHJ composed of a donor-acceptor combination with a high degree of electron and hole mobility mismatch. Nanotree indium tin oxide (ITO) electrodes were prepared via glancing angle deposition, structures that were previously demonstrated to be single-crystalline. A thin layer of zinc oxide was deposited on the ITO nanotrees via atomic layer deposition, followed by a self-assembled monolayer of C60-based molecules that was bound to the zinc oxide surface through a carboxylic acid group. Infiltration of these functionalized ITO nanotrees with the photoactive layer, the bulk heterojunction comprising PC71BM and a high hole mobility low band gap polymer (PDPPTT-T-TT), led to families of devices that were analyzed for the effect of nanotree height. When the height was varied from 0 to 50, 75, 100, and 120 nm, statistically significant differences in device performance were noted with the maximum device efficiencies observed with a nanotree height of 75 nm. From analysis of these results, it was found that the intrinsic mobility mismatch between the donor and acceptor phases could be compensated for when the electron collection length was reduced relative to the hole collection length, resulting in more balanced charge extraction and reduced recombination, leading to improved efficiencies. However, as the ITO nanotrees increased in height and branching, the decrease in electron collection length was offset by an increase in hole collection length and potential deleterious electric field redistribution effects, resulting in decreased efficiency.

Effect of porosity on optical properties of chiral films

Thin films with chiral or helical microstructures exhibit circular birefringence effects. Glancing angle deposition (GLAD) is a fabrication method capable of producing chiral thin films with controllable porosity and microstructure. In this paper, the effects of porosity on the circular birefringence exhibited by helical TiO2 films are presented. Transmittance measurements reveal two optimal film growth angles: one corresponding to a maximum in form birefringence and another corresponding to strong anisotropic scattering. Reflectance data support the transmittance measurements in the regime where scattering is minimized.