Nicholas G. Wakefield

Surface Area Characterization of Obliquely Deposited Metal Oxide Nanostructured Thin Films

The glancing angle deposition (GLAD) technique is used to fabricate nanostructured thin films with high surface area. Quantifying this property is important for optimizing GLAD-based device performance. Our group has used high-sensitivity krypton gas adsorption and the complementary technique of cyclic voltammetry to measure surface area as a function of deposition angle, thickness, and morphological characteristics for several metal oxide thin films. In this work, we studied amorphous titanium dioxide (TiO(2)), amorphous silicon dioxide (SiO(2)), and polycrystalline indium tin oxide (ITO) nanostructures with vertical and helical post morphologies over a range of oblique deposition angles from 0 to 86 degrees. Krypton gas sorption isotherms, evaluated using the Brunauer-Emmettt-Teller (BET) method, revealed maximum surface area enhancements of 880 +/- 110, 980 +/- 125, and 210 +/- 30 times the footprint area (equivalently 300 +/- 40, 570 +/- 70, and 50 +/- 6 m(2) g(-1)) for vertical posts TiO(2), SiO(2), and ITO. We also applied the cyclic voltammetry technique to these ITO films and observed the same overall trends as seen with the BET method. In addition, we applied the BET method to the measurement of helical films and found that the surface area trend was shifted with respect to that of vertical post films. This revealed the important influence of the substrate rotation rate and film morphology on surface properties. Finally, we showed that the surface area scales linearly with film thickness, with slopes of 730 +/- 35 to 235 +/- 10 m(2) m(-2) microm(-1) found for titania vertical post films deposited at angles from 70 to 85 degrees. This characterization effort will allow for the optimization of solar, photonic, and sensing devices fabricated from thin metal oxide films using GLAD.

Selective transmittance of linearly polarized light in thin films rationally designed by FDTD and FDFD theories and fabricated by glancing angle deposition

Columnar thin films exhibiting linear polarization selectivity have been theoretically investigated and fabricated using the glancing angle deposition (GLAD) technique. The film structure employed an s-shaped columnar morphology that may be either smoothly bent or composed of discrete segments. Finite-difference time-domain and frequency-domain methods have been used to model these thin film structures numerically. Simulation results have yielded optimal geometries in which the films exhibit either a single frequency band with polarization-selective transmittance or two separated frequency bands each transmitting one of two orthogonal linearly polarized modes. Based on these designs, a series of TiO2 films were grown by GLAD with continuous and discrete s-shaped columnar morphology. Experimental measurements by spectrophotometry verified the presence of selectivity for the orthogonal linearly polarized modes. Films with more then 24 periods were found to have polarization selectivity approaching unity. The agreement between the simulation and experimental results demonstrates the potential for future theoretical development of highly selective polarization filters based on GLAD thin films.Columnar thin films exhibiting linear polarization selectivity have been theoretically investigated and fabricated using the glancing angle deposition (GLAD) technique. The film structure employed an s-shaped columnar morphology that may be either smoothly bent or composed of discrete segments. Finite-difference time-domain and frequency-domain methods have been used to model these thin film structures numerically. Simulation results have yielded optimal geometries in which the films exhibit either a single frequency band with polarization-selective transmittance or two separated frequency bands each transmitting one of two orthogonal linearly polarized modes. Based on these designs, a series of TiO2 films were grown by GLAD with continuous and discrete s-shaped columnar morphology. Experimental measurements by spectrophotometry verified the presence of selectivity for the orthogonal linearly polarized modes. Films with more then 24 periods were found to have polarization selectivity approaching unity. Th...

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.

On the uniformity of films fabricated by glancing angle deposition

Films fabricated using the glancing angle deposition technique are subject to significant variations in several important film parameters across a sample due to geometric conditions that are not uniform across the substrate. This paper presents a method to quantify the non-uniformities in these quantities, starting from a generalized geometric framework, for low-pressure, physical vapor deposition of thin films on substrates of arbitrary size and position. This method is applicable to any glancing angle deposition setup including substrate tilt and rotation but focuses on the case of constant deposition angle and arbitrary azimuthal rotation. While some quantities, such as the effective deposition angle and the deposited mass per unit area at any given point on the substrate can be determined purely from the geometry of the deposition setup, obtaining further quantities, such as the film density and thickness, requires additional, material-specific information that is easily measured.

Three-dimensional alignment of liquid crystals in nanostructured porous thin films

Oblique evaporation of inorganic materials has long been used to induce alignment in liquid crystals, often for the purpose of controlling the pretilt angle in a liquid crystal cell. These alignment layers are relatively dense, keeping the liquid crystals above the surface of the inorganic layer. By evaporating at increasingly oblique angles (> 80°), the alignment layer can be made porous, allowing liquid crystals to infiltrate the film and to align to individual nanostructures. By coupling simultaneous computer controlled substrate motion during evaporation, a process known as glancing angle deposition (GLAD), the nanostructures can be grown in a variety of useful shapes, including helices, polygonal spirals, zigzags and periodically bent S-shaped columns. Alone, these films exhibit properties such as linear and circular polarization selective Bragg reflection, and full three-dimensional photonic bandgaps. By infiltrating liquid crystals into the voids of the film, one can align liquid crystals in three dimensions, as well as tune and switch the films optical properties. Additionally, the GLAD film can be used to template polymerizable liquid crystals for subsequent monomer infiltration. In this work, using spectroscopic ellipsometry, we examine the effects of liquid crystal infiltration on various film structures made from a variety of metal oxides, for both varying film thickness and deposition angle. Techniques for filling a porous film with a known volume of liquid crystals are also presented. Additionally, we examine the switching behaviour for these films under applied electric fields. Finally, we compare experimental and simulated results used to predict and optimize the optical properties of these hybrid films.

Alignment of Liquid Crystals Infiltrated into Porous Thin Films with Tailored Nanostructures Grown by Glancing Angle Deposition

Traditional liquid crystal alignment techniques influence mesogen alignment at the substrate alone, and rely on interactions between molecules to extend that alignment throughout a devices thickness. An alternative approach is the use of a nanostructured host that provides a large surface contact area with a guest liquid crystalline material and can extend for micrometres beyond the substrate. This permits a greater degree of control over the mesogens and allows alignment control throughout the thickness of the nanostructured host. We present the realization and experimental results using hosts fabricated through a specialized thin film process known as glancing angle deposition.

Optical behaviour of hybrid LC/inorganic nanostructures

Porous thin films of dielectric materials have been deposited using e-beam evaporation onto substrates held at highly oblique angles (> 80o), coupled with simultaneous computer controlled substrate motion about two independent axes. This technique, known as glancing angle deposition (GLAD), enables the formation of shaped, isolated nanostructures, including vertical posts, zig-zags, and both helical and polygonal spirals, which exhibit chiral optical properties. GLAD films form the backbone of liquid crystal (LC) hybrid optical materials and devices, and afford key advantages. The porous nature of the GLAD structures allows LCs to uniformly penetrate the film and modify its optical properties. Addition of LCs to GLAD films improves the properties of the films by reducing optical scattering, enhancing transmission, and accentuating existing chiral and linear optical anisotropies. Further, by mixing a dichroic dye with the LCs, polarization selective optical properties can be introduced into the film which can be used to augment the functionality of GLAD films. It has been found that addressing hybrid GLAD films with an electric field reorients the LCs, allowing one to switch the optical properties of the composite film. This behaviour extends to LCs mixed with dichroic dye, allowing one to switch the selective polarization properties with an applied voltage. Using results based on spectroscopic ellipsometry, we will examine the optical properties and switching behaviour of LC/dichroic dye hybrid GLAD films and discuss how the results allow one to infer the alignment of LCs in GLAD films, as well how the addition of dichroism to the film affects the selective transmission of both linearly and circularly polarized light.