Thin film/liquid crystal composite optical materials

The existence of three-dimensional photonic bandgaps in square spiral thin films, made using the Glancing Angle Deposition (GLAD) method, was recently verified. We further demonstrate the flexibility of the GLAD process to fabricate silicon photonic bandgap crystals with customizable bandgap centre frequencies. GLAD combines physical vapor deposition at highly oblique flux incidence angles with dual axis substrate motion control, creating porous thin films with three-dimensional submicrometer topographies. This makes it a near-ideal approach for diamond lattice based photonic crystal fabrication, with manipulation of the photonic properties through the deposition parameters. We have produced a range of different square spiral thin films, and present characterization results indicating bandgaps at wavelengths close to 2 micrometers. Such low wavelength bandgaps have not previously been achieved for square spiral architectures. Ongoing work towards optimization of the process holds the promise of square spiral photonic crystals with even lower bandgap centre wavelengths, approaching the telecommunications windows. In addition to its flexibility and mass-production suitability, we present how GLAD can be used to engineer defects inside the photonic crystals during the one-step growth process. Such defects may potentially be employed as stand-alone waveguides, or as elements of more complex photonic device and circuitry designs involving subsequent micromachining of the GLAD thin films.

Influence of zigzag microstructure on mechanical and electrical properties of chromium multilayered thin films

ABSTRACTPhotonic Band Gap (PBG) crystals are artificially engineered structures of periodically alternating dielectric materials that enable manipulation of light in unique ways. As proposed by O. Toader and S. John, large bandwidth PBG crystals based on the diamond lattice can be constructed by fabricating a two-dimensional periodic array of silicon square spirals. Furthermore, an improved photonic crystal can be constructed by fabricating the inverse of this structure, such that void square spirals are situated in a silicon backfill. This structure yields a theoretical maximum PBG of 24%, while the direct silicon square spiral structure yields a theoretical maximum of 16%. [1]The direct silicon square spiral structure has previously been successfully fabricated using the glancing angle deposition (GLAD) process [2]. The GLAD fabrication process uses advanced substrate motion and oblique incidence deposition to engineer precise nanostructures such as helices, chevrons, or polygonal spirals. These nanostructures are grown with lateral and longitudinal periodicity using a pre-patterned substrate consisting of a tetragonal array of relief structures.Upon successful fabrication of the direct silicon square spiral structure, it was proposed that fabrication of the inverse structure could be completed using a backfill process with a silica template [2]. A periodic array of silica square spirals would be fabricated using the GLAD technique. The template would then be filled with amorphous silicon using low-pressure chemical vapour deposition (LPCVD). To reveal the inverse silicon structure, the silica template would be removed by means of chemical etching with an isotropic chemical etchant. Creation of a suitable template for the inversion of silicon square spirals is an important step in the realization of the inverse silicon square spiral PBG crystal. However, we have seen empirically that silica square spiral films grown directly via the GLAD technique are structurally inferior to silicon films. We present an intermediate step for the fabrication of a suitable template, whereby a silicon structure is fabricated with appropriate geometries and subsequently oxidized. The procedure has been optimized to ensure a thorough oxidization of the silicon posts, while minimally distorting the structure of the square spiral template.

We report the application of solution- and vapor-phase siloxane-based methods for tailoring the surface chemistry/properties of highly porous, nanostructured thin films fabricated using glancing angle deposition (GLAD). The GLAD technique produces high surface area films consisting of isolated columns and provides complete control over the film/column morphology. In the present study, the chemical tunability of a variety of metal oxide GLAD films was investigated using solution-based and vapor-phase surface functionalization methodologies. The surface properties and structures of the treated and untreated films were investigated using scanning electron microscopy (SEM), advancing aqueous contact angle measurements, cyclic voltammetry, and X-ray photoelectron spectroscopy (XPS). Results indicate that the surface chemistry of metal oxide GLAD films could be tailored by either method; however, chemical reactivity depends strongly on the metal oxide film material. Chemical tunability is demonstrated through the covalent tethering of numerous chemical moieties onto the exposed and interior surfaces of metal oxide GLAD films of varied structural motifs. Through careful choice of surface modifier, the present derivatization methods afford a full range of aqueous wettability from hydrophilic to superhydrophobic without compromising film structure. These functionalized, nanoconstructed films demonstrate a high degree of tunability over both structural and surface properties, making them well suited for diverse applications such as optical filters or sensors.

It is known that glancing angle deposition (GLAD) utilizing extreme self-shadowing during film growth can produce periodic microstructures on a predefined seed layer using electron-beam evaporation. This deposition process has been applied to the fabrication of periodic arrays of magnetic pillars and has possible applications in optical devices. Critical to the production of these microstructures is adherence to a narrow angular flux distribution centered at an oblique incidence angle, and optimizing the seed pattern for the desired film characteristics. In this article, a low-pressure, long-throw collimated GLAD sputter deposition has been used to fabricate submicrometer scale periodic pillar and quasihelical microstructures over a range of seed separations (e.g., 150, 300, 600, 900, and 1200 nm) and deposition angles (82.5° and 86° with respect to the substrate normal). We have found that for fixed deposition parameters, periodic quasihelical growth degenerates into pillars as the seed separation increases, and that the transitional region over which this degeneration occurs shifts to high seed separations with increasing flux incidence angles. This and other growth trends are also observed and described using the three-dimensional (3D) ballistic simulator, 3D-FILMS, which has proven to be successful in simulating the growth of aperiodic and periodic GLAD films. The use of sputtering for periodic GLAD simplifies the process control, and should enable deposition of a broader range of materials for diverse applications including magnetics, optics, and sensors.

ABSTRACTWe report on fabrication, structural and infrared optical characterization of nanostructure aluminum sculptured thin films prepared by glancing angle deposition (GLAD) and controlled substrate motion on p-type silicon. We discuss two structures, one with plate-like and one with screw-like (chiral) morphology. While the plate-like sample possesses a metal Drude behavior in the infrared spectral range, the chiral nanowire sample behaves non-metallic and reveals a series of intriguing resonances, which are equally spaced in frequency by ∼7.5 THz. We suggest that formation of 3D nano resonator circuits consisting of inductances and capacitances has occurred within the screw-like conductive aluminum wire sample, which might be responsible for the observed resonances. We suggest conductive GLAD nanostructures in combination with Schottky diodes to facilitate active or passive THz detector and transmitter devices.

Molecular beam techniques are used to grow water films on Pt(111) with various incident angles and collision energies from 5 to 205 kJ/mol. The effect of the incident angle and collision energy on the porosity and surface area of the vapor-deposited water films was studied using nitrogen physisorption and infrared spectroscopy. At low incident energy (5 kJ/mol), the infrared spectra, which provide a direct measure of the surface area, show that the surface area increases with incident angle and levels off at angles > 65 degrees . This is in contrast to the nitrogen uptake data, which display a maximum near 65 degrees because of the decrease in nitrogen condensation in the larger pores that develop at high incident angles. Both techniques show that the morphology of vapor-deposited water films depends strongly on the incident kinetic energy. These observations are consistent with a ballistic deposition shadowing model used to describe the growth of highly porous materials at glancing angle. The dependence of film morphology on incident energy may have important implications for the growth of porous materials via glancing angle deposition and for the structure of interstellar ices.

A new approach to deposition of thin films for thermal barrier applications is described. During electron beam evaporation, the extreme shadowing effect that is present at highly oblique incidence is employed to introduce porosity into thin films of zirconia. Using controlled substrate motion a solid capping layer may be applied to these porous films. By depositing layers of porous material and capping in an alternating fashion a new structure is produced which warrants evaluation as an improved thermal barrier coating.

Liquid crystal (LC) is the promising material for the fabrication of high-performance soft, flexible devices. The fascinating and useful properties arise from their cooperative effect that inherently allows the macroscopic integration and control of molecular alignment through various external stimuli. To date, light-matter interaction is the most attractive stimuli and researchers developed photoalignment through photochemical or photophysical reactions triggered by linearly polarized light. Here we show the new choice based on molecular diffusion by photopolymerization. We found that photopolymerization of a LC monomer and a crosslinker through a photomask enables to direct molecular alignment in the resultant LC polymer network film. The key generating the molecular alignment is molecular diffusion due to the difference of chemical potentials between irradiated and unirradiated regions. This concept is applicable to various shapes of photomask and two-dimensional molecular alignments can be fabricated depending on the spatial design of photomask. By virtue of the inherent versatility of molecular diffusion in materials, the process would shed light on the fabrication of various high-performance flexible materials with molecular alignment having controlled patterns.

Investigating the effect of SiO2 thin films with sculptured topographies on the alignment properties of liquid crystals

Tailoring the structural and magnetic properties of Ni zigzag nanostructures using different deposition angles

This paper reports a thorough microstructural characterization of glancing angle deposited (GLAD) TiO2 thin films. Atomic force microscopy (afm), grazing-incidence small-angle x-ray scattering (GISAXS) and water adsorption isotherms have been used to determine the evolution of porosity and the existence of some correlation distances between the nanocolumns constituting the basic elements of the film’s nanostructure. It is found that the deposition angle and, to a lesser extent, the film thickness are the most important parameters controlling properties of the thin film. The importance of porosity and some critical dimensions encountered in the investigated GLAD thin films is highlighted in relation to the analysis of their optical properties when utilized as antireflective coatings or as hosts and templates for the development of new composite materials.

The analytical and physical properties are reported for nanohole arrays prepared with glancing angle deposition (GLAD) or plasma treatment of a nanosphere lithography (NSL) mask prior to the deposition of a thin Au film. The nanohole arrays obtained with a 450 nm nanospheres mask are characterized using atomic force microscopy (AFM) to determine the depth and the width of the nanoholes, and the periodicity of the nanohole arrays. The analytical properties are reported in terms of the surface plasmon (SP) excitation wavelength (500 nm to 1000 nm), sensitivity to refractive index (27 nm RIU(-1) to 487 nm RIU(-1)), sensitivity to monolayer formation (shift of the SP band by approx. 1 nm), and refractive index resolution (10(-4) RIU). These simple techniques produce well-ordered nanohole arrays with tunable analytical and physical properties for the development of biosensors.

Glancing angle deposition (GLAD) was used to fabricate nanostructured silicon (Si) thin films with highly controlled morphology for use in laser desorption/ionization mass spectrometry (DIOS-MS). Peptides, drugs and metabolites in the mass range of 150-2500 Da were readily analyzed. The best performance was obtained with 500 nm thick films deposited at a deposition angle of 85 degrees . Low background mass spectra and attomole detection limits were observed with DIOS-MS for various peptides. Films used after three months of dry storage in ambient conditions produced mass spectra with negligible low-mass noise following a 15 min UV-ozone treatment. The performance of the Si GLAD films was as good as or better than that reported for electrochemically etched porous silicon and related materials, and was superior to matrix-assisted laser desorption/ionization (MALDI)-MS for analysis of mixtures of small molecules between 150-2500 Da in terms of background chemical noise, detection limits and spot-to-spot reproducibility. The spot-to-spot reproducibility of signal intensities (100 shots/spectrum) from 21 different Si GLAD film targets was +/-13% relative standard deviation (RSD). The single shot-to-shot reproducibility of signals on a single target was +/-19% RSD (n = 7), with no indication of sweet spots or mute spots.

Sample preparation for mass spectroscopy typically involves several liquid and solid phase clean-ups, extractions, and other unit operations, which are labour-intensive and error-prone. We demonstrate a centrifugal microfluidic platform that automates the whole blood sample’s preparation and clean-up by combining traditional liquid-phase and multiple solid-phase extractions for applications in mass spectroscopy (MS)-based small molecule detection. Liquid phase extraction was performed using methanol to precipitate proteins in plasma separated from a blood sample under centrifugal force. The preloaded solid phase composed of C18 beads then removed lipids with a combination of silica particles, which further cleaned up any remaining proteins. We further integrated the application of this sample prep disc with matrix-assisted laser desorption/ionization (MALDI) MS by using glancing angle deposition films, which further cleaned up the processed sample by segregating the electrolyte background from the sample salts. Additionally, hydrophilic interaction liquid chromatography (HILIC) MS was employed for detecting targeted free amino acids. Therefore, several representative ionic metabolites, including several amino acids and organic acids from blood samples, were analysed by both MALDI-MS and HILIC-MS to demonstrate the performance of this sample preparation disc. The fully automated blood sample preparation procedure only took 35 mins, with a throughput of three parallel units.