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Anisotropic properties of columnar nanoporous thin films were utilized to design and fabricate interference mirrors with lossless omnidirectional reflection in the visible spectral range. Index graded columnar films with distributed Bragg reflector (DBR), sinusoidal, and Gaussian refractive index profiles were studied using finite-difference frequency-domain and finite-difference time-domain methods, with an emphasis on maximizing the omnidirectional reflection bandwidth. Titanium dioxide columnar films with sixteen period sinusoidal refractive index profile were fabricated using the glancing angle deposition technique and characterized by angle resolved transmittance measurements. Simulations and experimental measurements have shown the presence of the omnidirectional reflection band up to 5% wide for a film with a maximum refractive index nmax=2.3 and refractive index contrast Δn=0.8. Simulations further showed that with the optimal choice of the refractive index variation range, the omnidirectional reflection band can reach 10.5% width in TiO2 films with a sinusoidal index profile, 14.5% with a DBR index profile, and 12% with a Gaussian profile. Due to the optical anisotropy of the columnar films, the reflection bandwidth exceeded the corresponding value, observed in isotropic analogs, by a factor of three to four depending on the choice of the refractive index profile. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Oblique-incidence physical vapor deposition has been used to create optical thin films with a polygonal helix-shaped nanostructure. A series of titanium dioxide thin films are investigated, including triangle, square, pentagon, and star-shaped polygonal helices. Experimental optical measurements reveal a double-handed circular Bragg response: at one frequency band a polygonal helix reflects left-handed circularly polarized light, and at a second frequency band reflects right-handed circularly polarized light. The relative wavelength dependence of each reflection band is determined by the physical structure of the polygonal helix, a property that is set during the thin-film deposition process. Spectral-hole polarization filters, produced by adding twist and spacing layer defects to polygonal helix thin films, are also reported. [ABSTRACT FROM AUTHOR]

Presents information on a study which demonstrated the use of the transmission-line-matrix method for analyzing the grain boundary and interfacial diffusion problems in thin films. Methods; Results; Discussion.

Ultrathin-layer chromatography (UTLC) potentially offers faster analysis, reduced solvent and sample volumes, and lower costs. One novel technique for producing UTLC plates has been glancing angle deposition (GLAD), a physical vapor deposition technique capable of aligning macropores to produce interesting separation properties. To date, however, GLAD-UTLC plates have been restricted to model dye systems, rather than realistic analytes. This study demonstrates the transfer of high-performance thin-layer chromatography (HPTLC) sugar analysis methods to GLAD-UTLC plates using the office chromatography framework. A consumer inkjet printer was used to apply very sharp low volume (3-30 nL) bands of water-soluble analytes (lactose, sucrose, and fructose). Analytic performance measurements extrapolated the limits of detection to be 3-5 ng/zone, which was experimentally proven down to 60-70 ng/band, depending on the sugar. This qualitative analysis of sugars in a commercially available chocolate sample is the first reported application of GLAD-UTLC to food samples. The potential utility of GLAD-UTLC is further exemplified by successful coupling with electrospray ionization mass spectrometry for the first time to characterize underivatized sugars. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

With growing interest in nanostructured thin films produced by glancing angle deposition (GLAD), it becomes increasingly important to understand their overall growth mechanics and nanocolumn structure. We present a new method of isolating the individual nanocolumns of GLAD films, facilitating automated measurement of their broadening profiles. Data collected for α = 81° TiO2 vertical nanocolumns deposited across a range of substrate rotation rates demonstrates that these rates influence growth scaling parameters. Further, individual posts were found in each case that violate predicted Kardar-Parisi-Zhang growth scaling limits. The technique's current iteration is comparable to existing techniques in speed: though data were studied from 10,756 individual objects, the majority could not be confidently used in subsequent analysis. Further refinement may allow high-throughput automated film characterization and permit close examination of subtle growth trends, potentially enhancing control over GLAD film broadening and morphology. [ABSTRACT FROM AUTHOR]

We investigate the evolution of copper phthalocyanine thin films as they are etched with argon plasma. Significant morphological changes occur as a result of the ion bombardment; a planar surface quickly becomes an array of nanopillars which are less than 20 nm in diameter. The changes in morphology are independent of plasma power, which controls the etch rate only. Analysis by X-ray photoelectron spectroscopy shows that surface concentrations of copper and oxygen increase with etch time, while carbon and nitrogen are depleted. Despite these changes in surface stoichiometry, we observe no effect on the work function. The absorbance and X-ray diffraction spectra show no changes other than the peaks diminishing with etch time. These findings have important implications for organic photovoltaic devices which seek nanopillar thin films of metal phthalocyanine materials as an optimal structure. [ABSTRACT FROM AUTHOR]

Abstract: Ultrathin layer chromatography (UTLC) is a relatively new variant of thin layer chromatography, with a 10 m thick monolithic silica sorbent layer that gives faster separations with lower limits of detection and reduced analyte and solvent volumes. We have produced UTLC plates with controllable nanostructure and thickness, and show that the layer separation characteristics depends on the film nanostructure. We also show that layers made with in-plane anisotropic nanostructures will exhibit a decoupling effect, where the analyte spots do not develop in the same direction as the solvent front movement. The added layer morphology and material selection adds a degree of freedom to UTLC, and may have applications in multi-dimensional TLC. [Copyright &y& Elsevier]

Physical vapor deposition under conditions of obliquely incident flux and limited adatom diffusion results in a film with a columnar microstructure. These columns will be oriented toward the vapor source and substrate rotation can be used to sculpt the columns into various morphologies. This is the basis for glancing angle deposition (GLAD), a technique for fabricating porous thin films with engineered structures. The origin of the columnar structure characteristic of GLAD films is discussed in terms of nucleation processes and structure zone models. As deposition continues, the columnar structures are influenced by atomic-scale ballistic shadowing and surface diffusion. Competitive growth is observed where the tallest columns grow at the expense of smaller features. The column shape evolves during growth, and power-law scaling behavior is observed as shown in both experimental results and theoretical simulations. Due to the porous nature of the films and the increased surface area, a variety of chemical applications and sensor device architectures are possible. Because the GLAD process provides precise nanoscale control over the film structure, characteristics such as the mechanical, magnetic, and optical properties of the deposited film may be engineered for various applications. Depositing onto prepatterned substrates forces the columns to adopt a planar ordering, an important requirement for photonic crystal applications. [ABSTRACT FROM AUTHOR]

Glancing angle deposition (GLAD) is a physical vapour deposition method used to fabricate highly functional thin films with an engineerable columnar morphology. Recent developments in GLAD technology have produced columnar nanostructures of increased complexity, including periodic, nanofibrous, perforated, and graded porosity thin films for use in applications ranging from sensors and actuators to optical filters, microfluidics, and catalysis. A brief review of GLAD methodology and historical developments is followed by a discussion of the latest developments in this field. [ABSTRACT FROM AUTHOR]

Helical microstructures are of interest for MEMS devices because of their spring-like shape. However, helices with micron and submicron dimensions are difficult to engineer using conventional processing techniques where patterning is accomplished lithographically. In this paper, we report the fabrication of porous gold, nickel, and polystyrene thin films with helical pore architectures. All films were made using a replication process, in which a thin film comprised of independent helical microstructures acted as the template. Filling of the template with metals was achieved by electroplating through the microstructures, whereas filling with polystyrene was achieved by capillary action. Porous films were produced from these composites by wet etch removal of the template material. Typical helical pores were on the order of 100 nm in diameter and extended through a film 1 pm to 2 pm thick. These films were generally more robust than the films from which they were templated, since they consisted of a solid network with helical pores rather than individual structures. Polymer and metal films with helical pores could be used for sensor and catalytic devices that take advantage of the chemical properties of these materials. Polymer films are also of interest for mechanical sensor and actuator devices since they are expected to be more compliant than both traditional MEMS materials and the films from which they were templated. [ABSTRACT FROM AUTHOR]

We report the fabrication and optical characterization of a tetragonal square spiral photonic crystal with a three-dimensional relative band gap of approximately 10% using the glancing angle deposition (GLAD) technique. This thin film structure is produced in a one-step process that is highly versatile as a wide range of crystal structures can be created simply through the variation of deposition parameters. Measurements indicate upper and lower frequency band edges at vacuum wavelengths of 2.50 and 2.75 μm, in the infrared region of the spectrum. [Copyright &y& Elsevier]

Porous chiral thin films composed of a nearly hexagonal-close-packed array of helical nanorods of tris-(8-hydroxyquinoline) aluminum (Alq3) are fabricated using glancing angle deposition and exhibit a one-dimensional polarization-dependent photonic stop band. These chiral thin films transmit only one handedness of circularly polarized light and emit highly circularly polarized photoluminescence. The strong circular Bragg phenomena exhibited by these films suggest potential for use of these films in low-threshold mirrorless lasing and circularly polarized emission applications, while the porosity of the films allows for infiltration by dyes or liquid crystals to enable the formation of unique hybrid materials. [ABSTRACT FROM AUTHOR]

The article discusses the significance of glancing angle deposition (GLAD), a toolbox of processing techniques for nanostructure fabrication. GLAD is a combination of traditional thin-film vacuum deposition and controlled substrate motion. Accordingly, GLAD technique is used to deposit science materials on surfaces at a highly oblique angle resulting in a flexible and straightforward method of developing nanostructured and porous thin-film materials. The major advantage of GLAD process is its compatibility with many materials including dielectrics, metals and semiconductors.

Increasing the throughput of the powerful single-step glancing angle deposition (GLAD) method using a prototype simplified roll-to-roll (R2R) system has been explored. While the conventional GLAD technique is popular for fabricating nanostructured devices in a single deposition step, it is not a high-output process. To evaluate the feasibility of large area GLAD deposition, the authors examined the geometrical considerations required to eventually achieve GLAD in a roll-to-roll manufacturing system. Nominal deposition and rotation angles were mathematically translated to their effective R2R counterparts, allowing for deposition recipes of the archetype GLAD nanostructures (slanted posts, vertical posts, and square spirals) and the mechanics of the phi-sweep technique to be converted to this space. Representative structures were then deposited, and the phi-sweep technique successfully applied, in a prototype single barrel roller R2R experimental system. This prototype system provides a foundation for moving GLAD from the laboratory to mass production. [ABSTRACT FROM AUTHOR]

Abstract: Glancing angle deposition (GLAD) was used to deposit controlled-porosity TiO2 thin films of varying pore size and shape. Hybrid materials were fabricated by infiltrating the TiO2 films with poly(3-hexylthiophene) (P3HT) using a spin casting process, which were either employed as-cast or subjected to thermal processing. Photovoltaic devices fabricated using these hybrid materials were characterized as a function of TiO2 pore size and shape, and thermal processing. It was found that the optimum pore size was dependent on the thermal history of the sample. When as-cast hybrid materials were employed conversion efficiencies decreased with increasing pore size. When hybrid materials annealed above the melting point of P3HT were employed the trend was reversed, with conversion efficiency increasing with increasing pore size. Trends in conversion efficiency were found to correlate with P3HT crystallinity, demonstrating that optimum pore sizes for hybrid devices of this type are dependent on sample processing. [ABSTRACT FROM AUTHOR]

Porous chiral thin films composed of a nearly hexagonal-close-packed array of helical nanorods of tris-(8-hydroxyquinoline) aluminum (Alq3) are fabricated using glancing angle deposition and exhibit a one-dimensional polarization-dependent photonic stop band. These chiral thin films transmit only one handedness of circularly polarized light and emit highly circularly polarized photoluminescence. The strong circular Bragg phenomena exhibited by these films suggest potential for use of these films in low-threshold mirrorless lasing and circularly polarized emission applications, while the porosity of the films allows for infiltration by dyes or liquid crystals to enable the formation of unique hybrid materials. [ABSTRACT FROM AUTHOR]

Highlights: [•] Study of anisotropic SiO2, Al2O3, TiO2, and ZrO2 GLAD UTLC stationary phases. [•] Chromatographic merits evaluated using carotenoid and food dye mixtures. [•] Separation performance depended on material, oxidation heat treatments, and UV irradiation. [•] First successful GLAD UTLC–ESI-MS recording of dyes on inorganic ultrathin films. [•] New GLAD UTLC layers may separate samples better than traditional miniaturized sorbents. [Copyright &y& Elsevier]

Abstract: Iron pyrite is a promising material for photovoltaic power production due to low material extraction and processing costs and high optical absorption. Reliable production of photovoltaic grade iron pyrite thin films has, however, been challenging. One potential fabrication route is the direct conversion of iron-to-iron pyrite by sulfur annealing (sulfurization). Bulk iron thin films are used typically but they can suffer from cracking or delamination. Herein we report the sulfurization of porous, columnar Fe films deposited with Glancing Angle Deposition (GLAD), which allows us to control the inter-column spacing (void-fraction) of the precursor film. We show that the morphology and microstructure of the iron pyrite films are strongly affected by the void-fraction. By precisely tuning the void-fraction of the precursor film at 82° oblique angle incidence deposition we can produce iron pyrite films with increased crystallite sizes >100nm with a uniform, crack-free, facetted granular microstructure. Large crystallites may reduce carrier recombination at grain boundaries, which is attractive for photovoltaic devices. Further increasing the void-fraction produces a columnar iron pyrite structure. We also report composition, electrical and optical characterization including a 27ps lifetime of photocarriers measured with ultrafast optical-pump/THz-probe. Structured, porous precursors offer an alternate route to control microstructure and film stress during fabrication of iron pyrite thin films. [Copyright &y& Elsevier]

Abstract: Nb-doped TiO2 (TNO) has been widely explored as an alternative transparent conductor. TNO thin films have been fabricated by various deposition methods, and here we demonstrate fabrication by evaporation of sol–gel synthesized TNO source material. A range of Nb x Ti1− x O2 (x =0, 0.03, 0.06, 0.12, 0.24) compositions were synthesized, pressed into pellets, and deposited as thin films via electron beam evaporation. The thin films were characterized for composition by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy and for crystallinity by X-ray diffraction for the different Nb contents explored. Transparency, conductivity, and Haacke transparent conductor figure of merit values are reported for the evaporated TNO films as a function of Nb content, substrate heating temperature, and post deposition annealing conditions. Glancing angle deposition (GLAD) is used to demonstrate nanostructuring of evaporated TNO based on physical shadowing. Nanostructured GLAD TNO films of increased morphological complexity can be produced using this method, and could lead to unique transparent conductor device architectures. [Copyright &y& Elsevier]

Abstract: Nanopillar metal oxide thin films offer versatility as ultra high surface area supports and conductors. Metal oxide properties (e.g. stability, conductivity) can be tuned via phase and composition control to achieve desired application-specific functionality. Here we demonstrate phase control of high surface area thin films grown by glancing angle deposition and transformed to desired phases through high temperature annealing in a reducing environment. The post-annealed properties such as stoichiometry, phase, and morphology are shown to be largely dependent on initial film structure and hydrogen forming gas flow rate. Initially amorphous films of approximate stoichiometry Nb2O5 are transformed to NbO2 or NbN x O1−x through annealing. Transformation to oxygen-deficient phases is more easily achieved for films of higher initial porosity. Higher forming gas flow rates result in both increased oxygen removal and significantly less physical degradation of nanostructures. A phase map is included as a guide to phase formation and morphology control in annealed nanopillar niobium oxide films. [Copyright &y& Elsevier]

Abstract: A high-speed optical humidity sensor was fabricated from a nanostructured titanium dioxide thin film. A refractive index depth profile designed to yield a narrow-bandpass optical interference filter was obtained through nanoscale porosity variations produced by a physical vapor deposition technique known as glancing angle deposition (GLAD). The GLAD process provides film morphology control on the 10nm scale through a combination of deposition at highly oblique vapor incidence angles and controlled substrate motion. Under varying humidity conditions the transmittance spectrum of the filter shifts due to effective index changes of the porous structure resulting from water vapor adsorption/desorption. Performance analysis has shown that this device is highly sensitive, exhibits minimal hysteresis, and is extremely fast. The adsorption and desorption response times were measured to be 270ms and 160ms, respectively. [Copyright &y& Elsevier]