Michael J. Brett

Ultrathin layer chromatography on nanostructured thin films

Ultrathin layer chromatography (UTLC) is a relatively new variant of thin layer chromatography, with a 10mum 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.

Chapter 13 – Glancing Angle Deposition

Publisher Summary This chapter focuses on experimental aspects of the Glancing angle deposition (GLAD) technique – its methods and possibilities. It introduces the basic theory, and describes the canonical microstructures and apparatus required to produce GLAD films, describes advanced GLAD techniques, covering procedures, which occur before, during, and after a GLAD process. The chapter also discusses the properties of GLAD films at several length scales: single column properties, the properties of an ensemble of columns, and the ‘bulk’ properties of a GLAD film and reviews applications and devices made with GLAD films. GLAD is a highly flexible nanofabrication technique combining oblique angle deposition and precise manipulation of substrate position during deposition. By depositing at oblique angles, columnar structures arise owing to a combination of ballistic shadowing and limited surface diffusion. The resulting columns are tilted toward the incident flux direction, and can be sculpted into various morphologies: slanted and vertical posts, helices, chevrons, square spirals, and combinations thereof. GLAD can partially decouple the different length scales involved in thin film production, yielding novel materials with interesting properties. GLAD is compatible with a large number of materials, further increasing the combinatorial power of the GLAD technique. Advanced GLAD techniques may offer additional control or modification of the canonical GLAD structures. Such techniques include forced nucleation through seeding and advanced substrate motion control to combat film broadening. While GLAD has been used for many applications, the dominant use to date is in optics.

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...

Ambient humidity monitoring using a 1D photonic crystal sensor fabricated with glancing angle deposition

We present the design, fabrication, and characterization of a GLAD-fabricated photonic crystal sensor with a bandgap located in the visible optical spectrum. The photonic crystal is fabricated from TiO2 using electron-beam evaporation in a GLAD capable vacuum deposition system. Changes in humidity over a wide range (from 3% to 90% relative humidity) are detected by a colour change in the film due to movement of the photonic bandgap. The colour changes are quantified by measuring the transmittance of white light. Coupling the sensitivity of the film with a simple visual feedback system eliminates the need for complicated measurement techniques. This is desirable to minimize the cost and power consumption of the sensor device, making it amenable to large-scale production and deployment.

Morphology control and nanoscale patterning of small molecule organic thin films

We present methods for nanostructuring organic thin films for organic photovoltaic (OPV) devices. Using the glancing angle deposition (GLAD) technique, we fabricate a variety of columnar morphologies of metal phthalocyanine (MPc) materials that can be used as structured donor layers in OPV devices. We leverage this capability with block copolymer surface patterning techniques to achieve perfectly periodic columnar arrays while providing additional control over column dimensions, spacing, and density. Our investigation employs hexagonal seed patterns of platinum on silicon and we vary the seed spacing between 40 nm and 60 nm. We find that pattern resemblance begins to fade when the film thickness exceeds the seed spacing. We compare the film evolution between vertical post and slanted post morphologies, and use advanced substrate motion techniques to constrain column diameters. We conclude by addressing the compatibility of surface patterning with device fabrication. Patterned ITO surfaces and SiO2 seed patterns are shown.

Photonic crystal reflectance switching by dye electrophoresis

Reflectivity of a photonic crystal device fabricated by glancing angle deposition may be reversibly altered by infiltration with an absorbing dye solution. An electric field controls the dye ion motion through the photonic crystal. Rapid reflectance changes up to 0.4 in the crystal’s optical band gap are demonstrated. The time evolution of the dye movement process is examined and its operation described. This work may have applications for a passive optical display.

Using film nanostructure to control photoluminescence angular emission profiles

Luminescent thin films are used for many applications, including light-emitting diodes, lasers and flat panel displays. Glancing angle deposition (GLAD) is a physical vapor deposition technique which relies on highly oblique flux angles to create porous thin films. When combined with real-time substrate motion control and measurement of deposition rates, it is possible to produce high quality nanostructered thin films. A rugate filter uses a sinusoidally varying index profile to produce a stop band. Using the GLAD technique, it is possible to produce a rugate filter from a single material. The central wavelength, depth and width of the stop band can be designed by adjusting the film nanostructure. In this paper, rugates composed of Y2O3:Eu are used to control the angular emission profiles of the photoluminescent thin film. Confined, annular and isotropic emission profiles film is nearly uniform for emission angles between ~ -60° and ~60°.