Lab Report: Synthesis and Characterization of Silica Colloids for Photonic Materials, Lab Reports of Physical Chemistry

Download Lab Report: Synthesis and Characterization of Silica Colloids for Photonic Materials and more Lab Reports Physical Chemistry in PDF only on Docsity! Photonic Materials – Synthesis and Characterization of Crystals of Silica Colloids (i.e. Synthetic Opals) INTRODUCTION: Photonic materials are loosely defined as materials that interact strongly with light. All light absorbing or emitting materials are types of photonic materials. Another way to generate such materials is to produce a structure that has periodicity on the length scale of the wavelength of light (hundreds of nanometers for visible light). Such periodic materials will strongly scatter or reflect light, and the extent of the process will depend on the wavelength of the light, the periodic structure of the materials, and the direction of light propagation through the material. This basic approach has been highly fruitful and is the basis for dielectric stack mirrors and filters. For photons moving through a material with a periodic variation in refractive index, it turns out that one can describe something called a photonic band structure, which describes the wavelengths (energies) of photons that can move through the solid. This is the light analogy to an electronic band structure, which describes the allowed energies of electrons moving though a material with a periodic variation of charge (produced by the atomic cores). It turns out that it is even possible to produce the equivalent of a semiconductor for light – something called a photonic band gap material. As mentioned above, it is fairly easy to control the propagation of light in one direction by making a stack of layers with different refractive index where each layer is a few hundred nanometers in thickness. Photolithography can also be used to make patterns in 2 dimensions on the few hundred nanometer length scale (although it is not easy). Making a material that has a periodic variation of index of refraction in the 3 dimensions, however, is much harder, and for many applications it is the 3-dimensional structure that is the most interesting. Luckily, nature has already figured out how to do this. The exquisite colors seen in the gem stone opal are not due to absorption or reflectance, but rather due to interference effects in a photonic solid. Opals are composed of spheres of silica a few hundred nanometers in diameter, stacked together in a periodic array. In this lab, you will produce synthetic opal films and measure the optical properties of those films. The lab starts with the synthesis of monodisperse silica (glass) spheres that are a few hundred nanometers in diameter. Such nanometer size spheres are often called colloids. You will characterize the size of these spheres using dynamic light scattering (DLS) and, if possible, static light scattering, and then you will crystallize the colloids into a periodic array. Finally, you will measure the optical properties of the synthetic opal you have created. PROCEDURE: Overview -- Day 1, synthesize two sizes of silica colloids Day 2, measure colloid sizes using DLS, start film growth Day 3, measure transmission and reflectance of the films, if you have time, measure colloid size using static light scattering Synthesis of silica colloidal particles: You will make two different batches of colloidal silica using the two reagent lists described below. In the synthesis, mix ethanol, water, and ammonia in plastic bottles with vigorous magnetic stirring. Add the tetraethylorthosilicate (TEOS) in a single aliquot as quickly as possible (<10sec). Make sure that you maintain the vigorous stirring while the TEOS is being Copyright © 2003 by Sarah H. Tolbert. All rights reserved. Revised 2009. added. Cap the bottle and allow the sample to react for at least 2 – 3 hours with continuous vigorous stirring. Make 100ml total volume and assume all volumes are additive (for example for the first batch below use: 12 mL of 2M NH3 in ethanol, 13 ml of water, 71 ml of absolute ethanol and then add 4 mL of TEOS). Make sure that all reagents (the ethanol, the NH3 in ethanol, and the TEOS) are fairly new bottles or the reaction will not work well. For the NH3 in ethanol in particular, it is imperative that the bottle you use was opened the quarter you are taking the lab. The two syntheses shown below should produce two different sizes: First batch: 0.24 M ammonia 7 M water 0.18 M TEOS absolute ethanol to 100 ml Second batch: 0.24 M ammonia 7 M water 0.13 M TEOS absolute ethanol to 100 ml Growth of thin film colloidal arrays: Thin film colloidal arrays are grown on glass slides that are cut in half lengthwise and placed in a 20 ml scintillation vial containing a suspension of the silica particles in ethanol. Clean the slides thoroughly, first with acetone and then with ethanol. Also, you will need full length slides, so if they break horizontally during cutting, just start again. Although it is not pure ethanol, you can simply use your synthesis solutions to grow the films. The concentration of particles in the synthesis suspension may be too high, however resulting in very thick films. Optimal concentrations are 0.2 to 1% by volume. Estimate the vol% of silica in the solution by drying out a 2 or 3 ml aliquot in the oven (density of colloidal silica ~2.05g/mL). Then dilute the colloid solution with absolute ethanol to the desired vol%. You should try to use two different concentrations for each sample, one around 0.5% or higher and one closer to 0.25%. Fill the scintillation vial with the diluted colloid solution and immerse the glass slide in the vial as vertically as possible. Make sure that the top of the slide sticks out of the top of the vial. As the solvent evaporates, an opalescent film is deposited. You should let the solvent evaporate for at least 3 days (over the weekend). To produce the best films, try to find a place where the air is calm (not in the hood). Colloid Size Determination Using Static and Dynamic Light Scattering 1. This experiment makes use of the Coulter Beckman N4 Plus DLS 2. Rinse a plastic cuvette five times with DI water 3. Fill the cuvette with water to about 70% full 4. Add a drop of your colloidal silica solution into the cuvette. If your sample appears to have settled at all, you may want to sonicate it first to redisperse the colloids. 5. Cap the cuvette and invert it a few times to disperse the sample uniformly. DO NOT shake the cuvette, as this may create bubbles. 6. Place the cuvette in the sample compartment, and close the cover. 7. Click “123” in the menu bar to bring up the “Setup a Run” screen. 8. Click “check intensity” to verify the intensity of the selected angle(s). 9. Select the angles at which you wish to make measurements. Check 90° since it is the most commonly used angle. Copyright © 2003 by Sarah H. Tolbert. All rights reserved. Revised 2009.