Synthesis of Inverse Opal Photonic Crystals

Procedure modified by George Lisensky and Jacob Horger, Beloit College, from the Inverse Opal Photonic Crystals Laboratory Guide by R. Schroden and N. Balakrishnan, University of Minnesota MRSEC, 2001.

 Photonic crystals are periodic optical nanostructures that are designed to affect the motion of photons in a similar way that periodicity of a semiconductor crystal affects the motion of electrons. Photonic crystals occur in nature and in various forms have been studied scientifically for the last 100 years. Inverse Opal Photonic Crystals are three-dimensional structures formed from self-assembly -- essentially a mixture of dielectric nano-spheres to settle from solution into three-dimensionally periodic structures possessing photonic band-gaps. (From: http://en.wikipedia.org/wiki/Photonic_crystal)

Safety

  • Wear eye protection
  • Chemical gloves recommended
  • -Fume hood recommended

Procedure

Step 1. Use ethanol to wet a filter paper in a small Buchner funnel connected to a water aspirator.          

Step 2. Place 2.0 gram of dry polymethacrylate spheres in the funnel and spread them out to evenly cover the entire filter. Turn on the water aspirator and slowly wet the entire surface with a well mixed solution of 2.0 mL 100% ethanol, 3.0 mL tetraethylorthosilicate, 1.5 mL pure water, and 0.5 mL conc hydrochloric acid. Try to wet all the spheres before running out of liquid, but if not all of the polymethylmethacrylate is coated the yield will simply be smaller. On the other hand, once all the spheres are wet, stop adding liquid or the product may be too dense. Aspirate for 20 minutes to speed drying.

Step 3. Transfer the product to combustion boats.          

Step 4. Place the combustion boats into the quartz liner of a tube furnace (or a ventilated box furnace) in a hood. Ramp the temperature at 2 degrees C/minute from room temperature to 300 degrees C to complete the silica formation. Hold at 300 degrees C for 2 hours. Ramp the temperature at 2 degrees C/minute to 550 degrees C to decompose the polymethylmethacrylate spheres. Hold at 550 degrees C overnight (ten hours). Cool the oven to room temperature.

Step 5. High quality samples will be apparent by their opalescence.

Properties

What color is your product in air? (The index of refraction of silica is 1.460 and of air is 1.000)
Add a drop of ethanol to a small portion of your product. What color do you observe? (The index of refraction of ethanol is 1.360)
Add a drop of toluene to a small portion of your product. What color do you observe? (The index of refraction of toluene is 1.496)
Based on the sphere size used as a template, predict the absorption wavelength for each of these. How do you need to modify the equation for closed-packed spheres for the inverse material?

Characterization with a Scanning Electron Microscope

  • Samples are deposited on conducting graphite tape and gold coated.
    The coated samples are placed into an electron microscope.
  • Zooming in on a sample of monodispersed polymethylmethacrylate spheres. What is the diameter of the spheres?
  • Zooming in on a sample of inverse opal photonic silica crystal. What is the diameter of the holes.
  • Flying over the surface of a sample of inverse opal photonic crystal (2000x magnification.)

Materials

  • polymethacrylate spheres
  • 100% ethanol
  • concentrated hydrochloric acid
  • tetraethylorthosilicate
  • combustion boats
  • tube furnace

YouTube Link:

http://www.youtube.com/watch?v=VT77S8dSbFA