Unconventional Patterning at the Nanoscale
NANOSCALE DEVICES
Photonic Colloidal Crystals
In this lab, photonic crystals are assembled via convective self assembly of polystyrene
microspheres. They spontaneously form under ambient conditions of temperature and pressure.
Convective assembly uses capillary forces at the meniscus of a colloidal solution and substrate
to draw colloids into ordered arrays. These are then compared with the commercially available
synthetic opals. Gemstone opal diffracts visible and near IR as a result of sub micrometer
size of the colloids.
MATERIALS:
- Polystyrene sphere sizes: 0.3560 μ, 0.465 μ, 0.535 μ (Polysciences Inc) and
are about 2.6 wt % in solution.
- Glass Slides, Micropipette, Petri dish (to slow evaporation and tilt samples)
PROCEDURE: (Click on pictures to view the videos)
*You will need QuickTime Player installed on your computer to view the videos.
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The procedure is as simple as evaporation of a solution on a substrate, as outlined.
| Cleaning the glass slides |
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You will be provided with glass slides that have been treated with piranha etch
(1:5 ratio of sulphuric acid to hydrogen peroxide) for 30 min to obtain a clean and
hydrophilic surface.
To begin with, the microscope glass slides were placed in a beaker to which 50
ml of sulphuric acid was added in a fumehood. |
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10 mL of hydrogen peroxide was added to the beaker containing the glass slides
and sulphuric acid.
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The reaction is exothermic with gas release.
Warning: Piranha solution is very energetic and potentially explosive.
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After 30 minutes, the piranha solution was emptied into a waste container while
retaining the glass slides in the beaker. The piranha solution container remained
open in the hood until cool. NEVER ADD PIRANHA TO ORGANIC WASTE!!!!
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| Assembling Spheres on a Surface |
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Wash the glass slides with water multiple times and then dry with nitrogen. Glass
slides can be stored in ethanol until used.
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Polystyrene beads with various diameters (between 0.3 and 0.6 μm)
with no surface treatment and monodispersed in pure water are obtained from Polysciences
Inc . They are filtered using Wattman filters to ensure uniform size distribution.
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Place the clean glass slide in a petri dish and add about 6-8 ml of the polystyrene
mixture with a proper ratio of water / particles (depending on the deposition area
and the size of the spheres) using a syringe with the filter attached on the glass
slide.
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Tilt the solution around carefully so that it covers the slide without leaving
any gap.
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Cover the petri dish (small volume helps in slowing down the evaporation and also
the effect of external airflow). The whole system is tilted at about 9o to
ensure that the evaporation starts from the top of the sample and proceeds to the
bottom. It takes 2 hours or leave it overnight to dry completely.
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Within a minute or so, you can observe that excess solution accumulates at the
bottom of the slide.
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This can be drained with a syringe without disturbing the glass slide.
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Repeat the procedure for the different polystyrene diameters and compare the difference
in color with the sizes used.
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| Imaging the Photonic Crystal |
| Look at the sample using SEM. The sample needs to be sputter-coated (platinum/palladium
alloy) to provide a conducting path for the electrons and to avoid sample charging
effects before mounting in the SEM. |
QUESTIONS:
- Compare the SEM images of polystyrene microspheres of different diameters. What did
the ordering look like? Were you able to see multi-layer ordering?
- What colors were observed from the different sizes of spheres assembled into a crystal?
- What did the ordering look like for the solution with no dilution and a 1:3 dilution?
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Authors:
Prof. Teri W. Odom,
Dr. M. Viswanathan and Y. Babayan
Institution:
Northwestern University
Evanston, IL USA
Level:
College and above
In the classroom:
This Course is a video lab manual for hands on fabrication and characterization of materials at the nanoscale.
Materials requirements range from simple chemicals, benchtop tools and CDs to necessary access to advanced
characterization equipment such as an Scanning Tunneling Microscope.
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