Car paints changing with temperature
It's now common to build materials which can change colors depending on their surrounding environment because of progresses made in colloid chemistry. But now, German researchers have gone a step further. They've used ion bombardment and gold metallisation to produce new particles whose bonding behavior can be chemically tailored. This could lead to new shimmering car finishes which can change with temperature or humidity, new cosmetics, but more importantly, to new applications in optical data processing.
The team of researchers from the Max Planck Institute of Colloids and Interfaces, led by Dr Dayang Wang, has now produced particles that do not interact with their neighbours in spherically symmetric ways. So they placed a colloidal crystal on a surface and bombarded it with reactive ions, reducing the particles in the upper layer to the desired size and expanding the free surfaces between the colloids.
Below is a picture of these intentionally coated microparticles (Credit for image and caption: Max Planck Institute of Colloid and Interfaces).
Left: an electron microscopic image of the second layer of a colloidal crystal without ion etching. The gold metallised surfaces are light. Middle: a view of the third layer of a colloid crystal after etching and gold coating. Right: a rear image of the third layer of the colloid crystal. It contains gold spots (light) with an adjustable size between 20 and 80 nanometres in the deepest shadows of the etching.
Here is a link to a larger version of this image.
[The researchers] also metallised the crystal with gold. Part of the gold passed through the gaps in the upper layer as if through a stencil, all the way to the lower layers. In this way, patterns of metallisation of various symmetries and at nanoscale sizes are produced. Gold surprisingly also lodged itself in the deep layers on the underside of the particles.
Below is a scheme of the production of defined layered microparticle arrays (Credit for image and caption: Max Planck Institute of Colloid and Interfaces).
Left: a side view of a colloidal crystal being bombarded with reactive ions. Middle: a side view of a colloidal crystal whose size is being reduced by ion bombardment. The crystal is metallised by gold. Right: a colloid crystal with small colloids (pink) in its uppermost layer, a middle layer (blue) and a third layer (black), which contains gold deposits (orange) as triangular-shaped surfaces.
And here is a link to a larger version of this diagram.
Finally, for more information, this research work has been published by 'Angewandte Chemie' under the title "Decoration of Microspheres with Gold Nanodots - Giving Colloidal Spheres Valences" (Volume 44, Issue 47, Pages 7767-7770, December 2, 2005). Here is a link to the abstract.
Sources: Max Planck Society news release, via EurekAlert!, December 2, 2005; and various web sites
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