Feb 25, 2009
Bismuth nanostructures provide mechanism for automatic switching
Embedding bismuth nanostructures in amorphous Al2O3 thin films has been shown to produce size-dependent thermo-optical behaviour. Samples prepared by the Laser Processing Group at the Instituto de Optica, CSIC, Madrid, Spain, exhibit repeatable thermally induced optical changes with a contrast of up to 16%, which could provide a useful mechanism for automatic switching or data capture.
"The application that we had in mind is an optical switch that opens when the nanocomposite is heated above a certain temperature and closes once the material cools down," Rosalia Serna told nanotechweb.org. "It could be used as a safety shutter, but also as a temperature sensor for a measurement interval that depends on the dimension of the nanoparticles."
The Laser Processing Group has been studying materials that exhibit optical contrast for many years as part of a research program on data storage and discovered a change in transmission over a wide wavelength range in one of its nanocomposite samples when the material underwent stability testing.
"We decided to explore the result in greater detail and used optical transmission to check the changes," said Serna. "This led to the discovery of a wide hysteresis upon annealing with a large optical contrast and our next step was to investigate whether the dimensions of the nanoparticles could offer a means of controlling the hysteresis loop characteristics. The observed changes are related to solid -liquid phase changes of the Bi nanoparticles."
The scientists presented their findings in Nanotechnology and the team is now busy back in the lab working on the design of multiple nanostructures.
"From a fundamental point of view, it is very important to understand why bismuth nanoparticles have a higher melting temperature than bulk bismuth, and why nanoparticles from other metals, such as lead, indium or tin, exhibit the opposite behaviour," explained Serna. "In addition, we want to explore further whether this behaviour relates to the fact that bismuth contracts upon melting and fully understand how it relates to the material's optical properties."
The work was initiated during the sabbatical stay of Prof. Haro-Poniatowski (Universidad Autonoma Metropolitana Iztapalapa, Mexico D. F.) at the Spanish Laser Processing Group's laboratories and continues as a co-operative study between the researchers in Spain and Mexico.