Angewandte Chemie International Edition
Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2004, 43 (17), 2230—2234
"Drawing" with Nanoparticles
Defined patterns of gold nanoparticles in transparent materials—optoelectronic building blocks of the future?
Small but impressive: nanoparticles are so tiny that they are somewhere between individual atoms and "normal" solid particles. Their optical and electronic properties are correspondingly intriguing. Collaborating Japanese and Chinese researchers have now developed a method by which gold nanoparticles can be deposited so precisely that it is possible to use them to "draw" colorful, three-dimensional pictures in transparent materials.
However, the aesthetic, artistic aspect of this new technique is only a secondary feature. Materials doped with nanoparticles of precious metals are hot candidates for ultra fast optical switching elements in optoelectronics. The need for a precisely defined spatial distribution of nanoparticles within the material has thus far been a difficult hurdle. A team led by Jianrong-Qiu has now overcome this obstacle. The starting material for this new method is a silicate glass doped with gold oxide (Au2O3). When the glass is irradiated by a laser beam, tiny grey dots form wherever the beam strikes the glass. In this way, the researchers can “draw” an approximately 5 mm by 5 mm butterfly, in sharp detail, in the glass. If the glass is then tempered at 550 °C, the grey picture becomes colored; depending on the intensity of the initially implemented laser beam, the butterfly turns purple, red, or yellow. They also observed that the colour could be space-selectively deleted again by a second irradiation of the laser.
How does this work? The extremely high energy input from the short laser pulses causes electrons in the irradiated spots in the glass to separate from their atomic nuclei. Gold ions are capable of absorbing such free electrons and are thus reduced to gold atoms. The application of heat then gives the gold atoms enough energy to separate from the silicate network and wander about. If they bump into other gold atoms, they aggregate into tiny clumps. These gold nanoparticles are colored because they absorb light in the visible range of the spectrum. The wavelength of light absorbed depends on the size of the nanoparticles. And the greater the intensity of the laser, the more reduced gold atoms are formed per unit of volume. These atoms act as nucleating points. Heating thus produces a correspondingly larger number of correspondingly smaller gold nanoparticles. A second laser irradiation breaks the nanoparticles into tiny pieces or atoms.
This new technique could be an important step toward extremely fast three-dimensional optical storage with very high storage capacity. In addition, the team hopes to produce complete three-dimensional gold nanocircuits in glasses with a high gold-ion content.