BY ITS very nature, nanotechnology is too small to see with the naked eye. Even so, chemists have found a way to make it even less perceptible by creating a nanoscopic form of 鈥渋nvisible ink鈥�.
But it鈥檚 not about slipping secret messages across the lab, says at the University of Erlangen-Nuremberg in Germany. Instead, the technique offers a way of growing nanocrystals of a much higher purity than achieved to date.
Marbach and his colleagues used an electron beam to remove oxygen ions from a silicon oxide wafer, leaving nanoscopic dents in the surface. The process leaves virtually no visible trace. The dents facilitate chemical reactions, though, so a hidden message written onto the wafer with the electron beam can later be revealed by flowing iron pentacarbonyl gas across the surface. The gas reacts at the indentations to form carbon monoxide while leaving solid 鈥� and reflective 鈥� iron nanocrystals fixed to the surface (Angewandte Chemie International, DOI: ).
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鈥淭he process leaves virtually no visible trace, but the hidden message can later be revealed鈥�
Electron beams are already used commercially to deposit nanocrystals. In the semiconductor industry, for example, the technology is used to repair defects in the lithographic masks used to create integrated circuits. 鈥淚t鈥檚 a multimillion-dollar business,鈥� says Marbach.
Traditionally this involves targeting electron beams directly at molecules like iron pentacarbonyl, forcing them to decompose into iron nanocrystals, which stick to the surface. While this allows for the controlled growth of nanocrystals, the harsh process can lead to carbon and oxygen impurity levels as high as 60 per cent within the crystals, says Marbach.
Using the new technique, the crystals are over 95 per cent iron. 鈥淚nstead of using a sledgehammer approach to destroy the iron pentacarbonyl molecules, ours is gentler,鈥� says Marbach, because the decomposition of the molecules is purely chemical.
鈥淚t鈥檚 a nice technique,鈥� says at Imperial College London. 鈥淭he problem with existing electron-beam deposition is that the quality is not great,鈥� which affects the properties of the nanocrystals, he says.
