Current semiconductor technology is based on silicon, gallium arsenide (GaAs), and other opaque materials. But a promising alternative, metal oxide semiconductors (consisting of molecules with both metallic and oxygen atoms) are very efficient, and happen to be transparent. However, fabricating them by ordinary means requires extremely high temperatures, high enough to melt the types of polymers that structure the devices.
A new method for making metal oxide devices at much lower temperatures uses ultraviolet (UV) irradiation. Yong-Hoon Kim and colleagues used UV light to chemically activate metal particles in a chemical solution; the new metal oxide molecules condensed out of the solution, forming a thin semiconducting film. The process can be performed at room temperature -- far lower than the 350C temperatures typical of metal oxide fabrication.
Besides the awesomeness factor (itself probably a sufficient reason for most readers), flexible and transparent electronics are potentially useful in medical, transportation, and scientific applications. Conventional semiconductor materials don't work in the thin films required for flexible electronics because they are too brittle. Current organic semiconductors are electronically unstable when stressed and are not efficient at conducting charge, so they are not particularly useful for large-scale implementation.
Metal oxide semiconductors don't suffer from either of these difficulties. They have a high density of electric charge carriers (meaning they are efficient at carrying current) and are amorphous solids rather than rigid crystals. That makes thin metal oxide films very flexible.
Previous experiments have used what is known as a sol-gel technique, where metals (typically indium, gallium, and/or zinc) are dissolved in an organic substance such as 2-methoxyethanol (2-ME). The metal oxide films are formed by annealing: heating the solution (the "sol" part of "sol-gel") to high temperatures, which breaks the molecules of the liquid, allowing its component oxygen atoms to react with the metal atoms. Upon cooling, these metal oxide molecules precipitate out of the solution (the "gel"), forming a thin, transparent film, which can be guided into an etched surface to make circuits.
The high temperatures are the problem. 350C is above the melting point of most flexible, transparent substances (eg plastics), and real electronic devices need a substrate to give them shape. It doesn't matter how thin or transparent metal oxide devices are if they must be deposited on thick, opaque, rigid materials.
The present study bypassed the annealing process by using radiation. UV photons are sufficiently energetic to dissociate some molecules, so the researchers shone intense UV light on the solution (this sort of light-mediated activation is known as photochemistry). The solvent was chosen so this freed oxygen atoms, which combined with metal atoms just as in the annealing process, but the film formed at room temperature.
While the deposition of the film happened at room temperature, the mercury UV lamp (a high-intensity version of "black lights") they used heated the film to approximately 150C, much higher temperatures than expected, which may be a matter of some concern.
The researchers compared the performance of the metal oxides produced by annealing to those made using photochemistry. They found their performance to be comparable, and in some cases the UV-produced semiconductors actually did better in terms of electric charge carrier behavior.
This UV-based photochemistry method of metal oxide semiconductor fabrication is potentially very useful. Even with the unexpected heating, the temperatures were far lower than those required for annealing, which may allow the fabrication of inexpensive, flexible, transparent electronic devices.