Future’s bright for silicon

Researchers at the University of Texas have devised a method to make silicon shine. Their tiny, light-emitting spherical silicon crystals are said to hold great promise for future applications ranging from laser technology to flat panel displays.

‘Bulk silicon does not emit visible light, but our structures do,’ said Dr Brian Korgel, principal investigator on the project. ‘Our crystals have the added advantage of being tuneable, meaning that we can create a specific colour of light by adjusting the crystal’s size. Smaller crystals give off blue light — larger ones green, or even red.’

Silicon is the plentiful and inexpensive material that is the basis of transistor technology and the cornerstone of the electronics industry. But under normal circumstances, it does not emit light and this means costly alternative semiconductors such as gallium arsenide are used for LED’s, lasers, sensors and related products.

‘If you make silicon smaller, into nanostructures, you can force it to emit light in visible wavelengths,’ Korgel explained. ‘People have been struggling for about 12 years to come up with ways of doing that.’

The spherical silicon crystals Korgel and colleague Dr Keith Johnston have produced are called nanocrystals or quantum dots. The engineers have made crystals that emit the colours blue and green, and they say a crystal emitting red is not far off.

‘When we put them into devices, the physics changes from classical physics, which everybody understands to quantum-mechanical rules, which make things different,’ said Korgel. ‘It’s a great challenge to the industry. But it also presents opportunities, because you can create devices that work on entirely different principles.’

Korgel and Johnston grew the minute and colourful spheres by a relatively simple, inexpensive process called arrested precipitation.

They first heat a mixture of chain hydrocarbon ‘ligand’ (octanol) and organic solvent (hexane), in a highly pressurised titanium chamber to a temperature of 500 degrees. They then add pure silicon reagent, causing it to degrade to silicon atoms. Ordinarily, the same atoms would soon recombine to form large crystals. But the octanol chains bind to the silicon surfaces inhibiting crystal growth, said Korgel.

‘You end up with these sort of fuzzy particles of silicon that don’t stick to each other,’ said Korgel. ‘What controls their size is how many ligands you have. If you have a lot, the crystals will stay small. If you don’t have very many, they’ll continue to grow into very large crystals.’

Once the chemical reaction is complete, the nanocrystals are harvested by evaporating the solvent, and can be assembled into devices.

While other researchers have produced silicon emitters using different techniques, none are said to have achieved Korgel and Johnston’s combination of high efficiency, emission of light in the visible spectrum and the ability to ‘tune’ crystals to produce different colours.