Silicon spheres shine red, green or blue

17 Jun 2002

A new crystal growth process has resulted in silicon structures that emit tunable, visible light.

Tiny crystals of silicon, engineered to have a spherical shape, have been shown to emit light in the visible spectrum. The crystals emit at different wavelengths depending on their size, and thus offer a route to a tunable silicon-based light source. Scientists at the University of Texas, US, reported their breakthrough in the 25th April issue of the Journal of the American Chemical Society. They claim that their crystals promise greater efficiency and tunability than previously reported, light-emitting, silicon devices and thus have a great potential for use in future applications - ranging from lasers to displays.

Brian Korgel and Keith Johnston, both at the University of Texas, produce their spherical nanocrystals by a relatively simple process called arrested precipitation. In a highly pressurized titanium chamber, a mixture of octanol and hexane is heated to 500 degrees and a pure silicon reagent is added, which degrades to silicon atoms. The octanol chains act as a growth inhibitors - they bind to the silicon surfaces and prevent the atoms from recombining into large crystals. The crystals are harvested by evaporating the solvent and their photoluminescence is then tested. "Essentially, we shine light on them at one frequency, which the particles absorb, and they then re-emit light at a different frequency," explained Korgel. The high temperature and pressure conditions produce a high degree of crystallinity, which Korgel believes results in their high efficiency.

The Texas researchers's work is significantly different to recent research performed at Surrey University, UK, on light-emitting silicon (see March 9 news, Optics.Org). For one thing, the Surrey team stimulate their crystals using electricity, not light. "Also, the light from their crystals is not visible but is in the near infrared, " Korgel explained. "Their synthetic approach does not provide a rational way to tune the colour into visible wavelengths, which our synthesis does," he added.