New experimental method for
studying defects in crystal lattices

Academy Research Fellow Filip Tuomisto and researcher Jussi-Matti Mäki have developed a new experimental technique that can be used for studying opto-electronically active materials. Light-emitting diodes can be made more efficient by reducing lattice defects in the starting materials. In collaboration with the DTC Research Centre (UK), the method was used to solve the mystery of the brown colour in natural diamond.

Mäki and Tuomisto from the Department of Applied Physics at Aalto University have been studying natural diamonds using antimatter for a few years. Measurements of positron lifetimes in diamond have revealed that the crystal lattice of brown diamonds contains relatively large voids, that is, vacancy clusters formed by tens of missing atoms.

“We illuminated the samples with visible wavelengths of light during the positron experiments and found that the positrons became more sensitive to these vacancy clusters. This is due to electrons being excited from the bonds between the atoms in the lattice to the vacancy clusters. The phenomenon is visible also with bare eyes as absorption that reduces all visible wavelengths from the light spectrum (more at the blue end), causing the smoky brown colour,” says Tuomisto.

A “natural diamond” sounds like a brilliant and shiny gemstone. It is much less known that the majority of the highest-purity, naturally occurring diamonds have a brownish, smoky tint. This material looks much less brilliant and has a strongly reduced commercial value than its colourless counterpart. The origin of the smoky brown colour has been a mystery until now, while the blue colour has long been known to originate from boron impurities. Not all of the naturally occurring colours are yet understood – for instance, the origin of pink is not known. The positron experiments also found that by treating the brown diamond at high pressures and high temperatures (the so-called HPHT method, where the pressure can be up to 60 kbar and temperature up to 2,500 °C), the vacancy clusters causing the brown colour disappear.

“We've now developed a new experimental technique based on modulating the illumination conditions and measuring the time-dependent changes with positron annihilation spectroscopy. The same methodology can also be applied to studying materials that are more relevant from the opto-electronics industry's point of view, such as gallium-nitride-based thin films that are used in light-emitting diodes. It's typical of vacancy defects to hinder the performance of LEDs, making the understanding of the properties of these defects an important goal to ease the development of future energy-efficient lighting solutions and other opto-electronics devices,” Mäki explains.

Article: Time dependence of charge transfer processes in diamond studied with positrons, Physical Review Letters 107, 217403 (2011)

The research has been performed within the HIGHLIGHT project of the MIDE programme, in collaboration with the Department of Micro and Nanoscience and the DCT Research Centre (UK).

More information:

Dr Filip Tuomisto
Positron research group leader
Academy Research Fellow, Docent
Department of Applied Physics
Aalto University
tel. +358 9 4702 3144, fax +358 9 4702 3116

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