A large, single, hexagonal-shaped and high-purity crystal of boron nitride can be employed in many photonic as well as electronic devices.
Boron nitride in a polycrystalline, hexagonal form has been used for over half a century as a refractory material because it remains chemically stable at high temperatures. It is also an excellent thermal conductor with a high electrical resistance and a high mechanical strength. Recent research has shown that these properties exist even at a single crystal level and can be incorporated into a variety of optoelectronic, nano-photonic as well as electronic devices.
Examples of such applications are in LED lasers, neutron detection, chemical sensing, photonic interconnections and flat optoelectronic devices.
However, the challenge has been in producing single crystals that consist of layers of just a few atoms. Researchers started by dissolving powdered boron nitride in a solvent of molten chromium and nickel at a very high temperature. This mixture was maintained at a temperature of 1,550 degrees Celsius for 24 hours. When this solution has cooled at a rate of two degrees Celsius per hour, hexagonal boron nitride crystals precipitated randomly on the solution’s surface.
If the solution was cooled at a faster rate of 4 degrees Celsius per hour over a period of 50 hours, colourless transparent crystals were formed that had either a flat platelet shape or were in the form of a prismatic needle. The flat crystals had diameters of between one and two millimetres with a thickness of between 50 and 200 microns (micrometres). The needles had lengths of between 100 and 200 microns and widths between 20 and 50 microns.
On investigation, the main impurities found in both types of crystals were oxygen and carbon atoms. These were believed to have originated from the source material used in the synthesis. Nevertheless, the purities were extremely high.
Different types of boron isotopes were present in the crystals. In its natural state boron has 20% of the boron-10 and 80% of the boron-11 isotopes. The synthesis can produce crystals of a higher boron-10 concentration that can be used in many devices for neutron absorption. This isotope enriched type of crystal also has a thermal conductivity that is 40% higher than the polycrystalline material. Concentrations of boron-10 in the single crystals ranged between 1% and 99%.
Future research is concentrated on controlling crystal morphology, size and impurity concentrations so that their performance parameters can be incorporated into electronic and other devices.
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