A brilliant discovery

It’s been said that they last forever. Now, it seems, they may not. Diamonds, the world’s sparkliest and hardest naturally occurring material, have been shown to evaporate — slowly but surely — when exposed to the right kind of light.
The discovery, which may shock wearers and merchandisers alike of a girl’s best friend, came as a surprise to researchers at Macquarie University in Sydney.
‘‘Although this type of light-induced evaporation has been observed in some materials, this is the first time it’s been shown to occur for diamonds,’’ says lead researcher Richard Mildren, an associate professor in the university’s photonics research centre.
Diamonds, sometimes known as brilliants, derive their legendary hardness from the ingenious way in which their carbon atoms are arranged in an extremely rigid grid, known as a crystal lattice. The lattice’s rigidity results from the fact that each atom is bound tightly to four other carbon atoms.
Although diamonds are generally transparent to ultraviolet (UV) rays, a tiny sliver of the light is absorbed near the surface. The added energy, Professor Mildren says, is enough to break the chemical bonds that normally bind carbon atoms to the surface.
‘‘Our research has found that it takes the energy of two UV light particles, or photons, to dislodge one carbon atom,’’ he says. ‘‘Carbon atoms or carbon-containing molecules — we’re not sure which yet – are ejected from the surface one by one. The rate at which they are ejected is very predictable.’’
Exactly how the energy is absorbed, and leads to the bonds being broken, is not well understood, he says. ‘‘This is something we are working on.’’
Evidence so far suggests that the UV light is absorbed below the surface; the energy then moves to the surface where it breaks the carbon bonds. ‘‘The surface of a diamond is normally covered in oxygen atoms and we suspect the carbon is released in the form of carbon monoxide molecules,’’ Professor Mildren says.
Not any old light will do the trick. The diamonds his team worked on were exposed to a very specific form of UV — intense light pulses in the UV-C band. These are the sun’s harshest rays that are filtered out by the Earth’s ozone layer.
A few seconds after being bombarded with light pulses, the diamonds developed small pits on their surface. ‘‘The rate of mass loss in the diamond fell notably for lower light levels,’’ Professor Mildren explains. ‘‘But the etching process still continued — albeit at a slower and slower pace.’’
Because the rate of evaporation is so slow, it’s not noticeable under normal conditions. In fact, even under very bright UV conditions, such as intense sunlight or a UV tanning lamp, it would take approximately the age of the universe — almost 14 billion years — to make an appreciable impact.
So there’s no need to worry about the sparkler in your ring vanishing for some time yet.

Applications galore

In addition to being the most prized asset in jewellery, diamonds are used widely in industrial cutting machines and some forms of lasers.
Professor Mildren’s team is now studying how the range of potential applications for diamonds might be extended. ‘‘If we can make structures in diamonds that enable us to control the position of the light, we could make smaller and more efficient devices for use in quantum computing and high-performance lasers,’’ he says.
It is currently a major challenge to make small devices out of diamond because of its hardness, he says. ‘‘In future, we hope to be able to use finely tuned lasers and the UV evaporation technique to fabricate the microstructures needed to make such devices.’’

Other worldly brilliants

On Earth, diamonds are created over millions of years as a result of high temperatures and the crush of molten rock deep within the planet.
Might similar processes elsewhere in our solar system imply the existence of extraterrestrial diamonds?
Exposure to strong UV light is a factor that needs to be considered when trying to make predictions about where else in the solar system diamonds might lurk, Professor Mildren says.
‘‘People have conjectured about finding diamonds on the moon and on other planets,’’ he explains. ‘‘The probability of finding them depends on many factors. This includes having the right conditions to create them in the first place, and in having the right temperatures and pressures to keep them stable.’’
For instance, the high temperatures and pressures deep within Neptune, and perhaps Uranus, the solar system’s two outermost planets, would be sufficient to convert part of the methane in their lower atmospheres into diamonds, some astronomers have suggested. The diamonds, if they existed there, would most likely fall towards the planets’ centres.
Temperatures in the upper layers of Neptune’s atmosphere, for example, plummet to minus 220 degrees, and below. Its core heats to 7000 degrees or more. Moving inwards towards the centre, the temperature and pressure steadily rises to levels sufficient to create diamonds.
If they were to be found there, the chances of them evaporating are slim, to say the least.
‘‘UV evaporation won’t affect these planets — they are too far away from the sun,’’ Professor Mildren says. ‘‘The UV intensity would have to be more intense than what we experience on Earth.’’
So, apart from the Earth, the only other places that diamonds would evaporate over time would be on planets closer to the sun, or bodies without protective ozone layers, such as the moon, he says.

Surprise, surprise

Light-induced evaporation has been found to occur in other materials. So it’s not too surprising to find it works for diamonds, Professor Mildren explains.
Yet diamonds, he says, are extremely well known and have been studied for thousands of years. ‘‘So it was surprising that this phenomenon had not been observed before.’’
The UV evaporation process observed in other materials seems not to be the same for diamonds. ‘‘This is the first time we’ve noticed that the effect requires two photons to eject an atom,’’ Professor Mildren explains. ‘‘This gives us major clues about how the process works.’’
The scientists discovered the effect while conducting tests on a diamond laser designed to generate a UV beam.
Unfortunately, the surface evaporation caused by the beam interfered with the laser, which ceased to function after about 10 minutes. ‘‘So, one of our primary interests was to find a way to stop this evaporation,’’ says Professor Mildren. And that is what led to their discovery.

Links

A copy of the research team’s recently published paper can be found at: opticsinfobase.org/ome/abstract.cfm?uri=ome-1-4-576
Learn more about the chemical structure of diamonds at: www.avogadro.co.uk/structure/chemstruc/network/g-molecular.htm
Study the crystal structure of diamond at:
worldofmolecules.com/materials/diamond.htm
Discover more about UV light and how it relates the electromagnetic spectrum at: worldofmolecules.com/materials/diamond.htm

VCAA links

VELS
Level 6: vels.vcaa.vic.edu.au/vels/science.html
VCE 
Physics Unit 4 Area of Study 2: Interactions of light and matter: www.vcaa.vic.edu.au/vce/studies/physics/physicsd.pdf
Chemistry Unit 3: Area of Study 1: Chemical analysis: www.vcaa.vic.edu.au/vce/studies/chemistry/chemistry-sd-07.pdf

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