Beyond the cutting edge

Diamond's supreme hardness has made it invaluable for blades and drill bits. Now new synthetic versions have vastly widened its scope. Twenty-four carat TV screen, anybody? How about an implant?

Simon Hadlington
Friday 03 November 2000 01:00 GMT

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Louise Thomas

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Here's one that won't be appearing in Delia's next recipe book: "Take one part marsh gas and mix with 99 parts hydrogen. Microwave at high setting for a few hours. Upon hearing the ping, open microwave and remove diamond."

Here's one that won't be appearing in Delia's next recipe book: "Take one part marsh gas and mix with 99 parts hydrogen. Microwave at high setting for a few hours. Upon hearing the ping, open microwave and remove diamond."

That is a pretty much authentic recipe for making diamond. It is true that the mixture of gases needs to be in a vacuum, but for an outlay of a few thousand pounds, it is possible to synthesise artificial diamond.

The diamond created by this process, which is called chemical vapour deposition, or CVD, is unlikely to interest a jeweller. Rather than emerging as a large gemstone, the diamond is laid down as an extremely thin film, consisting of tens of thousands of tiny crystals - it is "polycrystalline".

However, such thin-film diamond is exciting technologists. Apart from being used for relatively low-tech applications such as coating drill bits and saws, diamond film could form the basis for flat-screen televisions, high-power microchips and even medical implants.

Diamond was first made artificially in the 1950s, when scientists essentially duplicated Nature's method for turning one form of carbon, graphite, into another, diamond. In graphite, the atoms of carbon are joined to each other to form sheets of flat hexagons which lie on top of each other.

In diamond, however, the carbon atoms are joined to form the four corners of a tetrahedron. This arrangement is very rigid and accounts for, among other things, diamond's supreme hardness.

In 1954, scientists at the General Electric Company in the US subjected black carbon powder to 50,000 times atmospheric pressure, and returned 16 hours later to find two small crystals of diamond. Millions of carats of such industrial- quality synthetic diamond are now manufactured each year by high- pressure techniques.

In the late 1960s, scientists in the then Soviet Union adopted a somewhat more subtle approach. They reasoned that if molecules that contained carbon, such as the gas methane, could be blasted into fragments in a vacuum, then the atoms of carbon might be encouraged to re-form as diamond.

The scientists had limited success: they created diamond but it took an inordinate amount of time to make only vanishingly small crystals. "The work was essentially forgotten for a decade before being revived by a group in the US," says Dr Paul May, a member of one of the leading groups of CVD diamond researchers in the UK, based at the University of Bristol. "The American group cracked the secret of growing diamond films. To make the process viable, you must grow the diamond in an excess of hydrogen."

In a CVD diamond chamber, a source of carbon - such as methane gas (carbon linked to four hydrogen atoms) - is released into the chamber, together with the excess of hydrogen. Sitting in the chamber is the substrate - the surface upon which the diamond will be deposited. Commonly this is silicon, or perhaps the cutting tool which is to receive a diamond coating.

A source of energy is introduced into the chamber: a hot metal filament, or microwaves, or an electrical discharge. This heats the gas mixture up to several thousand degrees, blasting the molecules of gas to single atomic bits and creating a plasma - a gaseous soup of highly reactive fragments of molecules.

As they fly around the chamber, some fragments of this molecular shrapnel will hit the substrate and stick there. Most will be plucked off by the hydrogen radicals, which are highly reactive. However, the occasional carbon atom that alights on the substrate will form the correct "diamond" type of chemical bond with a neighbouring carbon atom. This is a strong bond and as such is immune from attack by the hydrogen. Gradually, more and more carbons will link together in this way, slowly growing the diamond crystals.

Technologists are interested in diamond because of its extreme properties. Unlike graphite, which is a good conductor, pure diamond is one of the best electrical insulators on the planet. It is also possible to "dope" diamond with tiny impurities to make it a semiconductor.

Because it is so tough and can withstand extremes of temperature and is resistant to chemical corrosion and radiation damage, "doped" diamond would be an ideal electronic component for equipment operating in hostile environments: car engines, satellites or nuclear facilities.

Another property of diamond is that it will emit electrons from its surface very readily. Many groups around the world are trying to harness this phenomenon as the basis for display devices such as flat-screen televisions. At Heriot-Watt University in Edinburgh, Professor Phillip John, a chemist, and Professor John Wilson, a physicist, are leading a team that is attempting to grow diamond film suitable for flat-screen displays. "A TV works by heating a metal filament to release electrons and accelerating the electrons on to a phosphor screen," says Professor John.

However, it is conceivable that a flat panel of tiles made from diamond film could be positioned close to a flat phosphor screen. A small voltage could dislodge the electrons from the diamond at a specific point, be accelerated across the gap and impinge on the screen to create a pixel of light. The researchers at Heriot-Watt are experimenting with different gas mixtures in an attempt to produce CVD diamond which has the appropriate properties for display devices.

Professor John's team is also investigating how diamond might form the basis of medical implants. "One of the unusual properties of diamond is that it is tolerated remarkably well by the body," he says. The research team is exploiting this fact to create a tiny medical implant that can continuously monitor the concentration of glucose in the blood. This would be useful for people with diabetes, who must take regular samples of blood throughout the day to check their glucose level.

The researchers have succeeded in attaching an enzyme to a small chip of CVD diamond. The enzyme metabolises glucose and creates a small electric current, which can be measured: the more glucose the higher the current. "The sensor would be implanted in the body, and then communicate with an external device," says Professor John.

But before diamond can become a widespread material for these technological applications, the diamond-growers must overcome some hurdles. At the moment, diamond can be grown only at relatively high temperatures - the substrate reaches temperatures of between 700C and 900C. Scientists would like to make this a few hundred degrees lower, so that the diamond film can be put on more versatile surfaces, such as glass or aluminium.

Another problem is that the diamond is polycrystalline. For many high-tech applications, the diamond needs to be a single crystal, similar to the silicon from which chips are made. Furthermore, making diamond into the most useful kind of semiconductor entails introducing large foreign atoms such as phosphorus into the crystalline lattice. Because diamond is such a rigid crystal, it is proving almost impossible with current techniques to shoehorn these big molecules into the structure.

However, at University College London, Dr Richard Jackman is leading a group who may be the only people in the world to have succeeded in making a commercial device which relies on diamond's electronic properties.

Diamond does not absorb visible light, which is why it is so supremely transparent. However, it does absorb ultraviolet light: when UV light strikes diamond, the electrons in the crystalline lattice become excited and an electric current is produced. This can be measured, making diamond a very sensitive and selective detector of UV light. Significantly, UV light is crucial to the manufacture of computer chips.

As chips become more complex, the manufacturing process demands that the wavelength of the UV light used becomes shorter. This is difficult to detect accurately by conventional methods. Dr Jackman's group has developed a unique method for treating CVD diamond to make it suitable for detecting UV light and such detectors are now being manufactured commercially.

Is that the end of the story? Almost certainly not. The history of any material shows that as more uses are found for it, ways are also found to make it cheaper. Diamonds are for ever, and their uses are becoming as widespread, too.

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