Despite their close proximity in the periodic table, silicon and diamond might seem to occupy very different niches within the hierarchy of valuable materials. Silicon is one of the main components of common materials such as beach sand. On the other hand, although composed of nothing more precious than carbon, diamond has become the world’s most heavily marketed jewel. Add tiny amounts of other elements to silicon, however, and you have the essence of high technology — semiconductors capable of remarkable electronic feats. Diamond might rely primarily on its good looks to make a splash, but now researchers are exploring how addition of other elements to diamond can transform its properties as dramatically as for silicon, allowing diamond to become the basis of a wide range of applications.
The practice of doping materials — purposefully adding impurities to change a material’s characteristics — goes back more than a century. Naturally occurring diamonds can become doped during their geological formation, but so too can those produced artificially, using techniques such carbon vapour deposition. When this process occurs in the presence of hydrogen and boron-based gas, the resulting diamonds can acquire the features of a semiconductor or even a full-fledged conductor. In combination with diamond’s other outstanding qualities, such as corrosion resistance and high thermal conductivity, investigators are now exploring the potential of boron-doped diamond in a wide range of applications, from bionic eyes and other forms of in vivo electrochemistry to supercapacitors and advanced sensor systems.
Those possibilities have piqued the curiosity of Mary Anne White, FCIC, a professor emerita in Dalhousie University’s Department of Chemistry. She has spent much of her career examining the relationships between structure and properties of materials, including elemental boron, and she regards its impact on diamonds as just one more example of this unassuming element’s capacity for surprise.
Were you aware of boron’s doping abilities with respect to diamond?
“I already knew that the Hope diamond is a naturally occurring boron-doped diamond. Pure diamonds have no colour, but when you add a little bit of boron it becomes semi-metallic and you get this an electric blue colour. In the case of the Hope diamond that amount is around 5 parts per million. It’s actually more common to have nitrogen — carbon’s other neighbour — doped in there from the atmosphere. When they first started making artificial diamonds, they couldn’t do it without doping, because the nitrogen from the air got in too easily. Nitrogen gives rise to yellow diamonds and changes the properties to semi-conducting. If you add more boron to diamond, it can become metallic. That happens when you get about one boron atom in a thousand, but that’s still not a lot. Think of yourself inside this lattice, with 1000 carbon atoms all around you, carbon as far as the eye can see, and this one boron atom creeps in and makes diamond metallic!”
What is it about these elements that causes such major changes within a structure that seems to be as rigid and well defined as diamond?
“It’s really about electrons. Boron has one electron fewer than carbon and that modifies the electronic band structure. Nitrogen, on the other hand, adds negative charge because it has one electron more than carbon, so you’re messing with bands in a different way. In either case you’ve altered the wide band gap that made pure diamond an insulator. In pure diamonds visible light can’t cross that large gap and pure diamond is colourless. But boron or nitrogen add energy levels within that gap; visible light can now be absorbed and so you see colour. Electrons can get across the gap, too, which is how you get diamond to go from being a fabulous insulator to semi-conducting, and then even metallic.”
Why does doped diamond have such an impressive set of potential uses?
“You’re leaning on diamond’s excellent properties, especially its wide band gap, and very stiff lattice and consequent high speed of sound. Optically and acoustically diamond has special properties, but with these dopants you now get even more — different, tuneable electronic properties. Depending on how you control those properties, you can design a material for electrochemical reactions, microelectrodes, energy storage and more. The diversity of materials and applications is amazing.”