Diamonds are forever

As well as their appeal as gemstones diamonds have a remarkable range of physical properties. They are the hardest natural substance, with low friction, making them suitable for use as cutting tools. They have the highest thermal conductivity of any solid, making them ideal candidates for heat sinks in integrated circuits, and they are exceptional electrical insulators, chemically inert and biocompatible. In fact, the word diamond comes from the Greek adamas, meaning indestructible. Not surprisingly, scientists and engineers refer to diamond as the ‘ultimate engineering material’. However, its practical use in science and engineering has been limited because it is scarce and expensive. This has prompted researchers around the world to look at ways of making synthetic diamond in the laboratory.    

From graphite …   

Many attempts have been made to synthesise diamond from graphite, which is readily available and cheaper. However, at room temperature and pressure graphite is the thermodynamically stable allotrope, by ca 1–2kJmol^–1, and always forms in preference. Moreover, the activation barrier between graphite and diamond is of the order of 700kJmol^–1, which prevents interconversion at room temperature and pressure. Essentially the graphite sheet structure would have to be broken up before reforming into the tetrahedral diamond structure. However, it turns out that if graphite is compressed to tens of thousands of atmospheres and heated to 2000K, in the presence of a catalyst, diamond eventually crystallises. This high temperature, high pressure (HTHP) technique, is limited in that it produces single diamond crystals of differing sizes, typically around 1–2mm. Nevertheless, ‘industrial diamonds’, which are used for cutting tools and abrasives, are produced in this way.    

Researchers began to focus on producing diamond films by adding carbon atoms one at a time to a template to form the tetrahedrally-bonded network. The   breakthrough came in 1982 when Japanese researchers used ‘chemical vapour deposition’ (CVD) to grow diamond films on non-diamond substrates at a significant rate. This stimulated worldwide interest in diamond CVD.   

Introducing CVD   

Chemical vapour deposition is a crystal growth process used not only for diamond but also for a range of different semiconductor and other crystalline materials such as silicon or gallium arsenide. The technique generally involves growing a solid from a reactive gas mixture, which supplies the necessary active species (carbon in the case of diamond) onto a controlled surface (or substrate), usually a silicon wafer scratched with diamond. The latter encourages carbon atoms to line up in a diamond structure.    

In contrast to HTHP synthesis, CVD is generally done at or below (for diamond) atmospheric pressure. The activation of the gas phase can be done using thermal methods (a hot filament), electric discharge (eg DC or microwave) or a combustion flame (eg an oxyacetylene torch). Growth of diamond normally requires that the precursor gas – usually CH₄ – is diluted in excess hydrogen (see Fig 1). Also the temperature of the substrate should be above 700ºC. Gas phase hydrogen atoms help to break up any soot or polymers that may form; the high temperature ensures the mobility of the atoms on the surface, which encourages diamond growth.    

There are several research groups in the UK that focus on CVD diamond growth, including John Wilson and Phil John’s group at Heriot-Watt University, Richard Jackman’s group at University College London, John Foord’s group at Oxford University, and Paul May’s group at the University of Bristol. Although Bristol’s interest in single crystal diamond research goes back some 30 years, CVD diamond research began in 1991 as a collaborative venture between the chemistry and aerospace engineering departments to investigate diamond films for aerospace applications.   

May explains: ‘We use a variety of different CVD techniques, including hot filament reactors, microwave plasma reactors and a DC arcjet. Our main activities have been in studying the gas phase chemistry occurring in these very hot balls of plasma directly above the growing diamond surface. We use various plasma diagnostic tools to do this, including molecular beam mass spectrometry and a variety of laser spectroscopic methods’.   

And the chemistry of CVD diamond growth? The precursor gases first mix in the reaction chamber before diffusing towards the substrate surface. As the gas diffuses towards the substrate it passes through the activation region (ega hot filament or electric discharge). This causes the gaseous molecules to fragment into reactive radicals and atoms, creates ions and electrons, and heats up the gas. These reactive fragments continue to undergo complex reactions until they come into contact with the surface of the substrate (see Scheme 1). When this happens there are several possibilities. The fragments can adsorb and react with the surface, desorb back into the gas phase, or diffuse around close to the surface until a suitable reaction site is found. If all the conditions are favourable – the right ratio of H:C and the right number of nucleation sites – a surface reaction can lead to diamond formation.    

Applications of synthetic diamonds   

The Bristol team was the first in the world to make diamond fibres on a large scale using CVD, and to demonstrate their use as reinforcing agents in metal and plastic matrix composites. These light yet very stiff composites make ideal materials for aerospace components. May and his colleagues are now looking to dope diamond to make semiconducting materials suitable for biosensors. Such sensors are being designed to detect DNA fragments (ie genes), which could help clinicians detect various inherited diseases.    

Diamond is inert and biocompatible – ie the body doesn’t have an immune response to it – and semiconducting diamond sensors could, in theory, be implanted into the body to monitor levels of certain chemicals in the blood. For example, diamond sensors are being developed for detecting glucose blood levels, which could be read remotely by using a radio signal. This would free diabetic patients from the annoyance and pain of taking blood samples several times a day.    

Diamond is also being used to encapsulate electronic microcircuits, allowing them to be implanted directly into areas of the body that would be corrosive to most materials. In the US, for example, artificial retina chips, encapsulated in diamond, are being developed to implant into human eyes in an effort to restore the sight of patients with degenerative blindness.   

Chemical vapour deposition techniques and synthetic diamond films have come a long way in the past decade. As research continues we can expect to see diamond films appearing in many more applications.    

John Johnston