Diamond as a Technological Material

Peter Taborek

Carbon has been recognized as an element ever since the time of the alchemists, so it is somewhat surprising that new forms of carbon are making scientific headlines this late in the twentieth century. The alchemists (and your Chem I instructor) taught that the stable form of carbon at room temperature and pressure is graphite, familiar from pencil lead and charcoal briquettes. It has also long been understood that diamond, which bears no physical resemblance to graphite, is nevertheless just a different crystallographic organization of carbon atoms. Technically speaking, diamond is metastable, and will eventually convert to graphite, but the time required for this process is so long that it behaves like an ordinary stable solid. Recently, scientists have come to realize that rapid quenching techniques can be used to synthesize completely new families of metastable carbon solids and to grow novel forms of diamond.

The existence of efficient nonequilibrium methods of growing diamond and diamond-like films is not only of academic interest; several companies are also pursuing the technology. The possibility of forming large quantities of diamond and diamond-like materials quickly and cheaply opens up many applications which would have been previously impossible. Diamond is, of course, the hardest material, and the possibility of making a spray-on super hard surface coatings for items ranging from drill bits, cutting tools and turbine blades to sunglasses and disk drives is being actively investigated. In addition to hardness, diamond is the world champion in many other categories as well. It is the best thermal conductor of any solid at room temperature by nearly an order of magnitude. This is an attractive property for electronic chip designers who are faced with the problem of removing more and more heat from smaller and smaller packages. Diamond is one of the most transparent materials, and is used in demanding optical applications for electromagnetic waves ranging from X-rays to the infrared. Diamond is structurally identical to silicon, and is a wide gap semiconductor which can be used to make active electronic devices including transistors which could work at very high temperatures, and lasers that emit blue light. Low wavelength (blue) light sources are important for optical storage technology such as CDs, since shorter wavelengths translate into more bits per square cm.

One of the most famous new forms of carbon is C60, buckminsterfullerene, often referred to as "bucky balls". When carbon vapor is formed, either in an electric arc or by intense laser pulses, and then rapidly cooled by collisions with a cold, inert background gas, the atoms spontaneously form perfect soccer ball -like spheres. These balls can be crystallized to form a black solid called fullerite. One of the many fascinating properties of fullerite is that it can be doped with small amounts of metal to form a high temperature superconductor. Carbon quenched from the vapor will not only form spheres, but also footballs and long sausage-shaped molecules known as "bucky tubes".

The key to forming all of these new materials is to create an environment far from equilibrium with variations in temperature that happen so fast that the carbon atoms get stuck in a phase which is a local minimum of the free energy, rather than the true ground state. My students and I are pursuing this theme to explore new techniques of making diamond and diamond-like materials.

Artificial diamonds have been produced since the 1950's in processes that simulate the extreme conditions of temperature and pressure in the earth's crust where natural diamonds are formed. Just as in Nature, however, growing a large crystal takes geological times, so only small crystallites can be produced economically. The nonequilibrium techniques being developed in my lab are very different: they utilize gas or plasma at very low pressures and high velocities, which leads to high growth rates. One particularly efficient technique developed as part of Derrek Russell's PhD thesis is a plasma jet. In this device, hydrogen gas with a small amount of hydrocarbon added is heated to temperatures of 8000 degrees Kelvin, hotter than the surface of the sun. The resulting plasma is injected into a vacuum chamber at speeds of Mach 4. The plasma is quenched onto a metal plate where the temperature drops from 8000 degrees K to 1200 degrees K within a few microseconds. If conditions are just right, a diamond film 3 inches across and nearly a millimeter thick can be grown on the plate. This process converts methane into diamond simply by heating and cooling the gas, a trick that would make even the alchemists jealous.

The success of this and related techniques prove that it is relatively easy to crystallize carbon in the "wrong" phase, but the details of how and why this happens are still obscure. In an attempt to simplify the process as much as possible, my graduate student Eric Maiken and I have devised a way to make a high density, high energy beam of pure carbon which can be condensed on a substrate to form a film. The patented process utilizes a heated crucible of C60 to form "bucky vapor", which is ionized and broken into carbon fragments. The films formed in this way are an amorphous mixture of the planar bonds characteristic of graphite and the tetrahedral bonds of diamond. By controlling the beam energy and the substrate temperature, the properties of the films can be tuned through a remarkably wide range from very diamond-like (hard, transparent and highly insulating) to graphite-like (soft, black, and conducting).

The electronics industry has spent the last 50 years working its way up the fourth column of the periodic table, starting from the first germanium diodes and later advancing to silicon. Each of these materials required thousands of people-years to develop from a laboratory curiosity into a thriving industry, based on control of impurities at the parts per billion level. Since carbon is at the top of the column, it represents the final frontier in this series of materials. The different bonding configurations available to carbon make diamond a particularly difficult material to control, but the many possible applications will justify an intense research effort for years to come.