Turning Diamond into Metal
Normally an insulator, diamond becomes a metallic conductor when subjected to large strain.
Long known as the hardest of all natural materials, diamonds are also exceptional thermal conductors and electrical insulators. Now, researchers have tweaked tiny needles of diamond in a controlled way to transform their electronic properties, dialing them from insulating, through semiconducting, all the way to highly conductive or metallic. This can be induced dynamically and reversed at will, with no degradation of the diamond material. The research may open up a wide array of potential applications including new kinds of broadband solar cells, highly efficient LEDs and power electronics, and new optical devices or quantum sensors.
The team used a combination of quantum mechanical calculations, analyses of mechanical deformation, and machine learning to demonstrate that the phenomenon, long theorized as a possibility, really can occur in nanosized diamond.
The concept of straining a semiconductor material such as silicon to improve its performance found applications in the microelectronics industry more than two decades ago; however, that approach entailed small strains on the order of about 1%. The new method uses the concept of elastic strain engineering, which is based on the ability to cause significant changes in the electrical, optical, thermal, and other properties of materials simply by deforming them — putting them under moderate to large mechanical strain — enough to alter the geometric arrangement of atoms in the material's crystal lattice but without disrupting that lattice.
Key to this work is a property known as bandgap, which essentially determines how readily electrons can move through a material. This property is thus key to the material's electrical conductivity. Diamond normally has a very wide bandgap of 5.6 electron volts, meaning that it is a strong electrical insulator that electrons do not move through readily. In their latest simulations, the researchers show that diamond's bandgap can be gradually, continuously, and reversibly changed, providing a wide range of electrical properties, from insulator, through semiconductor, to metal.
The ability to engineer and design electrical conductivity in diamond without changing its chemical composition and stability offers flexibility to custom-design its functions. For example, a single tiny piece of diamond, bent so that it has a gradient of strain across it, could become a solar cell capable of capturing all frequencies of light on a single device — something that currently can only be achieved through tandem devices that couple different kinds of solar cell materials together in layers to combine their different absorption bands. These might someday be used as broad-spectrum photodetectors for industrial or scientific applications.
The process can also make diamond into two types of semiconductors, either “direct” or “indirect” bandgap semiconductors, depending on the intended application. For solar cells, for example, direct bandgaps provide a much more efficient collection of energy from light, allowing them to be much thinner than materials such as silicon, whose indirect bandgap requires a much longer pathway to collect a photon's energy.
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