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Carbon is everywhere around us. Its stable crystalline phase is graphite, but under some circumstances, it can be converted into its most interesting phase for applications—diamonds. Naturally, a diamond is produced inside the Earth's mantle by a high-pressure–high-temperature phase transformation of graphite. It is metastable and that is the reason why it can be recovered in mines after having migrated toward the most external shells of the Earth.
Its unsurpassed hardness, excellent transport properties, transparency, and inertness make a diamond a material of choice for many industrial applications. Synthetic diamond, produced by a high-pressure–high-temperature treatment of graphite, or by ion bombardme nt, is now commonly used in industries. But what happens to the carbon phase diagram when the sample size reaches the order of several nanometers?