Diamond Growth at Low Pressures

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Abstract
Diamond synthesis has attracted attention ever since it was established in 1797 that diamond is a crystalline form of carbon. Initially, synthesis was attempted at high pressures because diamond is the densest carbon phase. As understanding of chemical thermodynamics developed through the 19th and 20th centuries, the pressure-temperature range of diamond stability was explored. These efforts culminated in the announcement in 1955 of a process for diamond synthesis with a molten transition metal solvent-catalyst at pressures where diamond is thermo-dynamically stable. Worldwide sales of synthetic diamond now approach 330 million carats (73 tons) with a market price of between $500 million to $1 billion. Over the past 40 years a parallel effort has been directed toward the growth of diamond at low pressures, where it is metastable. Although diamond was successfully produced, low-pressure synthesis was plagued by extremely low growth rates. Recent developments have led to much higher growth rates, creating great interest in the field. Polycrystalline diamond films can now be produced on a variety of substrates at linear growth rates of tens to hundreds of micrometers per hour. In addition, the recognition of an entirely new class of solids, the so-called “diamondlike” carbons and hydrocarbons has arisen from this work. This article will discuss both crystalline diamond grown at low pressure and the diamondlike phases. The interest in diamond is driven by its extreme properties, summarized in Table I. Diamond stands alone as the densest (number density), strongest (elastic modulus), and hardest known material.