Ultrananocrystalline Diamond in the Laboratory and the Cosmos
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Ultrananocrystalline Diamond in the Laboratory and the Cosmos Dieter M. Gruen The following article is an edited transcript of the MRS Medal address presented on November 28, 2000, at the 2000 MRS Fall Meeting in Boston. Gruen received the Medal for “the low-pressure synthesis of nanocrystalline diamond films from fullerene precursors.”
Synthesis Diamond is one of the most intriguing and potentially useful materials known to science.1 It is the hardest substance that we know, and it has the highest sound velocity and the highest thermal conductivity of any material. Because diamond is so difficult to fabricate, the challenge is to take meaningful advantage of its extraordinary properties. The chemical vapor deposition (CVD) of diamond overcomes many of the fabrication problems and has become the focus of an important research and development effort worldwide.2 Although the General Electric Corp. succeeded about 50 years ago in the synthesis of diamond by high-pressure, high-temperature techniques, the low-pressure, or CVD, methods were developed only about 20 years ago in the former Soviet Union, Japan, and the United States.1,2 This presentation will deal with a more recent development in diamond CVD that allows one to control the crystallite size in such a way as to synthesize phase-pure nanocrystalline diamond films, which have many unique and fascinating properties not shared by other forms of diamond.3 The conventional CVD method of synthesizing diamond films relies on hydrocarbon precursors in the presence of a large excess of hydrogen.4 For example, with methane as the precursor, it is thought that the first gas-phase step abstracts a hydrogen atom from the methane molecule to form a methyl radical. Using a hot fila-
MRS BULLETIN/OCTOBER 2001
ment or a plasma discharge to produce atomic hydrogen, a subsequent series of hydrogen-abstraction reactions occur and allow the carbon in the methane molecule to form carbon–carbon bonds and thus, after an adsorption step, to participate in the growth of the diamond lattice. The microstructure of a diamond film grown from such hydrocarbon–hydrogen mixtures typically consists of 1–10-m-sized crystallites. Embryonic diamond crystallites that may initially form by heterogeneous renucleation on a seed layer of diamond are regasified by atomic hydrogen. Thus, only the larger crystallites survive, becoming ever larger as the film thickness increases. This growth mode has sometimes been characterized as the “survival of the largest.”5 The discovery of buckminsterfullerene, C60, in 1985 by Kroto, Smalley, Curl, and their collaborators6 inspired a new approach to diamond CVD synthesis. Five years later, when Krätschmer and Huffman and their collaborators described a method of synthesizing large quantities of C60, the new ideas could be implemented.7 Their work, published in Nature in September 1990, gave rise to a kind of “Woodstock of fullerenes” at the November 1990 meeting of the Materials Research Society. We gave a paper at that meeting on the photofragmentation of C60
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