Experimental measurement of the diamond nucleation landscape reveals classical and nonclassical features

Authors

M. A. Gebbie, H. Ishiwata, P. J. McQuade, V. Petrák, A. Taylor, Ch. Freiwald, J. E. Dahl, R. M. K. Carlson, A. A. Fokin, P. R. Schreiner, Z. X. Shen, M. Nesládek, N. A. Melosh

Laboratory / Department
Department of Functional Materials
Citation
Proceeding of the National Academy of Sciences of the United States of America …
DOI
10.1073/pnas.1803654115
Year
2018

Nucleation is the limiting step for thermodynamic phase transitions. While classical models predict that nucleation should be extremely rare, nucleation is surprisingly rapid in the gas-phase synthesis of diamond, silicon, and other industrial materials. We developed an approach for measuring nucleation landscapes using atomically defined precursors and find that diamond critical nuclei contain no bulk atoms, which leads to a nucleation barrier that is four orders of magnitude lower than prior bulk estimations. Our findings suggest that metastable molecular precursors play a key role in lowering nucleation barriers during materials synthesis and provide quantitative support for recent theoretical proposals of multistep nucleation pathways with much lower barriers than the predictions of classical nucleation theory.

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Schematic of diamond nucleation reaction coordinates. (A) Nucleation of condensed carbon phases from supersaturated carbon vapor, where relative chemical potentials are per carbon atom with two distinct states: μvapor and μsolid. Estimating nucleation barriers from the bulk cleavage energy leads to a barrier exceeding 1,000 kBT under PECVD conditions; this approach inherently assumes that nucleation is a single-step process into the bulk crystalline diamond phase. In contrast, the measured nucleation barrier is in the order of several values in kBT. (B) Two-step diamond growth mechanism as the simplest example of a multistep diamond nucleation and growth pathway, with three distinct states: μvapor, μsurface, and μbulk. The critical nucleus is composed entirely of surface atoms with diamond-like bonding, requiring an additional bulk transformation step to form bulk diamond. The carbon supersaturation in the plasma drives nucleation, and the nucleation barrier is determined by the plasma–nucleus interfacial energy, which is strongly influenced by the diamond surface termination