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Edited by Catherine J. Murphy, University of Illinois at Urbana–Champaign, Urbana, IL, and approved July 9, 2018 (received for review March 1, 2018)
Significance
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.
Abstract
Nucleation is a core scientific concept that describes the formation of new phases and materials. While classical nucleation theory is applied across wide-ranging fields, nucleation energy landscapes have never been directly measured at the atomic level, and experiments suggest that nucleation rates often greatly exceed the predictions of classical nucleation theory. Multistep nucleation via metastable states could explain unexpectedly rapid nucleation in many contexts, yet experimental energy landscapes supporting such mechanisms are scarce, particularly at nanoscale dimensions. In this work, we measured the nucleation energy landscape of diamond during chemical vapor deposition, using a series of diamondoid molecules as atomically defined protonuclei. We find that 26-carbon atom clusters, which do not contain a single bulk atom, are postcritical nuclei and measure the nucleation barrier to be more than four orders of magnitude smaller than prior bulk estimations. These data support both classical and nonclassical concepts for multistep nucleation and growth during the gas-phase synthesis of diamond and other semiconductors. More broadly, these measurements provide experimental evidence that agrees with recent conceptual proposals of multistep nucleation pathways with metastable molecular precursors in diverse processes, ranging from cloud formation to protein crystallization, and nanoparticle synthesis.
Footnotes
- ↵1To whom correspondence should be addressed. Email: nmelosh{at}stanford.edu.
Author contributions: Z.-X.S., M.N., and N.A.M. designed research; M.A.G., H.I., P.J.M., V.P., A.T., and C.F. performed research; J.E.D., R.M.K.C., A.A.F., and P.R.S. contributed new reagents/analytic tools; M.A.G., M.N., and N.A.M. analyzed data; and M.A.G. and N.A.M. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1803654115/-/DCSupplemental.
Published under the PNAS license.
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Experimental measurement of the diamond nucleation landscape reveals classical and nonclassical features
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