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.

Department of Materials Science and Engineering Stanford University Stanford CA 94305

Department of Materials Science and Engineering Stanford University Stanford CA 94305 Stanford Institute for Materials and Energy Sciences SLAC National Accelerator Laboratory Menlo Park CA 94025

Institute of Materials Research University of Hasselt B 3590 Diepenbeek Belgium Institute for Materials Research in Microelectronics Interuniversity Microelectronics Centre B 3590 Diepenbeek Belgium

Institute of Organic Chemistry Justus Liebig University D 35392 Giessen Germany

Institute of Organic Chemistry Justus Liebig University D 35392 Giessen Germany Department of Organic Chemistry Igor Sikorsky Kiev Polytechnic Institute 03056 Kiev Ukraine

Institute of Physics of the Czech Academy of Sciences CZ 18221 Prague Czech Republic

Stanford Institute for Materials and Energy Sciences SLAC National Accelerator Laboratory Menlo Park CA 94025

Stanford Institute for Materials and Energy Sciences SLAC National Accelerator Laboratory Menlo Park CA 94025 Applied Physics Stanford University Stanford CA 94305

Stanford Institute for Materials and Energy Sciences SLAC National Accelerator Laboratory Menlo Park CA 94025 Department of Electrical and Electronic Engineering Tokyo Institute of Technology Meguro ku Tokyo 152 8550 Japan

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