(167c) Experimental Assessment of Nucleation Theory at the Molecular Level

Nucleation is a core scientific principle, describing the formation of new phases and materials. Nevertheless, the applicability of classical nucleation at the molecular level remains open to debate. Currently, the direct observation of molecular proto-nuclei is an extreme challenge, leaving experimental studies of nucleation energetics restricted to colloids, proteins, and macroscopic systems. Here, we surpass this challenge by using atomically-defined sub- and post-critical nuclei to investigate the energetics of diamond nucleation during plasma-enhanced chemical vapor deposition and observe critical nucleation from 26 carbon clusters, confirming a key implication of classical theory. However, the nucleation barrier is two orders of magnitude lower than prior estimates from bulk properties. Rationalizing this discrepancy resolves long outstanding questions surrounding the growth of several industrially important materials and provides direct experimental verification of classical nucleation at the molecular level.