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

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.

Diamond growth from organic compounds in hydrous fluids deep within the Earth

“…The formation mechanism for crustal diamonds that emerges from the present study is consistent with nucleation models via metastable molecular precursors recently proposed for nano-crystal synthesis 51–54 . In particular, a nucleation pathway with tetrahedral H-terminated C groups acting as templates, before forming the sp 3 -structured C phase, resonates with Gebbie et al 51 experiments of diamond gas synthesis using diamondoid molecules as protonuclei.…”

“…The formation mechanism for crustal diamonds that emerges from the present study is consistent with nucleation models via metastable molecular precursors recently proposed for nano-crystal synthesis 51–54 . In particular, a nucleation pathway with tetrahedral H-terminated C groups acting as templates, before forming the sp 3 -structured C phase, resonates with Gebbie et al 51 experiments of diamond gas synthesis using diamondoid molecules as protonuclei. Their observation that diamond nucleation occurred through metastable, hydrogen-terminated diamond-like carbon clusters (e.g., pentamantane) led to propose a different molecular nucleation mechanism where the nucleation energetic barrier is orders of magnitude lower than that generally assumed by classical models 51 .…”

“…In particular, a nucleation pathway with tetrahedral H-terminated C groups acting as templates, before forming the sp 3 -structured C phase, resonates with Gebbie et al 51 experiments of diamond gas synthesis using diamondoid molecules as protonuclei. Their observation that diamond nucleation occurred through metastable, hydrogen-terminated diamond-like carbon clusters (e.g., pentamantane) led to propose a different molecular nucleation mechanism where the nucleation energetic barrier is orders of magnitude lower than that generally assumed by classical models 51 . The similarity with laboratory synthesis seemingly supports that a part of crustal diamonds may have nucleated at pressures outside the actual stability field of diamond 55 .…”

“…It can be assumed that diamond nuclei formed in precursor amorphous—presumably hydrogenated—carbon clusters with the structure of mixed sp 3 + sp 2 bonding. As the concentration of carbon atoms in the fluid gradually increased, the conversion of precursor phases was possibly favored by tetrahedral sp 3 -bonded (CH x ) n structural groups acting as a template, leading to the formation of diamond-structured carbon lattices 50,51 . At this initial step of nucleation (stage II in Fig.…”

Does Local Structure Bias How a Crystal Nucleus Evolves?

“…Such a reality is close at hand. Recently, Gebbie et al 10 experimentally prepared nucleus precursors for diamond crystallization from plasma-enhanced chemical vapour deposition, and observed that isomeric precursor structures can have differing nucleation rates. The challenge for future work is to rationally design precursors to, for example, enhance nucleation rates; such work has the potential for broad impact.…”