A Perspective on Multistep Pathways of Nucleation
Jim De Yoreo
Abstract
While views on crystal nucleation throughout the 20th century were dominated by classical nucleation theory (CNT), in which the ordered nucleus forms directly from monomeric species, observations over the past two decades across scientific fields have revealed a plethora of multistep nucleation processes, often involving more complex species. These processes have come to be known as nonclassical, reflecting distinctions from the expectations of CNT. This work considers the various classes of nonclassical nucleation pathways and examines the extent to which they can be easily placed within the CNT framework modified to include well-understood complexities in the free energy landscape and/or kinetic factors. I highlight recent examples that illustrate these pathways before turning to processes that cannot readily be included in this framework. I conclude by describing future research directions needed to fill the knowledge gaps that prevent the development of quantitative models for multistep nucleation processes.