Pinned Ad-colloids Disfavors Nucleation in Colloidal Vapor Deposition

Crystallization through vapor deposition is ubiquitous, and is inevitably influenced by impurities, which often impact the local structure. Interestingly, the effect of immobilizing some of the depositing particles themselves, which would still preserve local structural symmetry, remains largely unexplored. Herein, we perform colloidal vapor deposition on a substrate with a few pinned ad-colloids, termed "mobility impurities". Through thermodynamic and kinematic measurements, we demonstrate that these pinned ad-colloids, even though they share identical geometry and interaction with depositing particles, are disfavored as nucleation centers. We reveal that entropic contributions, rather than energetic ones, govern nucleation physics in the presence of mobility impurities. Moreover, tuning the mobility of colloids on the substrate adjusts the nucleation likelihood at pinned sites. In later stages of growth, pinning induces mode localization and alters the thin film's vibrational spectrum. Our work, thus, underscores the potential of strategically incorporating mobility impurities to engineer material properties.