Pinning Disfavors Nucleation in Colloidal Vapor Deposition

Layer-by-layer growth processes, like vapor deposition, are ubiquitous in natural and synthetic materials, offering precise control over structure formation. However, their effectiveness and impact on material properties can be significantly influenced by impurities. These impurities, often distinct from the depositing material, may disrupt the local structure. Interestingly, the effect of immobilizing some of the depositing particles themselves, which would still preserve local structural symmetry, remains largely unexplored. Here, by introducing a small fraction of pinned sites during colloidal vapor deposition and employing a combination of thermodynamic and kinematic measurements, we offer direct evidence that pinned sites, termed "mobility impurities," are disfavored as nucleation centers. Our experiments, supported by molecular dynamics simulations and a theoretical model, reveal that entropic contributions, rather than energetic ones, govern nucleation physics in the presence of mobility impurities. Furthermore, tuning the mobility of colloids on the substrate can adjust the nucleation likelihood at pinned sites. In the 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.