Mechanism of nonrandom pattern formation of polar-conjugated molecules in a partial wetting regime

An original nucleation and growth process has been investigated for vacuum-deposited films of a polar-conjugated molecule, tris-(8-hydroxyquinoline) aluminum (III) $\left({\mathrm{Alq}}_3\right),$ onto the apolar H-terminated Si(100). Homogeneous nucleation of amorphous ${\mathrm{Alq}}_3$ clusters is restricted to the early stage of deposition and is characterized by a large critical nucleus size $i=5,$ as determined from the dependence of the density of stable nuclei N on deposition rate κ. ${\mathrm{Alq}}_3$ clusters grow in a partial wetting regime to form correlated droplet patterns. For moderate deposition rates around 1 nm/min, patterns exhibit both (i) a typical scale invariance of the droplet size distribution with coverage and substrate temperature and (ii) strong correlations between the size and position of the droplets. Both these characteristics result from the absence of coarsening—e.g., Ostwald ripening, secondary nucleation, and coalescence during growth. Spatial correlations are analyzed by using Voronoi tesselation, which demonstrates the validity of a phenomenological capture zone model for correlated growth. Correlations emerge in the early stage of growth via direct ripening and further develop during growth by diffusive interactions between domains. Direct ripening manifests itself by the introduction of a minimum cutoff distance between droplets, which causes a significant narrowing of the droplet size distribution. During growth, diffusive interactions between droplets cause their centers of mass to move towards empty depleated areas, which results in enhanced spatial correlations. This peculiar nucleation and growth mechanism allows one to obtain droplet patterns where the interdroplet distance and droplet size can be tuned independently via deposition temperature and time.