Molecular semiconductors are increasingly used in devices, but understanding of elementary nanoscopic processes in molecular film growth is in its infancy. Here we use real-time in situ specular and diffuse X-ray scattering in combination with kinetic Monte Carlo simulations to study C60 nucleation and multilayer growth. We determine a self-consistent set of energy parameters describing both intra- and interlayer diffusion processes in C60 growth. This approach yields an effective Ehrlich–Schwoebel barrier of EES=110 meV, diffusion barrier of ED=540 meV and binding energy of EB=130 meV. Analysing the particle-resolved dynamics, we find that the lateral diffusion is similar to colloids, but characterized by an atom-like Schwoebel barrier. Our results contribute to a fundamental understanding of molecular growth processes in a system, which forms an important intermediate case between atoms and colloids.

Hybrid systems consisting of organic molecules at inorganic semiconductor surfaces are gaining increasing importance as thin film devices for optoelectronics. The efficiency of such devices strongly depends on the collective behavior of the adsorbed molecules. In the present paper we propose a novel, coarse-grained model addressing the condensed phases of a representative hybrid system, that is, para-sexiphenyl (6P) at zinc-oxide (ZnO). Within our model, intermolecular interactions are represented via a Gay-Berne potential (describing steric and van-der-Waals interactions) combined with the electrostatic potential between two linear quadrupoles. Similarly, the molecule-substrate interactions include a coupling between a linear molecular quadrupole to the electric field generated by the line charges characterizing ZnO(10-10). To validate our approach, we perform equilibrium Monte Carlo simulations, where the lateral positions are fixed to a 2D lattice, while the rotational degrees of freedom are continuous. We use these simulations to investigate orientational ordering in the condensed state. We reproduce various experimentally observed features such as the alignment of individual molecules with the line charges on the surface, the formation of a standing uniaxial phase with a herringbone structure, as well as the formation of a lying nematic phase.

Using large-scale kinetic Monte-Carlo (KMC) simulations, we investigate the non-equilibrium surface growth of the fullerene C60. Recently, we have presented a self-consistent set of energy barriers that describes the nucleation and multilayer growth of C60 for different temperatures and adsorption rates in quantitative agreement with experiments [Bommel et al., Nat. Comm. 5, 5388 (2014)]. We found that C60 displays lateral diffusion resembling colloidal systems, however it has to overcome an atom-like energetic step-edge barrier for interlayer diffusion. Here, we focus on the particle-resolved dynamics, and the interplay between surface morphology and particle dynamics during growth. Comparing C60 growth with an atom-like system, we find significant differences in the evolution of the surface morphology, as well as the single-particle dynamics on the growing material landscape. By correlating the mean-squared-displacement of particles with their current neighborhood, we can identify the influence of the different time scales that compete during growth and can pinpoint the differences between the two systems.

Using kinetic Monte Carlo simulations, we show that molecular morphologies found in non-equilibrium growth can be strongly different from those at equilibrium. We study the prototypical hybrid inorganic-organic system 6P on ZnO(10-10) during thin film adsorption, and find a wealth of phenomena including re-entrant growth, a critical adsorption rate and observables that are non-monotonous with the adsorption rate. We identify the transition from lying to standing molecules with a critical cluster size and discuss the competition of time scales during growth in terms of a rate equation approach. Our results form a basis for understanding and predicting collective orientational ordering during growth in hybrid material systems.Hybrid inorganic-organic systems (HIOS) have revolutionized opto-electronic semiconductor technologies by combining the high charge carrier densities and high tunability of conjugated organic molecules with stable, well controlled inorganic substrates. 1,2 At the same time, the design of efficient devices poses fundamental physical questions on a multitude of length-and time-scales,

Understanding fundamental growth processes is key to the control of nonequilibrium structure formation for a wide range of materials on all length scales, from atomic to molecular and even colloidal systems. While atomic systems are relatively well studied, molecular and colloidal growth are currently moving more into the focus. This poses the question to what extent growth laws are size scalable between different material systems. We study this question by analyzing the potential energy landscape and performing kinetic Monte Carlo simulations for three representative systems. While submonolayer (island) growth is found to be essentially scalable, we find marked differences when moving into the third (vertical) dimension.