Hybrid Interfaces in Thermodynamic Equilibrium

The structure in which organic molecules assemble is crucial for their properties. This is exploited in many modern applications, especially in the context of organic electronics. One example for this is the performance of OLED displays (e.g., the screen of your mobile phone), where the polymorph in which the organic molecules crystallize critically determine the power consumption, i.e. how fast an active screen drains the battery. In principle, the structure of the organic material can be influenced by the growth conditions, e.g., by using a different temperature, pressure, solvent, or similar. Because trying out all these different conditions is tedious, ideally, the choice of the best would be guided be theoretical predictions. Although these predictions, driven by a combination of quantum-mechanical calculations and machine-learning algorithms, have made great progress in the past years, their accuracy has practically hit a dead-end. This is in part because the analysis of these data rely on several assumptions that have originally be made for relatively simple inorganic materials (such as silicon), but never be validated for more complex organic matter. The situation is further complicated by the fact that good experimental data that allow to validate these assumptions are scarce. The fundamental target of this project, therefore, is twofold. First, we will create an experimental benchmark dataset, where we collect the structure of various organic molecules, deposited at different organic materials, for different deposition conditions. Here, care must be taken that the material can find its correct structure. To achieve this, we will build a new, dedicated experimental chamber, that allows unprecedented control over the conditions while simultaneously measuring the structure. In parallel, we will assess all the different approximations made for inorganic materials and see to which point they are also appropriate for organic materials. This will provide us with new insight into the physics that govern the structure of organic polymorphs, and help us to design new, better materials for technological applications.

The HI-TEq project investigated how organic molecules arranged themselves on metal surfaces and how such hybrid interfaces could be predicted more reliably using a combination of advanced simulations and precision experiments. The overarching goal was to establish benchmark data for molecular phase formation and to improve theoretical methods that predict which interface structures become stable under different experimental conditions. During the project, the consortium combined theoretical developments in Graz with dedicated surface-science experiments in Jena. On the theoretical side, the work systematically evaluated assumptions commonly made in ab initio thermodynamics and developed improved strategies to account for effects that are often neglected, including configurational and vibrational contributions to free energies. New computational workflows enabled these quantities to be treated more efficiently for complex organic-metal interfaces. Experimentally, a dedicated ultra-high-vacuum platform was established to study molecular layers under conditions closer to thermodynamic equilibrium than typically accessible in conventional growth experiments. Initial benchmark systems provided detailed structural information and allowed direct comparison between experiment and simulation. The scientific outcomes extended beyond the original objectives. Recent publications demonstrated how substrate lattice constants influence interface polymorphism independently of chemical effects, clarified the role of configurational and vibrational contributions in phase stability, and established how collective molecular kinetics determine whether thermodynamically preferred phases can actually form under realistic conditions. Additional work addressed metastable interface structures and strengthened the connection between equilibrium thermodynamics and experimentally accessible growth pathways. Overall, HI-TEq provided a substantially improved understanding of how thermodynamic and kinetic effects jointly govern structure formation at hybrid interfaces. The project established methodological foundations and benchmark concepts that are expected to improve the predictive design of complex interfaces and accelerate future research in computational materials discovery and interface engineering.