FWF - Monolayers - Defects in Organic Monolayers

Inorganic-organic interfaces present a versatile class of systems, providing the opportunity to achieve intriguing functionalities, e.g. as thermoelectrics, memories, or transistors. Currently, the common bottleneck for all these applications is the interface at the hybrid inorganic/organic material, over which charge or energy has to be transported. So far, most theoretical studies that consider these interfaces from an atomistic perspective have mainly focused on idealized, perfectly well-ordered interfaces. However, in reality, even if every effort is made to keep the interface well-defined, temperature and entropy will cause the formation of defects in the organic material. Although the crucial impact of defects and disorder for, e.g., the conduction in organic bulk materials has been recognized, a systematic assessment of the impact of defects for transport properties at the interface from first-principles has not yet gained appropriate attention. The aim of the present project is to close this gap and obtain an in-depth understanding of the nature, equilibrium concentration, and charge distribution of defects in organic materials deposited onto metallic and semiconducting substrate at finite temperature. The largest challenge of this endeavor is the vast configurational space spanned by the various adsorbate morphologies. For this project, we attempt to tackle this issue using a “divide-and-conquer”-approach: First, possible adsorption structures for single, isolated molecules on the surfaces will be determined. Then more complex, densely packed layers will be modelled starting from a regular arrangement of the various individual adsorption geometries. The various permutations for such arrangements serve as guess for basins of the potential energy surface, which can then be sampled using a basin hopping algorithm. Unambiguously assigning the different basins allows for a particular efficient screening that avoids recalculating known structures while allowing to cross parts of the potential energy surface that have already been visited. For each of the morphologies, the closest local minimum and its energy will be determined using density functional theory. The properties of the organic material can then be obtained as a Boltzmann-weighted average of the properties of each local minimum. Using feedback from experiments (mainly via scanning tunneling microscopy as well as core level and valence band spectroscopy), we will then investigate to which extent various observables, in particular the interface dipole, the density of states, and interfacial alignment of the transport level, are modified by the presence of defects compared to perfectly ordered materials, and how this may affect charge- and energy-transport across the interface. Understanding the nature and the influence of these unavoidable imperfections in the organic material will be a further, crucial step in finding the perfect organic material for real-world applications.