FWF - Monolayers - Defects in Organic Monolayers

Project: Research project

Description

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.
StatusActive
Effective start/end date1/03/1629/02/20