Our research concentrates on molecular structure formation at electrically insulating surfaces. We study the physico-chemical mechanisms responsible for molecular self-assembly on surfaces, tuning the subtle balance between reversible intermolecular and molecule-surface interactions. We also study means to covalently link organic molecules on surfaces.
Our main measurement tool is non-contact atomic force microscopy (NC-AFM). We develop this method for high-resolution imaging both in ultra-high vacuum and at the solid-liquid interface.
I. Ultra-High Vacuum Environment
Direct imaging of insulating surfaces at the molecular and atomic level requires state-of-the-art non-contact atomic force microscopy (NC-AFM). Especially when aiming for investigating processes at room temperature, special measurement protocols need to be developed, e.g., for compensating thermal drift.
We constantly improve the signal-to-noise ratio of our instruments and have implemented new methods such as the atom-tacking technique [1,2]. The latter method allows for compensating thermal drift, enabling three-dimensional force mapping even at room temperature. The high-resolution capability of our instruments allows for collecting Kelvin-probe force microscopy (KPFM) data with high resolution, which gives valuable insights into the charge distribution within molecular islands on surfaces.
 P. Rahe et al., Rev. Sci. Instrum. 82 (2011) 063704
 P. Rahe et al., Rev. Sci. Instrum. 85 (2014) 043707
Self-Assembly on Insulating Surfaces
Uni-directional molecular structure on calcite [P. Rahe et al. J. Phys. Chem. C 114 (2010) 1547]
Often, molecular self-assembly on insulating surfaces is hampered by the comparably weak molecule-surface interaction. Thus, strategies need to be developed for enhancing and precisely tuning the influence of the substrate in molecular self-assembly on insulting surfaces [3-5]. We have systematically changed the functionality of molecular building blocks for, e.g., varying the intermolecular interaction while at the same time maintain a given interaction strength with the supporting substrate .
As an alternative approach complementing our direct imaging capability, we have recently established a setup for temperature-programmed desorption (TPD), which allows for determining the change in enthalpy and entropy upon molecule adsorption and desorption.
 P. Rahe et al., Adv. Mater. 25 (2013) 3948
 M. Kittelmann et al., J. Phys.: Condens. Matter 24 (2012) 354007
 M. Körner et al., Phys. Rev. Lett. 107 (2011) 016101
 C. Hauke et al., ACS Nano 7 (2013) 5491
Investigating Kinetics on Surfaces
For gaining a comprehensive understanding of molecular self-assembly on surfaces, it is important to investigate the kinetics of fundamental processes at surfaces and quantify relevant energy barriers. A very important parameter for successful self-assembly is, e.g., the diffusion barrier. Yet, this fundamental parameter has basically only been studied on metals. We have quantified the diffusion barrier for C60 on CaF2(111), again demonstrating the rather weak influence of the insulating substrate on the molecular self-assembly .
Recently, we have followed the deprotonation kinetics of a benzoic acid on calcite by collecting NC-AFM movies (> 10 h) that provide detailed information on the deprotonation kinetics .
 F. Loske et al., Phys. Rev. B 82 (2010) 155428
 M. Kittelmann et al., ACS Nano 6 (2012) 7406
Covalent linking of benzoic acid derivatives on calcite 
On-surface synthesis provides a promising strategy for creating functional molecular structures on surfaces that go beyond molecular self-assembly. Especially when aiming for stable structures that can be brought outside the ultra-high vacuum environment, covalently linked structures are desired. Moreover, covalently linked conjugated molecules allow for efficient electron transport and are, thus, particularly interesting for future molecular electronics applications. When having these applications in mind, insulating substrates are mandatory to provide sufficient decoupling of the molecular structure from the substrate surface. However, on-surface synthesis had been achieved only on metallic substrates.
We have demonstrated the first proof-of-principle for successful covalent linking of a benzoic acid derivative directly on a bulk insulator . As a next step, we have extended this concept by a sequential and site-specific linkage, providing the possibility for hierarchical structure formation . Recently, we have presented the first example for photochemical initiation of an on-surface reaction on a bulk insulator. In the latter example, the substrate was found to guide the reaction direction, which constitutes an interesting strategy for controlled fabrication of oriented structures on surfaces .
This work is funded by an EU large-scale integrating project in FET Proactive – Planar Atomic and Molecular Scale devices (PAMS) – which is one out of only five projects funded in this area.
 M. Kittelmann et al., ACS Nano 5 (2011) 8420
 M. Kittelmann et al., ACS Nano 7 (2013) 5614
 R. Lindner et al., Angew. Chem. Int. Ed. 53 (2014) 7952
Charge Transfer at Surfaces
 M. Nimmrich et al., Phys. Rev. B 81 (2010) 201403 (rapid communication)
 M. Nimmrich et al., Phys. Rev. B 85 (2012) 035420
II. Solid-Liquid Interfaces
High Resolution in Liquids
High-resolution imaging of the solid-liquid interface requires the consequent identification and reduction of all possible noise sources in order to increase the signal-to-noise ratio as much as possible. We have modified commercial instruments for enabling frequency modulation atomic force microscopy operation in liquids . Recently, we have implemented a photothermal excitation method that allows for removing the so-called "forest of peaks", which is a major obstacle in high-resolution imaging under liquid conditions . Currently, we investigate means to obtain electrostatic information similar to Kelvin probe force microscopy also in liquids.
 S. Rode et al., Rev. Sci. Instrum. 82 (2011) 073703
 H. Adam et al., Rev. Sci. Instrum. 85 (2014) 023703
Molecular Self-Assembly at the Solid-Liquid Interface
pH dependent structures of Alizarine Red on calcite 
A key aspect of many application-relevant questions such as scale inhibition or mimicking biomineralization is investigating the structure formation of organic molecules at the solid-liquid interface and understanding the impact on crystallization.
We have studied the pH-dependent self-assembly of organic molecules on calcite  and compared the surface structures with the macroscopic effect on crystallization.
 M. Schreiber et al., Soft Matter 9 (2013) 7145
Solvation Layer Mapping
Solvation layer structure above a calcite surface 
 C. Marutschke et al., Nanotechnology 25 (2014) 335703