Method Development

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.

[1]    P. Rahe et al., Rev. Sci. Instrum. 82 (2011) 063704
[2]    P. Rahe et al., Rev. Sci. Instrum. 85 (2014) 043707

Self-Assembly on Insulating Surfaces

Controlling molecular self-assembly on surfaces depends upon an in-depth understanding of both, the intermolecular as well as the molecule-surface interactions. Compared to metal surfaces, rather little is known about the molecule-surface interaction when considering insulating surfaces. This is mainly due to the fact that many standard surface science techniques rely on conducting surfaces and are, therefore, not suited to study insulating materials.
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 [6].
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.

[3]     P. Rahe et al., Adv. Mater. 25 (2013) 3948
[4]     M. Kittelmann et al., J. Phys.: Condens. Matter 24 (2012) 354007
[5]     M. Körner et al., Phys. Rev. Lett. 107 (2011) 016101
[6]     C. Hauke et al., ACS Nano 7 (2013) 5491 

Investigating Kinetics on Surfaces

Coexistence of neutral and deprotonated molecules [8]

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 [7].
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 [8].

[7]     F. Loske et al., Phys. Rev. B 82 (2010) 155428
[8]     M. Kittelmann et al., ACS Nano 6 (2012) 7406

On-Surface Synthesis

Covalent linking of benzoic acid derivatives on calcite [9]

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 [9]. As a next step, we have extended this concept by a sequential and site-specific linkage, providing the possibility for hierarchical structure formation [10]. 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 [11].
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.

[9]     M. Kittelmann et al., ACS Nano 5 (2011) 8420
[10]   M. Kittelmann et al., ACS Nano 7 (2013) 5614
[11]   R. Lindner et al., Angew. Chem. Int. Ed. 53 (2014) 7952

Charge Transfer at Surfaces

Within a project funded by the Volkswagen foundation – Spin quantum computing based on N@C60 / NV centers in diamond – we explore the possibility to use endohedral fullerene molecules atop nitrogen-vacancy centers in diamond for a future quantum computing device. For this project, we study the charge transfer between hydrogen-passivated diamond surfaces and adsorbed fullerene molecules [12] and investigate the influence of charge transfer on the self-assembly process [13].

[12]   M. Nimmrich et al., Phys. Rev. B 81 (2010) 201403 (rapid communication)
[13]   M. Nimmrich et al., Phys. Rev. B 85 (2012) 035420

 

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 [14]. 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 [15]. Currently, we investigate means to obtain electrostatic information similar to Kelvin probe force microscopy also in liquids.

[14]   S. Rode et al., Rev. Sci. Instrum. 82 (2011) 073703
[15]   H. Adam et al., Rev. Sci. Instrum. 85 (2014) 023703

Molecular Self-Assembly at the Solid-Liquid Interface

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 [16] and compared the surface structures with the macroscopic effect on crystallization.

 

 

[16]  M. Schreiber et al., Soft Matter 9 (2013) 7145

Solvation Layer Mapping

For a comprehensive understanding of structure formation at the solid-liquid interface, the influence of the solvent molecules is pivotal. We have developed a routine that allows for solvation layer mapping on surfaces using amplitude modulation atomic force microscopy [17]. Currently, we use this technique for evaluating the influence of specific ions on the formation of hydration layers atop carbonate surfaces, namely calcite (CaCO3), dolomite CaMg(CO3)2 and magnesite (MgCO3). Here, we collaborate with the group of A. Foster, Aalto University, Helsinki, funded by a DAAD project.

[17]  C. Marutschke et al., Nanotechnology 25 (2014) 335703