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Research

Current and past research projects focus on the fabrication and manipulation of materials at the single molecular level in insulating environments. Using scanning probe methods, especially high-resolution atomic force microscopy, we study molecular and atomic adsorption on surfaces of insulating materials or on thin insulating films, where the electronic properties of the adsorbates are rather unaffected by the underlying substrate. Our current research topics contain:

• Fundamental understanding of mineral surfaces
• 2D molecular and metal-organic structures on insulator surfaces
• Molecular manipulation and charge-state control
• Nanoscale charge and potential mapping
• High-precision atomic-scale force measurements
• Method development

Fundamental understanding of mineral surfaces

Minerals are involved in many geological, chemical, or biological processes including weathering, dissolution-precipitation, or biomineral formation. We investigate the microscopic properties of mineral surfaces with a particular focus on understanding the surface structure and chemical termination. Our focus currently lies on two systems: The calcite(104)-(2×1) and the CaF₂(111) surface.

Calcium carbonate is one of the most abundant minerals in earth’s crust and a key component in geochemical processes. Initially, we studied the contrast formation and force interaction on calcite(104) in NC-AFM at the atomic scale using room temperature experiments [1,2,3]. We recently investigated the adsorption of carbon monoxide (CO) on this surface and confirmed the adsorption position on the calcium atoms [4]. Furthermore, we have now established that the pristine calcite(104) surface is (2×1) reconstructed, belongs to the planar space group pg, and generates two different adsorption geometries for CO, see Figure 1(a,b) [5]. Currently, we are studying the adsorption of water on this surface where we identified a particular reconstruction lifting mechanism due to water adsorption [6].

The second substrate of interest, Calcium fluoride, is by itself a technologically most relevant material with vast application, is available as both bulk material and insulating thin film [7], and offers sites with strong anchoring for functional molecules. We are studying the properties of both CaF₂/CaF₁/Si(111) thin films as well as of CaF₂(111) bulk crystal surfaces [7] with NC-AFM as well as DFT.

2D molecular and metal-organic structures on insulator surfaces

We are interested in the fabrication, characterisation, and utilisation of functional molecule-on-insulator nanosystems for novel concepts in quantum nanoscience, in particular for novel concepts in information processing. Building upon nanoscience, chemistry, and molecular electronics, we aim for arriving at nanosystems that are build from single atoms and individual molecules, and that deliver processing elements with significantly reduced energy consumption. Our focus currently lies in generating a fundamental understanding for assembly of metal-coordination materials on insulator surfaces.

We have previously studied molecular self-assembly on calcite(104) employing weak interactions such as van-der-Waals [1], hydrogen bonds [2], or combinations thereof (see Figure 2(a)) [3]. As a key result, molecular anchoring was identified as an important mechanism for molecular assembly on insulator surfaces [4]. This understanding has allowed for the realisation of on-surface synthesis on mineral surfaces [5, 6].

Currently, we investigate the formation of metal-organic structures as a key enabler for atomic-scale functionality. To name one example, we have recently characterised a quadruped binding mechanism for a functionalised ferrocene molecule on CaF₂(111) (see Figure 2(b)) [7, 8] that now allows us to precisely steer the molecular structure formation. In addition to the experimental investigations, we apply density functional theory (DFT) using CP2K (see Figure 2(c)) to unravel atomic-scale details of molecular adsorption [9].

Molecular manipulation and charge-state control

We develop and implement routines for vertical and lateral manipulation of single atoms and molecules and investigate charging effects in single molecules on insulator surfaces. This research interest is motivated by performing advanced measurements with functionalised tips, follows the idea of bottom-up fabrication, and forms the basis of molecular charge-based approaches for field-coupled technologies.

Recently, we have established high-resolution imaging with CO-functionalised tips of the calcite(104) mineral surface [1] by implementing measurement protocols for vertical CO manipulation (see Figure 3(a)) [1]. Along the same line, we currently use a combination of lateral and vertical manipulation of PTCDA molecules on Ag(111), see Figure 3(b), to isolate single molecules and attach them to the tip to act as a quantum sensor. 

Previously, we pioneered charge-state modification of small molecular assemblies on calcite(104) and in particular revealed the existence of a charge stabilisation mechanism within molecular islands self-assembled on calcite(104), see Figure 3(c) [2].

Nanoscale charge and potential mapping

Electrostatic quantities such as charge distribution or the electrostatic potential are straightforwardly accessible on the macroscopic scale, but their measurement is challenging when approaching atomic dimensions. We are following two routes for quantitative measurements with atomic resolution: Charge force microscopy [5] and scanning quantum dot microscopy.

We derived a consistent theory for charge force microscopy, a technique based on the method Kelvin probe force microscopy, to relate the experimental measurement data to charge magnitudes with the central equation shown in Figure 4(a) [1, 2, 3, 4]. This derivation ultimately led to the introduction of charge force microscopy (CFM) [5]. Recently, we performed a detailed analysis of contributing factors and unravelled many influences to the absolute measurement signal, see Figure 4(b) for one example [5].

Currently, we implement scanning quantum dot microscopy (SQDM) as a highly-sensitive technique with sub-molecular spatial resolution and the ability to perform quantitative measurements at the single-molecule level. This technique uses a quantum dot attached to the tip of an atomic force microscope acting as a quantum sensor.

High-precision atomic-scale force measurements

The core capability of frequency-modulated (also known as 'dynamic' or 'non-contact') atomic force microscopy is to quantitatively measure forces with pico-metre resolution and pico-Newton sensitivity, enabling the exciting possibility to investigate chemical or physical interactions at the atomic scale.

In the past, we applied this technique to measure the force field above surfaces such as calcite(104), see also Figure 5(a) [1], or above molecular structures such as hydrogen-bonded assemblies [2].
Furthermore, we critically analysed approaches to separate background (“long-range”) from site-specific (“short-range”) interaction forces [3] and studied the influence of an inclined tip oscillation path [4]. A recent breakthrough result is the introduction of force curve alignment (FCA) to significantly increase the precision of quantitative force measurements and to deliver an inherent consistency check, see Figure 5(b) [5].

Within the BMBF-funded project “VRnano” (https://www.vrnano.de), VR-based techniques are currently developed to allow for an efficient navigation within the three-dimensional space above a surface at the atomic scale, to manipulate single molecules by a manual “haptic” access, to visualise the many data dimensions in scanning probe microscopy with scientific rigorousness (see also Figure 5(c)), and to measure nanoscale forces between complex, three-dimensional molecules along free trajectories.

Method development

The study of insulator mineral surfaces as well as molecular structures at the atomic scale in real space requires an optimised, well-characterised, and low-noise scanning probe microscope operated at the physical limits.

Thermal drift – the result of minute temperature changes within a scanning probe microscope – is a well-known nuisance, and especially hinders a reliable absolute tip positioning. It especially forbids long-term data acquisition in constant-height mode as well as the acquisition of dense force volume data. An elegant solution is to "lock" the scanning probe tip to one surface site and to track the virtual movement, a technique known as atom tracking. To measure and compensate for these deviations, we have developed an atom-tracking system, see Figure 6(a) [1]. The latest version of this system is based on an ARM Cortex-M4 microcontroller that enables force measurements over the course of many hours.

The performance of scanning probe microscopes is ultimately defined by the magnitude of measurement noise. We have previously performed a detailed analysis of NC-AFM noise figures (see Figure 6(b)), including a mathematical description of the whole detection system [2] including the tip-sample interaction [3].

Furthermore, we introduced the “dip-df” control mode [4] and developed tools and protocols for the efficient high-resolution study of mineral surfaces at low temperatures such as a double sample holder, see Figure 6(c) [5].