Dr. Udo Schwarz

Abstract:

Entire scientific disciplines are governed by the interactions between atoms and molecules. On surfaces, forces extending into the vacuum direct the behavior of many scientifically and technologically important phenomena such as corrosion, adhesion, thin film growth, nanotribology, and surface catalysis. To advance our knowledge of the fundamentals governing these subjects, it would be useful to simultaneously map electron densities and quantify force interactions between the surface of interest and a probe with atomic resolution. When attempting to use scanning probe microscopy (SPM) towards this goal, significant limitations in both imaging and mapping persist despite their ability to image surfaces and map their properties down to the atomic level. Most commonly, SPM qualitatively records only one property at a time and at a fixed distance from the surface. To overcome these limitations, we have integrated significant extensions to existing SPM approaches, which we will shortly summarize in this talk. 

The work started in 2009, when we expanded noncontact atomic force microscopy (NC-AFM) with atomic resolution to three dimensions by adding the capability to quantify the tip-sample force fields near a surface with picometer and piconewton resolution [1, 2]. In 2013, we added electronic information through the recording of the tunneling current simultaneously with the force interaction. Using copper oxide as an example of a catalytically active surface, this allowed to study the role of surface defects as active sites [3]. With the goal of yielding information on energy barriers in on-surface chemical reactions, we further extended this approach in 2022 to gain insight into the energetics of molecular motions on surfaces, with benzene and iodobenzene as model systems. And most recently, we introduced the method to study single-molecule chemistry with the example of cobalt phthalocyanine (CoPc) molecules, which have shown great potential to favorably catalyze the formation of methanol from CO₂ and hydrogen [4, 5]. Thereby, the binding strength of the intermediate CO to the cobalt atom at the center of the CoPcs catalyst molecule has been recognized as a key descriptor affecting catalytic efficiency, with the ideal CO-Co binding strength being neither too strong nor too weak. Using a CO-terminated tip, the CO-CoPc equilibrium distances and potential energies at equilibrium distances were recovered across the molecule [6]. Currently ongoing work aims at systematically changing the substituents/side chains of the CoPc or the substrate the CoPc molecules sit on to clarify the effect of these changes on the CO-Co binding strength and eventually enable a fine tuning of the binding strength, which may open new avenues to optimize the catalytic reaction.