Bulk metallic glasses (BMGs) have been intensively investigated because of their special mechanical properties as amorphous materials and their unique glass transition state. However, the atomic-scale origins of their behavior have not been unequivocally clarified. To explain their properties, structural models of BMGs and their small-scale deformation behavior have been proposed but not yet confirmed due to the inability of conventional measurement approaches to characterize samples at the relevant scale. For example, local structural analysis of glasses at the atomic scale using methods such as transmission electron microscopy or neutron/x-ray scattering is challenging due to the material’s disordered nature. In contrast, scanning probe microscopes and nanoindenters possess the potential of direct nanometer-scale observation local glass structure and mechanical properties. But even with these approaches, extraction of meaningful data is challenging due to the difficulty to prepare clean, atomically flat surfaces of BMG. This is because a surface roughness of some nanometers, standard with most sample preparation techniques, may alter the results of local testing if the volumes probed are nanometer-sized as well.

In this talk, we will be reviewing our recent progress in developing novel imprinting and fabrication methods of metallic glasses that can produce both atomically flat surfaces with sub-nanometer-scale features and samples with well-defined nanometer- and micron-sized total volumes as well as their subsequent use for the study of their nanometer-scale structural and mechanical properties. Imprinting is realized via thermoplastic forming of BMGs [1,2] and, alternately, by magnetron sputtering of general metallic glasses [3]. The capability of imprinting at an atomic scale enriches the range of applications of BMGs and brings a new way to directly characterize heterogeneity, relaxation, and crystallization in BMGs [4, 5]. It also allows to study onset of yielding and the local plastic flow mechanisms of BMGs in the limit of very small activation volumes (about 1000 atoms). The experiments revealed a much higher yield stress compared to the value obtained by conventional nanoindentation testing, followed by homogeneous plastic flow [6]. These atomic-scale results are contrasted to the larger-scale model that explains plastic deformation of BMG as originating from the finite STZs activation. Finally, current work is aimed at producing large numbers (>1000) of well defined, uniform micron- or nanometer-scaled pillars that can be used to explore the deformation behavior of BMGs under compression as a function of sample volume and compression rate in a statistically relevant manner. 

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.