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Dynamik monodisperser Cluster unter dem schnellen Rastertunnelmikroskop

Fachliche Zuordnung Physikalische Chemie von Festkörpern und Oberflächen, Materialcharakterisierung
Förderung Förderung von 2012 bis 2021
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 230413507
 
Erstellungsjahr 2022

Zusammenfassung der Projektergebnisse

The project “Dynamics of monodisperse clusters under the fast scanning probe microscope” aimed at resolving the effects of temperature, adsorbates and ongoing reactions on the structural dynamics of atomically precise supported clusters. How fluctional are clusters, how do they sinter – and can we deduce from such findings concepts for stabilization? To that purpose, a scanning tunneling microscope (STM) has been accelerated to movie-rate image acquisition by addition of an external module, maintaining atomic resolution. Difficulties in this constant height measurement approach arise with cluster heights above two atomic layers. To overcome this intrinsic problem, we successfully implemented tracking algorithms that work under constant current conditions. These technological developments resulted in several scientific highlights. We started with following the ethene surface polymerization to graphene at moderate temperatures and found the surprising formation of size-specific coronene-like carbon cluster intermediates (consisting of 7 C6 rings). We succeeded in developing general concepts for cluster stabilization by systematically varying wetting and non-wetting sites on surfaces. We applied this concept to boron nitride films with known pore characteristics and managed to follow details on reversible cluster isomerization and diffusion in situ. While atoms diffuse along the rim of a pore, a small Pd~3 cluster experiences the corrugation in the potential energy landscape that leads to a discrepancy between adsorption site symmetry (sixfold – film) and jump dynamics symmetry (threefold – underlying support) which could clearly be revealed. We identified for clusters on graphene supports isomer-specific diffusivities, as well as isomer interconversion, and could follow the individual cluster diffusion path of a Pd12 cluster with atom tracking – clearly indicating a path that avoids non-wetting surface areas and connects the wetting ones. On studying these dynamics we realized that the dynamics of the support itself is essential for the complete understanding of these cluster-assembled materials. On one hand, we managed to investigate the Arrhenius-activated diffusion of hydrogen atoms on a magnetite (001) support, hereby revealing how the proximity to surface defects affects hopping rates. On the other hand, we discovered a new opportunity to apply our fast STM techniques: Instead of using single movie frames, averaging of several hundreds of images helps to reveal surface structures that persist under heavily dynamic adsorbate movements that make the single images appear fuzzy. In this way, we could demonstrate how the subsurface cation vacancy reconstruction of the magnetite support still exists as a local structural motif at the orderdisorder phase transition around 720 K. Summarizing, this project successfully pursued the defined research goals. We use this technology in a new proposal where we study cluster dynamics under defined reaction conditions in UHV systems. We are also heading for an application in air and liquid phase, by implementing the Fast STM technique in further microscopes; topic of a further DFG/DACH proposal.

Projektbezogene Publikationen (Auswahl)

 
 

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