For most applications, clusters need to be deposited on a substrate. A typical example is the platinum nanoparticle in catalytic convertor of automobiles, which are attached to a ceramic support. Clusters are much smaller than the platinum particles in common catalytic convertors, and their properties vary with each atom. The preparation of surfaces with clusters of one size is complex. The clusters are generated as ions with a suitable source and separated according to their mass in a mass spectrometer (e.g., sector magnet). The cluster ions of a selected size are softly landed on a substrate (experimental setup see figure below). Such an experiment was assembled in collaboration with the group of Kit Bowen at the Johns Hopkins University in Baltimore, Department of Chemistry.
The laboratory in Baltimore (picture below) enjoys a great popularity by the students from Konstanz. Sometimes many students (see photo collage) complete their bachelor or master thesis or a placement in the laboratory at the Johns Hopkins University at the same time.
The work follows two goals:
- Interaction with the surface changes the geometric and electronic structure of the clusters. A first goal of the research is a comparison of the properties of deposited clusters with those of free clusters. This change is also dependent on the kind of substrate; a gold cluster is, for example, on one surface catalytic active and is not active on another.
- A pursuing goal is the determination of the chemical properties of clusters. This includes, for example, the catalytic activity of selected model reactions, such as the oxidation of carbon monoxide to carbon dioxide.
Because the investigations on mass-selected clusters on surfaces are a novel research area, surprising discoveries are made beyond the set goals of research. An example is the “baby-crystal” (PbS)32 (see figure below).
- X. Tang, J. Schneider, A. Dollinger, Y. Luo, A. S. Wörz, K. Judai, S. Abbet, Y. D. Kim, G. F. Ganteför, D. H. Fairbrother, U. Heiz, K. H. Bowen and S. Proch, "Very small ‘‘window of opportunity’’ for generating CO oxidation-active Aun on TiO2", Phys. Chem. Chem. Phys., 2014, 16 (14), 6735 - 6742.
- X. Li, K. Wepasnick, X. Tang, D. H. Fairbrother, K. H. Bowen, A. Dollinger, C. H. Strobel, J. Huber, T. Mangler, Y. Luo, S. Proch, and G. Gantefoer, "A new nanomaterial synthesized from size-selected, ligand-free metal clusters", J. Appl. Phys. 155 (2014) in press.
- X. Tang, X. Li, Y. Wang, K. Wepasnick, A. Lim, D. H. Fairbrother, K. H. Bowen, T. Mangler, S. Noessner, C. Wolke, M. Grossmann, A. Koop, G. Gantefoer, B. Kiran, A. K. Kandalam, "Size Selected Clusters on Surfaces", Journal of Physics: Conference Series 438, 012005 (2013).
- Xiang Li, Kevin A. Wepasnick, Xin Tang, Yi Wang, Kit H. Bowen, D. Howard Fairbrother, and Gerd Gantefoer, “Ion induced modification of size-selected MoO3 and WO3 clusters deposited on HOPG”, J. Vac. Sci. Technol. B 30, 031806 (2012).
- K. Boggavarapu, A. Kandalam, R. Rallabandi, P. Koirala, Xiang Li, Xin Tang, Yi Wang, D.H. Fairbrother, G. Gantefoer, and K. Bowen, “(PbS)32: A baby crystal”, J. Chem. Phys. 136, 024317 (2012).