Ab Initio Statistische Mechanik Untersuchung dotierter Silizium Cluster: Von isolierten Käfigen zu Clustern in komplexen Umgebungen
Zusammenfassung der Projektergebnisse
The central objective in the first funding period was to establish ground state structures of gas-phase endohedrally doped silicon clusters by means of first-principles global geometry optimization. Initially targeted were transition metal-doped M@Sin cages, which were found to be stabilized via strong metal-cage interaction, generally leading to spin quenching of the dopant. As this is unfavourable for perspective applications, the investigation then focused on the hydrogenated counterparts. Stoichiometric or sub-stoichiometric hydrogenation of the silicon cages fully decouples the dopant from the cage, thus preserving the magnetic properties. Such nanoforms of silicon are thus identified as promising building blocks for cluster-assembled, functionalized nanomaterials for magnetoelectronic applications. The perspective of making such intriguing objects available to real-life synthesis and applications, however, requires the investigation of their properties in complex environments, such as when supported by extended surfaces, and/or in relation to the external experimental conditions. The ensuing increased complexity with respect to the focus of the first funding period generated challenges which required substantial method development in order to enhance the efficiency of the employed approaches. In particular, the employed global geometry screening techniques become rapidly cumbersome with increasing system size (e.g. introduction of support surfaces) and complexity (e.g. variable size and composition). To address this, we have developed computational routes to generate better trial structures in global geometry screening algorithms by means of “smart” moves, which greatly enhance the sampling efficiency by naturally driving the structural search towards chemically relevant regions of the Potential Energy Surface (PES). In addition, we extended the global optimization framework to sample directly the Grand Canonical Ensemble, to account for systems of unknown or varying composition. Significant additional efficiency can be gained by performing the structural sampling at lower levels of theory, and only subsequently re-ranking the best candidate structures with more accurate methods. Thus, in parallel, we carried out systematic assessment of the possibilities and limitations of approximate methods involved in such hierarchical approach, with the broader objective of identifying strategies to minimize the inevitable shortcomings. The developed computational protocol was applied to the theoretical prediction of the soft-landing behaviour of hydrogenated silicon clusters, focusing on two differently relevant cage sizes adsorbed on silicon substrates, where the main objective was to identify possible “evolution” routes for the smallest, and to assess the long-term stability for the largest. Additionally, two collaborations have been established within the research unit (project B and D), where theoretical calculations have been employed to support and interpret the experimental findings. Since one possible route to the fabrication of functionalized materials consists in employing pre-formed cages as building blocks, effective synthesis routes for “well-formed” cages are of crucial importance. This—in particular, precisely controlling the degree of hydrogenation—proved to be a difficult task, as observed in experiments and confirmed by calculations (with project B). An alternative route to explore is the direct silicide synthesis on templated surfaces. In this context, the properties of rareearth silicide clusters were investigated experimentally by means of Scanning Tunneling Microscopy, with calculations shedding some light on fine details of the structures and mechanisms involved (with project D).
Projektbezogene Publikationen (Auswahl)
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Structural Metastability of Endohedral Silicon Fullerenes, Phys. Rev. B 81, 201405, 2010
A. Willand, M. Gramzow, S.A. Ghasemi, L. Genovese, T. Deutsch, K. Reuter, S. Goedecker
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On the stability of “Non-magic” Endohedrally Doped Silicon Clusters: A First-Principles Sampling Study of MSi16+ (M=Ti, V, Cr), J. Chem. Phys. 134, 244705, 2011
D. Palagin, M. Gramzow, K. Reuter
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Evaluation of Endohedral Doping of Hydrogenated Si Fullerenes as a Route to Magnetic Si Building Blocks, Phys. Rev. B 86(4), 045416, 2012
D. Palagin, K. Reuter
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MSi20H20 Aggregates: From Simple Building Blocks to Highly Magnetic Functionalized Materials, ACS Nano 7(2):1763–1768, 2013
D. Palagin, K. Reuter
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Vibrational Spectra and Atructures of Bare and Xe-tagged Cationic SinOm+ Clusters, J. Chem. Phys. 141(10):104313, 2014
M. Savoca, J. Langer, D.J. Harding, D. Palagin, K. Reuter, O. Dopfer, A. Fielicke
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Global Materials Structure Search with Chemically Motivated Coordinates, Nano Lett. 15 (12), 8044-8048, 2015
C. Panosetti, K. Krautgasser, D. Palagin, K. Reuter, R.J. Maurer
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Global Structure Search for Molecules on Surfaces: Efficient Sampling with Curvilinear Coordinates, J. Chem. Phys. 145, 084117, 2016
K. Krautgasser, C. Panosetti, D. Palagin, K. Reuter, R.J. Maurer
