Properties of Highly Oriented Pyrolytic Graphite Supported TaSi16 Clusters: A Density Functional Investigation

Adsorption of the TaSi16 cluster on highly oriented pyrolytic graphite (HOPG) is studied using density functional methods. These calculations resolve some of the issues raised by the recent experiments of Shubita et al. (Shibuta, M.; Ohta, T.; Nakaya, M.; Tsunoyama, H.; Eguchi, T.; Nakajima, A. Chemical Characterization of an Alkali-Like Superatom Consisting of a Ta-Encapsulating Si-16 Cage. J. Am. Chem. Soc. 2015, 137, 14015-14018) and provide additional insights into the system. For the first time, the ground-state structure of TaSi16 is obtained through a global search employing an evolutionary algorithm. A symmetric structure of the Si cage, together with a small charge transfer from Ta to the Si atoms, explains the observed core-level photoemission spectra. The ground-state isomer is deposited on HOPG at different points and in different random orientations. The cluster is found to be physisorbed on HOPG and retains the Ta-encapsulating cage structure, with little charge transfer between the cluster and the substrate. At finite temperatures the cluster does not diffuse over picosecond time scales. The time scale for diffusion is estimated to be a few tenths of a millisecond from the calculated barriers. No coalescence between the cluster and the substrate is found even at 700 K, ruling this out as a possible cause for the observed core-level shift at elevated temperatures. Both at 400 and 700 K, the cluster oscillates about its lowest energy structure after deposition and is not found to transform to other isomers within the time scale of the simulations. However, these oscillations cause large variations in the energies of the electronic levels, which may explain the absence of definite peaks in the valence-level photoemission spectra.