Metal atoms and clusters on two-dimensional insulators: reactive adsorption, incorporation and doping effects
Prof. Gianfranco Pacchioni
Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, via Cozzi, 55 - 20125 Milano, Italy
17:00 - 18:00 Tuesday 30 June 2015 KFU HS 5.01 Ultrathin films of insulating materials grown on a metal, also called two-dimensional insulators, have a wide range of applications [1]. Here we will address a few examples based on DFT calculations performed in our group combined with experimental evidences, mostly from scanning tunneling microscopy. The first example is related to Ni nanoclusters grown on one mono-layer MgO/Ag(100) film [2]. The Ni clusters are not three-dimensional as expected for Ni tetramers, pentamers or hexamers in gas-phase. Both the shape of the clusters and the Ni-Ni distances indicate that the nanoparticles do not consist of pure metal atoms but that a NixOy structure has formed thanks to the availability of atomic oxygen accumulated at the MgO/Ag interface [2]. The second example is related to the self-doping that occurs when Co or Cr atoms are deposited on the surface of two-layer NaCl/Au(111) films [3,4]. The Cr and Co atoms spontaneously replace Na and Cl ions in the films changing the electronic properties of the system. This effect is peculiar of NaCl and does not occur on other two-dimensional insulators like MgO/Ag(100) films [5]. Self-doping (and in general doping) an oxide with transition metal atoms has important consequences on the surface properties [6-11]. To show this we will discuss the ability of Mo (Cr) impurities in a CaO (MgO) matrix to act as charge donors to adsorbed gold. Whereas CaOMo features a robust electron donor characteristic, MgOCr is electrically inactive due to the different stability of various oxidation states. Based on our findings, we develop general rules on how to optimize the electron-donor characteristics of doped oxide materials and thin films and how this can be used to tune the catalytic properties of supported nanoparticles.
[1] G. Pacchioni, “Two-dimensional oxides: multifunctional materials for advanced technologies”, Chemistry a European Journal, 18, 10144-10158 (2012).
[2] M. Smerieri, J. Pal, L. Savio, L. Vattuone, R. Ferrando, S. Tosoni, L. Giordano, G. Pacchioni, M. Rocca, "Spontaneous Oxidation of Ni Nanoclusters on MgO Monolayers Induced by Segregation of Interfacial Oxygen", in preparation.
[3] Z. Li, H.-Y. T. Chen, K. Schouteden, K. Lauwaet, L. Giordano, M. I. Trioni, E. Janssens, V. Iancu, C. Van Haesendonck, P. Lievens, G. Pacchioni, “Self-doping of ultrathin insulating films by transition metal atoms”, Physical Review Letters, 112, 026102 (2014).
[4] Z. Li, H.-Y. T. Chen, K. Schouteden, E. Janssens, C. Van Haesendonck, P. Lievens, G. Pacchioni, “Spontaneous doping of two-dimensional NaCl films with Cr atoms: aggregation and electronic structure”, Nanoscale, 7, 2366-2373 (2015).
[5] H.-Y. T. Chen, G. Pacchioni, "Properties of two-dimensional insulators: a DFT study of Co adsorption on NaCl and MgO ultrathin films", Physical Chemistry Chemical Physics, 16, 21838-21845 (2014).
[6] X. Shao, S. Prada, L. Giordano, G. Pacchioni, N. Nilius, H.-J. Freund, “Shape control of metal adparticles via doping of the oxide support: An STM and DFT study”, Angew. Chem. Int. Ed., 50, 11525 (2011).
[7] F. Stavale, X. Shao, N. Nilius, H.-J. Freund, S. Prada, L. Giordano, G. Pacchioni, “Transition metal dopants and the electron donor characteristic of oxide surfaces”, J. Am. Chem. Soc., 134, 11380 (2012).
[8] S. Prada, L. Giordano, G. Pacchioni, “Charging of gold atoms on doped MgO and CaO: identifying the key parameters by DFT calculations”, J. Phys. Chem. C, 117, 9943 (2013).
[9] Y. Cui, N. Nilius, H.-J. Freund, S. Prada, L. Giordano, G. Pacchioni, “Controlling the charge state of single Mo dopants in a CaO film”, Phys. Rev. B, 88, 205421-5 (2013).
[10] Y. Cui, X. Shao, S. Prada, L. Giordano, G. Pacchioni, H.-J. Freund, N. Nilius, “Surface defects and their impact on the electronic structure of Mo-doped CaO films: an STM and DFT study”, Phys. Chem. Chem. Phys. 16, 12764 (2014).
[11] S. Prada, L. Giordano, G. Pacchioni, “Nb-doped CaO: an efficient electron donor system”, J. Physics: Conden. Matter, 26, 315004(6) (2014).
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