A Monte Carlo method has been developed in order to investigate dynamic configurations of nanometer sized Au nanocrystallites (NCs) supported on the MgO(100) surface[1,2]. We have found significant concentrations of Au atoms which transiently occupy adatom positions on Au(111) facets. Their concentration increases from 10-4 per NC at 250 K to 10-2 per NC at 550 K. A complex roughening transition involving the creation of steps on Au(111) facets is observed close to 500 K. The appreciable numbers of various local atom configurations which may transiently appear on NC at finite temperature may be important for numerous applications, such as heterogeneous catalysis, for example[3,4].
The identification of transient configurations that may occur and importantly the quantification of their probability is a non-trivial matter. Molecular dynamics (MD) methods are not suitable due to the disparity between timescales that can be simulated, ~10-11s, and those which are often relevant physically, >1s. The MC method we have developed uses an efficient trial move to explore nanocrystallite configurations. This move involves random rearrangements surface atoms and ensures detailed balance. An embedded atom model potential is used for Au[5] and the energy for Au adsorption onto the MgO surface is based upon ab initio calculations[4]. Good statistics can be acquired using this method allowing quantitative statements about low probability configurations to be made.
[1] K. P. McKenna, P. V. Sushko and A. L. Shluger, J. Phys. Chem. C Lett. 111, 2823 (2007)
[2] K. P. McKenna, P. V. Sushko and A. L. Shluger, (Submitted to J. Phys. Chem.)
[3] N. Lopez et al., J. Catal. 223, 232 (2004)
[4] L. M. Molina and B. Hammer, Phys. Rev. B 69, 155424 (2004)
[5] A. P. Sutton and J. Chen, Philos. Mag. Lett. 61, 139 (1990)
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