Simulation of shape distribution in different FCC systems

FLORIDIA ADDATO, M. A.; CORTHEY, G.; ZELAYA, E.; FONTICELLI, M. H.; SALVAREZZA, R. C.

Kiel

Congreso; Microscopy Conference 2011 (MC2011); 2011

Nanosystems (NS) of Pt, Au, Ag and Au-Ag are of special interest due to their small size and consequent large surface to volume ratio which changes their optical and mechanical properties respect to the bulk samples. The properties of the NS are highly dependent on the atomic arrangement in a build block units. Unit blocks of FCC crystals like Pt, Ag and Au are reported with icosahedra, decahedra or truncated octahedron morphology [1]. The sizes of these units are commensurable, with discrete values, and changing the morphology according to the atomic component and the number of atoms involved in each unit [2]. Besides the morphological change of each unit block, they could combine with each other showing a rod, triangular or hexagonal like shape [2,3]. The objective of this work is to show that not every combination of unit blocks are possible for different shapes of NS and all the factors that should be taken into account to produce an accurate simulated image of each system. The synthesis of each NS was previously reported. The HRTEM images were taken in a FEI CM200, operated at 200 keV and the simulations were performed with JEMs. First, different kinds of construction of the NS are analyzed using truncated octahedron as a unit. Figure 1b and 1c shows two different ways to set two truncated octahedron of Pt. Even when the disposition between the unit blocks, the distance between then, the rotation and defocus for the final NS were varied, only the simulation of the construction shown in Figure 1b seems to match with the HRTEM image (Figure 1a and 1d.) As the number of atoms involved in each unit block increases, not only the relative disposition between them should be taken into account but also the different morphologies of the unit block should be analyzed. Figure 2a shows and example where not only two icosahedrons bonded by the edge but also two decahedrons joined in one edge could match with the HRTEM image shown in Figure 2a. The interpretation of the simulation results of the systems is more complex since they do not show a unique result. Another variant that is analyzed is the kind of faults involved in each system. The staking faults could be identified in all the three systems studied in this work. Figure 2b shows a HRTEM image of a Pt system in [110]FCC zone axis for the upper an the lower part of the system, however, the dashed line indicates that there is mismatch between both areas of the particle divided by the fault. This system as well as the system presented in Figure 2c present a simulation that match with the HRTEM using only two unit blocks. However, one presents a staking fault and the other presents a fault that tends to compensate a mismatch between the join of the two unit blocks. In Figure 2d one of the most complicated situations to simulate could be seen. The Au NS seems to be composed by three different unit blocks and also a fault is present in the system. References1.T. Ling, L. Xie, J. Zhu, H. Yu, H. Ye, R. Yu, Z. Cheng, L. Liu, G. Yang, Z. Cheng, Y. Wang, X. Ma, Nano Lett. 9,4, (2009) 1572-1576. 2.S. Sato, S. Wang and K. Kimura, J. Phys. Chem. C 111 (2007) 13367-13371. 3.J. Guerra, ? J.L. Burt, D.A. Ferrer, S. Mejía, M.J. Yacamán, J Nanopart Res, in press. 4.S. Chen and K. Kimura, Langmuir 15 (1999) 1075-1082. 5.F.J. Ibañez and F.P. Zamborini, ACS Nano 2 (2008) 1543-1552. 6.G. corthey et. al., ACS Nano 4 (2010) 3413-3421.