Faceting Transition of Gold Nano Materials Yanting Wang Advisors: Prof. Stephen Teitel, Prof. Christoph Dellago May 22, 2003 Department of Physics and Astronomy University of Rochester Introduction Simulation of morphological and structural change of gold nanorods Faceting Transition of gold cluster Faceting Transition of gold nanorods Conclusion and future work

Wide range of applications Laser heating experiments of rods Shorter and wider inter. Particles Internal point and planar defects MD simulation of heating rods Meta-stable intermediate state Fcc-hcp structural change Faceting transition of ih gold cluster Facets vanish below T m Faceting transition of gold nanorods Some evidence of connection between faceting and T s between faceting and T sIntroduction  Wang, Z. L. et al., Surf. Sci. 1999, 440, L809.  Link, S. et al., J. Phys. Chem. B 2000, 104,  Link, S. et al., J. Phys. Chem. B 2000, 104, 7867.

Simulation of Morphological and Structural Change of Gold Nanorods Experiments: more than 10 6 atoms Simulation: atoms Colored by local structure: yellow --fcc green --hcp red --others

Faceting Transition of Icosahedral Gold Cluster  Cooled from liquid obtaining ih cluster, then heated up.  Equilibrated at each temperature by constant T MD. Colored by local structure: yellow --fcc green --hcp gray --decahedron T m ~1090K T=100K T=1500K ?

Average shape of ih gold cluster 400K 600K 900K 1000K 1050K 1075K Cone algorithm Multi-layers Local curvature fitting Average shape Colored by larger local curvature with warmer color represents larger curvature.

Surface diffusion of ih gold cluster Mean squared displacements Interlayer diffusion Surface structure Bulk structure

Faceting transition point? Definition of faceting transition point Larger local curvature distribution Geometrical difficulties of defining faceting transition point for nano clusters Standard deviation Normalized distribution  Small cluster size  Asymmetric facets and edges  Macroscopic shape fluctuation  Large influence of surface atom's thermal fluctuation to the macroscopic shape No angular discontinuities. Not a good interpretation!

Faceting Transition of Gold Nanorod N=11076 T s ~990K T m ~1280K Colored by larger local curvature with warmer color represents larger curvature.

Conclusion Icosahedral gold cluster The vertices, edges and facets get rounder with higher temperature and totally disappear below T m. Faceting transition occurs before bulk melting. Surface becomes unstructured at higher temperature than that of surface diffusion. Gold nanorod Macroscopic shape transformations are intimately connected with the faceting transition occurring on the sides of the rod.

Future Work Icosahedral gold cluster Define faceting transition for small size nano cluster. Quantify faceting transition geometrically and find the transition temperature. Gold Nanorod Detailed analysis of surface atom behavior. Quantify faceting transition geometrically. Find out the energetic relation between surface behavior and internal structural change.