Faceting Transition of Gold Nano Materials

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

2. Introduction • 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 Tm • Faceting transition of gold nanorods • Some evidence of connection • between faceting and Ts • Wang, Z. L. et al., Surf. Sci.1999, 440, L809. • Link, S. et al., J. Phys. Chem. B2000, 104, 6152. • Link, S. et al., J. Phys. Chem. B2000, 104, 7867.

3. Simulation of Morphological and Structural Change of Gold Nanorods Experiments: more than 106 atoms Simulation:1404-11076 atoms Colored by local structure: yellow --fcc green --hcp red --others

4. 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=1500K T=100K Tm~1090K

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

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

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

8. Faceting Transition of Gold Nanorod N=11076 Ts~990K Tm~1280K Colored by larger local curvature with warmer color represents larger curvature.

9. Conclusion • Icosahedral gold cluster • The vertices, edges and facets get rounder with higher temperature and totally disappear below Tm. • 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.

10. 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.