Molecular Dynamics Simulations of Gold Nanomaterials

1. Molecular Dynamics Simulations of Gold Nanomaterials Yanting Wang Ph.D. Defense Dept. Physics and Astronomy University of Rochester August 09, 2004 Supervised by Prof. Stephen L. Teitel In cooperation with Prof. Christoph Dellago Institute for Experimental Physics University of Vienna

2. Outline of This Talk Some applications of gold nanomaterials. Backgrounds of slab-like gold surfaces and nanocluster structures. Melting of Mackay icosahedron gold nanocluster. Continuous heating of gold nanorods. Quasi-equilibrium heating of gold nanorods. Future work.

3. Applications of Gold Nanomaterials Ion detection Molecular electronics Electronic lithography Gold nanowires Chemical etching Larger Au particles change color J. Zheng et al., Langmuir 16, 9673 (2000) S. O. Obare et al., Langmuir 18, 10407 (2002) R. F. Service, Science 294, 2442 (2001) Both size and shape have effects in experiments!

4. Thermal Stability and Melting Behavior of Gold Nanomaterials Which nanocrystal structure? Liquid Melting Tm vs. size Thousands of atoms How ? Liquid Nanocrystal Ph. Buffat and J.-P. Borel, Phys. Rev. A 13, 2287 (1976) N<1000, energy barrier between different structures is small We focus on thousands of atoms, showing results for N=2624 (d ~ 4nm) How ? Large surface-to-volume ratio, surface plays a very important role Liquid Nanorod

5. Slab-like Gold Surfaces Slab-like Gold Surfaces Bulk gold with FCC structure and Tm=1337K Possible surface transformations Melting surface: Roughens at 680K and premelts at 770K T=0 Relaxation Reconstruction Partially melting surface: a thin, disordered film at T=1170K, but the thinckness does not grow with T T Deconstruction Roughening at T=TR (“solid disordering”) Wetting (surface premelting) Non-melting surface: ordered up to bulk Tm Bulk melting at T=Tm Gold {111} surface is always energetically preferred!

6. Typical Structures of Gold Nanoclusters HCP edges {111} facets Tetrahedron unit Mackay Icosahedron (Ih) Energetic competition: Mostly covered by gold {111} surface Small total surface area Extra strain or grain boundary energy inside pure FCC boday {111} facets Pure FCC body Octahedron Truncated octahedron Cuboctahedron Pure FCC body {111} facets {100} facets {111} facets {100} facets Very spherical Decahedron Truncated decahedron Including entropy at finite T, which is preferred by gold nanoclusters with thousands of atoms? {111} facets Internal strain T. P. Martin, Phys. Rep. 273, 199 (1996) {100} facets

7. Surface Bulk Cooling and Heating of Mackay Icosahedron Cooling from a liquid Same as left with 3 layers peeled away Empirical glue potential model Constant T molecular dynamics (MD) From 1500K to 200K with T=100K, keep T constant for 21 ns Obtained Ih at T=200K Mackay Icosahedron with a missing central atom Asymmetric facet sizes Colored by local curvature Colored by local structure Heating to melt Potential energy vs. T Keep T constant for 43 ns T=1075K for N=2624 Magic and non-magic numbers Cone algorithm to group atoms into layers

8. Structural Change of Gold Ih Cluster N=2624 Bond order parameters to quantify the structural change All have vanishing values for liquid state Surface Q6(T) / Q6(T=400K) Bulk Interior keeps ordered up to melting Tm Surface softens but does not melt below melting Tm

9. Atomic Diffusion of Ih Cluster Mean squared displacements (average diffusion) Interlayer Diffusion Number of moved atoms All surface atoms diffuse just below melting Surface premelting?

10. Surface Atom Movements and Average Shapes of Gold Ih Cluster Movement t=1.075ns Movement 4t Average shape Colored by local curvature Vertex and edge atoms diffuse increasingly with T Facets shrink but do not vanish below Tm Facet atoms also diffuse below Tm because the facets are very small !

11. Conclusions for Gold Nanoclusters Macky icosahedral structure has been found to be the preferred structure upon cooling from the melt for gold nanoclusters with thousands of atoms. The obtained Ih structure has a missing central atom. No surface premelting below Tm due to the stable gold {111} facets. No seperate faceting transition below Tm is suggested, since the surface softening T seems to be size dependent, and atomic diffusion is involved. Surface softening takes place about 200K below Tm. “Melting” of vertex and edge atoms: vertex and edge atoms diffuse at lower temperature, rounding the average crystal shape. It leads to inter- and intra-layer diffusion, and shrinking of the average facet size, so that the average shape is nearly spherical at melting.

12. Continuous Heating of Gold Nanorods T=5K T=515K T=1064K T=1468K T vs. time Energy change Shape transformation Experimental model Increasing total E linearly with time to mimic laser heating Aspect ratio of 3.0 Pure FCC body Z. L. Wang et al., Surf. Sci. 440, L809 (1999)

13. Internal Structural change of rod Slower heating HCP Small increase of FCC Different sizes and different heating rates result in different duration of hcp states FCC->HCP (!) HCP->FCC(?) Stable HCP intermediate state?

14. Cross Sections from the Continuous Heating Yellow: fcc Green: hcp Gray: other Sliding movement Surface disorder and reorder Crystal orientation changed Experiments: planar defects, shorter and wider intermediate state

15. More from Continuous Heating Ts and Tm vs. N Aspect ratios of the intermediate states Size, initial shape, and heating rate all have effects Motion of atoms during the shape transformation

16. Quasi-Equilibrium Heating of Gold Nanorods T=900K T=0K Heat up temperature by temperature with T=100K, 43 ns at each T Better relaxed and have more data to average at each T Shape change Surface at T=0K Yellow: {111} Green: {100} Red: {110} Gray: other Surface at T=900K Internal structural change Crystal orientation Yellow: fcc Green: hcp Gray: other {100} plane {111} plane Equilivalent to very slow continuous heating

17. Surface Change from Quasi-Equilibrium Heating Second sub layer Average cross-sectional shape Surface Surface curvature distribution Surface disorder and reorder Surface roughens at T~400K {111} facets formed after roughening Large {111} surface area Yellow: {111} Green: {100} Red: {110} Gray: other

18. Cross Sections from Quasi-Equilibrium Heating Yellow: fcc Green: hcp Gray: other Interior structure changed by sliding movement Interior change is induced by surface change Almost pure fcc after shape transformation Crystal orientation changed

19. Conclusion for Gold Nanorods Continuous heating found planar defects and shorter and wider intermediate state corresponding to experimental results. Quasi-equilibrium heating is qualitatively equivalent to very slow continuous heating. Shape transformation is induced by the surface energy minimization, and initiated by the roughening of the initial {110} facets at T~400K. The intermediate rod has very large {111} surface area. Internal structure changed from one pure fcc to another pure fcc with the crystal orientation changed. This change is accomplished by first sliding {111} plane from their fcc positions to form the hcp local structure, then sliding {111} plane along another direction to come back to fcc local structure. As gold Ih clusters, thermal stability is achieved by the surface minimization.

20. Future Work Check the hysteresis and the freezing mechanism of gold Ih cluster. Simulations with bigger sizes to determine the upper limit of the size when the Ih structure is perferred. Study the aggregation of gold nanoclusters and their binding mechanism to organic molecules. Simulate much bigger nanorods. Check the equilibrium properties of the intermediate state. Study other experimental nanorods to draw a more common shape transformation mechanism. Simulations with more complicated experimental conditions.