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

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.

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

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

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

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

Cooling and Heating of Mackay Icosahedron 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 Colored by local curvature Colored by local structure Mackay Icosahedron with a missing central atom Asymmetric facet sizes Cooling from a liquid Surface Bulk Cone algorithm to group atoms into layers Heating to melt Potential energy vs. T Keep T constant for 43 ns T=1075K for N=2624 Magic and non-magic numbers Same as left with 3 layers peeled away

Surface Bulk Structural Change of Gold Ih Cluster N=2624 Interior keeps ordered up to melting T m Surface softens but does not melt below melting T m Bond order parameters to quantify the structural change All have vanishing values for liquid state Q 6 (T) / Q 6 (T=400K)

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

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

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 T m due to the stable gold {111} facets. No seperate faceting transition below T m is suggested, since the surface softening T seems to be size dependent, and atomic diffusion is involved. Surface softening takes place about 200K below T m. “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. Conclusions for Gold Nanoclusters

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

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

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

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

Quasi-Equilibrium Heating of Gold Nanorods 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 Internal structural change Equilivalent to very slow continuous heating Surface at T=0K Yellow: {111} Green: {100} Red: {110} Gray: other Surface at T=900K Crystal orientation T=0K T=900K Yellow: fcc Green: hcp Gray: other {100} plane {111} plane

Surface Change from Quasi-Equilibrium Heating Surface Second sub layerAverage cross-sectional shape 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

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

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.

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.