Molecular Dynamics Simulations of Gold Molecular Dynamics Simulations of Gold NanomaterialsNanomaterials

Yanting Wang

Dept. Physics and Astronomy University of Rochester

Ph.D. Defense

Supervised by

Prof. Stephen L. Teitel

In cooperation with

Prof. Christoph DellagoInstitute for Experimental Physics

University of Vienna

August 09, 2004

Outline of This Outline of This TalkTalk

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 NanomaterialsApplications of Gold Nanomaterials

Molecular electronicsIon detection

S. O. Obare et al., Langmuir 18, 10407 (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 etchingGold nanowiresLarger Au particles change color

Thermal Stability and Melting Behavior of Thermal Stability and Melting Behavior of Gold NanomaterialsGold Nanomaterials

Melting Tm vs. size

Thousands of atoms

N<1000, energy barrier between different structures is small

Liquid Which nanocrystal structure?

LiquidNanocrystal

How ?

We focus on thousands of atoms, showing results for N=2624 (d ~ 4nm)

How ?

LiquidNanorod

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 SurfacesSlab-like Gold Surfaces

T

T=0

Relaxation

Reconstruction

Deconstruction

Roughening at T=TR

(“solid disordering”)

Wetting (surface premelting)

Bulk melting at T=Tm

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 Tm

Gold {111} surface is always energetically preferred!

Bulk gold with FCC structure and Tm=1337K

Typical Structures of Gold NanoclustersTypical Structures of Gold NanoclustersEnergetic 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

{111} facets

{111} facets

Mostly covered by gold {111} surface

Small total surface areaExtra strain or grain boundary

energy inside pure FCC boday

Cooling and Heating of Mackay IcosahedronCooling and Heating of Mackay Icosahedron

Empirical glue potential modelConstant 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 meltPotential energy vs. T

Keep T constant for 43 nsT=1075K for N=2624Magic and non-magic numbers

Same as left with 3 layers peeled away

SurfaceBulk

Structural Change of Gold Ih Cluster N=2624Structural Change of Gold Ih Cluster N=2624

Interior keeps ordered up to melting Tm

Surface softens but does not melt below melting Tm

Bond order parameters to quantify the structural changeAll have vanishing values for liquid state

Q6(T) / Q6(T=400K)

Atomic Diffusion of Ih ClusterAtomic Diffusion of Ih ClusterMean squared displacements (average diffusion)

All surface atoms diffuse just below melting

Interlayer Diffusion

Number of moved atoms

Surface premelting?

Surface Atom Movements and Average Surface Atom Movements and Average Shapes of Gold Ih ClusterShapes of Gold Ih Cluster

t=1.075ns

4t

Movement

Movement

Average shape

Vertex and edge atoms diffuse increasingly with TFacets shrink but do not vanish below Tm

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 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.

Conclusions for Gold NanoclustersConclusions for Gold Nanoclusters

Continuous Heating of Gold NanorodsContinuous Heating of Gold Nanorods

Shape transformationEnergy changeT 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 rodInternal 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 HeatingCross Sections from the Continuous Heating

Sliding movementSurface disorder and reorderCrystal orientation changedExperiments: planar defects, shorter

and wider intermediate state

Yellow: fccGreen: hcpGray: other

More from Continuous HeatingMore from Continuous Heating

Ts and Tm 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 NanorodsQuasi-Equilibrium Heating of Gold NanorodsHeat up temperature by temperature with T=100K, 43 ns at each TBetter 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: fccGreen: hcpGray: other{100} plane

{111} plane

Surface Change from Quasi-Equilibrium HeatingSurface Change from Quasi-Equilibrium Heating

Surface Second sub layer Average cross-sectional shape

Surface curvature distribution

Surface disorder and reorderSurface roughens at T~400K{111} facets formed after rougheningLarge {111} surface area

Yellow: {111}Green: {100}Red: {110}Gray: other

Cross Sections from Quasi-Equilibrium HeatingCross Sections from Quasi-Equilibrium Heating

Interior structure changed by sliding movementInterior change is induced by surface changeAlmost pure fcc after shape transformationCrystal orientation changed

Yellow: fccGreen: hcpGray: other

Conclusion for Gold NanorodsConclusion 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 WorkFuture 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.