Molecular Dynamics Simulations Molecular Dynamics Simulations of Gold Nanomaterials 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
20
Embed
Molecular Dynamics Simulations of Gold Nanomaterials
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 - PowerPoint PPT Presentation
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
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.