1. Molecular Dynamics Study on Deposition Behaviors of Au Nanocluster on Substrates of Different Orientation
S.-C. Leea, K.-R. Leea, K.-H. Leea, J.-G. Leea, N.M. Hwangb,c
aSupercomputational Materials Simulation Lab.,KIST, Korea
bCenter for Microstructure Science of Materials, SNU, Korea
cKorea Research Institute of Science and Technology, Korea

2. Charged clusters of a few nanometers
CHARGED CLUSTER MODEL: Mechanisms of Thin Film Growth
Hwang et al. JCG 162(1996) 55, Hwang et al. JCG 198/199(1999) 945
Charged clusters of a few nanometers
Gas phase nucleation
CONFIRMED
Transport to Surface
Growth unit of film ?
Film formation
The spontaneous formation of charged clusters in the gas phase in a typical CVD process has been confirmed by many systems: CVD diamond, Si, ZrO2, thermal evaporation of Au and Cu.

3. Confirmation of the 1st Hypothesis of the CCM
Wien Filter
Small Clusters
Size Distributions of C Clusters
with CH4 Concentration
2100oC, 6 Torr, 1 hr
I.D. Jeon et al. JCG, 213(2000) 79.
Large Clusters
1. The charged clusters are REALLY EXIST in a typical CVD process!!
2. The size distribution of the charged clusters can be changed with
the operating conditions
3. When the clusters are large, cauliflower-shaped microstructures are formed.

4. Confirmation of the 2nd hypothesis of the Charged Cluster Model
Objectives of This Work
Confirmation of the 2nd hypothesis of the Charged Cluster Model
The direct observation of the cluster deposition dynamics is practically impossible.
Molecular dynamics simulation is used as an alternative of the conventional experiment
The effects of cluster sizes and substrate temperatures on the
epitaxial rearrangement of the deposited clusters

5. Interatomic Potential Embedded Atom Method: FCC Metals
Methodology:
Molecular Dynamics Au O
Time Evolution of Each Atom
Extract Thermodynamic
And Kinetic Properties of
The System
Interatomic Potential
Embedded Atom Method: FCC Metals
Molecular dynamics simulation is deterministic when the interatomic potential is accurate.
The only input parameters are position, velocity of atoms in the system. Time evolution of of each atom is collected and manipulated by statistical methods.

6. Calculation Procedures
Initiation
321, 1055, and 1985-atom Au cluster
Au(001), (011), (111) Substrate
FCC Crystal and Amorphous Cluster
Equilibration
300 Cluster & Substrate
240 ps, t = 4 fs
Time Evolution
Total Evolution Time = 240~400 ps
Isothermal : Canonical Ensemble
Periodic Boundary Condition: x, y

7. Deposition of Clusters on (001) Substrate 300 K Substrate Temperature
321-, 1055-, and 1985-atom Cluster
300 K Substrate Temperature

8. S.C. Lee et al., JCG( Accepted)
Verification
MD Result
HRTEM
<1 0>
<001>
(110)
35.2o
619 atoms
Au cluster
(001) MgO
<010>
<001>
(100)
S.C. Lee et al., JCG( Accepted)
B. Pauwels et al., PRB 62(2000) 10383
MD simulation is agreed well with experiments, especially for nano-scaled materials due to its deterministic features.

9. 321 Atoms (2 nm), 300 K,Crystalline Cluster
<1 0>
<001>
(110)
Facet
Before Deposition
8 ps
32 ps
50 ps
When clusters are small in size, the deposited clusters shows a epitaxial rearrangement with the substrate at the early stage (50 ps) of deposition. The fast epitaxial rearrangement of the small cluster is due to the collective rotation of the cluster atoms, which was not observed in large clusters

10. 1055 Atoms (3.2 nm), 300 KCrystalline Cluster
Collective Motion
<1 0>
<001>
(110)
40 ps
150 ps
Before Deposition
{111} Twin Boundary
200 ps
When cluster sizes are become larger, the deposited clusters didn’t show epitaxial rearrangement with the substrate and {111} twin boundary was evolved, which degraded the degree of epitaxy. The {111} twin was fixed by the (100) facet near the cluster-substrate interface.

11. Structure Factors at 300 K after 320 ps
As predicted by the Charged Cluster Model, the cluster can be deposited epitaxially on the substrate by the collective motion of the cluster atoms when the cluster sizes are small. As the size of the cluster increases, the degree of epitaxy is made worse. In the case of 1985-atom cluster, most of the atoms didn’t epitaxial rearrangement with the substrate.

12. Deposition of Clusters on Various Substrate Orientations
Au(001), Au(011), and Au(111) Substrates

13. Substrate Orientation Effect, 300 K
321-atom Clusters
<011>
(100)
<001>
(110)
<111>
(011) Surface
(001) Surface
(111) Surface
The epitaxial rearrangement was completed after 25 ps of deposition for the three orientations.
All the deposited clusters show a good epitaxial relation with the substrate.
The fast epitaxial rearrangement of the 321-atom cluster was attributed to the collective motion of the cluster atoms, which was revealed by analyzing the snapshots of atomic configuration with time.
The mechanism of epitaxial rearrangement of the cluster is not by the interaction with the substrate but by the collective motion of the cluster atoms.

14. Substrate Orientation Effect, 300 K
1055-atom Clusters
<011>
(100)
<001>
(110)
<111>
(011) Surface
(001) Surface
(111) Surface
The shapes of the 1055-atom cluster were drastically changed with the substrate orientation.
The effects of the substrate were the smallest on the (011) substrate compared to other orientations.
Though the surface energy of the (011) substrate was the highest among three low index surfaces, the epitaxial rearrangement of the cluster atoms was observed to be the worst.
In the case of (001) surface, the cluster evolves to nanograins with a 3 twin boundary.
Almost all the atoms in the cluster showed the epitaxial relation with the (111) substrate except the small fraction of misfit atoms in the upper left part of the cluster.

15. The Effects on the Substrate Orientation on the Degree of Epitaxy at 300 K
In the case of 321-atom cluster, all the clusters had high values of the degree of epitaxy.
When the 1055-atom clusters were deposited on the various substrates, the degree of epitaxy was drastically diverged.
The (111) substrate showed the highest value of the degree of epitaxy, the (001) substrate showed the next and the (011) substrate showed the lowest value of the degree of epitaxy, as had been predicted from the previous snapshots.
The (111) orientation is concluded to be the most favorable substrate in epitaxial rearrangement of the cluster.

16. Conclusion
Molecular dynamics simulations on the deposition behaviors with three different cluster sizes and substrate orientations were conducted.
For a small 321-atom cluster, the epitaxial rearrangement of the cluster atoms could be achieved even at 300 K, irrespective of the substrate orientation by the collective motion of the cluster.
For larger 1055-atom and 1985-atom clusters, the substrate orientation drastically changed the morphology of the deposited cluster.
On a (011) substrate, no epitaxy was observed except the region where the cluster and the substrate were in contact.
When the substrate was replaced by the high symmetry orientation, the degree of epitaxy was improved. In the case of (111) substrate, the 1985-atom cluster still had a high value of the degree of epitaxy.