Molecular Dynamics Study on DepMolecular Dynamics Study on Deposition Behaviors of Au Nanoclustosition Behaviors of Au Nanocluster on Substrates of Different Orieer on Substrates of Different Orie

ntationntation

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, KoreacKorea Research Institute of Science and Technology, Korea

Gas phase nucleation

Transport to Surface

Film formation

Charged clusters of a few

nanometers

Growth unit of film

CHARGED CLUSTER MODEL: Mechanisms of Thin Film GrowthHwang et al. JCG 162(1996) 55, Hwang et al. JCG 198/199(1999) 945

CONFIRMED

?

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.

Confirmation of the 1st Hypothesis of the CCM

0 5 10 15 20 25 30 35 40

0

-10

-20

-30

-40

-50

-60

1.0% 1.5% 3.0% 5.0%

dI/d

m (A

/am

u)

Mass (X1000amu)

Size Distributions of C Clusterswith CH4 Concentration2100oC, 6 Torr, 1 hr

I.D. Jeon et al. JCG, 213(2000) 79.

Small Clusters

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 conditions3. When the clusters are large, cauliflower-shaped microstructures are formed.

Wien Filter

Confirmation of the 2nd hypothesis of the Charged Cluster Model

Objectives of This Work

The direct observation of the cluster deposition dynamics is practically impossible.

Molecular dynamics simulation is used as an alternative of the conventional experimentThe effects of cluster sizes and substrate temperatures on the epitaxial rearrangement of the deposited clusters

Au

O

zyxR ,,

zyxv ,,

Interatomic Potential

Embedded Atom Method: FCC Metals

Time Evolution of Each Atom

Extract ThermodynamicAnd Kinetic Properties of The System

Methodology: Molecular Dynamics

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.

Initiation

Equilibration

Time Evolution

321, 1055, and 1985-atom Au cluster

Au(001), (011), (111) Substrate

300 Cluster & Substrate

240 ps, t = 4 fs

Total Evolution Time = 240~400 ps

Isothermal : Canonical Ensemble

Periodic Boundary Condition: x, y

FCC Crystal and Amorphous Cluster

Calculation Procedures

Deposition of Clusters on (001) Substrate

321-, 1055-, and 1985-atom Cluster300 K Substrate Temperature

B. Pauwels et al., PRB 62(2000) 10383

(001) MgO

619 atomsAu cluster

HRTEMMD Result

<1 0>

<001>

(110)1

35.2o

<010>

<001>

(100)

S.C. Lee et al., JCG( Accepted)

MD simulation is agreed well with experiments, especially for nano-scaled materials due to its deterministic features.

Verification

8 psBefore Deposition

<1 0>

<001>

(110)1

50 ps

321 Atoms (2 nm), 300 K,Crystalline Cluster

32 ps

Facet

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

<1 0>

<001>

(110)1

200 ps

Before Deposition40 ps

{111} Twin Boundary

150 ps

Collective Motion

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.

1055 Atoms (3.2 nm), 300 KCrystalline Cluster

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.

Deposition of Clusters on Various Substrate

OrientationsAu(001), Au(011), and Au(111) Substrates

<011>

(100)011

<001>

(110)110

<111>

112 110

Substrate Orientation Effect, 300 K

321-atom Clusters321-atom Clusters

(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 mo

tion 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.

1055-atom Clusters1055-atom Clusters

Substrate Orientation Effect, 300 K

<001>

(110)110

<011>

(100)011

<111>

112 110

(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 ex

cept the small fraction of misfit atoms in the upper left part of the cluster.

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.

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.