1. Computational Nanomaterials and Nanomechanics Laboratory RCAS, Academia Sinica
Members:
Dr. Chun-Wei Pao (PI)
Dr. Grzegorz Gajewski (Post-doc)
Dr. Cheng-Kuang Lee (Academia Sinica Post-doc, will join this August)
Mr. Shih-Di Chen (Ph.D. candidate, co-advise with Prof. Chien-Cheng Chang)
Mr. Te-Huan Liu (Ph.D. candidate, co-advise with Prof. Chien-Cheng Chang)

2. Research Activities from July 2009 to Present
Atomistic simulations of tensile loading of Ag nanowires with strain rates close to experiments (manuscript in preparation)
Research conducted from July 2009 to September 2009 at Los Alamos National Lab., before joining RCAS
First atomistic simulation that can simulate nanoscale plasticity with atomistic resolution and experimental time scale
One contributed talk during TMS 2010 in Seattle, and one invited presentation in World Congress in Computational Mechanics in Sydney, Australia this coming July
Stress evolution during homoepitaxial growth of Cu with deposition rate close to experiments (manuscript in preparation)
Research conducted after joining RCAS, collaborating with Los Alamos and Univ. of Toledo
Demonstrate substantial compressive thin film growth stress even during homoepitaxial growth
Two invited seminars at National Taiwan Univ. and National Sun Yat-Sen Univ. this year

3. Parallel-Replica Dynamics Simulation of Ag Nanowire Stretching
Ohnishi et al. Nature (1998)
0.0 s
0.47 s
1.23 s
1.33 s
1.80 s
2.17 s
<110>
Rodrigues et al. PRB (2002)
Ag <110> nanowire
Nanowire thinned out uniformly
Au nanocontact formed by putting tip and sample together
# of rows of Au atoms reduced from 5 to 1
Performed massive parallel-replica dynamics simulations on Roadrunner supercomputer in Los Alamos National Lab. to simulate nanowire stretching processes with strain rates close to experiments

4. Parallel-Replica Dynamics Simulation of Ag Nanowire Stretching
t=0, dL=0
Replicate system
into M replicas
Dephasing
trajectories
Running independent
trajectories
t=0.02ms
dL=2Å
t=0.04ms
dL=4Å
t=0.06ms
dL=6Å
t=0.08ms
dL=8Å
t=0.14ms
dL=14Å
t=0.12ms, dL=12Å

5. Stress Evolution during Homoepitaxial Growth Of Cu (001)
during homoepitaxy growth, film growth
Stress should be
Friesen and Thompson, PRL 2004
However, it is reported that there exists correlations between surface roughness and thin film stress during homoepitaxial growth of Cu

6. Stress Evolution during Homoepitaxial Growth Of Cu (001)
Making use of the check board approach to extend the length scale of accelerated MD simulations
Will develop our own version this year
Shim et al., PRB 2007
Deposition flux
Run large-scale temperature accelerated dynamics simulations to simulate Cu homoepitaxial growth with deposition rate a million times slower than that in a typical direct MD simulation and monitor film stress evolution
60°
Shim et al., PRL 2008; Pao et. al., in preparation

7. Stress Evolution during Homoepitaxial Growth Of Cu (001)
Asymmetrical film stress during 60° deposition simulation
Distribution of atomic virial stress shows relaxation of surface stress fxx at free surface in 60° deposition simulation

8. Stress Evolution during Homoepitaxial Growth Of Cu (001)
Since there is no absolute starting point for homoepitaxy, the film stress-thickness can be expressed as
Since homoepitaxy, no mismatch stress
Compute changes in surface stresses during growth and compare with simulations results and obtain excellent agreements

9. Ongoing and Future Research Projects
Surface chemical reactions during CVD growth of graphene on Cu substrate
Dr. Grzegorz Gajewski
Graphene microstructures evolution
Mr. Te-Huan Liu
Nanoscale morphology evolution in the active layer materials of bulk heterojunction organic photovoltaic cells (NSC funded)
Dr. Cheng-Kuang Lee
Morphology evolution during annealing of C60 film
Mr. Shih-Di Chen
Development of accelerated MD codes for semi-empirical force fields and ab initio MD

10. Surface Chemical Reactions during CVD Growth of Graphene
It is possible to fabricate large area of few-layer graphene by methane decomposition on Cu surface (Cu acts as catalysis)
However, the surface chemical reaction pathways are not yet clear. Therefore, we are performing a series of ab initio calculations to study the surface chemical reactions
Li et al., Science 2009
Li et al., Science 2009
Adsorption energy of a CH4 on Cu(111) is negligible (less than eV), but, as will be shown later, once CH4 decomposes on Cu surface, the adsorption energies become much lower

11. Surface Chemical Reactions during CVD Growth of Graphene
Adsorption energy and binding position on Cu(111) surface
Position of CH
Ea d s.(eV)
Position of CH2
Ea d s. (eV)
Position of CH3
fcc
fcc-top
hcp
fcc-hcp
top
hcp-top
hcp-fcc
CH2 in fcc-top position
d(Cu-C) = 2.000/2.079 Å
d(C-H) = 1.104/1.117 Å
CH in fcc position
d(Cu-C) = Å
d(C-H) = Å
CH3 in hcp-top position
d(Cu-C) = Å
d(C-H) = Å

12. Surface Chemical Reactions during CVD Growth of Graphene
Adsorption energy and binding position on Cu(111) surface
Position of H atom
Ea d s. (eV)
Position of C atom
fcc
hcp
top
add atom adsorbed on Cu(111) fcc position
add atom
dist (Å) H
Cu-H 1.746 C
Cu-C 1.851

13. Surface Chemical Reactions during CVD Growth of Graphene
d(Cu-C)=2.137 Å
d(Cu-H)=1.559 Å
d(C-H) = 1.091/1.100 Å
d(C.....H)=1.827 Å
Eact=2.22 eV
ΔH(CH4(g)→CH3(s)+H(s))= 0.78 eV
ΔH(CH4(g)→CH3(s)+H(g))= 3.26 eV
ΔH(CH4(g)→CH3(g)+H(g))= 4.72 eV
Barrier of CH4 decomposition is tremendously lower on Cu surface
Our preliminary results demonstrate:
Adsorption energies decreases monotonically during CH4CH3CH2CH
CH4 decomposition barrier is much lower on Cu surface than in the gas phase
We will continue computing all the relevant transition state calculations and studying the reason why self-limiting growth happens

14. Grain Boundaries in Graphene
Graphene domain boundaries can be used as metallic wire
What about transport properties of other types of domain boundaries??
We plan to study the structures and transport properties of various domain boundaries in graphene, and study the migration mechanisms of these boundaries using accelerated MD in the future

15. Nanoscale Morphology Evolution in Bulk Heterojunction Organic Photovoltaic Cells
Phase separation of PCBM:P3HT from preliminary coarsed-grain MD simulation result
PCBM
P3HT