Computational Nanomaterials and

Nanomechanics LaboratoryRCAS, 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)

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

Parallel-Replica Dynamics Simulation of Ag Nanowire Stretching

Rodrigues et al. PRB (2002)

• Ag <110> nanowire • Nanowire thinned out uniformly

Ohnishi et al. Nature (1998)

0.0 s

0.47 s

1.23 s

1.33 s

1.80 s

2.17 s

<110>

• 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

Parallel-Replica Dynamics Simulation of Ag Nanowire Stretching

Replicate system into M replicas

Dephasing trajectories

Running independenttrajectories

t=0.02msdL=2Å

t=0.04msdL=4Å

t=0.06msdL=6Å

t=0.08msdL=8Å

t=0.14msdL=14Å

t=0, dL=0

t=0.12ms, dL=12Å

Stress Evolution during Homoepitaxial Growth Of Cu (001)

σ f = E fas − a fas

= 0

during homoepitaxy growth, film growthStress should be

However, it is reported that there exists correlations between surface roughness and thin film stress during homoepitaxial growth of Cu

Friesen and Thompson, PRL 2004

Stress Evolution during Homoepitaxial Growth Of Cu (001)

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

Making use of the check board approach to extend the length scale of accelerated MD simulations

• Will develop our own version this year

60°

Deposition fluxShim et al., PRB 2007

Shim et al., PRL 2008; Pao et. al., in preparation

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 σ xx

atomic ,0o σ xxatomic ,60o σ yy

atomic ,60o

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

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

Surface Chemical Reactions during CVD Growth of Graphene

Adsorption energy of a CH4 on Cu(111) is negligible (less than 0.005 eV), but, as will be shown later, once CH4 decomposes on Cu surface, the adsorption energies become much lower

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

Position of CH Ea d s.(eV) Position of CH2 Ea d s. (eV) Position of CH3

Ea d s. (eV)

fcc -5.1492 fcc-top -3.9084 fcc-top -1.4567

hcp -5.0934 fcc-hcp -3.9035 fcc-hcp -1.2592

top -3.3836 hcp-top -3.8771 hcp-top -1.4652

hcp-fcc -3.8807 hcp-fcc -1.2708

Surface Chemical Reactions during CVD Growth of Graphene

Adsorption energy and binding position on Cu(111) surface

CH in fcc positiond(Cu-C) = 1.910 Åd(C-H) = 1.100 Å

CH2 in fcc-top positiond(Cu-C) = 2.000/2.079 Åd(C-H) = 1.104/1.117 Å

CH3 in hcp-top positiond(Cu-C) = 2.239 Åd(C-H) = 1.109 Å

Position of H atom Ea d s. (eV) Position of C atom Ea d s. (eV)

fcc -2.5035 fcc -5.1119

hcp -2.5030 hcp -5.0541

top -0.0736 top -3.1093

Surface Chemical Reactions during CVD Growth of Graphene

Adsorption energy and binding position on Cu(111) surface

add atom adsorbed on Cu(111) fcc position

add atom dist (Å)

H Cu-H 1.746

C Cu-C 1.851

Surface Chemical Reactions during CVD Growth of Graphene

Eact=2.22 eVΔH(CH4(g)→CH3(s)+H(s))= 0.78 eVΔH(CH4(g)→CH3(s)+H(g))= 3.26 eVΔH(CH4(g)→CH3(g)+H(g))= 4.72 eV

d(Cu-C)=2.137 Åd(Cu-H)=1.559 Åd(C-H) = 1.091/1.100 Åd(C.....H)=1.827 Å

Barrier of CH4 decomposition is tremendously lower on Cu surface

Our preliminary results demonstrate:1. Adsorption energies decreases monotonically during CH4CH3CH2CH2. 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

, 5,1C n m C 38.59 GB eV A 112.75 GB eV A

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

Nanoscale Morphology Evolution in Bulk Heterojunction Organic Photovoltaic Cells

PCBM P3HT

Phase separation of PCBM:P3HT from preliminary coarsed-grain MD simulation result