SELF-ASSEMBLY AND DIRECTED-ASSEMBLY OF MULTI-COMPONENT METAL NANOCLUSTERS

Yong Han1, B. Unal2*, D. Jing3**, D.-J. Liu, Pat Thiel2,3, Jim Evans1,4 Departments of Physics & Astronomy1, MSE2, Chemistry3, and Mathematics4

Iowa State University, Ames, Iowa 50011

A. Engstfeld, R.J. Behm (Ulm U), L. Roelofs (Colgate), C.Z. Wang (Ames Lab)…for DA

Funding: National Science Foundation Grant CHE-1111500 Current addresses: *MIT - ChemE; **Syracuse - Physics

Al then Ni on NiAl(110)

Al

Al

Ni

PNAS 108 (2012) 989 J. Chem. Phys. 135 (2011)

SELF-ASSEMBLY OF BIMETALLIC 2D EPITAXIAL NANOCLUSTERS (NC’s)

Aux + Agy on Ag(100), Nix + Aly on NiAl(110)

PREDICTIVE ATOMISTIC-LEVEL MODELING and KMC simulation of far-from-equilibrium growth structure (shapes, composition profiles)

Challenge: precise description of edge diffusion + attachment-detachment for a vast number of local step-edge configurations and compositions

Model validation: check that recover observed behavior for single-component systems

DIRECTED-ASSEMBLY OF BIMETALLIC NC’s ON TEMPLATED SUBSTRATES

Pt + Ru on monolayer graphene supported on Ru(0001)

OUTLINE

ATOMISTIC-LEVEL MODELING & KMC simulation for periodically modulated substrate energetics: spatial locations and compositions

Co then Cu on Ru(0001) 300K Maria Bartelt, Schmid, Evans, Hwang PRL (1998)

BACKGROUND: FAR-FROM-EQUILIBRIUM MULTICOMPONENT NANOSTRUCTURES

Core- Ring nano- structures

Pt then Co on Pt(111) 220K + 250K Brune (2008)

Ag core – Au shell for SERS Kumar et al JPCC 111(2007) 4388

Au core – Ag shell nanotriangles Rai et al Mat Res Bull 42 (2007) 1212

ATOMISTIC PICTURE of EPITAXIAL NANOCLUSTER FORMATION during DEPOSITION DIFFUSION-MEDIATED GROWTH VERSUS SHAPE/STRUCTURE RELAXATION

DEPOSITION

NUCLEATION

GROWTH

EDGE DETACHMENT TERRACE ATTACHMENT

DIFFUSION DIFFUSION

Surface diffusion: Arrhenius form for

hop rates: h = exp[-Eact /(kBT)]

KMC simulation of stochastic atomistic

lattice-gas model

…atoms deposited

at and hop between

discrete array of

epitaxial adsorption

sites with appropriate

rates h / barriers Eact

Non-equilibrium 2D growth shapes vs. 2D Equilibrium Shapes

Reflects inhibited shape

relaxation/equilibration on

time-scale of (unstable)

diffusion-limited growth

Determined from a

2D Wulff construction

based on orientation

dependence of

step (free) energy. Ag/Ag(111) @ 150 K…ISU studies

PRB 71 (2005) 115414; 77 (2008) 033402 Ag/Ag(111) @ >300 K

flux F

Michely & Krug (Springer 2004) Islands. Mounds & Atoms Evans, Thiel & Bartelt (2006) Surface Science Reports 61

Hopping barrier selection: General “Exact” Treatment

ETS

Einit

thermally activated hop

Eact = ETS - Einitial

Einitial = Eads(initial) + ad int(initial-ad)

ETS = Eads(TS) + ad int(TS-ad)

for pairwise int. int summed over atoms on nearby adsorption sites (ad)

but can readily generalize to include trio, quatro, etc. interactions.

initial

ad

int(initial-ad)<0

for attraction

TS

ad

int(TS-ad)<0

for attraction

h = exp(-Eact/kT)

initial

state

final

state

transition

state (TS)

Key ingredient: Determine unconventional adatom interactions int(TS-ad) with one atom

at TS as well as conventional interactions int(initial-ad) with both atoms at adsorption sites.

PRL 108 (2012) 216102; PNAS 108 (2011) 989;

PRB 84 (2011); JCP 135 (2011) - ISU NSF project

Au and Ag on Ag(100): “simple” system

Factors controlling nanocluster growth & relaxation:

Rapid edge diffusion: barrier Ee 0.24 eV well below Ed = 0.43 eV for Ag/Ag(100)

Significant additional barrier for kink rounding: EKESE ~ 0.18 eV for Ag/Ag(100)

Yong Han, Da-Jiang Liu, Selena Russel, Holly Whalen, Pat Thiel & JE, in preparation; Yong Han, DJL & JE, in preparation

initial

kink rounding edge diffusion edge diffusion corner rounding

Ag

Initial TS TS

Parameters: Ag-Ag = 4+4; Au-Au = 4+4; Ag-Au = 10+10 chosen to recover “exact” DFT adlayer thermodynamics (e.g., alloy mixing) and exact kinetics for key edge processes

Ag

Ag Au

Ed

Growth shapes (deposition studies

~0.1 ML in ~10 sec.)

TRANSITION from square to irregular 2D island growth shapes below ~200 K

25 x 25 nm2

equilibrium

shape

near-square (2D Ising model)

…like growth shape

Post-deposition relaxation:

coalescence / sintering @ 295K

PRB 66 (2002) 155435 ISU + Ulm collaboration

Ag on Ag(100): data from STM experiments

PRL 81 (1998) 2950 Stoldt et al ISU PRL 86 (2001) 3088 – ISU + ORNL collaboration

PRB 66 (2002) 165407 – Liu & JE J Chem Phys C 113 (2009) 5047 Centennial Feature

(a) 0 min. (b) 180 min.

(a) 0 min. (b) 160 min.

islands

pits

50 x 50 nm2

50 x 50 nm2

180 K 200 K 225 K

13 x 13 nm2

Ag on Ag(100): KMC of atomistic LG model

00002 (0.0185361 s) 00100 (0.4921492 s) 00300 (1.5906460 s) 00600 (3.5459846 s)

Deposition of 0.1 ML with flux F = 0.006 ML/s

Restructuring @ 295 K of two 20x20 atom ISLANDS meeting corner-to-corner

Strong Kink ES barrier relaxation time ~ L3 for linear size L < LKESE vs. continuum theory ~ L4

First, 0.1 ML Ag deposition at 280 K

Then, 0.1 ML Au deposition

12 nm x 12 nm

12 nm x 12 nm

12 nm x 12 nm

250 K

235 K

Au + Ag on Ag(100)

MULTICOMPONENT NANOSTRUCTURES OF NixAly ON NiAl(110)

1:1 Ni:Al stoichiometric co-deposition: alloy self-growth (fundamentals)

3:1 Ni:Al codeposition creating new class if Ni3Al surface adlayers

Ni and Al on NiAl(110): 2 distinct ads. sites (Ni-br, Al-br) Distinct diffusion paths for Ni & Al

Central Challenge is the reliable treatment of the formation of non-equilibrium nanostructures:

Precisely describe edge diffusion and attachment-detachment for a vast number of local step-edge configurations and compositions (for multi-component systems)

T. Duguet, Y. Han, C. Yuen, D. Jing, B. Unal, J.W. Evans, and P.A. Thiel, Proc. National Academy Sciences 108 (2011) 989 Han et al., J. Chem. Phys. 135 (2011) 084706; PRB 84 (2011) 113414; MRS Proc. Vol. 1318 (2011); MRS Proc. (2012)

Al-br

Ni-br

DFT-GUIDED SELECTION OF KEY ENERGETIC INPUT PARAMETER FOR ATOMISTIC LG MODEL

16 “CONVENTIONAL” ADATOM INTERACTIONS (Ni-Ni, Al-Al, Ni-Al) with both adatoms on adsorption sites (determine thermodynamics)

24 “UNCONVENTIONAL” ADATOM INTERACTIONS (Ni-Ni, Al-Al, Ni-Al) with one adatom at TS and another at ads. sites (determines kinetics) PRB (2011)

MRS Proc. (2012)

Ni on NiAl(110): Island Shapes at 300-500 K (KMC vs. Expt.)

Model-aided analysis of behavior:

Model allows assessment of edge diffusion

(faster on diagonal vs. horiz./vert. edges)

and of a key anisotropy in corner rounding

(easier from diagonal to top + bottom edges)

Han et al. J Chem Phys 135 (2011) 084706

Xc & Yc = horizontal & vertical caliper lengths; R = Yc/Xc = Aspect Ratio

At 400 & 500K, fit to distorted octagons: Xav=(X1+X2)/2, Yav=…, Dav=…

Ni on NiAl(110): quantitative analysis of island shapes

EXPT: Aspect Ratio R = 1.43 ± 0.05 @ 300K R= 1.02 ± 0.03 @ 400 & 500 K KMC: Aspect Ratio R 1.2 @ 300K R 0.9 @ 400K and 500K

EXPT: Yav/Dav 0.9, Xav/Dav 0.4 @ 400K Yav/Dav 1.2, Xav/Dav 0.6 @ 500K KMC: Yav/Dav 0.8, Xav/Dav 0.4 @ 400K similar to experiment

Equil. Shape Analysis requires modified Wulff construction

…given 2 distinct ads. sites & energies …changing shape at const. size changes relative populations

D : Y : X = 8 : 3 : 1

J. Chem. Phys. (2011)

Al on NiAl(110): Island Shapes at 300 K (Expt vs. KMC)

MRS Proc. 1318 (2011) uu02-07

Han et al. PRB 84 (2011) 113414 STM

KMC

Ni

Al

D

A B C

FE

Al then Ni

Ni then Al

STM STM KMC

300 K deposition with F ~ 10-2 ML/s Images: 25x25 nm2

Duguet et al., PNAS 108 (2011) 989

SEQUENTIAL CO-DEPOSITION OF Ni & Al ON NiAl(110): STM VS KMC

Equilibrium alloy

island structure +

octagonal shape

= Ni

= Al

SEQUENTIAL CO-DEPOSITION OF Ni & Al ON NiAl(110): STM VS KMC

Duguet et al., PNAS 108 (2011) 989

Al-core Ni-ring islands:

Al core is robust against

extraction of Al aided

by peripheral Ni

Ni-core Al-ring islands:

Ni core is susceptible

to extraction of Ni

aided by peripheral Al

Sequential co-deposition of Ni then Al on NiAl(110)

400K: limited intermixing with poor Ni3Al & NiAl ordering

Ni : Al = 1 : 1/3 Ni : Al = 1 : 1

500K: intermixing with improved Ni3Al & Ni Al ordering

Ni : Al = 1 : 0

Han et al PRL 108 (2012) 216102

SIMULTANEOUS 1:1 STOICHIOMETRIC CODEPOSITION OF Ni & Al ON NiAl(110)

300K

400 K 500 K 600 K

Alloy order poor at 300K but impoves with T …almost perfect at >500K (cf. Tm = 1910 K)

300 K

Perfect Ni3Al overlayer by

codeposition on NiAl(110)

…structure is different

from that of any layer

in bulk Ni3Al !

500 K

400 K

600 K

SIMULTANEOUS 3:1 CODEPOSITION OF Ni & Al ON NiAl(110): Ni3Al ISLANDS

total

Ni3Al/NiAl(110) Ni3Al(111)

Han, Unal & JE PRL 108 (2012) 216102

Ni-Ag core-ring nanostructures on NiAl(110): catalysis apps.

12 nm

Sequential deposition: Ni then Ag on NiAl(110) @ 300 K J Chem Phys 135 (2011) 084706

Ag(110) bilayer ring Ni(100) R45-like monolayer core

Ni = inexpensive catalyst for steam-reforming methane+H2O CO+H2 (syn-gas) chemicals Decorating Ni steps with Ag or Au prevents coking Besenbacher… Science (98); Nature (05); Rostrup-Nielsen…, J Cat (09)

Ni-Ag CORE-RING

CALDERA Bilayer Ag on NiAl(110)

Han et al. PRB 81 (2010) 115462 PRL 100 (2008) 116105

Unal et al. PRB76 (2007) 195410

300 K

250 K 0.3 ML Ag on Ag(100)

No control over nanocluster formation For homogeneous nucleation & growth by deposition on crystalline surfaces…

DIRECTED-ASSEMBLY OF NANOCLUSTERS ON Ru(0001)-SUPPORTED GRAPHENE: STM studies and atomistic modeling of formation kinetics & ordering

25 x 50 nm2

STM: Ru NC’s on MLG/Ru(0001) Ru = 0.05 ML, FF = 34% Schematic: NC’s in fcc regions

fcc

hcp

graphene moire cell

2.98 nm

Engstfeld, Hoster, Behm, Roelofs, Liu, Wang, Han, and Evans PRB 86 (2012) 085442 Editor’s Suggestion

ML Graphene/Ru(0001)

Wang, Bocquet, Marchini, Gunther, Wintterlin, PCCP 10, 3530 (2008)

Lattice-mismatch between graphene and the supporting transition metal results in a periodically-modulated moire structure of the graphene sheet with a parallelogram moire unit cell (see schematics)

Miranda group STM PRL 100, 056807 (2008)

STM: Ru NC on MLG/Ru(0001) (c) 0.005 ML FF=13.5%; (d) 0.01 ML, FF = 17.5% (e) 0.03 ML, FF=26.1%; (f) 0.05 ML, FF=34.5% (g) 0.15 ML, FF=48.1% most images: 100x100 nm2

MODELING: PES FOR AN ADATOM ON MLG/Ru(0001)

Niu, Vardavas, Caflisch, Ratsch PRB 74 (2006) Saum, Schulze, Ratsch, CommCompPhys 6 (2009) Modeling strategy:

Develop atomistic lattice-gas model including:

(i) … random deposition of Ru adatoms; (ii) …diffusion by hopping between neighboring adsorption sites on the graphene sheet in a periodically modulated potential (above); (iii) …irreversible NC nucleation (when two diffusing adatoms meet) and irrev. aggregation

POINT-ISLAND MODEL analysis by KMC simulation.

Detailed 2D form based on (12x12)C/(11x11)Ru model

Parameters matching expt:

Ed = 0.62, =0.28, =0.20, *=0.15

or Ed = 0.58, = 0.40, * = 0.00

DFT for Ru free-standing graphene:

Ishii et al Ed=0.72 (2008), 0.96 (2011) …not consistent with experiment Ed = 0.62 eV (our work – CZ Wang) match experiment

COMPARISON OF KMC AND EXPT: NC Filling Factor (FF)

Filling Factor = % occupied moire cells

Average NC size (in atoms)

Short-range-order (SRO) and motifs (e.g., line formation) within NC array

STM KMC simulation

Analyze and compare: (i) Standard short-range-order parameter for pairs of NC’s with various separations r (ii) Populations of NN dimers (D), and of linear (LT), bent (BT) and triangular (TT) trimers (iii) “line formation” within the NC array

COMPARISON OF KMC AND EXPT: SRO and motifs for NC

SRO: (r) [probability of a pair of NC’s separated by r] – [probability of an NC]2

>0: clustering <0: anti-clustering

COMPARISON OF KMC AND EXPT: NC Short-Range-Order

EXPT KMC

COMPARISON OF KMC AND EXPT: D, LT, BT, TT populations

D = LT = BT = TT = (FF)2 D2/FF D2/FF (D/FF)3

random approx Kirkwood approximation

…but D is slightly below (FF)2 reflecting anti-clustering

STM: 0.05 ML Pt then 0.06 ML Ru on MLG/Ru(0001)

FF = 21%

FF = 41%

some intermixing possible

70 x 70 nm2 0.05 ML Pt + 0.06 ML Ru

Y. Han, A. Engstfeld, R.J. Behm, J.W. Evans, J Chem Phys subm. (2012)

KMC: 0.05 ML Pt then 0.06 ML Ru on MLG/Ru(0001)

0.05 ML Pt + 0.06 ML Ru

0

10

20

30

40

Counts [a.u.]N

um

ber

of

Ru

ato

ms

in

mix

ed

NC

s

20 40 60 80 100 120

Pt

All

Pure Ru

Mixed

Co

un

ts [

a.u

.]Number of atoms

+ Ru

Ru

Ru

Pt

PtKMC: 0.05 ML Pt

then 0.06 ML Ru

on MLG/Ru(0001)

Distribution of Ru atoms in mixed islands reflects Capture Zone distribution for mixed islands = Generalized Gamma Distr. Li, Han, Evans PRL (2010)

0.05 ML Pt then 0.06 ML Ru on MLG/Ru(0001)

Pt: S 9, 38, 70 for h = 2,3,4 Ru: S 7, 25, 60 for h=2,3,4 Ru@Pt: S 9, 34, 46,7 4 for h=2,3,4,5

STM: 0.05 ML Ru then 0.06 ML Pt on MLG/Ru(0001)

FF = 30%

FF = 34%

no intermixing expected

70 x 70 nm2 0.05 ML Ru + 0.06 ML Pt

Y. Han, A. Engstfeld, R.J. Behm, J.W. Evans, J Chem Phys subm. (2012)

KMC: 0.05 ML Ru then 0.06 ML Pt on MLG/Ru(0001)

0.05 ML Ru + 0.06 ML Pt

0

10

20

30

40

50

N

um

ber

of

Pt

ato

ms

in

mix

ed

NC

s

Counts [a.u.]

KMC: 0.05 ML Ru

then 0.06 ML Pt

on MLG/Ru(0001)

Distribution of Ru atoms in mixed islands reflects Capture Zone distribution for mixed islands = Generalized Gamma Distr. Li, Han, Evans PRL (2010)

0.05 ML Ru then 0.06 ML Pt on MLG/Ru(0001)

Ru: S 7, 25, 60 for h=2,3,4 Pt: S 9, 38, 70 for h = 2,3,4 Pt@Ru: S 7,20,42,56,77 for h=2,3,4,5,6

SUMMARY

2D bimetallic epitaxial metal NC: co-dep.n at lower T rich variety of far-from-equilibrium nanostructures

Multi-site LG modeling with precise diffusion barriers

Energetic input (ads energies, conventional adspecies int. and unconventional int. with an adatom at TS)

Bimetallic metal NC on ML graphene on Ru(0001)

Atomistic LG modeling incorporating periodically modulated energetics for interaction with substrate

Point-island models capture spatial distribution, etc.

Yong Han