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Seed induced growth of binary Ag/Au nanostructures on a graphite surface

Applied Physics Letters

October 2008
93(12):123107 - 123107-3

DOI:10.1063/1.2988188

Source
IEEE Xplore

Authors:

Nathalie Lidgi-Guigui

Université Sorbonne Paris Nord

Paul A. Mulheran

University of Strathclyde

Richard Edward Palmer

Swansea University/Nanjing University

The growth of Ag on a graphite surface decorated by size selected Au “seed” nanoclusters is investigated. Compared with the behavior on bare graphite, the deposition of the Au clusters decreases the lateral diffusion of Ag atoms and enables the growth of Ag/Au nanostructures on/around the top of the initial Au clusters. Depending on the Au cluster shape, which can be tuned by the cluster deposition energy, Ag deposition either leads to 2 ML high platelets or three-dimensional nanoclusters. This cluster seeding technique shows potential for the rapid production of binary model catalysts, biochips, and optical films.

Color online a AFM image of Au 2000 clusters soft-landed on Ar + sputtered graphite after silver evaporation 3 m 3 m 2. b Height distribution of the clusters before red and after green, as a 0.01 Ag monolayer evaporation.

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͑ Color online ͒ Cluster mass distributions from three simulations: growth only on the seed particles ͑ black ͒ , nucleation and growth without seeds ͑ blue ͒ , and growth on seeds plus significant nucleation ͑ red ͒ .

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Color online a STM image of Au 250 clusters self-pinned on graphite. b Height distribution of self-pinned Au 250 clusters, as a, after Ag evaporation green and height distribution of Au 250 clusters soft-landed on Ar + sputtered graphite after Ag evaporation red.

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Seed induced growth of binary Ag/Au nanostructures on a graphite surface

N. Lidgi-Guigui,1,a兲P. Mulheran,2and R. E. Palmer1

1Nanoscale Physics Research Laboratory, School of Physics and Astronomy, University of Birmingham,

Birmingham B15 2TT, United Kingdom

2Department of Chemical and Process Engineering, University of Strathclyde, James Weir Building,

Glasgow, G1 IXJ, United Kingdom

共Received 2 June 2008; accepted 27 August 2008; published online 23 September 2008兲

The growth of Ag on a graphite surface decorated by size selected Au “seed” nanoclusters is

investigated. Compared with the behavior on bare graphite, the deposition of the Au clusters

decreases the lateral diffusion of Ag atoms and enables the growth of Ag/Au nanostructures on/

around the top of the initial Au clusters. Depending on the Au cluster shape, which can be tuned by

the cluster deposition energy, Ag deposition either leads to 2 ML high platelets or three-dimensional

nanoclusters. This cluster seeding technique shows potential for the rapid production of binary

关DOI: 10.1063/1.2988188兴

Noble metal nanostructures are the focus of growing in-

terest for several reasons. The main one is that these objects

combine the physics emerging at the nanoscale with the in-

trinsically high stability of their constituent element. This

explains why the use of such nanostructures is reported in an

increasing number of publications centered on very different

topics, such as biomedical imaging,1,2surface enhanced Ra-

man scattering,3–5or catalysis.6–8Among noble metal nano-

structures, nanoparticles made from Au and Ag are of par-

ticular interest since their localized surface plasmon

resonance peaks lie in the visible or near UV regions. Com-

binations of these two elements show even more diverse

properties. As one example, bimetallic particles have proved

to be more efﬁcient in catalysis than their individual

elements.9–12 Bimetallic nanoclusters can be synthesized by a

variety of chemical methods.13–15 however, some physical

approaches also exist, mainly laser ablation from an alloy

target.16,17 In this latter approach, the clusters generated most

often are nanoalloys. In the chemical approach, the concept

of seeding is often exploited,18–20 e.g., Au colloids are syn-

thesized in solution with a shell of ligand molecules having

afﬁnity to Ag+; after a ﬁnal reduction step, the molecular

shell is removed leaving a AucoreAgshell structure. There is a

recent report of seeding in the context of a physical method,

but Cazayous et al.21 did not employ size selected clusters

nor investigate the underlying growth mechanisms.

Here, we investigate the evaporation of Ag atoms on a

graphite surface predecorated by the deposition of size se-

lected Au nanoclusters, with the aim of growing bimetallic

nanostructures on the surface. The model graphite 共highly

oriented pyrolytic graphite兲surface is chosen because the

defect density is very low and so the probability of growing

Ag only clusters by nucleation at natural defects is reduced

to a minimum.

The size selected Au clusters were produced with a

radio-frequency magnetron sputtering, gas condensation

cluster beam source described previously.22,23 The clusters

were mass selected by a lateral time-of-ﬂight mass selector24

operated with a resolution of ⫾2%independent of the cluster

mass. Two sizes of clusters were used, Au250 and Au2000 共i.e.,

clusters consisting, respectively, of 250 and 2000 atoms of

Au兲. They were deposited on freshly cleaved graphite

samples in one of two ways. The ﬁrst consists of depositing

Au clusters at high energy 共typically 20 eV/atom兲in order to

self-pin the clusters on the surface:25–28 the cluster impact on

the substrate displaces a surface carbon atom from the graph-

ite lattice, and the Au cluster is then trapped by this defect.

The maximum available energy in our setup is 5 keV/cluster,

so as a consequence Au250 is one of the biggest Au clusters

that can be efﬁciently self-pinned. The second technique is

soft-landing the clusters onto an assembly of defects made

by in situ sputtering of the graphite surface with an Ar+beam

at 500 eV.29 In both cases, immediately after the Au cluster

deposition, the nanostructured graphite samples were trans-

ferred into a home made thermal evaporator,30 where Ag was

evaporated. The evaporation rate was calibrated by measur-

ing the thickness of a Ag layer deposited on glass and was

about 1012 atoms/s.

The characterization of the samples was carried out

mainly using an atomic force microscope 共AFM兲共Digital

Instruments Dimension 3100兲equipped with a Nanoscope

IIIa controller. The tips used were standard oxide-sharpened

silicon nitride tips with a spring constant of 0.6 N/m. How-

ever, the Au250 clusters self-pinned on graphite were below

the resolution limits of the AFM. So the corresponding scan-

ning tunneling microscope 共STM兲head was employed in-

stead 共also in ambient conditions兲. Typical imaging param-

eters were a sample bias of +0.4 V and a tunnel current of

+0.4 nA. Mechanically cut PtIr tips were employed. For

both the AFM and STM measurements, the maximum cluster

height was measured, after height calibration on graphite

steps, using the grain analysis module of the SPIP 共Ref. 31兲

image processing program.

Figure 1共a兲shows a tapping mode AFM scan of Ag 共0.01

ML兲evaporated on a submonolayer ﬁlm of immobilized

Au2000 clusters 共with mean spacing of 162 nm兲. The behavior

is different from the case when Ag is evaporated in the ab-

sence of Au clusters. Quite different features are observed32

because the lateral Ag diffusion length is reduced by the

clusters. To minimize the probability of Ag cluster growth on

a兲Present address: Laboratoire d’Electronique Moléculaire, SPEC / IRAMIS

bat 125 CEA, Saclay 91125 Saclay Cedex, France. Electronic mail:

nlidgi@gmail.com.

APPLIED PHYSICS LETTERS 93, 123107 共2008兲

Downloaded 30 Oct 2008 to 166.111.38.226. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

isolated Ar+defects, we have deposited as many Au clusters

as Ar+defects. The height distribution of Au2000 soft-landed

on Ar+sputtered graphite before and after Ag evaporation, as

shown in Fig. 1共b兲, presents a mean height shifted upward by

almost 1 nm after Ag evaporation. No peak is found at the

Au2000 mean height after Ag evaporation.

To understand the nature of the clusters observed after

Ag evaporation, we have performed simulations based on a

point-island model of cluster nucleation and growth during

vapor deposition.33,34 The substrate is represented by a lattice

that contains randomly chosen sites to represent the seed

clusters. Monomers are deposited at random at a given rate.

Free monomer diffusion is simulated by the nearest-neighbor

hopping of free monomers. New clusters are assumed to

nucleate if two or more monomers occupy the same lattice

site. Results are shown in Fig. 2. Ag growth only on the Au

cluster seeds creates a distribution of cluster masses centered

on the average value of 5000 atoms. Growth without seeds

produces a broader asymmetric distribution of sizes centered

on the average size of 3000 atoms, but which extends from 0

to about 6000 atoms.35 The case of growth on the seeds

alongside an equivalent number of nucleation events pro-

duces a bimodal size distribution. If we assume that the clus-

ters are growing in a three-dimensional fashion, then the

cluster height correlates with the cube root of mass and a

bimodal distribution of heights would be observed experi-

mentally if there were signiﬁcant nucleation of Ag only

clusters—but this is not the case. Alternatively, we might

suppose that the clusters grow to a constant height, once they

are large enough. In this case, we need to consider how many

nucleated clusters would be observed in the AFM. As can be

seen from Fig. 2, many of the nucleated clusters in the simu-

lation are of comparable size to the initial Au2000 seeds, and

so would appear in the data. In this case, the density of

clusters observed after Ag deposition would be signiﬁcantly

greater than that found before, and this is also not the case.

We can conclude that the majority of the deposited Ag is

being captured by the Au seeds.

To extend the experimental investigation further the

same experiment was performed using Au250 clusters instead

of Au2000. The results are illustrated by the STM image and

the corresponding height distribution shown in Fig. 3. As for

Au2000, a shift of 1 nm toward larger sizes is observed for

Au250 after Ag evaporation. The growth of Ag on Au seeds is

thus a general feature independent of cluster seed size. As

previously mentioned, Au250 can either be soft-landed on Ar+

sputtered graphite or self-pinned at sufﬁciently high cluster

impact energy 共which also leads to ﬂattening of the clusters兲.

The height of Ag clusters grown on both kinds of samples is

shown in Fig. 3共b兲. 3 ML high Ag platelets 共i.e., 0.4 nm兲are

observed when Ag is deposited onto self-pinned Au clusters.

This contrasts with the, on average, 1.5 nm high clusters

found when Ag is evaporated on the graphite surface deco-

rated with Au clusters soft-landed on Ar+defects. Evidently,

the atomic structure of the Au seed particles regulates the

morphology of the binary structure generated from it. It

seems likely that this behavior reﬂects the different facets

and edge atoms presented by the two different cluster

shapes.18,26,36–39

In summary, the growth of Ag/Au binary nanoparticles

on graphite by evaporation of Ag atoms on size selected Au

clusters 共seeds兲has been demonstrated through a combina-

tion of experiment and computer simulation. Moreover, our

results show that the morphology of the nanostructures pro-

duced is strongly inﬂuenced by the shape of the seed clus-

ters. The results suggest several avenues for additional ex-

perimental investigations of these original systems, e.g.,

atom-resolved electron microscopy studies.28 From an ap-

plied perspective, the regulation by size selected cluster

seeds of the size and morphology of binary nanoparticles in

which most of the atoms are simply deposited by evaporation

共a cheap and efﬁcient method兲suggests a practical route to

the generation of nanostructured functional ﬁlms.

We acknowledge ﬁnancial support by the UK Technol-

ogy Strategy Board “Clusterbeam” project as well as the

EPSRC-GB.

FIG. 1. 共Color online兲共a兲AFM image of Au2000 clusters soft-landed on Ar+

sputtered graphite after silver evaporation 共3

␮

m⫻3

␮

m2兲.共b兲Height dis-

tribution of the clusters before 共red兲and after 关green, as 共a兲兴 0.01 Ag mono-

layer evaporation.

FIG. 2. 共Color online兲Cluster mass distributions from three simulations:

growth only on the seed particles 共black兲, nucleation and growth without

seeds 共blue兲, and growth on seeds plus signiﬁcant nucleation 共red兲.

FIG. 3. 共Color online兲共a兲STM image of Au250 clusters self-pinned on

graphite. 共b兲Height distribution of self-pinned Au250 clusters, as 共a兲, after

Ag evaporation 共green兲and height distribution of Au250 clusters soft-landed

on Ar+sputtered graphite after Ag evaporation 共red兲.

123107-2 Lidgi-Guigui, Mulheran, and Palmer Appl. Phys. Lett. 93, 123107 共2008兲

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Mar 1996
Dalton Trans

The growth of silver and gold clusters following atomic vapour deposition on highly oriented pyrolytic graphite has been studied using scanning electron microscopy and scanning tunnelling microscopy (STM). Three-dimensional clusters were grown on the terraces and quasi-one-dimensional chains of clusters along the surface steps. An STM study was made on the effect of the step height on cluster nucleation. Charge-density modulations on the substrate surface around the silver clusters were analysed. A preliminary study of the deposition of mass-selected silver clusters from a beam onto the graphite surface has been made. The effect of the impact energy of silver clusters on the deposition process is explored.

Laser-Induced InterDiffusion in AuAg Core−Shell Nanoparticles

Article

Dec 2000

Role of surface science in catalysis. Surf Sci 500(1-3):923-946

Article

Mar 2002
SURF SCI

J. H. Sinfelt

Around the time of World War I, Langmuir advanced a simple theory of chemisorption and showed how it could be used to formulate rate laws for reactions occurring on surfaces. From that time on, surface science has played an important role in heterogeneous catalysis. Between the two world wars, simple studies of extents of adsorption by catalyst surfaces led to the concept of activated adsorption and to a universally used method for determining the high surface areas associated with the pore structures of catalytic materials. After World War II, the application of various spectroscopic and structural probes made it possible to investigate catalyst surfaces at a more microscopic level. Studies with idealized surfaces such as the faces of single crystals in ultra-high vacuum apparatus also made their appearance. By the end of the twentieth century, direct information was being obtained on the rates of elementary reactions of well-defined surface species. The results of such work are beginning to put “finishing touches” on the great insight of early pioneers in surface science and heterogeneous catalysis. Much has been accomplished, but exciting opportunities still remain.

Rate equation approaches to thin film nucleation kinetics

Article

Mar 1973

J. A. Venables

The nucleation and growth of crystals on a substrate are discussed in terms of rate equations for the atom cluster concentrations as a function of time. Simple approximations allow this general set of equations to be reduced to three coupled equations. Many physical processes can be incorporated into these rate equations, including coalescence of clusters, and cluster mobility. The problem of increasing correlation between single atoms and stable clusters as growth proceeds is studied. It is shown that the problem can be solved self-consistently using an auxiliary diffusion equation and that approximations may be obtained which give upper and lower bounds for the cluster growth rates. These diffusion equations also give expressions which enable the cluster-cluster correlations and cluster size distributions to be discussed. With these approximations, expressions are derived for observable quantities and the expressions are compared with one experimental example. In this case, that of gold on alkali halides, it is shown that cluster mobility must be included to obtain agreement with experiment, and that the material parameters required to describe the nucleation behaviour are physically reasonable.

Controlling the surface enhanced Raman effect via the nanoshell geometry

Article

Jan 2003

Systematic variation of the internal geometry of a dielectric core-metal shell nanoparticle allows the local electromagnetic field at the nanoparticle surface to be precisely controlled. The strength of the field as a function of core and shell dimension is measured by monitoring the surface enhanced Raman scattering (SERS) response of nonresonant molecular adsorbates (para-mercaptoaniline) bound to the nanoparticle surface. The SERS enhancement appears to be directly and exclusively due to nanoparticle geometry. Effective SERS enhancements of 106 are observable in aqueous solution, which correspond to absolute enhancements of 1012 when reabsorption of Raman emission by nearby nanoparticles is taken into account. © 2003 American Institute of Physics.