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

  • Université Sorbonne Paris Nord
  • Swansea University/Nanjing University

Abstract and Figures

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.
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Seed induced growth of binary Ag/Au nanostructures on a graphite surface
N. Lidgi-Guigui,1,aP. 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
Received 2 June 2008; accepted 27 August 2008; published online 23 September 2008
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. © 2008 American Institute of Physics.
DOI: 10.1063/1.2988188
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,35or catalysis.68Among 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.912 Bimetallic nanoclusters can be synthesized by a
variety of chemical methods.1315 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,1820 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 highly
oriented pyrolytic graphitesurface 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 i.e.,
clusters consisting, respectively, of 250 and 2000 atoms of
Au. They were deposited on freshly cleaved graphite
samples in one of two ways. The first consists of depositing
Au clusters at high energy typically 20 eV/atomin order to
self-pin the clusters on the surface:2528 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 AFM兲共Digital
Instruments Dimension 3100equipped 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 STMhead was employed in-
stead also in ambient conditions. 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 Ref. 31
image processing program.
Figure 1ashows a tapping mode AFM scan of Ag 0.01
MLevaporated on a submonolayer film of immobilized
Au2000 clusters with mean spacing of 162 nm. 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
aPresent address: Laboratoire d’Electronique Moléculaire, SPEC / IRAMIS
bat 125 CEA, Saclay 91125 Saclay Cedex, France. Electronic mail:
0003-6951/2008/9312/123107/3/$23.00 © 2008 American Institute of Physics93, 123107-1
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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. 1b, 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 which also leads to flattening of the clusters.
The height of Ag clusters grown on both kinds of samples is
shown in Fig. 3b. 3 ML high Ag platelets i.e., 0.4 nmare
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
In summary, the growth of Ag/Au binary nanoparticles
on graphite by evaporation of Ag atoms on size selected Au
clusters seedshas 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
a cheap and efficient methodsuggests 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
FIG. 1. Color online兲共aAFM image of Au2000 clusters soft-landed on Ar+
sputtered graphite after silver evaporation 3
m2.bHeight dis-
tribution of the clusters before redand after green, as a兲兴 0.01 Ag mono-
layer evaporation.
FIG. 2. Color onlineCluster 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.
FIG. 3. Color online兲共aSTM image of Au250 clusters self-pinned on
graphite. bHeight distribution of self-pinned Au250 clusters, as a, after
Ag evaporation greenand height distribution of Au250 clusters soft-landed
on Ar+sputtered graphite after Ag evaporation red.
123107-2 Lidgi-Guigui, Mulheran, and Palmer Appl. Phys. Lett. 93, 123107 2008
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... The physical deposition method has been generally used to produce nanoclusters pinned on graphite. [12][13][14][15][16][17][18][19][20] Palmer's group suggested two main techniques of producing pinned clusters by varying the kinetic energy of the clusters before they land on the graphite. [13][14][15][16][17][18][19][20] With the first technique, called cluster pinning, clusters with high kinetic energy (typically 20 eV/atom) collide with the surface in order to self-pin the clusters. ...
... [12][13][14][15][16][17][18][19][20] Palmer's group suggested two main techniques of producing pinned clusters by varying the kinetic energy of the clusters before they land on the graphite. [13][14][15][16][17][18][19][20] With the first technique, called cluster pinning, clusters with high kinetic energy (typically 20 eV/atom) collide with the surface in order to self-pin the clusters. [13][14][15][16][17] The cluster impact on the substrate breaks the graphite lattice and causes small defects; clusters are then trapped by these defects. ...
... [13][14][15][16][17][18][19][20] With the first technique, called cluster pinning, clusters with high kinetic energy (typically 20 eV/atom) collide with the surface in order to self-pin the clusters. [13][14][15][16][17] The cluster impact on the substrate breaks the graphite lattice and causes small defects; clusters are then trapped by these defects. The second technique is called the soft-landing method. ...
We present a new approach to retard undesirable cluster aggregation. Using molecular dynamics simulations, we found that a large Au cluster that collides with a small cluster of metals other than Au has a locally distorted structure. Because of the lattice mismatch with a graphite surface, the distorted region of the Au cluster acts as a pinning center during the cluster diffusion process. Through the pivotal rotation caused by the pinning center, the Au cluster significantly reduces lateral diffusion. We also found that the most effective factor in the distortion of the structure is the difference in atomic size. The results of an AuNi system which has a large difference in atomic size confirm that a collision with a small Ni cluster reduces the diffusion of the large Au cluster. On the basis of these results, we expect that the use of cluster collision leads us to promising applications in the production of immobilized clusters.
... So far, the experimental data on the cluster height and size have been reported only in a few publications, e.g. [62,63]. In reference [62], the height of Au 2000 clusters soft-landed on highly oriented graphite was determined as 3.2 ± 0.4 nm. ...
... [62,63]. In reference [62], the height of Au 2000 clusters soft-landed on highly oriented graphite was determined as 3.2 ± 0.4 nm. The diameter of free Au 2000 clusters is approx. ...
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Structure and stability of nanometer-sized Ag 887 , Au 887 and Ti 787 clusters soft-landed on graphite (at deposition energies E dep = 0.001 − 5.0 eV per atom) are studied by means of molecular dynamics simulations. Parameters for the cluster–surface interactions are derived from complementary ab initio calculations. The shape and the contact angle of deposited clusters are systematically analyzed for different deposition energies and temperature regimes. The Ag 887 cluster deposited at E dep ≲ 0.1 eV/atom undergoes collision-induced plastic deformation, thus acquiring an ellipsoidal shape with the contact angle close to 180°. In contrast, Au 887 and Ti 787 clusters undergo a collision-induced melting phase transition followed by their recrystallization; these processes lead to the formation of the droplet-like shapes of the clusters in a form of truncated spheroids. At larger deposition energies all clusters flatten over the surface and eventually disintegrate at E dep ≈ 0.75 − 1.0 eV/atom (for Ag 887 and Au 887 ) and ≈3 eV/atom (for Ti 787 ). It is found also that the shape of deposited clusters is strongly influenced by the strength of cluster–substrate interaction and the corresponding interaction mechanism, namely the weak van der Waals interaction between metal and carbon atoms or the van der Waals interaction with an onset of covalent bonding. Similar phenomena should arise in the deposition of clusters made of other elements, which interact with a substrate by one of the above-described mechanisms. Graphical abstract
... Bimetallic clusters can be produced by a variety of techniques such as cluster beam generation, chemical reduction, electrochemical synthesis, etc. (Abdelsayed et al. 2008;Binder 2005;Ferrando et al. 2008;Li et al. 2002;Neukermans et al. 2004;Rao et al. 2000;Yin et al. 2011). The coating of clusters, pre-grown on a surface, by vapour deposition is a standard method (Burda et al. 2005;Ferrando et al. 2008;Lidgi-Guigui et al. 2008;Park et al. 2008;Schmidt et al. 1990). Normally, the metal which binds more strongly to the substrate would be first deposited onto the substrate. ...
... However, due to the limitations of the structure of the substrate and the higher preparation temperature, the size distribution of the bimetallic clusters prepared by this method is rather broad. Although previous work shows that this problem can be overcome by a technique of coating pinned size-selected clusters on selected substrate (Lidgi-Guigui et al. 2008). Moreover, it is very difficult to control the element arrangement distribution of the bimetallic clusters with this method. ...
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Bimetallic clusters display new characteristics that could not be obtained by varying either the size of pure metallic systems or the composition of bulk bimetals alone. Coating of pre-deposited clusters by vapour deposition is a typical synthesis process of bimetallic clusters. Here, we have demonstrated that hierarchical, gold cluster-decorated copper clusters as well as both heterogeneous and homogeneous Cu–Au bimetallic clusters (4.6 to 10.7 nm) can be prepared by coating pre-deposited, size-selected Cu5000 (4.6 ± 0.2 nm) with Au evaporation at various temperatures. These bimetallic clusters were analyzed by aberration-corrected scanning transmission electron microscopy and associated electron energy loss spectroscopy. The results indicate that the growth of bimetallic clusters is controlled by a competition between nucleation and diffusion of the coating Au atoms.
... on suitable surfaces and on controlling their composition, size and distribution: PdAu on TiO 2 , 1 PdAu and AgAu on carbon, 6 CuPd on NaCl, 7 AgAu on TiO 2 , 8 PdAu on graphite, 9 PtRh on TiO 2 , 10 PdAu on nanostructured alumina, 11 AgAu on graphite. 12 The key point is, obviously, the reaching of a complete understanding of the kinetic growth mechanisms of the NPs as a function of process parameters during preparation and/or postfabrication processes parameters. In this way, a correlation between the structural properties of the NPs (composition, structure, size, distribution, etc.) and the process parameters could be reached obtaining practical methodologies to control such properties for desired applications. ...
PdAu nanoparticles have been grown on SiO2 by room-temperature sequential sputtering depositions. The nucleation and growth kinetics have been determined crossing atomic force and scanning electron microscopy measurements. From these measurements the mechanisms of the nucleation and growth have been determined. In particular: (1) during the deposition of the first metal (Pd), atoms adsorbed on the substrate are readily trapped on the substrate defects, forming stable nuclei which grow further. During the deposition of the second metal (Au), adsorbed atoms are captured by the clusters formed during the first deposition, before they have time to form a stable nucleus of pure second metal on the surface sites. So, the nucleation is mainly controlled by the Pd and the Au atoms are incorporated essentially by direct impingement of the vapour atoms on the already formed particles. (2) fixing the amount of Pd and Au, during post-deposition thermal treatments, a surface diffusion limited ripening of the NPs occurs. Applying the standard ripening growth theory several parameters characterizing the process were determined, in particular, the growth exponent n and the activation energy E(a). n was found to be dependent on temperature and amount of Au deposited. E(a) was found to linearly increase with the amount of Au deposited. Such a dependence is discussed separating E(a) in two terms: one describing the activation energy for atomic surface diffusion (independent on the amount of Au deposited), the other one describing the activation energy for the film clustering process (dependent on the amount of Au deposited).
... [2][3][4][5] So far, a large number of well-defined noble metal nanostructures have been fabricated and synthesized by nanolithography and chemical methods. [6][7][8][9][10] Among them, ring-shaped nanostructures, as a special type of cavity, are particularly attractive and exhibit some fascinating phenomena such as plasmon focusing effect 11,12 and dark multipolar plasmon excitation. [13][14][15][16] With these properties, the ringshaped nanostructures are considered to be very useful in ultrabiosensor, nanofilter, nanoantenna, and negative index of refraction. ...
We investigate the far-field scattering property of a single Ag nanoring. Under oblique excitation, two-focus scatterings with distinct intensities were observed. We show that the two-focus scatterings result from the interference of far-field scattering light from the ring circumference, and the local field enhancement effect of surface plasmons plays the key role in the focus intensity. By finite difference time domain and numerical integer methods, we calculated far-field scattering and surface plasmons’ distributions, and the results are in good agreement with the experiment.
We study the diffusion of Ag based bimetallic nanoclusters supported on graphite. Using a molecular dynamics simulation, we reveal that the Ag clusters show rapid diffusion because of their hexagonal bottom layer. In order to decrease the rate of diffusion, we added Pt and Ni to distort the structure of the alloy cluster (i.e., the alloying method). We expected Pt to provide a stronger force on Ag atoms, and Ni to shorten the bond length and thereby change the structure of Ag cluster. However, the attempt was unsuccessful, because Pt and Ni atoms formed cores inside the Ag clusters. We therefore designed a collision system where large Ag clusters collide with small Pt or Ni clusters. Upon collision with Pt clusters, the diffusion showed little change, because Pt atoms are substituted at the Ag atomic site and form a perfectly ordered structure. The collision with Ni, however, deforms the bottom layer as well as the overall cluster structure and decreases diffusion. This outcome appoints toward the possibility of further application to the manufacture of durable nanocatalysts.
Adsorption of pre-formed Agn clusters for n = 1 - 8 on a graphite substrate is studied within the density functional theory employing the vdW-DF2 functional to treat dispersion interactions. Top sites above surface layer carbon atoms turn out to be most favorable for a Ag adatom, in agreement with experimental observations. The same feature is observed for clusters of almost all sizes which have the lowest energies when the Ag atoms are positioned over top sites. Most gas phase isomers retain their structures over the substrate, though a couple of them undergo significant distortions. Energetics of the adsorption can be understood in terms of a competition between energy cost of disturbing Ag-Ag bonds in the cluster and energy gain from Ag-C interactions at the surface. Ag3 turns out to be an exceptional candidate in this regard that undergoes significant structural distortion and has only two of the Ag atoms close to surface C atoms in its lowest energy structure.
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Vibration modes of metal core-shell nanoparticles have been measured by confocal micro-Raman spectroscopy. Compared to a standard macroscopic scale, the number of analyzed nanoparticles is reduced to 103–104. We observed two distinct contributions in the same Raman spectrum originating from Cu-Ag core-shell and pure Ag nanoparticles. The nanoparticle sizes are calculated and successfully compared to the ones obtained by electron microscopy on the same micron-scale area. The bond matching between Cu core and Ag shell is pointed out from a conjugated Raman and transmission electron microscopy study.
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Size-selected gold and nickel nanoclusters are of interest from an electronic, catalytic, and biological point of view. These applications require the deposition of the clusters on a surface, and a key challenge is to retain the cluster size. Here controlled energy impact is used to immobilize the size-selected clusters on the graphite surface at room temperature. The threshold energy for pinning of ionized AuN (N=20–100) and NiN (N=10–300) clusters, over the impact energy range 350–2000 eV, is shown by scanning tunneling microscopy to scale with the cluster mass. This behavior is consistent with a previous study of silver clusters and demonstrates the more general applicability of the cluster pinning model.
Full-text available
We report on a source for producing size-selected nanoclusters based on the combination of radio frequency magnetron plasma sputtering and gas condensation. The use of plasma sputtering to vaporize a target is applicable to a large range of materials; Ag, Au, Cu, and Si have been attempted to date. The source, combined with a time-of-flight mass filter, can produce clusters in the size range from 2 up to at least 70 000 atoms, depending on the target material, with a constant mass (M) resolution (M/ΔM ∼ 25) at an intensity that produces atomic monolayer coverage in as little as a few minutes. The source is also attached to an ultrahigh vacuum analysis chamber, which allows in situ surface chemical and structural analysis. Examples of cluster deposition experiments with the source are also presented.
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We describe the construction and performance of a gas condensation cluster source. The source was designed for deposition of mass-selected metal clusters with controlled landing energy. We have produced clusters of Pb-n (n =2- similar to 300) and Ag-n (n= similar to 20- similar to 300) with sufficient intensity to deposit size-selected clusters to a density of 10(12) clusters/cm(2) in 10 min. The landing energy of the clusters can be controlled from similar to 25 to 800 eV. (C) 1997 American Institure of Physics.
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A new mass selection technique has been developed, which allows one to size-select charged particles from atoms to nanoparticles of almost unlimited size. It provides a mass resolution of m/Δm = 20–50 and a transmission of about 50% for the selected size, both independent of mass. The technique is based on the time-of-flight principle, but differs fundamentally from time-of-flight mass selection normally used. The basic idea is to use time-limited high voltage pulses to displace laterally a preaccelerated ion beam, without changing its direction or shape. As the movement of the ions perpendicular to their original beam direction is independent of their forward velocity, mass resolution and calibration does not depend on the ion beam energy. A mass selector of this type has been implemented successfully into a cluster deposition experiment and has proven to be reliable and simple to operate. © 1999 American Institute of Physics.
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.
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.
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.
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.