Page 267 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN009M-428 July 18, 2001 1:6

524 Metal Particles and Cluster Compounds

The importance of these unusual growth characteristics Unfortunately, until very recently experimental studies
has stimulated theoretical work on the kinetics of surface of small metal particle structure usually lacked the reso-
cluster growth, and often cluster growth has been treated lution necessary to observe differences between various
as a polymer growth problem. The growth process be- geometries. In most cases only average particle sizes or
comes governed by an equilibrium between impingement size distributions are obtained and these usually assume
and desorption which is indicated by a constant value of simple cubic or spherical geometries. Technology was not
monomer concentration over a considerable time period. available (and is still not very accessible) to study local
So at a certain substrate temperature it should be possible surface disorders on an ordered core of a small particle.
to establish equilibria with very small cluster sizes. How- Some structural differences between small metal particles
ever, at low substrate temperatures no such equilibria can produced in different ways have been observed, however.
be established, and cluster concentration and nucleus con- These may be compared to large cluster compounds whose
centrations rise very steeply. Generally the cluster growth detailed structure can be determined by X-ray crystallo-
for Au under low-temperature conditions (80 K) on a clean graphic methods (see Section III).
surface involves an induction period of 10 −5 sec, twin for- New technology is now available, however, and dra-
mation until 10 −3 sec, constant growth to 10 −1 sec, and matic recent advances in microscopy techniques now al-
then coalescence to a film. low imaging of surfaces at the atomic level. Scanning
Small clusters on surfaces can also be affected elec- Tunneling Microscopy (STM) especially has made great
tronically, and so supported clusters have been exten- strides in this area. This technique depends on the move-
sively investigated. Of particular interest is the behavior ment of a probe tip over a surface, not touching but very
of the d electrons. X-ray photoelectron spectroscopy indi- close. An electrical potential between the probe tip and
cates that the d electron binding energy decreases and the the surface causes electrons to tunnel through the space
d electron bandwidth increases with increasing cluster between them. Automatic movement of the probe to main-
size. These effects can be explained in terms of changes in tain a constant tunneling current as the surface is scanned
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the d–s(p) orbital hybridization. Thus, the 3d 4s config- essentially plots out, with atomic resolution, the geogra-
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uration of atomic nickel becomes 3d 4s in bulk nickel. phy of the surface. These exciting developments allow the
Similar increases in d orbital occupation with increas- imaging of atoms and clusters, rows, and arrays of atoms.
ing cluster size are expected for other metals. Because of Truly, the age of “seeing” atoms is now with us.
their more localized nature, d electrons are repelled more Metal atoms nucleating on a [100] NaCl plane take
by the core electrons and the electron binding energy is on one of two structural orientations, depending on
expected to decrease with increasing d orbital occupancy the metal. These are the [001] plane initially for Ni >
(cluster size). From a simple molecular orbital picture, the Cu > Ag > Au > Al. Upon further deposition this expitax-
larger the number of like neighbors, the larger should be ial growth preferred the [111] orientation. High-resolution
the d electron bandwidth. Because of overlap integral ef- electron microscopy showed that twinned particles with
fects, this increasing width is asymmetric with the weakly external shapes of either decahedron or icosahedron were
bonding and antibonding orbitals shifting more; i.e., the formed. No gaps or dislocations were observed. Slightly
binding energy decreases with increasing size (number of diverging lines in the lattice images were interpreted as
neighbors). An UPS study of silver and iodine-covered particles being formed from a nucleus of several atoms
silver clusters has led to a similar interpretation of cluster forming the smallest unit of either a decahedron or icosa-
size effects upon the d electron bandwidth and ionization hedron. Growth proceeded layer by layer of atoms on the
potential. nucleus in the most closely packed form. These structures
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Gold clusters on weakly interacting substrates, such as are present for particles even less than 20 A in diameter,
carbon or alkali halides, have been most extensively stud- but they no longer kept their initial structures when they
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ied. In this case, changes in d electron binding energy as grew beyond 150 A in size.
a function of cluster size are exactly paralleled by similar A supersonic free jet mixture of metal particles in ar-
changes observed in gold–cadmium alloys as a function of gon has been studied by electron diffraction. Particles of
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gold concentration. The only important parameter appears Bi, Pb, and In of 40–95 A were measured. Changes in
to be the average number of like nearest neighbors. Dilute crystal structure from that of bulk metal were observed
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gold alloys behave like small clusters. Thus, there is a lin- for clusters in the 50–60 A diameter range (2000–4000
ear increase in d electron binding energy with decreasing atoms/particle). Indium growing particles changed from
concentration. When the substrate has localized p or d tetragonal to face-centered cubic as size increased. Lattice
orbitals which overlap the d orbitals of the cluster there is parameters of the microcrystals were found to decrease as
a strong interaction which usually leads to a decrease in the cluster size increased. Apparently a high proportion
the d electron binding energy. of surface atoms in small clusters favors crystal defects