Page 266 - 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

Metal Particles and Cluster Compounds 523

infrared spectroscopy. The CO stretching frequencies on Clusters on surfaces, or supported clusters, lend them-
clusters of one to four nickel (or copper) atoms rapidly selves to a variety of X-ray and electron spectro-
approach the values found for CO chemisorbed on the scopies, such as extended X-ray absorption fine structure
polycrystalline bulk metal. Thus, the CO to metal bond- (EXAFS), X-ray and ultraviolet photoelectron spec-
ing appears to be local as expected. troscopy (XPS and UPS), and transmission electron mi-
In mixed deposits of metal, methane, and argon only croscopy (TEM) to name a few. Most studies of model
A1 atoms of some 18 metals tried appears to react at 10 K systems have used sodium chloride or carbon substrates
without photolysis. The reactivity of A1 appears to be and ultrahigh-vacuum techniques.
2
unique and probably due to its radical like P state. The- Several authors have modeled the kinetics of cluster
oretical studies lend support to the idea that atoms with growth. Nucleation occurs at specific sites, often associ-
partially filled p orbitals would be most reactive in C H ated with lattice defects on the substrate. Cluster growth
insertion processes. commences as atoms impinge near each other; in a se-
The question as to whether clusters or atoms are more ries of experiments it was found that gold atoms will be
reactive in carbon-halogen bond breaking processes was captured if they land within 6.5 A of a growing cluster.
recently considered. It was found that in an argon matrix As cluster growth continues the energetics regarding
at 10 K Mg 2 and Mg 3 reacted with CH 3 Br (presumably to two- or three-dimensional growth must be considered.
form CH 3 Mg 2 Br and CH 3 Mg 3 Br), but Mg atoms did not. Calculations indicate that if the heat of vaporization of
This higher reactivity was attributed to the lower ioniza- the metal 0 is greater than three times the heat of des-
tion potential of these small clusters and to the fact that orption of the metal atom from the surface, E des , three-
in the free state Mg 2 and Mg 3 are very weakly bound, but dimensional clusters should form. However, if the heat
in the product should be strongly bound. Similar results of metal vaporization is less than three times the energy
have been found for Ca, Ca 2 , and Ca 3 . Ab initio theoret- of desorption minus the energy of diffusion E diff ,two-
ical studies convincingly support these experimental re- dimensional cluster growth is favored. Depending on the
sults. However, even after numerous attempts, these clus- surface, E des and E diff will vary:
ter compounds have eluded isolation, and are apparently
quite unstable. 0 3E des
Kinetic studies of metal atom aggregation in cold matri- (three-dimensional growth favored)
ces has received some attention. A statistical frozen matrix
approach (calculating probability that M and M are neigh- 0 3E des − E diff
bors and react to give M 2 ), and a highly mobile metal (two-dimensional growth favored)
atom approach (diffusion being rapid in quasiliquid layer)
have been used. It was found that the diffusion mecha- These theoretical considerations predict that metal clus-
nism was supported best by the experimental results. The ters growing on most clean metal surfaces and semicon-
eventual M 2 concentration was found to be proportional ductors would grow initially in two dimensions, and this
to the square of the M/substrate ratio. Concentrations of is found experimentally.
higher metal aggregates vary as some higher power of the Small clusters on surfaces have some unusual proper-
M/substrate ratio. ties. Their geometrical shapes usually do not resemble the
Since the statistical approach underestimates the for- bulk element. For example, using Moire interference pat-
mation of clusters in the matrix, further analysis has been terns and TEM the smallest colloids of gold are shown
done. Both dilute and high-concentration matrices have to have pentagonal symmetry. The presence of multiple
been dealt with experimentally and mathematically. The twinned tetrahedra such as icosahedra, again suggests that
best model was found to be one which simulates freeze- icosahedral structures are preferred over the bulk structure
out by assuming the reactions to stop abruptly after a cer- during the initial growth phase.
tain time. It should also be pointed out that the trend in If perfect crystallites are not formed in the initial stages
product distribution as a function of metal concentration of growth, there will be some strain energy in the cluster.
is adequately described by this model and greatly aids As metal thickness increases, the desorption energy of a
spectroscopic assignment of bands to metal atoms and metal atom on the surface changes. For Na on a tungsten
clusters. surface, E des is initially 2.5 ± 0.2 eV, but after four layers
A film produced by deposition of atoms or particles are deposited becomes 1.06 eV, which corresponds closely
on a surface forms in several stages: (1) nucleation, (2) to the bulk heat of vaporization of Na. Since the Na Na
cluster growth, (3) coalescence, (4) further thickening, and bonding energy is so high initially, lattice relaxation via
(5) recrystallization (perhaps). We will be concerned with two-dimensional dislocations is difficult so more strain
steps (1)–(3). energy is created in the growing Na cluster.