Page 261 - 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

518 Metal Particles and Cluster Compounds

Substitution the surface, rapidly loses kinetic and thermal energy. The
loss of kinetic energy is a result of an inelastic collision
2Ag
atom + (CF 3 ) 2 CFI → AgI + AgCF(CF 3 ) 2 of the atom with the surface. It loses most of its kinetic
Disproportionation and ligand transfer energy normal to the surface but may have some kinetic
energy parallel to the surface. Depending upon how “hot”
2Ni atoms (CH 2 CHCH 2 ) 4 Sn
the impinging atom is and how effective the substrate is
toward dissipating the heat of the atom, it loses a certain
CH 2 CH 2
amount of thermal energy. The efficiency of the substrate
H C Ni C H (Sn) n
to thermally stabilize an atom striking its surface is influ-
CH 2 CH 2 enced by the heat of desorption Q des . This is the energy
that a thermally equilibrated atom on the surface must ac-
Cluster formation quire in order to escape to the vapor phase. The degree
CH 3 Br of thermal equilibration actually achieved is also depen-
Mg atom + Mg atom → Mg Mg −−−→ CH 3 MgMgBr
dent on many other parameters as well. A measure of this
In the absence of some added reactant, condensation of equilibration efficiency is given by the accommodations
free atoms does not lead to new chemical compounds, but coefficient α T as
instead to metal clusters (particles) and then to metal films.
T 1 − T R E 1 − E R
With the availability of excellent high-vacuum technology α T = =
and high-temperature sources, industry has made great use T 1 − T S E 1 − E S
of this process. where T 1 , T S , and T R are the temperatures of the inci-
dent atoms, the substrate, and the desorbed atoms, re-
spectively, and E refers to their kinetic energies. Since
B. Atom Agglomerates (Particles/Clusters)
Q des depends upon the interaction between an impinged
Attention will now be given to small metal particles (atom atom and the surface, its value is very sensitive toward
agglomerates) M 2 through M 1,000,000 . We will be con- the condition of the surface and its cleanliness. If Q des is
cerned first with smaller particles M 2 –M 100 , and in Sec- much greater than kT , where T is the temperature of an
tion IV with larger particles (M 100 –M 1,000,000 ). incident atom, an atom rapidly becomes thermally equili-
The general topic of small metal particles has received brated and its mean stay time on the surface, before being
a great deal of attention recently. Much of this interest is desorbed, is long. During its stay time on the surface an
due to the fact that these particles bridge the gap between atom is considered to be absorbed and is called an adatom.
the chemist’s detailed knowledge of small molecular sys- If Q des ∼ RT , thermal equilibration is approached slowly
tems and the physicist’s understanding of the collective and an adatom remains “hot.” This “hot” atom has a very
behavior of matter. short stay time. The thermally equilibrated atom spends
Historically, Michael Faraday must be credited with it stay time diffusing about on the substrate surface. The
some of the earliest work on small particles, his gold col- distances X − that an equilibrated adatom diffuses over the
loid preparations are still used today over 200 years later. surface are given by the Einstein relation for Brownian
In 1925, Richard Zsigmondy received the Nobel Prize for movement:
his study of colloidal metal solutions. Today microclusters
√ Q des − Q d
of metals constitute one of the most important of man’s X − = 2a exp
2kT
synthetic materials; they are vital in heterogeneous catal-
ysis on which 20% of the GNP of the United States is where a is the distance between absorption sites on the sur-
dependent. Among other numerous applications of small face and Q d is the activation energy for a surface diffusion
metal particles are thin films and coatings, latent image jump. This theory assumes that the adatom migrates in the
development, and photographic films. form of jumps between potential energy wells on the sur-
The theory and applications of nucleation and cluster face. Therefore, Q d is another parameter that is sensitive
growth have taken great strides in recent years. First the to the surface condition. If there are other adatoms within
theory will be considered followed by further discussion X − of a given adatom, two of these may form a pair which
of experimental evidence. may or may not be stable depending mainly on the sub-
strate temperature. The pairing of two adatoms marks the
beginning of the nucleation process (Fig. 4). Nucleation
1. Theoretical Studies
theories can now be used to evaluate the nucleation of
Consider metal atoms in the vapor state striking normal to adatoms. Two nucleation theories, yielding the same qual-
the surface of a substrate. An individual atom, upon hitting itative results, will be used. The first is the capillarity