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Nanosized Inorganic Clusters 305

the 17 planes in the 3.60-nm clusters are formed by 2057 stabilized bimetallic clusters that offer other opportuni-
atoms 15 planes by 1415 atoms and the 11 planes by 561 ties to tune the properties of the overall cluster formed.
atoms. Overall, these numbers correspond, for example, The aim of such work in the future lies in both stabiliz-
to the eight-, seven-, and five-shell clusters with formulas ing and geometrically linking such cluster entities into
equating to: Pd 2057 Phen 84 O 1600 ,Pd 1415 Phen 60 O 1100 , and 2-D and 3-D networks. If such work was to succeed then
Pd 561 Phen 36 O 200 , respectively. new types of storage devices and electronic components
Naturally electron microscopy cannot determine the ex- of “minute dimensions” may become accessible that prob-
act number of metal atoms. However, considering the ob- ably will never be reached by established methods such as
servation that more than 90% of the particles detected nanolithography.
with the electron microscope occur with one of the dis-
cussed number of planes, it would appear that the natural
packing distribution of these assemblies tends toward the II. FROM CLUSTERS TO SEGMENTS
descriptiongivenbythemagicnumbers.However,incases OF SOLID-STATE STRUCTURES
where the number of metal atoms in the cluster approaches
several hundreds, a set of imperfect clusters/colloids are An alternative class of metal clusters which may also
formed with a certain distribution with respect to size and provide routes to interesting systems of scientific and
chemical composition. technological relevance are derived from metal chalco-
The use of colloidal dispersions in the synthesis of genides. During the last few years interest in this class
metal-based clusters has also afforded routes to ligand- of compounds has increased dramatically, as they can
be used as precursors in the production of semicon-
ducting metal selenides and tellurides. A considerable
number of multinuclear metal selenide cluster com-
plexes are known now which are protected by a lig-
and shell thus avoiding further reaction to stable bi-
nary selenides. Examples include [Ni 34 Se 22 (PPh 3 ) 10 ],
[Cu 70 Se 35 (PEt 3 ) 22 ], and [Cu 146 Se 73 (PPh 3 ) 30 ]. These com-
pounds are formed by the reaction of PR 3 complexes
(R = organic group) of metal halides with Se(SiMe 3 ) 2 ,
Scheme 1.
The mechanism for cluster formation, and thus the
molecular structure of the products, is strongly influ-
encedbythespecialreactionconditions(temperature,type
of copper salt used, type and size of the PR 3 ligand).
As expected very often the thermodynamically stable
metal chalcogenides are formed, however, calculations
have shown that the PR 3 -stabilized cluster complexes are
metastable.
It is possible to obtain copper chalcogenide clusters
which can be approximately described as a section of the
structure of the binary Cu 2 E phase (E = S, Se, Te) sur-
rounded by PR 3 ligands. Though spherical cluster cores
with up to 62 copper atoms do not permit a direct compar-
ison with the binary copper chalcogenides, with increas-
ing cluster size, a tendency toward a layered Cu 2 E-type
skeleton can be seen. Fragments of the structure of the
binary Cu 2 Se phase can be recognized for the clusters
[Cu 70 Se 35 (PEt 3 ) 22 ] and [Cu 146 Se 73 (PPh 3 ) 30 ] (Fig. 2). In
particular, the relation to the Cu 2 Se structure can be seen
by comparing the Se sublattices of the two cluster com-
pounds. In both clusters a layered segment is formed by
the Se ligands, consisting of layers with 10, 15, and 10 Se
FIGURE 2 Structure of [Cu 70 Se 35 (PEt 3 ) 22 ] and [Cu 146
Se 73 (PPh 3 ) 30 ] (without Et and Ph groups). The Cu atoms ({Cu 70 }) and 21, 31, and 21 Se atoms ({Cu 146 } cluster),
are shown as empty spheres, the Se atoms are shown as respectively (Fig. 3). Most of the Cu atoms are positioned
hatched spheres, and the P as black spheres. in the tetrahedral surroundings spanned by the Se atoms.