Page 182 - Carbon Nanotubes

et
174 U. ZIMMERMAN al.
Note that the structures depicted in Fig. 5 are not produce the multilayered clusters discussed above, so
self-similar because the angle of rotation of the faces high that large quantities of pure metal clusters may
differs for each layer. The layers should, therefore, also be formed. The great variety of isotopic compo-
not be called 'shells' as they are called in the case of sitions to be found in large clusters makes it impossi-
pure alkaline earth-metal clusters. With increasing ble, beyond some size, to distinguish between these
size, the shape of the cluster will converge asymptot- pure metal clusters and clusters containing a fullerene
ically to that of a perfect icosahedron. molecule. This complication limits the amount of
With C70 at the center of the cluster, we observed metal atoms that can be placed on one fullerene and,
the completion of layers at x = 37, 114, and 251. For thus, the number of layers observable. This maximum
completion of the observed three layers around C70, amount differs for each alkaline earth metal and is
each layer requires 5 atoms more than the correspond- lowest in the case of Ba coating. For this reason, it is
ing layer around c60. The arrangement of atoms in desirable to suppress pure metal-cluster formation.
the first layer is again obvious: place one atom above This is more easily achieved with certain metals, such
each of the 37 rings of the fullerene. as Ca and, as we will see below, Cs, making these el-
Attempting to preserve the D,,,-symrnetry of C70 ements particularly favorable coating materials.
molecule and of the first layer when constructing the At the end of this section, let us return briefly to
second and third layer, results in some ambiguity of the spectra shown in Fig. 3. Notice the structure in the
placing the atoms on the equator around the five-fold mass spectrum of C60Cax between the completion of
axis. Also, we found no structure that was sufficiently the first metal layer at 32 and the second at 104. This
close packed to be convincing. Lowering the demand structure is identical in the fragmentation mass spec-
on symmetry by removing the symmetry elements con- tra of fullerenes covered with Ca and with Sr. It is
taining a reflection (as was done in the case of the reminiscent of the subshell structure of pure Ca clus-
coated c60) leads to the point group D,. Similar to ters. The subshells could be correlated with the for-
c60, close-packed layers can be obtained by rotating mation of stable islands during the growth of the
the 10 remaining triangular faces around their normal individual shells[ 10,111. The 'sublayer' structure we
by 19". The remaining atoms can be placed in a close- observe here may also give some clue to the building
packed arrangement on the remaining faces on the process of these layers. However, the data is presently
equator. Fig. 7 shows these first three layers. For the insufficient to allow stable islands to be identified with
third layer, shown from two different directions, one certainty.
spiral of atoms is indicated by a dark grey shading.
Again, the layers can be envisioned to consist of five
spirals of atoms around the five-fold axis. 4. COATING WITH ALKALI METALS
Very high metal vapor pressures are required to The structures observed in the mass spectra of ful-
lerene molecules covered with alkaline earth metals,
as described in the previous section, all seem to have
a geometric origin, resulting in particularly stable clus-
ter configurations every time a highly symmetrical
layer of metal atoms around a central fullerene mol-
ecule was completed. When replacing the alkaline
earth metals by an alkali metal (i.e., Li, Na, K, Rb,
or Cs), a quite different situation arises.
Let us begin with clusters having a low metal content
but containing several fullerene molecules. Figure 8
shows a fragmentation mass spectrum of (C60)nRbx
C70M37 (a weak background has been subtracted). Mass peaks
C70M114 belonging to groups of singly ionized clusters with the
same number of fullerenes have been joined by a con-
nection line to facilitate assigning the various peaks.
This spectrum is clearly dominated by the peaks cor-
responding to (C,Rb6), Rb+. Of the peaks correspond-
ing to doubly ionized clusters, also visible in Fig. 8,
the highest peak of each group (C60Rb6)nRb:+ with
odd n, has been labeled '++' (note that every other
peak of doubly ionized clusters with an even number
of fullerenes coincides with a singly ionized peak).
Writing the chemical formula of these particularly sta-
c70M251 ble clusters in this way makes the systematics behind
these magic peaks immediately clear: one or two Rb
Fig. 7. Proposed arrangements of the atoms in the first three atoms are needed to provide the electrons for the
layers of an alkaline earth metal around a C70 molecule: the
atoms at the icosahedral vertices are drawn in black. Note charged state of the cluster, the remaining cluster con-
the spiral of atoms shaded in the third layer. sists of apparently exceptionally stable building blocks