Page 177 - Carbon Nanotubes

METAL-COATED FULLERENES

U. ZIMMERMANN, N. MALINOWSKI," A. BURKHARDT, and T. P. MARTIN
Max-Planck-Institut fur Festkorperforschung, Heisenbergstr. 1, 70569 Stuttgart, Germany

(Received 24 October 1994; accepted 10 February 1995)
Abstract-Clusters of c60 and C,, coated with alkali or alkaline earth metals are investigated using photo-
ionization time-of-flight mass spectrometry. Intensity anomalies in the mass spectra of clusters with com-
position C,M, and C70Mx (x = 0. . .500; ME f Ca, Sr, Bal) seem to be caused by the completion of
distinct metal layers around a central fullerene molecule. The first layer around Cs0 or C,o contains 32
or 37 atoms, respectively, equal to the number of carbon rings constituting the fullerene cage. Unlike the
alkaline earth metal-coated fullerenes, the electronic rather than the geometric configuration seems to be
the factor determining the stability of clusters with composition (c60)"Mx and (C70),M,, M E (Li, Na,
K, Rb, Cs]. The units CsoM, and C70M6 are found to be particularly stable building blocks of the clus-
ters. At higher alkali metal coverage, metal-metal bonding and an electronic shell structure appear. An
exception was found for C60Li12, which is very stable independently of charge. Semiempirical quantum
chemical calculations support that the geometric arrangement of atoms is responsible for the stability in
this case.
Key Words-Fullerenes, mass spectrometry, clusters, electronic shells, icosahedral layers.

1. INTRODUCTION each time a new layer of metal atoms has been com-
pleted around a central fullerene molecule, the stabil-
In their bulk intercalation phase compounds of c60 ity of these clusters seeming to be purely geometric in
and alkali or alkaline earth metals have been studied origin. The first layer contains exactly the same num-
intensively, spurred particularly by the discovery of su- ber of metal atoms as there are rings in the fullerene
perconductivity in several of these metal fullerides, cage. In growing additional layers, the metal might be
such as C6&, cs0Rb3, C60Ca,, etc.[l-5]. However, expected to prefer the icosahedral shell structure ob-
despite the wealth of information that could yet be ex- served in pure alkaline earth cIusters[lO,l I]. However,
tracted from these fullerene compounds, we would still
like to return briefly to looking at some interesting ex- our measurements suggest a different growth pattern.
Coating the fullerenes with alkali metals (section 4),
periments that can be done by bringing just one sin- the resulting structures seem to be primarily governed
gle fullerene molecule in contact with atoms of the
metals commonly used for the doping bulk fullerite. by the electronic configuration. For example, the
charge transfer of up to 6 electrons to the lowest un-
The properties of these very small metal-fullerene sys- occupied molecular orbital (LUMO) of C60 observed
tems then termed clusters, can be studied quite nicely in bulk alkali fullerides[5] is also observed in our ex-
in the gas phase[6]. We observed that, in the gas phase, periments, leading to the very stable building block
such a single fullerene molecule can be coated with lay- C&& for clusters, where M is any alkali metal. An
ers of various alkali and alkaline earth metals[7,8]. In exception to this is the cluster C60Li12. Supported by
lhis contribution, we will focus primarily on the struc- semiempirical quantum chemical calculations, we find
ture, both electronic and geometric, of this metal coat- the high stability of C60Li,2 to be caused by the geo-
ing of the fullerenes c60 and C70. metric arrangement of the metal atoms rather than by
The method we use to study these metal-fullerene the electronic configuration[l2]. As predicted by ab
clusters is photoionization time-of-flight mass spec- initio calculations, this arrangement most likely has
trometry (section 2). The clusters are produced by perfect icosahedral symmetry[ 131. At higher alkali
coevaporation of fullerenes and metal in a gas aggre- metal coverage, the coating becomes increasingly me-
gation cell. By ionizing and, in some cases, heating the tallic and an oscillating structure caused by the suc-
clusters with a pulsed laser, various features appear in cessive filling of electronic shells shows up in the mass
the mass spectra that contain the information neces- spectra, if photon energies near the ionization thresh-
sary to suggest a geometric or electronic configuration old are used.
for the cluster investigated. Note that we always speak of c60 and C70 as a moi-
When building clusters by coating the fullerenes ecule and not a cluster. We reserve the word 'cluster'
with metal, features similar to the electronic and geo- to refer to units composed of several fullerenes and
metric shells found in pure metal clusters[9] are ob- metal atoms.
served in the mass spectra. In the case of fullerene
molecules coated with alkaline earth metals (section 3),
we find that a particularly stable structure is formed 2. EXPERIMENTAL
*Permanent address: Central Laboratory of Photopro- Figure 1 shows a schematic representation of the
cesses, Bulgarian Academy of Sciences, 1040 Sofia, Bulgaria. experimental setup used to study the metal-fullerene
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