Carbon Nanotubes Episode 10 pptx

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 photoionization time-of-flight mass spectrometry. Intensity anomalies in the mass spectra of clusters with composition 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 clusters. 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 In their bulk intercalation phase compounds of c60 and alkali or alkaline earth metals have been studied intensively, spurred particularly by the discovery of su- perconductivity in several of these metal fullerides, such as C6&, cs0Rb3, C60Ca,, etc.[l-5]. However, despite the wealth of information that could yet be ex- tracted from these fullerene compounds, we would still like to return briefly to looking at some interesting ex- periments that can be done by bringing just one single fullerene molecule in contact with atoms of the metals commonly used for the doping bulk fullerite. The properties of these very small metal-fullerene sys- tems then termed clusters, can be studied quite nicely in the gas phase[6]. We observed that, in the gas phase, such a single fullerene molecule can be coated with layers of various alkali and alkaline earth metals[7,8]. In lhis contribution, we will focus primarily on the structure, both electronic and geometric, of this metal coating of the fullerenes c60 and C70. The method we use to study these metal-fullerene clusters is photoionization time-of-flight mass spectrometry (section 2). The clusters are produced by coevaporation of fullerenes and metal in a gas aggre- gation cell. By ionizing and, in some cases, heating the clusters with a pulsed laser, various features appear in the mass spectra that contain the information neces- sary to suggest a geometric or electronic configuration for the cluster investigated. When building clusters by coating the fullerenes with metal, features similar to the electronic and geometric shells found in pure metal clusters[9] are observed 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 *Permanent address: Central Laboratory of Photopro- cesses, Bulgarian Academy of Sciences, 1040 Sofia, Bulgaria. each time a new layer of metal atoms has been completed around a central fullerene molecule, the stability of these clusters seeming to be purely geometric in origin. The first layer contains exactly the same number of metal atoms as there are rings in the fullerene cage. In growing additional layers, the metal might be expected to prefer the icosahedral shell structure observed in pure alkaline earth cIusters[lO,l I]. However, our measurements suggest a different growth pattern. Coating the fullerenes with alkali metals (section 4), the resulting structures seem to be primarily governed by the electronic configuration. For example, the charge transfer of up to 6 electrons to the lowest unoccupied molecular orbital (LUMO) of C60 observed in bulk alkali fullerides[5] is also observed in our ex- periments, leading to the very stable building block C&& for clusters, where M is any alkali metal. An exception to this is the cluster C60Li12. Supported by semiempirical quantum chemical calculations, we find the high stability of C60Li,2 to be caused by the geometric arrangement of the metal atoms rather than by the electronic configuration[l2]. As predicted by ab initio calculations, this arrangement most likely has perfect icosahedral symmetry[ 131. At higher alkali metal coverage, the coating becomes increasingly metallic and an oscillating structure caused by the suc- cessive filling of electronic shells shows up in the mass spectra, if photon energies near the ionization threshold are used. Note that we always speak of c60 and C70 as a moi- ecule and not a cluster. We reserve the word 'cluster' to refer to units composed of several fullerenes and metal atoms. 2. EXPERIMENTAL Figure 1 shows a schematic representation of the experimental setup used to study the metal-fullerene 169 170 U. ZIMMERMAN et al. REF 7 C LERATION REOlON " CLUSTER PUMPING TOF MASS CONDENSATION STAGE SPECIXOMETER CELL Fig. 1. Experimental setup: the clusters are emitted from the cluster condensation cell, passing as a particle beam through a differential pumping stage into the focus of a time-of-flight mass spectrometer, where they are ionized by a laser pulse. clusters. The cluster source on the left is a low- pressure, inert gas condensation cell filled with approximately 1 torr He gas and cooled by liquid nitrogen flowing through the outer walls of the cell. Inside the cell, two electrically heated ovens, one containing a fullerene and one containing a metal, produce inter- penetrating vapor clouds of the two materials. This mixture is cooled by collisions with the He gas, thereby supersaturating the vapor and causing clusters to con- dense. The size distribution of the clusters thus produced is rather broad, but the mean composition of the clusters depends on the relative density of the vapor components and can be adjusted by the tempera- tures of the ovens. However, the range of cluster compositions that can be studied using mass spectrometry is limited, despite the high resolution of the mass spectrometer employed: Due to the various natural isotopes exhibited by most of the metals studied, the mass spectra become increasingly confused with rising metal content, making exact identification of the peaks im- possible. For metals with more than one significant isotope we can, therefore, only study clusters with ei- ther small metal and high fullerene content or high metal content and just one fullerene per cluster. The formation of pure metal clusters has to be avoided for the same reason. By keeping the temperature of the metal oven below the threshold for formation of pure metal clusters and introducing only small amounts of fullerenes as condensation seeds into the metal vapor, it is possible to generate cluster distributions consisting almost completely of compositions C60Mx or C70Mx with M E (Li, Na, K, Rb, Cs, Ca, Sr, Ba) and x=o 500. After condensation, the clusters are transported by the He-flow through a nozzle and a differential pumping stage into a high vacuum chamber. For ionization of the clusters, we used excimer and dye laser pulses at various wavelengths. The ions were then mass an- alyzed by a time-of-flight mass spectrometer, having a two-stage reflector and a mass resolution of better than 20,000. The size distribution of the clusters produced in the cluster source is quite smooth, containing no information about the clusters except their composition. To obtain information about, for example, the relative stability of clusters, it is often useful to heat the clusters. Hot clusters will evaporate atoms and molecules, preferably until a more stable cluster composition is reached that resists further evaporation. This causes an increase in abundance of the particularly stable species (Le., enhancing the corresponding peak in the mass spectrum, then commonly termed 'fragmentation spectrum'). Using sufficiently high laser fluences (=50 pJ/mm2), the clusters can be heated and ionized simultaneously with one laser pulse. 3. COATING WITH ALKALINE EARTH METALS In this section, we will investigate the structure of clusters produced when the metal oven is filled with one of the alkaline earth metals Ca, Sr, or Ba. A mass spectrum of C60Bax is shown in Fig. 2. The mass peaks corresponding to singly ionized clusters have been joined by a connecting line. Note that the series of singly ionized clusters shows a very prominent peak at the mass corresponding to C60Ba32, implying that this cluster is particularly stable. In searching for an explanation for the high stability of this cluster, we can obtain a first hint from looking at the doubly ionized clusters also visible in Fig. 2 (the peaks not con- nected by the line correspond to doubly ionized clusters). Again, the peak at x = 32 is particularly strong. This seems to indicate that the stability of CmBa,, is not caused by a closed-shell electronic configuration. Instead, the high stability is expected to be of geometric origin. Remembering that the total number of faces or rings constituting the cagelike structure of the c60 molecule is 32 and, thus, equal to the number of Ba-atoms required to form this highly 800 - : 9 -3 -2 8 Y c 1 0 1000 3000 5000 7000 mass [amu] Fig. 2. Mass spectrum of photoionized C,Ba, clusters containing both singly and doubly ionized species: the solid line connects peaks of singly ionized clusters. The sharp edge oc- curs at 32 metal atoms, equal to the total number of hexagonal and pentagonal rings of the C60 molecule. Metal-coated fullerenes 171 stable cluster, the arrangement of metal atoms in this cluster becomes obvious. By placing one Ba atom onto each of the 12 pentagons and 20 hexagons of the c60 molecule, a structure with full icosahedral symmetry (point group I,,) is obtained that can be visualized as an almost close-packed layer of 32 Ba-atoms coating the c60 molecule. It seems reasonable that this structure exhibits an unusually high stability, somewhat similar to the geometric shells observed in pure alkaline earth clusters[lO,l I]. Any additional metal atoms situated on this first metal layer are likely to be only weakly bound to the layer underneath and, thus, evaporate easily, causing the mass peaks of C60Bax with x greater than 32 to disappear almost completely. The small peaks at x = 35,38, and 43 might signal the com- pl’etion of small stable metal islands on the first metal layer. We can, however, presently only speculate on the nature of these minor structures. For a rough estimate of the packing density of this first metal layer, assume the atoms to be hard spheres having the covalent radii of the respective atoms (0.77 A for C; 1.98 A for Ba[14]). Placing the carbon spheres at the appropriate sites of the Cm structure with bond lengths 1.40 -4 and 1.45 A[ 151 and letting the Ba spheres rest on the rings formed by the carbon atoms, the Ba spheres placed on neighboring hexagons will almost touch, spheres on neighboring pentagons and hexagons will overlap by a few tenths of an hgstr~m. The distance of the metal atoms to the center of the molecule is almost equal for atoms on hexagonal and pentagonal faces. In this simple picture, the packing of the metal layer is almost perfectly dense, the Ba atoms having an appropriate size. Incidentally, this argument also holds in a similar manner for Sr- and Ca-atoms. Of course, this simple picture constitutes only a crude approximation and should be valued only for showing that the completion of a metal layer around C60 with 32 Ba-atoms is, indeed, plausible. More pre- cise predictions would have to rely on ab initio calculations, including a possible change in bond lengths of C60, such as an expansion of the double bonds of C,jo due to electron transfer to the antibonding LUMO (as was found in the case of C60Li,2[12,13]). The significance of the magic number 32 found in the experiment may also be stated in a different manner. If a cluster containing Ba and a fullerene molecule will be stable and, thus, result in a clearly discernible structure in the mass spectra every time there is exactly one Ba-atom situated on each of the rings of the fullerene molecule, this property might be used to ‘count the rings’ of a fullerene. Of course, such a proposal has to be verified using other fullerenes, for example, C70 which is available in sufficient quantity and pu- rity for such an experiment. In investigating the metal coating of C70, we will also replace Ba by Cain the data presented. The coating of the fullerenes with the latter material is basically identical but exhibits additional interesting features that will be discussed below. Figure 3 shows two mass spectra, the upper one of C,,Ca:, the lower of C70Ca;, both obtained under similar conditions as the spec- I x = 32 300 1 - 104 0 50 100 130 X Fig. 3. Mass spectra of photoionized C,,Ca; (top) and C7&a: (bottom): the lower axis is labeled by the number of metal atoms on the fullerene molecule. The peaks at x = 32 for CmCa, and x = 37 for C&a, correspond to a first metal layer around the fullerenes with one atom located at each of the rings. The edges at x = 104 and x = 114, respectively, signal the completion of a second metal layer. trum in Fig. 2 but with a higher metal vapor density. A slight background caused by fragmentation of clusters inside the drift tube of the mass spectrometer has been subtracted. The lower axis is labeled with the number of metal atoms on the respective fullerene. Again, the coverage of C6,, with 32 Ca atoms leads to a pronounced peak in the fragmentation mass spectrum. In the spectrum containing C70, a very strong peak at C70Ca:7 is observed. Note that C70, just as C60, has 12 pentagons but 5 additional hexagons on the equator around the remaining fivefold axis, totaling 37 rings. The ‘ring-counting’ thus seems to work for C70 also. However, the applicability of this ‘counting method’ to even higher fullerenes has to be verified as these become available in sufficient quantities for per- forming such an experiment. If it is possible to put one layer of metal around a fullerene molecule, it is tempting to look for the completion of additional layers also. In the spectra in Fig. 3, the sharp edges at C60Ca:04 and C70Ca~,, would be likely candidates for signaling the completion of a second layer. As we will see below, there is, in fact, a very reasonable way of constructing such a second layer with precisely the number of metal atoms observed in the spectrum. In proposing an arrangement of the atoms in the second layer, we will focus first on the metal coating of C60. Note that we speak of layers, not shells. The term ‘shell’ implies self-similarity which, as we will see 172 U. ZIMMERMAN et al. later, does not apply in our case. In the following paragraphs we will often specify the positions of the metal atoms relative to the central CW molecule. This is done for clarity and is not meant to imply any di- rect interaction between the c60 and the atoms of the second layer. In constructing the second layer, it seems reasonable to expect this layer to preserve some of the char- acteristic symmetry elements of the first layer (Le., the fivefold axes). The second layer on c60 contains 72 atoms, a number being indivisible by 5. This requires that each of the five-fold symmetry axes passes through two metal atoms. Consequently, in the second layer there must be one metal atom situated above each of the 12 pentagonal faces of c60. Let us first assume that the second layer has the full icosahedral symmetry I,, of the first layer. The remaining 60 atoms may then be arranged basically in two different ways. The first would be to place the atoms such that they are triply coordinated to the atoms of the first layer (i.e., placing them above the carbon atoms of the C6, molecule as shown in Fig. 4 on the upper left). The atoms above the pentagons of c60 (black) consti- tute the vertices of an icosahedron, the other atoms (white) resemble the C,,-cage. This structure can also be visualized as twelve caps, each consisting of a 5-atom ring around an elevated central atom, placed at the vertices of an icosahedron. This structure, however, does not result in an even coverage: there are 20 large openings above the hexagonal faces of Cm while neighboring caps overlap above the double bonds of C,,. Pictured on the upper right in Fig. 4 is a second way to arrange the 60 atoms with Ih symmetry, obtained by rotating each of the caps described above by Fig. 4. Three possible geometries for arranging the 72 atoms of the second layer: the atoms above the pentagons of Cs0 are shaded. The structure on the upper left can be trans- formed into the more evenly distributed arrangement of atoms on the upper right by 36" turns of the caps around the five-fold axes. From this, the structure on the bottom can be obtained by rotating each triangular face of atoms by 19". one-tenth of a turn (36") around the 5-fold axis through its center. The coordination to the atoms of the first layer will then be only two-fold, but the coverage will be quite even, making the latter of these two structures the more probable one. The latter structure could be described as an 'edge- truncated icosahedron' with 20 triangular faces, each face consisting of the three atoms at the icosahedral vertices with a smaller, almost densely packed triangle of three atoms set in between (exemplarily, one of these triangles has been shaded). Note that this layer, having no atoms right on the edges, is not identical to a Mackay icosahedron[l6] which is formed by pure alkaline earth metal clusters[lO,l l]. However, in this structure the two rows of atoms forming the truncated edges are not close-packed within the layer. This might be a hint that with the structure depicted on the upper right in Fig. 4 we have not yet found the most stable configuration of the second layer. Up to this point, we have assumed that the second layer of atoms preserves the full symmetry (Ih) of the fullerene inside. Let us now allow the second layer to lower its symmetry. This can be done in a simple way: model the interaction between metal atoms by a short-range pair potential with an appropriate equilibrium distance and let the atoms of the second layer move freely within this potential on top of the first layer. This allows the atoms to move to more highly coordinated positions. Starting with atoms in the arrangement with Ih-symmetry, the layer will relax spontaneously by rotating all 20 triangular faces of atoms around their three-fold axes by approximately 19". The resulting structure is shown at the bottom of Fig. 4. One of the rotated triangles has been shaded and the angle of rotation marked. In a projection on a plane perpendicular to the threefold axis, each pair of atoms at the edges of the triangle lie on a straight line with one of the three atoms on the surrounding icosahedral vertices. The two rows of atoms along the former truncated edges have now shifted by the radius of one atom relative to each other in direction of the edge, leading to close packing at the edges. Of course, the triangles could have been rotated counterclockwise by the same angle, resulting in the stereoisomer of the structure described above. This structure no longer has Ih-symmetry. There are no reflection planes and no inversion symmetry. Only the two-, three-, and five- fold axes remain. The structure belongs to the point group I (order 60). I is the largest subgroup of I,,. The layer has, thus, undergone the minimum reduc- tion in symmetry. Of the three arrangements of atoms in the second layer shown in Fig. 4, we find the one on the bottom (symmetry I) the most probable. It optimizes the coordination of neighboring atoms within the layer and, as we will see further down, this arrangement can also be well extended to C,, coated with metal. Of course, after having observed two complete layers of metal around a fullerene, we searched for evidence for the formation of additional layers. However, before looking at experimental data, let us try to con- Metal-coated fullerenes 173 C60M32 K=3 C60M104 C60M236 C60M448 Fig. 5. Proposed arrangements of the atoms in the first four layers of an alkaline earth metal around a Cm molecule: the atoms at the icosahedral vertices are drawn in black and one of the triangular faces of atoms has been shaded in each layer. Note the spiral of atoms (dark grey) in the fourth layer. struct the third and fourth layers around c60 in a manner similar to the second layer with I-symmetry: place one atom above each of the icosahedral vertices; for each additional layer, increase the length of the edges of the triangles between the vertices by one atom with respect to the underlying layer; rotate the triangles so that each edge points toward a different icosahedral vertex. For the second layer, this angle of rotation is 19". For the third and fourth layer it is approximately 14" and 1 lo, respectively. These angles are measured relative to the position with full Ih- symmetry. The atoms stacked as triangular faces above the hexagonal rings of c60 resemble a tetra- hedron with one tip pointing towards the center of the cluster, having a slight twist due to the difference in orientation of a few degrees between consecutive layers. The resulting structures of the first four layers are depicted in Fig. 5. For clarity, one of the triangular faces has been shaded. The atoms at the icosahedral vertices are drawn black. The number of atoms required to complete the third and fourth layer in this manner are 236 and 448. At the bottom of Fig. 5, the fourth shell is shown from two directions. Note the spiral of atoms that are emphasized by a dark grey. This spiral can be wound around any of the five-fold axes from tip to tip. Sim- ilar spirals exist in the other layers, too. Each layer can be envisioned to consist of five such spirals of atoms. For each layer, there is also the stereoisomer with the opposite sense of chirality. To express the number of atoms needed to complete such layers mathematically, let us introduce a layer index K. Define K as the number of atoms along the edge of a triangular face without including the atoms on the vertices above the C60-pentagons. The first layer then has K = 1, the second K = 2. The number of atoms in the Kth layer can easily be calculated to 10K2 + 10K + 12. (1) The total number of atoms N(K) in a cluster composed of K complete layers around c60 becomes N(K) = i(10K3 + 30K2 + 56K). (2) Note that the coefficient of the leading order in K, determining the shell spacing on an N1'3 scale, is equal to that of an icosahedral cluster of the Mackay type[l7]. Inserting K = 1 . . .4 into eqn (2), we find N( 1) = 32, N(2) = 104, N(3) = 236, N(4) = 448, N(5) = 760, etc. Did we predict the number of atoms required to complete additional layers around the metal-coated c60 correctly? Figure 6 shows a spectrum of c60 covered with the largest amount of Ca experimentally possible (note the logarithmic scale). Aside from the edges of x = 32 and x = 104 which we have already discussed, there are additional clear edges at x = 236 and x = 448 (completion of a third layer was also observed at C6OSr236). Note that these values are identical to the ones just predicted above for the completion of the third and fourth layer of metal atoms. We, therefore, feel confident that the alkaline earth metals studied do, in fact, form the distinct layers around a central c60 molecule with the structures depicted in Fig. 5. It should be pointed out again that these layers would, of course, contain identical numbers of atoms if the triangular faces had not been rotated and, thus, the Ih-symmetry had been preserved[7]. The reason for preferring the arrangement with I-symmetry (which can still be called icosahedral) is that it leads to higher coordination of the atoms at the borders between the triangular faces. 0 10000 mass [amu] 20000 Fig. 6. Mass spectrum of photoionized C&a, clusters with high metal content: additional edges, interpreted as completion of a third and fourth layer, are observed at x = 236 and x = 448. 174 U. ZIMMERMAN et al. Note that the structures depicted in Fig. 5 are not self-similar because the angle of rotation of the faces differs for each layer. The layers should, therefore, not be called 'shells' as they are called in the case of pure alkaline earth-metal clusters. With increasing size, the shape of the cluster will converge asymptot- ically to that of a perfect icosahedron. With C70 at the center of the cluster, we observed the completion of layers at x = 37, 114, and 251. For completion of the observed three layers around C70, each layer requires 5 atoms more than the corresponding layer around c60. The arrangement of atoms in the first layer is again obvious: place one atom above each of the 37 rings of the fullerene. Attempting to preserve the D,,,-symrnetry of C70 molecule and of the first layer when constructing the second and third layer, results in some ambiguity of placing the atoms on the equator around the five-fold axis. Also, we found no structure that was sufficiently close packed to be convincing. Lowering the demand on symmetry by removing the symmetry elements containing a reflection (as was done in the case of the coated c60) leads to the point group D,. Similar to c60, close-packed layers can be obtained by rotating the 10 remaining triangular faces around their normal by 19". The remaining atoms can be placed in a close- packed arrangement on the remaining faces on the equator. Fig. 7 shows these first three layers. For the third layer, shown from two different directions, one 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. Very high metal vapor pressures are required to C70M37 C70M114 c70M251 Fig. 7. Proposed arrangements of the atoms in the first three layers of an alkaline earth metal around a C70 molecule: the atoms at the icosahedral vertices are drawn in black. Note the spiral of atoms shaded in the third layer. produce the multilayered clusters discussed above, so high that large quantities of pure metal clusters may also be formed. The great variety of isotopic compositions to be found in large clusters makes it impossi- ble, beyond some size, to distinguish between these pure metal clusters and clusters containing a fullerene molecule. This complication limits the amount of metal atoms that can be placed on one fullerene and, thus, the number of layers observable. This maximum amount differs for each alkaline earth metal and is lowest in the case of Ba coating. For this reason, it is desirable to suppress pure metal-cluster formation. This is more easily achieved with certain metals, such as Ca and, as we will see below, Cs, making these elements particularly favorable coating materials. At the end of this section, let us return briefly to the spectra shown in Fig. 3. Notice the structure in the mass spectrum of C60Cax between the completion of the first metal layer at 32 and the second at 104. This structure is identical in the fragmentation mass spectra of fullerenes covered with Ca and with Sr. It is reminiscent of the subshell structure of pure Ca clusters. The subshells could be correlated with the formation of stable islands during the growth of the individual shells[ 10,111. The 'sublayer' structure we observe here may also give some clue to the building process of these layers. However, the data is presently insufficient to allow stable islands to be identified with certainty. 4. COATING WITH ALKALI METALS The structures observed in the mass spectra of fullerene molecules covered with alkaline earth metals, as described in the previous section, all seem to have a geometric origin, resulting in particularly stable cluster configurations every time a highly symmetrical layer of metal atoms around a central fullerene molecule 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 (a weak background has been subtracted). Mass peaks 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 corresponding to (C,Rb6), Rb+. Of the peaks corresponding 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 stable clusters in this way makes the systematics behind these magic peaks immediately clear: one or two Rb atoms are needed to provide the electrons for the charged state of the cluster, the remaining cluster con- sists of apparently exceptionally stable building blocks Metal-coated fullerenes 175 300 3 0 2000 4000 ' 6000 ' 8000 mass [amu] Fig. 8. Mass spectrum, with background subtracted, of photoionized (C,),Rb, clusters containing both singly and doubly ionized species: the solid line connects peaks belonging to groups of singly ionized clusters with a fixed value of n. Note the dominant peaks corresponding to (c,&b6),Rb+ and (C60Rb6),Rb$+ (marked 'I++"). C6,Rb6. The corresponding building block can be found in the mass spectra of clusters containing any alkali metal and Cm. Only Na is a minor exception to the extent that the clusters (c60Na6),,Naf do not show up as especially strong peaks in the fragmentation mass spectra. They do, however, mark a sharp fall- ing edge and a distinct change in the character of the spectra, as we will see later. It seems quite obvious that the origin of the stability of these building blocks is not geometric. More likely, the electronic configuration of this unit is responsible for the stability, the six valence electrons of the metal transferred to the six-fold degenerate t, , LUMO of the c60 molecule. Such a transfer of six electrons to the LUMO of Cm has also been observed in the bulk intercalation phases of C60M6 with M E (K, Rb, Cs)[5]. These alkali metal fullerides become in- sulators due to the complete filling of the t,, derived band (which was found to be only slightly disturbed by the presence of the alkali ions[5]). The appearance of such a building block is not limited to clusters containing c60. Mass spectra of (C70)nMx show exactly the same intensity anomalies at (C70M6)nM+ and (C70M6)nM:+. An explanation similar to the one given for c60 regarding the stability of the building block observed holds for C,,[18]. Adhering to this interpretation, the bonding of the first six or seven alkali metal atoms will be primarily ionic in nature. How will additional atoms attach to the c60 molecule? Will they continue transferring their valence electrons to the next unoccupied orbital of Cmr again showing high stability when this six-fold degenerate tl, orbital becomes filled? Looking for information supporting this hypothesis, we will begin with an investigation of clusters having the composition CbOLix. Based on ab initio calculations, it has been suggested that the cluster C60Li12 should be stable with the valence electrons from the Li atoms filling both the t,, and the t,, orbitals[l3]. Figure 9 shows fragmentation mass spectra of sin- 2000 v) Y FI 2 0 300 v) Y 0 1 8 0 720 1 LixC,+, so0 mass [amu] 900 Fig. 9. Mass spectra of singly (top) and doubly (bottom) ionized C,Li, clusters: note the prominent features at x = 7 for singly ionized and x = 8 for doubly ionized clusters and at x = 12 in both spectra. gly and doubly ionized CmLiw clusters. Mass peaks are, again, joined by a connecting line. The fine structure of the peaks is caused by the two natural isotopes of Li. Again, we find prominent peaks at x = 7 for singly ionized and x = 8 for doubly ionized clusters. Ad- ditionally, there are prominent peaks at x = 12 in both spectra. Twelve is exactly the number of electrons needed to fill the t,, and t,, orbitals, so it seems, at first, that we have found what we were looking for. However, remember that these clusters are charged, so the tl, orbital obviously cannot be filled completely. Since the appearance of the magic number 12 is independent of charge, it seems more promising to try a geometric interpretation. Ab initio calculation shows that the twelve Li atoms have their equilibrium position above each of the twelve pentagonal faces and, thus, retain the icosahedral symmetry[l3]. It seems likely that this highly symmetrical arrangement of atoms is responsible for the high stability of C60LilL, independent of the state of charge, rather than a complete occupation of vacant molecular orbitals. To support this interpretation, we performed semiempirical quantum chemical calculations using the modified-neglect-of-diatomic-overlap (MNDO) method[19,20]. For x = 1 . . . 14, we searched for the most stable ground state geometries of C,,Li,. We found that for x = 1 . . .8 for Li atoms preferred to be centered above the hexagonal faces of c60[12]. Exem- plarily, the geometry of C60Li8 is shown in Fig. 10 on the left. The eight Li atoms are situated at the corners 176 U. ZIMMERMAN et al. Fig. 10. Most stable ground-state geometries found for Cdi, and C&i14 by the MNDO calculations: the Li atoms are represented by the filled black circles. of a cube. The bonds between the Li atoms (black) and the carbon atoms (white) were drawn merely to clarify the geometry and are not meant to imply any specific bonds. After a transition at x = 9, all Li atoms are found to be most stable when centered above the pentagonal rings for x = 10. . . 12. For C6,,Li12, the icosahedral arrangement of Li atoms proved to be significantly lower in energy than all other isomers, independent of the charge of the cluster, while for clusters with x around 7, the number of electrons in the cluster dominated over the geometry in determining the total binding energy of the cluster. Interpreting the magic numbers x = 7 and x = 8 to be of electronic and x = 12 to be of geometric origin thus seems reasonable. For CsoLi13, the most stable geometry has 12 Li atoms above the pentagons and one above a hexagon. If a fourteenth atom is placed near the Li atom above a hexagon, the arrangement of Li atoms becomes un- stable. The two Li atoms initially not above a pentagon of c6(, will then slide on top of a pentagon. The resulting most stable geometry of C60Li,4 has one equilateral Li trimer (Li-Li bond length of 2.23 A) lying flat above a pentagon and 11 Li atoms centered above the remaining pentagons of C,o as shown in Fig. 10 on the right. For comparison: MNDO calcu- lates a bond length of 2.45 A for the isolated Li: (equilateral triangle) and 2.19 A for the two short bonds of neutral Li3. From the binding energies calculated for the different cluster compositions, we determined abundance mass spectra for heated C6,LiX clusters from a simple Monte Carlo simulation. Figure 11 shows the simu- lated mass spectra resulting from these calculations, including the Li and C, isotope distributions. The peaks at x = 12 and at x = 6 + n (where n is the cluster charge) observed in the experiment (Fig. 9) are well reproduced. For more details, see ref. [12]. For values of x greater than 14, a strong even-odd alternation becomes visible in the spectra shown in Fig. 9, peaks corresponding to clusters with an even number of available metal valence electrons being stronger. We suggest that this even-odd alternation, similarly observed in pure alkali metal clusters, signals the onset of metal-metal bonding of the metal atoms 1 LiXC60". ' "' '""' 12 I 12 j 4 4 x=7 n I Li,C6," ? J 0 2 4 6 8101214 # of Li-atoms on c,, Fig. 11. Abundance mass spectra of differently charged hot C,,Li, clusters evaporating atoms calculated with a Monte- Carlo simulation (the Li and C,, isotope distributions are included). Energies required to remove Li atoms were calculated using the MNDO method. The peaks at x = 12 and at x = 6 + n (where n is the cluster charge) observed in experiment (Fig. 9) are well reproduced. on the surface of Cs0 (remember that the MNDO calculations already show the formation of a metal trimer for x = 14). The electronic configuration of the clusters would, then, again determine their relative stability just as it does for pure alkali metal clusters. Con- sistent with this 'electronic'interpretation, the even-odd alternation displayed by the doubly ionized clusters is shifted by one atom with respect to the singly ionized clusters, an additional Li ion required to supply the charge of the cluster. Such an even-odd alternation is observed to a different degree for all alkali metals covering fullerene molecules (see also Fig. 8). It is especially strong for Na. Fig. 12 shows a fragmentation mass spectrum of singly charged C&ax. A strong even-odd alternation starts above x = 7, the point at which we suggested the metal-metal bonding to begin, and extends up to approximately x = 66. Note that x = 12 does not appear as a magic number in these spectra. In fact, Li is the only metal for which this magic number is observed. One possible explanation as to why Li behaves differently is the ability of Li atoms to form covalent bonds with carbon because the Li 2s orbital is close enough in energy to the carbon valence orbitals. Other than Li, the higher alkali metals form essentially ion pairs 0 20 40 60 No. of Na-atoms on CG0 Fig. 12. Mass spectra of singly charged clusters composed of a single C, molecule coated with a large amount of Na (background subtracted). The even-odd alternation extends up to approximately x = 66. Note that x = 12 does not appear as a magic number in these spectra. in the gas phase (a Li' ion is exceptionally small and has, therefore, an exceptionally high charge-radius ra- tio, comparable to that of Mg2+). A neighboring neg- atively charged fuIlerene would be polarized to such an extent that the description as ion pair would not be justified. The configuration of Li atoms around Cb0 might, therefore, be influenced more strongly by the structure of the fullerene molecule than is the case for other alkali metals, resulting in the unique configuration and stability of C6,,Li12. TJnfortunately, in the case of fullerenes covered with alkali metals, clear evidence is lacking regarding the geometry of the clusters. We can, therefore, only present speculation that may appear plausible but cannot be proven presently. The first seven Na ions of the C,Na: clusters arrange themselves as far from each other as possible to minimize coulomb repulsion while adhering to the C, molecule. Additional Na atoms might successively attach to these 7 ions in pairs of two, forming Na: trimers similar to the one calculated for Cs0Lil4. Every time such a stable trimer, each containing two metal valence electrons, is completed, a strong peak is observed in the spectrum, resulting in an even-odd alternation. The abrupt change in the strength of this alternation at x = 21 = 3 X 7 Na atoms fits this speculation. When coating fuIlerenes with larger alkali metal atoms, the even-odd alternation is interrupted before reaching x = 21, so the structural sequence must be different for these. Nevertheless, we do suggest that the first 6 alkali metal atoms, having transferred their valence electron to the fullerene molecule, will remain distributed over the surface of the fullerene, gather- ing additional metal atoms around them as the cluster increases its metal content. This would result in at least one metallic layer coating the molecule (so speak- ing of metal-coated fullerenes seems justified). How- ever, we do not have any evidence from the spectra indicating when this layer will be completed (a rough estimate shows that a first metal layer, for example of Cs, would require around 30 atoms for completion). As we have already mentioned, the stability of the alkali-fullerene clusters seems to be primarily determined by the electronic configuration. Therefore, it is not too surprising that completion of a Payer of atoms, which would be a geometrically favorable structure, does not lead to any pronounced features in the mass spectra. Furthermore, it should be emphasized that to obtain these fragmentation spectra, the clusters have been heated up to a temperature at which they evaporate atoms on a psec time scale. This cor- responds to a temperature at which bulk alkali metals are molten. Incidentally, a similar behavior is observed in pure metal clusters: small alkali clusters (less than 1500 atoms) show electronic shells and alkaline earth clusters show geometric shells[9,10]. When the cluster, containing one fullerene, contin- ues to grow by adding more metal, it will probably assume the more or less spherical shape observed for pure alkali metal clusters. It could, then, be viewed as a metal cluster with a large 'impurity': the fullerene. Alkali metal clusters containing small impurities, such as (SO,), or On, have already been studied[21,22], showing that the main influence of the impurity is to shift the number of atoms at which electronic shell closings are observed upwards by 2n, 2 being the num- [...]... vapo-grown 1 carbon nanotubes see nanotubes; pyrolitic carbon nanotubes carbon- carbon intralayer distance 59 catalysis growth mechanism 87 nanotubule production 15 single-layer nanotubes 47 chiral nanotubes 27 clusters, metal-fullerenes 169 cobalt nanocrystals 153 cobalt particles 47 coiled carbon nanotubes 87 fullerenes 87 growth pathway 65 metal-coated 169 multi-shell, synthesis 153 nanotubes comparison... networks 105 mechanical properties 143 nanoparticles 153 single-layer 47, 187 STM 65 structural properties 37 thermal properties 143 vibrations, theory of 129 natural resonance 143 nickel filled nanoparticles 153 normal modes 129 onion-like graphitic particles 163 open tips 11 PCNTs see pyrolyk carbon nanotubes pitch angle 59 Subject Index pyrolitic carbon nanotubes (PCNTs) hemi-toroidal networks 105 vapor... synthesis 153 nanotubes comparison 15 fundamental parameters 27 geometry carbon nanotubes 59 metal-coated fullerenes 169 glow discharge, buckybundles 111 graphene layers, flexibility 149 graphene model 37 graphite structure 1 graphitic carbon 77 graphitic particles, onion-like 153 helical forms 77 helix angle 59 hemi-toroidal nanostructures 105 high-resolutiontransmission electron microsopy (HREM) 1,37, 111,... studies 129 interlayer distance 59 iron nanocrystals 153 diameters, Mlerene-scale 15 dekcts 7 1 disordered carbons 129 knee structures 87 electric field, nanotube growth 11 electrical properties 47 electrical resistivity 121 electron irradiation 163 electronic bands 27 electronic properties 111 carbon nanotubes 121 structure 37 electronic shells 169 magnetic properties, buckytubes 111 magnetoresistance... fullerenes 169 molecular dynamics 77 multi-shell fullerenes 163 multi-shell tubes 65 multi-wall nanotubes 27 fiber-reinforced composites 143 fibers 47 structures 65 nanocapsules 153 nanocones, STM 65 182 nanofibers 87 nanoparticles 153 nanostructures 65, 163 nanotubes bundles 47 catalytic production 15 coiled carbon 87 defect structures 7 1 electric effects 11 electronic properties 37, 121 fullerene-scale... 6 P Weis, R D Beck, G Brauchle, and M M Kappes, J Chem Phys 100 , 5684 (1994) 7 U Zimmermann, N Malinowski, U Naher, S Frank, and T P Martin, Phys Rev Lett 72, 3542 (1994) 8 T P Martin, N Malinowski, U Zimmermann, U Naher, and H Schaber, J Chem Phw 99 4 210 (1993) 9 T P Martin, T Bergmann, H Gohlich, and T Lange, J Phys Chem 95, 6421 (1991) 10 T P Martin, U Naher, T Bergmann, H Gohlich, and T Lange,... Olk, C H 121 Ebbeson, T W.71 Eklund, P C 129 Endo, M vii 1: 105 Fonseca, A IS, 87 Goddard III, W A 47 Heremans, J 121 Hernadi, K 87 Holden, J M 129 Ihara, S 77 Iijimla, S vii Issi, J.-P 121 Itoh, S 77 Ivanov, V 15 Jishi, R A 129 Ketterson, J B 111 Kiang, C.-H 47 Kobori, K 1 Kroto, H W 1, 105 Ruoff, R S 143 Saito, R 27 Saito, Y 153 Sarkar, A 1, 105 SattIer, K 65 Setton, R 59 Smalley, R E 11 Song, S N 11... single-layer walls 47 single-wall nanotubes 27 spectroscopy 121 stiffness constant 143 STM see scanning tunneling microscopy strain energy 37 structuralproperties 37 thermal properties 143 topological defects 7 1 topology 77 toroidal cage forms 77 toroidal network 1 torus form 77 transport properties, buckytubes 111 tubes, growth pathways 65 tubule arrays 27 vapor growth 65 vapor-grown carbon fibers 1 vibrational... Experiment c @ 6 , 12 f 0 27 f 1 33 f 1 44 f 0 61 f 1 Potential well M, [21,23] With barrier Without barrier 8 20 8 20 32 8 20 34 40 58 34 40 58 50 98 1 146 f 2 92 138 90 130 178 198 i 0 255 f 5 352 f 10 445 f 10 198 i 2 263 f 5 341 f 5 443 a 5 * 80 252 330 428 92 138 186 196 254 338 440 *See text The first two columns give the numbers of metal atoms at which electronic shell closings have been observed... Lange, Chem Phys Lett 183, 119 (1991) 1 1 T P Martin, T Bergmann, H Gohlich, and T Lange, Chem Phys Lett 176, 343 (1991) 12 U Zimmermann, A Burkhardt, N Malinowski, U Naher, and T P Martin, J Chem Phys 101 , 2244 (1994) 13 J Kohanoff, W Andreoni, and M Parinello, Chem Phys Lett 198, 472 (1992) 14 L Pauling, J Am Chem SOC 69, 542 (1947) 15 C S Yannoni, P P Bernier, D S Bethune, G Meijer, and J K Salem, . 111 cage forms 77 cahon fibers 87 carbon nanotubes see nanotubes; pyrolitic carbon carbon- carbon intralayer distance 59 catalysis vapo-grown 1 nanotubes growth mechanism 87 nanotubule. particles 163 open tips 11 PCNTs see pyrolyk carbon nanotubes pitch angle 59 pyrolitic carbon nanotubes (PCNTs) hemi-toroidal networks 105 vapor grown 1 Raman scattering studies 129. production 15 single-layer nanotubes 47 chiral nanotubes 27 clusters, metal-fullerenes 169 cobalt nanocrystals 153 cobalt particles 47 coiled carbon nanotubes 87 diameters, Mlerene-scale

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