Doping the Fullerenes - ACS Symposium Series (ACS Publications)

Chapter 10

Doping the Fullerenes

Downloaded by PENNSYLVANIA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: January 6, 1992 | doi: 10.1021/bk-1992-0481.ch010

R. E. Smalley Rice Quantum Institute and Departments of Chemistry and Physics, Rice University, Houston, TX 77251 Fullerenes are a new class of carbon molecules, the first truly molecular form of pure carbon yet isolated. Consisting of hollow cages composed of three connected networks of carbon atoms arranged to form 12 pentagons and a varying number of hexagons, these spheroidal molecules may be most useful when they are mixed with small numbers of other atoms. These dopant atoms may be located (1) outside the cage, producing fulleride salts; (2) inside the cage, producing a sort of superatom; or (3) as part of the cage itself, replacing one or more of the carbon atoms in the cage network. Examples of all three types of doping have already been demonstrated.

For most observers, on first learning of the proposed soccer-ball structure for C^Q, there often is an almost irrepressible urge to ask "Can you put something inside?" Here is a hollow molecule with a sort of holier-than-thou sphericity and symmetrical perfection. Inside is a perfect void. Perhaps it is natural to the human spirit, perhaps even healthy, to immediately think of messing up this sphere: changing just one of the carbon atoms for some other, and/or filling the void. The phrase, "Let's dope the bucky ball!" seems rather enticing. There may also be considerable virtue in learning to do just that. In semiconductor technology and solid-state physics, doping an otherwise pure and pristine semiconductor lattice is what gives the material its vital electrical properties. For example, doping is what makes silicon work. Using such techniques as ion implantation, modern microelectronics companies add small, but controlled amounts of boron or phosphorus. Without these critical impurities, elegant perfect crystals of silicon would be useless insulators at room temperature. The English—American word "dope" stems from the Dutch word "doop", which means sauce. In turn this doop is derived from earlier Dutch, German, and French usage of similar-sounding words for dipping in various fluids, and in particular, the rite of baptism. In Christianity this baptism is an act of cleansing in preparation for receipt of the life force of the Holy Spirit. For semiconductors and fullerenes, it's not immediately clear what this kind of doping would mean, but it certainly sounds like something worthwhile. So how do we, following this etymology, go about baptizing C^? Calculations (i, 2) and detailed experiments (3, 4) have revealed that crystalline films 0097-6156/92/0481-0141$06.00/0 © 1992 American Chemical Society In Fullerenes; Hammond, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


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of pure are fee (face-centered cubic) lattices that, in bulk form, are direct band-gap semiconductors, with a gap of roughly 1.7 eV. At room temperature this gap energy is many times the average thermal energy, kT, and hardly any electrons have enough energy to jump from ball to ball. At room temperature is, therefore, effectively an insulator, just like pure silicon. But like silicon, it should be possible to dope the pure lattice with balls that rather more readily give up, or take up, electrons. Once these extra electrons (or holes) travel far enough from their original modified-bucky home, they are screened from the charge they left behind, and they will serve as free carriers of electri city. In other words, it should be possible to produce η-type (electron carrier) or p-type (hole-carrier) doped films, if we could only figure out a means of producing fullerenes with varying electronegativities. There can only be three distinct ways of doing this. We can modify a fullerene like (1) on the outside, (2) on the inside, or (3) on the side itself. As long as we (teal with three-dimensional Euclidean space, these are the only possibilities. Examples of the feasibility of all three methods have already been published (5-9), although only the first has so far been used to produce interesting new materials in the laboratory. By mixing alkali metals such as rubidium and cesium into the fee lattice, the superconducting alkali fulleride crystals are made (10), and a vast literature is growing about such externally doped fullerene materials. This short chapter deals, instead, with the other two distinct ways of doping. Unlike the first, they constitute new molecular forms of fullerenes. In some cases they will be electronically closed-shell species, in many ways just like the pure, empty fullerenes themselves. But in most cases, these doped ful lerenes, or "dopy balls" will be rather reactive: They will either be open-shell species like C B , or closed-shell singlets with close-lying, open-shell triplet diradicals. In many cases, these open-shell or nearly-open-shell species will be the most interesting and the most technologically worthwhile. But their syn thesis and manipulation will be somewhat more of a challenge. 59

Nomenclature and Symbolism These internally doped fullerenes are, in fact, new molecules, and therefore it will soon be necessary to evolve a systematic way of referring to them both by name and with symbols in a molecular formula. To chemists who have to con front the problem of systematizing myriads of possible molecules, the naming and representing of compounds is an important unifying discipline, which is not to be taken lightly. Ultimately, the IUPAC committee will have to consider this problem in detail, but for now we need to at least make a trial start. My colleagues and I have considered this problem rather extensively, and at least within our laboratory, have found the following symbolism to be expressive and nicely concise. We have here a new class of molecules that are roughly spheroidal. They have enough room inside to house at least one atom of any element in the periodic table, and a number of substitutions are feasible for the carbon

In Fullerenes; Hammond, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


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atoms in the cage itself. In a useful sense, these complex fullerenes are superatoms, and it would be nice to have the symbolism represent this fact. We therefore use a set of parentheses to group the relevant atom symbols together, and we use the "at" symbol, @, to denote that these atoms will make a fullerene. Atoms that are located in the central cavity inside the fullerene cage are grouped to the left of this @ symbol, whereas atoms that are part of the cage are listed to the right. For example, the archetypical fullerene, C^, is then explicitly written as ( © C ^ ) . This convention seems needlessly complex until we consider how to represent a 60-cage fullerene with a potassium on the inside, several potassium atoms on the outside, and one boron substituting for carbon in the cage. This molecule has actually been synthesized in small amounts as a positive ion levitated in an magnetic field (11). It takes a lot of words to describe it, but with the @ symbolism, it is easily expressed as K^K®^). Most likely, chemists in general will respond to this suggestion with some healthy conservatism—why bring in an entirely new symbol to a way of writing chemical formulas that has been perfectly satisfactory for millions of other compounds over the past many decades? The answer is that we don't have to. We do need some sort of single character delimiter to distinguish those atoms that are inside the cage, and those that make up the cage itself. But we could use a comma: Κ^Κ,Ο^Β), or a period: K ^ I C C ^ B ) , or K^(KC B ) , or K2(K>C^ B), or subscripts, or superscripts, etc. But none of these serves to suggest tne nature of the object being specified nearly so well as K2(K@C B). The @ symbol is an almost perfect picture of a central atom surrounded by a spheroidal cage. It is a standard character available on all modern keyboards, readily printed and transmitted electronically. We need something that says the atoms to the left are to be found at the center of the fullerene cage made from the atoms following on the right. Why not use the symbol for "at"? The more complex question of systematic names for these complex ful lerene molecules has been left alone for now. In what follows a few new names such as "borofullerene" are tried out, but no attempt at a systematic nomencla ture has yet been attempted. 59

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Doping the Cage So how do we dope the cage? Thus far only one technique has been proven effective, and not in high yield. But it is a start. It is a generalization of the original method (12) for generating C ^ . A short, high-energy laser pulse is used to vaporize a graphite target, producing a carbon plasma that rapidly con denses in a pulse of helium to produce small clusters. When this cluster distri bution is examined, it is discovered that to a remarkable degree, only the even clusters are present for sizes greater than roughly 40 atoms. These even C clusters all turn out to be fullerenes, and the distribution extends out to well past 600 atoms in size (13, 14). Of these clusters, as is now well known, some are specially stable chemically—particularly C^, C g, and to a lesser extent C ^ , C , and some selected higher fullerenes (15). This stability pattern has been 7

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Figure 1. Boron-doped fullerenes, (©C^B^, produced by laser vaporization of a boron—graphite composite target disc, and monitored by FTICR mass spec troscopy of the positive cluster ions levitated in a magnetic trap. (Reproduced from reference 9. Copyright 1991 American Chemical Society.) convincingly explained as a combined result of these molecules' particular abil ity to adopt smoothly curved structures where no carbon atom has a dispropor tionate amount of bond-angle strain (and sp character), and the quantum mechanics of forming a closed-shell electronic structure from an edgeless delocalized network. By symmetry, CLQ in the ( © C ^ ) soccer-ball structure is the smoothest, roundest possible molecule. It also turns out to be closed-shell, with the largest H O M O - L U M O (highest occupied molecular orbital-lowest unoccupied molecular orbital) gap of any of the fullerenes (16). To incorporate some other heteroatom as part of the cage, bond strength, valency, and size need to be considered. Two elements, boron and nitrogen, are nearly perfect replacements for carbon in the fullerene cage by all these criteria. Accordingly, when boron powder is mixed at a level of 15% by weight with graphite powder, and the composite is pressed into a dense pellet, laser vaporization produced B-doped fullerenes (9). Figure 1 shows the sort of mass spectral evidence that has been obtained for these species. This is a Fourier transform ion cyclotron resonance (FTICR) mass spectrum of the positively charged clusters recorded as they are levitated in the magnetic field of a superconducting magnet. Only clusters with an even number of atoms appear to be present in abundance, and there is extensive substructure within the mass spectrum for each cluster size. Careful analysis of this structure, for the 60-atom clusters, for example, reveals that it results from a mixture of boron-doped fullerenes, ( @ ε ^ Β ) , where χ ranges between 0 and at least 6. For the particular doping experiment probed in Figure 1, the approximate amounts of the various 60-atom clusters were measured to be 22% 3

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C ^ , 21% C B , 24% C^B 18% C B , 9% C B , 4% C , B , and 2% a B . Similar doping compositions were measured for the other clusters in this 50- to 70-atom size range. The fact that only even-numbered clusters are present here together with the relative importance of the 60-atom clusters suggests that all these species are, in fact, fullerenes. But, aside from the pattern of their masses, how can we tell this for sure? The answer involves probes of their sur face chemistry and measures of their photochemistry.


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The Cluster FTICR Apparatus. Figure 1 is a mass spectrum, but the apparatus used to generate this spectrum can be used for considerably more than simply examining the masses of a distribution of clusters. Most of the evi dence that currently exists for successful doping of fullerenes comes from these more elaborate experiments, and so it is useful to consider this apparatus in more detail. My colleagues and I have been developing this cluster FTICR apparatus for nearly a decade now as one of the most versatile and powerful means of studying the chemistry and physics of clusters (17). Its latest incarnation has been described in detail in a number of publications (17—19). A schematic cross section is shown in Figure 2. Briefly, it consists of a pulsed supersonic nozzle—laser-vaporization cluster beam source (17, 20) mounted so that the supersonic cluster beam is aimed directly down the central axis of a supercon ducting magnet. Cluster ions are produced either directly as residual ions from the laser-vaporization event itself within the supersonic nozzle, or as a result of photoionization of the neutral cluster beam just before it passes into the bore of the magnet. Because they are entrained in an intense supersonic beam of helium, all these cluster ions are traveling at speeds close to the terminal beam velocity of 1.9 χ 10 cm/s. The cluster's translational energy going into the magnet is then linearly dependent on their mass. For example, a 1000-amu cluster at this velocity has a translational energy of roughly 19 eV, and a 2000amu cluster moves into the magnet with a 38-eV energy. This supersonic clus ter ion beam is mounted so close to the central axis of the magnetic field that the ν χ Β components of the Lorentz force that are responsible for the mag netic mirror effect turn out to be negligible, and the cluster ions travel smoothly along the periaxial field lines into the center of the magnet. As shown in the schematic of Figure 2, the cluster ion beam passes through a "deceleration tube" on its way into the magnet. This tube is pulsed under control of a computer to a negative voltage so as to slow down the clus ters as they approach the ICR analysis cell. By adjusting the magnitude of this decelerating voltage, one can control the size range of the clusters that will be trapped in the ICR cell. This analysis cell of the FTICR cell is a 15 cm long with a 4.8-cmdiameter cylinder, fitted with separate front and rear "door" electrodes that are normally kept at a small positive potential in order to trap the cluster ions. In order to inject new clusters into the cell, the front door is typically dropped to 2 V, the rear door is held at 10 V, and the "screen door" shown in the figure is dropped to 0 V. Cluster ions exiting the decelerator tube with translational energies slowed to between 2 and 10 V then pass into the cell and are reflected 5



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Figure 2. Schematic view of FTICR apparatus used in used to study cluster ions. The clusters are produced via laser vaporization within the small pulsed supersonic beam source in the left-most chamber. The cluster ion beam is then decelerated and ultimately trapped in the ICR analysis cell (the Excitation and Detection Cylinder) mounted in the bore of a 6-T superconducting magnet. (Reproduced with permissionfromreference 17. Copyright 1990.)

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back from the rear-door electrode. Under computer control the screen door is pulsed back up to 10 V before these bounced cluster ions can escape, thereby trapping them in the cell. Collisions with a low-pressure helium or argon thermalizing gas then remove the 2-10-eV translational energy of these trapped clusters so that they cannot escape when the screen door is dropped to 0 V to let in the next batch of clusters. The result is that cluster ions can be accumu lated in the ICR cell until there are enough to study. Cluster ions are detected in the ICR cell by coherently exciting their cyclotron motion with a computer-crafted rf (radio frequency) waveform, and then monitoring the time-varying tiny image currents induced in the sides of the cell as this cyclotron motion continues for many thousands of cycles. Fourier transformation of this time-domain signal then shows sharp intensity spikes in the frequency domain at the cyclotron frequencies of the cluster ions in the cell—resulting in extremely high resolution, broad-range mass analysis of the contents of the cell. A critical feature of this apparatus is the fact that the trapping front, rear, and screen door electrodes have large holes (2-cm diameter) through their centers. This feature permits extremely intense laser beams to be directed through the cell to excite or fragment the trapped clusters without excessive scattered light hitting the trapping electrode surfaces. It also permits rapid introduction of reactant gases, and their subsequent evacuation. The result is that any cluster ion that can be made in the supersonic beam source can be probed both for its surface chemistry and for its detailed photophysics and pho tochemistry while it is levitated in the apparatus, and this can all be probed at a mass resolution between 10 and 10 to 1. In the discussion to follow, these capabilities of the ICR apparatus are used extensively. 4

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Ammonia Titration of Boronated Fullerenes (©C^BJ. If, in fact, the laser vaporization of the boron—graphite composite disc did produce true boron-doped fullerenes, the clusters detected in the mass spectrum of Figure 1 should show a very distinct chemistry. Substituting a boron for a carbon atom on a fullerene cage will produce an electron-deficient site at the boron position on the cage. This site should behave as a Lewis acid, and, perhaps even be titrated by moderate pressures of ammonia gas. Figure 3 shows the result of such a titration experiment on the 60-atom B-doped clusters. In this experiment, the same cluster formation and injection sequence was followed as that used to generate the spectrum of Figure 1, but all clusters except those with 60 atoms were selectively ejected from the cell. This selective ejection was accomplished by a specially crafted rf waveform technology (18) known in the trade as SWIFT (stored waveform inverse Fourier transform). The top panel of Figure 3 shows how effective this ejec tion was in removing all other clusters from the analysis cell. The bottom panel shows the result of exposure to ammonia gas at 1 χ 10" torr (133 χ 10"" Pa) for 2 s. This is enough time and ammonia gas pressure for the aver age cluster to have received roughly 300 collisions with ammonia molecules. The cluster FTICR mass spectrum separates into distinct clumps that, as labeled in the figure, correspond to increasing numbers of chemisorbed 6

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Doping the Fullerenes - ACS Symposium Series (ACS Publications)

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