Figure 3. Figure 4. Three regions of a negative ion mass spectrum of HNDs doped with C 60 and copper. Direct attachment of the incident electron to the dopant is another process, and the only viable process for electron energies below 21 eV.
However, this process is statistically unlikely in large droplets. A , , 21 , More by Paul Martini. More by Marcelo Goulart. More by Lorenz Kranabetter. More by Norbert Gitzl. More by Bilal Rasul. More by Paul Scheier. More by Olof Echt. Cite this: J. A , , 21 , — Article Views Altmetric -. Citations 1. Abstract High Resolution Image. Interest in fullerene—metal complexes has a long history. The possibility to cage lanthanum atoms in C 60 was already envisioned in the seminal paper by Kroto et al.
Several groups have studied the interaction between C 60 and coinage metals by vapor deposition on films; most of these studies pertain to gold.
Xiao and Zhang have prepared copper clusters coated with C 60 by pulsed laser ablation at the interface between a copper target and a flowing fullerene solution of hexane. Very few studies pertain to small, isolated complexes of C 60 and coinage metals. Palpant et al. Lyon and Andrews have recorded infrared spectra of C 60 embedded in an argon matrix, codoped with Cu, Ag, or Au.
Theoretical studies of the interaction between neutral or charged fullerenes and coinage metals have been either limited to a single metal atom 19,20 or to C 60 sandwiched between two small, identical clusters of silver or gold. In a recent letter, we reported experiments in which C 60 —gold complexes were grown inside cold 0. Density functional theory DFT calculations confirmed the conclusion; they predict that the two C 60 molecules are located on opposite sides of the Au atom in a dumbbell-like arrangement.
The present contribution presents a full account of our experimental data for C 60 —gold complexes. Furthermore, gold is rather special among coinage metals; its ability to undergo covalent bonding derives from relativistic effects which lower the energy of the 6s orbital while destabilizing the 5d orbital. Experimental Section.
HNDs were produced by expanding helium Linde, purity Nozzle temperatures ranged from 9. The droplets then passed through another pickup cell filled with copper or gold vapor produced in a resistively heated oven. The temperature of the metal oven could not be measured directly; it was adjusted in order to obtain optimal conditions for the formation of mixed fullerene—metal cluster ions. The doped HNDs passed through a differentially pumped vacuum chamber where they were crossed with an electron beam of variable energy.
Ions were accelerated into the extraction region of a commercial orthogonal time-of-flight mass spectrometer equipped with a reflectron Tofwerk AG, model HTOF. The ions were detected by a microchannel plate operated in single-ion counting mode and recorded via a time-to-digital converter. Additional experimental details have been described elsewhere. Mass spectra were evaluated by means of a custom-designed software.
The natural abundance of 13 C is only 1. Gold is monoisotopic Au , but the presence of two copper isotopes 63 Cu and 65 Cu, natural abundance Furthermore, the software corrects for experimental artifacts such as background signal levels, the mass shift of the mass spectra, non-Gaussian peak shapes, and mass drift over time.
These onsets are indicated in Figure 1 a by their m , n values, although their exact values are, admittedly, a matter of judgment. High Resolution Image. Experimental parameters used to generate and dope the HNDs were similar to those used for the positive ion spectrum see Section 2 for details. By and large, the spectrum of anions resembles that of cations above the mass of the fullerene dimer. The bottom panels display several representative stacked distributions for larger values of m.
Distributions of cations that are not shown in panel 2 e i. Abundance distributions of cations and anions extracted from mass spectra of HNDs doped with C 60 and Au are displayed in Figure 3 ; they are arranged similarly to those in Figure 2 for copper. The vapor pressure of gold in the measurement of anions was significantly lower than in the measurement of cations, explaining the rapid decline in abundance with increasing n. Close inspection of the mass spectrum of negatively charged C 60 —copper complexes reveals the appearance of another series of mass peaks that are positioned exactly midway between peaks due to C 60 m Cu n —.
Figure 4 shows sections of a mass spectrum starting with the bare fullerene monomer, dimer, and trimer panels a, b, and c, respectively. The only other intense mass peaks in Figure 4 are the aforementioned peaks located midway between C 60 m Cu n —. Those positions are marked by vertical lines; corresponding mass peaks are most prominent near the center of panel b and toward the end of panel c.
What is the nature of those mass peaks? A mass spectrometer measures the mass-to-charge- ratio of ions, hence mass peaks that are midway between C 60 m Cu n — ions might be due to C 60 2 Cu n 2— dianions n odd in panel a, C 60 4 Cu n 2— in panel b, and C 60 6 Cu n 2— in panel c. A word of caution is in order, though. A definite proof of our assignment would require a comparison of the shape of each group of mass peaks with the characteristic pattern of C 60 m Cu n 2— isotopologues.
Even more telling would be the identification of mass peaks at nominally half-integer mass because of dianions whose nominal mass number is odd. The statistical quality of the strongly smoothed mass spectrum shown in Figure 4 is insufficient for this task; acquisition of spectra with the required statistical quality is currently not feasible.
The abundance distributions in Figures 2 and 3 reveal several local anomalies i. We begin our discussion with pure copper or gold clusters. The evaporative model originally proposed by Klots provides a link between cluster abundance and cluster stability or, more specifically, their dissociation energy D n also referred to as evaporation or separation energy, i. The model was initially proposed for and tested with van der Waals or hydrogen-bound clusters for which ionization is followed by intramolecular reactions that release a large amount of energy, more than enough to rapidly shed several monomers.
This scenario is less obvious in the present case: the cohesive energies of bulk copper and gold are 3. In the latter case, about 15—20 eV the difference between the ionization energies of helium, Even if the basic assumption of the evaporative model is fulfilled, the link between cluster abundance and dissociation energy is intricate.
Their signatures in mass spectra will be different, but all of them are accompanied by abrupt changes in the abundance relative to the envelope of the abundance distribution.
The abundances of pure, positively charged copper and gold clusters are presented in Figures 2 a and 3 a, respectively. According to the spherical jellium model, applicable to free-electron-like metals, the first two electronic shells the 1s and 1p shell are filled when the number of delocalized valence electrons in the neutral or charged cluster equals 2 and 8, respectively.
The spherical jellium model is, of course, a gross simplification. For negatively charged Cu and Au clusters, the number of atoms needed to fill the 1s and 1p shells in the spherical jellium model would be 1 and 7, respectively. The very low abundance of Cu n — anions Figure 2 f precludes a critical analysis, but features consistent with the jellium model were observed in previous mass spectra of Cu n — anions formed by sputtering.
The inset in Figure 3 f displays the abundance of Au n — on a logarithmic scale; it reveals an odd—even oscillation with a hint of enhancement for Au 7 —. The enhancement of clusters that are predicted to be magic within the spherical jellium model Au — and Au 7 — is weak at best, nor does Au 3 — appear to be magic in those earlier reports.
For energetically excited anions, there is always a competition between dissociation and electron detachment. When energetically allowed, photodetachment is usually the dominant process and photofragmentation is a minor process. We now turn to a discussion of mixed clusters.
These conjectures need to be confirmed by calculations which are beyond the scope of the current work. Our recent DFT study shows that this channel is energetically favored over emission of Au. In the absence of any theoretical work, what can we conclude? Of course, reality may be more complicated.
One may also have to consider dissociation into two clusters rather than sequential emission of two monomers. These channels are less common, but they do occur. For anions, one should also consider the possibility of electron detachment. The channel is not likely to become competitive for pure, large Au n — because the detachment energy converges to the work function 4. For homogeneous cluster ions, this channel is closed because the ionization energy tends to decrease with increasing size; energetics thus favor emission of a neutral monomer.
There is no obvious way to estimate how the ionization energy of C 60 m Au n will change with increasing size m , n. However, how about emission of neutral or charged Au as opposed to C 60?
Will Au loss become competitive for larger complexes? Definitely yes if gold coats C 60 as predicted in the first-principles study by Batista et al. Even partial wetting may tip the balance in favor of Au loss; the layer may also cage the fullerene.
True, some metals including the alkaline earth metals do wet C 60 ; 28,68 thulium and holmium even seem to coat individual C 60 within a positively charged fullerene aggregate. The experimental evidence summarized in the previous paragraph may be insufficient to rule out that gold wets C 60 , but our recent theoretical work 25 clearly does.
Our discussion now turns to copper. What causes this sudden change, and the difference between copper and gold? For a possible clue, let us consider previous reports on the copper—C 60 system.
Furthermore, the cohesive energy of copper is 0. However, in that scenario, one would expect magic numbers that are reminiscent of those of bare copper clusters, the opposite of what we are observing. A smooth distribution may also be expected if the copper atoms are dispersed on a cluster of fullerenes, that is, if copper wets C C 60 wetted by other metals alkalis, alkaline earths, thulium, and holmium exhibits magic numbers as well but at sizes larger than those covered in the present work.
For the C 60 trimer, a similar trend exists, but there are also distinct differences between the charge states, and between copper and gold. The observation of C 60 2— dianions in the gas phase was first reported in In the gas phase, sequential attachment of one or more electrons to an anion is impeded by the repulsive Coulomb barrier for the incoming electron s.
Schweikhard and co-workers have managed to sequentially add up to five electrons to aluminum clusters containing about atoms by judiciously adjusting the electric potential of their ion trap.
The difference is possibly due to different dopant levels of the HNDs, but the presence of metal atoms and the concomitant increase in the stability of dianions may also play a role. Surprisingly, we do not observe C 60 m Cu n 2— dianions with odd values of m. We cannot offer a compelling explanation for this odd—even effect. More work is needed to identify the nature of the perceived odd—even effect in our present data.
We have synthesized clusters of C 60 and gold or copper in HNDs and recorded mass spectra of positive and negative ions formed by electron ionization.
Their abundance distributions, presented here versus n for fixed values of m , feature several local anomalies. On the other hand, the abundance distributions of C 60 3 Cu n — and C 60 3 Au n — anions are dissimilar, possibly because of differences in the fragmentation channels that lead to these ions. Theoretical work is needed to better understand the origin of similarities and differences between cations and anions, and between copper and gold.
It would be highly desirable to directly identify the ions produced by unimolecular dissociation. Time-of-flight mass spectrometers equipped with a reflectron are, in general, well suited for that task. How would another C 60 bind to that complex? Another gold atom? Does copper form similar structures? Hopefully, the experimental data presented here will stimulate work that addresses these questions.
Author Information. The authors declare no competing financial interest. C 60 : Buckminsterfullerene. Nature , , — , DOI: Laser-induced vaporization of graphite produced a remarkably stable cluster, consisting of 60 C atoms.
A truncated icosahedron is suggested, a polygon with 60 vertexes and 32 faces, 12 of which are pentagonal and 20 hexagonal. The C60 mol. American Chemical Society. A review, with refs. The extent to which widely accepted simple models of the metallic and superconducting behavior in the A3C60 compds.
The search for higher superconducting transition temps. American Physical Society. We explore theor. Our first-principles d. The strong binding is attributed to an intriguing charge transfer mechanism involving the empty d levels of the metal elements. The charge redistribution, in turn, gives rise to elec.
Hydrogen storage properties of Li-coated C60 fullerene have been studied using d. Hydrogen atoms are found to bind to Li6C60 in two distinct forms, with the first set attaching to C atoms, not linked to Li, in at. Once all such C atoms are satd. The corresponding hydrogen gravimetric d. Desorption of hydrogen takes place in succession, the ones bound quasi-molecularly desorbing at a temp.
The results are compared with the recent expt. RSC Adv. Royal Society of Chemistry. In this work we present a d. We have found that the interaction between the hydrogen mols. Furthermore, the hydrogen satn. Hydrogen Energy , 42 , — , DOI: Elsevier Ltd. Hydrogen storage by physisorption in carbon based materials is hindered by low adsorption energies. In the last decade doping of carbon materials with alkali, earth alkali or other metal atoms was proposed as a means to enhance adsorption energies, and some expts.
We investigate the upper bounds of hydrogen storage capacities of C60Cs clusters grown in ultracold helium nanodroplets by analyzing anomalies in the ion abundance that indicate shell closure of hydrogen adsorption shells. Doping C60 with a single cesium atom leads to an increase in relative ion abundance for the first 10H2 mols. We emphasize the large effect of the quantum nature of the hydrogen mol. Five normal modes of libration and vibration of H2 physisorbed on the substrate contribute primarily to this large decrease in adsorption energies.
A similar effect can be found for H2 physisorbed on benzene and is expected to be found for any other weakly H2-binding substrate. C60 fullerene is an icosahedrally shaped nanosized mol. To date, fullerenes have found applications in various fields such as the prepn.
A review. The energy level and absorption profile of the active layer can be tuned by introduction of an addnl. Careful design of the addnl. This article reviews the recent progress on ternary org.
Gold Cluster Formation on a Fullerene Surface. American Institute of Physics. Due to its highly corrugated surface fullerene films provide a wide range of bonding sites which could be exploited as mol. To gain insight into the fullerene-Au interaction two types of expts. In both expts. The deposition of submonolayer amts.
In addn. In the reverse expt. This is in contrast to observations with Si clusters, which prefer to reside in the troughs between the fullerene mols.
The Au clusters grow continually from a size of about 55 atoms for the early stages of growth up to atoms for the deposition of a nominal coverage of 1. These data are derived from an anal. The thermal stability of the Au-clusters-covered fullerene film was investigated by annealing in situ up to temps. For temps.
This may be due to a ripening of the clusters. The presence of Au apparently delays fullerene sublimation. Nanotechnology , 19 , , DOI: Institute of Physics Publishing. Create a personalised ads profile. Select personalised ads. Apply market research to generate audience insights.
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She has taught science courses at the high school, college, and graduate levels. Facebook Facebook Twitter Twitter. Updated December 24, Srivastava UC Molecular orbital energies and x-ray K -absorption spectra of copper in metal and its oxides. Indian J Pure Appl Phys — Suzuki T Electron distribution in Cu 2 O crystal. J Phys Soc Jpn — Wyckoff RWG Crystal structures, vol 1.
J Wiley, New York p Download references. You can also search for this author in PubMed Google Scholar. Reprints and Permissions. Hafner, S. The electric field gradient at the position of copper in Cu 2 O and electronic charge density analysis by means of K -factors. Phys Chem Minerals 9, 19—22 Download citation. Received : 18 March Issue Date : January Anyone you share the following link with will be able to read this content:.
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