J. Phys. Chem. 1994,98, 1810-1818
1810
Annealing Carbon Cluster Ions: A Mechanism for Fullerene Synthesis Joanna M. Hunter, James L. Fye, Eric J. Roskamp,' and Martin F. Jarrold' Department of Chemistry, Northwestern University, 21 45 Sheridan Road, Evanston, Illinois 60208 Received: November I O , 1993'
We describe studies of the annealing and dissociation of even-numbered carbon cluster ions containing 50-70 atoms. The polycyclic polyyne ring isomers for cluster ions in this size regime can be annealed almost entirely into the fullerene geometry, with a small fraction being converted into an isomer which appears to be a large monocyclic ring. Suprisingly, we find that Cao+ behaves essentially the same as other similarly-sized clusters (such as CSS+). This has important implications for understanding the mechanism of these structural transformations, as well as the overall scheme for fullerene synthesis. The activation energies for conversion of the polycyclic rings to fullerenes are low and relatively independent of cluster size, though the efficiency of forming a fullerene (rather than a large monocyclic ring) increases with cluster size. Based on our experimental results, a detailed mechanism is proposed to account for conversion of the polycyclic polyyne rings into fullerenes. According to this mechanism, a fullerene fragment is prepared by a Bergman enediyne cyclization followed by a radical-induced ring closure and a retro [2 21 process. The polyyne chains are then configured to spiral around the fullerene fragment and zip up to form a spheroidal fullerene. We also consider how these processes fit into an overall scheme for fullerene synthesis from small carbon fragments and describe a scheme that is consistent with the experimental results presented here.
+
Introduction In this paper we describe experimentalstudies of the annealing and dissociation of even-numberedcarbon cluster ions containing 50-70 atoms. The objectives of these studies are to identify the structural transitions that occur between the different isomers, estimate activation energies, and assess what role, if any, these processes may play in the gas-phase synthesis of fullerenes.1*2In a recent paper we reported detailed studies of the annealing and dissociation of even-numbered carbon cluster ions containing 3050 atoms.3 von Helden et al. have also recently reported some annealing studies for clusters with 30-40 atoms! These studies show that it is possible to convert some of the polycyclic polyyne ring isomers of the smaller clusters into fullerenes. The work reported here covers the range where macroscopic quantities of some specific fullerenes can be generated using the arc synthesis method. Thus, these studies may provide mechanistic insight into why some specific fullerenes predominate in the arc synthesis. We have previously reported a preliminary account of the annealing behavior of c60+ ions.s In this paper we report a full account of the behavior of C a + along with the results of similar studies of the behavior of other carbon clusters with 50-70 atoms. An important issue we address in these studies is whether the annealing behavior of Ca+is different to that of other similarlysized clusters. The experimentalresults described here (along with our previous studies of clusters with 30-50 atoms3) provide important clues about the detailed mechanism of the conversion of the planar polyyne ring isomers into spheroidal fullerenes. A detailed mechanism, consistent with the available experimental results,' is proposed. We also consider how our experimental results fit into an overall scheme for fullerene synthesis that can account for the preponderance of certain fullerenes such as c 6 0 and c70. The experiments were performed using the injected ion drift tube technique. Size-selected cluster ions are injected at various energies into a drift tube containing an inert buffer gas. As the clusters enter the drift tube, they are rapidly heated by collisions with the buffer gas. By increasing the injection energy, it is possible to heat the clusters to the point where they undergo structural transitions or ultimately even fragment. After the Abstract published in Advance ACS Abstracts, January 15, 1994.
cluster's kinetic energy is thermalized, further collisions with the buffer gas cool them. Their structure is then probed by measuring their mobility. Compact geometries such as the fullerene travel across the drift tube (under the influenceof a weak electric field) more rapidly than the less compact isomers.6 Any fragmentation that occurs as the clusters enter the drift tube can be monitored by measuring the mass spectrum of the ions which exit the drift tube.
Experimental Methods The experimental apparatus and techniques have previously been described in detai17-12 so only a brief account will be given here. The carbon cluster ions are generated by pulsed laser vaporization of a graphite rod in a continuous flow of helium buffer gas. A 1.2-kV electron beam is injected into the buffer gas flow (approximately 1 cm from where the clusters exit the source) to enhance the abundance of cluster ions generated by the source. Previous experience with a variety of different clusters suggests that the temperatureof thecluster ions exiting the source is close to that of the source b l o ~ k . ~Cluster - ~ ~ ions which exit thesource are focused into a quadrupolemass spectrometerwhere a particular cluster size is selected. These size selected cluster ions are then focused into a low-energy ion beam and injected at various energies into the drift tube. The drift tube is 7.6cm long with 0.025-cm-diameter entrance and exit apertures. It was operated at room temperature, with helium as the buffer gas at a pressure of -5 Torr, and with a drift field of 13.12 V/cm. The drift tube has guard rings to ensure a uniform electric field.. After traveling across the drift tube, a small fraction of the ions exit and are focused into a second quadrupole mass spectrometer. By setting the second quadrupoleto transmit ions of different masses, the mobilities of the annealed parent ion or any of the fragment ions can be recorded. After passing through the mass spectrometer, the ions are detected by an off-axis collision dynode and dual micrcchannelplates. High-resolution mass spectra show isotope distributions in good agreement with those expected from the I3C/12C natural abundance ratio. However, all other studies were performed with low mass resolution to minimize mass discrimination. The mobility measurements were performed by using an electrostatic shutter to inject a 5 0 - b pulse ~ of size-selected cluster
0022-3654f94f 2098-18 10%04.50/0 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 7, 1994 1811
Annealing Carbon Cluster Ions
I
I
I
I
fraction surviving
d
k+. p.olycyclic rings
\--4 -3
'v?
100 eV. fullerene 150 eV 0.2 Y
0.0
200 eV
0
100
200
300
400
500
Injection Energy, eV
250 eV,
300 eVP I
I
I
I
500 1000 1500 2000 Time, microseconds
I
2500
Figure 1. Arrival time distributions recorded for C@+ with injection energies ranging from 100 to 300 eV. ions into the drift tube and recording the arrival time distribution at the detector with a multichannel scaler using 10-psresolution. These measurements are performed both with and without the buffer gas in the drift tube, and the average drift time, tD, is obtained from the difference between these two measurements (plus some small corrections). The reduced mobility is then determined fromI3 L P 273.2 KO = --t&l60 T where E the electric field, L is the length of the drift tube, and P is the buffer gas pressure in Torr. Figure 1 shows arrival time distributions recorded for c 6 0 + with injection energies ranging from 100 to 300 eV. The flight time from the drift tube to the detector has been subtracted from the time scale of these distributions so they represent the amount of time spent traveling across the drift tube. For injection energies less than 100 eV there are no significant changes in the arrival time distributions, so the 100-eV distribution shown in Figure 1 is representative of the isomer distribution exiting the source. Two main features are apparent: the sharp peak at -800 ps, which is due to the fullerene, and a broad component extending over 1200-2000 ps. The mobility of the fullerene isomer is identical, within the reproducibility of these measurements (&OS%), with the mobility of the Cm+ ion generated by laser desorption of a fullerene film. (The fullerene is the only c60+ isomer observed from the fullerene film.) The broad component at longer times is due to a mixture of unresolved polycyclic polyyne ring isomers. Only around 40% of the Cao+ ions which exit the source are in the fullerene geometry. Mass spectra of the carbon cluster ions generated by the source show the usual strong oddeven alternation in the relative intensities. However, the relative intensities of C a + and C ~ Oare + only slightly larger than their neighbors. This observation, along with the low fraction of fullerene Cm+ ions generated by the source, is consistent with cluster growth occurring under relatively low-temperature conditions where a significant fraction of the clusters can be trapped in unannealed geometries. In addition to the two main features in the arrival time distribution, there is a small peak at -600 ps which is assigned to fullerene CI2o2+. (The abundance of this feature varies significantly from day to day.)
Figure 2. Relative abundance of the isomers and fragmentsobserved for C52+ as a function of injection energy. The figure shows the fraction of C52+ ions which survive injection into the drive tube (0)along with the relative abundancesof the ring fragments (A)and the fullerene fragments ( 0 )(see text). The variations in the relative abundances of the resolved C S ~isomers + determined from the arrival time distributions (the fullerene (O), the monocyclic ring (A),and the other unresolved polyyne rings (B) are also shown. As the injection energy is raised, the arrival time distribution changes dramatically. The polycyclic ring isomers gradually disappear, and a new feature grows in at 1900 ps. As we have shown elsewhere,14this new feature has a mobility which suggests that it is due to a large monocyclic ring. We have now observed this isomer for clusters containing more than 80 atoms.l5 As the injection energy is raised further, the relative abundance of the monocyclic ring decreases, and ultimately only the fullerene remains. At intermediate injection energies (200 eV in Figure 1) a reproducible feature is apparent at 1800 ps. This feature has a mobility which suggeststhat it is due to bicyclic ring isomers (two fused polyyne rings)? At the higher injection energies another small feature becomes apparent at -1100 MS. This feature exists, at the higher injection energies, for all clusters with 30-70 atoms. It has previously been observed, for clusters with around 30-40atoms, in studies of unannealed carbon clusters, and it has been tentatively assigned to three-dimensionalpolyyne ring isomers? However, the resilienceof this isomer arguesagainst it being a polyyne ring isomer. Its mobility over the 30-70-atom size range is close to, but not in exact agreementwith, that expected for a roughly circular graphite sheet. Comparison of the measured arrival time distributions with those predicted by solution of the transport equation for ions in the drift tube can indicate whether the observed features are due to a single isomer or a mixture of isomers with similar mobilities. For the features assigned to the fullerene and the monocyclic ring, the measured arrival time distributions were in good quantitative agreement with the calculated distributions. This suggests that these features are due to a single isomeric form. However, it is unlikely that we would be able to separate fullerene isomers (such as those with adjacent pentagons) from icosahedral &+. And in the case of the monocyclic ring, we would probably not be able to resolve a 59-atom ring with one atom stuck to the outside from the 60-atom ring. We have examined in detail the annealing and dissociation of carbon cluster ions containing 52,58,60,64, and 70 atoms, and we have performed less detailedstudiesof theother even-numbered clusters containing 50-70 atoms. First we will present the results for the clusters studied in detail. Figures 2-6 show the relative abundances of the different isomers resolved in these studies (the fullerene, the monocyclic ring, and the other polyyne ring isomers which are grouped together) plotted against the injection energy. The results shown in these figures are the average of two or more independent measurements. Generally, the agreement between measurements performed on different days was reasonably good.
-
-
Hunter et al.
1812 The Journal of Physical Chemistry, Vol. 98, No. 7 , 1994 1.2 1
I
I
I
I
I
r
f raction surviving
e,
h,
-
k
U
20.6-
0.8
2 0.6
L*a* \
\
e,
.-> zo.4-e,
c*,*_(
\
r Y
0.2 -
A-C+-f/
100
0
-
~'P 7 '
monocyclic ring
/
0.0
I
.
0.2
ring -
1.2,
A
I
200 300 400 Injection Energy, eV
I
I
f u I lerene fragments
fragments
% ,
I - 2 h - A
I
0.0
500
Figure 3. Same as Figure 2 for C58+.
0
I
I n
5 0.05 2 -e,
500
+
I
I
-6 I
I
I
,
Mass, a m u
I
\
Cf
fullerene Y fragments ring k / fragments
\
0.2
-
0.0
- ring
'\
mon ocyc Iic
\A- /
