C60 interactions with surfaces, gaseous species and photons: an overview

Nuclear

Instruments

and Methods

in Physics Research

B78 (1993)

R!lOM B

118- 12.5

North-Holland

Beam Interactions with Materials (L Atoms

C,, interactions with surfaces, gaseous species and photons: an overview Dictcr M. Gruen Materials Scienc~e/ Chemistry Lhisiom,

The behavior processes

leading

fragmentation. detail if C,

of C,,

Argonne .VaGonal I.aboratory. Argonne, IL 60439, USA

in collisions with surfdccs, with other gaseous species, and with photons is discussed. The focus is on the

to tragmentation

of

the

molrculc,

the

identification

of fragmentation

products,

and

the

mechanism

The nature of the fragmentation

process of C,,, is of considerable interest in itself and needs to bc understood cluster ion beams are to be fully cxploitcd technologically. In addition. one may anticipate that an understanding

C,4, fragmentation

will contrihutc

mutatis mutandis

to an elucidation

of the elementary

steps leading to the assembly of C,

of in of

from

smaller carbon clusters.

1. Introduction

steps leading to the carbon clusters.

.

The availabthty of C,,, and other fullercncs in macroscopic quantities [ 1,2] opens up the possibility for generating high-intensity carbon cluster ion beams uscful for a variety of fundamental studies and potentially useful for applications in the field of surface modification. The propcrtics of C,, arc remarkable in many ways. The vapor pressure of the solid is 10 ’ Torr at 800 K cvcn though the C-C bond cncrgy is - 4 eV, comparable to that of graphite and diamond. Equally intercsting is the thermodynamic instability of C,, with respect both to diamond and graphite by - 0.37 eV under ambient conditions of temperature and pressure [3]. Buckminstcrfullerenc and the other fullerencs appear to bc unique among elemental clusters studied up to now in majntainjng their “cluster” characteristics in the condcnscd state. The stability of the fullercncs as “kinetic” entities has led to an explosion of scientific inquiry into the physical, physicochemical, and chemical properties of this fascinating new class of molecules. This paper concerns itself with the behavior of C,, in collisions with surfaces, with other gaseous spccics, and with photons. The focus is on the proccsscs leading to fragmentation of the molcculc, the identification of fragmentation products, and the mechanism of fragmcntation. The nature of the fragmentation process of C, is of considerable intcrcst in itself and, as will become apparent, needs to be understood in detail if C,, cluster ion beams are to be fully exploited tcchnologically. In addition, one may anticipate that an understanding of C,, fragmentation will contribute mutatis mutandis to an elucidation of the elemcntaty 0168-.583X/93/$06.0

1

1993 - Elsrvier

Science Publishers

2. Multiphoton of c,

assembly

excitation,

of C,,,

dissociation,

from

smaller

and ionization

The first systematic study of C, fragmentation was done by Smalley and co-workers in photodissociation/tandem time-of-flight (TOF) mass spectrometric experiments involving C, produced by laser desorption from a graphite rod [4]. They found the primary photodissociation channel for CM, to be the loss of neutral C, units with the activation energy for the initial C, loss estimated to bc 18 cV. With the availability of macroscopic amounts of C,,,, the low-energy collisional fragmentation of C, in MS/MS cxperimcnts was studied 151. When Xe was used as a colljsion gas, Cz loss was observed to begin at center-of-mass collision energies of - 23 cV, representing the upper limit necessary to initiate this process. With macroscopic amounts of pure C,,, it has also recently become possible to do quantitative studies of multiphoton-induced dissociation whose interpretation is uncncumbercd by problems associated with fragments originating from higher mass fullerencc constitucnts. The three major proccsscs occurring after absorption of several photons by gas phase C, arc direct ionization, dissociation into ionic and neutral fragmcnts, and delayed ionization. The neutral channel distribution appears very similar to the ion fragments and is probably the dominant channel. Only cvcn-numbercd fragments are obsctvcd in the range C,, to C,?

B.V. All rights reserved

D.M. Gruen / CbO inteructims with surfaces, gaseous species and photons [4,7]. The neutral low-mass fragments arc assumed to be C, molecules, although this conjecture still awaits experimental verification [lo]. Ph~)Loioniza~ion spectra arc displayed in fig. 1 whcrc some propensity is found for fragments with the “magic” numbers C& and C&. The C,, ion peak shows a broadening towards longer flight times, an effect attributed to delayed ionization resulting from thermionic emission [8,11]. The distribution of fragments can be understood on the basis of the stepwisc process C, ---)C,, + C, -+ C,, Dissociating cluster ions have been +2c,-, *... treated by Klots as evaporating species [12]. This approach regards the evaporation of hot clusters as a statistical unimolecular decay of a canonical ensemhlc. If

k(E)

=I((T,),

(1)

where k(E) is a microcanonical rate coefficient and is a canonical rate coefficient which defines an equivalent bath tcmperaturc Tb, then

k(T,)

20.0

40.0

30.0

In eq. (21, AE,,, is the vaporization (i.e., binding) energy of the cluster and y = 23.5 + 1.5. Wurz and Lykke found rate constants (fig. 2) for thermionic emission of electrons (dashed line) and fragmentation (full line) of excited C,. Since delayed electron emission and fragmentation arc strongly correlated, their mcasured delayed electron emission rate constants (open circles) serve to “fix” the vaporization or binding or

Fig. 2. Kate constants for thermionic emission (dashed line) and fragmentation (full line) of C,. (From ref. L9])

activation energy for cluster fragmentation. Thus, A&,, = 5.7 eV is required to give a fit to the measured data. A “temperature” characteristic of the internal energy distribution of the molecule for 4000 K is also given in fig. 2 {right-hand scale). Interestingly, evaporation of C, from C, has rccently been studied by kinetic energy release distribution measurements [13]. Again, the Klots model was used to interpret the data. For C, -+ C,, + C,, A&,,, was found to hc 6.05 eV with Tb = 3000 K. The kinetic temperature, r,, is not directly comparable to the internal energy “tempertlture” of the molecule.

3. Gas phase collisionally

induced endohedra1 complex formation

I

60.0

(2)

Tt, = Qap,ykB.

300

50.0

internal energy (eV)

/

I

400

500

600

1

700

J

/

800

900

1000

m/z Fig. 1. Time-of-flight spectra of photoionization of C,, provided from an effusive source. The mass peaks are the parent molecule C, (on the right) and the even-numbered fragment clusters C,,, C,,, . . down to C,,. Spectra have been selected for laser fluences that give approximately the same signal for the parent molecule at the detector. (From ref. [9])

fragmentation

and

A number of studies had in fact shown [14] that highly accelerated C& and C& ions lose C, unimolccularly and presumably sequentially upon collisional activation (CA) [15]. Marc recently, Wciskc ct al. 116,171 have shown that 8 keV C& and C&, when allowed to react with H,, D,, AX, or SF, in a collision cell, produce exclusively fragments in which C, (n = 2, 4, 6, 8) has been split off. A fundamentally different result was obtained when He was used as the collision gas. New signals appear at higher masses than the corresponding C,, loss. The mass difference is 4 and 3 when ‘He or 3H~ is used as the collision gas. It is thought that Hc is in~rporatcd inside the cage forming the endohedral C&/He complcx. Ross and Callahan [18] were also able to identify the complex and to show that fragmentation does not II. MULTIPLY CHAKGED/IlEAVY

120

IL+i. Gruen / C& interactions with surfaces, guseou~ species and photons

necessarily accompany adduct formation. Furthermore, it was shown the He is retained in the cage even after Xe collision-indu~cd fragmentation of C,, He resulting in loss of C, units. Wan, Christian, and Anderson [20] collided atomic beams of NC+ with neutral CGOvapor in an experiment where collision energies could be varied from - 0 to - 200 cV. The primary products were again fragments with variable C, loss. However, CMO_2nN~’ endohedral complexes were seen at about the 1% abundance level. The activation energy of at least 20 eV required for complex formation was found to increase with the number of C, fragments lost. The dominant mechanism would appear to be insertion to give C,,Ne’ which then “boils off’ variable numbers of C, units, depending on the collision energy. Wan et al. [21] went on to study collisions of Li+ and Na+ with neutral C,. With Li+, the main product is C,,Li+ which appears at 6 eV collision energy. At energies above 20 eV, the C,,Li+ signal decreases and a series of C,_,,Li+ fragment ions appear. At collision energies above 30 cV, CA, is observed with a cross section about 0.25 that of C,,Li+. As collision energy is increased above 40 cV, the C& signal decreases and is replaced by a series of C, loss fragments. Similar results were found with Na+ and the formation of Li and Na endohcdral complexes is a reasonable explanation of the phenomena encountered. For the high energy collisions ( ECE - 30 cV) where a large increase in the C& signal is observed, it is assumed that the alkali ion simply undergoes an elastic collision with a single carbon atom. In this limit, the C, internal energy after collision can be shown to be

Collision

formally equivalent to a temperature of - 3200 K. This is the temperature range where thermionic emission can lead to the efficient production of C& [7-9,111. Christian et al., in a revealing study of CA, showed fragmentation patterns resulting from collisions of energetic C- with a gas phase neutral Cc,,. Fig. 3 gives the cross sections as a function of collision energy for the major product ions observed.

The dominant product channel at all collision energies is C&, which can result from two mechanisms, both of which are 3.65 cV exoergic [IP(C,) = 7.61 cV]: c-+C,+C&+C (charge transfer) or c++c,+(c,:)*-,c;,+c (adduct formation/decay). At low collision energies, a significant fraction of the C& adduct survives the long flight time to the detector ( - 3 ms at 3 cV) and can be observed directly. The C& “cross section” appears to peak at low collision energies and decreases rapidly as increasing energy reduces the complex lifetime. The energy dcpcndence suggests that there is no activation energy for the C++ C, -+ C,+, process. The appearance energies of approximatcly 16, 24, 33, 37, 39 and 44 eV for Cc, through C 4+a,respectively, give the energy required to cause measurable fragmentation on the experimental timescale. Dissociation may also bc competing with radiative or other relaxation processes. In addition to the even-numbered fullerene fragment ions, one also obSCNCS a small amount of C:,, presumably resulting . . . from C, chmmatron from C,t,. The cross section for

Energy (eV)

Fig. 3. Cross sections (relative) for products of C+ f C,, collisions, plotted as a function of callision energy. The collision energy dependence for Cz6 and for CT n < 44 were not measured. (Righthand scale - C& only) (From ref. 1221)

D.M. Gruen / C,, interactions with surfaces, ~u:useou~ species and photons

C& increases with collision energy, but is approximately two orders of magnitude smaller than the evennumbered fragment channels.

4. Energetic

C, + ion beam interactions

with surfaces

In a prescient series of molecular dynamics simulations, Mowrey et al. studied collisions of C,, with a

121

hydrogen-terminated diamond(l11) surface. The positions and vcIocities of the atoms in C, and the surface were determined as a function of time. The potential energy function used permitted bond breaking and formation between C,, and the surface. Analysis of the trajectories gave detailed information about the dynamics of the collision process enabling energy thresholds for chemical reactions with the surface and the distribution of products as a function of impact energy to be calculated.

Fig. 4. Atomic positions for a trajectory resulting in nonreactive scattering at various times: (a) 14, (b) 54, Cc) 114, (d) 174, (e) 215, and (f) 294 fs. The initial collision energy is 250 eV. (From ref. [23]) II. MULTIPLY CHARGED/HEAVY

122

D.M. Gruen / C,, ~nte~act~~ with surfaces, gaseous species and photons

The efficiency of conversion of the center-of-mass translai .ional energy of the incident molecule to internal ene :rgy (heating) was of interest since a high conversion efficiency would result in highly vibrationally

excited molecules that could dissociate. The average increases in the internal energy of molecules that have undergone an unreactive collision are 43.4, 51.5, and 60.7 eV for incident energies of 150, 200, and 250 eV,

Fig, 5. Atomic positions for a reactive trajectory resulting in formation of C,s H at various times: (a) 114, (b) 174, (c) 241, (d) 299, (e) 354), and (f) 381 fs. The initial collision energy is 250 eV. (From ref. [231)

D.M. Gruen /

c,o inteructions

with surfaces, gaseous species and photonv

respectively, mostly in vibrational degrees of freedom corresponding to temperatures in the range 3~0-4000 K. It seems likely that such molecules will dissociate at time scales longer than the pro~~ation times used in the simulations in view of the results of Wm.?. and Lykke [7,9], for example, where multiphoton laser absorption processes lead to fragmentation of C, molecules excited to similar levels of internal energy. For 150 eV normal collisions, only nonreactive trajectories were seen in the simulations, while at 200 cV 14% and at 250 eV 72% of the trajcctorics were reactive. Fig. 4 shows the atomic positions at various times for a typical 250 eV nonreactive trajectory. The lower half of the molecule (4b) and the upper half (4~) flattens while the surface compresses. As the top layers of surface atoms return to their equilibrium positions, they cxcrt forces on C& causing dcsorption (4d, e). The rebounding molecule is in a vibrationally excited state (4f). Experimental observations bear out the ability of C,, to experience severe structural distortions during colhsions up to - 200 eV and to return to its original shape. McElvany et al. 1243colhdcd CL, and Ci: with a stainless steel surface at 60 and 120 eV, respectively, and observed only inelastic nonreactive scattering. Beck et al. [25] studied colhsions of C& and C,, at energies up to 200 eV with Si(100) and graphite(O~1) surfaces. Again, the collisions appeared to be highly inelastic and f~~mcntati~)n was not observed, at least on the us time scale allowed by the TOF spectrometer used in this study. As already discussed, at collision energies of 2.50 cV, the MD simulations of Mowrey et al. [23] showed the majority of the trajectories to be reactice. Fig. 5, taken from their paper, shows the atomic positions at several times beginning near the midpoint of one such reaction of C6(r with the hydrogen terminated diamond(l11) surface in which C&H was the product. In fig. Sa, a hydrogen atom has been abstracted from the surface by the buckminsterfulierenc, a process that created a radical carbon atom at the surface. Fig. 5b shows that this surface carbon atom reacted with a carbon atom in the molecule. In fig. 5c-e, the molecule rebounded from the surface while the atoms in the ncighborh(?(~d of the bond started to “unravel” forming a chain. The bond linking the molecule to the surface stretched as the molecule continued moving away from the surface and cvcntua~ly broke allowing the molecule to desorb (fig. 5f). Collisions occurring via this mechanism created molecules with relatively low ccnter-ofmass translational energies but high internal energies. Note that the C, fragment eliminated from the C,,, as a result of the collision has remained attached to the surface. Experimental results in the reactice collision regime have now been obtained by Busmann et al. [26] who

123

collided C, with HOPG graphite surfaces at energies between 138 and 412 cV, While Beck et al. cooled their C, beams in a supersonic He beam expansion, Busmann et al. used uncooled and therefore “hotter” cluster beams with a consequent reduction in charge exchange probability and therefore larger secondary ion fractions. Busmann et al. find that the intensity of the unfragmcntcd scattered C, clusters decreases with incrcasing energy between 250 and 450 eV. In fact, the intensity of the fragmented chtstcr ions when all masses bctwccn Cj, and C, are summed and normalized to the primary beam surpasses that of the unfragmcntcd C, at collision energies of about 400 cV. The prcdictions of the simulations of Mowrey et al. are thus strikingly confirmed. In a final observation in their paper, Mowrcy et al. go on to state that, at 250 cV, chemisorption is the most likely outcome because colliding C,, molecules retain insufficient center-of-mass translati~~nal energy to break the bonds linking them to the surface. In the event, the nature of such “chemisorbed” layers would be of considerable interest. Very recently, Hiss et al. [27] have impacted C, ion beams accelerated to 1 kcV onto NaCl and Si. Under these conditions, the scattered ion yield has vanished. Instead, carbon films were deposited which contain partly spa-bonded amorphous carbon and graphite nanostructurcs which arc preferentially oriented with the hexagonal sheets perpendicular to the substrate surface.

5. Concluding

remarks

Unimoiccular rate constants for the fragmentation of C, clusters for 5 32 can II. MULTIPLY C~ARGED/~~EAVY

124

D.M. Gruen / C,, interactions with surfaces, guseou~ species and photom

Acknowledgment

This work was supported by the U.S. Department of Energy, BES-Materials Sciences, under Contract W31-109ENG-38. I wish to thank Drs. Keith Lykke and Peter Wurz for several hclp~l discussions.

References K. Fostiropoulas and D.R. Huffman, Chem. Phys. Len. 170 (1990) 167. [2] W. Krgtschmcr, L.D. Lamb, K. Fostiropoulas and D.R.

[l] W. Kr&chmcr,

01

0

10

20

CA cluster

30

size,

number

40 of

50 carbon

60

I

70

atoms

Fig. 6.

be rationalized on the basis of free energy maximization leading to a compromise between thermodynamic stabilities and entropy considerations. Between C, and C,,, C, is the dominant fragmcnration channel while between C, and C,, the fragmcnts C,, C,, Cl,,, and C,, are also observed. It must be remembered that C,, is thermodynamically unstable so that once its special kinetic stability has been breached by loss of a single C,, further fragmentation appears to proceed quite readily. Finally, von Helden et al. 1331have been able, in an ingenious experiment, to obtain infor~tion on the structure of the C, fragments and by inference from these observations to gain insight into the mechanism resulting in the synthesis of C,. The experiment involved measuring mobilities of carbon clusters on a drift cell from arrival time distributions (ATDs) at a mass spectrometer. The results are displayed in fig. 6. The ATDs for C;-Cishowed only a single peak due to linear chains. Two peaks are seen between C: and C:,, most likely due to two isomeric (linear and cyclic) forms of the clusters. (Calibration mobility measurements were made using cyclic alkancs and linear alkenes.) For C;,-C&, a single peak is observed which correlates to the mobilities of the cyclic Ct-CT0 clusters since, for structurally related clusters, the mobility is expected to be a smoothly varying function of cluster size. A second, probably planar, ring appears at Cl, and persists to C& (Ring II). The various families of rings arc attribl;ted by the authors [33] to isomeric forms with mono-, bi-, and tricyclic planar structures as well as to three-dimensional structures. The mobility of pure C& correlates to the very high mobility structures which first appear at C$,. Therefore, the highest mobility fragments between C& and C& are assigned cage or fullcrene structures.

Huffman, Nature 347 (1990) 354. [3] D.R. Kirklin, AL. Smith, Y.-W. Hui, A. McGhie, W.J. Romanow and G. Zimmerman, Abstract 690 FUL, 18lst Meeting Program, The Electrochemical Society, May Ii’22, lY92, St. Louis, MO, USA. [4] S.C. O’Brien. J.R. Ileath, R.F. Curl and R.E. Smalley, J. Chem. Phys. 88 (1988) 220. [S) S.W. McElvany and J.11. Callahan, J. Phys. Chem. Y.5 (1991) 6186. 161K.R. Lykkc, M.J. Peilin, P. Wurz: D.M. Gruen, J.E. Hunt and M.R. Wasiclewski, Mater. Res. Sot. Proc. 206 (1991) 679. t71 P. Wurz, K.R. Lykke, M.J. Pellin and D.M. Gruen, J. Appl. Phys. 70 (1991) 6647. [81 P. Wurz. and K. Lykkc, J. Chem. Phys. 95 (1991) 7008. WI P. Wurx and KR. Lykke, J. Phys. Chem., submitted. [lOI P.P. Radi, L. Bunn, P.R. Kemper, M.E. Malchan and M.T. Bowers, J. Chem. Phys. 88 (1988) 280Y. Ill1 E.E.B. Campbell, G. Ulmer and I.V. Hcrtel, Phys. Rev. Lett. 67 (1991) 1986. WI C.E. Klots, 2. Phys. D21 (1991) 335; Chem. Phys. Lett. 186 (1991) 73; J. Chem. Phys. 90 (1989) 4470; Z. Phys. D20 (1991) 105; Act. Chem. Res. 21 (1988) 16; J. Phys. Chem. 92 (1988) 5864. [131 P. Sandier, T. Peres, G. Wcissman and C. Lifshitz, Proc. Schliersee Meeting, March 1992. [141(a) P.P. Radi, M.T. Hsu, J. Brodhclt-Lustig, M. Kincon and M.T. Bowers, J. Chem. Phys. 92 (1990) 4817; (b) AR. Young, L.M. Cousins and A.G. Harrison, Rapid Commun. Mass Spectrom. 5 (1991) 226; (c) D.R. Luffer and K.H. Schram, Rapid Commun. Mass Spectrom. 4 (1YW) 552; (d) P.P. Radi, M.T. Hsu, M.E. Rincon, P.R. Kemper and M.T. Bowers, Chcm. Phys. Lett. 174 (19YOf223; (e) C. Lifshitz, M. Iraqi, T. Peres and J.E. Fischer, Int. J. Mass. Spectrom. Ion Processes, in press: (0 C. Lifshitz, M. Iraqi, T. Peres and J.E. Fishor, Rapid Commun. Mass. Spectrom. 5 (1991) 238; (g) D. Schriider and D. Siilzie, J. Chem. Phys. 94 (lY91) 6Y33; (h) R.J. Doyle, Jr. and M.M. Ross, J. Phys. Chem., in press. [15] Reviews: (a) R.G. Cooks (ed.), Collision Spectroscopy (Plenum, New York, 1978); (b) K. Levscn and H. Schwarz, Mass Spectrom. Rev. 2 (1983) 77; (c) J. Bordas-Nagy and K.R. Jennings, Inst. J. Mass Spectrom. Ion Processes IO0 (1990) 105. [16] 1’. Weiskc, D.K. Biihme, J. Hrusak, W. Kdtschmer and H. Schwarz, Angew. Chem. Int., Ed. Engl. 30 (1991) 884. 1171T. Weiske, J. Hrusak, D. Biihme and Ii. Schwarz, C%em. Phys. Lett. 186 (1991) 459.

D.M. Gruen / C,, interactions

with surfaces, gaseous species and

[18] M.M. Ross and J.H. Callahan, J. Phys. Chem. 95 (1991) 5720. [19] K.A. Caldwell, D.E. Giblin, C.S. Hsu, D. Cox and M.L. Gross, JACS 113 (1991) 8519; K.A. Caldwell, D.E. Giblin and M.L. Gross, JACS (1991), in press. [20] Z. Wan, J.F. Christian and S.L. Anderson, J. Chem. Phys. 96 (1992) 3344. [21] Z. Wan, J.F. Christian and S.L. Anderson, J. Chem. Phys., in press. [22] J.F. Christian, Z. Wan and S.L. Anderson, J. Phys. Chem. 96 (1992) 3574. 1231 R.C. Mowrey, D.W. Brenner, B.I. Dunlap, J.W. Minhvire and C.T. White, Mater. Res. Sot. Symp. Proc. 206 (1992) 357; J. Phys. Chem. 95 (1991) 7138. 1241 S.W. McElvany, M.M. Ross and J.H. Callahan, Mater. Rcs. Sot. Symp. Proc. 206 (1991) 697. [25] R.D. Beck, P. St. John, M.M. Alvarez, F. Diederich and R.L. Whitten, J. Phys. Chem. 95 (1991) 8402. [26] H.-G. Busmann, Th. Lill and I.V. Hertel, Chem. Phys. Lett. 187 (1991) 459.

p~tons

12.5

[27] R. Hiss, H. Gaber, H.-G. Busmann and I.V. Hertel, Nature, submitted. [28] K. Raghavachari and S. Binkley, J. Chem. Phys. 87 (1987) 2191. [29] KS. Pitzer and E. Clementi, J. Am. Chem. Sot. 21 (1959) 4477; D.W. Ewing and G.V. Pfeiffer, Chem. Phys. Lett. 86 (1982) 365; K. Raghavachari, R.A. Whiteside and J.A. Pople, J. Chem. Phys. 85 (1986) 6623; S.J. Strickler and KS. Pitzer, in: Molecular Orbitals in Chemistry, eds. B. Pullmann and P.O. Lilwdin (Academic, New York, 1964) p. 281. [30] M.M. Kappes, Chem. Rev. 88 (1988) 369. [31] M. Kappes and S. Leutwyler, in: Atomic and Molecular Beam Methods, ed. G. Stoles (Oxford University, Oxford, 1988). [32] J. Drowart, R.P. Burns, G. De Maria and M.J. Inghram: J. Chem. Phys. 31 (1959) 1131. [33] G. von Helden, M.T. Hsu, P.R. Kemper and M.J. Bowers, J. Chem. Phys. 95 (1991) 3835.

II. MULTIPLY CHARGED/HEAVY