510
J . Phys. Chem. 1992, 96, 510-513
The thermochemistry also shows that the enthalpy change for methanol addition to H+T2to form H+MT2is more exothermic than for the analogous association of water to generate H’WT2. However, this trend is reversed in considering the addition to H+T3 to form H+MT3or H+WT3. This condition is also reflected in the ternary cluster ion distribution resulting from the expansion of a 1:l:l mixture (Figure 3), which indicates that the ion intensities for H+MT2 and H+WT3 are greater than those for H+WT2 and H+MT3, respectively. The energetic effects associated with cluster composition are consistent with (CH3)3Nmolecules binding to a H30+core ion. The cluster distributions and magic numbers observed in the beam expansion are also consistent with clusters incorporating H 2 0 in the center and (CH3)3Non the periphery. Therefore, the present results provide evidence for the thermodynamic origin of magic numbers in this system. The distributions are therefore affected by small differences, 1-5 kcallmol, in the relative stabilities of the clusters, despite the substantial energy deposited during electron impact ionization. This probably reflects the results of the last steps in consecutive evaporation, where the first steps remove most of the energy. The last step that results in the observed cluster distributions occurs from clusters with small internal energies, and small differences in the required endothermicity for the various competitive channels may be significant. The correlation between cluster distributions and energies can be used to calculate the rate constants for consecutive cluster evaporation and to test different evaporation m0de1s.l~ (13) Klots, C. E. J. Phys. Chem. 1988, 92, 5864.
The distributions also reveal that the (n + 2) rule breaks down for large clusters. In these cases, the H20core may start to form cyclic structures, making less hydrogens available for bonding to (CH3),N. This effect is observed in the distributions given in Figure 2a. For clusters containing five or more water molecules, sharp decreases in ion intensities occur when the cluster contains more than five (CHJSN molecules, suggesting the formation of a cyclic water pentamer with an extra labile proton. The formation of cyclic structures in large ions was suggested by Newton and Ehrenson,I4and Deakyne calculated that many isomeric structures, presumably also cyclic ones, can be very close in energy to the most stable structure in large clusters2 More recently, Castleman and co-workers have made similar observations on cluster distributions following MP17 and have suggested the formation of cyclic structures for H+W20and H+W2,. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the Thomas F. and Kate Miller Jeffress Memorial Trust for the partial support of this research. L.W.S. was supported by the Division of Chemical Science, Offrce of Basic Energy Science, US.Department of Energy. (14) Newton, M. D.; Ehrenson, S. J . Am. Chem. SOC.1971, 93, 4971. Newton, M. D. J . Chem. Phys. 1977,67, 5535. (15) Lias, S. G.; Liebman, J. F.; Levin, R. D. J . Phys. Chem. Ref Data 1984, 13, 685. (16) El-Shall, M. S.; Marks, C.; Sieck, L. W.; Meot-Ner (Mautner), M. J . Phys. Chem., in press. (17) Meot-Ner (Mautner), M. J . Am. Chem. Sac. 1989, 106, 1265.
Scanning Tunneling Microscopy of Cgoand CT0on Ordered Au( 111) and Au( 110): Molecular Structure and Electron Transmission Yun Zhang, Xiaoping Cao, and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (Received: October 1 1 , 1991; In Final Form: November 25, 1991)
Scanning tunneling microscopy (STM) images are reported for Csoand C70 layers on ordered Au( 111) and Au( 110) surfaces in air and in aqueous 0.1 M HC104. Hexagonal closepacked layers were obtained for both Csoand C70on Au( 111); however, the latter features distortions consistent with the presence of groups of ‘standing-up” and “lying-down” C70 orientations. While the fullerene layers are less ordered on Au( 1lo), the molecules are more rigidly held and can yield STM images with resolved intramolecular carbon rings. The likely modes of tipsurface electron tunneling via the adsorbed fullerenes, involving ‘superexchange” coupling, are discussed briefly in light of the observed STM imaging properties.
Recent reports of a straightforward procedure for synthesizing macroscopic quantities of C60(buckminsterfullerene) along with C701’2have triggered an intense broad-based effort aimed at elucidating the physical and chemical properties of these remarkable new allotropes of carbon. Substantial interest has been generated from an early stage in the scanning tunneling microscopy (STM) of Ca layers on metal and semiconductor surfaces, in both (1) (a) Kratschmer, W.; Fostiropoulous, K.; Huffman, D. R. Chem. Phys. Lett. 1990, 170, 167. (b) Kratschmer, W.; Lamb, L. D.; Fostiropoulous, K.; Huffman, D. R. Nature 1990, 347, 354. (2) (a) Haufler, R. E.; Conceicao, J.; Chibante, L. P. F.; Chai, Y.; Byrne, N. E.; Flanagan, S.; Haley, M. M.; OBrien, S. C.; Pan, C.; Xiao, 2.;Billups, W. E.; Cuifolini, M. A,; Hauge, R. H.; Margrave, J. L.; Wilson, L. F.; Curl, R. F.; Smalley, R. E. J . Phys. Chem. 1990, 94, 8634. (b) Ajie, H.; Alvarez, M. M.; Anz, S. J.; Beck, R. D.; Diederich, F.; Fostiropoulos, K.; Huffman, D. R.; Kratschmer, W.; Rubin, Y.; Schriver, K. E.; Sensharma, D.; Whetten, R. L. J . Phys. Chem. 1990,94,8630. (c) Parker, D. H.; Wurz, P.; Chatterjee, K.; Lykke, K. R.; Hunt, J. E.; Pellin, M. J.; Hemminger, J. C.; Gruen, D. M.; Stock, L. M. J. Am. Chem. SOC.1991, 113, 7499.
0022-365419212096-5 10$03.00/0
air and ultrahigh-vacuum (uhv) environments.” Related studies utilizing atomic force microscopy (AFM) have also been rep ~ r t e d . ~Besides ,~ seeking information on intermolecular packing (3) Wilson, R. J.; Meijer, G.; Bethune, D. S.; Johnson, R. D.; Chambliss, D. D.; de Voies, M. S.; Hunziker, H. E.; Wendt, H. R. Nature 1990,348,621. (4) Wragg, J. L.; Chamberlain, J. E.; White, H. W.; Kritschmer, W.; Huffman, D. R. Nature 1990, 348, 623. ( 5 ) (a) Li, Y. 2.;Patrin, J. C.; Chandler, M.; Weaver, J. H.; Chibante, L. P. F.; Smalley, R. E. Science 1991,252,547. (b) Li, Y. 2.;Chander, M.; Patrin, J. C.; Weaver, J. H.; Chibante, L. P. F.; Smalley, R. E. Science 1991, 253, 429. (6) (a) Chen, T.; Howells, S.; Gallagher, M.; Yi, L.; Sarid, D.; Lichtenberger, D. L.; Nebesny, K. W.; Ray, C. D. J . Vac.Sci. Technol., in press. (b) Chen, T.; Howells, S.; Gallagher, M.; Yi, L.; Sarid, D.; Lichtenberger, D. L.; Nebesny, K. W.; Ray, C. D. Mater. Res. Sac. Symp. Proc. 1991, 206, 721. (7) Snyder, E. J.; Anderson, M. S.; Tong, W. M.; Williams, R. S.; Anz, S. J.; Alvarez, M. M.; Rubin, Y.; Diederich, F. N.; Whetten, R. L. Science 1991, 253, 171. (8) Sand, D.; Chen, T.; Howells, S.; Gallagher, M.; Yi, L.; Lichtenberger, D.L.; Nebesney, K. W.; Ray, C. D. Appl. Phys. Lett., in press.
0 1992 American Chemical Society
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The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 511
and molecular stereochemistry,” STM can be anticipated to shed light on the spatial electronic properties of Cm Some preliminary information of the latter type has indeed beem reported.6 We have recently obtained highquality atomic-resolution STM images of ordered low-index gold surfaces in aqueous media under potential control? While surface reconstruction is observed to occur at all three low-index faces at negative electrode charges, essentially unreconstructed (1 X 1) surfaces are present at more positive potential^.^ This high degree of structural definition, together with the excellent stability of gold surfaces in air as well as in electrochemical environments, has led us to investigate the STM properties of monolayer (and submonolayer) films of Cm and C70 on Au(ll1) and Au(ll0) in air and in aqueous 0.1 M HC104. Unlike Cm, the C70 molecule is anticipated to be nonspherical;2 however, no STM images of pure C70 layers have apparently been reported previously. The results, presented herein, provide information on the electronic and surface conformational properties of these fullerenes, as well as their intermolecular packing.
Experimental Section Most details of the STM measurements are available elsewhere.9-’’ The microscope is a commercial Nanoscope I1 instrument (Digital Instruments, Inc.), with a bipotentiostat for in-situ electrochemicalSTM. The images shown here were obtained in the so-called “height mode” (i.e., constant-currentmode) in air, except where noted otherwise. The acquisition time for each image was typically 15-20 s (400 scans, each ca. 200 A). The set-point current was typically 0.5-10 nA,with bias voltages from 0.1 to 0.7 V, so that the effective ‘gap resistance”R, varied from ca. lo7 to IO9 ohm. While the form of the images is dependent somewhat on the magnitude of the current and bias voltage (vide infra), it is insensitive to the bias direction (Le., whether the tip has a negative or positive potential with respect to the surface). The STM tips were made from 0.01-in.-diameter tungsten wire etched electrochemically in 1 M KOH. Calibration of the x-y distances for a given piezo driver was achieved from the measured interatomic spacings on the clean Au( 11 1) and Au( 1 10) substrates. The Au( 1 11) and Au(ll0) crystals (hemisphere, 7-mm diameter) were grown, cut, and polished in Laboratorie d’Electrochimie Interfaciale du CNRS, Meudon, France, by Dr.A. Hamelin.I2 The crystals were flame annealed immediately prior to each experiment and cooled in ultrapure water. This pretreatment procedure was found to yield largely unreconstructed Au( 1 10) as well as Au( 1 1 1) surfaces when examined subsequently by STM in air. The Cm and C70 films were prepared by evaporating on the gold surfaces a few microliters of a fresh ca. 0.01 mM solution of the fullerene in dichloromethane (cf. ref 13). The solid Cm and C70 samples were prepared and purified chromatographically according to procedures in refs 1 and 2. Electrode potentials were measured versus an oxidized gold wire but are reported versus the saturated calomel electrode (SCE). Results and Discussion Not surprisingly, the solution-evaporation method of preparing the fullerene films did not yield entirely uniform adlayers. Nevertheless, regions of uniformly close-packed Cm molecules could readily be obtained on Au( 11 1). Figure 1A shows a typical unfiltered top view image of such a region in air. Similarly to the results of Wilson et al. obtained on Au(l11) in u ~ v a, hex~ (9) (a) Gao, X.; Hamelin, A.; Weaver, M. J. Phys. Reu. Lett. 1991, 67, 618. (b) Gao, X.; Hamelin, A.; Weaver, M. J. Phys. Reu. R 1991,44, 10983. (c) Gao, X.; Hamelin, A.; Weaver, M. J. J . Chem. Phys. 1991, 95, 6993. (IO) Chang, S.-C.; Yau, S.-L.; Schardt, B. C.; Weaver, M. J. J . Phys. Chem. 1991, 95,4787. (1 1) Yau, S.-L.;Gao, X.; Chang, S.-C.; Schardt, B. C.; Weaver, M. J. J. Am. Chem. Sue. 1991,113,6049. (12) See Appendix A in: Hamelin, A.; Morin, S.; Richer, J.; Lipkowski, J. J . Electroonal. Chem. 1990, 285, 249. (13) Zhang, Y.; Edens, G.; Weaver, M. J. J . Am. Chem. Soc. 1991,113, 9395.
Figure 1. (A, top) Lnfiltercd top view molecular-resolution STM images of close-packed Cm layer on Au( 1 1 1 ) in air. Imaging conditions: bias voltage (VJ = 100 mV, tunneling current (i,) = 1.5 nA. (€3, bottom) Filtered height-shaded image of Cm domain similar to (A). V, = 200 mV, i, = 1.4 nA.
agonal close-packed structure is clearly observed, where the Cm molecules appear as near-circular spots of enhanced tunneling current (depicted, as usual, as brighter regions in the gray scale STM images). The intermolecular spacing is 11 f 1 A, a p proximately consistent with the anticipated van der Waals diameter of Cm (cf. ref 3). A smaller region of a similar hexagonal domain is shown in the form of a mildly filtered height-shaded STM image (60’ off surface normal) in Figure 1B. The z displacement between the Cm centers and surrounding minima, Le., the z corrugations within these close-packed domains obtained at constant tunneling current, is 1.4-1.8 A for lower gap resistances (cf. ref 3), although smaller corrugations tended to be obtained at the largest R, values. The corresponding z displacement between the Cmcenters and nearby bar Au( 1 1 1) terrace atoms is 3.5-4 A. These values are distinct1 smaller than the corresponding height differences, ca. 5 and 10 ,anticipated from the Cm diameter. This finding indicates that electron tunneling between the STM tip and a given adsorbed C60molecule is less efficient than to the gold substrate at the same spatial separation (vide infra). In addition to such hexagonal domains, surface regions on Au( 1 1 1) were obtained that featured apparently random packing
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Letters
512 The Journal of Physical Chemistry. Vol. 96, No. 2, 1992
Figure 2. Unfiltered ring-resolution images of individual Cnomolecules on Au(1 IO) in 0.1 M HCIO, at -0.2 V vs SCE. v b = 100 mV, i, = 1 . 1 nA; 'current mode" (i.e., constant-height mode).
of Cmmolecules at lower coverages. Such images tended to be less stable than those for the close-packed layers, especially at low gap resistances (i.e., for relatively small substratetip distances), consistent with translational movement of Ca caused by the tip scan. On Au(1 lo), however, stable images of isolated Ca molecules at lower coverages could reproducibly be obtained. This finding suggests that the Cmcould be wedged between the parallel rows of Au atoms, spaced 4 A apart, which constitute the (1 X 1) Au( 110) surface?b Alternatively or additionally, Ca may be immobilized at the domain edges or other undulations that abound on the 'atomically rough" Au(l10) ~urface.'~ Furthermore, unlike the Au(l11) images, the internal ring structure of Cmcould often be resolved in STM images obtained on Au( 1 lo), both in air and in aqueous solution under electrode potential control. A typical unfiltered image of randomly packed Ca molecules featuring such apparent ring resolution is shown in Figure 2, obtained in 0.1 M HCIO4 at an electrode potential of -0.2 V vs SCE. Two distinct, yet reproducible, image patterns can be distinguished. The first, as evident (albeit not clearly) in the top right-hand comer of Figure 2, contains one central "dark" ring surrounded by six others. The second, exemplified by the molecule seen in the central left-hand edge, features instead five rings surrounding the central one. Examination of the C , icosahedral structurel4 indicates that two alternate pictorial views, consistent with these images, are indeed predicted. One involves a central hexagon (C,)ring encircled by three pentagons and three hexagons; the other features a central pentagon framed by five hexagons. Such observations imply a remarkable conformational rigidity for Ca on Au(1 IO). The corresponding images of C70 layers show some significant differences to Ca. Figure 3A shows a typical STM image (height-shaded plot, 30" off surface normal) for a close-packed C70 layer on Au( 1 1 1) in air. While the structure is near-hexagonal, some asymmetry is evident in that the rows from bottom left to top right have a slightly longer intermolecular spacing (12 f 1 A) than in the other two hexagonal directions ( 1 1 f 1 A). In addition, some clusters of molecules, such as in adjacent rows, exhibit significantly different heights (z displacements). A clase-up illustration of this nonuniformity is given in Figure 38. Rows containing 'higher" and 'lower" C70 molecules are evident, with an effective height difference of 0.6-1.1 A. The z displacements between the CT0centers and surrounding minima, and the bare Au(l11) terrace regions, were comparable to those noted above for Cm. ~
~~
~
~
(14) For example: Curl, R. F.; Smalley, R. E. Sci. Am. 1991 (Oct), 54.
Figure 3. (A, top) Height-shaded molecular-resolution image of closepacked Clb layer on Au( 1 1 1 ) in air. V, = 430 mV, it = 4.5 nA. (B, bottom) Filtered close-up image of Ct0 domain as in (A). v b = 750 mV, it = 10nA.
The C70 molecule is believed to be a prolate spheroid, related to Cmby the addition of a central cylindrical I0-atom belt.I5 On this basis, the z corrugation observed in the STM images most probably arise from two (or more) surface conformations, involving "standing" or "lyingdown" C70orientations correspondingto the minor and major axes being parallel to the surface. The height difference between these conformations as estimated from molecular models, 1.7 A, is indeed comparable to (although somewhat greater than) the z corrugation observed in the STM images. The propagation of a given molecular conformation along a row in the hexagonal structure, as suggested by the z cormgation patterns (Figure 3), also can account for the observed asymmetry in the intermolecular distances noted above. The STM images of close-packed C70 on Au(l10) in air exhibit some degree of order, favoring a quasi-hexagonal arrangement, but with less regularity than observed on Au( 1 11). These differences are not surprising given the rectangular, rather than hexagonal, packing of the gold substrate atoms on unreconstructed Au( 1 10). Although less clear-cut than for Ca on Au( 1 lo), several ~~
~
( 1 5) For example: (a) Raghavachari, K.; Rohtfing, C. M. J . fhys. Chem. 1991, 95, 5768. (b) Manolopulos, D. E.; May, J. C.; Down, S. E. Chem. Phys. Leu. 1991, 181, 105. ( c ) Wasielewski, M. R.; O"ei1, M. P.; Lykke, K. R.; Pcllin. M. J.; Gruen, D. M. J . Am. Chem. Sm. 1991, 113. 2774.
Letters
The Journal of Physical Chemistry, Vol. 96, No.2, 1992 513 can occur even for bridging orbitals having energies up to several electronvolts above (orbelow) the redox orbital energies.2021 This concept can be transposed readily to substratetip electron tunneling mediated by adsorbate molecular orbitals, including those lying energetically far from the metal Fermi level. Given the availability of a relatively low-lying LUMO (1.9 eV above the HOMO energy in CmZ2),one could readily anticipate either electron (or hole)-driven superexchangeZ1being responsible for the observed molecular (and ring)-resolution STM images, Le., the facile electron tunneling observed via the Cm and C70adsorbates. Such a mechanism can account readily for the observed insensitivity of the STM images to the sign of the potential bias. The present results prompt two more specific comments along these lines. As noted above, the measured z displacement, Azm, for moving the tip (at constant tunneling current) from, say, the center of an adsorbed Cmmolecule to the bare Au( 1 1 1) surface, 3.5-4 %(, is rather smaller than the "true" displacement, Aq 10 A, anticipated from the Cm diameter. However, Azm will approach Az,only in the limit where the electronic transmission coefficient, Kel, through the adsorbate is unity (as it is through the metal). Nevertheless, the observation that L\zm> 0 (Le., that z corrugation is obtained) infers that the tunneling probability a,I decays less steeply with increased distance from the surface, d,, via the fullerenes than through air. The extent, ca. 2-fold, to which this ~ , ~ - dependence d, is attenuated for tunneling via the fullerenes is roughly consistent with the diminished slopes of log K J ~plots , anticipated for superexchange via hydrocarbon bridges as compared with through-space electronic coupling.20 The apparent resolution in the STM images of carbon ring structures for Cm and C70 on Au(l10) implies further that the through-bond mechanism of electron transmission is operative. Theoretical treatments for sp3 hydrocarbon linkages emphasize the likely dominant role of through-bond coupling;20long-range electron transmission should be even more effective through the sp2-hybridized fullerene frameworks.23 Chen et a1.6 have also reported some STM images for Cm,although on polycrystalline Au surfaces, that show intramolecular structural details. Their images appear to show unequal imaging from alternate ring carbons? Such detail, however, could not be discerned clearly in the present data. Overall, then, besides structural information obtainable for the fullerenes from STM, these molecules provide intriguing systems with which to explore the nature of electron transmission in STM experiments. Further studies of this type, such as for metal fullerene complexes and other derivatives, are well worth pursuing from both standpoints.
-
Figure 4. Top view image of CT0on Au( 1 10) in air. V , = 100 mV, it = 3.0 nA.
images of CT0on Au( 1 10) also showed evidence of resolved ring structures. An example is given in Figure 4. A number of the molecular images are seen to contain distinct internal "patches", although it is not possible to discern clearly reproducible patterns as seen for Cm on Au( 1 10) (Figure 2). It is appropriateto discuss briefly the ring resolution and other topographical STM features in terms of likely electron-tunneling mechanisms. The nature of tipsurface electron tunneling, especially via molecular adsorbates, might be expected to share common features with bridge-assisted electron transfer in purely molecular systems.16 Perhaps surprisingly, however, most discussions of electron transmission in STM have emphasized, and even been restricted to, simple concepts of tunneling through a square-well barrier," so that the electronic properties of the molecular adsorbate tend to be neglected. The perturbations caused by molecular as well as atomic adsorbates on the STM tunneling tend to be interpreted in terms of electronic densityof-state arguments, involving consideration of adsorbate-surface charge sharing via orbitals of the former lying energeticallyclose to the metal substrate Fermi le~eI.~'-l~ A central concept employed in bridge-assisted molecular electron transfer, yet strangely neglected so far in discussions of STM electron tunneling, is that of "superexchange". In this mode of electron transmission, either unfilled or filled ligand molecular orbitals mix (as virtual states) with those of the donor and acceptor redox sites so to facilitate through-bond electronic coupling and hence the Occurrence of nonadiabatic electron It is important to recognize that significant superexchange mixing (16) For explanative reviews, see: (a) Mikkelsen, K. V.; Ratner, M. A. Chem. Reu. 1987.87. 113. (b) Newton, M. D. Chem. Reo. 1991, 91,767. (c) Sutin, N. Ado. Chem. Ser. 1991, No. 228, 25. ( 1 7) For recent overviews, see: (a) Ogletree, F.; Salmeron, M. Prog. Solid Srare Chem. 1990.20.235. (b) Wu, X. L.; Lieber, C. M. Prog. fnorg. Chem. 1991,39,431. ( 18) For molecular adsorbate STM imaging, see for example: (a) Lippel, P. H.; Wilson, R. J.; Miller, M. D.; Wbll, Ch.; Chiang, S. Phys. Reo. Len. 1989.62. 171. (b) Gimzewski, J. K.; Stoll, E.; Schlittler, R. R. Surf. Sci. 1987, 181, 267. (19) (a) Lang, N. D. Phys. Reo. Leu. 1987,58,45. (b) Lang, N. D. f B M J . Res. Deu. 1W6,30, 374.
Acknowledgment. We are grateful to Dr.Antoinette Hamelin for preparing the ordered Au( 111) and Au( 110) surfaces. We thank Prof. Bart Kahr for providing a sample of purified C70 and Joe Roth and Lance Safford for preparing purified Cm Prof. Dennis Lichtenberger kindly supplied refs 6 and 8 in advance of publication. This work is supported by the Office of Naval Research and the Natioual Science Foundation. (20) For example: (a) Larsson, S. Chem.Scr. 1988,28.4,15. (b) Onuchic, J. N.; Ekretan, D. N. J . Am. Chem. Soc. 1987, 109, 6771. (c) Beretan, D. N.; Onuchic, J. N. Ado. Chem. Ser. 1991, No. 228, 71. (2 1 ) The superexchange is considered to be electron- or hole-type transfer, depending on whether high-lying unfilled, or occupied bridging orbitals, respectively, are involved.16 (22) Curl, R. F.; Smalley, R. E. Science 1988, 242, 1017. (23) Some deviations from pure sp2 hybridization are anticipated for C, resulting from its nonplanar structure.*' (24) Hadden, R. C.; Brus, L. E.; Raghavachari, K. Chem. Phys. Leu. 1986. 125, 459.
