9025
J. Phys. Chem. 1992,96,9025-9028 would only have to displace the ends of the alkyl chains rather than the entire chain. The decalins are more rigid, requiring that the whole molecule be displaced in the course of acridine orange reorientation. Isopropylcyclohexane represents an intermediate case. The correlation between in-plane reorientation time and interfacial roughness is thus likely to owe to the fact that flexible molecules, which present a smoother interface, have lower interfacial friction due to their flexibility. Conclusions Liquid alkane/water interfaces are quite smooth on the molecular scale. Rigid alkanes on the order of the size of the probe have a rougher interface, as sensed by the probe. Liquid/liquid interfaces comprising n-hexadecane and water are smoother than those comprising n-octadecylsioxanemonolayer/water interfaces'5 by a factor of 3, as shown by out-of-plane reorientation of the probe. For these same two interfaces, in-plane reorientation times are faster on the liquid/liquid interface by a factor of approximately 2. The decalin/water interfaces behave virtually the same as the n-octadecylsiloxane monolayer/water interface for both in-plane and out-of-plane reorientati~n.'~ This indicates that the covalently bonded monolayer alkyl chains are less flexible than those of liquid n-hexadecane.
Acknowledgment. This work was supported by the National Science Foundation under Grant CHE-9 1 13544.
References and Notes (1) Weber, T. A.; Hefland, E. J . Chem. Phys. 1980,72,4014. (2)Kloubek, J. Colloids Surf 1990,48, 323. (3)Kloubek, J. Colloids Sur/. 1991,55, 191. (4)Heinz, T.F.;Chen, C. K.;Ricard, D.; Shen, Y. R. Phys. Rev. Lett. 1982,48, 478. ( 5 ) Goh, M. C.: Hicks. J. M.:Kemnitz. K.: Pinto. G. R.:Bhattacharvva.. K.;'Ehenthal, K. B. J. Phys. Chem. 1988,92,5074. (6)Xiaolin, Z.;Goh, C.; Eisenthal, K. B. J. Phys. Chem. 1990.94,2222. (7)Higgins, D. A.; Byerly, S. K.;Abrams, M. B.; Corn, R. M.J . Phys. Chem. 1991,9.5,6984. ( 8 ) Moaz, R.; Sagiv, J. J. Colloid. Inr. Sci. 1884,100, 465. (9)Nuzzo, R.G.; Dubois, L. H.; Allara, D. L. J . Am. Chem. Soc. 1990, 112. 558. (10)Duevel, R. V.; Corn, R. M.; Lui, M. D.; Leidner, C. R. J . Phys. Chem. 1992,96,468. (1 1) Pershan, P. S. Furaduy Discuss. Chem. SOC.1990,89,231. (12)Lofaren, H.: Neuman. R. D.: Scriven. L. E.: Davis. H. T. J . Colloid Interface SG.1984.-98.175. (i3) Sauer, B. B.; Chen, Y.; Zografi, G.; Yu, H. hngmuir 1986,2,683. (14)Zanker, V. Z.Phys. Chem. 1952,199,225. (15) Burbage, J. D.; Wirth, M. J. J . Phys. Chem. 1992,96, 5943. (16)WirthfM. J.; Burbage, J. D. Anul:Chem. 1991,63,1311. (17)Viswanath, D. S.; Natarajan, G. Dura Book on the Viscosity of Liquids; Hemisphere Publishing: New York, 1989. (18)Wirth, M. J.; Chou, S.-H. J . Phys. Chem. 1991, 95, 1786. (19)Matsuoka, Y.; Yamaoka, K. Bull. Chem. Soc. Jpn. 1979,52,3163. (20)Kinosita, K.; Kawato, S.; Ikegami, A. Biophys. J . 1977,20, 289. (21)Chou, S.-H.; Wirth, M. J. J. Phys. Chem. 1989,93,7694. (22) Buff, F. P.; Lovett, R. A.; Stillinger, F. H. Phys. Rev. Lett. 1965,15, 621.
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Order and Disorder in CBoLangmuir-Blodgett Films: Direct Imaging by Scanning Tunneling Microscopy and High-Resolution Transmission Electron Microscopy Ping Wang, Mohammad Shamsuzzoha; Xiang-Li Wu, Wan-Jin Lee, and Robert M. Metzger* Departments of Chemistry and Metallurgical and Materials Engineering and School of Mines & Energy Development, University of Alabama, Tuscaloosa, Alabama 35487-0336 (Received: July 7, 1992)
c 6 0 forms very rigid insoluble films ("monolayers") at the air-water interface (collapse pressure n, = 62.5 mN/m). The "area per molecule" A. = 21.6 A2 (at zero pressure) and A, = 11.5 A* (at n,) are too small for a single Cmmolecule and suggest 4-7.5 Cmmolecules piled on top of each other, per "monolayer". Cmforms Langmuir-Blodgett (LB) films on glass and HOPG and Langmuir-Schaefer (LS) films on a Cu grid. The ellipsometric LB film thickness is about 2 molecules per "monolayer". The STM (LB) and HRTEM (LS) images show that the Cmfilms have 2-dimensional and 3-dimensional crystalline regions. The STM image shows a slightly distorted (0131projection of an face-centered cubic (fcc) pattern of roundish, atomically unresolved peaks, with a cell constant of 14.2 A. The HRTEM micrographs and a selected-areaelectron diffraction pattern show that, at optimum crystallinity, the Cbomolecules pack in an fcc pattern with a cell constant of 14.20 f 0.05 A. No hexagonal close-packed structures were found.
Introduction The breakthroughs in synthesis of c 6 0 and related fullerenes have stimulated a series of studies of the structural, electronic, and electrical properties of these novel forms of carbon, in both pure and compound form.'" The structural studies of c 6 0 by X-ray diffraction (XRD), scanning tunneling microscopy (STM), atomic force microscopy (AFM), and high-resolution transmission electron microscopy (HRTEM) have firmly established that the Cmmolecule is a polyhedron with 20 six-membered rings and 12 five-membered rings (which gave it the trivial name 'fussballene" in Germany and "buckminsterfullerene" in the US), with the carbon atoms roughly distributed on the surface of a sphere. In a crystal, the Cm molecules form a close-packed array, which, with equal packing density, could have either a face-centered or a close-packed cubic (fcc) structure, with a series of (1111 To whom correspondence should be addressed.
'Department of Metallurgical and Materials Engineering and School of
Mines and Energy Development.
closepacked planes stacked in ...ABCABC...order, or a hexagonal close-packed (hcp) structure, with (0001) close-packed planes stacked in ...ABAB order. Whereas the early work on c 6 0 involved crystals or vapor-deposited films, Cmalso forms7-'O an insoluble film at the air-water interface;11J2this film, or "monolayer", can be transferred onto solid substrates by the vertical or Langmuir-Blodgett (LB)13J4 method or by the horizontal or Langmuir-Schaefer (LS)15transfer method. LB and LS films of e 6 0 have been studied by Obeng and Bard,' by Zhu and @workers,8 by Kawabata and @workers? and by Bryce, Petty, and co-workers.lo These films form because the spherical c 6 0 molecules are hydrophobic and rigid?JO Kratschmer and co-workers studied powders of Cmby X-ray powder diffraction and reported a hexagonal crystal system (hcp) with unit cell parameters a = 10.02 A and c = 16.36 A.I Also, for selected area electron diffraction and HRTEM studies on LB films, a hexagonal projection with a = 10 A was reported by Zhu and co-workers.* On the other hand, X-ray powder diffraction show at room temperature an fcc lattice, with lattice spacing a
...
0022-3654/92/2096-9025$03.00/0 0 1992 American Chemical Society
9026 The Journal of Physical Chemistry, Vol. 96, No. 22. 1992
= 14.11' or 14.04 f 0.01 which imply an intercluster spacing of 9.984or 9.93 A.S (Below 249 K, the structure transforms to simple cubic, with a = 14.04 i 0.01 A at 11 IC5 The singlecrystal X-ray structure of the osmylated derivative of C, shows an average radius of 3.51 A.I6 Several studies of the surface of C, particles and vapor-deposited films on different substrates have been done by scanning tunneling microscopy (STM). Films of C, sublimed onto Au (1 1 1 ) substrates, studied in ultrahigh vacuum, reveal a closepacked ~tructure.!~J~ The STM intercluster spacing of 11.0 0.5 A has been explainedi7 as close to the sum of either the calculated diameter of C, (7.1 A) or the average diameter (= internuclear distances across the ball = 7.02 A) of the osmylated derivative of C, plus the van der Waals diameter C (= 'outerfringe distance of the *-electron cloud extending outward from the moleculen7= interlayer spacing in graphite = 3.35 A).'' C, monolayers and multilayers deposited epitaxially on GaAs (1 10) substrates by sublimation in ultrahigh vacuum also show a slightly distorted closed-packed array. To minimize the influence of substrates, Whetten and co-workerssublimed 1500-A-thick films of pure Ca onto CaF2(1 1 1) substrates and studied them by X-ray diffraction and AFM, which show fcc (31 1) planes and not an hcp struct~re.!~ This paper focuses on LB film structures of C, on several substrates, studied by both STM and high-resolution transmission electron microscopy (HRTEM). One goal of this study has been to determine the packing of the C, molecules within an "LB monolayer"; another goal has been to establish whether these films are fcc or hcp. We present below firm evidence that our films are fcc.
Wang et al.
*
Experimental Procedures Ca (stated purity 99.9%)was obtained from Texas Fullerenes Corp. and used without further purification. A C , solution in benzene (1.297 X 10-4 mol/L) was carefully spread on a purified water subphase (Millipore Milli-Q, conductivity 16 Ma) at room temperature (20 "C) in a Lauda film balance (Langmuir trough) set on a vibration-free table in a room provided with filtered air. The insoluble film of C , at the air-water interface (called a Langmuir film or a Pockels-Langmuir (PL) film*O)was formed by waiting for 20 min for the benzene to evaporate. The PL films were transferred onto several substrates: glass, a oopper grid for electron microscopy, and highly oriented pyrolytic graphite (HOPG, Union Carbide grade ZYH). For the films on glass and HOPG, vertical (LB) transfer was used. For HRTEM, the PL film was transferred onto a Cu grid by quasi-horizontal (LS) transfer, with the film pressure held at 20 and 45 mN/m, respectively. The transfer was assumed to be Ztype?Jo The films were transferred at different constant pressures, such as 10,20, and 45 mN/m, to detect resulting differences in film structure or orientation, but none were found. The thickness of the C , LB film on a glass substrate was measured by ellipsometry (Rudolph Auto-EL-111, with a H e N e laser operating at 632.8 nm). Two samples of 91 and 100 layers were used. The real (n) and complex (k)parts of the index of refraction of the glass microscope slide substrate were measured as n, = 1.424 and k, = 0.001. For the yellowish films of Cm k = 0 was assumed; the index of refraction nfand the thickness tf for the film varied somewhat in different regions of the films. XRD of the films (using a Rigaku thin-film diffractometer) revealed no characteristic peaks for multilayer films deposited on either glass or graphite. The orientation of the C, LB film on HOPG was studied by STM in air at room temperature, using a Digital Instruments Nanoscope I1 quipped with a type A head (0.6-pm maximum scanning range), and a Pt/Ir tip, using bias voltages between 0.2 and 1 V, the constant current mode, and set point currents of 2 or 5 nA. The STM images were automatically corrected for tilt by a plane-fitting procedure. The C, LS film growth on Cu electron microsoope grids was examined in a 200 KV Hitachi H-800transmission electron microscope. High-resolutionmicrographs were recorded near the
Figure 1. A typical surface prtssurtarca isotherm of Cmon pure water at 20 OC.
Figure 2. STM image (after one low-pass filtration of the data) of a Cm LB 20-layer film on H O E : a 4 X 4 nm2area, showing a (0131 projection
of an fcc lattice, with distances of 7.1 A ([200] lattice direction in the horizontal direction) and 5.0 A (( 13 11 direction).
optimum defocusing, typically at a magnification of 2 X lo5 diameters. The Cm atomic clusters appear black under these experimental conditions.
ReSdQ A typical pressurearea isotherm for a PL film of C, is shown in Figure 1. The collapse pressure was & = 62.5 mN/m, which agrees with previous ~ t u d i e s and ~ - ~indicates ~ a remarkably rigid film at full compression. From the surface pressurearea isotherm, one obtains two measures of the area per molecule: A,, = 2 1.6 A2/molecule (at the "foot" of the isotherm, extrapolated to zero pressure) and A, = 11.5 A*/molecuIe (at &). These estimates are smaller than those reported by Zhu and co-workers ( - 31 A2)*and by Bryce and Petty and co-workers (Ao = 3A'for Figure 1bio)and much smaller than that reported by Obeng and Bard (Ao = 96 A2).7 The ellipmetric results (averages of 52 readings of two pale yellow C, LB films on glass and four positions) are index of refraction nf = 1.80 f 0.06 and film thickness tf = 19.0 f 1.2 A/layer. The STM images of the C, LB films presents different structures in different regions of the substrate: amorphous,
Order and Disorder in CsoLangmuir-Blodgett Films
The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 9021
Figure 3. High-resolution transmission electron micrograph of C, LS onelayer film on a Cu grid, showing a considerable amount of apparently faulty crystallinity.
0
.
Figure 5. (top) High-resolution transmission electron micrograph of a C, film. The micrograph shows a large amorphous region mixed with a crystalline region, with twins marked T. (bottom) Schematic showing stacking of CM columns in the crystal.
Figure 4. (top) High-resolution transmission electron micrograph of C, LS one-layer film on a Cu grid, showing a three-dimensional crystalline region at position A. (bottom) Schematic showing the stacking of CM columns in the crystal, as viewed along the [ 1 101 direction.
polycrystalline, and crystalline. An STM image of the more crystalline part is shown in Figure 2; the image is not atomically resolved but shows a parallelogram pattern of roundish peab with sides 7.1 f 0.1 and 5.0 f 0.1 A and angle 72 f 2O. The image is consistent with a sli htly distorted (013) facecentered pattem, with distances of 7.1 (attributed to the [200] lattice direction) and 5.0 A (attributed to the [ 131 J direction): these index well for a fcc cell constant of 14.20 A, which may be due to attempts to epitaxy over the graphite substrate. A 7.1-A separation in the horizontal axis of Figure 2 is half the f a lattice spacing; this cannot be explained as a "new and short" lattice period. If, somehow, the STM tip did not discriminate the height difference, e.g., between Csoat the face comers and the Ca atom in the face center (7.1 A below!), then Figure 2, and our assignment, can be understood. Represemtative HRTEM images are given in Fwres 3-5. The most interesting feature of the CmLB films is the considerable variation in morphology of the specimen seen from area to area. Figure 3 shows an HRTEM image of a region of apparently faulty crystallinity. A close inspection of the micrograph reveals
x
that the structure is in fact composed of rather closely packed Ca molecular columns, normal to the plane of the micrograph. These column are aranged along well-defined lattice lines (not always straight), extending along the direction indicated by the arrow. Neighboring lattice lines are displaced randomly with res* to each other. But, within a single lattice line the Csocolumns show periodicity. This indicates a one-dimensional pattern in the micrograph (a two-dimensional (2D) structure, if the existence of the Cmcolumns is assumed). Small regions, such as region A in Figure 4, are also noted, where the atomic columns show periodicity in two directions, indicating a three-dimensional (3D) structure in these regions. Region A (Figure 4) shows two (1 1 1) and (200)lattice fringes: this indicates 3D periodicity, as observed for a (1 10) plane of an fcc crystal. It is suggested that a layered 2D Csocrystal, in its search for the lowest-energy configuration, undergoes this closest packing and transforms into a 3D fcc structure. Elsewhere in the specimen, larger regions of apparently amorphous material are visible (Figure 5), yet such amorphous regions contain occasional patches of crystalline islands, in which well-defined lattice fringes with spacing 8.20 and 10.05 A are found. The crystalline island present in the backgrounds of amorphous regions of the micrograph in Figure 5 shows one (200) and two (1 11) lattice fringes, indicating it to be the { 110) plane of an fcc crystal. The image also shows parallel twin boundaries along the same {l 11) plane, extending along a (1 12) direction. It is now worth noting that the 3D crystalline region of Figure 4, along with its neighboring 2D counterpart, upon viewing on the other orthogonal {l10) plane, is expected to yield a similar HRTEM image. That is, the 2D and 3D crystalline regions of Figure 4 will respectively appear amorphous and crystalline in this image.
9028 The Journal of Physical Chemistry, Vol. 96, No. 22, 1992
Wang et al. (at film collapse). Thus, the film thickness is probably variable, and the estimates from ellipsometry and from packing at the air-water interface are roughly compatible. It is not unusualIO for nonamphiphilic molecules, e.g., porphyrins and phthalocyanines,21to form films more than one molecule thick. What may occur at the air-water interface in PL films is the incipient formation of the ...ABCABC...stacking of spheres in an idealized 3D f a lattice. The STM and HRTEM images show that the C, LB and LS films have polycrystalline and crystalline regions. In the more crystalline region, the STM image shows a slightly distorted (131)f a pattern, with a cell constant of 14.20 A. In the HRTEM micrographs, there are regions of 2D and 3D crystallinity. In the latter, the C, molecules pack unambiguously in an fcc pattern, with cell constant of 14.20 f 0.05 A. We did not find any evidence for hcp structures. The behavior of LB and I Sfilms of C , is consistent with what most other workers have found on vapordeposited and powdered films of Cm.
Figure 6. Selected area diffraction pattern of a Ca LS one-layer film,
Acknowledgment. We are grateful to the School of Mines and Energy Development (P.W. and M.S.) and to JVC America (P.W.) for partial support and to Dr. M. Bersch for useful discussions. Registry NO. HOW, 7782-42-5; C- 99685-96-8; CU,7440-50-8.
which is characteristic for the [21 I ] plane of an f a crystal, with unit cell side of 14.20 i 0.05 A.
References and Notes
Selected-area diffraction and microdiffraction patterns, taken from crystalline regions of the specimen, were found to be those of a typical fcc crystal, as shown in Figure 6. The cell parameter of the corresponding fcc unit cell calculated from this diffraction pattern is 14.20 f 0.05 A.
Discussion In accordance with previous studies, we found that C , forms very rigid PL “monolayers” at the air-water interface, with a large collapse pressure (62.5 mN/m). The “area per molecule” values (either A. or A,) obtained from our LB films are too small to be due to a single Ca molecule. A plane primitive hexagonal lattice (space group p6, closest packed, which corresponds to the (1 11) lattice plane of an fcc lattice) has a packing fraction of 1Oo?r/243 = 90.69%. From the X-ray diffraction data on C, powders, the van der Waals diameter of Ca is 10.0 A;s the van der Waals area is then AvdW= 78.5 A2, which, for a planar close packing of spheres, bemmes 86.6 A*. Then our A. = 21.6 A2 suggests that, at zero film pressure, when the “islands” of C , floating on water meet to create a measurable film pressure, there are Avdw/Ao= 4 C, molecules piled on top of each other, on the average, in the “monolayer”);this increases to Avdw/A,= 7.5 at the collapse pressure. Cm forms LB films on glass and HOPG, and LS films on a Cu grid. Since no XRD peaks were found for LB films, there seems to be no characteristic distance, i.e., no periodicity in the successive films deposited on the same substrate. The measured index of refraction for C, seems reasonable. The film thickness measured by ellipsometry suggests that each “monolayer” is 19.0/10.0 k: 2 molecules thick, while the PL film thickness ranges from 4 molecules/layer (at zero pressure) to 7.5 molecules/layer
(1) KrBtschmer, W.; Lamb, L. D.;Fostiropoulos, K.; Huffman, D. R. Nature 1990,347, 354. (2) Kroto, H. W.; Heath, J. R.; O’Brien, S.C.; Curl, R. F.; Smalley, R. E. Nature 1985,318, 162. ( 3 ) Benning, P. J.; Martins, J. L.; Weaver, J. H.; Cbibante, L. P. E.; Smalley, R. E. Science 1991, 252, 1417. (4) Stephens, P. W.; Mihaly, L.;Lee, P. L.;Whctten, R. L.;Hung, S.-M.; Kaner, R.; Deiderich, F.; Holczer, K. Nature 1991, 351,632. (5) Heiney, P.; Fisher, J. E.; McGhie, A. R.;Romanow, W.; Denenstein, A. M.; McCauley, Jr., J. P.; Smith, 111, A. B. Phys. Rev.Lett. 1991,66,2911. (6) Hebard, A. F.; Rosseinsky, M. J.; Haddon, R. C.; Murphy, D. W.; Glarum, S. H.;Palstra, T. T. M.; Ramirez, A. P.; Kortan, A. R.Nature 1991, 350,600. (7) Obeng, Y. S.; Bard, A. J. J. Am. Chem. Soc. 1991, 113,6279. (8) Long, C.; Xu, Y.;Gou, F.; Li,Y.; Xu, D.; Yao, Y.;Zhu, D.Solid State Commun. 1992,82,381. (9) Nakamura, T.; Tachibana, H.; Yumura, M.; Matsumoto, M.; Ammi, R.; Tanaka, M.; Kawabata, Y. Langmuir 1992,8,4. (10) Williams, G.;Pearson, C.; Bryce, M. R.; Petty, M. C. Thin Solid Films 1992, 209, 150. (1 1) Gaines, Jr., G.L. Insoluble Monolayers at Liquid-Gas Interfacs; Interscience: New York, 1966. (1 2 ) Roberts, G.G.,Ed. Lungmuir-Blodgett Films; Plenum: New York, 1990. (13) Blodgett, K. B. J . Am. Chem. Soc.1935,57, 1007. (14) Langmuir, I.; Blodgett, K. B. Phys. Rev. 1937, 51,964.’ (15) Langmuir, I.; Schaefer, V. J. J. Am. Chem. Soc. 1938,60, 1351. (16) Hawkins, J. M.; Meyer, A.; Lewis, T. A.; Loren, S.;Hollander, F. J. Science 1991, 252, 312. (17) Wilson, J. R.; Meijer, G.; Bethune, D.S.;Johnson, R. D.;Chambliss, D.D.; de Vries, M. S.;Huziker, E. H.; Wendt, R. H. Nature 1991,348,621. (18) Wragg, J. L.; Chamberlin, J. E.; White, H. W.; Kritschmer, W.; Huffman, K. Nature 1991,384,623. (19) Snyder, E.J.; Anderson, M. S.;Tong, W. M.; Williams, R. S.;An&
S.J.; Alvarez, M.M.; Rubin, Y.; Diederich, F. N.; Whetten, R. L.Science 1991, 253, 171. (20) Metzger, R. M.; Schumaker, R. R.; Cava, M. P.; Laidlaw, R. K.; Panetta, C. A.; Torres,E. Lungmuir 1988, I ,298-304. (21) See review in ref 12, pp 59-65.
