Langmuir 1996, 12, 5625-5629
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Scanning Tunneling Microscopy Images of a Novel Dimeric Liquid Crystal on Graphite F. Stevens, D. J. Dyer, U. Mu¨ller, and D. M. Walba* Department of Chemistry and Biochemistry and Optoelectronic Computing Systems Center, Campus Box 215, University of Colorado, Boulder, Colorado 80309-0215 Received March 11, 1996. In Final Form: August 6, 1996X Scanning tunneling microscopy (STM) images of a side-by-side dimeric liquid crystal adsorbed on graphite suggest an unusual structure for the monolayer. Instead of the contact with the graphite being maximized, two of the alkyl tails appear to be nearly normal to the surface. Calculations suggest that this may be due to the substitution of one of the aromatic rings in the core, which causes an alkoxy tail to prefer an orientation perpendicular to the aromatic core rather than the usual coplanar orientation. STM images taken at various bias voltages correlate strongly to the aromatic “cores” of the molecules, but do not appear to correlate to the highest occupied moleculear orbital or lowest unoccupied molecular orbital.
Introduction In scanning tunneling microscopy (STM) an atomically sharp metal tip is scanned over a conducting surface while a voltage is applied between the tip and the surface. In typical experiments, the tip height is adjusted during scanning to maintain the current at a constant value. In this way, an atomic-scale map of the surface conductivity and topography can be obtained. Furthermore, it has been found that if a smooth, flat, conducting surface is covered with an organic compound, the molecules sometimes adsorb to the surface, forming a monolayer 2-D crystal. This adsorbed monolayer typically modulates the tunneling current, and images of the individual adsorbed organic molecules at up to atomic resolution can be obtained.1 STM allows adsorbates to be observed at molecular resolution and in real space and provides a unique opportunity to study surface structure. We are particularly interested in studying the interactions of liquid crystals (LCs) with solid surfaces, as these interactions are important for LC device fabrication. STM provides a method for characterizing the solid surface that is in contact with the liquid crystal films in certain circumstances. Specifically, it is known that when an LC sample is placed on highly oriented pyrolytic graphite (HOPG), 2-D crystals of LC molecules often grow on the HOPG.2 These crystal monolayers can then be imaged by STM through the LC film in situ. In these preparations, the 2-D crystal of LC molecules is the solid surface that is in contact with the bulk LC. Thus, STM imaging can provide an approach for obtaining a detailed characterization of the LC/solid interface. In such adsorbed monolayers, it is widely accepted that, in general, the aromatic moieties image brighter (that is, exhibit more efficient tunneling) by STM than the aliphatic regions.2,3 However, examples of nonaromatic moieties imaging brightly are known,4 and a general theory of STM imaging of organic molecules is still lacking. We recently reported on STM observations of a series of nitrobiphenylbenzoate liquid crystals.5 A further extension of the series has now been synthesized, consist* To whom correspondence may be addressed: phone, (303) 4926750; fax, (303) 492-5894; e-mail,
[email protected]. X Abstract published in Advance ACS Abstracts, October 1, 1996. (1) Frommer, J. Angew. Chem., Int. Ed. Engl. 1992, 31, 1298-1328. (2) Smith, D. P. E.; Ho¨rber, H.; Gerber, C.; Binnig, G. Science 1989, 245, 43-45. (3) Iwakabe, Y.; Kondo, K.; Oh-hara, S.; Mukoh, A. Langmuir 1994, 10, 3201-3206. (4) Jung, T. A.; Schlittler, R. R.; Gimzewski, J. K.; Tang, H.; Joachim, C.; Science 1996, 271, 181.
S0743-7463(96)00227-2 CCC: $12.00
Figure 1. Structures of the two “dimeric” liquid crystals observed (1, 2) and the related monomeric LC studied previously (3).
ing of pairs of liquid crystal molecules linked between the aromatic rings with a conjugated spacer (Figure 1). Linking two LC molecules in this manner creates a large polarizability and hyperpolarizability that is nearly perpendicular to the average molecular long axis. Although these molecules were designed for their optical properties, their behavior at surfaces is also of great interest. In addition, the donor-[conjugated spacer]acceptor dye functionality was of interest for the STM study. Experimental Section The scanning tips were prepared from mechanically cut 0.25 mm platinum/iridium (80:20) wire. The typical scanning conditions were 1.0 nA and -0.4 V (tip positive), constant current mode, 5.8 Hz (scan line/s, 400 lines/image). Images were taken in air using a commercial STM (Nanoscope II, Digital Instruments, Inc.) and are raw data unless otherwise noted. Both mesogens 1 and 2 are crystalline or glassy solids near room temperature, so 2-D crystals were formed from solutions in 1-phenyloctane (Aldrich). A fresh drop of solution was placed on (5) Stevens, F.; Dyer, D. J.; Walba, D. M. Langmuir 1996, 12, 436440.
© 1996 American Chemical Society
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Stevens et al. Table 1. Average Unit Cell Values compound
[0]
[1]
angle (deg)a
1 2 model
2.46 ( 0.17 2.40 ( 0.20 2.53
3.06 ( 0.18 2.96 ( 0.20 2.98
82, 102 72, 99 74, 106
a The average angle does not reflect the true unit cell (see text). The two most frequent angles are reported instead.
Figure 3. Plot of unit cell dimensions found by autocorrelation analysis of STM images of 1 (9) and 2 (+). Each data point is an average of the values for the consecutive up and down scans, to reduce drift effects.
Figure 2. STM images of the two dimeric liquid crystals, voltages are tip positive: (A) 1, 15 nm, -500 mV, 1 nA; (B) 2, 15 nm (cropped from 20 nm), -600 mV, 1 nA. the HOPG sample before each scanning session to avoid crystallization of the bulk. All of the observations were consistent with monolayer (rather than multilayer) formation. LC phase behavior and optical properties of similar materials have been published elsewhere.6,7
Results Both compounds 1 and 2 produced similar images (Figure 2) showing bright rows of molecular cores. The measured length of the observed bright regions (1.3-1.6 nm) correlates well to the length of the phenylbenzoate cores, 1.3 nm measured between end carbons or 1.6 nm measured between end oxygen atoms. As in previous cases, the rows appear to be composed of pairs of molecules (6) Walba, D. M.; Dyer, D. J.; Sierra, T.; Cobben, P. L.; Shao, R.; Clark, N. A. J. Am. Chem. Soc. 1996, 118, 1211-1212. (7) Walba, D. M.; Dyer, D. J.; Cobben, P. L.; Sierra, T.; Rego, J. A.; Liberko, C. A.; Shao, R.; Clark, N. A. In Thin Films for Integrated Optics Applications; Wessels, B. W., Marder, S. R., Walba, D. M., Eds.; Materials Research Society: Pittsburgh, PA, 1995; Vol. 392; pp 157-162.
arranged in an antiparallel fashion. Because the molecules in this case contained two aromatic cores, the images show “pairs of pairs” of molecular cores (bright areas), as expected. The exact appearance of the images varied, probably due to tip variations, but in the best images the individual aromatic cores of each molecule could be observed. The unit cell for each image showing a single-crystal domain was determined by autocorrelation analysis.8 Because the tip drift caused consistent variations between the up and down scans, whenever possible pairs of consecutive up and down scans were taken and the results averaged. Only these “pair averaged” values were used in the subsequent analysis. At least 130 pairs of images from eight different sessions were taken for each compound. As the maximum number of pairs taken in a single session was 30, we believe that the results are not dominated by any one session. Compound 3, a liquid crystal molecule very similar to one-half of these dimeric molecules, was studied previously5 and was found to form unit cells with an area of 4.1 nm2 that contained two molecules. Simple modeling of the monolayers of 3 suggested that the molecules were close-packed. As each half of the dimeric LCs 1 and 2, bears a close resemblance to 3, the monolayers of the dimeric LCs might be expected to form a unit cell that is twice as large, or 8.2 nm2. In fact, both 1 and 2 form monolayers having unit cells of about 7.0-7.5 nm2 (Table 1), which is too small to contain a pair of molecules if the molecules are extended flat on the surface. The monomeric LC 3 clearly showed polymorphism,5 and if polymorphs occur, then the average unit cell values are not appropriate. A plot of the dimer unit cells (Figure 3) does not show obvious polymorphism, but the plot does show a somewhat elongated distribution of values, which may indicate the presence of several unit cells that are too similar to be resolved or could simply be due to thermal drift or other (8) Saxton, W. O.; Frank, J. Ultramicroscopy 1977, 2, 219-227.
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Figure 5. Model of a monolayer of 1 (essentially the same as a monolayer of 2); aromatic cores which show up as bright in STM images are marked with dotted ovals.
Figure 4. Low angle domain wall (13°) in a monolayer of 1, low-pass filtered, 22 nm (cropped from 30 nm), -500 mV, 1 nA.
variability. Observation of low-angle domain walls (Figure 4) supports the possibility of real variations in the unit cell. Although most of the domains met at angles of 60° or 120°, as expected for epitaxial growth on graphite, some other angles were seen. If both of the domains in Figure 4 are epitaxial, then the low angle must represent either a change in the underlying graphite crystal structure or a change in the monolayer unit cell. The frequency distributions for the unit cell vectors showed reasonably bell-shaped distributions centered on the average values. However, the distribution plots for the unit cell angles showed isolated sharp maxima which were located on either side of the average value. We suspect that the unit cell program has picked sometimes one and sometimes the other angle of an oblique unit cell, which results in the average value being meaningless. The average angle values were discarded, but the most frequent angles found are reported in Table 1. The two dimeric LCs show slightly different average unit cells, with the alkyne linked LC 1 appearing slightly larger, but the difference is within experimental error. The most frequent unit cell angles for 1 and 2 are also identical within experimental error. The two materials produced indistinguishable STM images. Initial attempts to produce a molecular model of the monolayer (POLYGRAF, Molecular Simulations, Inc., Burlington, MA) produced monolayers which appeared nearly close-packed but had unit cell areas between 8 and 9 nm2, which is close to the 8.2 nm2 predicted from the monomer results but larger than the areas of about 7 nm2 actually observed. These models also reproduced the appearance of the STM images poorly. A conformational study of the LCs (Spartan, Wavefunction, Inc., Irvine, CA) revealed that while the “anisole effect” typically causes the alkoxy groups that are connected to the aromatic rings to reside in the plane of the aromatic ring (dihedral angle of 0°),9 when an o-amino substituent is present, alkoxy groups prefer to make a dihedral angle of about 100° to the aromatic ring. Since the ring with the amino group is constrained to be parallel to the surface due to its linkage to the other aromatic system, if the short tail makes an (9) Schaefer, T.; Sebastian, R. Can. J. Chem. 1989, 67, 1148-1152.
angle of nearly 90° to this ring, then tail would be bent off the surface at a steep angle. Modeling a monolayer of 2, where both short tails were bent off the surface, produced a model (Figure 5) having a significantly smaller area per molecule (7.2 nm2), which was a close match to both the STM images and the unit cell data (Table 1). Because of the similarities between the molecules, this model also matched the unit cell and STM images of 1; therefore a separate model for 1 was not created. Overlaying the molecular model on the STM images showed a nearly perfect match between the aromatic cores in the model and the bright regions in the STM image for both compounds. If the short alkoxy tails are indeed bent off the surface, this might be expected to have two consequences. First, these tails might be visible to the STM by affecting the topography of the surface, similar to what has been seen with tert-butyl porphyrins.4 And second, the protruding tails might cause nucleation of a second layer, as has been seen by Parkinson et al. when dye molecules with long alkyl chains were doped into monolayers of 4-cyano-4′hexylbiphenyl.10 We have seen no evidence of these phenomena and have no independent evidence that the short tails are indeed oriented off the surface. However, no satisfactory model could be made which had the molecules completely flat on the graphite. It may be that in this monolayer, the tails sticking off of the surface are flexible enough that the STM tip simply pushes them aside without imaging them, but are “concentrated” enough to hinder the formation of a second layer, in contrast to the system of Parkinson where the “vertically oriented” tails were fairly dilute. A different LC previously studied by these labs11 appeared to have a tail which was not in contact fully with the graphite, but in that case the tail was quite short and contained a trans-epoxide unit which prevented the tail from making a good contact with the surface in any orientation. For both of the dimeric LCs, the highest occupied molecular orbital and lowest unoccupied molecular orbital (LUMO) are localized on the conjugated amino-nitro system which runs across the molecular axis (Spartan), and the LUMO is nearly 1 eV lower in energy than the next nearest unoccupied orbital. As unoccupied orbitals have been implicated in STM imaging,12 we predicted that the conjugated system normal to the molecular axis might image differently from the rest of the aromatic core. However, in all of the STM images, the only clearly observed bright areas appeared to be parallel to the molecular axis and in the location that is expected for the biphenylbenzoate cores (compare Figure 2 and Figure 5). Even reducing the bias voltage (which should increase the preference for tunneling through the orbitals closer (10) Sampson, D. L.; Parkinson, B. A. Poster presented at STM ‘95, Snowmass, CO; submitted to Langmuir. (11) Walba, D. M.; Stevens, F.; Parks, D. C.; Clark, N. A.; Wand, M. D. Science 1995, 267, 1144-1147. (12) (a) Lippel, P. H.; Wilson, R. J.; Miller, M. D.; Wo¨ll, C.; Chiang, S. Phys. Rev. Lett. 1989, 62, 171-174. (b) Ludwig, C.; Gompf, B.; Petersen, J.; Strohmaier, R.; Eisenmenger, W. Z. Phys. B 1994, 93, 365-373. (c) Richter, S.; Manassen, Y. J. Phys. Chem. 1994, 98, 29412949.
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Figure 6. Change in appearance of 1 with changes in bias voltage, images low-pass filtered: (A) -200 mV, 0.2 nA; (B) +200 mV, 0.2 nA; (C) -1000 mV, 5 nA; (D) -50 mV, 0.05 nA. All of the images are 15 nm, negative biases are tip positive.
to the Fermi level) caused little change. Images could be obtained at biases as low as 0.05 V (Figure 6D) by using low currents (0.05 nA), although much noise was present. Images taken with tip negative bias (Figure 6B) looked essentially the same as images taken with tip positive bias. Changing the magnitude and sign of the bias changed the appearance of the STM images to some degree, but no consistent variation in the STM image appearance with the bias voltage could be identified. The inability to observe the amino-nitro system by STM made the detailed analysis of these STM images difficult, as the inability to distinguish the two ends of the molecules made it impossible to tell with certainty how the molecules are arranged on the graphite; the model shown in Figure 5 fits the available data, but it is not the only possibility. However, this nonobservation of the amino-nitro system does aid in the analysis of our previous images of the monomer LC 3. In the STM images of 3, the two ends of the molecule were also indistinguishable, although it seemed reasonable that the nitroaromatic ring might image differently from the rest. It was difficult to draw any conclusions from the nonobservation of the nitroaromatic ring in 3 because the orientation of the nitroaromatic
ring was unknown, so the fact that this ring appeared the same as the other aromatic rings could be due to some fortuitous orientation that happened to make all of the rings appear the same. In the case of LCs 1 and 2, the linkage between the cores requires the conjugated nitroaromatic system to lie flat and parallel to the graphite surface, and the aromatic rings of the amino-nitro system are observed (as seen by the length of the bright areas in the STM images), so the nonobservation of the nitro group, dimethylamino group, and conjugated spacer is difficult to understand. One other comparison between the dimeric and monomeric LCs is in the appearance of the images. While the overall appearances are very similar (rows of tilted molecules), the images are chiral in 2-D, and the nature of the chirality differs between the monomer and dimer. Monomeric LC 3 (as well as four analogous unsaturated LCs) has a (S) stereocenter and produced STM images where the molecules were tilted clockwise in the rows.13 The enantiomer of 3 produced enantiomeric images where (13) Stevens, F.; Dyer, D. J.; Walba, D. M. Angew. Chem., Int. Ed. Engl. 1996, 35, 900-901.
Novel Dimeric Liquid Crystal on Graphite
the molecules were tilted counterclockwise in the rows. This tilt was extremely consistent in both cases, with no counterexamples observed. The dimeric LCs posses an (S) stereocenter but show the molecules tilted counterclockwise in the rows, the opposite of that seen in the (S) monomer. Also, the tilt of the dimers is less consistent, with a few dimer images showing the opposite (clockwise) tilt. It is not yet possible to draw firm conclusions from these intriguing differences. Conclusions Two different dimeric liquid crystals, having different functional groups connecting the monomer units, were
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studied by STM. Comparison of these results to the data from a similar monomeric LC showed general similarities, but also significant differences. The small molecular area found for the dimer appears to show that the two short tails are bent off of the surface, and molecular modeling provides limited support for this. Also, the conjugated amino-nitro system, which is the location of the lowest unoccupied molecular orbital, could not be clearly observed, apparently indicating that the STM imaging of organic molecules is not always dominated by the frontier molecular orbitals. LA9602271
