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J. Phys. Chem. B 1998, 102, 5298-5302
Epitaxy at Non-60° Angles: An STM Study of 7CB on HOPG Jo1 rg Schulze,† Forrest Stevens, and Thomas P. Beebe, Jr.* Department of Chemistry, The UniVersity of Utah, Salt Lake City, Utah 84112 ReceiVed: February 11, 1998; In Final Form: April 13, 1998
Studies of homologues are often useful for correlating material properties to molecular structure. The liquid crystal 4-heptyl-4′-cyanobiphenyl (7CB) has received very little study, although the homologous 8CB has been studied extensively by STM. Here we report several unusual properties of 7CB monolayers on graphite, particularly the fact that angles between contacting domains of this liquid crystal were observed to favor angles of 17° rather than the expected 60° angles. This and other observations, coupled with an experimentally determined unit cell of A ) 3.85 ( 0.37 nm, B ) 1.10 ( 0.10 nm, and ΘAB ) 92 ( 6°, lead us to propose an unusual monolayer structure for 7CB, where only half of the molecules make the expected alkyl-graphite contact in any domain. The 7CB monolayer shows great complexity, including chirality effects and domaindomain interactions.
Introduction Although the interaction of liquid crystals with surfaces is critical for device applications, basic information on these interactions at the molecular scale is scant. Due to its importance for technology and for scientific understanding, the surface ordering in thin films has been studied for a number of years as a model for a variety of fundamental and applied interfacial phenomena.1 For technological applications, improved knowledge of how to predict and control thin-film chemical and physical properties is of great concern. The connection between molecular structure and film properties is of particular interest. One way of studying this connection is to study the monolayers formed by a series of related molecules. Scanning tunneling microscopy has proven to be an outstanding tool for the structural analysis of self-assembled, ordered organic monolayers.2,3 In particular, the homologous 4-n-alkyl4′-cyanobiphenyl (mCB, m ) 6-12) series of liquid crystals has been studied with success on several different substrates.4-9 Most of the studies have focused on a single member of this series, 8CB, with the other members receiving relatively little attention. In this paper, we report molecular-scale structural studies of ordered domains and domain boundaries for 7CB films grown on highly oriented pyrolytic graphite (HOPG). Although 8CB and 7CB have been reported to form very similar structures on MoS2,10 we find that these two molecules form quite different monolayer structures on HOPG. Experimental Section The STM used in these studies is custom-built and has been described elsewhere.11 All images were acquired in air at room temperature using mechanically cut Pt/Ir (80:20) or Pt/Rh (90: 10) tips which produced similar results. 4-Heptyl-4′-cyanobiphenyl (7CB), shown in Figure 1, forms crystalline monolayers on HOPG and is a nematic liquid crystal at room temperature. 7CB was used as-received from Aldrich, and ZYB grade HOPG was supplied by Dr. Arthur W. Moore of Union Carbide. * Corresponding author: phone (801) 581-5383; FAX (801) 581-8433; e-mail
[email protected]. † Visiting from Technische Universita ¨ t Braunschweig, Germany.
Figure 1. Molecular structure of the liquid crystal 4-heptyl-4′cyanobiphenyl (7CB). Dimensions shown are for the energy-minimized structure (HyperChem) and include a van der Waals radius of 0.12 nm.
Samples were prepared by depositing a small droplet of 7CB onto a cleaved and etched HOPG surface (see below). 7CB tends to crystallize near room temperature (mp ) 30 °C),12 and the bottle sometimes had to be heated gently before use to remelt the material. All images reported here were acquired using constant-height mode, a bias voltage of -0.6 V (sample positive), and a tunneling current of 160 pA. The images are unprocessed except for y-offset subtraction to remove periodic vertical artifacts sometimes seen at the edges of images. Figure 2 is a pixelwise average of several consecutive images of the 7CB monolayer, corrected for drift. As is customary, the images presented here use a false-color scale with brighter colors corresponding to a higher tunneling current. Model structures were generated using a commercially available visualization and modeling software package (HyperChem by Autodesk, Inc., Marin, CA). After exposing a fresh graphite surface, graphite substrates were heated in air at 650 °C to produce circular etch pits one monolayer deep.13-15 Previous work by this group used these etch pits to contain small ensembles of molecules for study, and hence the etch pits were termed “molecule corrals”.16 These molecule corrals are useful for observing many different domains of a monolayer in the same image, since the domains in each corral nucleate separately from each other and separately from domains on the surrounding terrace. Results and Discussion Organic compounds deposited onto HOPG surfaces sometimes form 2-dimensional crystalline monolayers3 composed of some number of domains separated by grain boundaries. Typically, the molecules in each domain are related to the
S1089-5647(98)01056-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/11/1998
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Figure 3. A histogram of angles formed by adjacent domains on the same HOPG terrace at their grain boundaries (N ) 312). Angles below 10° were ignored. It is notable that a large peak centered at ∼17° can be observed, while no significant peak is observed at 60°. Figure 2. A high-resolution image of 7CB (10.0 × 10.0 nm) obtained by pixelwise averaging of five sequential images, correcting for drift between images. Biphenyl moieties are known to image as “bright” relative to the darker alkyl tails. Because aromatic-aromatic and aliphatic-aliphatic interactions are favored, a structure of bright rows of aryl groups and darker rows of alkyl groups is formed (see Figure 6). Two different types of row structures (type I and type II) can be seen in an alternating pattern. Four molecules in each row have been shown schematically in the proposed monolayer structure, although image quality is not good enough to determine molecular position exactly. Unlike 8CB, where higher resolution is routinely achieved on the same instrument, 7CB molecules in the monolayer apparently undergo a greater degree of molecular thermal motion, resulting in lower image resolution.
molecules in other domains by a rotation angle of 60°. This phenomenon can be explained by a molecular alignment with the underlying graphite lattice, causing the adsorbed molecules to align in certain directions. The 6-fold symmetry of the HOPG, coupled to the 2-fold symmetry of the monolayer, leads to the observed 60° angles. This behavior (epitaxy) is observed with self-assembled monolayers of most organic compounds, including the liquid crystal 8CB (4-octyl-4′-cyanobiphenyl).7 Unlike the 8CB molecules, which tend to form single large domains, 7CB was frequently observed to form many domains. Each domain appeared as a group of alternating bright and dark bands, or stripes. The stripes result from rows of lined-up molecules. When two domains met, the angle between the stripes in the two domains could be measured. If only one structure is present, the domains are expected to reflect the symmetry of the substrate and meet at angles of (60°. Instead of this expected result, 7CB monolayers are dominated by domains meeting at angles of near 17°, as shown in Figure 3 (angles of ∼0° were ignored). Also unusual is the fact that no significant peak was observed at 60°, even though angles of greater than 60° were sometimes observed. These studies were done on HOPG surfaces having molecule corrals in order to immobilize the domain boundaries17 and to increase the number of domains, since domains inside and outside corrals nucleate separately. Domains of the longer homologue 8CB have been shown to favor angles of 60°, plus a few other angles due to domain chirality.18 Only a few studies of the shorter homologue 6CB have been published,9,19-22 and the images show a high degree
of variability, with several different structures reported. However, one study reported that domains of 6CB also favored 60° angles.20 In an attempt to gain an understanding of the observed strong preference for 17° domain angles, nanometer-scale STM images of grain boundaries were acquired. Pictures of the ∼17° grain boundaries showed a distinct structure in the grain boundary, as seen in Figure 4A. Grain boundaries at other angles did not show this structure, instead showing only a poorly resolved swath between the two domains, as seen in Figure 4B (40° angle between rows). This disordered region indicates a lack of any distinct molecular structure within non-17° grain boundaries, so over the time scale of the STM image acquisition these boundaries appear blurred because of translational movements of the molecules. Similar low-contrast regions where molecular adsorbates were present but not ordered have been reported previously.16 Grain boundaries of 0° (translational grain boundaries) were also observed to possess a distinct structure. At these boundaries the two domains are not rotated in comparison to each other but are shifted by a certain distance along the direction of the grain boundary, as seen in Figure 4C. The preference for 17° angles between domains could be a consequence of interactions between the domains and the substrate or of interactions between the domains, or some combination of both. To test this, angles were measured between domains that were next to each other but separated by a step in the HOPG surface (∼3.35 Å height difference), and the results are shown in Figure 5. The step between the domains should greatly reduce, or eliminate, domain-domain interactions, so the fact that the prevalence of 17° angles persists even when the two domains are separated by a step in the graphite surface indicates that this angle is not determined primarily by domain-domain interactions, but rather by domain-substrate interactions. The additional intensity seen below 17° in Figure 5 will be discussed below. The unusual behavior of 7CB might be understood if the structure of the monolayer was known. Few structures of the 7CB monolayer have been proposed.8,9 They are all different and do not appear to be based on experimental unit cell measurements but only on a visual “fit” between a proposed structure and the STM images. To create a model of the monolayer structure, we first determined the unit cell parameters by acquiring 98 different
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Figure 4. Different types of grain boundaries observed in 7CB; all images 12.0 × 14.0 nm. (A) A grain boundary with the characteristic ∼17° angle. Note the distinct structure observable even at the boundary. (B) A grain boundary with an angle of 40°. A disordered region with no visible structure is observed between the domains (indicated by arrow and dashed lines on right side). (C) A translational grain boundary with no change of angle (noted by arrows). This kind of grain boundary also exhibits a distinct structure.
high-resolution STM images of 7CB taken on 13 different days using different samples and tips. The images typically showed an alternating pattern of two rows as seen in Figure 2 (type I row and type II row). The unit cell of each image was obtained by Fourier transform analysis, and the average values from the 98 images so analyzed are given in Table 1. Comparison of the unit cell values to the dimensions of the 7CB molecule shown in Figure 1 suggests that the unit cell is two molecules
wide and two molecules long and thus contains four molecules. The fact that the unit cell is consistently twice as long as a 7CB molecule indicates that the alternating row structure observed is not an artifact of limited resolution and that there really are two different types of rows in the monolayer. If adjacent rows of molecules were identical, then the unit cell would include only one row. This alternating row structure can be seen clearly in Figures 2 and 4C.
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Figure 5. A histogram of angles formed by adjacent domains separated by a HOPG step (N ) 96). The preference for 17° angles is retained, showing that domain-domain interactions are not the only reason that this angle is preferred. The additional intensity seen at ∼13° arises from the chirality of the monolayer (see text).
TABLE 1: 7CB Unit Cell Data experiment model
vector A (nm)
vector B (nm)
ΘAB
3.85 ( 0.37 3.94
1.10 ( 0.10 1.06
92 ( 6° 96.6°
To model the unit cell structure, a commercially available computer program (HyperChem) was employed to create 7CB monolayer structures that matched both the unit cell data and the best molecular-scale images of 7CB which had been obtained. Initial modeling attempts made use of the assumption that the methylene hydrogens of each 7CB molecule were positioned over the centers of the graphite rings.3,23,24 Structures were also assumed to possess a local symmetry such that the dipole moments of the 7CB molecules canceled within the unit cell. Modeling was complicated by the fact that different STM images often showed contrast differences visually, even when Fourier transform analysis showed that the images had identical unit cells. Much of this variation between images is probably due to differences in tip shape. Structural assignment was also difficult due to the only moderate resolution of the images. The lack of any high-resolution images is likely due to molecular motions within the monolayer, since we have achieved higher resolution when imaging monolayers of other molecules with the same instrument. Despite extensive attempts, no molecular model could be found that matched the STM image and unit cell data and adhered to all of the structural assumptions stated above. We then reviewed the data and created a new model, shown in Figure 6 and indicated in Figure 2. The key feature of this new model is that only half of the 7CB molecules per unit cell have their alkyl hydrogens positioned over the centers of the graphite rings. In fact, the double-row structure observed was reproduced by rotating molecules in every other row by 17°. Thus, within any domain half the 7CB molecules have their alkyl groups aligned with the graphite, as is expected for longchain hydrocarbons on HOPG, while half are rotated off of alignment by 17°. For example, in Figure 6 the alkyl tails of the molecules in the type II rows are aligned with the HOPG lattice vector, while the molecules in the type I rows have their alkyl tails rotated (17° from the HOPG lattice vector. This produced a model that appears reasonably close-packed and contained minimal steric or dipolar repulsions. Although the
Figure 6. Proposed structure for the 7CB monolayer showing a horizontal boundary (indicated by horizontal arrow) between domains of opposite chirality. Unit cell dimensions, obtained by assuming epitaxy and calculating distances between graphite lattice sites, were found to be A ) 3.94 nm, B ) 1.06 nm, and ΘAB ) 96.6°. The molecules in the type I rows are rotated by (17° relative to the molecules in the type II rows. The type I and type II rows can be identified in the STM images because the type I rows are narrower than the type II rows (see Figures 2 and 4C). The top and bottom halves of the structure have opposite chiralities, and the bright stripes in the two domains form an angle of 13°, as indicated by the dashed lines.
STM images were not clear enough to unambiguously determine whether this proposed structure is correct, its unit cell (Table 1) is within experimental error of the experimentally determined unit cell, and the proposed structure possesses the observed double-row structure and is a good visual match for many of the STM images obtained. This model also explains the observed preference of 17° angles at domain boundaries. When two domains which have structures as shown in Figure 6 meet at a 17° angle, both domains are equally epitaxial, but in one domain the type I rows are aligned with the graphite, while in the other domain it is the type II rows which are aligned with the graphite. Furthermore, the structure shown in Figure 6 is chiral (not superimposable on its mirror image) in two dimensions, and structures of both chiralities are expected to be formed.18,25 Although the domain chirality could not be easily determined by inspection of the STM images, the rows of aromatic groups (these rows lie along the B unit cell vector and show up as bright in the STM images) are tilted by 96.6° to one set of alkyl chains (type II row in Figure 6). So if two domains of opposite chiralities meet, both with their type II rows aligned with the same HOPG vector, the bright and dark rows seen by STM will make an angle of ∼13° (6.6° + 6.6°) as shown in Figure 6. However, modeling indicates that while angles of 17° can form with a “good” boundary structure (few gaps or crowding between molecules), angles of 13° cannot. The boundary shown in Figure 6 was the best that could be found while keeping the registry between the type II rows and the substrate. Because 13° angles between domains must contain gaps or unfavorable steric or dipole interactions, we would expect domain-domain interactions to favor angles of 17° over angles of 13°. This is confirmed by experiment, as Figure 3 shows that angles between domains on the same HOPG level (where domain-domain interactions can occur) show many 17° angles and very few 13° angles, while Figure 5 shows that angles between domains
5302 J. Phys. Chem. B, Vol. 102, No. 27, 1998 separated by an HOPG step (which minimizes domain-domain interactions) occur with similar frequency for 17° and 13° angles. This explains the extra intensity below 17° in Figure 5 as compared to Figure 3. Since rotations of both 17° and 13° can occur in addition to the 60° symmetry of the substrate, we would expect to see domains meeting at angles of 0°, 13°, 17°, 30° (17° + 13°), 43° (60° - 17°), 47° (60° - 13°), 60°, 73° (60° + 13°), 77° (60° + 17°), and 90° (60° + 13° + 17°). Although sharp histogram peaks are observed only at 13° and 17°, the spread of observations across a wide range of larger angles qualitatively agrees with predictions. The fact that small angles were observed much more frequently than larger angles strongly suggests that the surface domains are being aligned by a bulk ordering of the liquid crystal, as has been seen in 8CB.26 The presence of an ordered bulk material overlying the monolayer favors monolayer domains that are aligned in directions similar to the bulk and disfavors domains that are misaligned to the bulk. Thus, the monolayer domains, all of which are in contact with the same bulk, will tend to be aligned along similar directions and show small domain-domain angles. The mCB series of liquid crystals has proved extremely productive for STM study, not only because these molecules tend to make monolayers that are readily observed by STM but also because of the variety of structures formed. Both of these features are likely related to the strong dipole moment of the cyano group, which can produce both strong attractive forces to stabilize the monolayer or strong repulsive forces which prevent some potential structures from forming. Moleculesubstrate interactions are also important, as shown by the fact that different structures are formed on different substrates.21 While a detailed explanation of why the observed 7CB structure forms is not at hand, it is evidently nature’s solution to the problem of reconciling these competing interactions. Conclusions Domains of the liquid crystal 7CB do not favor the expected angle of 60° at their grain boundaries, instead showing a strong preference for a 17° angle. This is difficult to explain based on the commonly observed molecular epitaxy on HOPG but can be explained by a novel “rotated row” model. This model matched the accurate unit cell data that were acquired and was a good match to the observed properties of the monolayer. The observed absence of 13° angles when two domains (of different chirality) met on the same HOPG terraces demonstrates that domain-domain interactions play a significant role in monolayer development. It is striking that 7CB differs so greatly
Schulze et al. from its close homologue 8CB, which has received much greater study. This demonstrates that monolayer structure can be very sensitive to small changes in molecular shape or functionality and that analysis of domain angles can provide important insights into monolayer properties and structure. Acknowledgment. Jo¨rg Schulze would like to thank the Deutscher Akademischer Auslandsdienst (DAAD) for funding his exchange studies at the University of Utah. This work was supported by the National Science Foundation (CHE-9357188), the Camille and Henry Dreyfus Foundation, and the Alfred P. Sloan Foundation. References and Notes (1) Brown, G. H.; Doane, J. W. Appl. Phys. 1974, 4, 1. (2) Frommer, J. Angew. Chem., Int. Ed. Engl. 1992, 31, 1298. (3) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. Chem. Mater. 1996, 8, 1600. (4) Foster, J. S.; Frommer, J. E. Nature 1988, 333, 542. (5) Spong, J. K.; Mizes, H. A.; LaComb, L. J. J.; Dovek, M. M.; Frommer, J. E.; Foster, J. S. Nature 1989, 338, 137. (6) Hara, M.; Iwakabe, Y.; Tochigi, K.; Sasabe, H.; Garito, A. F.; Yamada, A. Nature 1990, 344, 228. (7) Smith, D. P. E.; Ho¨rber, J. K. H.; Binnig, G.; Nejoh, H. Nature 1990, 344, 641. (8) Shigeno, M.; Mizutani, W.; Suginoya, M.; Ohmi, M.; Kajimura, K.; Ono, M. Jpn. J. Appl. Phys. 1990, 29, L119. (9) Shigeno, M.; Ohmi, M.; Suginoya, M.; Mizutani, W. Mol. Cryst. Liq. Cryst. 1991, 199, 141. (10) Iwakabe, Y.; Hara, M.; Kondo, K.; Tochigi, K.; Mukoh, A.; Yamada, A.; Garito, A. F.; Sasabe, H. Jpn. J. Appl. Phys. 1991, 30, 2542. (11) Zeglinski, D. M.; Ogletree, D. F.; Beebe, T. P. J.; Hwang, R. Q.; Somorjai, G. A.; Salmeron, M. B. ReV. Sci. Instrum. 1990, 61, 3769. (12) Morsy, M. A.; Oweimreen, G. A.; Hwang, J. S. J. Phys. Chem. 1996, 100, 8331. (13) Evans, E. L.; Griffiths, R. J. M.; Thomas, J. M. Science 1971, 171, 174. (14) Chang, H.; Bard, A. J. J. Am. Chem. Soc. 1990, 112, 4598. (15) Chu, X.; Schmidt, L. D. Carbon 1991, 29, 1251. (16) Patrick, D. L.; Cee, V. J.; Beebe Jr., T. P. Science 1994, 265, 231. (17) Patrick, D. L.; Cee, V. J.; Purcell, T. J.; Beebe Jr., T. P. Langmuir 1996, 12, 1830. (18) Smith, D. P. E. J. Vac. Sci. Technol. B 1991, 9, 1119. (19) Brandow, S. L.; DiLella, D. P.; Colton, R. J. J. Vac. Sci. Technol. B 1991, 9, 1115. (20) Magonov, S. N.; Wawkuschewski, A.; Cantow, H.-J.; Whangbo, M.-H. Appl. Phys. A 1994, 59, 119. (21) Smith, D. P. E.; Heckl, W. M.; Klagges, H. A. Surf. Sci. 1992, 278, 166. (22) Mizutani, W.; Shigeno, M.; Ohmi, M.; Suginoya, M.; Kajimura, K.; Ono, M. J. Vac. Sci. Technol. B 1991, 9, 1102. (23) Groszek, A. J. Proc. R. Soc. London A 1970, 314, 473. (24) Findenegg, G. H.; Liphard, M. Carbon 1987, 25, 119. (25) Stevens, F.; Dyer, D. J.; Walba, D. M. Angew. Chem., Int. Ed. Engl. 1996, 35, 900. (26) Patrick, D. L.; Cee, V. J.; Baker, R. T.; Beebe Jr., T. P., manuscript in preparation.
