Langmuir 2006, 22, 7507-7511
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Multiple Two-Dimensional Structures Formed at Monolayer and Submonolayer Coverages of p-Sexiphenyl on the Au(111) Surface C. B. France,† F. Andrew Frame, and B. A. Parkinson* Colorado State UniVersity, Department of Chemistry, Fort Collins, Colorado 80523 ReceiVed October 31, 2005. In Final Form: June 2, 2006 The two-dimensional structures formed by monolayers and submonolayers of p-sexiphenyl (p-6P) molecules evaporated onto the Au(111) surface are investigated using ultrahigh vacuum scanning tunneling microscopy (UHV-STM). Five different 2D structures corresponding to different surface coverages are discovered and their 2D structures solved. The trends in the molecular alignment with respect to the underlying gold lattice are discussed. An unusual structure that consists of paired rows of p-6P molecules was discovered. A surface structure with alternating domains of slightly differently packed p-6P molecules was also found. The boundary between these two domains contains systematic molecular vacancies.
Introduction Electronic devices under development, such as field effect transistors and light-emitting diodes, use organic molecules as the active components. The devices use thin films of organic molecules deposited onto metal substrates or metal layers evaporated onto the organic films to make electrical contact with the films. The structure of the initial monolayer of the organic molecules is crucial to device operation; charge transport across the metal-to-organic interface has been shown to be injectionlimited because of the presence of interfacial dipoles inducing intermediate states that participate in the injection process.1a The initial surface organization can also template the growth of the bulk film,1b,2 influencing the grain structure, orientation, and even the polytype of the organic layer.3,4 These factors will then determine many aspects of device performance. p-Sexiphenyl (p-6P) is an aromatic molecule that has been used in the active layer of an organic blue-light-emitting diode.5-8 The electronic structure of the metal/p-6P interface has been measured for samarium,9 graphite,10 Al(111),11 Mg,12 SnS2,13 and Au(111).12,14 The physical ordering of p-6P layers has been * Corresponding author. Fax: (970) 491-1801. E-mail: bruce.parkinson@ colostate.edu. † Present address: Portland Technology Development, Intel Corp., 5200 Elam Young Parkway, RA3-301, Hillsboro, OR 97124. (1) (a) Baldo, M. A.; Forrest, S. R. Phys. ReV. B 2001, 64, 8520. (b) Resel, R.; Koch, N.; Meghdadi, F.; Leising, G.; Unzog, W.; Reichmann, K. Thin Solid Films 1997, 305, 232-242. (2) Resel, R.; Leising, G. Surf. Sci. 1998, 409, 302-306. (3) Siegrist, T.; Kloc, C.; Schon, J. H.; Batlogg, B.; Haddon, R. C.; Berg, S.; Thomas, G. A. Angew. Chem., Int. Ed. 2001, 40 (9), 1732-1736. (4) Venuti, E.; Della Valle, R. G.; Brillante, A.; Masino, M.; Girlando, A. J. Am. Chem. Soc. 2002, 124 (10), 2128-2129. (5) Tasch, S.; Brandstatter, C.; Meghdadi, F.; Leising, G.; Froyer, G.; Athouel, L. AdV. Mater. 1997, 9, 33. (6) Mikami, T.; Yanagi, H. App. Phys. Lett. 1998, 73 (5), 563-565. (7) Koch, N.; Pogantsch, A.; List, E. J. W.; Leising, G.; Blyth, R. I. R.; Ramsey, M. G.; Netzer, F. P. App. Phys. Lett. 1999, 74 (20), 2909-2911. (8) Yanagi, H.; Okamoto, S.; Mikami, T. Synth. Met. 1997, 91, 91-93. (9) Koch, N.; Zojer, E.; Rajagopal, A.; Ghjisen, J.; Johnson, R. L.; Leising, G.; Pireaux, J.-J. AdV. Funct. Mater. 2001, 11 (1), 51-58. (10) France, C. B.; Schroeder, P. G.; Parkinson, B. A. Nano Lett. 2002, 2 (7), 693. (11) Ivanco, J.; Winter, B.; Netzer, F. P.; Ramsey, M. G. AdV. Mater. 2003, 15 (21), 1812-1815. (12) Seki, K.; Hayashi, N.; Oji, H.; Ito, E.; Ouchi, Y.; Ishii, H. Thin Solid Films 2001, 393 (1-2), 298-303. (13) Schroeder, P. G.; France, C. B.; Parkinson, B. A.; Schlaf, R. J. Appl. Phys. 2002, 91 (11), 9095. (14) France, C. B.; Parkinson, B. A. App. Phys. Lett. 2003, 82 (8), 1194-1196.
studied on TiO2(110),15 alkali halide crystals,16-18 Ag(111),19,20 Al(111),21-23 Ni(110)-2 × 1-O,24 muscovite25 GaAs(001)-2 × 4,26 and Au(111).14 Despite the extensive surface studies of this molecule on many substrates, there is little information on the appearance of coverage-dependent structures. In this contribution we concentrate on the molecular ordering of p-6P on the Au(111) surface in ultrahigh vacuum (UHV). We previously used photoemission to measure the band line-up between p-6P and Au(111) and reported one interesting structure imaged with STM.14 Herein we report additional ordered structures in the submonolayer coverage regime for p-6P adsorbed on Au(111). Au(111) is chosen as a substrate since it is often used as a contact metal in organic electronic devices, in addition the well-known 23 × x3 reconstruction greatly aids in solving the 2D structures of the ordered molecular overlayers. Experimental Section Experiments were performed in a commercial Omicron Multiprobe ultrahigh vacuum (UHV) system (base pressure 5 × 10-11 mbar). This system is equipped with variable temperature scanning tunneling microscopy (VT-STM), low-energy electron diffraction (LEED), and X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). Electron detection is achieved using a VSW EA125 single-channel hemispherical analyzer. Physical vapor deposition was performed (15) Koller, G.; Berkebile, S.; Krenn, J. R.; Tzvetkov, G.; Hlawacek, G.; Lengyel, O.; Netzer, F. P.; Teichert, C.; Resel, R.; Ramsey, M. G. AdV. Mater. 2004, 16 (23-24), 2159-2162. (16) Kintzel, E. J., Jr.; Gillman, E. S.; Skofronick, J. G.; Safron, S. A.; Smilgies, D.-M. Mater. Res. Soc. Symp. Proc. 2002, 708, 293-297. (17) Kintzel, E. J., Jr.; Smilgies, D.-M.; Skofronick, J. G.; Safron, S. A.; Van Winkle, D. H. J. Vac. Sci. Technol., A 2004, 22 (1), 107-110. (18) Yoshimoto, N.; Sato, T.; Saito, Y.; Ogawa, S. Mol. Cryst. Liquid Cryst. 2004, 425, 1-10. (19) Braun, K.-F.; Hla, S.-W. Nanoletters 2005, 5 (1), 73-76. (20) Hla, S.-W.; Braun, K.-F.; Wassermann, B.; Rieder, K.-H. Phys. ReV. Lett. 2004, 93 (20), 208302-1-208302-4. (21) Winter, B.; Ivanco, J.; Netzer, F. P.; Ramsey, M. G. Thin Solid Films 2003, 433 (1-2), 269-273. (22) Resel, R.; Salzmann, I.; Hlawacek, G.; Teichert, C.; Koppelhuber, B.; Winter, B.; Krenn, J. K.; Ivanco, J.; Ramsey, M. G. Org. Electron. 2004, 5 (1-3), 45-51. (23) Winter, B.; Ivanco, J.; Netzer, F. P.; Ramsey, M. G.; Salzmann, I.; Resel, R. Langmuir 2004, 20 (18), 7512-7516. (24) Koller, G.; Surnev, S.; Ramsey, M. G.; Netzer, F. P. Surf. Sci. 2004, 559 (2-3), L187-L193. (25) Plank, H.; Resel, R.; Sitter, H.; Andreev, A.; Sariciftci, N. S.; Hlawacek, G.; Teichert, C.; Thierry, A.; Lotz, B. Thin Solid Films 2003, 443 (1-2), 108114. (26) Mu¨ller, B.; Kuhlmann, T.; Lischka, K.; Schwer, H.; Resel, G.; Leising, G. Surf. Sci. 1998, 418, 256-266.
10.1021/la052924b CCC: $33.50 © 2006 American Chemical Society Published on Web 07/29/2006
7508 Langmuir, Vol. 22, No. 18, 2006 in a chamber (base pressure 1 × 10-9 mbar) attached to the analysis UHV system, allowing samples and films to be prepared in situ while the cleanliness of the analysis system is maintained. The gold film was prepared by heating a 1 cm × 1 cm mica sample, attached to the sample plate using molybdenum clips, at 300 °C for 24 h in UHV. Gold was then evaporated from a resistively heated tungsten basket onto the heated mica substrate to produce a film several microns thick. The Mo clips provided electrical contact to the gold surface during STM imaging. Argon ion sputtering (3 keV) and annealing (350 °C) cycles were used to clean and prepare the Au(111) surface. The chemical purity of the surface was determined with XPS (Mg KR, 50 eV pass energy) and the presence of the 23 × x3 reconstruction was confirmed with STM. p-Sexiphenyl (p-6P) was obtained from Aldrich Chemical Co. and placed in the deposition chamber without further processing or purification. Thin films of p-6P were deposited under UHV (base pressure 1 × 10-9 mbar) from a resistively heated boron nitride crucible (source temperature, 193°C). The source was maintained at 120 °C for 12 h prior to deposition to remove any contaminants more volatile than p-6P. Deposition rates of p-sexiphenyl were determined and monitored using a Leybold quartz crystal microbalance (QCM). The Au(111) substrate was maintained at room temperature during the organic deposition. Prior to each experiment, the Au(111) surface was renewed with a sputter and anneal cycle. p-Sexiphenyl films with varying thicknesses were then deposited on the cleaned surface at rates of several angstroms per minute. All STM images were obtained at room temperature in a constant current mode with sample biases ranging from -2.0 to +2.0 V and tunneling currents between 0.1 and 0.5 nA. The proposed 2D molecular structures were the result of the analysis of many STM images where distances and angles between molecules and rows of molecules were averaged between forward and backward scans to obtain good statistics for the reported unit cell values. Often, but not always, the 23 × x3 reconstruction of the Au(111) surface was visible through the molecules in the STM images and could then be used to orient the proposed unit cells or molecular directions with the underlying Au substrate.
Results and Discussion The STM image in Figure 1A shows an STM image taken at a low coverage of p-sexiphenyl on the Au(111) surface. At low coverages the structures tended to be more fluxional and unstable to scanning with the STM tip; thus, the STM images have lower resolution compared to higher coverages, where the molecules have restricted movement on the surface. The image shows a lack of long-range order; however, areas with local order can be observed, with a lower packing density structure at the top of the image and a higher packing density structure at the bottom. The poor resolution of the STM image precludes detailed analysis of the unit cell structure; however, several trends can be extracted from analysis of the image. A number of nearly isolated molecules show end-to-end nearest neighbor interactions and form linear chains of molecules. The unusual end-to-end configuration of this lower density structure is similar to the end-to-end row structures formed by pentacene molecules on Au(111) at lower coverages27 and end-to-end rows observed in STM images of p-6P on a Ni/NiO surface.24 We have no detailed explanation for this preferred low-coverage interaction, although it may be due to the interface dipole formed and distribution of positive charge in the molecules as a result of charge transfer from the p-6P to the Au substrate.14 The p-6P end-to-end molecular rows (upper structures in Figure 1A) contain on average two to four molecules and align with the three [110] related directions perpendicular to the Au(111) reconstruction lines. The [110] and symmetryrelated vectors are shown in Figure 1B, and these vectors are aligned with the image and diagram in the figure to demonstrate (27) France, C. B.; Schroeder, P. G.; Forsythe, J. C.; Parkinson, B. A. Langmuir 2003, 19, 1274-1281.
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Figure 1. (A). Low-coverage STM image of p-6P on Au(111) (75 × 75 nm; tunneling conditions, -0.5 V, 0.1 A, max z-height ) 0.37 Å). (B). Directional vectors with which the p-6P molecules align on the surface. (C). Models for the orientations of p-6P molecules on the Au(111) surface.
the orientation of the end-to-end structures. A higher packing density structure for p-6P molecules can be seen in the lower section of the STM image in Figure 1A. The image shows that the molecules from the two different packing densities are not aligned along the same lattice directions, and a 30° angle difference between the polymorphs is evident. In contrast to the endto-end molecular rows, the higher packing density molecules align with the long axis of the molecule parallel to the Au(111) reconstruction or another symmetry-related [112] direction (vectors also shown in Figure 1B). It is worth noting that the Au(111) reconstruction was not imaged simultaneously with these low coverage structures due to the degraded resolution discussed above. However, the Au(111) reconstruction was imaged on sections of the substrate containing no p-6P, and those images near to these structures provide general knowledge of the substrate orientation and allow us to determine the relationship between the organic overlayer and gold surface. The images of the wellestablished clean Au(111) reconstructed surface were not included in this report. Models for the interactions between the p-6P molecules and Au(111) substrate are presented in Figure 1C. The model is orientated so that it is aligned with the directional vectors presented in Figure 1B as well as the STM image in Figure 1A. The dark substrate atoms represent the elevated Au(111) atoms of the 23 × x3 reconstruction along the [112] direction. The six p-6P molecules on the right of the model represent the orientations of the low-coverage structure, aligned with the three [110] symmetry-related directions of the gold. The model also shows what we believe to be the preference for aromatic molecules to place phenyl rings on atop sites of the Au(111), a trend deduced from solving the structures for many ordered aromatic molecules on Au(111).10,14,27-30 The 12 molecules on the left of Figure 1C represent all the possible molecular orientations on the substrate of the higher coverage structure of p-6P shown in Figure 1A. The alignment of the organic molecules with the metallic substrate in these low-coverage structures is determined on the basis of
p-Sexiphenyl on the Au(111) Surface
Figure 2. (A). Close up STM image of the p-6P paired row structure. (Tunneling conditions, -0.5 V, 0.5 nA, max z-height ) 0.63 Å) (B). Paired row model of p-6P on Au(111). Dark substrate atoms represent the reconstructions lines on the surface. This model is aligned with the directional vectors as well as with the STM image in part A.
the observation of their alignment with the Au(111) reconstruction. At higher coverages of p-6P another 2D polytype appears that we call the “paired row structure”. A small-area STM image of the paired row structure is shown in Figure 2A. In this structure the molecular rows pair up leaving a larger gap between each pair of molecular rows, demonstrating the side-by-side preference of this higher packing density structure. The molecules are aligned with the gold substrate at angles of 30° with respect to the gold reconstruction lines that aid in creating a model for this structure (It is important to note that the reconstruction can be observed through p-6P molecules in the image of Figure 2A manifested as an apparent rippling effect on the molecular rows. Larger images of this exact section more clearly show the direction of the reconstruction.) The model of the structure is presented in Figure 2B and oriented in the same direction with the image and directional vectors. The experimental and modeled data are compared in Table 1, and a good agreement is evident. Parameter (28) France, C. B.; Parkinson, B. A. J. Am. Chem. Soc. 2003, 125, 1271212713. (29) France, C. B.; Parkinson, B. A. Langmuir 2004, 20 (7), 2713-2719. (30) Schroeder, P. G.; France, C. B.; Park, J. B.; Parkinson, B. A. J. Appl. Phys. 2002, 91 (5), 3010.
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Figure 3. (A). Close up STM image of the p-6P paired offset and paired row. (tunneling conditions, -0.5 V, 0.25 nA, max z-height ) 0.91 Å) (B). Combined domains of the offset (light gray) and end-centered (black) model of p-6P on Au(111). Dark substrate atoms represent the reconstructions lines on the surface. This model is aligned with the directional vectors as well as with the STM image in part A. Table 1: Experimental and Modeled Parameters of p-Sexiphenyl Paired Row Structure on Au(111) parameters
experimental
modeled
a, Å bwithin, Å bbetween, Å R, deg angle to reconstruction, deg
11.2 ( 1 32.5 ( 1 39 ( 3 80 ( 5 45 ( 5
10.4 33.2 38.6 83 44
a represents the distance between the side-by-side molecules, bwithin is the distance between the two molecules within the paired row, bbetween is the distance between paired rows, -R is the angle of the unit cell and -areconstruction is the angle between the direction of the paired rows and the Au(111) reconstruction. At slightly higher p-6P coverages an interesting set of surface structures is observed. Figure 3A reveals molecules organized into three domains of two different structures. The middle domain, which we will call the end-centered structure, consists of rows of molecules that are nearly perpendicular to the row direction. The other two domains present in Figure 3A, which we call the offset structure since across rows the molecules are offset by one
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Table 2: Experimental and Modeled Parameters of p-Sexiphenyl Offset and End-Centered Structures on Au(111) offset domain parameter
experimental
modeled
a, Å b, Å R, deg boundary angle, deg
11.1 ( 1 29 ( 2.5 118 ( 3 158 ( 3
10.4 32.1 115 155-156
end-centered domain experimental modeled 20.8 ( 1 26 ( 2.5 95 ( 3 same
20.2 28.8 98 same
Au atom, also consists of rows of molecules but are tilted with respect to the row direction. The models for these structures are shown in Figure 3B, where the insets show the unit cells for the two structures. The experimental data for both of the structures is compared to their modeled values in Table 2. The two domains alternate across the surface with the offset structure having larger domain sizes than the end-centered structure (20-35 nm vs 5-20 nm, respectively). Figure 3C shows parts of five domains that alternate between these two structures. Analysis of many images, many containing only partial domains, revealed that the surface contained statistically equivalent numbers of the two domain types. The images reveal dark regions, corresponding to missing p-6P molecules, arranged in a zigzag along the domain boundaries. The missing molecules occur in every other row and at every other domain boundary if one follows a row to the next boundary (Figure 3A,C). Figure 3B shows the model for the intersection of the two structures shown in detail in the insets with the end-centered structure shown with black molecules and the offset structure shown in red. It can be seen that, when the two structures intersect, and one row is made continuous, there is a molecule from the end-centered domain that must be removed from every other row to make room for the offset structure. In Figure 3B the molecules shown in blue are not present in the structure and correspond to the dark line observed in the STM images. All the features of the domains are closely modeled by our proposed structure, including the angle between the rows of the two structures. This is the first case that we are aware of where the detailed structure of a 2D domain boundary was solved. Table 2 summarizes all the experimental and modeled data for the structures shown in Figure 3. One can only speculate about the reasons that at this particular coverage the p-6P molecules cover the surface with two alternating structures. One possibility is that there is a slight incommensuration between the preferred surface packing and the underlying Au lattice. One reasonable explanation is that both structures are incommensurate with the underlying Au lattice. The offset domains are on average about 25 nm wide, whereas the endcentered domains are about 15 nm wide, and since the p-6P distances are about 1 nm in each structure, the lattice mismatch in the two structures would be about 4% and 6%, respectively. Therefore, the more commensurate domain grows until its strain energy forces a shift in the position of the next molecule that is then oriented to grow in a different 2D polytype until it reaches its critical size and forces another reorientation back to the original 2D phase. In alternating rows the new phase grows continuously by only a reorientation of the molecules in a row, whereas this is not possible in every row, so a systematic molecular vacancy occurs in alternate rows. In this speculative model the domain sizes and free energies of the two phases must be nearly equal. Otherwise, simply covering the surface with large islands of the most commensurate or low-energy domain, with molecular vacancies at the grain boundaries, would minimize the total energy. We have previously found that there can be many coexisting ordered 2D structures for polyaromatic molecules, especially pentacene, on different areas of the surface, corresponding to slightly different coverages.27 The unusual aspect of these coexisting domain structures is their regular alternating
Figure 4. (A) STM image of the alternating p-sexiphenyl structures. The Au(111) reconstruction is visible through the organic film. (tunneling conditions, -0.5 V, 0.1 nA, max z-height ) 0.53 Å). (B) Models for the rectangular and oblique structures. (C) Close up STM image of the alternating structure (tunneling conditions, +0.4 V, 0.2 nA, max z-height ) 0.6 Å).
structure and similar packing densities over the same area of the surface. A high-coverage STM image of p-6p on Au(111) is presented in Figure 4A. This image shows two different structures we generally term alternating structures. In these structures every other molecule along the row direction is rotated 90° along its major axis, as is the case in the bulk crystal structure of p-6P.31 This is seen in the STM image in Figure 4A,C as the light and dark alternating molecules. We cannot definitively correlate whether bright or dark molecules correspond to flat or edge-on orientation, as tunneling efficiencies may be influenced by the orientation of the molecule and its interaction with the Au(111)
p-Sexiphenyl on the Au(111) Surface
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Table 3: Experimental and Modeled Parameters of Rectangular Alternating p-Sexiphenyl Structure on Au(111) parameter
experimental
modeled
a, Å b, Å R, deg angle to reconstruction, deg
14.6 ( 1 32.3 ( 1 90 ( 5 31 ( 3
14.4 33.2 90 30
Table 4: Experimental and Modeled Parameters for Oblique Alternating p-Sexiphenyl Structure on Au(111) parameter a, Å b, Å R, deg
experimental
modeled
14.6 ( 1 36.3 ( 1 60 ( 4
14.4 34.5 60
Table 5: 2D Crystallographic Parameters for the Five p-6P Structures Investigated structure
plane group
molecules/ unit cell
packing density (molecules/cm2)
double row end centered offset alternating oblique alternating rectangular
p2 p2 p1 p2 pmm2
2 2 1 2 2
1.45 × 1013 3.4 × 1013 3.0 × 1013 4.0 × 1013 4.2 × 1013
substrate. It could be that the hydrogen atoms at the ring edges protrude toward the tip and reduce the tunneling distance, thus increasing the tunneling current in the edge-on orientation. Hydrogen atoms have been invoked to increase the tunneling and produce bright areas in images of alkanes on HOPG.32 Conversely, the efficient overlap between the π states of the aromatic molecule and the substrate may make the tunneling between the tip and the extended π states in the flat-lying molecules to the substrate more facile. In either case the effects of the alternating structure can be seen in many images. Figure 4A shows two distinct alternating structures, one that has the molecules perpendicular to the row direction and the other with the molecules tilted with respect to the row direction. The perpendicular structure, seen clearly in the lower center and upper right of the image, was presented in detail in our previous paper,14 but the oblique structure, seen in the lower right of the image, was not solved. Figure 4C shows a close up STM image of the oblique structure. There appears to be some flexibility in the tilt of the molecules within a row, since a transition between the two structures can be seen between the upper right and lower right of Figure 4A, where the molecules in the rows have intermediate tilt angles with respect to the row direction. The models for the two alternating domain structures are shown in Figure 4B and the values of the experimental values compared to the modeled data are seen in Tables 3 and 4. Table 5 summarizes all of the p-6P structures that we have thus far observed and characterized on the Au(111) surface along with the calculated packing densities, plane groups for the identified structures, and the number of p-6P molecules in the 2D unit cell. Figure 5 shows a schematic with all the molecular orientations observed in the five structures reported herein. The molecules have one end ring centered over a single Au atom and, as discussed, adopt orientations where they can center as many rings as possible over substrate Au atoms. We believe this placement maximizes the van der Waals interactions between the molecules and the substrate, resulting in more close atom-atom contacts. Recently, however, Hla et al. have published several papers using STM imaging at 6 K and (31) Baker, K.; Fratini, A.; Resch, T.; Knachel, H.; Adams, W.; Socci, E. Polymer 1993, 34, 1571. (32) Liang, W.; Whangbo, M. H.; Wawkuschewski, A.; Cantow, H. J.; Magonov, S. N. AdV. Mater. 1993, 5 (11), 817-21.
Figure 5. Representation of all the molecular orientations adopted by the p-6p molecule centered at one Au atom. Three different sets of related orientations with respect to the substrate are shown: rectangular alternating and paired row structures (blue), offset structure (red), and the end-centered and oblique structures (black).
DFT calculations to show that individual p-6P molecules on a Ag(111) surface have an alternating twisted conformation like that found for p-6P in the gas phase.19,20 In the condensed phase, however, packing forces are responsible for producing a planar configuration of p-6P molecules31 and we believe will also produce a flat structure on a surface. On Ag(111) the rings of the twisted p-6P molecules are postulated to have the two carbon atoms on one side of a ring located above a single atom and the two carbons on the other side of the ring above two surface Ag atoms. Hla et al. attribute the zigzag appearance of the molecule in the STM image to be the result of one edge of alternate rings being tilted upward toward the tip.19 It is possible that this is the low-energy configuration, but at temperatures well above 6 K the molecules may be rapidly alternating between tilting rings to both sides of a Au or Ag atom, resulting in an STM image that shows an average position just above the substrate atom. It is also possible that, due to electronic structure differences, despite the similarity in lattice constants between Ag and Au, that p-6P molecules adsorb differently on Ag(111) than on Au(111). A tilted orientation also may only occur on isolated molecules, as were observed in the STM images of Hla et al., where, as in the gas phase, there are no molecule-molecule interactions.
Conclusions We have shown that p-6P forms a rich variety of packing structures on the Au(111) surface as a function of its coverage. Several unusual structures were discovered: two structures with alternating flat and tilted molecules, which have a similar packing structure to the bulk p-6P structure, and a set of alternating coexisting surface phases with ordered domain boundaries. The structures of both coexisting phases as well as the domain boundary were solved, perhaps the first 2D molecular domain boundary to have its detailed structure determined. The orientation of adsorbed p-6P molecules on Au(111) surfaces and the relation to their appearance in STM images were discussed. Acknowledgment. This work was supported by NSF grant # CHE-0518563. We appreciate the help of Adam Matzger in assigning plane group symmetries. LA052924B
