STM Investigations of the Two-Dimensional Ordering of

Langmuir 2001, 17, 6185-6195

6185

STM Investigations of the Two-Dimensional Ordering of Perylenetetracarboxylic Acid N-Alkyl-diimides on HOPG and MoS2 Surfaces Yasuo Kaneda,‡ Michele E. Stawasz,† David L. Sampson,§ and B. A. Parkinson*,† Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, Mitsubishi Paper Mills, Ltd., Research Center, 46 Wadai, Tsukuba-City, Ibaraki, Japan 300, and Digital Instruments/Veeco, 112 Robin Hill Road, Santa Barbara, California 93117 Received January 9, 2001. In Final Form: April 30, 2001 The 2D structures of a variety of n-alkyl-substituted perylene diimides adsorbed onto HOPG and MoS2 surfaces from phenyloctane solutions were studied using scanning tunneling microscopy (STM). Both rectangular, or herringbone-like, structures and row structures were observed. Surprisingly, the lattice constants, and thus the area per molecule of the rectangular structures, did not increase as expected when the alkyl chain length was increased. Protrusion of the alkyl tails into the solvent above the 2D layer is proposed to account for this behavior. Row structures, where the alkyl tails lie flat on the substrate surface, were also observed wherein the area per molecule increases as expected for the increase in the length of the alkyl tail. The formation of domains with a particular orientation with respect to the underlying lattice was observed for many of the 2D structures. The alignment of the molecular layers with the substrate could be explained with a point-on-line coincidence model. Formation and filling of missing molecule defects within the oriented domains was observed during continuous scanning of the STM.

Introduction Organic thin films are currently attracting much attention due to their applicability to future electronic devices.1 Interfacial crystal structures are often different from the bulk crystal structure.2,3 The interfacial region between a substrate and the bulk of the organic thin film is often crucial to the performance of devices. The interfacial layer may control the rate of carrier transport between the film and the substrate as well as template the bulk structure of the thin film. It is therefore important to understand the structures of molecules at interfaces to subsequently control the performance of the devices and to understand basic molecule-substrate interactions. Scanning tunneling microscopy (STM) has become a powerful tool for the investigation of organic thin films because it is capable of atomic scale resolution of organic molecules on conducting and semiconducting substrates. STM has provided information on the two-dimensional ordering of many types of molecules including alkanes,4-11 substituted alkanes,5,12-14 and liquid crystals.15-18 * To whom correspondence should be addressed. Phone: 970-491-0504. Fax: 970-491-1801. E-mail: Parkinson@ mail.chm.colostate.edu. † Colorado State University. ‡ Mitsubishi Paper Mills, Ltd. § Digital Instruments/Veeco. (1) Reed, M. A. Proc. IEEE 1999, 87, 652-658. (2) Yanagi, H.; Chen, S.; Lee, P. A.; Nebesny, K. W.; Armstrong, N. R.; Fujishima, A. J. Phys. Chem. 1996, 100, 5447-5451. (3) Hoshino, A.; Isoda, S.; Kurata, H.; Kobayashi, T. J. Appl. Phys. 1994, 76, 4113-4120. (4) McGonigal, G. C.; Bernhardt, R. H.; Thomson, D. J. Appl. Phys. Lett. 1990, 57, 28-30. (5) Rabe, J. P.; Buchholtz, S. Science 1991, 253, 424-427. (6) Cincotti, S.; Rabe, J. P. Appl. Phys. Lett. 1993, 62, 3531-3533. (7) Askadskaya, L.; Rabe, J. P. Phys. Rev. Lett. 1992, 69, 1395-1398. (8) Rabe, J. P.; Buchholz, S.; Askadskaya, L. Phys. Scr. 1993, T49, 260-263. (9) Wawkuschewski, A.; Cantow, H.-J.; Magonov, S. N. Langmuir 1993, 9, 2778-2781. (10) Watel, G.; Thibaudau, F.; Cousty, J. Surf. Sci. Lett. 1993, 281, L297-L302.

There have been a number of previous STM studies of perylene-based dyes. Primarily, these studies have been performed in ultrahigh vacuum (UHV),19-27 but herein we introduce a very simple way of imaging perylene dyes at the liquid-solid interface. Previously, perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) and perylene3,4,9,10-tetracarboxylic acid diimides (PTCDI) were imaged at the solid-liquid interface only when the liquid crystal octylbiphenylcarbonitrile (8CB) was used as a solvent.28,29 In these studies, the perylene dyes were (11) Uosaki, K.; Yamada, R. J. Am. Chem. Soc. 1999, 121, 40904091. (12) Giancarlo, L.; Cyr, D.; Muyskens, K.; Flynn, G. W. Langmuir 1998, 14, 1465-1471. (13) Claypool, C. L.; Faglioni, F.; Goddard, W. A., III; Gray, H. B.; Lewis, N. S.; Marcus, R. A. J. Phys. Chem. B 1997, 101, 5978-5995. (14) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. Chem. Mater. 1996, 8, 1600-1615. (15) Foster, J. S.; Frommer, J. E. Nature 1988, 333, 542-544. (16) Smith, D. P. E.; Horber, H.; Gerber, C.; Binng, G. Science 1989, 245, R43-R45. (17) Stevens, F.; Dyer, D. J.; Muller, U.; Walba, D. M. Langmuir 1996, 12, 5625-5629. (18) Patrick, D. L.; Cee, V. J.; Beebe, T. P., Jr. J. Phys. Chem. 1996, 100, 8478-8481. (19) Ludwig, C.; Gompf, B.; Glatz, W.; Petersen, J.; Eisenmenger, W.; Mobus, M.; Zimmermann, U.; Karl, N. J. Phys.: Condens. Matter 1992, 86, 397-404. (20) Schmitz-Hubsch, T.; Fritz, T.; Staub, R.; Back, A.; Armstrong, N. R.; Leo, K. Surf. Sci. 1999, 437, 163-172. (21) Fritz, T.; Hoffman, M.; Schmitz-Hubsch, T.; Leo, K. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1998, 314, 279-284. (22) Glockler, K.; Seidel, C.; Soukopp, A.; Sokolowski, M.; Umbach, E.; Bohringer, M.; Berndt, R.; Schneider, W.-D. Surf. Sci. 1998, 405, 1-20. (23) Schmitz-Hubsch, T.; Fritz, T.; Sellam, F.; Staub, R.; Leo, K. Phys. Rev. B 1997, 55, 7972-7976. (24) Seidel, C.; Awater, C.; Liu, X. D.; Ellerbrake, R.; Fuchs, H. Surf. Sci. 1997, 371, 123-130. (25) Kendrick, C.; Kahn, A. Appl. Surf. Sci. 1998, 123, 405-411. (26) Seidel, C.; Schafer, A. H.; Fuchs, H. Surf. Sci. 2000, 459, 310322. (27) Umbach, E.; Glockler, K.; Sokolowski, M. Surf. Sci. 1998, 404, 20-31. (28) Freund, J.; Probst, O.; Grafstroem, S.; Dey, S.; Kowalski, K.; Neumann, R.; Woertge, M.; Putlitz, G. z. J. Vac. Sci. Technol., B 1994, 12, 1914-1917.

10.1021/la0100587 CCC: $20.00 © 2001 American Chemical Society Published on Web 08/31/2001

6186

Langmuir, Vol. 17, No. 20, 2001

imaged in defects created in the adsorbed 8CB layer since 8CB has a greater surface affinity than PTCDA or PTCDI. It is unknown whether the observed 2D structure of the PTCDA and PTCDI layers was the freely forming structure or whether it is a byproduct of the boundary conditions imposed by the 8CB layer. To avoid this ambiguity, we have used phenyloctane as a solvent. Phenyloctane has been widely used to selectively deposit long-chain alkanes and substituted alkanes for STM imaging and has recently been used to deposit aromatic squaraine dye molecules30 for the same purpose, but its use for depositing polyaromatic compounds such as PTCDA and PTCDI has not been reported. Phenyloctane is well suited as a solvent for STM imaging because it has a relatively weak interaction with the graphite and MoS2 substrates and so does not compete with the adsorbing species of interest. It also has a low vapor pressure. As a result, convection currents and thermal gradients, which cause thermal drift in STM images due to evaporative cooling, are minimized. PTCDI dyes are very promising candidates for use in electronic devices due to their photo and thermal stability and their electro-optic properties. In particular, n-alkylsubstituted perylene diimide films are n-type semiconductors31,32 and may be useful as electron-accepting materials in organic electronic devices. A solar cell utilizing this characteristic was constructed using N,N′-dimethyl3,4,9,10-perylene tetracarboxylimide as the electron acceptor and zinc phthalocyanine as the electron donor.33 Recently, a study of the electronic characteristics for several n-alkyl-substituted perylene diimides has been published by Struijk et al.34 In addition, they investigated the bulk ordering of these perylenes using X-ray diffraction and molecular modeling. They found that these alkylated perylene diimides form smectic and columnar layers with interdigitating alkyl tails. However, bulk ordering alone does not determine device characteristics. As mentioned earlier, the interfacial layer may control the rate of carrier transport between the film and the substrate, as a result of substrate registry and defect sites. In addition, the interfacial layer may act as a template for the structure of the rest of the film. Therefore, understanding the mechanism of molecular alignment on a surface is critical to proper and reproducible device manufacturing. In this work, we report on STM investigations of the ordered adsorption of n-alkyl derivatives of PTCDI of various tail lengths on highly ordered pyrolytic graphite (HOPG) and molybdenum disulfide (MoS2). We will generally refer to these alkylated perylene-3,4,9,10tetracarboxylic acid diimides as PTCDI-Cn’s. See Figure 1 for the chemical structure of PTCDI-Cn as compared to that of PTCDA. The length of the hydrocarbon tail on the molecules was varied to see if it is possible to observe the transition between surface structures driven by intermolecular core interactions and interactions between the hydrocarbon tails and the substrate. Previous studies of liquid crystalline molecules with hydrocarbon tails of various lengths (nCBs) on HOPG have shown that there (29) Grafstrom, S.; Schuller, P.; Kowalski, J.; Neumann, R. Appl. Phys. A 1998, 66, S1237-S1240. (30) Stawasz, M. E.; Sampson, D. L.; Parkinson, B. A. Langmuir 2000, 16, 2326-2342. (31) Meyer, J.-P.; Schlettwein, D.; Wohrle, D.; Jaeger, N. I. Thin Solid Films 1995, 258, 317-324. (32) Horowitz, G.; Kouki, F.; Spearman, P.; Fichou, D.; Nogues, C.; Pan, X.; Garnier, F. Adv. Mater. 1996, 8, 242-245. (33) Wohrle, D.; Kreienhoop, L.; Schnurpfeil, G.; Elbe, J.; Tennigkeit, B.; Hiller, S.; Schlettwein, D. J. Mater. Chem. 1995, 5, 1819-1829. (34) Struijk, C. W.; Sieval, A. B.; Dakhorst, J. E. J.; Dijk, M. v.; Kimkes, P.; Koehorst, R. B. M.; Donker, H.; Schaafsma, T. J.; Picken, S. J.; Craats, A. M. v. d.; Warman, J. M.; Zuilhof, H.; Sudholter, E. J. R. J. Am. Chem. Soc. 2000, 122, 11057-11066.

Kaneda et al.

Figure 1. PTCDA and PTCDI-Cn chemical structures.

is a transition from rowlike structures in which headgroups and tailgroups alternate directions (head-driven structure), thereby precluding interaction between the alkyl tails, to rowlike structures with alignment of hydrocarbon tails of adjacent molecules. This transition occurs when the carbon chain lengths exceed six carbons.35 Furthermore, on HOPG the hydrocarbon tails are commensurate with the underlying HOPG structure. This commensuration has been shown to be important for ordering simple hydrocarbon chains on HOPG surfaces.4,5,10,36,37 We also report on the ordering of PTCDICn molecules on MoS2 substrates to examine the influence of the substrate lattice on this adsorbate system since previous studies of PTCDI in ultrahigh vacuum showed only slight differences between the 2D unit cell parameters of PTCDI adsorbed on HOPG as compared to those of PTCDI on MoS2.38 Experimental Section PTCDA and phenyloctane were purchased from Aldrich and used as received. PTCDI-C13 was received from the N. R. Armstrong group at the University of Arizona. The remaining PTCDI-Cn derivatives were synthesized through condensation reactions between PTCDA and the appropriate alkylamine according to procedures described in the literature.39 STM measurements were performed with a commercial Nanoscope III scanning probe microscope with mechanically cut Pt/Ir (90: 10) or electrochemically etched W tips. All samples were imaged in air at room temperature (22-25 °C). PTCDA or PTCDI-Cn was added to phenyloctane to produce a saturated solution, since some suspended undissolved material was often present. The substrate (HOPG or MoS2) was heated on a hot plate to ∼50 °C for several hours. Immediately before deposition, the substrate was cleaved. A droplet of the supernatant solution was deposited on the HOPG or MoS2 surface. The substrate, with the deposited solution, remained on the hot plate for several more hours, and then the heat was turned off and the sample was allowed to cool. Several hours to several days after turning off the hot plate, the sample was imaged with STM. An alternative method of sample preparation did not require heating of the sample and allowed the sample to be imaged in situ. In this method, the solution was applied directly to a room temperature, freshly cleaved substrate surface. The tunneling tip was then immersed directly in the droplet, and the (35) Smith, D. P. E.; Heckl, W. M.; Klagges, H. A. Surf. Sci. 1992, 278, 166-174. (36) Groszek, A. J. Proc. R. Soc. London, Ser. A 1970, 314, 473-498. (37) Rabe, J. P.; Buchholtz, S. Phys. Rev. Lett. 1991, 66, 2096-2099. (38) Ludwig, C.; Gompf, B.; Petersen, J.; Strohmaier, R.; Eisenmenger, W. Z. Phys. B 1994, 93, 365-373. (39) Demmig, S.; Langhals, H. Chem. Ber./Recl. 1988, 121, 225230.

2D Ordering of Alkyl-Substituted Perylene Diimides

Langmuir, Vol. 17, No. 20, 2001 6187

Table 1. 2D Lattice Constants and Epitaxy Data of PTCDA or PTCDI-Cn Monolayer on HOPG or MoS2a structure

molecule

substrate

a [Å]

b [Å]

γ [deg]

molecular area [Å2]

θexp [deg]

nearly rectangular lattice

PTCDA PTCDI-C4 PTCDI-C8 PTCDI-C12 PTCDI-C13 PTCDI-C18 PTCDI-C4 PTCDI-C13 PTCDI-C18 PTCDI-C18 PTCDI-C4 PTCDI-C13

HOPG HOPG HOPG HOPG HOPG HOPG MoS2 MoS2 MoS2 HOPG MoS2 MoS2

12.6 ( 1.3 15.8 ( 0.2 15.1 ( 0.5 15.3 ( 0.3 16.2 ( 0.3 15.4 ( 0.2 15.4 ( 0.2 15.9 ( 0.2 15.7 ( 1.0 7.0 ( 0.7 12.0 ( 1.0 9.8 ( 0.7

19.7 ( 0.2 16.5 ( 0.2 16.7 ( 0.3 16.7 ( 0.5 16.9 ( 0.3 16.0 ( 0.2 17.0 ( 0.2 17.1 ( 0.1 16.7 ( 1.0 43.0 ( 0.7 22.0 ( 1.0 30.0 ( 0.7

85.4 ( 5.6 89.0 ( 0.5 87.5 ( 0.1 87.4 ( 1.2 88.2 ( 0.2 88.2 ( 1.1 87.5 ( 0.7 87.6 ( 0.4 88.8 ( 1.5 * 57 ( 5 *

124 ( 3 130 ( 3 126 ( 7 128 ( 5 133 ( 5 123 ( 1 130 ( 5 135 ( 4 131 ( 23 302 ( 49 264 ( 30 296 ( 40

10.3(0.8 8.4 ( 1.0 8.4 ( 1.2 9.6 ( 0.8 9.0 ( 0.4 8.8 ( 0.7 8.9 ( 1.1 19.0(1.0 * * * *

row

a

θcalc [deg]

supercellcalc [na × nb]

9.00 8.75 9.00 9.00 9.00 9.25 20.0

2×7 1×5 1 × 20 1 × 18 2×5 1×3 5 × 17

An asterisk (*) represents insufficient data due to experimental difficulty.

sample surface was imaged wet. No significant differences were observed in the STM images of samples prepared by either method. Sample biases ranged from 1.0 to 2.3 V (sample positive), and the tunneling current was set to between 30 and 300 pA. Before and/or after imaging the adsorbate, the substrate atoms were imaged to calibrate the lateral distances in the frame by the known lattice constants of the substrate. This was accomplished by lowering the bias voltage and increasing the tunneling current with respect to the tunneling parameters for imaging the adsorbate molecules (typically 100 mV and 200-300 pA for both HOPG and MoS2). This provided information about not only the lattice constants of the ordered adsorbate structure but also the relationship between the adsorbate lattice and the substrate lattice. Commensuration with the underlying lattice and the orientation angle of the monolayer in relation to the substrate were also determined from this observation. Voltage pulses (3 V for 0.5 s) were sometimes used to clean the tip and sometimes resulted in improved images. All images presented in this manuscript were collected in constant current mode. To reduce the influence of thermal drift, the images were taken after at least 1 day of thermal equalization, scans were collected at high tip speeds (>100 nm/s), and lattice constants were derived from multiple images collected from different scan directions. Only flattening of the images was performed to compensate for tilting of the substrate and scan line artifacts. Determination of the mode of epitaxy was accomplished using EpiCalc, a program developed by Ward and co-workers40,41 and available on the web at http://www.wardgroup.umn.edu/software.html. EpiCalc uses an analytical function that enables the calculation of a “dimensionless potential”, V/Vo, which evaluates the misfit between the periodic overlayer and substrate lattices. The value of V/Vo relates the degree of commensuration of the overlayer with the substrate at the azimuthal (rotation) angle. Values of 1.0 indicate an incommensurate overlayer; 0.5, pointon-line coincidence; and -0.5 (for hexagonal substrates), a commensurate overlayer. Substrate unit cell parameters (a1, a2, and Ro) and a user-defined range of overlayer lattice parameters (b1, b2, and βo) are input into the program, and V/Vo is calculated over a specified range of azimuthal angles (at an increment of ∆θ ) 0.25°). Lattice parameters input for the substrates used in this study were a1 ) a2 ) 2.46 Å and Ro ) 60° for HOPG and a1 ) a2 ) 3.16 Å and Ro ) 60° for MoS2. Lattice parameters input for the PTCDI-Cn overlayers were taken from Table 1, with indicated error defining the parameter range. The azimuthal angle was set to vary from 0° to 60° at a constant interval of 0.25°. An overlayer size of 51 × 51 unit cells was used for each of the calculations, with geometric solutions up to 25 unit cells. Periodic contrast variation was evident in the STM images of PTCDI-C8 layers on HOPG and was used to verify the calculated supercell sizes for coincident orientations of this overlayer/ substrate system. This verification was not possible for the other PTCDI-Cn systems due to a lack of periodic contrast modulation data. (40) Hillier, A. C.; Ward, M. D. Phys. Rev. B 1996, 54, 14037-14050. (41) Last, J. A.; Hooks, D. E.; Hillier, A. C.; Ward, M. D. J. Phys. Chem. B 1999, 103, 6723-6733.

Figure 2. STM images of PTCDA adsorbed on the basal plane of HOPG from phenyloctane solution: (a) 37 nm × 37 nm scan and (b) 10 nm × 10 nm scan. Figure 8’s are overlaid on the images to elucidate the positions of single PTCDA molecules.

Results and Discussion STM Imaging with Phenyloctane. Figure 2 shows STM images of PTCDA on HOPG as deposited from a phenyloctane solution. The molecules exhibit a similar contrast to the “double-ring-like”19 or “figure 8”3 shaped contrast obtained in vapor-deposited PTCDA in which the STM experiment was conducted in air. The lattice constants derived from our STM images are consistent with values reported by Ludwig et al.19 and Hoshino et al.3 We have also sometimes observed modulated contrast where nearly every second PTCDA molecule of translationally equivalent orientation is brighter, which has also been reported in Hoshino et al.’s STM analysis and corresponds to their designation of Type II orientation with respect to the graphite lattice. Several different orientation angles between the PTCDA overlayer and the

6188


Kaneda et al.

Figure 3. Adsorption, annealing, and defect formation of a -C4 monolayer on HOPG: (a and b) 100 nm × 100 nm STM images showing PTCDI-C4 domain formation over a 14 min timespan and (c-e) 50 nm × 50 nm STM images showing the evolution of a domain in terms of dynamic adsorption of PTCDI-C4 molecules and subsequent defect shape changes.

substrate lattice have been observed and are summarized in Table 1. Our observation of the angles between the PTCDA oriented molecular domains and the HOPG substrate are again similar to other reported values from previous STM studies on graphite. We therefore conclude that the same herringbone-packed structure is formed when PTCDA is deposited from solution (phenyloctane) as from vacuum deposition. An advantage of investigating molecular order at the solid-liquid interface is the ability to observe the dynamics of adsorption. Investigation of adsorbed layers at the solid-gas interface (with the exception of the interfacial layer of adsorbed water) after vacuum deposition is a relatively static process at low temperatures except for the movement of molecules from interactions with the tip. Real-time observation of adsorption, defect nucleation and growth, and annealing processes are not possible without introducing the scanning tip into the deposition system. With vacuum deposition, this is problematic due to the likely deposition onto the tip, which necessarily decreases resolution and makes imaging difficult. Thus, STM investigations of vacuum-deposited layers are typically conducted postdeposition or between sequential adsorbate doses. We have shown elsewhere, however, that it is possible to observe all of these dynamic processes at the interface of the substrate and phenyloctane solution.30 Observation of the growth of ordered domains of PTDCI-Cn on HOPG or MoS2 was possible after the application of a voltage pulse (3 V, 0.5 s) to an adsorbate layer that appeared amorphous even ∼24 h after solution deposition. It is possible that this was caused by disordered material within the tunneling gap that may have interfered with tunneling through an ordered adsorbate layer. Immediately following the pulse, small isolated domains of ordered adsorbate were observed randomly scattered over the substrate. Subsequent growth of these small, isolated domains occurred approximately radially outward

from the domain edges across the substrate surface. This sequence is illustrated in Figure 3a,b. Domains at substrate step edges, however, grew anisotropically along the step edge. Molecular vacancies were common within any given domain. 2D growth of isolated domains resulted in coalescence of neighboring domains into a larger domain if they were of the same orientation on the substrate surface. Domains with different orientations maintained a boundary between them as they grew together. Occasionally, Ostwald ripening was observed where a domain with a dominant orientation adjacent to another domain of differing orientation would convert it to the dominant orientation. More complete discussion of the azimuthal orientation of adsorbate domains with respect to the orientation of the substrate lattice will be addressed in a following section. Domain annealing tended to stop when domain sizes of several hundred nanometers were reached, even after 24 h of equilibration. However, by this time the vacancies within a domain had filled in and no further defect diffusion or creation was observed. Only after completion of the monolayer were multiple layer structures observed. Figure 3 displays a series of STM images showing some of the dynamic processes described above occurring during the regrowth of a monolayer of PTCDI-C4 molecules on HOPG after the application of a voltage pulse. Images a and b show a time sequence for the adsorption process. Image b was captured 14 min after image a and illustrates the growth of ordered domains across a bare area of graphite. Images c, d, and e are at later times and show the adsorption dynamics of PTCDI-C4 molecules on the HOPG surface. Blue arrows show where molecules are observed to adsorb and fill in empty areas on the surface. The green arrows point to several sites where a missing molecule defect has diffused to a new site in the monolayer. It is unlikely that the seemingly “new” defect sites observed are due to the desorption of ordered molecules since the


energetic penalty for desorption from the ordered 2D array into the solvent would be relatively high. Yellow arrows point out areas where a combination of both adsorption and defect diffusion have occurred to change the shape of an originally larger vacancy defect in the domain. Adsorbate Polytypes. More than one structure or polytype has been observed for several of the PTCDI-Cn molecules adsorbed on both HOPG and MoS2. Often, more than one polytype is found simultaneously on adjacent areas of the same substrate. The polytypes can be classified into two distinct classes of structures: (1) nearly rectangular, or “herringbone-like”, lattice structures and (2) row structures. Rectangular structures are defined as those where the orientation of the long axis of the perylene core switches from molecule to adjacent molecule. Row structures are defined as those in which the perylene cores are arranged in lamellae, with the same orientation of the long axes of all the perylene cores within the lamellae. More detail on the structures and interactions involved in the formation of each of the polytypes will be given within the following sections that discuss each PTCDICn compound. Ordering of PTCDI-C4. Figure 4 shows a typical STM image of PTCDI-C4 on HOPG. A two-dimensional array of small, bright parallel lines can clearly be seen. For PTCDA and PTCDI, the STM contrast has been reported to be dominated by the shape of the lowest unoccupied molecular orbital (LUMO) of the molecules, which is essentially the same for both PTCDA and PTCDI despite their varying functionality.38 Partial electron density calculations predict that STM images of PTCDA and PTCDI on weakly interacting substrates such as HOPG and MoS2 should ideally contain 10 bright spots, in the shape of a figure 8, corresponding to the LUMOs of these molecules. Since the alkyl tails do not contribute significantly to the frontier orbitals of the PTCDI-Cn, the tunneling contrast of an alkylated PTCDI should resemble the LUMO of an unsubstituted PTCDI. By the same argument, the LUMO of the whole series of PTCDI-Cn’s should be essentially the same as that for PTCDA and PTCDI. Thus neglecting such effects as modulation of contrast due to overlayer registry with the substrate lattice, PTCDI-Cn’s adsorbed on HOPG and MoS2 should be imaged by STM as a figure 8 consisting of 10 bright spots. In our studies of perylene derivatives imaged under phenyloctane solution, we did not obtain this high level of resolution in which individual spots are resolved. STM image resolution is primarily dependent upon the tip radius and tip-sample distance. Decreased resolution in our case has resulted in a merging of the individual spots of the LUMO to form two solid parallel lines, represented in Figure 4 in transparent green. In the case of Figure 4, we therefore assign each pair of lines to one PTCDI-C4 molecule lying with the perylene core flat on the substrate. It can be seen that every other “row” of molecules has the same molecular orientation. Also observed are two different types of single missing molecule sites that are mirror images of one another, as illustrated in Figure 4. On the basis of these STM observations and the measured 2D lattice constants (Table 1), we propose the structure for the PTCDI-C4 monolayer on HOPG shown in Figure 4b. This structure is very similar to that reported in the literature of PTCDI on HOPG, where the perylenes form a rectangular, or herringbone, type structure. Due to the similarity between this structure and those reported in the literature, a complete analysis will not be included here. However, compared with the lattice constants of the major structure of the parent PTCDI, those of the PTCDIC4 adsorbate are expanded. This structural change is

Langmuir, Vol. 17, No. 20, 2001 6189

Figure 4. (a) 17.1 × 17.1 nm STM image and (b) corresponding molecular model showing the PTCDI-C4 rectangular polytype on HOPG. The pair of green parallel lines in (a) represents a single PTCDI-C4 molecule. Their proposed structural correlation is shown in (b). Several missing molecule defects are present, two of which are noted. Their labels (type 1 or 2) are reflected in the molecular model.

believed to be a result of the introduction of the butyl tail. Hydrogen bonding between the oxygen and hydrogens on the nitrogens from adjacent PTCDI molecules is the interaction which controls the intermolecular spacing in the PTCDI packing structure.38 The substitution of the butyl tail for the hydrogen in PTCDI-C4 precludes the formation of this hydrogen bond and thus increases the intermolecular spacing. The increased intermolecular spacing, however, is not sufficient to allow all of the butyl tail to lie on the surface. As a result, we propose that the butyl tails are either fully or partially desorbed from the surface and possibly solvated by the phenyloctane tails. A side-view model of this conformation is shown in Figure 4c. The occurrence of the two different types of single-

6190


Figure 5. 15 × 15 nm STM image showing the PTCDI-C4 row polytype on MoS2 with its corresponding molecular model.

molecule defects can be understood by removing a molecule from the adsorbate structure in either of the two possible molecular orientations. This is shown in the molecular model (Figure 4b), which represents the structure imaged in Figure 4a. When adsorbed on MoS2, PTCDI-C4 was found to exhibit both rectangular and row polytypes. As mentioned above, typically the row structure was observed to eventually convert to the rectangular structure. As shown in Figure 5, the row structure consists of bright lamellae with dark channels running between the lamellae. Within the lamellae, bright oblong structures oriented uniformly in the same direction can be seen. The long axis of this structure was found to measure 20 ( 1.1 Å in length, in good agreement with the length of the PTCDI-C4 molecule from the terminal carbon of one butyl tail to the other (20.9 Å). The measured width of this structure, 6.5 ( 1.5 Å, is likewise in good agreement with the 7 Å width of the perylene core. Thus, we assign the bright oblong structures to the PTCDI-C4 molecules lying flat on the MoS2 surface. The lower quality of the PTCDI-C4 on MoS2 row structure images does not allow us to resolve the butyl tails from the PTCDI core. In addition, the core no longer appears as two distinct parallel lines as it did in the rectangular polytype on HOPG. The distance of adjacent perylene molecules within the same lamella, or row, was measured to be 12.0 ( 1.0 Å. The angle of the long axis of the molecule to the edge of the bright lamella was measured to be 57

Kaneda et al.

( 5°. This angle allows a staggering between adjacent molecules within the same lamella, offsetting the polar areas of the adjacent molecules to mitigate their repulsion. The butyl tails presumably lie flat on the MoS2 surface. The shortness of the butyl tails does not allow for full interdigitation. However, as can be seen from the molecular model in Figure 5, it may be possible for pairs of molecules neighboring one another in adjacent lamellae to experience attractive dispersion interactions if their alkyl tails lie alongside one another. These flat-lying row structures are dissimilar to the “rod” structures observed via STM by Schmitz-Hubsch et al.20 for PTCDA adsorbed on Au(100). In that system, bright rods were found to consist of stacks of PTCDA molecules oriented with the molecular plane perpendicular to the plane of the substrate. This orientation is presumably driven by π-stacking interactions between adjacent molecules. In our system, as mentioned above, “rows” consist of lines of PTCDI-Cn molecules oriented with the molecular plane parallel to the plane of the substrate. In addition, the system reported by Schmitz-Hubsch et al. was investigated in a vacuum without solvent. These contrasts are worth mentioning to prevent any confusion between the previously reported rods and the row structures that we observe here. We can speculate on why a row structure occurs for PTCDA-C4 adsorbed on MoS2 and not on HOPG. NAlkanes on HOPG orient in registry with the substrate lattice, due to the similarity of the HOPG [011h 0] direction with the alkane backbone.4,10,36 We would expect that PTCDI-C4 alkyl tails would orient in a similar manner on HOPG to maximize dispersion interactions. It is clear, due to the observation of a lone rectangular structure in which only a portion of the tail adsorbs on the surface, that molecule-molecule interactions between adjacent PTCDI-C4 molecules on HOPG were strong enough to preclude a complete interaction of the butyl tails with the surface. We have reported a similar instance of partial tail interaction with the substrate and dominant molecule-molecule interactions for a dibutyl-aminophenyl squaraine on HOPG.30 In that system, alkyl tail length played a large role in adsorbate structure, and longer alkyl tails were found to lie fully adsorbed on the substrate surface, while shorter butyl tails partially desorbed from the surface. A second possibility is that the slightly more polarizable MoS2 surface exerts a stronger influence on the orientation of the perylene cores than the less polarizable HOPG surface. The polarization disrupts the quadrupolar interaction between adjacent cores that dominates the packing interactions in rectangular perylene adsorbate structures. The rectangular structure for PTCDI-C4 observed on MoS2 was found to be slightly different from that observed on HOPG (see Table 1). The a lattice parameter was found to be within experimental error for both HOPG and MoS2, whereas the b parameter was found to be slightly larger for MoS2. This corresponds to a slightly larger distance between adjacent PTCDI-C4 molecules with the same long axis orientation that are separated by a molecule with an opposite long axis for PTCDI-C4 on MoS2. Previous investigations of PTCDI on HOPG and MoS2 have observed the same trend.38 No explanation was offered, however. We believe that this slight difference in the unit cells between HOPG- and MoS2-adsorbed PTCDIC4 structures may be due to a phase coherence driven interaction41 with the MoS2 lattice. Observed epitaxy of the adsorbed perylene diimide unit cells with the substrates will be discussed below.


Langmuir, Vol. 17, No. 20, 2001 6191

Figure 7. Graph of molecular area vs alkyl tail length for PTCDI-Cn’s on both HOPG and MoS2. Data points for adsorbed structures on MoS2 are denoted with an asterisk (*).

Figure 6. Table showing STM images and molecular models of adsorption polytypes observed for PTCDI-C4, -C8, -C12, -C13, and -C18 on HOPG and MoS2. z-Ranges for all images (except the two -C18 on HOPG: z ) 2.0 nm) are 0.3 nm.

Ordering of Other PTCDI-Cn’s. Figure 6 shows a compilation of some typical STM images and their corresponding molecular models of the adsorbed structures exhibited by PTCDI-C4, PTCDI-C8, PTCDI-C12, PTCDIC13, and PTCDI-C18 on both HOPG and MoS2. The molecules PTCDI-C8, PTCDI-C12, and PTCDI-C13 were found to exhibit only the rectangular polytype on HOPG. PTCDI-C18 is the only alkylated perylene diimide that exhibited the row polytype while adsorbed on HOPG. On MoS2, PTCDI-C8 and PTCDI-C18 were found to form rectangular structures, while PTCDI-C13 formed a row structure. As mentioned earlier, PTCDI-C4 formed both row and rectangular structures on MoS2. Lattice parameters for each of these molecules and their different polytypes on both HOPG and MoS2 are listed in Table 1. For PTCDI-Cn’s exhibiting both rectangular and row polytypes on the same substrate (-C4 on MoS2 and -C18 on HOPG), the row structure was often the initial structure observed and was sometimes observed to convert to the rectangular polytype. These observations suggest that the row structure is kinetically stable but the rectangular polytype is thermodynamically more stable. The area per molecule from the various observed structures, plotted as a function of the number of carbon atoms in the alkyl tail, is shown in Figure 7. Data for the rectangular structure are represented as filled circles, while that for row structures are represented as open squares. Data points corresponding to structures adsorbed on MoS2 are designated by an asterisk (*) next to the point. It can easily be seen that the rectangular PTCDI-Cn polytype shows no increase in the measured molecular area with increasing tail length, whereas molecular areas for the row polytype increase linearly with alkyl tail length. In addition, the measured molecular areas for the rectangular polytype are approximately the same for both HOPG and MoS2. The approximately constant molecular area of the rectangular polytype indicates that the alkyl tails do not play a role in this adsorbate structure. We infer from this trend that the alkyl tails of the PTCDI cores protrude above the adsorbate layer and into the phenyloctane solvent. There are essentially two extremes for the conformations for the desorbed tail. First, as shown in Figure 8, the alkyl tail may protrude up from the adsorbate layer at an angle which is approximately perpendicular to the plane of the perylene core. We refer to this conformation as “tail-up”. Second, as shown in

6192


Figure 8. Molecular models of the rectangular adsorbate structure exhibited by PTCDI-C18 on HOPG showing two possible conformations which allow the tails to protrude above the adsorbate layer. (a) Top and side views of a molecular model approximating the “tail-up” conformation. (b) Top and side views of a molecular model illustrating the “tail-twist” conformation.

Figure 8, the tail may contain one or more gauche defects causing it to twist into a conformation that protrudes from the adsorbate layer but is not perpendicular to it. We refer to this conformation as “tail-twist”. Intuitively, one may expect that the ability of the phenyloctane to solvate the hydrocarbon tails in a nearly perpendicular tail-up conformation would act to stabilize this adsorbate structure, whereas the tail-twist conformation will not be as readily solvated due to its being essentially parallel to the molecular plane. There is precedent in the literature for nonplanar adsorbate conformation involving alkyl tails. Gorman et al. have proposed a geometry similar to our tail-up conformation for the system of a dialkylamine, DAPDA, adsorbed on HOPG.42 In addition, we have proposed a similar solvent-mediated adsorbate conformation for dialkylamino squaraine molecules adsorbed on HOPG under phenyloctane.30 It would be interesting to investigate a “dry” monolayer of these PTCDI-Cn’s, preferably vacuum-deposited at the solid-gas interface. Assuming that the protrusion of the alkyl tails from the plane of the molecule is solvent-mediated, row structures should preferentially form in the absence of mediating solvent. Unfortunately, the larger molecular weight of these molecules, as compared to PTCDA and PTCDI which are routinely vacuum-sublimated, lowers their vapor pressure to a point that makes sublimation problematic. An alternate method may be that of solution deposition with a high vapor pressure solvent which evaporates, leaving several layers of PTCDI-Cn molecules adsorbed on the substrate surface. In addition, it may be possible to test (42) Gorman, C. B.; Touzov, I.; Miller, R. Langmuir 1998, 14, 30523061.

Kaneda et al.

the solvent-mediated hypothesis by investigating this PTCDI-Cn system at the liquid/solid interface using a low vapor pressure liquid in which PTCDI-Cn is sparingly soluble (thus allowing for solution deposition of the molecule onto a surface via partitioning) but unable to solvate the alkyl tails once the molecule has adsorbed onto the substrate surface. Additional indirect evidence for this type of partially desorbed structure is found from the image quality obtained with rectangular structures formed by longertailed PTCDI-Cn’s. The STM images of the PTCDI-C18 rectangular lattice and other longer-tailed PTCDI-Cn’s are consistently noisier and less stable than images of PTCDI-Cn’s with shorter tails, as can be seen in comparing Figure 6 with Figure 4. The noise may be due to an interference of the flexible, protruding alkyl tails with the STM tip. Images of the PTCDI-C18 rectangular lattice and those of other longer-tailed PTCDI-Cn’s were acquired with many different tips, of both the mechanically cut PtIr and electrochemically etched W types. In all of the images of the longer-tailed perylenes, a difference in quality was noted as compared to images taken of shortertailed perylenes. Thus, it is unlikely that the difference in image quality is just due to tip shape and quality alone. It is also unlikely that molecular diffusion is the cause of image fuzziness. Adsorbate diffusion tends to cause streakiness in the image and an instability in the ordered layer. These characteristics were not observed in our study. A second possibility for the cause of the decrease in image quality with longer tail lengths may be that the alkyl tails do not protrude into the solvent but instead lie on top of the perylene cores. This conformation of alkyl tails would thus interfere with the signal derived from tunneling into the LUMO (which resides primarily in the core of the molecule), causing the cores to appear “fuzzy” in STM images. In either case, it is clear that the alkyl tails do not make substantial contact with the substrate surface in the rectangular packing structure. The dashed line through the points representing PTCDI-Cn row structures in Figure 8 shows the increase in molecular areas observed for increases in the alkyl chain length. Row structures were observed only for PTCDIC18 adsorbed on HOPG and for -C4 and -C13 on MoS2. Although these structures were observed on two different substrates, the row-structure molecular areas for adsorption on HOPG and MoS2 are lumped together in this analysis for the sake of determining the tail length driven trend. Admittedly, the error bars on the row structure data points are quite large due to the lower resolution in STM images of these structures. Nevertheless, we will note that the theoretical molecular area of a PTCDI molecule oriented within a row structure, 210 ( 15 Å, falls within the error bars of the calculated y-intercept (214 ( 30 Å). Likewise, the slope of the line, 5.3 ( 1.8 Å2 carbon-1, is within the error limit of the theoretical increase in molecular area (6.8 ( 1.2 Å2 carbon-1) that would occur with a one-carbon increase to each of the tails of a PTCDI-Cn molecule oriented in a row packing structure. The fact that the row polytype does not occur for every PTCDI-Cn molecule, and indeed did not occur on HOPG until PTCDI-C18, suggests a stability difference between row and rectangular structures. Preliminary molecular mechanics calculations comparing the relative energies of row structures to those of rectangular structures were performed. These calculations did not take into consideration the interaction between the adsorbate layer and the substrate lattice. It was found that rectangular structures tended to be more energetically stable than


row structures for PTCDI-Cn’s. This may explain the polytype conversion from row to rectangular that was often observed with STM for molecules that exhibited both row and rectangular structures on HOPG or MoS2. The absence of row structures on HOPG until -C18 and the observance of row structures for -C4 and -C13 (but not for -C18) on MoS2 are somewhat confusing. On HOPG, the possibility for alkyl tail commensuration along its carbon chain backbone and the length of the tail may finally provide more stability for the row structure. Why is it then possible for shorter tail lengths to form row structures on MoS2 but not on HOPG? It is likely that our investigations were not complete and that the row structures for other PTCDICn’s were kinetically unstable such that they were not observed. The row structure exhibited by PTCDI-C18 is somewhat confusing in itself. PTCDI-C18 is the only molecule in our study which forms a row structure in which the adsorbed alkyl tails do not interdigitate (see the molecular model in Figure 6). One may expect a large dispersion interaction between interdigitating octadecyl tails driving those tails to interdigitate when a row structure is formed. In fact, interdigitation of octadecyl tails on HOPG has been observed in the system of dialkylamino-hydroxyphenyl squaraines30 and dialkylaminophenyl squaraines43 imaged under phenyloctane. However, no interdigitation is observed in the PTCDI-C18 adsorbate structure. It is possible that the larger molecular core of PTCDI-C18 as compared to that of the octadecylated squaraines plays a part in the conformation of the octadecyl tails, preventing them from attaining the highly rigid and restrictive packing required for interdigitation. Influence of the Substrate. The adsorbed structures of PTCDI-Cn’s on HOPG and MoS2 differ greatly from their bulk crystal structures. As mentioned earlier, Struijk et al.34 recently investigated the bulk ordering of several of these alkylated perylene diimides using X-ray diffraction and found that they form smectic and columnar layers with interdigitating tails between the layers and random rotation of the perylene cores along the column. Obviously, competition between overlayer-substrate noncovalent interactions and molecule-molecule intralayer interactions has caused a significant distortion of the bulk PTCDI-Cn structure. In this situation, molecular overlayers tend to be stabilized by coincidence of the adsorbate supercell with the substrate lattice points at periodic intervals. To investigate the presence of such a substratemediated stabilization, we compared experimental measurements of the orientation of the overlayers with respect to the substrate lattice with theoretical calculation of commensuration and molecular orientation. To experimentally determine the degree of commensuration of the PTCDI-Cn overlayers with the HOPG and MoS2 lattices, we measured the rotation (or azimuthal) angle, θ, of the a lattice vector of the perylene overlayer to that of the HOPG or MoS2 two-dimensional lattice vector. This measurement was performed for domains consisting of the rectangular polytype of PTCDI-C4, -C8, -C12, -C13, and -C18 on HOPG and of PTCDI-C4 and -C13 on MoS2. Due to experimental difficulties, rotation angles could not be determined for row structures. For the rectangular structure PTCDI-Cn’s adsorbed on HOPG, we found that the direction of the a-axis of the PTCDI-Cn unit cell is always rotated by ((8 ( 3)° with respect to the HOPG [011h 0] lattice vector (see Table 1 for exact azimuthal angles of each of the PTCDI-Cn’s). Figure 9a shows several domains of PTCDI-C4 adsorbed on HOPG. Arrows (43) Stawasz, M. E.; Parkinson, B. A. Manuscript in preparation.

Langmuir, Vol. 17, No. 20, 2001 6193

Figure 9. (a) 70 × 70 nm STM image showing several domains of PTCDI-C4 adsorbed on the HOPG surface. Arrows indicate the direction of the a vector of the adsorbate unit cell. (b) Schematic representation of the relationship of the domain directions shown in (a) with the HOPG lattice. Arrow colors correspond with those of the domains in (a). The [011h 0] and symmetry-equivalent directions of the HOPG lattice are delineated by black lines. The angle θ represents the angle of the adsorbate a vector to the closest HOPG [011h 0] direction. For all domains shown in (a), θ is ((8 ( 3°).

overlaid on the domains show the direction of the a-axis of the perylene unit cell for each domain. Arrows of the same color represent domains with the same unit cell orientation: there are three domains with the same unit cell orientation (pink arrows) and four different orientations total. The rotation angle was measured for each domain direction with respect to the nearest HOPG [011h 0] symmetry-equivalent vector. Figure 9b illustrates the relationship of the overlayer domain directions shown in Figure 9a to the HOPG lattice. The [011h 0] direction and symmetry-equivalent directions of the HOPG lattice are shown as black lines. The colored arrows correspond to the domain directions shown in the image in Figure 9a. θ is the angle between an arrow and the nearest HOPG [011 h 0] symmetry-equivalent vector (azimuthal angle). It can be seen that when symmetry considerations for the HOPG lattice are made, the θ values for each of the domain orientations are equivalent and equal to (8°. Surprisingly, despite the differing lattice constants of HOPG and MoS2, the θ of PTCDI-C4 on MoS2 was also found to be 8 ( 3°. PTCDI-C13 on MoS2, however, shows a θ of 19°. It is not surprising that all of the PTCDI-Cn rectangular polytypes on HOPG exhibited the same

6194


rotational angle to the HOPG lattice: the very similar unit cells of these structures and the lack of influence of the length of the alkyl tails make it likely that the PTCDICn’s experience nearly the same interaction with the HOPG substrate. However, it seems counterintuitive that a rectangular polytype (of the same dimensions on both HOPG and MoS2) would exhibit the same θ on both substrates. This would suggest that the physicochemical driving force for PTCDI-C4 overlayer formation is not a phase coherence between the overlayer unit cell and the HOPG lattice. To further examine whether the azimuthal angles that we measured experimentally for each of the PTCDI-Cn unit cells on both HOPG and MoS2 do correspond to coincident overlayer structures, we utilized model epitaxy calculations. Epitaxy calculations were performed utilizing an analytical algorithm developed by the Ward group at the University of Minnesota, called EpiCalc.40,41,44 The algorithm analyzes the geometrical fit of an input overlayer lattice with respect to the input substrate lattice as a function of (1) small variations in the overlayer lattice constants and (2) the rotation angle between the overlayer lattice and the substrate. The algorithm utilizes a sinusoidal potential to obtain analytical solutions for a dimensionless interface energy. The value of the interface energy gives the degree of commensuration of the overlayer with the substrate. This method of epitaxy determination is significantly less computationally intensive than both the method of best degree of fit by Hoshino et al.3 and complete potential energy calculations. EpiCalc has been proven to reliably detect the existence of coincidence for a given set of overlayer and substrate parameters as well as predict coincident azimuthal orientations of the overlayer. In addition, it has shown good agreement with experimental data and potential energy calculations.41 EpiCalc was used to determine a number of azimuthal orientations and commensurate supercell sizes that produce overlayer point-on-line coincidence with our unit cell data. Experimentally measured azimuthal angles were very close to angles determined by EpiCalc to be pointon-line coincident. Supercells corresponding to coincident structures were calculated at azimuthal angles within the experimental error of the measured angle for all of the rectangular structures produced on HOPG and MoS2. Table 1 shows these calculated coincident azimuthal angles and their minimum supercell. Minimized supercells for the coincident PTCDI-Cn structures, corresponding to the angles we measured, varied in size from 1 × 3 unit cells for the rectangular structure of PTCDI-C4 on MoS2 to 5 × 17 for the rectangular structure of PTCDI-C13 on MoS2. An experimental corroboration of the supercell size predicted by EpiCalc would be the existence of some superstructural periodic contrast within the STM images of ordered PTCDI-Cn. Such periodic contrast is characteristic of overlayer-substrate registry and has been used itself to determine epitaxy.3 We clearly observed such periodic contrast in the PTCDI-C8 adsorbate system. Figure 10 shows an 83 × 83 nm image of the intersection of two ordered domains of PTCDI-C8 on HOPG, both domains of which display one-dimensional modulation of contrast in the form of periodic stripes. These stripes are oriented at 60° angles to one another and have a modulation distance of 8.1 ( 1.2 nm. This period corresponds well with the commensurate supercell calculated by EpiCalc (1a × 5b), with 5b ranging from 8.2 to 8.5 nm. The 60° angular orientation reflects the symmetry of the (44) EpiCalc, available at http://www.wardgroup.umn.edu/software.html.

Kaneda et al.

Figure 10. 83 × 83 nm STM image showing the domain intersection between two rectangularly ordered PTCDI-C8 domains adsorbed on HOPG. Visible in both of the domains is a 1D periodic contrast modulation which is much larger than the unit cell of the adsorbate. The period of the modulation measures 8.1 ( 1.2 nm, which corresponds well with the predicted size of the commensurate supercell (∼8.3 nm).

HOPG substrate. Thus, we have experimentally corroborated the supercell prediction made by EpiCalc for PTCDI-C8 on HOPG. The size of the supercell indicates the relative strength of the overlayer-substrate interaction and the corresponding influence of the substrate lattice on the formation of the overlayer. The existence of larger supercells infers a weaker overlayer-substrate interaction relative to the intralayer interactions. By reference to Table 1, it is apparent that overlayers consisting of the rectangular polytype exhibited by PTCDI-C13 experience a decreased interaction with the MoS2 lattice (supercell ) 5 × 17) relative to the interaction it experiences with HOPG (supercell ) 1 × 18). The fact that the unit cell dimensions of the rectangular polytype of PTCDI-C13 on both HOPG and MoS2 are within experimental error of one another provides additional evidence that the influence of the MoS2 lattice is weaker than that of HOPG (assuming interactions between PTCDI-C13 molecules are the same). Conversely, the unit cell dimensions for the rectangular polytype of PTCDI-C4 on HOPG and MoS2 are not the same, yet the experimentally determined azimuthal angles for PTCDI-C4 overlayers on both HOPG and MoS2 are within experimental error of one another. In this case, the interaction experienced between the PTCDI-C4 overlayer and the substrate lattices may be slightly stronger than the interaction between molecules, causing a slight change in the unit cell on substrate lattices with different periodicities. It is interesting to ponder why the rectangular structure of PTCDI-C4 changes when it is so similar to that of -C13 (see Table 1), which does not change when switching from HOPG to MoS2. It may be that differences in unit cell dimensions between the rectangular structures of PTCDI-C4 on HOPG and on MoS2 are driven by a stabilizing commensuration between the PTCDI-Cn supercell and the substrate lattice. Whereas PTCDI-C13 exhibits an azimuthal angle which has been calculated to correspond to 1 × 20 and 5 × 17 supercells, respectively, calculated supercells for PTCDI-C4 are much smaller 2 × 7 and 1 × 3 unit cells, respectively. This trend in supercell size indicates stronger interactions between the PTCDI-C4 and the HOPG and MoS2 lattices than those of PTCDI-C13. Therefore, a model of adsorbate-substrate coincidence involving overlayer supercells commensurate with the substrate lattice has proven to be useful in


addressing the above trends in PTCDI-Cn overlayer structure that result as a function of substrate periodicity. Conclusion We have studied the 2D structures of a variety of substituted perylene diimides adsorbed onto HOPG and MoS2 surfaces from phenyloctane solutions. We find that both rectangular or herringbone-like structures and row structures can be observed. Surprisingly, the lattice constants, and thus the area per molecule of the rectangular structures, do not increase as expected as the alkyl chain length is increased. Protrusion of the alkyl tails into the solvent above the 2D layer is proposed to account for this behavior. Kinetically unstable row structures were also observed where the alkyl tails lie flat on the substrate surface and the area per molecule increases as expected for the increase in the length of the alkyl tail. The formation of domains with a particular orientation with respect to the underlying lattice was observed for many of the 2D structures. The angular orientation of the molecular layers with respect to the substrate could be explained with the point-on-line coincidence model, determined by an analytical misfit algorithm. Due to the in situ nature of this

Langmuir, Vol. 17, No. 20, 2001 6195

investigation at the solid-liquid interface, dynamics of adsorption and the annealing of the oriented domains could be observed in real time. Acknowledgment. The authors thank A. K. Rappe´ for helpful discussions on the molecular mechanics calculations and Neal Armstrong for providing PTCDIC13. The Mitsubishi Paper Mills is thanked for its support of Y. Kaneda while at Colorado State University. This work was supported by the U.S. Department of Energy, Division of Chemical Sciences, through Contract DE-G0396ER14625. Note Added in Proof. HOPG unit cells noted here differ from those appearing in our previously published paper “Scanning Tunneling Microscopy Investigation of the Ordered Structures of Dialkylamino Hydroxylated Squaraines Adsorbed on Highly Oriented Pyrolytic Graphite” (Langmuir 2000, 16, 2326-2342). Our determination of the HOPG lattice vectors appearing in that paper is in error. Vectors listed as [012 h 0] and [0010] in that paper are correctly noted as [011h 0] and [112h 0], respectively, in this paper. LA0100587

STM Investigations of the Two-Dimensional Ordering of

Recommend Documents