2748
Langmuir 1994,10, 2748-2756
Epitaxial Thin Films of Large Organic Molecules: Characterization of Phthalocyanine and Coronene Overlayers on the Layered Semiconductors MoS2 and SnS2 C. D. England,? G. E. Collins,$T. J. Schuerlein, and N. R. Armstrong* Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received February 7, 1994. I n Final Form: May 20, 1994@ Ordered overlayers of large organic molecules have been studied on the basal planes of freshly cleaved metal dichalcogenides such as MoS2 or SnS2. Low-energy electron diffraction (LEED) has been used to characterizethe growth of copper (CuPc)and chloroindium (InPcC1)phthalocyanine and coronene on these substrates, at coverages up to ca. two monolayers. InPcCl and CuPc films form square lattice nets on both SnS2 and MoS2, with lattice vectors aligned along (a = OD,MoS2) or near (a = f4”,SnS2) the principal lattice vectors of the metal dichalcogenide (0001)surface,resultingin 3-6 equivalentPc domains. Ordered coronene films on MoS2 consist of two hexagonal lattice domains rotated by h13.9” from each principal axis of the (0001) surface of MoS2. Modeling studies were conducted for adsorbed coronene on MoS2 and for InPcCl on both MoSz and SnS2, where purely van der Waals forces were presumed to be responsible for the orientationofthe epitaxiallayers ofthese organic materials. By summing van der Waals interactions over surface lattices of up to nine molecules, it is possible to show that small differences in the binding site, repulsive and attractive interactions between molecules, and sulfur-sulfur spacing in the MoS2 or SnS2 basal plane may be responsible for the orientations of the first close-packed monolayers. generally not commensurate, lattices can be observed for these same dyes, and the coronenes and fullerenes, on substrates which will interact with these molecules through weak forces In contrast to the strong interactions with metal surfaces, the (0001)basal planes of the metal dichalcogenides such as MoSz and SnS2,and the basal plane of highly ordered pyrolytic graphite (HOPG), apparently interact with close-packed monolayers of these dyes principally through van der Waals forces.1a,2Epitaxial layer growth for these dyes has also been observed on the (100)cleavage planes of single crystal KC1 and KBr, where additional weak electrostatic forces may help to orient monolayers of these same d y e ~ . l ~It- “ ~ is the size of these molecules, and the symmetry mismatch * To whom correspondence should be addressed. with the weakly interacting substrates, which often + P r e s e n t address: Digital Equipment Corp., 77 Reed Rd., precludes the formation of commensurate surface lattices. Hudson, MA 01749. Present address: Geo-Centers, 10903 Indian Head Hwy., Ft. Multiple domain formation therefore often occurs in the Washington, MD 20744. first monolayer, which affects the molecular architecture Abstract published in Advance A C S Abstracts, July 15,1994. in the next deposited monolayer^.^^^ Large molecule (1)(a)Hara, M.; Sasabe, H.; Yamada, Y.; Garito, A. F. Jpn. J. Appl. systems which form commensurate surface lattices on Phys. 1989,28,1306.(b) Tada, H.; Saiki, K.; Koma, A. Jpn. J. Appl. metal substrates have therefore been used as a starting Phys. 1991,L306,30.(c) Dann, A. J.;Hoshi, H.; Maruyama, Y. J . Appl. (d)Yanagi, H.; Ashida, M.; Elbe, J.; Worhle, Phys. 1990,67,1371,1845. point for the characterization of more complicated epitaxial Yanagi, H.; Kouzeki, T.; Ashida, M. D. J . Phys. Chem. 1990,94,7056; deposits on other substrate^.^,^ J.Appl.Phys. l990,73,3812.(e)Yanagi,H.;Dauke,S.;Ueda,Y.;Ashida, M.; Warhle, D. J . Phys. Chem. 1992,96,1366. (0 Terasaki, A.; Hosoda, We have focused most of our attention on macrocyclic M.; Wada, T.; Tada, H.; Koma, A.; Yamada, A.; Sasabe, H.; Garito, A. molecular systems that have bulk structures which make F.; Kobayashi, T. J. Phys. Chem. 1992,96,10534.(g) Fritz, T.; Knoll, them amenable to layered growth along at least one plane. W.; Hara, M.; Sasabe, H. Mol. Cryst. Liq. Cryst., in press, preprint These materials therefore lend themselves to layer-bykindly supplied.
Introduction There is currently considerable interest in the epitaxial growth of dye molecules and various other “supramolecular assemblies’’ on single crystal insulator, semiconductor, and metal surfaces, because of the potential for unique optical and electrical properties imparted to such highly ordered thin Commensurate lattices for monolayers of phthalocyanines (Pc),naphthalocyanines (NPc), and certain perylenes have been observed for low index metal single crystals, where strong chemisorption interactions control the packing structures (e.g., C ~ ( l o O ) . It l~,~ has recently been shown, however, that coincident, but ~~~
*
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(2)(a)Nebesny,K. W.; Collins, G. E.; Lee, P. A.; Chau, L.-K;Danziger, J . L.; Osburn, E.; Armstrong, N. R. Chem. Mater. 1991,3,829. (b) Armstrong, N. R.; Collins, G. E.; Nebesny, K. W.; England, C. D.; Chau, L.-K.; Lee, P. A.; Parkinson, B. A. J . Vac. Sci. Technol., A 1992,10, 2902. (c) Armstrong, N. R.; Arbour, C.; Chau, L.-K.; Collins, G. E.; Nebesny, K. W.; Lee, P. A.; England, C. D.; Parkinson, B. A. J . Phys. (d) Armstrong, N. R.; Chau, L.-K.; England, C. Chem. 1993,97,2690. D.; Chen, S.-Y. J . Phys. Chem. 1993,97,2699. (e) Anderson, M. L.; Collins, G. E.; Williams, V. S.; England, C. D.; Chau, L.-K.; Schuerlein, T. J.; Lee, P. A,; Nebesny, K. W.; Armstrong, N. R. Surf. Sci., in press. (0 England, C. D.; Collins, G. E.; Schuerlein, T. J.; Anderson, M. E., Armstrong, N. R. In Chemically Selective Interfaces; Mallouk, T. E., Harrison, J. E., Eds.; ACS Symposium Series; American Chemical Society: Washington, DC, in press. (g) Chau, L.-K; Chen, S.-Y.; Armstrong, N. R.; Collins, G. E.; England, C. D.; Williams, V. S.; Anderson, M. L.; Schuerlein, T. J.; Lee, P. A.; Nebesny, K. W. Mol. Cryst. Liq. Cryst., in press. (h) Collins, G. E. Ph.D. Dissertation, University of Arizona, 1992. (3)(a) Buchholz, J. C.; Somorjai, G. A. J . Chem. Phys. 1977,66,573. (b) Lippel, P.H.; Wilson, R. J.;Miller, M. D. Wo11, Ch.; Chiang, S. Phys. Rev. Lett. 1989,62, 171.
(4)(a) Fryer, J. R.; Kenney, M. E. Macromolecules 1988,21, 259; Fryer, J. R. Mol. Cryst. Liq. Cryst. 1989,137,49. (b) Ashida, M. In Electron Crystallography of Organic Molecules; Freyer, J. R., Dorset, D. L., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1990,pp 227-240. (5)(a)Zimmerman, U.; Karl, N. Surf. Scz. 1992,268,296. (b) Ludwig, C.; Gompf, B.; Glatz, W.; Petersen, J.; Eisenmenger, W.; Mobus, M.; Zimmerman, U.; Karl, N. Z. Phys. B 1992,86,397. (c)Jung, M.; Gaston, U.; Shnitzler, G.; Kaiser, M.; Papst, J.;Ponvol, T.; Freund, H. J.; Umbach, E. J . Mol. Struct. 1993,293,239. (6)(a) Haskal, E. I.; So, F. F.; Burrows, P. E.; Forest, S. R. Appl. Phys. Lett. 1992,60, 3223.(b) So,F. F.; Forrest, S. F. Phys. Reu. Lett. 1991,66,2649. (c)So, F. F.; Forrest, S. F.; Shi,Y. 4.; Steier, W. H.App1. (d) Haskal, E. I.; Burrows, P. E.; Forrest. S. Phys. Lett. 1992,56,674. R. Appl. Phys. Lett. 1992,60, 3223 and Burrows, P. E.; Forrest, S. R. Appl. Phys. Lett. 1993,62,3102. (e) Zhang, Y.; Forrest, S. R. Phys. Rev. Lett. 1993,71, 2765. (7)Weaver,J . H.Acc. Chem.Res. 1992,143,25andreferences therein. (8)Schuerlein, T. J.;Armstrong, N. R. J . Vac. Scz. Technol., in press.
0743-746319412410-2748$04.50/00 1994 American Chemical Society
Epitaxial Thin Films of Large Organic Molecules
layer growth of epitaxial films, with the plane of the molecule parallel to the substrate plane.2 The trivalent and tetravalent metal Pc's and NPc's fall into this category. The trivalent metal Pc's can pack in the first monolayer with the axial halogen substituents pointed up. The second layer in such an assembly can pack in several different orientations. The two most important packing structures are those (a) where the second layer halide is pointed toward the first layer and the Pc rings are staggered to accommodate this packing and (b)where the halide in the first layer interacts strongly with the metal in the second layer to form an eclipsed, cofacial packing arrangement.2c-h Clearly the packing structure of the
first Pc monolayer in such systems will have a significant influence on the packing structures for the second and subsequent monolayers. Ordered organic superlattices (e.g., alternating multilayers of dissimilar dyes) have also recently become of interest, where the ordering in each layer is controlled principally by van der Waals and weak electrostatic forces between mo1ecules.2eJ1J2 Organid organic multilayers and superlattices have been created from two molecular materials with complementary spectral properties, such as phthalocyanines interleaved with coronenes, perylenes, or fullerenes. The long-range ordering in each molecular layer affects both the optical and electrical properties of the entire assembly,2eJ1and has been the subject of recent modeling studies to determine how the weak interactions between organic layers can control these properties.6e The adsorption of coronene, perylene, and substituted versions of both of these molecular systems, to the basal planes of the layered metal dichalcogenides and HOPG, is therefore also of i n t e r e ~ t . ~The . ~ bulk structures of molecules like perylene and coronene are not amenable to layer-by-layer growth with the molecules lying parallel to the substrate. Recent studies have indicated, however, that at least coronene and closely related molecules will still form flat-lying monolayer deposits on substrates such as M o S ~ . ~ ~ , ~ ~ Both commensurate and coincident surface lattices of these types of large molecular assemblies may eventually provide for the study of small molecule chemisorption on ordered ultrathin dye molecule films.s This is an area of particular relevance for the application of these organic dyes in thin film gas sensors where such chemisorption processes are related to those sites which control dark and photoconductivity on much thicker, polycrystalline fiims.9 We review here low-energy electron diffraction (LEED) studies for two phthalocyanines (InPcC1, CuPc) and coronene, for which epitaxial layer growth has been
Langmuir, Vol. 10,No. 8,1994 2749
demonstrated on the metal dichalcogenides MoS2 and SnS2.215 LEED data are discussed elsewhere for comparable monolayers of a perylene dye and C ~on O these same substrates,2fand for the deposition of CuPc, FePc, demetalated and metalated naphthalocyanines, and coronene and perylenes on metals like Cu(100).8Jo For deposition on weakly interacting surfaces the Pc's exhibit alignment of their square lattices near or along the principal axes of the metal dichalcogenide basal planes2bpc while coronene and the perylenes form ordered lattices whose principal axes are displaced by ca. f12-14" from the principal axes of the MoS2 or SnS2 ~ u r f a c e . ~ ~ , ~ Modeling studies reported here show that subtle differences in the accumulation of dispersive (van der Waals) forces for particular orientations of the first monolayer of coronene on MoS2 may explain the observed domain structures and the relative alignments of those domains. Modeling of the InPcCl lattices on MoS2 and SnS2 also suggests that the summation of van der Waals forces and the sulfur-sulfur spacing in the metal dichalcogenide lattice are important in guiding the orientation of the first ordered monolayer.
Experimental Section
The deposition and characterization of all of the films grown occurred in a two-chamberultrahigh vacuum (UHV)system (base pressure 5 1 x Torr), similar to previous reports.2 Organic deposition and LEED analysis occurred in separate chambers, with freshly cleaved metal dichalcogenide substratestransported on a single rod between two UHV chambers (transfer rod: Kurt Lesker, O M " translator). Deposition occurred from resistively heated Knudsen cell sources designed here. The sublimation temperature was accurately controlled (&2 "C) using a Eurotherm temperature controller. Deposition rates were monitored using a quartz crystal microbalance (QCM), which could be positioned line-of-sight to the deposition sources. Deposition rates were controlled to be ca. 3-6 equivalent monolayers per hour. Substrate temperatures during deposition were controlled between room temperature and ca. 90 "C. Annealing at 90 "C following deposition was generally carried out for up to 8 h. Because of the volatility of coronene, the substrate crystals were kept at room temperature during and after deposition of this molecule. The LEED analysis was carried out using an Omicron SPECTALEED/Auger system. A lanthanum hexafluoride electron filament was used, with a 100-pm-diameter spot size. Typical beam energies for the LEED analysis of MoSz and SnSz single crystals were ca. 50-60 eV, while the beam energies used for the characterization of monolayer deposits of the large organic molecules ranged from 10 to 15 eV, to provide clear images of multiple Laue zones. These excitation energies are dictated by the sizes ofthese moleculesand their reciprocal lattice geometries, in relation to the geometry of the LEED optics. Typical beam currents were on the order of 1 PA. All the LEED images presented were obtained using a CCD camera (Burle TC-651EC) and a Data Translation 2853 Frame Grabber. The sensitivity of this system was such that it allowed capture of LEED images in periods of time of less than 2-5 s, so that, coupled with the low excitation beam currents, no detectable damage occurred to the organic overlayers. This was ascertained by monitoring the irradiation times necessary to see structural changes occurring in the LEED data andor loss of ordering, as evidenced by blurring of the diffraction spots. All electron beam exposures were kept (9)Collins, G.E.; Pankow, J. W.; Odeon, C.; Brina, R.; Arbour, C.; below those limits. The CCD image ofthe LEED phosphor screen Dodelet, J.-P.; Armstrong, N. R. J. Vac. Sci. Technol. 1993,11, 1383. was output to a laser printer, rather than using photographs. (10)Schuerlein, T.J.;Armstrong, N. R. Manuscript in preparation. The dynamic range of the laser printer is not as good as (11)Collins, G. E.; Williams, V. S.; Chau, L.-K.; Nebesny, K. W.; photographic emulsion, but line scans through the major England, C.; Lee, P.A.; Lowe, T.; Fernando, Q.Synth. Met. 1993,54, diffraction spots are shown to indicate the real contrast between 351. (12)(a)Forrest,S.R.;Leu,L.-Y.;So,F.F.;Yoon,W.Y.J.Appl.Phys. the center of the diffraction spot and the secondary emission 1989,66,5908. (b)Danziger, J.;Dodelet, J.-P.; Lee, P. A.; Nebesny, K. background. W.; Armstrong,N.R. Chem. Mater. 1991,3,812. (c)Danziger,J.;Dodelet, Surfaces of interest in these studies were either naturally J.-P.; Armstrong, N. R. Chem. Mater. 1991,3,821.(d) Popovic, Z.D.; occurring, highly conductive samples of bulk MoSz or specially Hor, A. M.; Loutfy, R. 0. Chem. Phys. 1988,127,451.
England et al.
2750 Langmuir, Vol. 10,No. 8,1994 atom S
c1 C Sn
E
Table 1. van der Waals Parameters (kcavmol) R (A) atom E (kcavmol) 1.8 In 0.1 0.314 0.314 1.75 Mo 0.1 0.107 1.7 N 0.095 2.459 H 0.042 0.1
eV) (hexagonal close-packed (hcp) lattice; a1 = a 2 = 3.16
R (A) 2.124 1.844 1.55 1.5
prepared bulk SnSz crystals doped with chlorine to ca. 10l6cm-3.13 Prior to the introduction of a single crystal into the deposition chamber, the surface was freshly cleaved to produce basal planes that appeared flawless over dimensions of up to 0.5 cm. The samples were then heated overnight at 350 "C in order to ensure atomically clean surface conditions, as ascertained by Auger electron spectroscopy. Coronene was purchased from Kodak, and then purified using extraction and repeated sublimation. InPcCl was synthesized in our laboratory and purified in a similar fashion.2 In this work the crystallographic features of a common molecular modeling program (Sybyl) were used to construct simplified MoSz(0001)and SnSz(0001)surfaces consisting of two sulfur planes and one metal plane (S-Mo-S or S-Sn-S), whose dimensions were up to 20 x 20 arrays of surface sulfur atoms. Bulk crystallographic data for the molecules in question were retrieved from the Cambridge Crystallographic Database and loaded into Sybyl. Using bulk atom positions and crystallographic functions, various organic overlayer structures, not necessarily those of the bulk structures, were created. The Tripos 5.2 force field was used for subsequent energy calculation^.^^ The calculated "system" energy (kcavmol)can include several terms; however, the only energy term of interest for our calculations to date is the total van der Waals energy, which is readily modeled using this software package. The Tripos force field van der Waals energy term is a Leonard-Jones (6-12) potential,
A). The LEED data obtained for a full monolayer of CuPc on this surface (Ebam= 15 eV) have been discussed previously and are reproduced in Figure 1B(an equivalent As image is obtained for the first monolayer of InPcC1).2b*c discussed earlier, these data have suggested multiple domain formation for the Pc overlayers, and a schematic of these LEED data is shown in Figure 1D based upon three equivalent square lattice domains.2cA model of one domain of the real space lattice is shown in Figure 1C. The diffraction image seen in Figure 1Bcould be faithfully reproduced as the electron beam was scanned over most of the exposed MoSp basal plane. Occasionally regions could be found where some of the diffraction spots decreased in intensity, and a single square set of spots in the first Laue zone dominated the image. Such diffraction data are consistent with a single Pc domain with dimensions near the electron beam diameter of ca. 50-100 pm. This type of diffraction data was also reproduced for comparable coverages of InPcCl on the MoSz basal plane, confirming a commonality in the packing structures of these two different Pc's in the first monolayer. As shown in the schematic of the LEED data (Figure lDj, and the real space lattice model (Figure 1 0 , these Pc's pack in a square lattice geometry, with alignment along a principal axis of the MoS2 basal plane (bl = (13/3)al, bz = (5/2)al + 5az, bl x b2 13.7 A,r = go", = 0", area of the unit cell 187 Az).A single domain of CuPc or InPcCl on MoSz appears to form a coincident 3 x 2 superstructure, represented as
MoS,(OOOl)-[ -512 1313 0 5]-CuPc
and effectively reaches zero for any pair of atoms when their separation exceeds 8.0 A. Table 1contains the van der Waals parameters of energy, E , and radius, R , for all the atoms used in the energy calculations for the coronenelhlosz, and PdMoSz, and PdSnSz systems. The first step in these modeling experiments was to determine the optimum position (minimizing the van der Waals energy of the system) of two or more molecules, adjacent to each other in the substrate plane, assuming that they took on orientations parallel to the surface plane. This was carried out by creating two or more molecules in the same molecular area by using the crystallographic symmetry operators, defining each molecule as an aggregate, and then calculating the interaction energy. Once this optimization was obtained, the interaction energy between one molecule, and then a complete organic overlayer, and the metal disulfide substrate was calculated. The overlayer and substrate structures were first defined in separate molecular areas in the Tripos force field calculation program. The MoSz or SnSz substrate and each overlayer molecule were next defined as separate aggregates, and the overlayer and substrate were oriented into some desired configuration (e.g., some particular azimuthal angle of the overlayer with respect to the substrate). These structures were then merged into a new molecular area, and a new energy was calculated. This process was first carried out for overlayers of one molecule and then overlayer sizes of up to nine molecular units. Our calculations were limited to such overlayer aggregate sizes due to availability of RAM and processor speed.
Results and Discussion CuPc, InPcCl/MoS~(OOO1). Freshly cleaved,naturally occurring MoS2 crystals, annealed overnight a t ca. 350 "C, yield the LEED data shown in Figure 1A @beam= 51 (13)Parkinson, B. A. Langmuir 1988,4 , 967. (14)(a) Sybyl, Version 5.5, Tripos Associates, Inc. (b) Clark, M.; Cramer, R. D., 111;Van Odendenbosch, N. J. Comput. Chem. 1989,10, 988.
Scanning tunneling microscopy images of such Pc lattices have shown that the phthalocyanines are packed in an interleaved fashion, with the benzenoid portions interdigitated.1cse,3b Acomparable packing structure, with nearly the same lattice dimensions, has recently been observed by RHEED characterization ofthe CuPc system on Au(lll)/mica.lg The scanning tunneling microscopy (STM) images for such a monolayer are the most compelling evidence to date to support packing structures like those shown in Figure 1C. This type of packing leads to three equivalent domains of the first Pc monolayer, aligned along the three principal axes of the MoSz substrate. Earlier RHEED studies of the CuPc/MoSz system had suggested that there was a single square lattice geometry associated with the first CuPc monolayer on MoS2.la In our hands, however, Pcl MoSz deposits are formed where rotation of the sample through different azimuthal directions brings one of the Pc domains into alignment with the incident electron beam every 15",so that diffraction streaks with approximately the same spacing are seen at several azimuths. This is in contrast to those cases where ordered Pc layers have been produced on substrates such as single crystal KBr, where only one domain is formed, and RHEED characterization a t different azimuthal directions produces the changes expected in streak spacing for a unidirectional square surface lattice.lbZf InPcCVSnS2(0001). LEED studies of the first monolayer of CuPc and InPcCl deposition on the SnSz surface have suggested that the packing structures may be similar to those seen on the MoSz, but with some important differences (Figure 2). Figure 2A represents the LEED = 64 eV) for the bare SnSz(0001) surface, after data (Ebeam an overnight anneal a t 350 "C (a1 = a2 = 3.64 A), and Figure 2B represents the LEED data (&am = 15 eV) for a full monolayer of InPcC1. The schematic for these LEED
Langmuir, Vol. 10,No.8,1994 2751
Epitaxial Thin Films of Large Organic Molecules
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data is in Figure 2D. The diffraction data and the corresponding schematic demonstrate multiple square lattice domain formation, with each Pc square lattice lying parallel to the SnS2 basal plane (Figure 2C). The additional splitting in the diffraction spots and the schematic of the LEED data suggest rotation of these domains away fiom the principal axis of the SnS2 substrate by ca. 4". The increased sulfur-sulfur spacing on the SnS2 basal plane suggests that a 3 x 2 superstructure unit cell could form, as on MoS2, but with lattice parameters bl = (11/3)al f (1/3)a2, b2 = (2 f 1/2)al (9/2)a2(the f values indicate the possibility for rotation of the lattice by two equivalent directions), I'= go', and a) = f4':
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The overlap of difiaction spots, the additional secondary electron scattering in LEED data involving these Pc's, and the limited dynamic range in contrast available for reproduction of these images using laser printers make the exact determination of diffraction spot position problematic. Line scans were therefore used to help delineate these difiaction spots, over multiple deposition trials (Figure 2B). In the first Laue zone, the LEED data of the InPcCl deposits on SnS2 are barely distinguishable from those seen on MoS2. In the third and fourth Laue zones, however, an additional set of diffraction spots is
observable. These additional spots, substantiated by line scans, can be rationalized as shown in the schematic in Figure 2D, by formationof a 3 x 2 coincidentlattice rotated by 4" with respect to each principal axis in the SnS2 basal plane. CoroneneMoSz (0001). The bulk structure for coronene does not possess parallel planes of the molecule. The unit cell is monoclinic with parameters a = 16.119 A,b = 4.702 A,c = 10.102 A,and p = 110.9" and space group P21/a.15 Nevertheless, coronene has shown itself to be amenable to formation of a flat-lying monolayer packing structure when deposited on such substrates as MoS2 and Cu(100).2h95aIn additionto its desirablespectral properties as a transparent spacer molecule in phthalocyanine or perylene dye multilayers, coronene also forms a convenient model molecule with which to characterize the cumulative van der Waals interactions with the basal plane of a material like M o S ~ . ~ ~ ~ ~ ~ ~ ~ Figure 3A shows the LEED data for a closest packed monolayer of coronene on MoS2, along with the proposed packing structure (Figure 3B) and a schematic version of the LEED data (Figure 3C). Coronene forms a commensurate, hexagonal lattice, MoS2(0001)-p( 2/13x 413)Rf13.9", possessing two equivalent domains. The structure shown in Figure 3B is slightly different from previously proposed coronene/MoS2 and is based on that configuration which would relieve as much (15)(a) Fawcett, J. K.; Trotter, J. Proc. R. SOC.London 1965, A289, 366. (b) Robertson, J. M.; White, J. G. J. Chem. SOC.1945, 607.
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2752 Langmuir, Vol. 10,No.8,1994
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of the hydrogen interatomic repulsions as possible andmaintain the symmetry and dimensions necessary for this
organic unit cell (a conclusion drawn from the model studies described later). A slightly less attractive con-
Epitaxial Thin Films of Large Organic Molecules
figuration is also shown which relates to the modeling studies (below). Modeling of Molecular Epitaxy for Coronene on MoS2 and InPcCl on MoS2 and SnS2. Preliminary modeling has been carried out to determine whether van der Waals forces alone can predominate in determining the orientation of simple adsorbates such as coronene on MoSz, and more complex adsorbates such as InPcCl and CuPc on both MoSz and SnS2. The summation of van der Waals forces over a large enough array of molecules appears to impart certain preferred orientations to adsorbed long-chain alkanes and simple aromatic compounds on HOPG and on MoSz."j Other models of "van der Waals epitaxy" of metal dichalcogenides deposited on different dichalcogenides have also shown that the summation of similar weak interactions may begin to dictate crystallization of the layers of metal dichalcogenide nuclei at certain critical sizes of the over1ayer.l' The interactions of coronene and the Pc's with the MoSz and SnSz surfaces appeared to be additional candidates for epitaxy controlled by such weak interactions. Modeling of Coronene on MoS2. The optimum position of the coronene molecules with respect to each other was first determined using a two-dimensional hexagonal lattice consisting of one central coronene molecule, and six nearest neighbors, with a lattice constant for this flat layer consistent with the LEED data (a = 11.394A), and no presumed interaction with a substrate. The van der Waals energy of this surface lattice was next computed as a function of internal rotation, 4, for each coronene in the lattice, about the common axis ofthe center coronene molecule (as defined in Figure 3). Such rotations of each coronene molecule help to minimize repulsive interactions, and small-energy minima can be produced which optimize the slight attractive forces between molecules. The most attractive lattice model (most negative van der Waals energy) occurs when 4 = f 6 . 5 " ) but even those structures resulting from no rotation of the coronene molecules produce only a slightly less attractive lattice. Within 4 = f 1 0 " there are several configurations that would be possible for the coronene surface lattice which would not produce interactions that are more than 10% less attractive (i.e., whose total van der Waals energy is more than 10% higher) than this optimum configuration. The interaction energies were next calculated for asingle coronene molecule centered above four different surface sites on the MoSz surface (S-atop, S-Mo, S-hollow, and S-bridge), such that the molecular plane of the coronene was parallel to the basal plane. The S-atop site places the center aromatic ring directly over a sulfur surface atom, while the S-Mo site represents a point on the surface directly above the Mo atom in the metal cation plane directly below the s u r f a ~ e . ' ~A J ~S-hollow site represents a 3-fold sulfur coordination site, not directly over a Mo atom, and a S-bridging site represents the placement of (16)(a)Rabe, J.P.; Buchholz, S.; Askadskaya, L. Synth. Met. 1993, 54, 339.(b) Askadskaya, L.;Boeffel, C.; Rabe, J. P. Ber. Bunsen-Ges. Phys. Chen. 1993,97, 517.(c) Rabe, J.P.; Buchholz, S. Science 1991, 253,424. (17)(a)Ohuchi, F.S.;Parkinson, B. A,; Ueno, K. J.Appl. Phys. 1990, 68, 2168. (b) Ohuchi, F. S.; Shimada, T.; Parkinson, B. A. J. Cryst. Growth 1991,111,1033.(c) Parkinson, B. A.; Ohuchi, F. S.; Ueno, K. Appl. Phys. Lett. 1991,58, 472.
(18)Balchin, A. A. In Crystallography and Crystal Chemistry of Materials with Layered Structures; Levy, F., Ed.; D. Reidel Publishing Co.: Dordrecht, The Netherlands, 1976. (19)(a) Tributsch, H.Struct. Bonding (Berlin) 1982,49, 127. (b) Physics and Chemistry of Materials with Layered Structures; Mooser, E., Ed.; Reidel: Dordrecht, The Netherlands, 1976-79;Vols. 1-6. (c) Electronic Structures and Electronic Transitions in Layered Materials; Groasso, V., Ed., Reidel: Dordrecht, The Netherlands, 1986.
Langmuir, Vol. 10, No. 8, 1994 2753
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Figure 4. Apparent van der Waals interaction energy for a single coronene molecule on MoS2, as a function of rotation angle (qj), with respect to a principal axis of the MoSz(0001)
basal plane, for the following surface sites: (0)S-atop site, (m) S-bridge site, (A)S-Mo site, (V)S-hollow site.
the central aromatic ring across two adjacent S atoms. The interaction energy of the system depends on the distance between the coronene plane and the top sulfur plane, z , the azimuthal rotation angle of the coronene, 4, and the size of the MoSz(0001) surface. We found that the interaction energy for these single coronene molecules converged to a fmed value as the size of the MoSz(0001) surface increased. For computational simplicity, the smallest possible MoSz(0001) surface was used, a 7 x 7 rhombic slab. An exact equilibrium distance, zeq,was not determined, since zeqdepends slightly on the rotation angle, 4, of the coronene. An approximate equilibrium distance, zeq 3.05 f 0.10 A, however, was found for all four surface sites. The approximate equilibrium distance was used for the calculation of the interaction energy as a function of the azimuthal rotation of the coronene molecule. Figure 4A shows the interaction energy as a function of 4 for the S-atop and S-bridge sites. The fluctuation in interaction energy with 4 for these sites exhibits a 6-fold rotational sy"etry,E($) =E(@ n60"). Figure 4B shows the interaction energy as a function of 4 for the S-Mo and S-hollow sites. The interaction energy for these sites exhibits a fluctuation with 4 with 3-fold rotational symmetry,E(#)=E(qj n120"). The calculated rotational symmetry for each site is a result of the 3-fold rotational symmetry of the coronene molecule and the rotational symmetry ofthe underlying site. There are obviously only small absolute and relative fluctuations in van der Waals energy for a single coronene molecule regardless of the type of site chosen for adsorption. From these plots, however, we would predict that there is a slight preference
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2754 Langmuir, Vol. 10, No. 8, 1994
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(0001)surface as a function of rotation of the coronene lattice (cb), for two different internal rotation angles (4 = 0" and 4 = 6.5"), for the four different sites described in Figure 4: (A) S-atop site, (B) S-bridge site, (C) S-Mo site, (D)S-hollow site.
for the S-atop site at the center of the molecule in the weak binding of a single coronene molecule. These calculations were next extended to a twodimensional lattice of seven coronene molecules with the experimental lattice constant a = 11.394 A,placed at the approximate equilibrium distancez,, x 3.05 A above a 24 x 24 (S-Mo-S atom pairs) rhombic MoS2(0001)surface. The total interaction energy was found to be a function of the major angle of rotation CP between the principal axes of the coronene overlayer and the MoSz substrate, and the surface site. The interaction energy for hexagonal lattices with two different internal azimuthal rotations was calculated. The resulting calculations are shown in Figure 5 for the four different sites discussed above, as sites for the binding ofthe coronene molecule at the center of the lattice. The energy minima observed depend on the initial azimuthal rotation of the coronene as well as the type of surface site chosen. For 4 = 0" the deepest energyminima occur for the S-hollow site with CP f13.9", which is close to the orientation observed in the LEED data above. For q5 = 6.5" (the most attractive coronene lattice configuration shown above), the deepest energy minima occurred for the S-atop site with CP x +16.0" and CP -10.0'. These energy minima are all close to the experimentally determined rotation of the coronene lattice with respect to the MoSz principal axes. Finding an exact minimum energy configuration will require calculations for much larger hexagonal coronene lattices and MoS2(0001) surfaces, which was not possible in these studies, due to computational limitations. The results obtained do show, however, that the accumulation of subtle differences in van der Waals forces may explain much of
the origin of the orientation of coronene and related molecules on the MoS2(0001) surface. Modeling of PhthalocyanineInteractionson SnSz and MoS2. A Pc-Pc interaction energy was first computed for a plane of the bulk lattice for InPcCl consisting of nine molecules with the central metal atom and halide pointed up, a central Pc and its eight closest neighbors (four nearest neighbors and four next nearest neighbors). The interaction energy for the bulk lattice plane was -29.457 kcal/mol, which is approximately equal to the value calculated from the nearest neighbor Pc interactions, E J E % 0.98. Since the two-dimensional lattice for a InPcCl monolayer on MoSz(0001)and SnSz(0001)surfaces has been observed by LEED to be a square lattice, the Pc-Pc interaction energy was calculated for various square lattice configurations. The interaction energy was found to be a function of the lattice constant, a, and the internal azimuthal rotation angle, 4. The perfect square lattice with benzenoid rings interdigitated is optimum and is slightly more attractive than the single layer configuration seen in the bulk (from single crystal X-ray studies), which is a slightly distorted square lattice201, for lattice constants in the range 13.8-14.0A. Similar results are obtained when a single, close-packed layer of CuPc is considered, which is a more planar molecule, minus the axial halide attached to the metal. The octahedral arrangement of sulfur atoms around the tin cation in SnSz leads to three distinct sites on the (0001) basal plane which are important to consider for adsorption of these cyclic aromatic molecule^.^^-^^ A S-atop site exists as for the MoSz surface, with 6-fold symmetry. Sites are also present which bridge three (20) Wynne, K.
J. Inorg. Chem. 1984,23,4658.
Epitaxial Thin Films of Large Organic Molecules
Langmuir, Vol. 10,No. 8, 1994 2755
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adjacent sulfur atoms, directly over a tin cation in the plane below, and have 3-fold symmetry (designated "3fold-Sn" sites). The other sites with similar symmetry, bridging sulfur atoms not situated directly over a Sn cation, are designated "3-fold-hollow" sites. Figure 6A shows the interaction energy as a function of 4 for the three surface sites on SnS2, for a single Pc molecule. The interaction energy for the InPcCl molecule above the S-atop or sulfur 3-fold-hollow site has a 6-fold rotational symmetry of the interaction energy, E(+)=E(# n60"). The interaction energy for the InPcCl molecule above the 3-fold-Sn site has a 3-fold rotational symmetry ofthe interaction energy, E(4)= E ( # n120"). Figure 6B shows similar interactions for a single Pc on three of the previously identified sites on the MoS2 surface. All three sites appear to show 6-fold symmetry, with the S-atop site once again being slightly less repulsive than the site directly over a Mo atom or the site over a Mo-vacancy. We also examined the interaction of a single InPcCl molecule over a site on the SnSz surface where a sulfur atom had been removed. These kinds of defects are routinely seen in STM images of both SnSz and MoS2 surfaces.2b I t appeared possible that the halometalated phthalocyanines might find their way to such sites, with the axial halide pointed toward the site, and halide-Sn cation interactions to bind them. We found that the interaction energy for the InPcCl molecule above such a S-vacancy site with the chloride pointed down (not shown) has several fluctuations with the angle (#), but no =E(0). rotational symmetry of the interaction energy, E(@) The movement of the C1 atom to a physically realistic separation distance, with respect to the underlying Sn atom, moves the Pc to within 1A of the SnS2 basal plane, and the accumulation of van der Waals forces no longer
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provides an attractive interaction. Other optimum configurations, with InPcCl lying in a "canted" orientation, axial halide down, are also possible on basal plane sites with no defects. For single molecules these orientations likewise have no particular rotational symmetry and seem less likely to form structures with long-range order than the flat-lying orientation with the halide up. The van der Waals interaction energy was next computed for a two-dimensional square lattice consisting of a central Pc molecule and its ei h t closest neighbors with the lattice constant a = 13.924 . The interaction energy was calculated for the three different sites (3-fold-S-atop, S-Sn and 3-fold-S-hollow). A 20 x 20 rhombic SnSz(0001) slab was used for these calculations. The S-Sn site had an approximate equilibrium distance for the flat-lying Pc of zeq x 3.08 A, while the other two sites had an approximate equilibrium distance of zeqx 3.13 A. The dependence of the interaction energy on the angle of rotation Q,, between principal axes of the Pc overlayer and SnS2 substrate, for each site, is shown in Figure 7 8 . Relative to the average van der Waals energy, it can be seen that rotation of the InPcCl lattice produces two significantly less attractive interactions a t Q, = ca. 116" and the most attractive interaction a t CP = -4". The S-atop site produces a slightly more attractive interaction for the Pc lattice, versus the single molecule. Whether the optimum value of Q, is +4" or -4" is clearly dependent upon the initial rotation of the Pc a t the origin ofthe lattice, a t the initiation of the calculation (see Figure 2). Similar calculations were done for InPcCl overlayers on the MoSz(0001) surface. A 24 x 24 rhombic MoSz(0001) slab was used for these calculations. All three surface sites had an approximate equilibrium distance of zeqx 3.11 A. The interaction energy for three sites (S-
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2756 Langmuir, Vol. 10, No. 8, 1994 atop, S-Mo, and S-hollow) is shown in Figure 7B. Two slight minima are observed for the Pc lattice centered over S-Mo sites, at @ = -12 and +8". The observed rotation of the Pc lattice from the LEED data, however, was @ = 0", where none of the modeled configurations show a particular minimum. Two maxima are observed, however, for the Pc lattice centered over the S-atop site, at @ = +9.5 and -10.5". The result is that the total energy a t @ = 0" rotation is bracketed by steep energy maxima which, in larger Pc lattices, may be sufficient to guide these overlayers to the observed orientation.
Conclusions The epitaxial growth of large molecules is clearly complicated by the problems in obtaining commensurate or coincident lattices for systems where considerable symmetry and size mismatch occurs between the overlayer and substrate. It is now clear that, for a number of large aromatic molecules, at least coincident, close-packed lattices will form, with the lack of commensurability extending by no more than 4-5 substrate unit cell periods. These conditions appear to be satisfied for the layered metal dichalcogenides and graphite as ~ubstrates.l-~ Where stronger interactions with the overlayer are likely (e.g., on single crystal metal halides or certain single crystal metal surfaces), it appears that commensurate lattices form, with sometimes significant changes in the packing density to satisfy commensurability requirements.2h,3,8 The initial characterization of Pc overlayers with the basal plane of MoSz suggested that the structure and orientation of square lattice phthalocyanine domains might be chiefly determined by (a) intermolecular forces which govern the geometry of the square lattice and (b) topographically directed nucleation ofthe square lattices, e.g., nucleation along step sites in the basal planes of such materials. Expansion of these studies to Pc's on SnSz, which has an hcp surface with lattice constants (sulfursulfur spacings) a1 = a2 = 3.6 (vs 3.1 A in MoSZ),shows that the summation of overlayer-substrate van der Waals forces, over large enough organic lattices, may play the dominant role in determining overlayer orientation and the number of equivalent domains. It is interesting to note that these same calculations do predict the observed orientation ofthe Pc lattice on SnSz, but fall short offinding a true energetic minimum for the same lattice on MoSz. The studies to date also suggest that S-atop sites provide the optimum point of origin for adsorption, and that intermolecular interactions dictate the packing density and internal rotation, @, of an adsorbate such as the
England et al. coronene lattice. These studies clearly need extrapolation to larger lattices and to a series of other chalcogenides with different sulfur-sulfur spacings and with different chalcogen atoms (e.g., Se) substituted into the basal plane. Topographically directed nucleation of certain of these molecular systems should not be ruled out, however, as an added factor in the determination of monolayer packing structures, especially in light of some recent studies of molecular crystal formation on low index planes of other molecular crystalline systems.21 It was shown that a combination of topography and dispersive forces on certain surface sites can control the formation ofthe earliest stages of thin film crystal growth. There is certainly the possibility that ledge sites in the basal planes of these metal dichalcogenides will play a similar role for some of the dye systems of interest here. The extent to which the architecture ofthese monolayers controls the packing of multilayer thin films is already apparent.1,2a5,6J0 Only those molecules whose bulk structures lend themselves to layered growth have shown themselves to be amenable to the formation of epitaxial single component and multiple component thin films with sufficient long-range order to produce optical andlor electrical properties which are uniquely attributable to that ordering.l-' These systems include the trivalent and tetravalent metal phthalocyanines and naphthalocyanines, certain fullerenes, perylenes, etc. The number of equivalent domains in the first monolayer, and their relative size, controls the number of equivalent crystalline domains in much thicker films. This has a significant impact on observable optical properties, where interactions over 10 or less molecule units dominate spectral line shapes.zd,e,gJ1Other recent modeling studies suggest how important these kinds of interactions are between organic layers, where weak interactions control the packing over two and three dimensions.6ee.22
Acknowledgment. This research was supported in part by grants from the National Science Foundation (Chemistry and Small Grants for Exploratory Research), by the Air Force Office of Scientific Research, and by the Materials Characterization Program, State of Arizona. N.R.A. wishes to acknowledge the very useful discussions with J. Rabe at the Max Planck Institute fur Polymerforschung, during the early stages of this work. (21) Carter, P. W.; Ward, M. D. J.Am. Chem. SOC.,in press. Carter, P. W.; Hillier, A. C.; Ward, M. D. J. Am. Chem. SOC.,in press. (22) Scaringe, R. P.In Electron Crystallographyof Organic Molecules; Fryer, J.R.,Dorset, D. L., Eds.; Kluwer Academic Press: Dordrecht, The Netherlands, 1990; pp 85-113.
