Langmuir 1998, 14, 4227-4231
4227
Molecular Order within Langmuir-Blodgett Films of Two Amphiphilic Octasubstituted Phthalocyanines Studied by Atomic Force Microscopy M. Ve´lez* and S. Vieira Laboratorio de Bajas Temperaturas, Departamento de Fı´sica de la Materia Condensada, C-III, Universidad Auto´ noma de Madrid, 28049, Madrid, Spain
I. Chambrier and M. J. Cook School of Chemical Sciences, University of East Anglia, Norwich, U.K. NR4 7TJ Received January 29, 1998. In Final Form: May 8, 1998 Y-type Langmuir-Blodgett films of the octasubstituted amphiphilic phthalocyanines 1 and 2 deposited onto hydrophobic glass have been investigated by atomic force microscopy (AFM). The technique has been used to measure the thicknesses of one, two, and three bilayer films, and the results show that each bilayer is of the same thickness. This characteristic thickness is the same as the repeat spacing measured using low-angle X-ray diffraction methods on thicker samples, and thus we demonstrate that the order of the molecules in the monolayers after several cycles of deposition remains constant. We have detected considerable differences with respect to the consistency of the films of 1 and 2 and their resistance to the pressure exerted by the AFM tip during the scanning process required to obtain the images. Films of 1 are more robust and we have observed the columnar arrangement of the molecules at the surfaces of one, two, and three bilayers. However, under the same imaging conditions it was not possible to observe molecular order before perturbing the surface of the films formed by 2. The lateral extent of the ordered regions at the surfaces of the films of 1 covers areas of at least 10 µm2. There is very clear evidence that there is a predominant alignment. Inspection of 82 images taken of different regions of the film indicates that ca. 83% show columns whose axes are aligned to within (30° of the dipping direction.
Introduction Phthalocyanine (Pc) molecules are macrocyclic organic compounds that show a variety of optical and electronic properties such as a variable electronic conductivity in different ambients, photovoltaic effects, and electrochromism.1 The sensitivity of their electrical conductivity to the presence of different gases makes them good candidates for their incorporation into gas-sensing devices.2 However, to take full advantage of these properties, it is essential to understand and control the structure of the films prepared with these materials. The elegance of the Langmuir-Blodgett (LB) technique for depositing thin films of ordered molecules in two and three dimensions has motivated extensive work on the synthesis and study of LB film forming properties of various substituted Pcs.3,4,5,6 One of our groups has previously investigated the Langmuir monolayer behavior and LB deposition properties of Pc compounds derivatized with eight substituent aliphatic chains, two of which terminate with a polar headgroup. A systematic study of some 20 derivatives of * Corresponding author: e-mail, Marisela.Ve´
[email protected]; fax, (34-1) 397-3961; tel, (34-1) 397-5552. (1) Leznoff, C. C., Lever, A. B. P. Eds. Phthalocyanines: Properties and Applications; VCH Publishers: New York, 1989. (2) Travis, J.; Ray, A. K.; Thorpe, S. C.; Cook, M. J.; James, S. A. Meas. Sci. Technol. 1995, 6, 988. (3) Wang, Hong-Ying; Mann, J. Adin, Jr.; Lando, J. B.; Clark, T. R.; Kenney, M. E. Langmuir 1995, 11, 4549. (4) Bourgoin, J. P.; Doublet, F.; Palacin, S.; Vandevyver, M. Langmuir 1996, 12, 6473. (5) Gupta, V. K.; Kornfield, J. A.; Ferencz, A.; Wegner, G. Science 1994, 265, 940. (6) Embs, F. W.; Thomas, E. L.; Gittinger, A.; Dulong, L. Thin Solid Films 1994, 237, 217.
this type, each with two groups bearing hydroxyl (-OH) headgroups, examined the efficacy of different substituent chains in promoting good film-forming properties. The molecular ordering within the films, typically constructed from 30 layers, was assessed using polarized visible region spectroscopy, low-angle X-ray diffraction methods,7 and, for some compounds, reflection absorption infrared spectroscopy (RAIRS).8,9 Films prepared using derivatives where the aliphatic substituents were attached by ether linkages were not highly ordered. However, analogues where the chains were attached by carbon-carbon bonds, as in structures 1 and 2 (Figure 1), showed superior monolayer behavior and proved to be excellent materials for Y-type deposition as LB films.7 Low-angle X-ray reflectivity studies and Fourier transform infrared (FTIR) measurements of films on gold indicated that the molecules are packed with their aromatic cores essentially perpendicular to the substrate surface.8 Furthermore, the type of molecular packing within a monolayer in this series was found to depend on chain length. Thus films prepared from compound 1 and its lower homologues where R are nonyl and octyl chains showed an absorption band envelope with λmax ca. 771 and 639 nm. In contrast, films of 2 showed a quite different absorption signature with just one main band, λmax 740 nm. Both types of spectra showed dichroism, implying anisotropic packing of the molecules. This was attributed to their deposition within columnar arrays aligned with (7) Cook, M. J.; McMurdo, J.; Miles, D. A.; Poynter, R. H.; Simmons, J. M.; Haslam, S. D.; Richardson, R. M.; Welford, K. J. Mater. Chem. 1994, 4 (8), 1205. (8) Poynter, R. H.; Cook, M. J.; Chesters, M. A.; Slater, D. A.; McMurdo, J.; Welford, K. Thin Solid Films 1994, 243, 346. (9) Chester, M. A.; Cook, M. J.; Gallivan, S. L.; Simmons, J. M.; Slater, D. A. Thin Solid Films 1992, 210/211, 538.
S0743-7463(98)00118-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/24/1998
4228 Langmuir, Vol. 14, No. 15, 1998
Ve´ lez et al.
Figure 1. Structures of compounds 1 and 2.
the column axes parallel to the substrate surface and organized preferentially along the dipping direction. However, it has not been possible to obtain unambiguous data concerning the uniformity of the columnar packing in terms of either column length or column alignment. The atomic force microscope (AFM) can provide detailed local information of the molecular arrangement of the surface of films.4,10,11,12 We have now utilized this emerging technique to investigate the films of 1 and 2 in more detail, focusing attention on both the construction of the first few layers and the exploration of their surfaces. Materials and Methods Metal-Free Alkyl Amphiphiles. The preparation and characterization of compounds 1 and 2 used in this study have been reported elsewhere.13 LB Film Deposition. Langmuir-Blodgett films were prepared on a KSV 5000 trough (KSV Instruments, Ltd., Helsinki, Finland). To prepare the LB films, the molecules were dissolved in 1,1,1-trichloroethane at known concentration of ca. 1 mg mL-1. The monolayer was formed on a subphase of distilled water (18 MΩ resistance). All experiments were carried out at room temperature (20-22 °C). The monolayers were transferred to glass rendered hydrophobic by silanizing the slides in a 4% solution of dichlorodimethylsilane in 1,1,1-trichloroethane for 10 min. Vertical dipping through the molecular monolayer was undertaken with the surface pressure maintained constant during the dipping process at 30 mN m-1 or 15 mN m-1. Dipping speeds were 8 mm min-1. The AFM images were taken with an inhouse built microscope under atmospheric conditions using commercial Nanoscope cantilevers (Digital Instruments, Santa Barbara, CA). The Z piezoceramic responsible for the height measurements was calibrated using a capacitance bridge. The X-Y calibration was done using the distance between mica atoms in atomically resolved images. All images were taken in contact mode with the tip exerting a force on the film surface on the order of 50 nN. The images shown have had the plane removed and, in the case of Figure 3, have also been smoothed.
Results Deposition for both compounds 1 and 2 (Figure 1) onto a hydrophobized glass substrate was Y-type with transfer (10) Josefowicz, J. Y.; Maliszewskyj, Idziak, S. H. J.; Heiney, P. A.; McCauley, J. P., Jr.; Smith, A. B., III Science 1993, 260, 323. (11) Frommer, J. E. Angew. Chem., Int. Ed. Engl. 1992, 31 (19), 1298. (12) Frommer, J. E. Thin Solid Films 1996, 273, 112. (13) Chambrier, I.; Cook, M. J.; Cracknell, S. J.; McMurdo, J. J. Mater. Chem. 1993, 3, 841.
Figure 2. Large scale topographic AFM images (3.4 µm × 3.4 µm) of the surface of one (A), two (B), and three (C) Y-type bilayers of Langmuir-Blodgett films of compound 1. (The total height range in the three images is 12, 18, and 25 nm, respectively.) The scan line across the holes is indicated to the right of each image.
ratios close to unity; cf. ref 7. Figure 2 shows areas of a few square micrometers of the surface of LB films of compound 1 deposited at 30 mN m-1 as a bilayer (one dip through the monolayer), two bilayers (two dips), and three bilayers (three dips). The holes shown in the films in Figure 2 were dug into the films with the AFM tip by scanning that small area at a higher force. The force was then reduced and a larger area was scanned to obtain the image. Using this procedure, it was possible to measure the thicknesses of the films. The height measured corresponds to 40 ( 1 Å (n ) 25) for a single Y-type bilayer, 80 ( 2 Å (n ) 25) for the two bilayers and 120 ( 2 Å (n ) 30) for the three bilayer assembly; n is the number of measurements made. Depositing the film at 15 mN m-1 gave more heterogeneous surfaces where domains of different thicknesses were observed (image not shown). Figure 3 shows the surface of a two bilayer film deposited at 30 mN m-1 at higher resolution and provides evidence for the columnar arrangement of the molecules in the surface layer. This columnar arrangement is observed on the surface of films with one, two, and three Y-type bilayers. Figure 3A depicts an area where columns with different orientations come together. The inset is the Fourier transform of the image showing spots that correspond to the spacing and orientation of the different columnar domains. The intercolumnar distance is measured in Fourier space and corresponds to 21 ( 2 Å (n ) 60 images). When the surface of these columns is scanned in the direction of their longitudinal axis, the friction images show the edges of individual molecules (Figure 3B,D). The distance between the molecules averaged 5.0 ( 0.6 Å from 118 individual measurements done on nine different images. In these higher resolution images if the
Y-Type LB Films of Phthalocyanines
Langmuir, Vol. 14, No. 15, 1998 4229
Figure 3. Higher magnification friction images of the surface of a two-bilayer film. The scanning direction was along the y axis in parts A and C and along the X axis in parts B and D. Panel A shows a 1000 Å × 1000 Å area with domains of different columnar orientations. The insert is the 2-D Fourier transform showing spots that correspond to the different domains with an intercolumnar distance of 21 ( 2 Å. The horizontal and vertical lines through the center and the spots are noise due to the raster scanning and the finite image size. (B) Smaller area where the columnar packing and individual molecules within the columns can be observed. (C) Small area scanned with the tip at more than 60° with respect to the columnar axis (see text for explanation). (D) A zone in (B).
scanning direction of the tip is more perpendicular to the columnar axis, a thinner column between the 21 Å spaced columns appears (Figure 3C), but there is no evidence of the individual molecules within the columns. After repeated scanning in this tip-column orientation, the distances between the columns and their integrity are perturbed. Figure 4 shows a topographical image of a large area of a two bilayer film before (Figure 4A) and after (Figure 4B) its central part has been scanned to observe smaller areas in which the columnar arrangement is manifest. When scanning with the tip moving perpendicularly to the columnar arrangement, each scan induced some damage on the film that accumulated after several images of the same area were taken. The result was removal of the topmost monolayer. The layer left exposed is 20 Å below the rest of the surface, a height consistent with the removal of one monolayer of the film. Furthermore, the friction image (Figure 4C) corresponding to this same area shows that the friction of this newly exposed surface is higher than the one encountered by the tip on the hydrophobic surface of the rest of the film. Figure 5 shows the surface of a two bilayer film of compound 2 deposited also at 30 mN m-1. From the depth of the hole the film thickness is 69 ( 3 Å (n ) 38); for three bilayers the film thickness is 100 ( 3 Å (n ) 80). However, attempts to observe the columnar arrangement of the molecules were unsuccessful because of the much greater fragility of the film of 2 compared with films of 1.
Figure 4. (A) AFM topographical image of a 4.8 µm × 4.8 µm area of a two-bilayer film before and after (B) its central part has been scanned (total height range is 10 nm). (C) Friction image of the same area shown in (B). Lighter regions represent areas that have a stronger interaction with the scanning tip.
Discussion
surface of organic films.11,12 However, the interpretation of the images needs to be made with care since the force exerted by the tip when scanning over the surface of a soft material can disrupt the structure of the film. The mechanical stability of the film to tip damage depends on the orientation of the scanning direction, particularly in materials that are constructed with a highly anisotropic arrangement of molecules.14 The interaction between the tip and the molecules that gives rise to the contrast in the
Atomic force microscopy is a powerful surface technique that can be used to obtain high-resolution images of the
(14) Tsukruk, V.; Foster, M. D.; Reneker, D. H.; Schmidt, A.; Wu, Hong; Knoll, W. Macromolecules 1994, 27, 1274.
4230 Langmuir, Vol. 14, No. 15, 1998
Figure 5. Large scale topographic AFM images (3.4 µm × 3.4 µm) of the surface of a two Y-type bilayer of Langmuir-Blodgett films of compound 2. (The total height range is 20 nm.)
friction images is also very sensitive to the orientation of the molecules with respect to the tip. Therefore these friction images can often reveal a higher degree of detail of the molecular arrangement15,16 than the corresponding topographic images. The present AFM results obtained for the LB films of the amphiphilic octasubstituted Pcs, 1 and 2, provide important new information regarding the vertical and lateral extent of the molecular arrangement previously described for these films using other techniques. Surface analysis of the LB films of compound 1 has shown that the molecules form a stable and homogeneous surface from the first bilayer onward when transferred at a surface pressure of 30 mN m-1 onto a hydrophobic glass surface. Deposition at the lower surface pressure, 15 mN m-1, gives incomplete coverage of the substrate surface. The surface of the films of compound 1 shows small lumps (Figures 2 and 4) even immediately after transfer. Their density is different in different monolayer preparations, and their appearance is not affected by scanning. We therefore think they could represent small aggregates of phthalocyanine molecules excluded from the monolayer, since their heights are compatible with one or two layers of material. The surface analysis of the films prepared with compound 2 at 30 mN m-1 also shows a homogeneous and stable surface from the first deposited bilayer transferred. However, in this case, the shorter aliphatic chains clearly have a profound effect on the stability and rigidity of the film toward the AFM tip. It was not possible to image any structural detail in films of 2 under the same conditions in which individual molecules could be observed in the films of 1; see below. The thickness of the Y-type bilayer in the films of 1 deposited at the higher pressure is constant (40 Å) in the one-, two-, and three-bilayer films and corresponds well to the d-spacing (41 Å) measured earlier for thicker films (30 layers) using low-angle X-ray diffraction methods. The height measurements for films of 2 transferred at a surface pressure of 30 mN m-1 are also consistent with the thickness of the bilayer deduced from X-ray scattering on 30-layer films, 35 Å. The bilayer thicknesses for 1 and 2 are consistent with the molecules in the component layers being arranged essentially vertical to the substrate surface. Importantly, the AFM results point to the fact that the packing type within the first molecular bilayer is not unique but is continued as the film is built up with further dips. The Y-type deposition of the molecules onto a hydrophobic surface implies that the first molecular monolayer (15) Overney, R. M.; Takano, H.; Fujihira, M.; Overney, Paulus W.; Ringsdorf, H. In Forces in Scanning Probe Methods; Gu¨ntherodt, H. J., et al., Eds.; Kluwer Academic Publishers: Dordrecht, 1995; pp 307312. (16) Santesson, L.; Wong, T. M. H.; Taborelli, M.; Descouts, P.; Liley, M.; Duschl, C.; Vogel, H. J. Phys. Chem. 1995, 99, 1038.
Ve´ lez et al.
deposited has the hydrophobic tails, the R groups, in contact with the surface with the hydroxyl headgroups exposed for H-bonding with the hydroxy headgroups of the second monolayer. The bilayer so formed then has a hydrophobic surface of R groups exposed at the commencement of the second dip. It was observed during the AFM experiments on films of 1 that the technique was able to distinguish between the hydrophobic surface of a bilayer and the surface remaining once parts of the outer monolayer were removed by the tip, exposing the hydroxy headgroups of the monolayer below.11 Thus the friction images (Figure 4C) indicate that the most external surface of the bi- or multilayers interacts less with the tip (more hydrophobic behavior) than the surface exposed after removing the outermost monolayer. Unsubstituted and lightly substituted phthalocyanine molecules show a strong propensity to form columnar structures in which the planar aromatic nucleus of one molecule is either fully cofacial or partially offset from the nucleus of its neighbor.17 No quantitative data are available concerning molecular packings within condensed phases of 1 and 2 though it is known that both generate columnar mesophases at elevated temperatures.13 The striations in the AFM images for the films of 1 provide convincing evidence for the presence of columnar assemblies and for the first time provide an indication of their extent. Several points emerge. The first is that the columns are essentially continuous, at least over the areas of the films imaged (ca. 1000 × 1000 Å). The second is that there are domains within the films in which the columns are aligned in different directions. Nevertheless, there is very clear evidence that there is a predominant alignment. Inspection of 82 images taken of different regions of the film indicates that ca. 83% show columns whose axes are aligned to within (30° of the dipping direction. Third, the distance between the striations is measured to be 21 ( 2 Å and is consistent in images taken at various places of the film. This compares with a width of 28 Å for individual molecules of 1 calculated by molecular modeling and thus implies either that the planes of the molecules are tilted with respect to the column axis or that there is an overlap of about 6 Å between the aliphatic chains of neighboring molecules. Earlier, we interpreted the visible region spectrum of the film of 1 in terms of a “herringbone” packing, which does, indeed, require the molecules to be tilted to the column axes. The angle between the macrocycle and columnar axis needed to account for such shortening of the intercolumnar distance is around 40°, so we would expect to see it in the AFM images. The fact that it was not seen cannot, however, be taken as evidence that it is not present. We have no detailed information as to which part of the molecule is the one responsible for the contrast observed. If only the central ring of the macrocycle is being imaged, we would not expect to see the relative tilt of neighboring molecules. As mentioned above, and illustrated in Figure 4, repeated scanning over an area produced irreversible damage of the upper monolayer of the film. Figure 3C shows the type of detailed columnar arrangement detected when a small surface was scanned with the tip moving at an angle greater than 60° with respect to the columnar axis. We observe the presence of a narrower spacing between the wider columns. These images can be interpreted as showing as wider columns the zone where the ends of aliphatic chains pertaining to two neighboring macrocycles come together and as narrow columns the (17) See for example: Snow, A. W.; Barger, W. R. In ref 1, pp 345392.
Y-Type LB Films of Phthalocyanines
edges of the central part of the macrocycles lying equidistant to the ends of the aliphatic chains. This intercalated thinner “column” was observed in the higher resolution images and also in some of the lower resolution ones, probably depending on the exact tip shape, angle, and integrity of the film. Consecutive high-resolution images taken when scanning in this configuration manifested the damage induced to the film as an apparent increase in the distance between the wider columns and a disruption of their integrity. These observations are in agreement with a previous study of “hairy-rod” type polymers,16 which revealed that their mechanical stability to the scanning tip in this direction is low. Contrarily, when the tip scans the column at a small angle with respect to its longitudinally axis, the hairy aspect disappears and the backbone of the column is revealed showing equally spaced individual molecules. The distance (ca. 5 Å) between molecules of 1 within the columnar structures detected in parts B and D of Figure 3 is stable to repeated scanning, consistent with the higher mechanical stability of the molecular arrays in this direction. This spacing is marginally more than the distance between rings of unsubstituted phthalocyanines in the crystal state measured along the columnar axes, 3.79 Å and 4.79 Å, for the R and β polymorphs, respectively,1 and may be a consequence of the presence of the decyl chains in 1. In both the polymorphic forms of the unsubstituted compound, the planes of the rings are tilted with respect to the columnar axes as proposed earlier for the molecular assembly in the LB film of 1. The different robustness of the LB films 1 and 2 detected by the AFM probe adds to the list of differences between the LB film forming properties of these compounds. Earlier structural evaluations using FTIR spectroscopy and visible region spectroscopy had shown that the length of the alkyl chain had a profound effect on the molecular packing and the thermal stability of the films.9,18 Films of compound 1 showed reversible changes upon heating, whereas the response of films of compound 2 to the same treatment was irreversible. The greater fragility of the film of 2 to the AFM tip encountered in this study implies weaker intermolecular packing interactions than those in the film of 1. 1,4,8,11,15,18,22,25-Octahexylphthalocyanine, a close analogue of 2 in which all eight alkyl chains are hexyl, forms a columnar structure in the crystal state in which there is a remarkable 8.5 Å interplane distance between neighboring molecules within a column.19 This arises because a hexyl chain on each side of the ring is oriented orthogonally to the aromatic core and effectively acts as a spacer. The crystals are soft and readily smeared when rubbed. It is plausible that the more fragile structure of the LB film of 2 arises because here the columnar packing is more similar to that in the crystal state of the octahexyl (18) Cook, M.; Mayes, D. S.; Poynter, R. H. J. Mater. Chem. 1995, 5 (12), 2233. (19) Chambrier, I.; Cook, M. J.; Helliwell, M.; Powell, A. K. J. Chem. Soc., Chem. Commun. 1992, 444.
Langmuir, Vol. 14, No. 15, 1998 4231
derivative than in the film of 1. It is unfortunate that the mechanical fragility of the films did not allow us to get clear enough images in which to distinguish the distances between molecules within the columnar structure. Conclusions In summary, we have confirmed that the LB films from monolayers of the octasubstituted amphiphilic phthalocyanines 1 and 2 on hydrophobic glass are deposited as Y-type bilayers with the molecules standing edge-on to the surface of the substrate. Evidence has been provided that they are homogeneous from the first transferred bilayer. As the thicknesses of the first few bilayers are the same as the repeat spacing measured using low-angle X-ray diffraction methods on thicker samples, we can confirm that the order of the molecules in the monolayers after several cycles of deposition remains constant. The information provided here, taken together with the structural analysis performed earlier on 30-layer films of these materials, indicates that the order in the vertical direction is mantained from a few angstroms up to 600 Å. We have also observed the molecular arrangement of the molecules at the surface of one-, two-, and three-bilayer films of compound 1. There is a high degree of anisotropic packing manifested in the formation of columns of molecules, a few hundreds of nanometers long, oriented preferentially along the dipping direction. This has also been described for some tetrafunctionalized phthalocyanines,4 but in contrast to the films formed by these other derivatives, films of 1 and 2 are transferred very efficiently without any coverage defects over areas of a few square micrometers. The lateral extent of the ordered regions is seen with the AFM to cover at least areas of up to 10 µm2. The AFM evidence for the preferred alignment along the dipping direction accounts for the dichroism in the visible region spectra observed hitherto7 and the anisotropic enhanced conductivity of films of these compounds when exposed to NO2 gas.20 We have observed, however, considerable differences with respect to the consistency of the films of 1 and 2 and their resistance to the pressure exerted by the AFM tip during the scanning process required to obtain the images. Whereas we were able to observe the molecular arrangement within the columns formed by the derivative 1, under the same imaging conditions it was not possible to observe molecular order before perturbing the surface of the films formed by the derivative bearing the shorter aliphatic chains. Acknowledgment. We thank the EU for financial support for I.C. and for travel (Contract CHRX-CT940558) and the Spanish Government for DGCYT Grant MAT951542. We also thank Dr. Sabarna Mukhopadhyay for her help in making this work possible. LA980118H (20) Crouch, D.; Thorpe, S. C.; Cook, M. J.; Chambrier, I.; Ray, A. K. Sensors Actuators 1994, 18-19, 411.
