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Imaging of a Fluorine-Substituted Isophthalic Acid Derivative on Graphite with Scanning Tunneling Microscopy A. Gesquie`re, M. M. Abdel-Mottaleb, and F. C. De Schryver* University of Leuven (KULeuven), Department of Chemistry, Laboratory of Molecular Dynamics and Spectroscopy, Celestijnenlaan 200-F, 3001 Heverlee, Belgium
M. Sieffert and K. Mu¨llen Max-Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany Received March 22, 1999 A semifluorinated isophthalic acid derivative and its mixtures with a number of nonfluorinated analogues have been investigated with scanning tunneling microscopy (STM) at the solution-graphite interface. The specific contrast arising from fluorine atoms in STM images allows us to use this functionality as a probe to aid in the analysis of the data obtained for the mixtures under investigation. Unlike the findings reported for other systems, we did not observe any segregation of fluorinated and nonfluorinated alkyl segments into microdomains that can be attributed to the incompatibility between the two components.
Introduction Semifluorinated n-alkanes, F(CF2)n(CH2)mH (abbreviated as FnHm in this paper) have been the subject of a number of studies in both the melt and solid state1 as well as in solution.2 Ho¨pken et al. found that adding a perfluorinated n-alkane to an n-alkane of the same number of carbons, both liquids, gives rise to a volume expansion (solution at 25 °C). At lower temperatures the solution separates into two macroscopic phases. This illustrates the strong incompatibility of the system’s components. This type of phase separation is of course impossible if the incompatible moieties form part of the same molecule. Ho¨pken et al. have shown that semifluorinated hydrocarbons crystallize in ordered bilayer lamellar structures, in which the hydrocarbon and perfluorinated segments segregate in to microdomains. The 3D crystalline structure of perfluoroalkylalkanes in their different solid and liquidcrystalline phases has been extensively investigated, while in solution formation of micelles and bilayer membranes of perfluoroalkylalkanes was observed.1,3-6 The phase behavior of binary systems of perfluoroalkylalkanes differing in segmental length was also described.7,8 The above studies demonstrate that the miscibility of fluorocarbon and hydrocarbon molecules is generally poor. Chain molecules consisting of hydrocarbon and fluoro* To whom correspondence should be addressed. (1) (a) Twieg, R.; Rabolt, J. F. J. Polym. Sci., Polym. Lett. Ed. 1983, 21, 901. (b) Rabolt, J. F.; Russell, T.; Twieg, R. Macromolecules 1984, 17, 2786. (c) Minoni, G.; Zerbi, G. J. Polym. Sci., Polym. Lett. Ed. 1984, 22, 533. (2) Ho¨pken, J.; Pugh, C.; Richtering, W.; Mo¨ller, M. Makromol. Chem. 1988, 189, 911-925. (3) (a) Twieg, R. J.; Rabolt, J. F.; Russell, T. P. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1985, 26, 234. (b) Twieg, R. J.; Rabolt, J. F. Macromolecules 1988, 21, 1806. (4) Viney, C.; Russell, T. P.; Depero, L. E.; Twieg, R. J. Mol. Cryst. Liq. Cryst. 1989, 168, 63. (5) Turberg, M. P.; Brady, J. E. J. Am. Chem. Soc. 1988, 110, 7797. (6) Ishikawa, Y.; Kuwahara, H.; Kunitake, T. J. Am. Chem. Soc. 1989, 111, 8530. (7) Dorset, D. L. Macromolecules 1990, 23, 894. (8) Russell, T. P.; Rabolt, J. F.; Twieg, R. J.; Siemens, R. L. Macromolecules 1986, 19, 1135-1143.
carbon segments of at least six to eight carbons in each segment tend to organize in ordered bilayer structures consisting of microdomains of fluorinated and hydrogenated segments. Very few SPM studies have been carried out on fluorinesubstituted n-alkanes. Stabel et al. investigated the properties of monolayers of a single fluorine atom substituted stearic acid physisorbed on graphite with STM.9 They observed the fluorinated methylene group with dark contrast, in agreement with theoretical calculations performed by Lambin et al.10 Moreover, they confirmed that the fluorinated moiety, although it has a large stereoelectronic impact, has little steric influence, in agreement with calculations and experimental results reported for single fluorine atom substituted enzymatic systems.11 Claypool et al. carried out scanning tunneling microscopy (STM) measurements on completely fluorinated alkanols but were unable to image monolayers, most likely because a crystalline monolayer of this type of molecules is not formed at room temperature on the graphite surface. They could however observe monolayers of partially fluorinated alkanols. A herringbone structure is visible in the images in which the molecules are oriented head to head with respect to each other. The perfluorinated part of the alkyl chain appears with dark contrast in the reported images at both positive and negative sample bias.12 Self-assembled monolayers of fluorinated alkanethiolate monolayers on gold, previously investigated with infrared absorption spectroscopy,13 have been characterized with atomic force microscopy (AFM) by Alves et al.14 Both studies confirm the larger rigidity of partially (9) Stabel, A.; Dasaradhi, L.; O’Hagan, D.; Rabe, J. P. Langmuir 1995, 11, 1427-1430. (10) (a) Lambin, G.; Calderone, A.; Lazzaroni, R.; Bre´das, J. L.; Clarke, T. C.; Rabe, J. P. Mol. Cryst. Liq. Cryst. 1993, 235, 75. (b) Lazzaroni, R.; Calderone, A.; Bre´das, J. L.; Rabe, J. P. J. Chem. Phys. 1997, 107, 99-105. (11) O’Hagan, D.; Dasaradhi, L. Bio. Med. Chem. Lett. 1993, 3, 1655. O’Hagan, D.; Rzepa, H. S. J. Chem. Soc., Perkin Trans. 1994, 2, 3. (12) Claypool, C. L.; Faglioni, F.; Goddard, W. A., III; Gray, H. B.; Lewis, N. S.; Marcus, R. A. J. Phys. Chem. B 1997, 101, 5778-5995. (13) Chidsey, C. E. D.; Lioacono, D. N. Langmuir 1990, 6, 682-691. (14) Alves, C. A.; Porter, M. D. Langmuir 1993, 9, 3507-3512.
10.1021/la9903393 CCC: $15.00 © 1999 American Chemical Society Published on Web 09/04/1999
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Figure 1. Chemical structure of (a) F6H11-ISA, and (b) HnISA, n ) 14, 16.
fluorinated alkyl chains with respect to nonfluorinated alkyl chains. This property was used by Overney et al. to use AFM as a tool to differentiate between the two molecular classes in Langmuir-Blodgett (LB) films deposited from mixtures of partially fluorinated and nonfluorinated fatty acids.15 Islands of nonfluorinated hydrocarbons could be distinguished from the surrounding flat film consisting of the partially fluorinated component. Thus, phase separation of the semifluorinated and nonfluorinated compounds into separate domains was observed. However, no molecular resolution was achieved. A near-field scanning optical microscopy (NSOM) study of dye segregation in a hydrocarbon-fluorocarbon mixed LB monolayer corroborates the aforementioned finding.16 These results indicate the high degree of incompatibility between fluorocarbons and hydrocarbons. We have investigated this phenomenon in two dimensions with STM at the submolecular level by imaging mixtures of a fluorinated isophthalic acid derivative with nonfluorinated analogues. The molecular structure of the compounds under investigation is shown in Figure 1. Experimental Section Prior to imaging, all compounds under investigation were dissolved in 1-phenyloctane, 1-octanol, or 1-heptanol (Aldrich, 99%) and a drop of this solution was applied on a freshly cleaved surface of highly oriented pyrolytic graphite. The STM images were acquired in the variable current mode (constant height) under ambient conditions, unless otherwise stated in the text. In the STM images, white corresponds to the highest and black to the lowest measured tunneling current. STM experiments were performed using a Discoverer scanning tunneling microscope (Topometrix Inc., Santa Barbara, CA) along with an external pulse/function generator (model HP 8111 A), with negative sample bias. Tips were electrochemically etched from Pt/Ir wire (80%/20%, diameter 0.2 mm) in 2 N KOH/6 N NaCN solution in water. The experiments were repeated in several sessions using different tips to check for reproducibility and to avoid artifacts. Different settings for the tunneling current and the bias voltage were used, ranging from 0.3 to 1.3 nA and -10 mV to -1.5 V, respectively. The unit cell parameters were not affected by the difference in experimental conditions. After registration of a STM image of a monolayer structure, the underlying graphite surface was recorded at the same position by decreasing the bias voltage, serving as an in situ calibration. All the presented STM images are raw data, without any filtering or image processing.
Results and Discussion A. 5-((ω-Perfluorohexyl)undecanyloxy)isophthalic Acid (F6H11-ISA). Images of the lamellar structure of (15) Overney, R. M.; Meyer, E.; Frommer, J.; Brodbeck, D.; Lu¨thi, R.; Howald, L.; Gu¨ntherodt, H.-J.; Fujihira, M.; Takano, H.; Gotoh, Y. Nature 1992, 359, 133-135. (16) Monobe, H.; Koike, A.; Muramatsu, H.; Chiba, N.; Yamamota, N.; Ataka, T.; Fujihura, M. Ultramicroscopy 1998, 71, 287-293.
Gesquie` re et al.
F6H11-ISA could be obtained by depositing a drop of a solution containing this compound on highly oriented pyrolytic graphite (HOPG). A STM image representative for the observed monolayer structure is shown in Figure 2a. The perfluorinated part of the alkyl chain can clearly be distinguished as a black band, due to a decreased tunneling current detected over the fluorinated methylene groups. This is in agreement with predictions that the fluorinated moiety will induce a change in contrast with respect to the hydrogenated moiety in the STM images when physisorbed on the basal plane of graphite. The observed bright spots correspond to the location of the isophthalic acid groups. The nonfluorinated part of the alkyl chain can be seen as a darker band with respect to the isophthalic acid groups. Bright and dark refer to the black/white contrast in the images. White corresponds to the highest and black to the lowest measured tunneling current in the image. The occurrence of a higher tunneling current above an aromatic moiety, as predicted by theoretical calculations,17 is a general finding, which has been observed for a large variety of organic adsorbates on graphite. Two different lamellar widths can clearly be distinguished. The wide lamellae consist of F6H11-ISA molecules while the narrow lamellae are built up by solvent molecules.18 On the basis of the observed Moire´ pattern, the parameters of the unit cell, indicated in the molecular model presented in Figure 2b, were determined to be 9.59 ( 0.05 Å, 52.3 ( 2.6 Å, and 81 ( 3°, respectively. The unit cell parameters a, b, and R obtained from molecular modeling are 9.60 Å, 52.1 Å, and 81°, respectively. The experimental parameters are in good agreement with the molecular model. From the presented data it is clear that the fluorinated alkyl segments do not segregate into microdomains. B. Mixtures of F6H11-ISA with H16-ISA. Upon mixing of F6H11-ISA with H16-ISA monolayers can again be observed. Both semifluorinated and nonfluorinated molecules can be recognized in the presented image by their characteristic STM contrast, arising from a modified tunneling current over specific areas of the monolayer. From the image it is clear that we do not observe separate domains of nonfluorinated and semifluorinated molecules. Even though both species differ in length by one methylene unit, the lamellae are built up by both fluorinated and nonfluorinated molecules which are interdigitating with each other, as can be seen in Figure 3. In the image presented in Figure 3 the nonfluorinated isophthalic acid derivative is the dominating species, although the mixture has a 1:1 concentration ratio. The same results were obtained for a 5:1 mixture of F6H11-ISA and H16-ISA, respectively (not shown). There is no clear dependence of the composition of the two-dimensional cocrystal on the relative concentration of the components in solution. This is probably due to the preferential adsorption of the CH2 groups on graphite compared to the CF2 groups. Moreover, the hydrocarbon chain has a stronger interaction with the graphite surface since it has an all-trans conformation while the fluorocarbon chain has a helical conformation. Fluorinated molecules are incorporated randomly in the lamellae of the nonfluorinated isophthalic acid derivative. The semifluorinated alkyl chains are fully interdigitated with the nonfluorinated alkyl chains. This was observed for all the STM data obtained for the mixtures discussed (17) Lazzaroni, R.; Calderone, A.; Lambin, G.; Rabe, J. P.; Bre´das, J. Synth. Met. 1991, 525, 41-43. (18) Solvent codeposition was previously observed within our research group: Vanoppen, P.; Grim, P. C. M.; Ru¨cker, M.; De Feyter, S.; Moessner, G.; Valiyaveettil, S.; Mu¨llen, K.; De Schryver, F. C. J. Phys. Chem. 1996, 100, 19636-19641.
Imaging an Isophthalic Acid Derivative
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Figure 2. (a) STM image of a monolayer of F6H11-ISA adsorbed from a solution in 1-octanol on to a freshly cleaved graphite surface. This submolecularly resolved image allows us to distinguish the different functional groups within a single molecule based on the characteristic contrast these groups exhibit in STM images. The green arrow points out the fluorinated part of the alkyl chain, while the nonfluorinated part is pointed out by the yellow arrow. Lamellae of F6H11-ISA are interspersed with lamellae built up by 1-octanol molecules. The Moire´ period is indicated by ∆M and contains 4 alkyl chains and 9 main graphite axis. The image size is 13.1 × 13.1 nm2. Iset ) 1.0 nA, and Vbias ) 696 mV. (b) Molecular model representing the area indicated in the STM image. Unit cell parameters a, b, and R are 9.59 ( 0.30 Å, 52.1 ( 0.7 Å, and 81 ( 2 °, respectively.
Figure 3. Monolayer of a mixture of F6H11-ISA with H16ISA adsorbed from 1-octanol on to the graphite surface. The image reveals a densely packed structure in which the molecules are all interdigitating, regardless of being perfluorinated or nonfluorinated molecules. Two fluorinated molecules are indicated by the green arrows, a H16-ISA molecule is indicated by the yellow arrow. The image size is 9.7 × 9.7 nm2. Iset ) 1.3 nA, and Vbias ) 1.35 V.
in this section. Separate domains of either compound were never observed during the different sessions using different tips. The perfluorinated part of the alkyl chains can again be observed with dark contrast at the end of the molecules. This allows us to point out the exact location of each semifluorinated molecule that has been coadsorbed in monolayers of H16-ISA. A fluorinated molecule incorporated in a H16-ISA lamella is indicated by an arrow in Figure 3. The fluorine subsituted methylene groups can thus serve as a nonperturbative probe for the analysis of the structure of two-dimensional crystalline systems observed with STM.
The observation of miscibility of the protonated and fluorinated isophthalic acid is corroborated by the crystal structures.19 The ISAs C12-C16 form an isostructural series in which the lattice parameters change in a regular fashion adding an increment for each (CH2-CH2) unit added. F6H11-ISA also crystallizes in this form. It should be noted that here the fluorinated segment has protonated next neighbors. Apparently the length of the F6H11 segment is not sufficient to cause segregation. C. Mixtures of F6H11-ISA with H14-ISA. The only type of segregation we could observe is most likely caused by a significant difference in chain length of the molecules. This segregation was observed when a mixture of F6H11ISA and H14-ISA in 1-octanol was investigated with STM. While in section B the components in the mixture differ in length by only one methylene unit, there is now a difference of three methylene units. One of the obtained images is shown in Figure 4a. Two types of lamella are visible. The wide lamellae (green arrow) are built up by F6H11-ISA molecules, as can be concluded from the typical features in the STM contrast that are characteristic to this molecule. The narrow lamella to the right of the image consists of H14-ISA molecules (red arrow). This is backed up through the analysis of the image. The width of the wider lamella is 35.42 Å; the width of the narrow lamella is 31.88 Å. This is in good agreement with values obtained from molecular modeling. In both types of lamellae the molecules are fully interdigitated. In contrast to the observations made for the mixture discussed in section B, in this case the dominating species in the monolayer are the fluorinated molecules. A small number of nonfluorinated molecules coadsorbed on to the surface in separate lamellae. For this mixture it was thus possible to observe lamellar segregation. An example of an area with domain boundaries is shown in Figure 4b. The lower resolution of this image in comparison to Figure 4a is caused by the mobility of the molecules at the domain boundaries. At this boundary, the domains show a slight (19) These data will be published elsewhere.
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Figure 4. (a) STM image of a monolayer of F6H11-ISA and H14-ISA adsorbed on graphite. The green arrow indicates a F6H11-ISA lamella. The narrow H14-ISA lamella is indicated by the red arrow. A H14-ISA molecule coadsorbed in the F6H11-ISA monolayer is indicated by the yellow arrow. The image size is 8.3 × 8.3 nm2. Iset ) 1.0 nA, and Vbias ) 696 mV. (b) STM image of a monolayer of F6H11-ISA and H14-ISA adsorbed on graphite. Three domains that make a small angle with each other are visible. The lamellar axis for each of the three domains is indicated by a solid line. In the domain in the center of the image a H14-ISA lamella can be recognized. No domains consisting only of H14-ISA molecules can be detected. The image size is 13.4 × 13.4 nm2. Iset ) 1.0 nA, and Vbias ) 796 mV.
angle (4° at the domain boundary in the top left of the image and 7° at the domain boundary in the bottom right of the image) with respect to each other. From this image one can clearly distinguish an isolated H14-ISA lamella incorporated in the F6H11-ISA domain in the center of the image. We never observed domains consisting of only H14-ISA lamellae. From these data, which was observed during several sessions using different tips, we conclude that H14-ISA and F6H11-ISA do not segregate into separate domains but that H14-ISA forms separate lamellae in F6H11-ISA domains. Conclusion In this paper we report on the two-dimensional organization of a semifluorinated isophthalic acid derivative physisorbed on the basal plane of HOPG. We find that the fluorinated and nonfluorinated alkyl segments do not segregate into microdomains and that fluorinated and nonfluorinated molecules do not adsorb on the surface in separate domains. These findings deviate from the
organization observed for most semifluorinated alkanes in three-dimensional crystals. Phase separation due to the incompatibility between fluorocarbon and hydrocarbon segments has thus not been observed. This suggests that the hydrogen bonding interactions between the isophthalic acid groups, the interactions between the alkyl chains in the monolayer, and the interactions of the monolayer with the surface are stronger interactions than the repulsive interactions between fluorinated and nonfluorinated alkyl segments. The segregation observed in STM images of mixtures of F6H11-ISA with H14-ISA most likely stems from the difference in chain length between the two compounds. Acknowledgment. The authors thank the DWTC, through IUAP-IV-11, and ESF SMARTON for financial support. Andre´ Gesquie`re thanks the IWT for a predoctoral scholarship. The collaboration was made possible thanks to the TMR project SISITOMAS. LA9903393
