Langmuir 2003, 19, 10997-10999
X-ray Reflectivity Study of an Amphiphilic Hexa-peri-hexabenzocoronene at a Structured Silicon Wafer Surface Stephan Kubowicz,† Andreas F. Thu¨nemann,*,‡ Thomas M. Geue,§ Ullrich Pietsch,§ Mark D. Watson,| Natalia Tchebotareva,| and Klaus Mu¨llen| Max Planck Institute of Colloids and Interfaces, Am Mu¨ hlenberg 1, 14476 Golm, Germany, Federal Institute for Materials Research and Testing, Richard-Willsta¨ tter-Strasse 11, 12489 Berlin, Germany, Fraunhofer Institute for Applied Polymer Research, Geiselbergstrasse 69, 14476 Golm, Germany, Institute of Physics, University of Potsdam, D-14415 Potsdam, Germany, and Max-Planck-Institut fu¨ r Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany Received July 5, 2003. In Final Form: September 29, 2003
Introduction There is a growing interest in the application of discotic materials as conducting layers in organic molecular devices. Much effort has been made to orient disklike molecules macroscopically,1-3 which has included the research on monolayers and Langmuir-Blodgett multilayers of discotic liquid crystals.4-16 The molecules of alkyl-substituted hexa-peri-hexabenzocoronenes (HBCs) self-assemble to columnar structures of face-to-face stacked aromatic cores surrounded by saturated hydrocarbons.17 Strong π-π interactions result in stable liquid crystalline phases and also allow the formation of well-organized layers with columnar ordered molecules. The mobility of the charge carriers along the * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Max Planck Institute of Colloids and Interfaces. ‡ Federal Institute for Materials Research and Testing and Fraunhofer Institute for Applied Polymer Research. § University of Potsdam. | Max-Planck-Institut fu ¨ r Polymerforschung. (1) Furumi, S.; Janietz, D.; Kidowaki, M.; Nakagawa, M.; Morino, S. Y.; Stumpe, J.; Ichimura, K. Mol. Cryst. Liq. Cryst. 2001, 368, 42854292. (2) Furumi, S.; Janietz, D.; Kidowaki, M.; Nakagawa, M.; Morino, S.; Stumpe, J.; Ichimura, K. Chem. Mater. 2001, 13, 1434-1437. (3) Bonosi, F.; Ricciardi, G.; Lelj, F.; Martini, G. J. Phys. Chem. 1993, 97, 9181. (4) Karthaus, O.; Ringsdorf, H.; Urban, C. Makromol. Chem., Macromol. Symp. 1991, 46, 409-413. (5) Vandevyver, M.; Albouy, P.-A.; Mingotaud, C.; Perez, J.; Barraud, A. Langmuir 1993, 9, 1561-1567. (6) Maliszewskyj, N. C.; Heiney, P. A. Langmuir 1995, 11, 16661674. (7) Scho¨nherr, H.; Kremer, F. J. B.; Kumar, S.; Rego, J. A.; Wolf, H.; Ringsdorf, H.; Jaschke, M.; Butt, H.-J.; Bamberg, E. J. Am. Chem. Soc. 1996, 118, 13051-13057. (8) Tsuruk, V. V.; Bengs, H.; Ringsdorf, H. Langmuir 1996, 12, 754757. (9) Vaes, A.; Van der Auweraer, M.; De Schryver, F. C., Laguitton, B.; Jonas, A. Henderson, P.; Ringsdorf, H. Langmuir 1998, 14, 52505254. (10) Chi, L. F.; Anders, M.; Fuchs, H.; Johnston, R. R.; Ringsdorf, H. Science 1993, 259, 213-216. (11) Laschewsky, A. Adv. Mater. 1989, 1, 392-395. (12) Angelova, A.; Ionov, R. Langmuir 1996, 12, 5643-5653. (13) Mindyuk, O. Y.; Heiney, P. A. Adv. Mater. 1999, 11, 341-344. (14) Janietz, D. J. Mater. Chem. 1998, 8, 265-274. (15) Roisin, P.; Wright, J. D.; Nolte, R. J. M.; Sielcken, O. E.; Thorpe, S. C. J. Mater. Chem. 1992, 2, 131. (16) Poynter, R. H.; Cook, M. J.; Chesters, M. A.; Slater, D. A.; McMurdo, J.; Welford, K. Thin Solid Films 1994, 243, 1994. (17) Watson, M. D.; Fechtenko¨tter, A.; Mu¨llen, K. Chem. Rev. 2001, 101, 1267-1300.
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main axis of the columns can be remarkably high. HBCs display, for example, charge carrier mobilities in the range of 0.5-1 cm2 V-1 s-1, which are the highest values for organic molecules observed to date.18 The combination of a high charge carrier mobility with a high thermal and chemical stability makes the columnar structures of HBCs promising for the formation of molecular wires. An important step for their application in electronic devices is to find techniques for an exact spatial alignment of the columns. Significant progress in this direction was made recently by Bjørnholm et al., who prepared Langmuir and Langmuir-Blodgett monolayers of an amphiphilic HBC containing linear alkyl chains.19 In a recent work, we were preparing mono- and multilayers of an amphiphilic HBC at solid surfaces and demonstrated that the HBC forms highly correlated layers (perpendicular to the surface).20 In these layers, the molecules have a preferred direction, in which the planes of the flat cores of the molecules are perpendicular to the dipping direction. This can be explained by the orientation of the disks during their compression in the Langmuir trough due to the shape anisotropy (flat surface) of the cores. In this study, we continued this work by preparing a film containing five layers of HBC at a regularly onedimensional structured silicon wafer surface. This grating will limit the lateral dimension of the film, which should lead to a higher orientation of the molecules along the grating vector. In our investigation, we will show that it is possible to form well-defined layers of HBC on a structured support by the Langmuir-Blodgett technique. The HBC that we used here is monofunctionalized with a carboxylic acid group [provides amphiphilicity and binding to poly(ethylene imine) (PEI)]. Branched alkyl chains, instead of n-alkyl chains that were used in the earlier study, enhance its solubility in common organic solvents. The branched alkyl chains have the advantage that they suppress side-chain crystallinity efficiently; that is, they avoid the problem of packing competition between π stacking and crystalline alkyl-chain packing, as it has been found for a HBC with linear chains.19 We used structured silicon wafers as a substrate and functionalized them with high-molecular-weight PEI. This provides the HBC molecules the possibility to anchor at the wafer by forming a PEI-HBC complex at the surface of the silicon wafer, as shown in Figure 1. PEI has been proved to be suitable as a complexing compound for the formation of thin films.21 A PEI-HBC complex has been further shown to form highly ordered HBC columns in the bulk material.22 It was, therefore, chosen as an appropriate candidate to connect the first layer to the wafer surface. Experimental Section Materials. The 2-(10-carboxy-undecyl)-5,8,11,14,17-(3,7-dimethyloctanyl) hexa-peri-hexa-benzocoronene, an amphiphilic HBC, was synthesized using a slight modification of a published (18) van de Craats, A. M.; Warman, J. M.; Fechtenko¨tter, A.; Brand, J. D.; Harbison, M. A.; Mu¨llen, K. Adv. Mater. 1999, 11, 1469-1472. (19) Reitzel, N.; Hassenkam, T.; Balashev, K.; Jensen, T. R.; Howes, P. B.; Kjaer, K.; Fechtenko¨tter, A.; Tchebotareva, N.; Ito, S.; Mu¨llen, K.; Bjørnholm, T. Chem.sEur. J. 2001, 7, 4894-4901. (20) Kubowicz, S.; Pietsch, U.; Watson, M. D.; Tchebotareva, N.; Mu¨llen, K.; Thu¨nemann, A. F. Langmuir 2003, 19, 5036-5041. (21) Chiarelli, P. A.; Johal, M. S.; Holmes, D. J.; Casson, J. L.; Robinson, J. M.; Wang, H. L. Langmuir 2002, 18, 168-173. (22) Thu¨nemann, A. F.; Ruppelt, D.; Ito, S.; Mu¨llen, K. J. Mater. Chem. 1999, 9, 1055-1057.
10.1021/la035210e CCC: $25.00 © 2003 American Chemical Society Published on Web 11/18/2003
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Langmuir, Vol. 19, No. 26, 2003
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Figure 1. Chemical structure of the amphiphilic HBC with branched side chains (left-hand figure). The HBC is bound ionically to a silicon wafer surface via the amino groups of high-molecular-weight PEI. The right-hand figure shows a sketch of columns of HBC molecules that are aligned at a silicon wafer surface.
Figure 2. Sketch of the used silicon grating (a ) 780 nm, b ) 42 nm) (left-hand figure). The right-hand figure shows an AFM picture of the substrate taken right after the edging process. procedure23 starting from 2-(bromo)-5,8,11,14,17-(3,7-dimethyloctanyl) hexa-peri-hexabenzocoronene.24 High-molecular-weight PEI (Lupasol WF) was supplied by BASF and used as received. The polymer is highly branched with a molar ratio of primary to secondary to tertiary amino groups of 34:40:26 and a molecular weight of M h w ) 25 000 g/mol.25 The structured silicon wafers with 10 mm × 10 mm size were supplied by Professor Bauer from the Institute of Semiconductor Physics at the Johannes Kepler University, Linz, Austria. The grating cross section (perpendicular to the grating vector) is of rectangular shape with a height of 42 nm and a period of 780 nm (see Figure 2). Film Preparation. The wafers were immersed in an aqueous solution of PEI [0.1% (w/w)] for 60 s. Afterward, they were rinsed with water (Millipore) and dried in an air stream. This procedure results in a thin homogeneous coating of the wafer with PEI with a thickness of about 0.5 nm. The HBC was dissolved in chloroform at a concentration of 1.16 g L-1, and 200 µL of this solution was spread on the water surface of a Langmuir-Blodgett trough. The solvent was allowed to vaporize, and the remaining HBC monolayer was then compressed with a velocity of 30 cm2 min-1 up to a surface pressure of 25 mN/m. The compression was monitored by recording the pressure-surface isotherm. The HBC was transferred to the PEI-coated silicon wafer at a surface pressure of 25 mN/m and with a transfer speed of 6 mm/min. The dipping direction was parallel to the grating. A film of five HBC layers was transferred onto the wafer by having the substrate, which was hydrophilic because of the PEI, in the subphase prior to spreading the monolayer. (23) Ito, S.; Wehmeier, M.; Brand, J. D.; Kubel, C.; Epsch, R.; Rabe, J. P.; Mu¨llen, K. Chem.sEur. J. 2000, 6, 4327-4342. (24) Fechtenko¨tter, A.; Tchebotareva, N.; Wantson, M. D.; Mu¨llen, K. Tetrahedron 2001, 57, 3769-3783. (25) Information supplied by the manufacturer, BASF, specialty chemicals, Ludwigshafen, Germany.
Measurement. Specular X-ray reflectivity measurements were carried out to investigate the vertical d spacing of the HBC film. These were performed at the EDR beamline at BESSY II in Berlin, Germany, with polychromatic X-rays with an energy range of 3-35 keV (which corresponds to a wavelength λ of 0.0350.413 nm). The incident angle Ri was equal to the exit angle Rf for the specular reflectivity measurements and was fixed to 1°. We calculated qz, the wave vector transfer in z direction (perpendicular to the film surface) by qz ) 4π/λ sin Ri with the wavelength λ (0.035-0.413 nm). The parallel white beam was guided through a Ti-bladed slit system, and the reflected beam was detected by a Ro¨ntec detector 1.34 m downstream. The Ro¨ntec detector is an energy dispersive detector with a resolution ∆E/E ) 10-2. It is sensitive for energies between 0 < E < 80 keV (divided into 4000 channels by a Multi Channel Analyzer) and has an active area of 2 mm × 2 mm. As an important parameter, the counting response of the detector is linear up to 105 cps. For better spatial resolution, a 10-µm pinhole was installed 40 mm in front of the detector. To reduce air absorption, an evacuated flight tube was equipped before the detector. Further experimental details of the energy dispersive small-angle X-ray scattering are described elsewere.26,27
Results and Discussion The sample was aligned with the grating perpendicular to the X-ray beam. Therefore, we get an interference between the scattering signal from the HBC layers and (26) Pietsch, U.; Grenzer, J.; Geue, Th.; Neissendorfer, F.; Brezesinski, G.; Symietz, Ch.; Mo¨hwald, H.; Gudat, W. Nucl. Instrum. Methods Phys. Res., Sect. A 2001, 1077, 467-468. (27) Panzner, T.; Leitenberger, W.; Grenzer, J.; Bodenthin, Y.; Geue, Th.; Pietsch, U.; Mo¨hwald, H. J. Phys. D: Appl. Phys. 2003, A93, 36.
Notes
Figure 3. X-ray reflectivity curve of a film with five HBC layers on a structured silicon wafer (solid line) and on a smooth silicon wafer (dashed line).
that from the grating. Figure 3 shows the diffractogram of the sample on the structured wafer in comparison with an HBC film with the same number of layers on a smooth wafer. It can be seen that the minima of both curves are almost at the same qz value, which indicates that they have the same film thickness. Furthermore, one can see a short-range oscillation, which comes from the grating of the substrate and the long oscillation from the HBC film. From the short-range oscillation, one can calculate the height of the grating with d ) 2π/∆qz,max, which yields 37.7 ( 0.4 nm. This is in agreement with 42 nm found from atomic force microcopy (AFM) measurements (see Figure 2). The long-range oscillations give the thickness of the film with D ) 2π/∆qz,max, which is 12.5 ( 0.1 nm. This value is almost identical with the layer thickness of HBC on a smooth wafer, which has been investigated in a recent work.20 From this, we can conclude that the HBC film is located in the grating valleys as well as on top of the grating. If the film would only be transferred, for example, between or on top of the grating, one would get a lower layer thickness that is due to the ensemble averaging of the measured X-ray signal on the sample surface. For the simulation of the scattering curve, a simplified model was used (see Figure 4). In this model, a regular grating with a height of 42 nm was applied for modeling input. The HBC film on top and between the grating was assumed to be a homogeneous layer with a mean electron density of 430 electrons/nm3. The layer thickness on top of the grating was set to 12.4 nm and that between the grating to 13.1 nm. Figure 5 shows the simulated scattering curve (simulation 1, curve b) together with the experimental results. As one can see, the simulated curve from this simple model fits quite well to the experimental data. The additional features of the experimental curve (exact intensity ratios) were mainly due to internal structuring and imperfections of the HBC film, which were neglected in the model. Further simulations were carried out for a better understanding of the data. In simulation 2 (curve c), it was assumed that the HBC layer is located between the grating only, and in simulation 3 (curve d), the HBC layer is only on top of the grating. It can be clearly seen that these two simulations are obviously not in agreement with the experimental data, which is a further indication for our interpretation that the HBC layer is deposited homogeneously on the structured wafer.
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Figure 4. Electron density profiles perpendicular to the surface of the five-layer HBC film, which were used for the simulation (curve b in Figure 5). For simpicity of the calcuation, we assumed that the layer is disrupted at the edges of the grating, but it is also possible that a cohesive layer is folded over the edges of the grating.
Figure 5. Comparison of the simulations and the experimental results (curve a): simulation 1 (curve b), HBC film on top and between the grating; simulation 2 (curve c), HBC film only between the grating; and simulation 3 (curve d), HBC film only on top of the grating.
For simpicity of the calcuation, we assumed that the layer is disrupted at the edges of the grating (see sketch in Figure 4). But it is also possible that a cohesive layer is folded over the edges of the grating. Both cases cannot be distiguished. Conclusion In conclusion, we have shown that it is possible to transfer an amphiphilic HBC via the Langmuir-Blodgett technique homogeneously to a structured silicon wafer. The ordering of the HBC film on the structured wafer is surprisingly high and the same as when HBC was deposited on a smooth wafer. This finding may be important for further developments, such as using HBC in electronic devices of nanoscopic scale. Acknowledgment. The authors thank Jo¨rg Grenzer for support at the EDR beamline at Bessy II, Petr Mikulı´k for his X-ray reflectivity program, which was used to simulate and fit the X-ray reflectivity data, and Professor Bauer for providing the structured silicon wafer. LA035210E
