Thin Layers of Columns of an Amphiphilic Hexa-peri

We present here the preparation and the structures of thin films of an amphiphilic hexa-peri-hexabenzocoronene (HBC) containing 1, 5, 9, and 15 layers...
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Thin Layers of Columns of an Amphiphilic Hexa-peri-hexabenzocoronene at Silicon Wafer Surfaces Stephan Kubowicz,† Ullrich Pietsch,† Mark D. Watson,‡ Natalia Tchebotareva,‡ Klaus Mu¨llen,‡ and Andreas F. Thu¨nemann*,§ Institute of Physics, University of Potsdam, D-14415 Potsdam, Germany, Max-Planck-Institut fu¨ r Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany, and Fraunhofer Institute for Applied Polymer Research, Geiselbergstraβe 69, D-14476 Germany Received December 9, 2002. In Final Form: April 4, 2003 We present here the preparation and the structures of thin films of an amphiphilic hexa-perihexabenzocoronene (HBC) containing 1, 5, 9, and 15 layers of columns. These films were prepared by the Langmuir-Blodgett technique on poly(ethylene imine) functionalized silicon wafers and investigated by X-ray reflectivity measurements using synchrotron radiation. Columns of HBC cores were aligned parallel to the silicon wafer surface. The thicknesses of the films, which were composed of stacks of highly coherent HBC layers plus a polymer layer, were 3.70-34.6 nm. Each HBC layer has a thickness of 2.4 nm. The transfer rate of the HBC monolayers from the water/air surface to the silicon wafer surface is close to 100%. A macroscopic in-plane orientation of the columns with their main axis parallel to the dipping direction was determined by polarized UV-vis spectroscopy.

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,1,2 which has included the research on monolayers and Langmuir-Blodgett multilayers of discotic liquid crystals.3-13 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.14 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 main axis of the columns can be remarkably high. HBCs * Corresponding author. E-mail: andreas.thuenemann@ iap.fhg.de. † University of Potsdam. ‡ Max-Planck-Institut fu ¨ r Polymerforschung. § Fraunhofer Institute for Applied Polymer Research. (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) Karthaus, O.; Ringsdorf, H.; Urban, C. Makromol. Chem., Macromol. Symp. 1991, 46, 409-413. (4) Vandevyver, M.; Albouy, P.-A.; Mingotaud, C.; Perez, J.; Barraud, A. Langmuir 1993, 9, 1561-1567. (5) Maliszewskyj, N. C.; Heiney, P. A. Langmuir 1995, 11, 16661674. (6) 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. (7) Tsukruk, V. V.; Bengs, H.; Ringsdorf, H. Langmuir 1996, 12, 754-757. (8) Vaes, A.; Van der Auweraer, M.; De Schryver, F. C.; Laguitton B.; Jonas, A.; Henderson, P.; Ringsdorf, H. Langmuir 1998, 14, 52505254. (9) Chi, L. F.; Anders, M.; Fuchs, H.; Johnston, R. R.; Ringsdorf, H. Science 1993, 259, 213-216. (10) Laschewsky, A. Adv. Mater. 1989, 1, 392-395. (11) Angelova, A.; Ionov, R. Langmuir 1996, 12, 5643-5653. (12) Mindyuk, O. Y.; Heiney, P. A. Adv. Mater. 1999, 11, 341-344. (13) Janietz, D. J. Mater. Chem. 1998, 8, 265-274. (14) Watson, M. D.; Fechtenko¨tter, A.; Mu¨llen, K. Chem. Rev. 2001, 101, 1267-1300.

display, for example, charge carrier mobilities in the range 0.5-1 cm2 V-1 s-1, which are the highest values for organic molecules observed to date.15 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 use 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 amphiphilic HBC containing linear alkyl chains.16 In this study we show how we continued this work by preparing mono- and multilayers of an amphiphilic HBC at solid surfaces (cf. Figure 1). The HBC that we used here is monofunctionalized with a carboxylic acid group (provides amphiphilicity and binding to poly(ethylene imine)). 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 has been found for a HBC with linear chains.16 We used silicon wafers as a substrate and functionalized them with high molecular weight poly(ethylene imine) (PEI). This gave 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.17 A PEI-HBC complex has been further shown to form highly ordered HBC columns in the bulk material.18 It was (15) 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. (16) Reitzel, N.; Hassenkam, T.; Balashev, K.; Jensen, T. R.; Howes, P. B.; Kjaer, K.; Fechtenko¨tter, A.; Tchebotareva, N.; Ito, S.; Mu¨llen, K.; Bjornholm, T. Chem.sEur. J. 2001, 7, 4894-4901. (17) Chiarelli, P. A.; Johal, M. S.; Holmes, D. J.; Casson, J. L.; Robinson, J. M.; Wang, H. L. Langmuir 2002, 18, 168-173. (18) Thu¨nemann, A. F.; Ruppelt, D.; Ito, S.; Mu¨llen, K. J. Mater. Chem. 1999, 9, 1055-1057.

10.1021/la026968l CCC: $25.00 © 2003 American Chemical Society Published on Web 05/10/2003

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Figure 1. (Left)Chemical structure of the amphiphilic hexa-peri-hexabenzocoronene (HBC) with branched side chains. The HBC is bound ionically to a silicon wafer surface via the amino groups of high molecular weight poly(ethylene imine). (right) Sketch of columns of HBC molecules that are aligned at a silicon wafer surface.

therefore chosen as an appropriate candidate to connect the first layer to the wafer surface. Experimental Methods Materials. The 2-(10-carboxyundecyl)-5,8,11,14,17-(3,7-dimethyloctanyl)hexa-peri-hexabenzocoronene, an amphiphilic HBC, was synthesized by slight modification of a published procedure19 starting from 2-bromo-5,8,11,14,17-(3,7-dimethyloctanyl)hexaperi-hexabenzocoronene.20 High molecular weight poly(ethylene imine) (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 Mw ) 25 000 g/mol.21 Polished silicon wafers of 50 mm in diameter (resistivity 180-210 Ω cm, orientation (1, -1, -1)) were supplied by Topsil, Denmark. Film Preparation. The natural oxidized silicon wafers were cleaned prior to use by the RCA method.22 The wafers were then immersed in an aqueous solution of poly(ethylene imine) (0.1% (w/w)) for 60 s. After this they were rinsed with water (Millipore) and dried in an air stream. This procedure results in a thin homogeneous coating of the wafers with poly(ethylene imine) with a thickness of about 0.5 nm. A similar value (ca. 0.5 nm) was reported for PEI as a primer layer on quartz and silicon substrates.23 The roughness of the wafers before and after coating with PEI was the same, namely < 0.2 nm, as was proved by X-ray reflectivity. 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 trough area was 1200 cm2, LB deposition film balance model RK3, R&K Ultrathin Organic Film Technology, Germany). The solvent was vaporized, 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 pressuresurface area isotherm, and the surface of the trough was observed simultaneously using Brewster angle microscopy. The HBC was transferred to the PEI coated silicon wafers at a surface pressure of 25 mN/m and with a transfer speed of 6 mm/min. Films of 1, 5, 9, and 15 HBC layers were transferred to the wafers by having the substrate, which was hydrophilic due to the PEI, in the subphase prior to spreading the monolayer. Measurements. Specular X-ray reflectivity and nonspecular (diffuse) X-ray scattering measurements were used to investigate (19) Ito, S.; Wehmeier, M.; Brand, J. D.; Kubel, C.; Epsch, R.; Rabe, J. P.; Mu¨llen, K. Chem.sEur. J. 2000, 6, 4327-4342. (20) Fechtenko¨tter, A.; Tchebotareva, N.; Wantson, M. D.; Mu¨llen, K. Tetrahedron 2001, 57, 3769-3783. (21) Information, supplied by the manufacturer, BASF, specialty chemicals, Ludwigshafen, Germany. (22) Kern, W. RCA Eng. 1983, 28, 99. (23) Koetse, M.; Laschewsky, A.; Jonas, A. M.; Verbiest, T. Colloids Surf., A 2002, 198-200, 275-280.

Figure 2. Surface pressure isotherm of HBC spread on a water surface. The insets (a-d) show Brewster angle micrographs at different points of the isotherm (indicated by arrows). HBC monolayer domains cover the bright areas while dark areas are water only. the vertical d spacing and the lateral height-height correlation of molecular order of the HBC films. These were performed at beamline ID1 at the ESRF in Grenoble, France. The incident angle Ri was equal to the exit angle Rf for the specular reflectivity measurements, and we calculated qz, the wave vector transfer in the z-direction (perpendicular to the film surface), by qz ) 4π/λ sin Ri with the wavelength λ (0.165 nm, the photon energy is 7.5 keV). The incident and exit angles were not equal in the diffuse scattering measurements, and we received the wave vector transfer qx that is in the x-direction (in the x-direction parallel to the film surface) with qx ) 4π/λ(cos Rf - cos Ri). Polarized UV-vis absorption spectra were recorded on a photodiode array spectrometer (HP 8452A) equipped with a polarizer. The scanning force microscopy (AFM) was performed with a Nano Scope IIIa microscope (Digital Instruments, Santa Barbara, CA), operating in tapping mode. The instrument was equipped with a 10 µm × 10 µm E-scanner and commercial silicon tips (model TESP, the force constant was 50 N/m, the resonance frequency was 300 kHz, and the tip radius was smaller than 20 nm).

Results and Discussion Initially we focused on the most appropriate conditions to transfer the amphiphilic HBC to the silicon wafers by measuring surface pressure isotherms of its Langmuir films on water. It can be seen in Figure 2 that the surface pressure isotherms of monolayers of the amphiphilic HBC are of a characteristic shape. The reduction of the mean area per molecule from 1.00 to 0.75 nm2 is accompanied by a pressure increase from zero to 25 mN/m while compressing the film at the surface. A sharp point of inflection is then observed in the curve at further

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Figure 3. X-ray reflectivity of a HBC monolayer which is anchored via poly(ethylene imine) to a silicon wafer surface (circles). The solid line is a simulation of the data based on an optimized density profile. The inset shows the density profile of the HBC monolayer perpendicular to the surface of the wafer in which the cores are separated in columns from the alkyl chains.

compression, followed by an increase of the surface pressure to 40 mN/m at a mean area of 0.25 nm2 per molecule. The process of compression was reversible for compressions up to 25 mN/m but irreversible when the pressure exceeded this value. It can be seen in Figure 2, for example, that the surface pressure drops sharply down to zero when expanding the film after its compression to 40 mN/m. The surface pressure measurement was simultaneously accompanied by Brewster angle microscopy to monitor the morphological changes in the monolayer. It was found that monolayer domains of HBC molecules formed and grew together while the pressure was increased (see Figure 2a). A closed and homogeneous film of HBC is formed at the water/air surface at a pressure of 25 mN/m (see Figure 2b). A collapse of the HBC monolayer was found for pressure values higher than 25 mN/m, accompanied by the formation of irregular stacks of HBC domains (not shown). These stacked domains ruptured immediately and formed irregular shaped domains when the film was expanded (see Figure 2c and d). We concluded from the combination of the surface pressure measurements and Brewster angle microscopy that the amphiphilic HBC requires an average molecular area of about 0.75 nm2 in a dense packed Langmuir monolayer. No indications of further phases were found. This is in contrast to the phase behavior of an analogous HBC with linear side chains (n-dodecyl).16 Its monolayers reveal a noncrystalline lowpressure phase and a crystalline high-pressure phase. We explain the absence of a crystalline phase in our amphiphilic HBC (no crystalline reflections were found in the bulk material by wide-angle X-ray scattering) as the result of the branched side chains that suppress crystallinity due to steric hindrance. We transferred HBC monolayers to silicon wafers (films of different layer numbers) at a pressure of 25 mN/m (the surface per HBC molecule was 0.75 nm2, and the temperature was 20 °C). These were the optimum conditions at which the most homogeneous HBC monolayers were observed at the water/air interface. Since we were interested in the internal structure of the films that we prepared by the Langmuir-Blodgett technique, X-ray reflectivity measurements with synchrotron radiation were performed on the mono- and multilayers for this purpose. Film Structure. The X-ray reflectivity of an HBC monolayer as prepared by the Langmuir-Blodgett technique is shown in Figure 3. Pronounced maxima and

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minima are present in the curve which indicate that the film is of uniform thickness and therefore transferred successfully from the water/air surface to the silicon wafer surface. The total film thickness was estimated from the distance of the first two minima to be 3.70 ( 0.05 nm. More information on the film structure could be extracted from a quantitative simulation of the reflectivity curve. We had first to find an appropriate electron density profile in the direction perpendicular to the surface of the wafer. Here we used a “box model” where boxes of uniform thickness and electron density, respectively, approximate the various structural parts of the molecule. The scattering intensity at a particular qz is obtained from the square of the sum of the individual reflectivity amplitudes of boxes, considering the respective path differences (phases) of the incoming and outgoing waves. In particular, we used a modified Parratt formalism.24,25 For organic material with low density contrast, the main problem consists of the ambiguity of the evaluated electron density profile.25 This problem was overcome using an a-priori structural model as input, where the parameters were refined by a comparison between simulation and experiment. This a-priori model assumes the HBC in which the aromatic cores of HBC are standing perpendicular to the wafer surface (cf. Figure 1). The middle region of the HBC layer contains the aromatic cores (mean electron density of Fel ) 450 nm-3). The thickness of this sublayer was assumed preliminarily to be 1.24 nm, which is close to the theoretical value of the aromatic HBC cores.26 The regions between the wafer surface and the HBC cores as well as between the cores and the air are filled with alkyl chains. We determined the electron densities to be Fel ) 350 and Fel ) 280 nm-3, respectively. The latter is lower than the former because of some roughness at the air/film surface. The thicknesses of the two alkyl-chainrich sublayers were set to 0.63 nm. Furthermore, we had a 0.68 nm thick PEI layer and a 0.5 nm thick layer of natural silicon oxide. We used the electron densities of each sublayer as fitting parameters for the simulation of the scattering curve. It can be seen in Figure 3 that the simulated curve (solid line) matches well with the measured curve (circles). Further, we calculated the mass density profile of the HBC monolayer from its electron density profile by F ) (FelM)/(NANE), where Fel is the electron density, NA is the Avogadro number, NE is the number of electrons per molecule, and M is the molar weight. The result is shown in Figure 3 (inset). We found that the mass density distribution within the Langmuir-Blodgett film is similar to that reported for the bulk material by Rabe and Mu¨llen et al.27 They determined the volume of the aromatic core to be Vcore ) 0.41 nm3 and the mass density to be 2.1 g‚cm-3. This value is about 40% higher than that of the cores in the HBC monolayer (1.46 g‚cm-3), which is understandable as resulting from a lower packing order of the cores in the monolayer. We determined the surface roughness of the HBC films to be σ ) 0.17 nm by simulation. The electron density of each sublayer was kept constant, and we varied only the thickness and the interface roughness of each sublayer. We also kept the total film thickness constant, which was (24) Parratt, L. G. Phys. Rev. 1954, 95, 359. (25) Poloucek, P.; Pietsch, U.; Geue, T.; Symietz, Ch.; Brezesinski, G. J. Phys. D: Appl. Phys. 2001, 34, 450. (26) Keil, M.; Samorı´, P.; dos Santos, D. A.; Kugler, T.; Stafstro¨m, S.; Brand, J. D.; Mu¨llen, K.; Bre´das, J. L.; Rabe, J. P.; Salaneck, W. R. J. Phys. Chem. B 2000, 104, 3967. (27) Ito, S.; Wehmeier, M.; Brand, J. D.; Ku¨bel, C.; Epsch, R.; Rabe, J. P.; Mu¨llen, K. Chem. Eur. J. 2000, 6, 4327.

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Table 1. Structure Parameters of the Films of HBC Multilayers at Silicon Wafers As Determined by X-ray Reflectivity 5 layers

9 layers

15 layers

thickness per layer d, nm film thickness D, nm vertical correlation length Lz, nm

(2.43 ( 0.02) (12.3 ( 0.1) (12.2 ( 0.8)

(2.42 ( 0.02) (22.3 ( 0.2) (22.1 ( 3.6)

(2.36 ( 0.02) (34.6 ( 0.3) (36.7 ( 3.3)

sublayer thickness (alkyl), nm sublayer thickness (core), nm mean electron density (alkyl), nm-3 mean electron density (core), nm-3 thickness per layer, nm film thickness, nm roughness σ, nm

Simulation Results 0.59 1.2 380 510 2.38 12.2 0.29

0.62 1.14 330 510 2.38 22.2 0.36

0.58 1.19 330 480 2.35 36.0 0.39

Figure 4. X-ray reflectivity curves of films (circles) with 5, 9, and 15 layers of HBC. The solid lines of a-c are the corresponding simulations.

known from the first simulation. Using these conditions, we calculated a thickness of 1.22 nm for the HBC cores and an electron density of Fel ) 490 nm-3. The thickness of each alkyl layer was 0.69 nm with an electron density of Fel ) 300 nm-3 for the top layer (between cores and air) and Fel ) 340 nm-3 for the bottom layer (between cores and wafer). The reflectivity curves of the films with 5, 9, and 15 HBC layers, as shown in Figure 4, display Kiessig fringes that indicated a smooth film/air interface. Bragg peaks were also found that result from the stacking of the single layers. The single layer thicknesses were calculated from the position qz,max of the peaks to be d ) 2π/qz,max, resulting in d ) 2.43 ( 0.02 nm (5 layers), 2.42 ( 0.02 nm (9 layers), and 2.36 ( 0.02 nm (15 layers). The total film thicknesses as determined from the distance of the minima of the Kiessig fringes were 12.3 ( 0.1 nm (5 layers), 22.3 ( 0.2 nm (9 layers), and 34.6 ( 0.3 nm (15 layers). Further, the vertical correlation lengths, Lz, were estimated from the fwhm, ∆qz, of the Bragg peaks using the approximation Lz . 2π/∆qz to be 12.2 ( 0.8 nm (5 layers), 22.1 ( 3.6 nm (9 layers), and 36.7 ( 3.3 nm (15 layers). A summary of the data is given in Table 1. These data prove that the vertical correlation lengths were close to the total film thicknesses. Therefore, we conclude that the HBC layers scatter coherently as a result of a complete transfer of the HBC monolayers from the water subphase to the silicon wafer. Note that the ratio of correlation length to the total film thickness is constantly close to one, which is independent of the number of the transferred layers. Any imperfect transfer would result in a ratio smaller than one, which further would decrease with the number of layers. We expanded our structure model of the monolayer to multilayers by adding an appropriate number of HBC layers to fit the reflectivity data. Again, we applied the Parratt procedure to refine the parameters of a box model. The best fits, in which the HBC cores were aligned

Figure 5. Density profiles of films with 5 layers (dotted line), 9 layers (dashed line), and 15 layers of HBC (solid line).

perpendicular to the wafer surface, are shown in Figure 4. A summary of the sublayer thicknesses and the mean electron densities of the films with 5, 9, and 15 HBC layers is given in Table 1. It can be seen in the corresponding simulations (cf. Figure 5) that the electron densities of the sublayers containing the HBC cores and also the ones with the alkyl chains fluctuate around a mean value. Furthermore, the top layer at the film/air interface has a slightly higher electron density than the layer below. One would expect intuitively that the electron density decreases slowly from the bottom (wafer) to the top of the film (air) due to an insufficient film transfer during the sample preparation. But this is not the case here, as proved by the very high vertical correlation of the layers. It must be said that the observed small fluctuations in the density profile around their mean values could be a result of the limitations of our model. The surface roughness of the films was also determined using the box model simulation and found to be 0.29 nm (5 layers), 0.36 nm (9 layers), and 0.39 nm (15 layers) (cf. Table 1). These values are very small, increase only slightly with the increasing number of layers, as expected, and are in agreement with the proposed high quality of the films. Further information on the film structures was obtained by the determination of the lateral correlation length, which is a measure for the height-height correlation of interface roughness along the film surface. As shown recently,28 the lateral correlation length Lx is proportional to the lateral extension of ordered domains. The diffuse scattered intensities of films with 5 and 15 HBC layers were measured by transversal scans for this purpose at qz ) 1.06 nm-1. An example is shown in Figure 6. The intensity of the central peak at qx ) 0 is composed of the specular and the diffuse scattered intensities which originate from the surface and interfacial roughness. It (28) Geue, T.; Schultz, M.; Englisch, U.; Sto¨mmer, R.; Pietsch, U.; Meine, K.; Vollhardt, D. J. Chem. Phys. 1999, 110, 8104.

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Figure 6. Transversal scan intensity of a film containing five HBC layers (qz ) 1.06 nm-1).

Figure 8. Scattering intensity of a film containing 15 HBC layers as determined by a transversal scan at qz ) 0.929 nm-1.

Figure 7. Half width ∆qx of the diffuse scattering intensity as a function of qz2 measured for 5 HBC layers (circles) and 15 layers (squares). The inset shows an example for the determination of the diffuse scattering intensity and the half width ∆qx.

can be seen that the diffuse scattered intensity of our samples is more than 2 orders of magnitude lower than the specular scattered intensity. One can see further (Figure 7, left) that it appears as the broadening at the feet of the central peak. We calculated the lateral correlation length, Lx, from the half width, ∆qx, by approximating the height fluctuations by Gaussian distributions using Lx ) 2σ2qz2/∆qx with the surface roughness σ.29 A plot of the half width ∆qx as a function of various measurements over qz2 is shown in Figure 7 (right). We obtained values of about 130 nm (5 layers) and 400 nm (15 layers) for the lateral correlation lengths from the slope of the straight lines. The scattering intensities vary with the variation of the incident and exit angles, as shown in Figure 8 for a transversal scan at qz ) 0.929 nm-1 (the number of HBC layers is 15). The Yoneda peak30 was found as a maximum in the region of the outer total reflection of the curve. It appeared for both the incident and exit angles. We obtained the critical angle for total external reflection of the film and of the silicon wafer from the structure of the left wing (incident angle). The electron density was calculated from the critical angle using Fel ) πRc2/(λ2r0) with the wavelength λ and the electron radius r0. We obtained an average electron density of 410 nm-3 for the films from the critical angle of the HBC film (Rc,film ) 0.18°). The average electron density is composed of the electron densities, Fel,i, and the thicknesses, di, of the sublayers i. This can be written as the sum Fel,film ) ΣFel,idi/Σdi. We calculated an average electron density of 400 nm-3 by using the structure (29) Sto¨mmer, R.; Englisch, U.; Pietsch, U.; Holy, V. Physica B 1996, 221, 284-288. (30) Yoneda, Y. Phys. Rev. 1963, 131, 2010.

Figure 9. UV-vis absorption spectra of films containing 1, 5, and 15 layers of HBC with maxima at 360 nm. The inset shows the normalized absorbance as a function of the number of layers and a linear regression.

Figure 10. Absorbance of a film containing 15 HBC layers as a function of the polarization angle.

parameters of the density profiles displayed in Figure 5, which is in good agreement with the experimental value. The electron density of silicon, which was calculated from the critical angle (Rc,silicon) 0.24°), yields 720 nm-3, which is also in agreement with the theoretical value (711 nm-3).31 To verify the low roughness as determined by the X-ray reflectivity measurements (0.29-0.39 nm), AFM measurements were carried out. A typical example of a height profile is shown in Figure 11, which represents the surface of a film of HBC with five layers. It can be seen that the (31) Tolan, M. X-ray scattering from Soft-Matter Thin Films; Springer Tracts in Modern Physics 148; 1999.

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this that the HBC 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 orientation of the disks during the compression, due to the shape anisotropy (flat surface) of the cores. Such orientation effects of disklike molecules were observed earlier for the assembly of amphiphilic phenylacetylene macrocycles32 and also for Langmuir-Blodgett films of phthalocyaninato-polysiloxane.33 Further investigations for optimizing the macroscopic orientation of the HBCs to ordered domains will be carried out in the future. Figure 11. AFM height image of a film containing five layers of HBC (inset) and the height profile of the film from a section of the AFM height profile (straight line, inset).

film has no defects and it is very smooth with a roughness of about 0.4 nm. This is in agreement with the results of the reflectivity measurements. UV-vis Spectroscopy. Optical investigations using UV-vis spectroscopy were performed on films with 1, 5, and 15 HBC layers that were transferred to glass slides using the Langmuir-Blodgett technique. Examples of the spectra in the range 300-500 nm are shown in Figure 9. The absorbance increases with the number of layers, and it was normalized for comparison with the average absorbance of one layer (cf. Figure 9, inset). It was found that the normalized absorbance decreases with the number of layers. This means that the preparation of the layers on glass is not as good as that on the silicon wafers. Nevertheless, we calculated the transfer rate from the Langmuir-Blodgett trough from the linear regression on average to be 0.98, which is still high, when compared to those of other amphiphilic molecules of similar size. The macroscopic orientation of the HBC molecules in the 15-layer sample was investigated with the help of polarized UV-vis spectroscopy. The absorbance at different polarization angles was measured at a wavelength of 360 nm. A polarization angle of 0° means that the polarization plane is parallel to the dipping direction of the Langmuir-Blodgett trough. The film shows a dichroism with a maximum of the absorbance at a polarization angle of 90°, as shown in Figure 10. We concluded from

Conclusions It has been shown that an amphiphilic hexa-perihexabenzocoronene can be used for the preparation of thin films using the Langmuir-Blodgett technique. The films were deposited on silicon wafers by poly(ethylene imine) as an anchor and consist of mono- and multilayers. Each layer is well defined in its thickness and lateral order. It contains columns that are formed by the aromatic cores of the molecules. The columns of one layer are separated from those of the next layer by alkyl chains. Preliminary experiments show that the formation of macroscopically ordered domains of HBC is possible. Acknowledgment. We would like to thank Petr Mikulı´k for his X-ray reflectivity program which was used to simulate and fit the X-ray reflectivity data. We thank Dimitri Grigoriev for performing the Brewster-angle microscopy and Thomas Fischer for the UV-vis measurements. We also thank the beamline scientists at ESRF for beam time and support. The financial support of the Max Planck Society and the Fraunhofer Society, the Zentrum fu¨r Multifunktionelle Werkstoffe und Miniaturisierte Funktionseinheiten (BMBF 03N 6500), EU-TMR project SISITOMAS, and EU project DISCEL (G5RD-CT-200000321) is gratefully acknowledged. LA026968L (32) Shetty, A. S.; Fischer, P. R.; Stork, K. F.; Bohn, P. W.; Moore, J. S. J. Am. Chem. Soc. 1996, 118, 9409-9414. (33) Silerova, R.; Kalvoda, L.; Neher, D.; Ferencz, A.; Wu, J.; Wegner, G. Chem. Mater. 1998, 10, 2284-2292.