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Preparation of Highly Stable Organic Steps with a Fullerene-Based Molecule Stephan Burghardt,† Andreas Hirsch,*,† Nicolas Medard,‡ Refahi Abou Kachfhe,‡ Dominique Aussere´,‡ Marie-Pierre Valignat,§ and Jean-Louis Gallani*,| Institut fu¨ r Organische Chemie, Lehrstuhl II, Universita¨ t Erlangen-Nu¨ rnberg, Henkestrasse 42, D-91054 Erlangen, Germany, NANORAPTOR S.A., Parc des Sittelles, 72450 Monfort-Le-Gesnois, France, Colle` ge de France, 11 place Marcelin Berthelot, 75005 Paris, France, and Institut de Physique et Chimie des Mate´ riaux de Strasbourg, UMR7504, 23 rue du Lœss, BP43, 67034 Strasbourg Cedex 2, France Received May 16, 2005. In Final Form: June 9, 2005 We report the formation of highly stable Langmuir-Blodgett (LB) organic steps made with a hexaadduct fullerene-based amphiphile. This amphiphile forms films of excellent quality, with a very low roughness, that are structurally stable: X-ray reflectivity spectra recorded on fresh and 12-month-old samples are undiscernible. Such a behavior contrasts with that of more traditional amphiphiles, which are unfortunately well-known for their instability in time. The stability of the films stems, among others, from the spheroidal shape of the constitutive molecules. These experiments show that it is possible to circumvent the major drawback of LB films and to prepare materials more suited for applications. We show that the LB film prepared with this fullerene derivative can successfully be used as thickness gauges for atomic force microscopy or light microscopy studies.
Introduction At the “LB9” international conference held in Potsdam, Germany, in 2000, J. Israelachvili gave a lecture on the history of Langmuir and Langmuir-Blodgett (LB) films in which he said: “For Langmuir-Blodgett films, applications are at the corner of the street but are going to stay there”, quoted from memory. True, all researchers in the field must have experienced that feeling that “something” very nice should be feasible with these elegant architectures and the molecules so precisely organized within but, unfortunately, so fragile and unstable at the same time. The sensitivity of LB films to scratches and physical damage is probably incurable because they are made from soft matter, but this fact should not prevent their use in protected setups or environments. The main drawback is the instability of multilayered films,1,2 in which most of the time, the whole structure is held together only by forces such as van der Waals and hydrogen bonds. Well are aware of the problem that K. Blodgett herself managed to improve with the stability of LB films using a process called skeletonization3 and, indeed, used these films for one of the very few applications that eventually went onto the market: color gauges for asserting the thickness of thin films. This quest for the stability of LB films has since led to numerous publications dealing with how molecules do rearrange with time. As should probably have been expected, there are as many mechanisms as amphiphiles but fatty acids have been especially scrutinized.4-7 Several rather efficient ways of stabilizing mul* To whom correspondence should be addressed. E-mail:
[email protected] (A.H.); gallani@ ipcms.u-strasbg.fr (J.-L.G.). † Universita ¨ t Erlangen-Nu¨rnberg. ‡ NANORAPTOR S.A. § Colle ` ge de France. | Institut de Physique et Chimie des Mate ´ riaux de Strasbourg. (1) Ulman, A. Ultrathin Organic Films; Academic Press: New York, 1991. (2) Collective Langmuir-Blodgett Films; Plenum Press: New York, 1990. (3) Blodgett, K. B.; Langmuir, I. Phys. Rev. 1937, 51, 964.
tilayered LB films have been proposed.8 One strategy is to pile up unorganized layers, that is, layers made of polymeric materials. With such films, the intralayer ordering of the molecules is obviously not so good. Therefore, it cannot degrade, and this can be a problem for some applications, e.g., nonlinear optics.9 A second strategy is to cross-link the layers, with this cross-linking process usually taking place only within the layers. The problem here is that it is not always possible to design a cross-linkable molecule bearing the desired function. D. Talham has nicely shown that the introduction of inorganic layers can bring a large gain of stability, in addition to bringing interesting properties such as ferromagnetism.10 More recently, the interesting concept of “glued LB films” has been developed, and this provides new opportunities, again, for LB films.11 The idea is to ionically cross-link a polycationic amphiphile with a polyanionic polymer. A minor inconvenience is the dilution effect of any property that the amphiphile has by the layer of polymeric glue. Such LB films are expected to be very stable against postdeposition molecular reorganization, contrary to fattyacid-based ones. Still, there are no data available to date on the aging process in such materials, and only bilayers have been deposited so far.12 The amphiphile, which behavior shall be described hereafter, forms inherently (4) Gyo¨rvary, E.; Peltonen, J.; Linden, M.; Rosenholm, J. B. Thin Solid Films 1996, 284-285, 368. (5) Kondrashkina, E. A.; Hagedorn, K.; Vollhardt, D.; Schmidbauer, M.; Ko¨hler, R. Langmuir 1996, 12, 5148. (6) Mikrut, J. M.; Dutta, P.; Ketterson, J. B.; MacDonald, R. C. Phys. Rev. B 1993, 48, 14479. (7) Takamoto, D. Y.; Aydil, A.; Zasadzinski, J. A.; Ivanova, A. T.; Schwartz, D. K.; Yang, T.; Cremer, P. S. Science 2001, 293, 1292. (8) We exclude from this discussion self-assembled polymeric films such as those invented by Decher, G. Science 1997, 277, 1232. (9) Bosshard, C.; Sutter, K.; Preˆtre, P.; Hulliger, J.; Flo¨rsheimer, M.; Kaatz, P.; Gu¨nter, P. Organic Nonlinear Optical Materials; Gordon and Breach: Amsterdam, The Netherlands, 1995; Vol. 1. (10) Culp, J. T.; Park, J. H.; Stratakis, D.; Meisel, M.; Talham, D. R. J. Am. Chem. Soc. 2002, 124, 10083. (11) McCullough, D. H.; Regen, S. L. Chem. Commun. 2004, 2787. (12) McCullough, D. H.; Janout, V.; Li, J.; Hsu, J. T.; Truong, Q.; Wilusz, E.; Regen, S. L. J. Am. Chem. Soc. 2004, 126, 9916.
10.1021/la051297n CCC: $30.25 © 2005 American Chemical Society Published on Web 07/07/2005
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Figure 1. Structure of the molecule.
stable LB films mostly because of its shape. The molecule uses fullerene C60 as a core and has a dendritic polar head at one pole and 10 alkyl chains attached around the equator and to the other pole of the fullerene sphere. The overall shape of the molecule is therefore spheroidal. This simple fact makes it very difficult for the molecules to destroy the layers upon rearranging, as compared to more traditional amphiphiles such as fatty acids with their cylindrical or conical shape. Organic structures with 10, 20, 100, and even 250 steps have been prepared successfully and characterized by X-ray reflectivity, atomic force microscopy (AFM), and a new promising optical technology called Sarfus. Experimental Procedures (1) Synthesis. The full description of the synthesis is given in the Supporting Information. A sketch of the molecule is given in Figure 1. (2) Langmuir Films. Isotherm data were collected using a Teflon trough and symmetrical hydrophilic barriers. The trough was set in a Plexiglas enclosure so as to be protected from drafts and dust, and temperature was controlled to (2 °C. All isotherms were taken at 20 °C. The ultrapure water (F ) 18.2 MΩ cm) used for the subphase was obtained from a Milli-RO3Plus/Milli-Q185 Ultrapurification system from Millipore. Surface pressure was measured by means of a platinum Wilhelmy plate. Solutions at ≈1 mg mL-1 concentration were prepared using chloroform (analysis grade from Carlo Erba). Usually 50 µL of these solutions were spread on the water surface using a microsyringe. Films were left to equilibrate for 15 min before any measurement started. The monolayers were compressed at typical speeds of 5 Å2 molecule-1 min-1, and compression isotherms showed little dependence on the compression speed. As long as the collapse point was not reached, the isotherms showed limited hysteresis, provided that the decompression was not too fast (less than 5 Å2 molecule-1 min-1). Last, the surface pressure stayed stable if the compression was paused. Brewster angle microscopy (BAM) pictures have been recorded on a Bam2Plus from NFT.13 The observations are made at Brewster angle (around 53.15° for an air-water interface) incidence; hence, the limited depth of focus of the objective makes it impossible to have an entirely sharp picture. When the film under study is motionless, it is possible to scan the entire field while adjusting the film-objective distance. Unfortunately, the films usually are quite mobile; hence, the photographs are usually taken as one snapshot. Consequently, the focus is more and more out when going from the center of a picture to the upper and lower edges, and this effect is clearly visible on some pictures. The images presented here typically show a 500 × 500 µm area. (13) NFT-Nanofilm; Goettingen, Germany; www.nanofilm.de.
(3) LB Films. LB films were obtained by transfer on hydrophobic silicon wafers (100) at a surface pressure of 40 mN m-1. Transfers started from above the water surface, with a typical vertical speed of 1 mm min-1. The silicon wafers were first cleaned by treatment in an oxidizing mixture HNO3-H2O2 (1:2, v/v), followed by several rinsings in pure water. Subsequent silanization makes the substrates hydrophobic. The cleaned substrates are exposed to vapors of octadecyltrichlorosilane for 2 h, heated to 100 °C in an oven for 2 h, and rinsed with chloroform. All treatments were done in an ultrasonic bath. (4) Grazing Incidence X-ray Analysis. The grazing incidence X-ray studies of LB films were performed on a spectrometer operating with a ceramic X-ray tube and equipped with a nickel β filter, a programmable divergence slit (1/32°), a parallel plate collimator, a flat Ge monochromator, and a proportional Xe detector. The Cu KR line (wavelength ) 1.541 Å) was used. (5) Optical Analysis. A new imaging technique (Sarfus, Nanoraptor, France) based on the combination of an optical microscope and contrast-enhancing supporting plates (called Surfs) has been employed. It increases by 1 or 2 orders of magnitude of the sensitivity of traditional optical microscopy, allowing direct visualization of nanometric structures. It provides real-time detection and imaging of particles as small as 10 nm in diameter and cylindrical objects down to 2-5 nm in diameter and images and measures films with a vertical resolution better than 1 nm. Three-dimensional visualization (Blossom technique) is possible through the calibration of the color/thickness correspondence of Sarfus pictures. Each color on the Sarfus picture then corresponds to a film thickness on the Surf. To ensure a correspondence between the color and thickness in any case, a calibration standard made of nanometric steps and specific software developed by Nanoraptor are used. Optical observations have been carried out on a reflection optical microscope Leica DM4000. The sample is deposited on the Surf in exactly the same way as on a classical microscope glass slide or silicon wafer.
Results and Dicussion The compression isotherm of the compound is given in Figure 2. The surface pressure π starts to rise at molecular areas A of ca. 350 Å2 and steadily increases until the collapse of the film, which occurs at A ≈ 235 Å2 and πc ≈ 44mN m-1. The final molecular area extrapolated to zero surface pressure is Af ≈ 300 Å2. A crude estimate of the minimal area that such a large molecule requires can be made as follows: the fullerene core needs ∼100 Å2; each of the 8 equatorial alkyl chains needs ∼25 Å2; therefore, the molecule as a whole needs around 300 Å2, which is what we observe. These numbers also are very similar to those previously reported for a parent molecule.14 The
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Figure 2. Isotherm of the compound. Inset shows reversibility of two successive compression-expansion cycles. Figure 4. BAM image taken at A ) 650 Å2.
Figure 3. BAM image taken at A ) 650 Å2.
high collapse pressure indicates that the molecules are strongly anchored at the interface and that the hydrophilic-hydrophobic balance is good.15 The dendritic polar head certainly helps to form nice films,14,16 as do the encapsulation of the C60 core with alkyl chains.17 The compressibility of the film and the reversibility of the isotherm (inset in Figure 2), corroborated by the BAM observations, indicate that the film is in a liquid condensed state. Last, the films are stable in the sense that the surface pressure remains constant if the compression is paused. This means that the molecules do not dissolve into the subphase and that the film withstands continuous pressure without buckling. BAM of the Langmuir films reveals that the film is always in a liquid state, with the molecules spontaneously forming large homogeneous assemblies, even at large molecular areas (A > 650 Å2). Figures 3-8 are BAM pictures taken at various stages of the compression isotherm Figure 9 is a picture taken during expansion. On these BAM pictures, the water surface appears black and the film is gray/white. The film domains often form unusual peduncles (Figure 3), maybe the sign that they could expand further and form a liquid expanded or gas phase over time. Water evaporation prevented us from testing this hypothesis; at most, our observations took place within 3 h after spreading. Figure 3 shows the typical aspect of a film at large molecular areas. The smooth curved boundaries of the domains are proof of the liquid state of the Langmuir film. (14) Maierhofer, A. P.; Brettreich, M.; Burghardt, S.; Vostrowsky, O.; Hirsch, A.; Langridge, S.; Bayerl, M. Langmuir 2000, 16, 8884. (15) Zhang, S.; Rio, Y.; Cardinali, F.; Bourgogne, C.; Gallani, J.-L.; Nierengarten, J.-F. J. Org. Chem. 2003, 68, 9787. (16) Nierengarten, J.-F.; Eckert, J.-F.; Rio, Y.; Carreon, M. d. P.; Gallani, J.-L.; Guillon, D. J. Am. Chem. Soc. 2001, 123, 9743. (17) Felder, D.; Gutie´rrez Nava, M.; Carreon, M.; Eckert, J.-F.; Luccisano, M.; Schall, C.; Masson, P.; Gallani, J.-L.; Heinrich, B.; Guillon, D.; Nierengarten, J.-F. Helv. Chim. Acta 2002, 85, 288.
Figure 5. BAM image taken at A ) 520 Å2.
Figure 6. BAM image taken at A ) 460 Å2. The fringes are artifacts of the laser illumination.
Figure 5, taken at 520 Å2, shows the smooth merging of domains, as does Figure 6, taken at 460 Å2. The film eventually becomes defectless at A ≈ 300 Å2, as can be seen in Figure 7. Figure 8 shows the collapse of the film, which seems to occur via expulsion of molecules from the water along fracture lines. This observation is consistent with the high collapse pressure and the large number of hydrophobic chains, which would cost too much energy to get into the water. Figure 9, taken during expansion of a film compressed up to πc, confirms that the film is in a liquid state and expands via the formation of peduncles, again. The edges have a sloping thickness, as indicated by their darker shade of gray. The molecule transfers very nicely onto hydrophobic substrates. Up to 250 layers have been easily deposited on silicon or silica plates coated with silane, at a pressure of 40 mN m-1. The transfer ratio is a steady 1, and the
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Figure 7. BAM image taken at A ) 300 Å2 and π ≈ 15mN m-1 showing a perfect homogeneous film.
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Figure 10. UV-vis absorption spectra of LB films having 1, 2, 6, 16, and 44 layers on each side of the fused silica plate. Inset is a plot of the absorption at 273 nm versus the number of layers on each side of the silica plate.
Figure 8. Collapse of the film, A ) 200 Å2 and π ≈ 52mN m-1.
Figure 11. X-ray reflectivity of LB films. The main graph is the spectrum of a 10-layer-thick LB film (black curve) together with the best fit (red curve). Inset is the spectrum of a 30layer-thick LB film taken immediately after deposition (black curve) and the spectrum of the same film after 12 months of storage (red curve).
Figure 9. Expansion of the film, A ≈ 380 Å2.
amount of deposited material grows nicely in a linear fashion with the UV-vis absorption, as can be seen in Figure 10. Preparation of the thicker samples took about 3 days, with several reloadings of the trough. The quality of the LB films can be better assessed with grazing incidence X-ray reflectivity. The results are summarized in Figure 11. The main graph shows the spectrum of a 10-layer-thick LB film on silanized silicon. The data have been fitted with a box model. Each molecular layer has been divided into three sublayers: hydrophilic head, core, and hydrophobic tail, with each sublayer being ascribed a specific thickness, electronic density, and roughness. One monomolecular layer is therefore described with nine parameters. The electronic density and roughness of the layers deposited during the up- and downstroke motions are different, but the thickness of each sublayer stays constant. Consequently, a set of 12 instead of 18 parameters is
required to fully describe each deposited bilayer. This number may look large, but still, these 12 parameters were kept identical in all layers, meaning that the 10layer film has been fitted as a whole with only 12 parameters instead of 90. The result, quite convincing as can be seen in Figure 11, means that the quality of the layers remains constant throughout the sample. In these LB films, each layer is, on average, 28.2 ( 0.6 Å thick. The rather large incertitude given corresponds to an interval containing 90% of the samples tested. For a given sample, the incertitude on the thickness of the layers is on the order of 0.1 Å; the different samples gave values between 27.6 and 28.7 Å. More details on the fit parameters are given as Supporting Information together with a schematic drawing of the structure of the film. The inset in Figure 11 compares two spectra recorded on the same LB film, immediately after deposition and after 12 months. The film has been stored without any precaution on a shelf, at room temperature (no air conditioning in the storage room). The two spectra are undiscernible, which proves the excellent stability of the film, because a change of 0.5 Å in thickness would have been easily detected. This excellent stability in time for a noncross-linked or covalently bond LB film is essentially the consequence of the spheroidal shape of the molecules (see the molecular dynamics simulations in the Supporting Information) that make them pack like marbles in a box.
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Figure 12. AFM scan of a multilayered LB film. The scan shows three steps, with each one corresponding to a bilayer. The picture is 75 × 60 µm, and the full vertical scale is 50 nm. The saw tooth appearance of the steps is an artifact from the plane compensation of the atomic force microscope.
In addition to that are the possibility of forming hydrogen bonds between the hydrophilic heads of adjacent layers and the possibility of dipole-dipole interactions between the carbonyle groups at the substitution site on C60. The first point is supported by the observation of a band at 3340 cm-1 in the IR spectrum of a multilayer LB film and has already been reported18 for other fullerene-based amphiphiles. For the molecule under study bearing no functional group of immediate interest, we have investigated its potential use as a thickness gauge for AFM or light microscopy studies. The intrinsic stability of the films makes them suited for such an application: the layers are flat to within a few angstroms, and their thickness is constant and has the right order of magnitude. AFM studies have been conducted on multilayer samples, just at the silicon/LB film boundary. In this region, the LB technique inherently create “steps”, essentially because the water of the Langmuir trough slowly evaporates when the electronic device13 controlling the water level is switched off. Selective reflection of white light by the layers produces a rainbow effect on this part of the film, easily seen with the naked eye (see the picture in the Supporting Information). The observations confirm the presence of these steps, with each one actually corresponding to a bilayer (Figure 12). The image shows that the steps are very flat and that their edges are clearly defined. After calibration of the AFM piezo tube with a calibration standard provided by the manufacturer, an average edge thickness of 60 Å is measured. This makes each monolayer 30 Å thick, a value in excellent agreement with the 28.2 Å measured with the X-ray. Conversely, the LB film itself could be used as a calibration tool. We have further tested our LB films regarding their potential use as thickness gauges with a new optical microscopy technique. As explained in the Experimental Procedures, this technique makes use of specific contrast enhancement substrates. Because the outmost surface of these substrates is amorph silica, they are responsive to silanization and consequently permit the deposition of our fullerene amphiphile. Figure 13 show a typical image obtained with the technique, with the edges of the bilayers being easily (18) Felder, D.; Gallani, J.-L.; Guillon, D.; Heinrich, B.; Nicoud, J.F.; Nierengarten, J.-F. Angew. Chem. Int. Ed. 2000, 39, 201.
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Figure 13. Microscopic observation of a multilayered LB film with the Sarfus microscopy technique. The edge of the bilayers is clearly seen.
Figure 14. Three-dimensional rendering of the microscopic observation.
observed. This effect can be enhanced using a 3D rendition software, as shown in Figure 14. Thanks to a large lateral visualization, Sarfus allows us to simultaneously observe more than 10 steps of LB bilayers. The height of the different steps can be extracted from the 3D image (see the Supporting Information), giving a value of 63 ( 7 Å (bilayer thickness), in good agreement with all other measurements. Similar to the AFM, such LB films can conversely serve as precise calibration tools for this observation technique. AFM and Sarfus analyses show that the LB films prepared with this fullerene derivative have the desired characteristics for serving as thickness gauges: low roughness, well-defined edges, and size of the steps in the good range. In our samples, we did not try controlling the lateral spacing of the steps, but should this feature be desirable, a precise control of the water level and/or mechanical dipper would be enough for controlling this parameter. Conclusion We have prepared and studied multilayered LB films made with a hexa-adduct fullerene-based amphiphile. These films are structurally stable for at least 12 months in a standard laboratory environment. The layers have a low roughness, and their edges can be used as thickness calibration gauges for techniques such as AFM or optical microscopy. Supporting Information Available: Synthesis of the molecule, details of X-ray analysis, schematic drawing of the multilayered films structure, molecular shape from computer simulations, visual aspect of LB film, and steps height calculation with the Blossom technique. This material is available free of charge via the Internet at http://pubs.acs.org. LA051297N
