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Direct Topographic Measurement of Multilayers on Water by Atomic Force Microscopy Fre´de´ric Dubreuil,†,§ Jean Daillant,†,‡ and Patrick Guenoun*,† Service de Physique de l’Etat Condense´ (SPEC), CEA/Saclay, 91191 Gif Sur Yvette Cedex, France, Max-Planck-Institut fu¨ r Kolloid und Grenzfla¨ chenforschung (MPIKG), 14424 Potsdam, Germany, and LURE, CNRS/CEA/ MJENR, Baˆ timent 209D, Centre Universitaire Paris-sud, B.P. 34, 91898 Orsay Cedex, France Received February 25, 2003. In Final Form: June 26, 2003 We report a new experimental method allowing the direct observation of Langmuir films at the air/water interface by atomic force microscopy. Using this method, a quantitative observation of multilayer formation in various systems, such as gold nanoparticles and polymerized octadecyltrichlorosilane layers, could be achieved. In particular, a high vertical resolution is reached (around 1 nm) which enables topographic measurements on Langmuir multilayers in the submicrometer range.
* Corresponding author. E-mail:
[email protected]. † SPEC. ‡ LURE. § Present address: MPIKG.
density magnetic storage.10 We then report on the mechanism of collapse of a monolayer.11 Here both the dynamics of the growth12 and the large variety of shapes13-15 of the observed multilayered domains remain poorly understood. In this respect, the use of an atomic force microscope (AFM) is very appealing. It offers the required in situ local and nonperturbative investigation, directly at the air/water interface, at a resolution beyond conventional light microscopy and gives access to reliable threedimensional topography of the surface. In principle, AFM could outline the existence of any structure in the film, and even provide temporal information about equilibration. Results obtained by this technique are complementary to surface scattering techniques (X-rays or neutrons) in reciprocal space, giving statistical results averaged over the structure of the film with a comparable vertical range in structural information. High-resolution topography scans, at both solid/air and solid/liquid interfaces, are now routinely obtained on various samples using AFM. Nevertheless, fluid surfaces or surfaces of soft materials are more delicate to observe because of the surface distortion exerted by the tip-surface interaction. An extreme and destructive case is the formation of a capillary neck induced by wetting of the tip.16 This problem can be partially overcome by using the so-called intermittent contact (“tapping”) mode where the oscillating tip touches the surface only briefly. This mode was used recently to image polymer droplets on hard surfaces17 but is quite limited to systems with low relaxation times. To go a step further and study surfaces of low-viscosity fluids, several attempts have been made with near-field scanning optical microscopy (NSOM) using interferometric
(1) Dutta, P.; Peng, J. B.; Lin, B.; Ketterson, J. B.; Prakash, M.; Georgopoulos, P.; Ehrlich, S. Phys. Rev. Lett. 1987, 58, 2228. (2) Lo¨sche, M.; Mo¨hwald, H. Rev. Sci. Instrum. 1984, 55, 1968. (3) He´non, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936. Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590. (4) Bourdieu, L.; Daillant, J.; Chatenay, D.; Braslau, A.; Colson, D. Phys. Rev. Lett. 1994, 72 (10), 1502-1504. (5) Fontaine, P.; Daillant, J.; Guenoun, P.; Alba, M.; Braslau, A.; Mays, J. W.; Petit, J. M.; Rieutord, F. J. Phys. II 1997, 7, 401-407. (6) Saint-Jalmes, A.; Gallet, F. Eur. Phys. J. B 1998, 2, 489-494. (7) Huang, S.; Tsutsui, G.; Sakaue, H.; Shingubara, S.; Takahagi, T. J. Vac. Sci. Technol., B 2000, 18 (6), 2653-2657. (8) Markovich, G.; Leff, D. V.; Chung, S.-W.; Soyez, H. M.; Dunn, B.; Heath, J. R. Appl. Phys. Lett. 1997, 70, 3107-3109. (9) Shimomura, M.; Sawadaishi, T. Curr. Opin. Colloid Interface Sci. 2001, 6, 11-16.
(10) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989-1992. (11) Nikomarov, E. S. Langmuir 1990, 6, 410-414. (12) Hatta, E.; Hosoi, H.; Akiyama, H.; Ishii, T.; Mukasa, K. Eur. Phys. J. B 1998, 2, 347-349. (13) Gourier, C.; Knobler, C. M.; Daillant, J.; Chatenay,D. Langmuir 2002, 18, 9434-9440. (14) Ibn-Elhaj, M.; Mo¨hwald, H.; Cherkaoui, M. Z.; Zniber, R. Langmuir 1998, 14, 504-516. (15) Buzin, A. I.; Godovsky, Y. K.; Makarova, N. N.; Fang, J.; Wang, X.; Knobler, C. M. J. Phys. Chem B 1999, 103, 11372-11381. (16) Luna, M.; Colchero, J.; Gil, A.; Gomez-Herrero, J.; Baro, A. M. Appl. Surf. Sci. 2000, 157 (4), 393-397. (17) Sheiko, S. S.; Muzafarov, A. M.; Winkler, R. G.; Getmanova, E. V.; Eckert, G.; Reineker, P. Langmuir 1997, 13, 4172-4181.
Introduction In recent years, investigations on Langmuir insoluble films at the air/water interface have benefited from the development of a wide range of new techniques such as surface X-ray scattering1 (thanks to new synchrotron sources) and microscopy techniques (fluorescence microscopy2 and Brewster angle microscopy3). The former technique averages over large domains to provide information in reciprocal space, whereas the latter methods are limited in resolution to several micrometers. However, more local investigations of such layers are needed, providing details in the nanometer to micrometer range and avoiding any transfer, which may alter the structure. For example, numerous cases of buckled monolayers have been recently reported4-6 in the literature. Amplitudes of a few angstroms and length scales varying from 20 nm to 20 µm have been reported. A nondestructive, local technique of investigation would yield lacking and valuable information about the dynamics of this phenomenon, the possible multiscale nature of the instability, and its homogeneity. It would also provide valuable information on possible defects in Langmuir films. In this paper, we discuss examples of two other active areas: first, we describe the organization of films formed by encapsulated metal nanoparticles before their transfer onto a solid substrate. These films have attracted a great deal of interest because of their potential application in nanotechnologies such as electronics,7,8 optics,9 and high-
10.1021/la0343268 CCC: $25.00 © 2003 American Chemical Society Published on Web 08/27/2003
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feedback18-20 or evanescent wave coupling.21 Although in a couple of cases good contrast18 or good resolution20 was achieved, NSOM has difficulty providing topographic information20 (distribution of heights). To prevent the formation of the capillary neck, the surface imaging by AFM can alternatively be performed from the water surface using an inverted AFM22 or air bubbles.23 Eng et al.22 thus reported interesting pictures of headgroup distribution, but here also the topography information cannot be obtained. We chose to use the AFM in true noncontact mode24 from the air side in order to get a maximum lateral and height resolution. This operating mode relies on the attractive part of the interaction forces between the tip and the surface and minimizes occurrences perturbing the surface compared to the tapping mode. A minimization of the interfacial movements was also found to be crucial for good control of the attractive force, and a special setup has been designed for this purpose. Measurement artifacts were carefully addressed, and two examples are given for illustration. For the first system, thiol-capped gold nanoparticles deposited at the air-water interface, we show the multilayer formation before transfer on a solid substrate. For the second system, polymerized silane multilayers, we demonstrate the existence of steps with different heights in agreement with the existence of trilayers. Experimental Setup Atomic Force Microscopy. For this study, a dedicated setup coupled to a commercial apparatus (M5 from TM Microscopes) was built. This apparatus was used in the noncontact mode by choosing rather rigid cantilevers (rigidity constants k ∼ 4.5-7.5 N/m) and working at frequencies slightly above the bare resonance value. We used commercially available cantilevers from NT-MDT (Russia) SC12 series, combining a low radius of curvature (∼15 nm) and a high tip length (∼10 µm) to minimize the air damping effects.25,26 Langmuir Trough. A homemade Langmuir trough equipped with commercial electronics (Riegler & Kirstein Gmbh) was specially designed to fit the space below the scanning head of the microscope. This trough has a large area (300 cm2) in addition to a high compression ratio (6.5); both these features are necessary for well-defined, reproducible compression of the monolayer. Film Preparation. Gold hydrophobic nanoparticles were kindly provided by the group of J. P. Bourgoin from the “Service de Chimie Mole´culairesCEA Saclay”. Synthesis of the nanoparticles is described in detail elsewhere.27 The hydrophobicity of these 3 and 5 nm diameter nanoparticles is due to the grafting of thiols at their surface. The nanoparticles were dissolved in chloroform with a concentration of about 10-5 mol/L and were then spread onto the pure water surface (Milli-Q Plus, Millipore). After complete evaporation of the solvent, the monolayer was compressed and the surface pressure was measured by a standard filter paper Wilhelmy balance. Figure 1a shows a common isotherm observed for these nanoparticle monolayers. Reflection (18) Shiku, H.; Dunn, R. C. J. Microsc. 1999, 194 (2-3), 461-466. (19) Kramer, A.; Hartmann, T.; Stadler, S. M.; Guckenberger, R. Ultramicroscopy 1995, 61, 191-195. (20) Kramer, A.; Hartmann, T.; Eschrich, R.; Guckenberger, R. Ultramicroscopy 1998, 71, 123-132. (21) Seaver, M.; Duncan, M. D.; Frost, A. E. Ultramicroscopy 1995, 57, 219-222. (22) Eng, L. M.; Seuret, C.; Looser, H.; Gu¨nter, P. J. Vac. Sci. Technol., B 1996, 14 (2), 1386-1389. (23) Knebel, D.; Sieber, M.; Reichelt, R.; Galla, H.-J.; Amrein, M. Biophys. J. 2002, 82, 474-480. (24) Checco, A.; Guenoun, P.; Daillant, J., to appear in Phys. Rev. Lett. (25) Fontaine, P.; Guenoun, P.; Daillant, J. Rev. Sci. Instrum. 1997, 68 (11), 4145-4151. (26) Leveˆque, G.; Girard, P.; Belaidi, S.; Cohen Solal, G. Rev. Sci. Instrum. 1997, 68 (11), 4137-4144. (27) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801.
Figure 1. (a) Compression isotherm of a gold nanoparticle monolayer recorded at a constant rate of compression (4.8 × 10-2 cm2/min) on pure water. Pictures 1, 2, and 3 are optical micrographs (reflection microscopy) of the monolayer at different compression stages. Each picture is 2.3 mm wide; the water appears as dark areas. (b) Compression isotherm of the octadecytrichlorosilane film compressed at 1.6 Å2/molecule/min on water at pH 2. The compression was stopped at a pressure of 30 mN/m and at an area per chain of 10 Å2 (point 1). After 4 h the pressure decreased to 16 mN/m (point 2). optical micrographs in the inset show the homogeneity of the film for a surface pressure around 10 mN/m (point 3), where the compression was stopped to perform the AFM experiment.28 For the silane film, octadecyltrichlorosilane (OTS) was purchased from Fluka and distilled before use. Several microliters of a chloroform solution of OTS (1 mg/mL), were spread onto the pure water surface acidified to pH 2 with hydrochloric acid (Sigma Normapur quality). After a short time (15 min) the monolayer was completely polymerized and then overcompressed at specific areas around 10 Å2. Since this value is less than the close packing area occupied by each headgroup (∼20 Å2), it indicates the collapse (28) Bourgoin, J.-P.; Kergueris, C.; Lefe`vre, E.; Palacin, S. Thin Solid Films 1998, 327-329, 515-519.
Measurement of Multilayers on Water by AFM
Figure 2. Schematic of the experimental setup providing the appropriate imaging conditions. Mobile barriers allow the compression of the Langmuir monolayer. The water level is adjusted via a capillary placed outside the compressed area to prevent any convection flows at the surface. The grazing metal ring that prevents the in-plane movements of the compressed monolayer is large enough to get a perfectly flat surface in its center.
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Figure 3. Topographic scans showing variations of wave amplitudes with thickness of the water layer. The thickness of the water was determined as the difference between the two imaging positions (above the water or the wafer) after the dewetting of the wafer. Back and forth traces are in dark gray and light gray, respectively. The y-scale of the grid is given on the bottom left corner and the water layer thickness on the bottom right one. The x-scale of the grid is 200 nm. For a thickness of 290 µm the amplitude is typically 50 nm and decreases below 10 nm for a thickness of 160 µm.
of the film and the formation of multilayers. In Figure 1b we report a typical isotherm showing the pressure relaxation due to the collapse of the monolayer.
Principle of the Measurements Precise conditions on the imaging parameters are needed in order to image a liquid surface. Moreover, interfacial movements (due to the approaching tip or external perturbations) must be minimized. AFM Imaging Conditions. To prevent the cantilever from touching the surface, the microscope was used in the true noncontact mode. Far above the surface, the cantilever bearing the tip oscillates at a working frequency slightly above its resonance value. Typical free amplitudes of 20 nm at frequencies in the 100 kHz range were observed. When approaching the surface, the reduction of the vibration amplitude due to the interaction of the tip with the substrate was used as a feedback signal for the vertical position of the scanning head. A maximum reduction of the amplitude of 20% of the free amplitude was allowed. Using higher values of the free amplitude or allowing higher reduction was found to induce possible contacts with the liquid surface (see below). The scan frequency was chosen to be at most 0.5 Hz, which results in a compromise between the efficiency of the feedback loop and the decrease of the water level due to evaporation. Interfacial Movements. Special care was taken to prevent any movement of the interface. A profiled stainless steel ring grazed the interface and suppressed most of the in-plane movements of the monolayer (Figure 2). To minimize the amplitude of the waves induced by the vibration of the cantilever, the depth of the water layer below the monolayer was reduced to a couple of hundred micrometers. This was obtained by inserting a clean hydrophilic piece of silicon wafer in the middle of the ring. This wafer also prevented any rapid dewetting of the thin film. The whole trough and the AFM head were enclosed in a box, reducing most of the water evaporation and suppressing acoustic disturbances. Approach Procedure. The initial approach of the AFM tip is made on a rather thick depth of water (some millimeters) bearing a monolayer. Thanks to the precautions explained above, the scanning head of the AFM stabilized above the interface and induced waves, due to the cantilever oscillation. It should be noted that with the M5 AFM only the head moves and not the sample (trough). At this stage, the phase signal (phase shift of the cantilever vibration with respect to its original value) and the topography signal (based on the amplitude feedback) are recorded. The topography revealed the existence of waves
Figure 4. Large in-plane displacement of a polymerized silane multilayer due to draining of the water film above the silicon wafer, before the complete dewetting. All scan lines were taken in the same direction (from left to right) and are therefore separated by the same time interval (10 s), which explains the stair shape of the observed domains.
at the interface (Figure 3), and the phase signal showed only some noise. The thickness of the water subphase was then reduced by steps of typically 50 µm by removing precise amounts of water in order to reduce the wave amplitude until the phase signal and the topography signal exposed the same features. Moreover, the back and forth traces of each scanned line superimpose. These two criterions were found to assess that the observation is meaningful (see below). Surface Waves and Dewetting. As explained under Approach Procedure, waves are generated by the coupling of the oscillating cantilever with the deformable soft surface. To overcome this undesired phenomenon, we limited the thickness of the water layer above the silicon wafer to a few tens of micrometers. On the other hand, the complete system becomes more sensitive to a possible dewetting of the silicon wafer, leading to unexpected measurements on layers deposited on the solid substrate. However, this dewetting was quite detectable since it led to a noticeable flow of liquid and global in-plane movements of the structures observed at the interface. The image in Figure 4 is a typical example of such movements of the interface prior to dewetting. It is sometimes possible to readjust the water level to stabilize the film and prevent the in-plane movements before dewetting. Consequences of Drifts. Other perturbations must be taken into account during the measurement because
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Figure 5. Instabilities occurring while performing a scan of the interface upon multilayered domains of nanoparticles (in gray) maintained at a constant pressure of 10 mN/m. Instabilities are due to small drifts in the imaging conditions leading the tip to briefly touch the interface. The top part of the image shows the recovery of the optimum imaging parameters despite the elapsed time during the instability.
of possible drifts in the nominal settings (cantilever excitation, vertical piezo displacement). These perturbations usually consist of brief contacts with the water surface. An instability starts to develop and leads, in some cases, to a complete wetting of the tip and the total failure of the experiment. The bright spots in the center of the image in Figure 5 are typical examples of such artifacts. In this case it was possible to regain the optimum imaging conditions (on top of the image) despite the time elapse in the instability. Multilayer Packing of Thiol-Capped Gold Nanoparticles Nanometer-size metal particles are of great interest in production of single-electron devices. A major challenge associated with their incorporation into electronic devices is the preparation of reproducible uniform films of such particles. Indeed, important conduction features such as a Coulomb blockade phenomenon have been observed in very well ordered arrays of nanoparticles deposited via a Langmuir-Blodgett transfer on a solid substrate.8,28 The quality of the deposited layer is crucial for the observation of this single electron tunneling effect at room temperature. In particular, it has been shown that this phenomenon depends dramatically on the number of the transferred layers.7 Since the size of the nanoparticles used in such a study must be a few nanometers, AFM appears to be perfectly suitable for the observation of the possible multilayer formation before transfer because it may provide a high vertical resolution (below 1 nm). Figure 6 presents the topographic and phase observations of a film of thiol-capped 3 nm gold nanoparticles maintained under a constant pressure of 10 mN/m. At this pressure, as shown on previous transferred films,28 we expected to get a compact monolayer with a few defects (holes or bilayer domains). On the topographic image, one can observe domains where well-defined and reproducible steps are evidenced. The step height is between 15 and 18 nm, a height that approximately corresponds to five or six layers. This figure also shows the good lateral and vertical resolutions (below 200 and 10 nm, respectively) that can be achieved. The phase image presents a clear
Figure 6. Topography and phase images of multilayered domains of 30 Å diameter gold nanoparticles, showing height variations of 176 Å (0-0) and 182 Å (1-1). Phase variations are in the 0.2° (20 mV) range. Note the good correspondence between the two pictures. The monolayer was compressed to an initial value of 10 mN/m.
contrast between the multilayered domains and the background. The similarity of the borders between neighboring domains can also be noticed. It may indicate that these domains were produced by the rupture of a larger one, and it is then reasonable to assume that the background of the image is water. Moreover, the phase signal is sensitive to the viscoelastic properties of the scanned area29 and reveals the expected slight differences (0.2°) between the background (water) and the domains (thin solid above water). These phase variations ensure that the measured steps truly reflect the existence of multilayered domains. Indeed, the formation of a microcapillary wetting neck between the tip and the water subphase would conduct to a much higher phase variation (more than a few degrees) and usually lead to an instability (presented in Figure 5). This confirms that the measured steps are not due to any wetting neck between the tip and the sample. The same result was confirmed by the observation of some transferred layers that present small multilayered domains.30 The formation of such multilayers may be due to the stability of the initial monolayer that has to stay compressed at a constant surface pressure for more than 5 h (time for lowering the water level) before the AFM (29) Couturier, G.; Aime´, J. P.; Salardenne, J.; Boisgard, R. J. Phys. D 2001, 34, 1266-1270. (30) Bourgoin, J.-P.; Werts, M. Private comunication.
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measurements start. This hypothesis needs to be crosschecked by further experiments since our results point to the sensitivity of the monolayer organization with equilibration time. In any case, this experiment demonstrates the possibility of investigating ordered domains at the air/water interface and of retrieving meaningful information at the nanometer scale (topography, phase). Multilayer Observation on Polymerized Silane Layers At high surface pressures, monolayers usually undergo irreversible changes leading either to partial dissolution in the subphase or to the so-called “collapse”. Fifty years ago, H. Ries31 proposed a mechanism where collapse proceeds by buckling and folding of the monolayer, but in fact little is known about the exact mechanisms leading to the formation of multilayers observed in collapsed monolayers. Optical in situ observations are limited in resolution and then remain phenomenological.32,33 AFM measurements on Langmuir-Blodgett films after transfer on solids13,34 (of course subject to caution because of the transfer process32,35,36) have pointed out the possible coexistence of bilayers and trilayers. Figure 7 shows a typical picture taken by AFM on a polymerized silane layer at the air-water interface, previously compressed to a pressure of 30 mN/m. A simple first-order baseline flattening has been performed to get rid of the small drift due to some unsuppressed water evaporation. To show the quality of the images, no further treatment was performed. On scan A one can estimate the topographic noise to be below 10 Å (peak-to-peak amplitude). According to the area per molecule reached at this compression (15 Å2 per molecule), it is reasonable to assume that the background (scan A) is at least a silane monolayer. Topographic scans B and C reveal well-defined steps of either 25 or 50 Å, which are measured on the borders of the different domains, and suggest the existence of steps of one or two layers since the fully extended OTS molecules have a length of 2.5 nm.4 Also interesting is the apparent curvature of the large central domain (line B), which seems to have elevated borders. These borders may result from the deformation of the domain, stressed by the compression and still unrelaxed. Although the right imaging conditions were achieved, the topographic pictures can, at least partially, be altered by the presence of some blurred lines due to other instabilities (Figure 8). In this case, it is then possible to get more reliable information on the image thanks to the observation of the phase variations. For instance, the remaining waves on scan 1 (bottom of Figure 8A, topography) and the topographic noise on scan 2 (top of the same figure) are greatly reduced in Figure 8B (phase). In such cases, phase pictures are able to recover a part of the lost information. On the other hand, the domain borders are sometimes poorly determined because of the low variations in the phase signal (0.2°), and the topographic image allows the observation of much better defined domain borders. Moreover, quantitative height information is also lost in the phase images. In any case, (31) Ries, H. E.; Kimball, W. A. J. Phys. Chem. 1955, 59, 94. (32) Gaines, G. L. Insoluble monolayers at liquid-gas interfaces; Interscience: New York, 1966. (33) Ybert, C.; Lu, W.; Mo¨ller, G.; Knobler, C. M. J. Phys. Chem B 2002, 106, 2004-2008. (34) Sheiko, S. S.; Buzin, A. I.; Muzafarov, A. M.; Rebrov, E. A.; Getmanova, E. V. Langmuir 1998, 14, 7468-7474. (35) Riegler, J. E.; LeGrange, J. D. Phys. Rev. Lett. 1988, 61 (21), 2492-2496. (36) Riegler, J. E.; et al. Europhys. Lett. 1994, 25, 211.
Figure 7. Topographic image of octadecyltrichlorosilane multilayered domains at the air/water interface. The monolayer was initially compressed to 30 mN/m. The gray level codes the local height of the interface. Scan A shows typical height variations on the monolayer (
