Langmuir 1997, 13, 6547-6549
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In-Situ X-ray Diffraction Study of Langmuir-Blodgett Deposition M. K. Durbin,*,† A. Malik,† A. G. Richter,† K. G. Huang,‡ and P. Dutta† Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, and Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 Received August 4, 1997. In Final Form: September 11, 1997X We have built a Langmuir-Blodgett (LB) deposition trough that mounts onto a Huber 4-circle diffractometer, so that the LB substrate and the water are in the same closed, temperature-controlled environment. We have used this system to study Langmuir-Blodgett films just after transfer using synchrotron X-ray diffraction. We were able to deposit from the L2, L2′, S, and RII phases of a fatty acid monolayer, and observe the same structures in the transferred monolayers. Three of these phases have not been observed before on a glass substrate. Once deposited, the LB monolayer structure was not affected either by pressure changes in the water monolayer (when part of the substrate was still immersed) or by the substrate remaining in contact with the water or not. The structures were stable over time but changed irreversibly under the effects of radiation damage and temperature cycling. These results show that the change from the structure observed on water to the final LB structure does not occur during deposition, but long afterward.
Introduction Langmuir-Blodgett (LB) films have been the subject of much study and interest since the technique was first developed. They are layered structures formed by repeatedly passing a solid substrate through a monolayer spread at the air-water interface, and they typically also have structure within the layer plane. Because of this easily created molecular organization, the potential of these films as devices ranging from biosensors to waveguides has been eagerly investigated.1 The realization of this potential will depend on several factors, including thermal stability, number of defects, order within the plane, and ability to modify the film structure in desired ways. Clearly it is important to understand the deposition process and how the structure is affected by and during the process. The deposition process is known to affect the in-plane structure and the domain size and shape.2-7 The final structure on the solid substrate is typically different from the structure of the precursor film on the water surface. One of the first experiments to show this was by Riegler and LeGrange,8 who studied the meniscus region of a phospholipid monolayer using fluorescence microscopy during transfer of a film. They found that the domain structure of the meniscus region differed substantially from the bulk and deposited regions at high pH values. They concluded that this was due to a pH gradient induced by the meniscus. Another possible cause of change during deposition is transfer-induced strain. Maruyama9 et al. found substantial orientation of polymer films (measured using linear dichroism) as a result of the flow induced by †
Northwestern University. Argonne National Laboratory. X Abstract published in Advance ACS Abstracts, November 1, 1997. ‡
(1) Roberts, G. G. Adv. Phys. 1985, 34, 475. (2) Brzezinski, V.; Peterson, I. R. J. Phys. Chem. 1995, 99, 12545. (3) Shih, M. C.; Peng, J. B.; Huang, K. G.; Dutta, P. Langmuir 1993, 9, 776. (4) Steitz, R.; Mitchell, E.; Peterson, I. R. Thin Solid Films 1991, 205, 124. (5) Engel, M.; Merle, H. J.; Peterson, I. R.; Riegler, H.; Steitz, R. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1514. (6) Sikes, H. D.; Woodward, J. T., IV; Schwartz, D. K. J. Phys. Chem. 1996, 100, 9093. (7) Mikrut, J. M.; Dutta, P.; Ketterson, J. B.; MacDonald, R. C. Phys. Rev. B 1993, 48, 14479. (8) Riegler, J. E.; LeGrange, J. D. Phys. Rev. Lett. 1988, 61, 2492.
S0743-7463(97)00872-X CCC: $14.00
LB deposition. LB deposition can affect more than just domain size and orientation; the molecular structure may change as well. Brzezinski and Peterson,2 Shih et al.,3 Steitz4 et al., and Engel et al.5 have all performed ex-situ studies of fatty acid monolayers on various substrates with a view to understanding the differences between deposited monolayers and precursor monolayers. In comparing their results, it is clear that the substrate has a stronger effect than the initial structure (on water) on the final (transferred) structure. Each group found differences between the known structures of the monolayers on water and the structures observed on solid substrates. Only Peterson and Brzezinski were able to observe structures that varied as a function of deposition pressure. While in-situ studies have been performed at the macroscopic and mesoscopic scales and a number of exsitu studies have been performed at the molecular level, there has been no direct observation of the molecular scale reordering that is expected to occur during transfer. In order to learn more about this aspect of the deposition process, we performed an in-situ X-ray diffraction study of Langmuir-Blodgett deposition. Experimental Details We designed an LB trough to fit onto a Huber 4-circle diffractometer. This allowed us to scan a freshly deposited film from just above the meniscus to several centimeters from the surface of the water. The substrate remained in contact with the water, in a closed, temperature controlled environment. We were able to observe the structure either immediately following deposition or up to several hours later. The trough was made of aluminum, and coated with Halar (a fluoropolymer). X-rays entered and exited through Kapton windows. Temperature was controlled by passing cooled/heated water through channels below the surface. Temperature gradients were minimized by cooling/heating the lid of the trough with a separate circulator. The trough was supported by a jack that rested on the table below the Huber 4-circle. The sample was mounted onto a Teflon holder, connected to a rod which passed from the 4-circle mounting plate (so that the incident angle and height could be controlled), through a hole in the lid of the trough. Thus the position of the trough relative to the beam and the position of the substrate relative to the beam were (9) Maruyama, T.; Friedenberg, M.; Fuller, G. G.; Frank, C. W.; Robertson, C. R.; Ferencz, A.; Wegner, G. Thin Solid Films 1996, 273, 76.
© 1997 American Chemical Society
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Figure 2. Phase diagram for a heneicosanoic (C21) acid monolayer. Phases are labeled in accordance with the notation of Stallberg-Stenhagen and Stenhagen.14 We deposited LB films at the phase diagram locations indicated by the circles. additional motion, we could access three dimensions of reciprocal space without moving the sample. Motor motions were controlled by utilizing the six-circle mode of SPEC. Resolution was determined by slits placed in front of the sample and in front of the detector, and was approximately 0.01 Å-1 in the plane of the film. Resolution out of the plane was 0.1 Å-1.
Results
Figure 1. Schematic diagram of our setup in (a) front view and (b) top view. The trough was mounted onto a Huber 4-circle diffractometer. The angle 2θ was controlled by the normal Huber 2θ motion and determined the magnitude of the wave vector parallel to the substrate surface (Kxy). The angle γ was controlled by two motors, one which controlled a rotation stage and one which controlled the distance along a horizontal track. The angle γ determined the magnitude of the wave vector perpendicular to the substrate plane (Kz). controlled separately. Deposition could be accomplished either by raising the sample or by lowering the trough. In order to maintain a closed environment while moving the substrate independently of the trough, we connected the two via a plastic sleeve. A slight overpressure of helium was maintained in the trough, to reduce radiation damage and to minimize air scattering. The subphase consisted of water purified to 18 MΩ cm by a Nanopure II system. Hydrochloric acid was added to adjust the water to a pH of 2 to prevent any residual metal contaminants from interacting with the acid monolayer. Pressure was measured using a Wilhelmy plate attached to a balance based on a linear voltage differential transformer (LVDT from Schaevitz). Constant pressure was maintained during deposition using electronic feedback. Heneicosanoic acid (obtained from Sigma Chemicals, stated purity 99+%) was spread from a 3 mM chloroform (obtained from Aldrich Chemical, stated purity 99.9+%) solution. Substrates were either window glass or highly polished silicon wafers (1 in. × 1.5 in. × 0.1 in.). They were sonicated in a dilute solution of detergent and pure water, rinsed, soaked for 24 h in a Nochromix solution, rinsed copiously, and then dried in an oven at ∼100 °C for 30 min. Deposition speeds were ∼3mm/minute. The X-ray diffraction experiments were carried out at beamline X6B of the National Synchrotron Light Source. The beam energy was 8 keV. The need for the sample to be dipped vertically into the water determined the scattering geometry, which is shown schematically in Figure 1. The substrate was attached to the φ motion of the diffractometer, and this was used to adjust the incident angle. Background scattering was reduced by maintaining an incident angle of 0.1-0.15°, which was less than the critical angle for total external reflection from glass. To avoid moving the sample, we mounted a rotating stage onto a track on the Huber 2θ arm. This allowed the detector to scan perpendicular to the 2θ motion (see Figure 1b). By including this
The phase diagram of a heneicosanoic acid (C21 acid) monolayer at the air-water interface is shown in Figure 2. There are seven phases in this region of the phase diagram, and we deposited films from four of them: L2, L2′, S, and RII (we label the phases according to the notation of Stallberg-Stenhagen and Stenhagen,10 except for the RII phase, which we name according to ref 11). We chose these phases because they are representative of the lattice structures and tilts observed in fatty acid monolayers. The high-pressure/high-temperature rotator (RII) phase is the most symmetric phase, with a hexagonal lattice of untilted molecules. It is also the only phase which has also been observed (ex situ) on glass substrates. The S phase is also untilted, but has a distorted hexagonal structure. The L2 and L2′ phases have distorted hexagonal lattices with molecules tilted toward different symmetry directions of the lattice. Some of the temperatures and pressures at which we deposited are marked on Figure 2. Sample data from monolayers transferred from each phase are shown in Figure 3. The data are plotted as intensity contours as a function of the in-plane scattering vector, Kxy, and the off-plane scattering vector, Kz. It is not possible to break Kxy down into x- and y-components because the monolayers are polycrystalline without a preferred orientation (this is demonstrated by the fact that we were able to observe reasonably intense peaks without rotating the sample). Each contour plot in Figure 3 also has a triangle or triangles marking the expected peak positions for a film at that temperature and pressure on the surface of water. These positions are based on our past studies of lipid monolayers at the surface of water (see ref 11 for the untilted phases and refs 12 and 13 for the tilted phases). In each case, the peak positions are qualitatively the same; for the most part they are also quantitatively the same. Thus, immediately after deposition, these LB monolayers retain the structures that they exhibit on the water surface. (10) Stallberg-Stenhagen, S.; Stenhagen, E. Nature 1945, 10, 357. (11) Shih, M. C.; Bohanon, T. M.; Mikrut, J. M.; Zschack, P.; Dutta, P. Phys. Rev. A 1992, 45, 5734. (12) Durbin, M. K.; Malik, A.; Richter, A. G.; Ghaskadvi, R.; Gog, T.; Dutta, P. J. Chem. Phys. 1997, 106, 8216. (13) Shih, M. C.; Durbin, M. K.; Malik, A.; Zschack, P.; Dutta, P. J. Chem. Phys. 1994, 10, 9132.
Langmuir-Blodgett Deposition
Figure 3. Sample X-ray diffraction data observed from LB films deposited from four different phases (see phase diagram in Figure 1). Data are plotted as contours of intensity as functions of the in-plane scattering wave vector (Kxy) and the out-of-plane wave vector (Kz). Marked on each plot is the phase observed and the pressure and temperature at which the scan was performed. Triangles indicate the positions where peaks would be observed for a monolayer on the surface of water.
It is possible that the monolayers were able to retain their structure because of a thin film of water remaining on the substrate. According to Petrov et al.,14 it would be very difficult to deposit a fatty acid monolayer onto glass slowly enough to prevent water entrainment. However, we were able to observe these retained structures even after removing the sample from contact with the water and after waiting up to 2 h for equilibration and drainage. Further, although these newly deposited films retained the structure of the water phase from which they were deposited, they were no longer in equilibrium with the water monolayer, even though the substrate remained in contact with the water. When the pressure of the water monolayer was altered after deposition, the phase on the solid substrate did not change (this agrees with the observations of Riegler and LeGrange8). Moreover, the structure of the film did not appear to be a function of the distance from the water meniscus. Scans performed on the area of the substrate just above the meniscus gave diffraction patterns similar to scans several centimeters from the water surface. We found that the monolayer structure did not even depend on whether or not the substrate remained in contact with the water. This is surprising given the results of Cira´k et al.,15 who observed that the surface acoustic wave (SAW) echo signal from a deposited monolayer vanished until the monolayer was removed from contact (14) Petrov, J. G.; Kuhn, H.; Mo¨bius, D. J. Colloid Interface Sci. 1980, 73, 66. (15) Cira´k, J.; Kosˇtial, P.; Vajda, J.; Tomcı´k, P.; Barancok, D.; Kelesˇi, L. Appl. Surf. Sci. 1997, 108, 53.
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with the water for >30 min. Their results can be explained if there is water entrainment (as Petrov et al.14 claim). Our results imply that while there may be a water layer on the substrate, the transferred monolayer is not connected to the monolayer on water. For instance, it is possible that defects on the substrate pin the monolayer domains, so that structural changes at the water surface are prevented from propagating onto the solid substrate. However, the fact that we observe the water monolayer structure implies that substrate-monolayer interactions are not strong enough, immediately after transfer, to force the structure of the monolayer into the expected LB structure. It was only through heating and/or radiation damage (which may cause local drying) that the in-situ monolayer was driven to the Rotator phase that is typically observed in ex-situ films. We found that the deposition process was difficult to control, and our results were plagued with irreproducibilities. Although we were able to observe the peaks expected from the known water phase on most occasions, there were also several depositions (∼25%) that resulted only in a single rotator-phase peak, and several (∼20%) that resulted in no observable peaks at all. Some of this irreproducibility may have been due to radiation damage. Although radiation damage is observed on monolayers on water, it does not occur as quickly or as drastically as it did on these LB films. After a single scan of only a few minutes, the peaks would begin to shift toward the rotator position. Further radiation damage lead to deterioration of the rotator peak, until no peaks at all were observed. If a similar (or even greater) amount of time was allowed to pass with no radiation, the peaks did not visibly move or deteriorate. The rate of radiation damage was not predictable, and we observed some instances where the film retained its structure over many scans and others in which it decayed as quickly as we could scan it. Conclusions Our results show that for fatty acid monolayers the change from the phase observed on water to the final LB structure does not take place immediately upon transfer of the film from the water to the substrate. Instead, this transition occurs unpredictably over a period of time following the macroscopic deposition. If the processes of drying and heating are carefully controlled, it may be possible to control the change from the known monolayer phase to the expected LB phase, which may influence, for instance, the number of defects, the domain size, and the orientation of the domains. Acknowledgment. This work was supported by the US Department of Energy under Grant No. DE-FG0284ER45125. The X-6B beamline and the National Synchrotron Light Source are supported by the Department of Energy. M.K.D. was supported by an AT&T Bell Laboratories PhD Scholarship. LA970872N
