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Langmuir 1997, 13, 1602-1606
Phase Transitions in Langmuir-Blodgett Films of Cadmium Stearate: Grazing Incidence X-ray Diffraction Studies J. B. Peng,† G. J. Foran,‡,§ G. T. Barnes,† and I. R. Gentle*,† Department of Chemistry, The University of Queensland, Brisbane, Queensland 4072, Australia, and Australian Nuclear Science and Technology Organisation, PMB 1, Menai, New South Wales 2234, Australia Received August 19, 1996X Variations in the structure of multilayer Langmuir-Blodgett (LB) films of cadmium stearate with temperature have been monitored by grazing incidence synchrotron X-ray diffraction. At room temperature the film structure comprises a centered rectangular unit cell in the plane of the substrate with the hydrocarbon chains normal to the substrate surface. As the temperature increases up to 90 °C, the diffraction pattern due to this in-plane structure becomes diffuse and the intensity decreases. Changes in the Q-values of the in-plane spots indicate an anisotropic expansion of the unit cell. At around 100 °C the film undergoes a phase transition from a layered three-dimensional crystalline structure to an array of rod structures packed hexagonally. The rods are oriented parallel to the surface of the film. The core of each rod is 6.7 Å in diameter and appears to be composed of cadmium cations and the carboxylate groups of the fatty acid chains. The aliphatic chains are randomly arranged around these rods. The rod structure is retained on cooling, but the observed correlation length is reduced.
Introduction An understanding of the effect of temperature on the properties of Langmuir-Blodgett (LB) films is important for their effective utilization as waveguides, chemical sensors and biosensors, and nonlinear optical devices.1,2 Further, such a study is of fundamental interest and importance in understanding the nature and kinetics of phase transitions in structured membrane-like systems. Approximately 30 years ago an X-ray diffraction study3 of pure cadmium soaps revealed a phase transition between 80 and 120 °C, well below the melting point. It was interpreted as a change from a three-dimensional lamellar crystalline form to a structure consisting of cylindrical elements packed in a hexagonal array. However, neither the X-ray diffraction patterns nor the intensity distributions were presented, despite the fact that the latter are correlated with the structure of the cylinders. Recently, Merle et al. observed the structures of LB films of some cadmium soaps by small angle X-ray scattering (SAXS)4,5 and electron diffraction6 and suggested that these films undergo this same phase transition. There remains, however, a degree of uncertainty about the structure of the high-temperature phase, owing to deficiencies in the reported data. For example, in the SAXS measurements,4,5 only the (00l) diffraction signal was recorded. Each of the three d-spacings revealed in the electron diffraction measurements6 are from different * Author to whom correspondence should be addressed. Fax: +61 7 3365 4299. Phone: +61 7 3365 4800. E-mail: i.gentle@ mailbox.uq.edu.au. † The University of Queensland. ‡ Australian Nuclear Science and Technology Organisation. § Current address: Australian National Beamline Facility, Photon Factory, Oho 1-1, Tsukuba-shi, Ibaraki-ken 305, Japan X Abstract published in Advance ACS Abstracts, February 15, 1997. (1) Fuchs, H. O.; Prass, W. Adv. Mater. 1991, 3, 10. (2) See the proceedings of the 5th-8th International Conferences of LB Films. (3) Spegt, P. A.; Skoulios, A. E. Acta Crystallogr. 1963, 16, 301. (4) Merle, H. J.; Yu, M; L’vov; Peterson, I. R. Makromol. Chem., Macromol. Symp. 1991, 46, 271. (5) Merle, H. J.; Metzger, H.; Pietsch, U. Phys. Scr. 1992, T45, 253. (6) Merle, H. J.; Steitz, R.; Pietsch, U.; Peterson, I. R. Thin Solid Films 1994, 237, 236.
S0743-7463(96)00826-8 CCC: $14.00
diffraction photos of different films. Moreover, as most of these measurements were performed at room temperature after the films had been heated, the structures were not necessarily the same as those existing at the high temperature where the new phase developed. During the last decade, the grazing incidence X-ray diffraction (GIXD) technique using synchrotron radiation has developed into a powerful tool for studying the structures of single layer and multilayer LB films.7-10 However, because of the weakness of the diffraction signal and the less than ideal performance of available detection systems, only a few diffraction peaks could be recorded for a given sample even when using the brilliant synchrotron radiation from a wiggler source.8 Recently, the introduction of imaging plate detection has greatly improved the quality of the data which may be recorded in a GIXD study of LB films.11,12 Imaging plate detection has been employed in the present work to observe the structure of cadmium stearate LB films over the range from room temperature to about 120 °C. Experimental Section LB films of cadmium stearate were formed from monolayers of stearic acid spread on a solution of CdCl2 by the vertical deposition mode. The trough and barriers were constructed of PTFE (Teflon).13 Polished silicon wafers ((111) surface, Semiconductor Processing Company, Boston, MA) were used as substrates after cleaning by RCA standard procedure B.14 Stearic acid (>99.5%, Fluka AG) was dissolved in chloroform (spectroscopy grade, Merck) at a concentration of 2.0 × 1015 molecule µL-1 for use as the spreading solution. CdCl2 (AR Grade, Sigma) was dissolved in Milli-Q water to a concentration of 5.0 × 10-4 mol (7) Shih, M.; Peng, J. B.; Huan, J.; Dutta, P. Langmuir 1993, 9, 776. (8) Tippmann-Krayer, P.; Kenn, R. M.; Mo¨hwald, H. Thin Solid Films 1992, 210/211, 577. (9) Barbeka, T. A.; Ho¨hne, U.; Pietsch, U.; Metzger, T. H. Thin Solid Films 1994, 244, 1061. (10) Malik, A.; Durbin, M. K.; Richter, A. G.; Dutta, P. Phys. Rev. B 1995, 52, R11654. (11) Foran, G. J.; Peng, J. B.; Steitz, R.; Barnes, G. T.; Gentle, I. R. Langmuir 1996, 12, 774. (12) Foran, G. J.; Gentle, I. R.; Peng, J. B.; Barnes, G. T.; Garrett, R. F.; Creagh, D. C. Submitted for publication. (13) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; Wiley Interscience, New York, 1990; p 121. (14) Kern, W. RCA Eng. 1983, 28, 99.
© 1997 American Chemical Society
Phase Transitions in LB Films of Cadmium Stearate
Langmuir, Vol. 13, No. 6, 1997 1603
Figure 2. Profile at constant Qxy ) 1.54 Å-1 for cadmium stearate film at room temperature.
Figure 1. Raw image from cadmium stearate film (31 layers) at room temperature. dm-3 and adjusted to pH 7.0 using KHCO3 (7.5 × 10-5 mol dm-3). LB deposition was performed at a surface pressure of 29 ( 1 mN m-1, a temperature of 25.0 ( 2 °C, and a dipping rate of 0.1 mm s-1, beginning with an upstroke. The transfer ratio was unity on both upstrokes and down strokes. Each prepared LB film consisted of 31 monolayers of cadmium stearate. The X-ray diffraction experiments were carried out at the Australian National Beamline Facility (ANBF) located at beamline 20B of the Photon Factory in Tsukuba, Japan. The experimental setup, the convention regarding coordinate frame choice, and the data reduction have been described elsewhere.11,12 The silicon wafer carrying the LB film was mounted horizontally on a heating stage located in the center of the diffractometer. The wafer was aligned such that the propagation direction of the incident X-ray beam was perpendicular to the dipping direction used during deposition of the LB film. The sample temperature was controlled remotely and manually by adjusting the power voltage supplied to the heating stage and monitored with a K-type thermocouple in mechanical contact with the sample surface. The measurements were carried out with the diffractometer evacuated to a pressure < 0.1 Torr. A monochromatic and horizontal beam (λ ) 1.117 Å, size (w × h) ) 1.0 mm × 0.05 mm) was incident on the surface of the sample at an angle of 0.1375° (ca. 0.8 of the critical angle for total external reflection). Imaging plates were mounted in the cylindrical diffractometer cassette at a distance of 570.4 mm from the center of the sample. In order to determine the instrumental resolution, we calculate that for a diffraction angle (2θ) of 15° a nondivergent beam of size defined by the slit dimensions given above would give a spot with a full width of 6.4 mm at the imaging plate position. The actual resolution was determined by measuring the diffraction cone from a powdered silicon sample, which gave a width at half maximum of 2.74 mm corresponding to Q ) 0.027 Å-1. The exposure time was 90 min for the high-Qxy measurements and 5 min for the low-Qxy measurements. All of the reported data are from the one 31-layer film, although similar results have been observed for many other samples.
Results and Discussion At temperatures less than 100 °C the LB films of Cd stearate have a three dimensional (3D) lamellar structure. Figures 1-3 show the diffraction patterns recorded at 25, 50, 75, and 90 °C. The values of the Q-vector and d-spacing calculated from the diffraction spots are listed in Table 1. Room Temperature. As can be seen in Figure 1, at room temperature there are strong sets of diffraction spots
with each set apparently split into a pair of subsets. One possible interpretation of the splitting of the sets of spots is the existence of two structures in the film, but this is thought unlikely for the following reasons. If the sets of spots are considered to arise from multiple structures and the centre of the sample is taken as the diffraction origin, then the derived unit cell dimensions for one of the structures result in an impossibly small area per molecule (17.7 Å2) for close packing of C-C (trans) chains.15 Furthermore, if the two sets of spots are considered to arise from the one structure taken to be located on the forward half of the film in the one case and the rearward half of the film in the other case, then the d-spacings calculated using revised sample-to-detector distances are identical for the two sets. Further experiments have been performed which confirm this interpretation. The exact reason that the spots split into two rather than merely smearing out is not well understood as yet, although it is not surprising, given that the use of area detection precludes the defining of a specific scattering area on the surface, as is normally done with Soller collimation and scanning detectors. The structures and dimensions described below are based on this interpretation. The main sets of diffraction spots are assigned as {11l} and {20l} reflections, respectively, where the z-direction is taken to be normal to the substrate. These reflections are characteristic of the diffraction from a centered rectangular in-plane structure with d110 ) 4.08 Å (Q110 ) 1.54 Å-1) and d200 ) 3.72 Å (Q200 ) 1.69 Å-1). The area per molecular chain for this centered rectangular structure is 18.2 Å2. The spots in these two subsets have identical Qz values (Figure 2) that correspond to d001 ) 50.4 ( 0.5 Å and indicate a double layer of stearate salts with the aliphatic chains fully extended and perpendicular to the surface of the film. At Qz ) 2.48 Å-1 the diffraction intensity is enhanced. This phenomenon has been attributed to diffraction from the planes formed by every second carbon atom along the aliphatic chains.11 Intensity enhancement due to this effect is observed as a spot of medium intensity at lower Qxy than the {11l} diffraction. This spot is due to the diffraction from the {01l} planes of the rectangular structure and hence d010 ) 4.88 Å. The observation of diffraction where (h + k) is odd has been interpreted previously as being due to a herringbone arrangement of the aliphatic chains in the unit cell.11 Another feature observed in the pattern is a weak diffraction spot at higher Q-value (see Figure 1) (Q ) 2.12 (15) Kitaigorodskii, A. I. Organic Chemical Crystallography; Consultants Bureau: New York, 1961; pp 181-215.
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a
b
Peng et al.
c
d
Figure 3. Raw images of the diffraction pattern from the same film as in Figure 1 at (a) 50 °C, (b) 75 °C, (c) 90 °C, and (d) 105 °C.
Å-1, d ) 2.96 Å) which corresponds to the diffraction from {210} and agrees well with the position predicted from the Q-values of the {110} and {200} sets (2.12 Å-1). The correlation lengths in the plane of the film and along the z-direction are 160 and 120 Å, respectively.15 In three-dimensional terms then, the film structure may be assigned as a perpendicular structure having unit cell parameters a ) 7.45 Å, b ) 4.88 Å, c ) 50.4 Å, and R ) β ) γ ) 90°. Intermediate Temperatures. As the temperature of the sample was increased from room temperature to about 90 °C, several changes occurred in the diffraction pattern: a general decrease in intensity and sharpness, the merging of the previously-split spots, and changes in the Q-values of some spots.
At 50 °C (Figure 3a) the diffraction spots are more diffuse and each pair of spots tends to overlap one another. With further heating the diffraction intensity gradually weakened as the film became more and more disordered. The d200 spacings became larger whereas the d110 spacings remained almost constant up to about 90 °C. The d001 spacing was also unchanged even at 90 °C. This behavior indicates that the thermal motion of the molecules in the crystal domains is clearly anisotropic. At 90 °C the remaining diffraction spots were quite weak and the spacings of the structure were d110 ) 4.12 Å and d200 ) 3.83 Å. Although the lamellar spacing in the vertical direction remained at 50.4 Å throughout, there was evidence at higher temperatures that the intensity maximum of the lowest order spots occurred at Qz > 0,
Phase Transitions in LB Films of Cadmium Stearate
Langmuir, Vol. 13, No. 6, 1997 1605
Table 1. Indexed Q- and d-Values at Different Temperatures (T) Calculated from the Corresponding Diffraction Spots and Resultant Areas per Acyl Chain
a
T/°C
hkl
Q/Å-1
d/Å
A ˆ /Å2 chain-1
room room room room 50 50 50 75 75 75 90 90 90 105 105 105 105 room, after heating to 105 room, after heating to 105 room, after heating to 105 room, after heating to 105
110 200 001 210 110 200 001 110 200 001 110 200 001 10, 01 20, 02 11 21, 12 10, 01 20, 02 11 21, 12
1.54 1.687 0.1247 2.12 1.54 1.68 0.1269 1.54 1.666 0.1267 1.525 1.640 0.1247c 0.233 0.467 0.403 0.615 0.242 0.484 0.420 0.641
4.08 3.72 50.4 2.96 4.08 3.74 50.4 4.08 3.77 50.4 4.12 3.83 50.4c 27.0 13.4 15.6 10.2 26.0 13.0 14.9 9.8
18.17
upright upright upright
18.22
upright upright upright tilted (∼1.1°) tilted (∼1.1°) tilted (∼1.1°) tilted (∼1.5°) tilted (∼1.5°) tilted (∼1.5°) 2D hexagonalb 2D hexagonalb 2D hexagonalb 2D hexagonalb 2D hexagonalb 2D hexagonalb 2D hexagonalb 2D hexagonalb
Q001 and d001 are averaged values. b Q10:Q11 Q20:Q21 ) 1:x3:x4:x7.
a
b
Figure 4. (a) Raw image of the diffraction pattern in the lowangle region from the film heated to 105 °C. The direct beam position is in line with the central vertical axis of the pattern. (b) Diffraction pattern from the above film after cooling to room temperature.
and hence the chains appear to have tilted slightly. The tilt is only of the order of 1-1.5° however, which is too small to be discernible as a change in lamellar spacing. Higher Temperatures. At approximately 100 °C the film underwent a dramatic phase transition. The diffraction signals at high values of Qxy, due to the multilayer lamellar structure described above, vanished entirely, and part of a new pattern appeared in the low-Q region (see Figure 3d). Figure 4a shows the entire diffraction pattern produced by the new phase at 105 °C, revealing a hexagonal pattern of reasonably sharp spots centered
c
18.27 18.7
structure
Very weak.
around, and relatively close to, the direct beam position. Rather than diffraction rings such as those reported by Spegt et al.,3 the pattern exhibits features which are characteristic of a sample composed of polycrystals in a particular ordered orientation. The positions of the spots were measured and indexed, and this information is summarized in Table 1, revealing that Q10 ) Q01 ) 0.233 Å-1 and Q10:Q11:Q20:Q21 ) 1:x3:x4:x7. The reciprocal (Q) lattice is therefore a two-dimensional (2D) hexagonal net, and this is consistent with a film structure of a hexagonal net located in a plane parallel to the 2D Q-space, i.e. in the yz plane (with the real net rotated through an angle of 30° to the net in Q-space). No spots other than the hexagonal pattern were detected, and hence no periodic structure is indicated in the x-direction. In their study of pure cadmium soaps, Spegt et al.3 identified a hightemperature phase which they designated the B phase, which consists of cylinders arranged hexagonally. Our data indicate an array of rods lying on the surface with their long axes parallel to the x-axis and packed hexagonally in the yz-plane (see Figure 5). The spacings of the 2D hexagonal net are d10 ) d01 ) 27 Å, which is much smaller than a bilayer thickness. This can be attributed to either a declination of the extended chains to the rod axis or the occurrence of gauche conformations.17-19 The correlation lengths calculated from the diffraction peak widths in the y- and z-directions are 329 and 80 Å, respectively,16 implying that on average each of the domains in the film contains 12 × 3 ) 36 rods. Another feature is that the spots show a wide slitlike diffraction intensity distribution which decreases to a minimum at Q ) 0.91 Å-1. This suggests that each rod has a diameter of about 6.70 Å. The dimensions of the rod core and the spacings between the rods imply that each rod consists of the polar head groups of the soap which contribute intensity to the diffraction spots, while the aliphatic chains are randomly oriented between the rods. Theoretically, (16) Here the correlation length ) (0.9 × 2π)/w and w ) (wm2 win2)1/2, where w is the intrinsic full width at half maximum (fwhm), wm is the measured fwhm, and win is the instrumental fwhm. (17) (a) Rabe, J. P.; Novotny, V.; Swalen, J. D.; Rabolt, J. F. Thin Solid Films 1988, 159, 359. (b) Rabe, J. P.; Swalen, J. D.; Rabolt, J. F. J. Chem. Phys. 1987, 86, 1601. (18) Gaber, B. P.; Prticolas, W. Biochim. Biophys. Acta 1977, 465, 260. (19) Buhaenko, M. R.; Grundy, M. J.; Richardson, R. M.; Roser, S. J. Thin Solid Films 1988, 159, 253.
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independent of the rotation of the sample about the normal to the film surface, i.e., the z-direction. This indicates that the domains are randomly arranged in the xy-plane and the diffraction observed is from those domains in which the rods are parallel (and almost parallel) to the incident beam (i.e. the x-direction), and that there is no apparent favored orientation. When the film was cooled to room temperature (Figure 4b), the rod structure was retained. The d-spacings (d10 ) d01 ) 26.0 Å) were, however, slightly smaller than those measured at the elevated temperature. Also, the diffraction spots became more diffuse and the correlation length in the y-direction decreased to 192 Å. The correlation length in the z-direction was unchanged (Figure 4 and Table 1). In addition, a pair of weak and diffuse arcs appeared at high Q (1.55 Å-1), due to some recovery of the lamellar structure. Conclusion Figure 5. (a) Schematic diagram of the surface after heating to 105 °C. The lines on the surface indicate randomly-arranged domains of rods lying on the surface. Each rod consists of a linear arrangement of Cd2+ ions and head groups (depicted as shaded circles) with the hydrocarbon chains surrounding them. (b) Cross section showing the hexagonal arrangement of the rods on the surface, when looking endways down a domain of rods (see dashed rectangle in part a above). The centers of the rods are spaced 27 Å apart. (c) Side view of one rod.
the length of the head group, COOCdOOC, calculated from semiquantitative quantum chemistry is about 6.98 Å,20 which is consistent with the size of the rods calculated above. The small difference between the observed and the calculated sizes is either due to the tilting of the polar group from the normal of the long axis of the rod or the deviation of the theoretical value from the experimental value. Spegt et al.3 also suggested that the 2D diffraction pattern is attributed to the polar groups of the soaps. Moreover, more recent results21 (not shown here) show that the orientation of the 2D diffraction pattern is (20) The length of the CsO bond in stearate is taken to be 1.28 Å, the OsCd bond to be 2.24 Å, the (OO)CsC bond to be 1.51 Å, the OsCdO angle to be 124.3 °, and the CsOsCd to be 98.4°. (See: Sasanuma, Y., Nakahara, H. Thin Solid Films 1995, 261, 280.)
At room temperature the LB multilayer film of cadmium stearate is comprised of a centered rectangular crystalline structure, with a herringbone arrangement of the upright aliphatic chains. As the temperature was increased, the film became more disordered, while retaining its original in-plane structure. At about 100 °C the film underwent a phase transition, from a 3D lamellar to an hexagonal arrangement of rods with their long axes parallel to the surface of the substrate (xy-plane). The rods constitute domains of an average size of a few hundred angstroms in diameter. The domains are randomly orientated in the xy-plane. Each rod consists of the polar head groups of the stearate molecules with the cadmium cations located at the center along the axis. The aliphatic chains of the stearate ions are randomly arranged around the rods. Acknowledgment. Financial support from the Australian Nuclear Science and Technology Organisation and the Australian National Beamline Facility is acknowledged. The ANBF is funded by a consortium comprising the ARC, DIST, ANSTO, CSIRO, ANU, and UNSW. LA9608268 (21) Peng, J. B.; Foran, G.; Barnes, G. T.; Gentle, I. R. To be published.
