Langmuir 1991, 7, 2298-2302
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Structural Changes before and during Desorption of Langmuir-Blodgett Films P. Tippmann-Krayer and H. Mohwald' Institut fur Physikalische Chemie, Universitiit Mainz, Welder Weg 11, 06500 Mainz, FRG
Yu. M. L'vov USSR Academy of Science, Moscow, USSR Received January 15, 1991. I n Final Form: April 23,1991 LangmukBlodgett (LB)f i b of cadmium arachidate are studied by opticalreflection, opticalmicroscopy, and small-angleX-ray diffraction with special emphasis on the temperature dependence between 100 and 150 "C. It is shown that at about 110 O C a transition into a state with uniform thickness but with loss of discrete layer structure occurs. The thickness of this film decreases when the temperature is increased and then detracts in a typical dewetting process. Droplets are later formed, leadingto strong light scattering. A special benefit of this study results from the refined methodology. Bragg diffraction as well as Kiessig fringes could be well resolved, and Nomarsky microscopic observations were quantified. This enabled correlation between optical images and X-ray scattering data. Introduction Studies of temperature-dependent structures of Langmuir-Blodgett films are important for different reasons: An essential prerequisite of possible technological applications of organic films is sufficient thermal stability. This, and also the need to understand basic interactions, makes it interesting to investigate the phase diagram of these systems. Ultrathin films may differ from bulk material, since the substrate may influence the structure. This influence and ita range are physically very interesting. It may affect not only the local equilibrium structure but also film morphology and nonequilibrium features. Melting of multilayers and desorption of part of them leads to inorganic particles or platelets, which themselves may have interesting properties.' This work considers a model system (cadmium arachidate multilayers) where much is known about temperaturedependent properties from electron diffraction2 and IR and Raman s p e c t r o ~ c o p y . A ~ ~transition ~ from an orthorhombic to a hexagonal phase at a temperature near 100 "Chas been characterized in detail. We elaborate on changes that occur at slightly higher temperatures, that lead to a phase with translational disorder within the plane5 and normal to it and then to desorption. We combine information from quantitative analysis of optical microscopy, reflection, and small-angleX-ray scattering (SAXS). The results are relevant not only for an understanding of these systems but also for the development of these methods. Experiments and Analysis In order to study thermodesorption of mono- and multilayers we have introduced an optical reflection technique.6 It derives its high sensitivity to film thickness (-0.2 A) from the fact that (1) Tippmann-Krayer,P.; Meisel, W.; MBhwald, H. Adu. Mater. 1990, 12, 589. ( 2 ) BBhm, C.; Steitz, R.; Riegler, H. Thin Solid Films 1989,178,511. (3) Neselli, C.; Rabolt, J. F.; Swalen, J. D. J . Chem. Phys. 1986, 82, 2136. (4) Rabe, J. P.; Swalen, J. D.; Rabolt, J. F. J . Chem. Phys. 1987,86, 1601. (5) Tippmann-Krayer, P.; Kenn, R.; MBhwald, H.; Kjaer, K.; AbNieleen. J. Hasvlab. Jahresbericht 1988. 249. (6) Lkhube;, L. A,; Rothenhilusler, B.; Schneider, G.; Mbhwald, H. J. Appl. Phys. A 1986,39, 173.
0743-7463/91/2407-2298$02.50/0
a 1600-A oxide layer on a Si wafer acts nearly like a X/4 plate for special optical conditions (wavelength633 nm, 70" incidence angle, perpendicular polarization). Thus the surface is weakly reflecting, and the reflection is strongly affected by an organic deposit. However, it turns out that the reflectivity is also influenced by light scattering, which therefore has to be studied separately. This can be done in one experiment,since one of the two polarization directions is more sensitive to deposit changes (the perpendicular one) than the other. In addition,desorption changes the reflection of the two polarizations in opposite directions,whereas scattering affectsboth polarizations the same way. Hence, by splitting the reflected beam into polarizations parallel and perpendicular to the plane of incidence and measuring the two components separately, we can distinguish the mechanisms of desorption and formation of scattering inhomogeneities. Nomarsky microscopy has proven to be a valuable tool for visualizingscattering centers.' To quantify the information,the video micrographs recorded with a precision TV camera (Hamamatsu, C2400, 8 bit) were processed by an image analysis system. Lateral areas of about 800pm2were selectedto determine an average intensity. Since the surface illumination was very inhomogeneous, the areas to be compared were laterally moved to identical positions within the field of view. Hence lateral variations of detection sensitivity could be eliminated as an error Bource. Duringthe measurement of a thermal cycle,the reflected intensity from an uncoated surface varied by up to 3 % The intensity differencesbetween coated and uncoated areas amounted to 20 & 0.2%. The temperatures of the heating stages were calibrated with known standards. Small-angle X-ray experiments were performed with a Siemens D500 diffractometer. For this geometry (incidence angle = scattering angle) we measure the intensitydistributionnormal to the surface. At this stage no attempt is made to calculate the full density profile, since we concentrate on two basic features: (1)For a periodic density distributionthe repeat length &I, equivalent to the thickness of a double layer, is related to the Bragg maxima &araccording to
.
i~ = 2dw, sin Bi,Br (1
- -) 6 sin'
eigr
where the film refractive index n = 1 - 6. Since the surface normal shifted slightly with temperature, dwl was determined from measurements of &gr and 655,. Bigrwas not taken because the maximum position is also influenced by overlap with the 0 1991 American Chemical Society
Structural Changes of LB Films
Langmuir, Vol. 7, No. 10,1991 2299
4.0
3.5 r: a
3.0
Y
2.5 2.0 1.5 r:
2
Y
2.0 1.5
0
50100150200"
50 pm
Temperature ["C]
T = 30°C
Figure 1. Reflected intensity for both polarization directions as a function of temperature at a constant heating rate (0.07 K s) for a film consisting of 11layers of cadmium arachidate on a i wafer with 1600-8,oxide thickness. The dashed lines indicate corresponding slope changes.
fl
Kiessig fringes and by refractive index dispersion.' dm1 could thus be measured to an accuracy of *0.2 A. Due to low angular resolution (0.25') the half width was not analyzed quantitatively. To determine the integrated Bragg intensity I,we summed over all observablepeaks (In) with a volume correction and had to use a correction factor K given by I = K C n 1,. The latter was necessary due to thermal misalignments and derived from a comparison of background signals, assuming that background and signal are changed proportionally by alignment errors. (2) The interference between a beam reflected from the air/ LB film and from the LB film/substrate interfaces, respectively, leads to the so-called Kiessig fringes. If one neglects deviation of the refractive index n = 1- 6 from unity, the fringe maxima dn are related to the total thickness t and to the X-ray wavelength by
'. *
x
sin 8, = n (2) 2t The slope of the sin 8, versus n plot thus yields t. For the more refined analysis performed in this work, 6 was measured from the critical angle in reflection measurements (not shown). We obtained 6 = 2.9 X lo* and for the analysis we used the modified eq 2' )
(I--&-)
sinO,=n-
x 2t
The thickness t was then calculated to an accuracy of f15 A using the fringes with n = 1,2. Fringes with higher indices could not be taken if the Bragg maxima were pronounced and hence overlapped (see Figure 3). They could, however, be used for the liquidlike films observed at elevated temperatures and showing no Bragg peaks. It turned out that multilayers consisting of 11 monolayers were optimally suited for the studies presented below, since different types of Kiessig fringes and Bragg peaks could clearly be discerned. The multilayers were prepared by LangmuirBlodgett technique. Arachidic acid ( N 1 mg/mL) was spread from a chloroform solution on a subphase existing of Millipore water with M CdC12 and NaHC03 (6 mg/L; pH -7). The silicon wafer with 1600-8,oxide, a generousgift of Wacker Chemie, Burghausen, used as a support was polished and purified before use by chromosulfuric acid.
Results and Discussions Figure 1 shows the optical reflection of a coated surface on increasing the temperature with a constant rate for the (7) Rieutord,F.;Benattar,J. J.;Boaio,L.;Robin, P.;Blot, C.; de Kouchkovsky, R. J. Phys. 1987,48,679.
P
Figure 2. Nomarsky photomicrographs of a film consisting of 11 layers of cadmium arachidate, heated very slowly to the indicated temperatures.
two different polarizations. One observes a good correspondence of reflection changes recorded for both polarization directions. For the beam with electric field vector in the plane of incidence, a decrease of the thickness yields
2300 Langmuir, Vol. 7, No. 10, 1991
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i
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0
12
Figure 3. Scattered intensity on a logarithmic scale in a SAXS experiment (X = 1.54A)of a cadmium arachidatefilm (11 layers) on silicon aa a function of scattering angle 28 with normal wave vector transfer at different temperatures. Each curve is displaced by a factor 10. 57.0
2
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-I-
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0.6 0.4 0.2 0.0
150 20
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Temperature ["Cl
Figure 4. Double layer spacing&I, integrated intensity of Bragg peaks and totalthickness t as a function of temperature for a LB film, consisting of 11 layers of cadmium arachidate extracted from temperature-dependent SAXS data. a signal increase. Hence the decrease of this signal on increasing the temperature above 140 "C is due to formation of scattering centers which later disappear, and desorption then causes a signal increase beyond the value at 30 "C. In a similar way the two steps in signal decay with temperature can be understood for perpendicular polarization. There both mechanisms yield a signal reduction. The main goal of this work is to understand the processes occurring between 100 and 150 "C. The formation of scattering centers can be followed by Nomarsky microscopy (Figure 2). Whereas below 110 "C
2
L JJ'fJf
0.0 0
C 97'C 1 W C 107'C 112'C 1 2 7 T
5
10 15 20
25 30
35
40
index of kiessig fringe
Figure 5. Maxima of Kiessig fringes versus their index for differenttemperatures. For each temperature the index is shifted by 5 units. only few dust particles on an otherwise homogeneous surface are observed (a), the film starta to form droplets at -115 OC (b). There are areas of uniform intensity separated by elongated regions of variable thickness. These regions migrate laterally leaving droplets behind. At a later stage (c) above about 150 "C the surface exhibits many droplets. Further increasing the temperature, these droplets shrink and only small particles remain. The latter have been identified as an inorganic Cd salt.l The images of Figure 2 should provide an overview of characteristic features but do not enable quantification. It turns out, for example, that the large dark areas in Figure 2 correspond to the uncoated surface whereas the LB film of uniform thickness appears bright in Figure 2a. Experiments dedicated to a quantification of film thickness will be presented below. The prime message of this work is contained in Figure 3, presenting the SAXS measurements of an LB multilayer on silicon for different temperatures. Near room temperature (30"C) one observes the Bragg diffraction lines (&I) due to the periodic layer structure of the fii. Between these dominant peaks one observes smaller maxima with higher periodicity corresponding to the Kiessig fringes and Bragg side maxima. They are only pronounced in every second interval between the Bragg peaks. This is expected for a film with odd numbers of layers,' because in the other intervals there is destructive interference between these fringes and the Bragg side maxima. The diffraction pattern is virtually constant for temperatures between 20 and 100 "C. At higher temperatures, however, drastic changes occur. The Bragg peaks are reduced in intensity (107,112 "C)and the Kiessig fringes are more pronounced. The latter is expected if the film loses its layer structure but maintains a uniform thickness. Note that Kiessig fringes appear in those intervals where they are not present at lower temperatures (e.g. 107 "C). To our knowledge this manifestation of the underlying X-ray theory has not yet been manifested that clearly. Upon further increasing the temperature, the Kiessig fringes also disappear and only a broad signal remains. It corresponds to a spacing of 35 A and may be attributed to the dto spacing of a hexagonalarray of tubular inverted micelles of Cd arachidate.* Such a structure is expected on heating the bulk material above 115 "C but will not be discussed further. (8)Spegt, P.A.; Skoulios, A. E.Acta Crystallogr. 1963, 16,301.
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Langmuir, Vol. 7, No. 10,1991 2301
a) T = 30°C b) T = 130°C Figure 6. Nomarsky photomicrographs of a coated silicon wafer with one to nine monolayers of cadmium arachidate a t two different temperatures. The dark spots in part a and a t the same positions in part b are due to dust on the camera lens.
A quantitative analysis of temperature-dependent diffraction measurements is iven in Figure 4. The measured value of dml= 55.4 f 0.2 is in accordance with literature data7~4-11and bulk observations,8 but we hesitate to conclude on a reduction of dml on approaching 100 "C, as reported in ref 11 (to 54.9 A). Our results differ from those of a temperature-dependent SAXS study on cadmium arachidate multilayers sandwiched between diacetylene polymer layers.12 There, the analysis yielded thicknesses on every individual monolayer, but the results appear unphysical. Probable explanations for this result from an inaccuracy of 3 A in measurement of molecular length. Taking this into account, one can rationalizelength measurements above 30 A. However, conclusions on tilt angles are very doubtful, since the length inaccuracy transforms into a tilt inaccuracy of f20". Therefore, we doubt their conclusions on chain untilting and premelting for temperatures approaching 80 "C. Probably there are also changes in the boundary diacetylenic layers which affectthe SAXS data. We additionally observe a reduction of integrated intensity with temperature. This is expected due to an increase in normal molecular mobility or single layer roughness. A very important and new result of this work is the measured temperature dependence of the total film thickness t given in the lowest graph of Figure 4. Please note the decrease in thickness for temperatures above 107 O C , where no Bragg peaks were observable. The validity of this result can be judged easily from an inspection of Figure 5. It presents a plot of the Kiessig fringe maxima versus index according to eq 2, where the abscissas have been shifted by 5 units for each temperature. At low temperatures the indices of the peaks near 4 O are somewhat uncertain relative to those near lo,because there is a large gap of unobservable fringes. However, one sees that the slopes of lines through neighboring points "are consistent" and that with proper indexing all measurement points are on one line. Deviations of points near an angle of 5.5" can be understood from an interference with Bragg side maxima. The temperature-independent slope of the lines
R
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(9) Feigin, L. A,; L'vov, Yu.M.; Troitsky, V.I. Phys. Rev. 1989,11,285. (10) Fromherz, P.; Oelschliigel, U.; Wilke, W. Thin Solid Films 1988, 159, 421.
(11) Fujui, T.;Sugi, M.; Iizima, S. Phys. Rev. B: Condens. Mutter 1980,22,4890.
(12)Richardson, W.; Blasie, J. K. Phys. Rev. B 1989,39,12165. (13)Pomerantz, M.;Segmiiller, A. Thin Solid Films 1980,68, 33.
Thickness t
Figure 7. Intensity for various temperatures of a cadmium arachidate film on silicon, determined from Nomarsky photomicrographs (as in Figure 6) with an image analysis system (averaged over the rectangular area indicated in Figure 6) as a function of film thickness (in monolayer and 8, units).
for T < 107 "C then proves unchanged thickness. One derives a value o f t = 303 f 15A at room temperature and this is in excellent agreement with the value expected from the Bragg spacings for 11 monolayers: t(theor) = (11/ 2)dml = 305 A. In Figure 5 slope changes for temperatures between 107and 127 "C are observed. It is especially gratifying that in this interval many neighboring fringes are observable in proving this unexpected result. Therefore, the measurement of a thickness decrease with temperature is well established and quantitative. An often very serious criticism against results derived from SAXS data is that the latter are obtained on the ordered fraction of a film which may not be a major part. Thus it is highly desirable to investigate which part of the surface the data are derived from. This has been possible within this work by quantitative analysis of Nomarsky micrographs. Figure 6 shows this for a wafer that was coated in lateral steps between one and nine monolayers. These steps can obviously be seen as intensity steps, but there is a problem due to inhomogeneous illumination. Therefore, only the intensity out of the indicated square was analyzed and the intensity out of this spot area served as a calibration. The intensity measured in this way near room temperature for different thicknesses yields the curve in Figure 7. At these temperatures one may assume the thickness given by the SAXS measurements, thus enabling
2302 Langmuir, Vol. 7,No.10, 1991
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1
U
200 150 100
1 20
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0
Optics
40
60
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120
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160
Temperature [ T I
Figure 8. Total thickness t of an 11layer cadmium arachidate film as a function of temperature. Data derived from (X) SAXS (Kiessigfringes) and (0) optics (quantitative image analysis of Nomarsky micrographs). a conversion from monolayer into angstrom units. The right side of Figure 6 shows that the contrast weakens a t 130 "C, indicating a reduction in thickness. The dark spot on the top of the figure shows an area where the film has ruptured. For the analysis we concentrate on those regions where the film is continuous, and the measurements at 100, 120, and 145 OC, respectively, yielded intensities given in Figure 7. Under the assumption of temperatureindependent refractive index, one may convert these values into thicknesses, using the continuous curve at room temperature as calibration curve for this experiment. As a result Figure 8 enables us to compare thicknesses obtained by optics and by SAXS. The nearly quantitative agreement proves that the continuous areas (the bright ones in Figure 2b) observed by optical microscopy are those that yield the Kiessig fringes. Hence a large fraction of the multilayer at temperatures between 110 and 150 "C is in a liquid state with no layer structure, but with uniform and temperature dependent thickness.14
Conclusions The combination of SAXS and optical data has provided new insight into the processes occurring on heating a film of a fatty acid salt before thermodesorption occurs. There is no thinning of a multilayer structure by partial desorption of molecules or of top layers with maintenance of the discreteness of 1 a ~ e r s . lInstead, ~ as sketched in Figure 9, the film becomes a liquid of uniform thickness that then forms droplets and desorbs. This model is also supported by thermodesorption experiments with films prepared with different ions or with different fatty acids in discrete layers. There no preferential time sequence of desorption takes place indicating mixing before evaporation.ls The model also explains the finding of a uniform Fe distribution after desorption of iron stearate.17 There the organic component desorbs without droplet formation and the extension of the curve in Figure 8 makes understandable the formation of an inorganic film of uniform thickness. We do not fully understand why the film is uniformly thick. One possible explanation is that the surface tension of the organic liquid maintains a flat surface. However, it is metastable. If there are ruptures, the film detracts by desorption or by forming larger
i
T >25OOC
2 Figure 9. Model for processes occurring during heat of a cadmium arachidate multilayer.
droplets. Thus the process is a typical dewetting process starting from an ultrathin organic liquid of defined thickness.18 Concerning the local structure of the "liquid" film, we have noted that it may exhibit liquid crystalline order as observed for bulk material8 and as also observed for cadmium behenate m~1tilayers.l~ Finally we should notice that the temperatures given in Figure 1derived while varying the temperature may differ from those in the other experiments that were obtained at quasistatic conditions. In particular, this means that droplet formation may start well below 140 "C. On the other hand, the phase transition near 110 "C to the liquid film, observable as a slight reflectivity change in Figure 1, is also observed in measurements of the evaporation rate under vacuum16as well as in X-ray studies5 and thus independent of heating rate.
Acknowledgment. We thank G. Decher and H. Riegler for help with the SAXS experiments. The work was supported by the Bundesministerium fur Forschung and Technologieand by the Deutsche Forschungsgemeinschaft through SFB 262. Registry No. Cadmiumarachidate, 14923-81-0;silicon,744021-3.
(14) Daillant, J.; Benettar, J. J.; Bosio, L.; Leger, L. Europhys. Lett.
1988. ~... , 6. -, 431. (15) Laxhuber, L. A.; MBhwald, H. Surf. Sci. 1987, 186, 1. (16) Schreck, M. Thesis Tijbingen, 1990.
(17) Meisel, W.; Tippmnnn-Krayer,P.;MBhwald, H.;Giitlich, P. Fresenius' 2.Anal. Chem,, in press.
(18) Redon, C.; Brochard-Wyart, F.; Rondelez, F. Submitted for publication in Phys. Reu. Lett. (19)Merle, H. J.; L'vov, Y. M.; Peterson, I. R. Proceedings of the 3rd European Conference on Organized Organic Films. Makromol. Chem. 1991, 46, 271.
