Langmuir 1987,3,837-845
a37
Thermodesorption Spectroscopy of Langmuir-Blodgett Films L. A. Laxhuber and H. Mohwald* T U Munchen, Physics Department E22 (Biophysics), D 8046 Garching, West Germany Received December 3, 1986. In Final Form: April 10, 1987 Thermodesorption spectra of mono- and multilayen of fatty acids and other lipids are reported for various preparation conditions. It is shown that thermostability most drastically depends on head group forces and thus on pH and the nature of the divalent counterion in the subphase during transfer of multilayers and on the support of monolayers. Ionic conditions influence desorption temperatures by up to 140 OC, corresponding to changes in binding energies of 30 kJ/mol. Comparable influence is also observed by increasing the size of the hydrophobic parts of the molecule by varying either the chain lengths or the number of aliphatic chains. The spectra show a variety of binding states reflecting the complicated and preparation-sensitive microstructure of LB films. It is demonstrated that, at least for this pioneering thermostability study, the newly developed optical reflection technique is superior to the conventional mass spectroscopic analysis. It is also shown that the increase in thermostability on photopolymerization can be used to laterally structure the film.
I. Introduction Ultrathin organic films, especially the well-structured Langmuir-Blodgett (LB) films, encounter increasing interest predominantly due to many possible applications.' Beyond that, they represent valuable models to study the coupling between different multilayers and between organic and inorganic solids. One critical issue in using organic films generally results from molecular and structural stability. This in turn requires the development of preparative and analytical techniques to control the film stability. In this work, we report on thermal stability studies with LB films predominantly of simple fatty acids focusing on thermdesorption (TD) experiments. We demonstrate the advantages of a new optical technique to measure surface reactions and quantify the influence of electrostatic forces and molecular structure on binding energies. It turns out that well-resolved TD spectra are much more sensitive to structural details as pressure/area isotherms or electron diffraction data. 11. Experiments and Methods
after cleaning. The metal f i served as a heating electrode. This f i was also evaporated onto the glass slideswhere it additionally served as a smooth support with little contamination. A1 films were prepared by the same procedure. The LB film was prepared by the conventional dipping technique2 using a Teflon trough with a Wilhelmy type pressuremeasuring system. Normally during transfer the pressure was kept constant at 30 mN/m, but the influence of other pressures was also tested. The dipping speed was 6 mm/min. For photopolymerization of the Diac multilayer an UV lamp (Oriel 6035, 300 mW, 90% intensity at wavelength X = 253.7 nm) was placed 6 cm in front of the surface. The excitation intensity was 1.5 mW/cm2, illumination time 10 min. 11.2. General. Usually thermally induced desorption is measured by preparing a surface in UHV and then heating it in a linearly programmed way and measuring the desorption r a k 3 Rd = -dN/dt where N is the number of molecules per surface area. We have shown previously that for monolayer deposits the desorption kinetics can be described by a first-order process.4 In that case, if the sample is heated with a constant rate p (T = To+ fit),the temperature T,, corresponding to the maximum desorption rate and the binding energy E are related according to
11.1. Materials and Sample Preparation. Arachidic acid
(Ara), palmitic acid, and L-a-dimyristoylphosphatidic acid (DMPA) were purchased from Fluka (Buchs, CH). Tetrakis(3eicosylpyridinium-1-y1)porphyrin(TPyP) and the polymerizable 12,8-diacetylenefatty acid (Diac) were gifts of Dr. A. RuaudelTeixier, Saclay, and Dr. B. Tieke, Ciba-Geigy, respectively. The diacetylene fatty acid was dissolved in methanol whereas the other lipids were dissolved in a 3:l chloroform/methanol mixture. The divalent ion salts of the lipids at the air/water interface were prepared by adding CdC12or MgC12 (pro analysis, Merck, Darmstadt) to the subphase. If desired, pH was adjusted with Tris buffer (Sigma, Munich). The water used was Millipore filtered. Flame-polished glass (Duran) slides or 3-in. silicon wafers supplied by Wacker-Chemie,Burghamen, were used as substrates. They were cleaned either for several hours in 80 "C concentrated chromosulfuric acid or, for less contaminated surfaces, with the cuvette cleaner Hellmanex (Hellma). After being rinsed in Millipore water in ultrasonic bath, they were dried for 2 h at elevated temperatures before being immersed into an UHV chamber (Varian) for electrode evaporation. Before being cleaned, the silicon wafers were polished on one side and thermally oxidized in the target laboratory of our department (oxide thickness 1600 A). On the nonpolished side, a 1500-AAu film was evaporated *New address: University of Mainz, Institute of Physical Chemistry, D 6500 Mainz, FRG. 0743-7463f 87/2403-0837$01.50/0
where lzo is the frequency factor and R the gas constant. The problem in applying this classical surface analytical technique to LB films basically results from the facts that the samples are prepared in atmosphere and not under vacuum and that the preparation is far from being controlled considering the microstructure reflected in the desorption spectra. This requires (a) especially clean ways of sample preparation and storage, (b) measurement techniques that are less sensitive to contamination but still surface sensitive, and (c) development of simple techniques enabling studies of many samples during short times. The latter disfavprs vacuum techniques that are time consuming due to sample transfer into the vacuum and establishment of UHV. However, it does not strictly argue against these, because many features can be studied only under vacuum. 11.3. TD Studies under Vacuum. For studies under vacuum, the pumping system of an UHV evaporation chamber was used. (1)Roberta, G. G.Adu. Phys. 1985,34, 475. (2) Kuhn, H.; Mobius, D. Angew. Chem. 1971, 672. (3)Menzel, D.Topics in Applied Physics; Gomer, Springer-Verlag: Berlin, Heidelberg, New York, 1975;p 110. (4)Laxhuber, L. A.; Rothenhausler, B.; Schneider, G.; Mohwald, H. Appl. Phys. A 1986, A39, 173. (5)Redhead, P.A. Vacuum 1962,2, 203.
0 1987 American Chemical Society
Laxhuber and Mohwald
838 Langmuir, Vol. 3, No. 5, 1987
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11.4. TD Studies in Atmosphere. As mentioned above, it is highly desirable to be able to measure desorption also under atmospheric conditions. This led us to develop a new technique, interference-enhancedreflectometry (IER), that was used for most of the experiments reported in this work. The theoretical foundations of the technique together with model experiments have been given previously.4 The technique makes use of the fact that the reflection of a surface is changed by a deposit. This effect can be enhanced by coating a reflecting surface with a transparent overlayer of thickness equivalent to one-quarter of the wavelength onto which the organic film is transferred. Being interested in LB films on silicon oxide, we used a Si wafer with an oxide overlayer of 1600 8, thickness. Light from a He-Ne laser (wavelength X = 632.8 nm, Uniphase Model 1104) directed at an angle of 70' onto the surface and being polarized perpendicular to the incidence plane is reflected only to lo%, and an additional organic film increases the reflection considerably. We have shown that the thickness can be determined with an error of 0.2 8, using this technique and that over the range of LB film thicknesses used in our experiments (0-300 A) the reflected intensity depends linearly on deposit thickness. Consequently, the time derivative of the optical signal is proportional to the desorption rate. The set up for reflection measurements is basically that of Figure l b where the vacuum chamber, pumping system, and mass spectrometer are removed. The sample is mounted on an optical bench in a purgeable environmental chamber. The polarized light beam is chopped and detected by a large area photodiode (Siemens, Munich, BPW 33) that is fast (time constant, approximately microseconds) and linear in intensity over 7 decades. The signal from the diode is analyzed by a lock-in amplifier (EG&G Princeton, 5206),AD converted (12 bit), and further processed by a microcomputer. 11.5. Auxiliary Techniques To Study Thermally Induced Structure Changes. Previously heated samples were investigated by Nomarsky light microscopy using a Zeiss photomicroscope 111. This method allows detection of deposits with depth sensitivity of only about 10 8, but yields information on lateral heterogeneities. To study structural differences by electron microscopy and by electron diffraction, the multilayer was removed from the solid support in a HF bath and transferred on an electron grid.s It was then placed on a variable-temperature sample holder inside the microscope (PhillipsEM 400)where it was heated with a rate p = 0.2 K/s. Electron diffraction was measured at various temperatures with a dose of 0.3 e-/A2per measurement. By reference experiments we confirmed that for these doses beam-induced damage was not responsible for changes in electron diffraction spots observed.
111. Results and Interpretation 111.1. Comparative Mass Spectroscopic and Optical Desorption Studies. Our experiments were started by trying the mass spectroscopic measurements of desorption rates until we invented the more convenient method of optical detection. The results presented in this section shall demonstrate that both techniques principally yield comparable information. Figure 2a shows the change in total pressure on heating the sample with a constant rate for an uncoated A1 surface or for an A1 surface with a monolayer of cadmium arachidate. In both cases, one measures the maximum desorption rate at 80 "C (for this heating rate) indicating that also the monolayer desorption spectrum is largely determined by contamination. To reduce the influence of contamination, one may concentrate on detecting single organic masses that are specific for the deposit and not measure the integral over all masses (0-200 amu). T o do (8) Fischer, A. Ph.D. Thesis, TU Munich, 1985. (9) Laxhuber, L.A.;Mohwald, H.: Hashmi, M. Int. J.Mass. Spectrom. Ion Phys. 1983, 51, 93.
Langmuir, Vol. 3, No. 5, 1987 839
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Figure 3. Mass spectroscopically detected TD spectra of Cd Ara multilayers on a Au support. Subphase before transfer: pH 5.5, no buffer, 2 X lo4 M CdC1,; p o = 4 X mbar, heating rate fi = 8 K/s. Curves a, b, and c: Au support uncoated or coated with two or four monolayers, respectively. Curve d was calculated for four monolayers by extrapolation of the optical measurements as described in the text. The dotted peak near 60 O C indicates a contamination signal that was often experimentally observed. this we investigated the mass spectrum of arachidic acid powder heated to about 150 "C. Besides lines from small molecules like H2, HzO, Nz,and CO it contains groups of lines similar to those observed in SIMS s p e ~ t r a .The ~~~ most prominent of the higher mass lines is the one a t 58 amu that was used as specific for arachidic acid TD. Figure 2b shows the TD spectrum recorded for this line. One realizes that the maximum near 130 "C corresponds to a shoulder (see arrow) in the integral desorption spectrum (Figure 2a, trace m). This is reasonable if one assumes that the latter spectrum is composed of a contamination-free spectrum, but due to breaking of the molecule in many different masses, the signal to noise ratio is rather small t o allow an analysis of details in the spectra. As we could not better decontaminate the A1 (basically Al,O,) support, we abandoned it to study films on the less contaminated Au support. For this case Figure 3 gives TD spectra for zero, two, and four monolayers deposited. This
-TEMPERATUR I'C 1 Figure 4. Optically detected TD of Ara mono- and multilayers M CdC1,; atmospheric on Si02 Subphase: pH 5.5, no buffer, environment. The coverage N(T) and the derivative dN/dT are derived as described in section 11.4 of the text. Parts a, b, and c refer to desorption spectra of LB films consisting of 1,5, and 11 monolayers with /3 = 0.5 K/s.
support is obviously rather clean and the signal, due to contamination (trace a), does hardly influence the spectra even if one integrates over all masses. Yet a low intensity maximum at 60-80 "C is often observed in the multilayer spectra. The spectrum seems to consist of two components shifting to higher temperatures with increasing LB film thickness. Although the component with lower desorption temperature exhibits its maximum between 100 and 130 "C, i.e., a t about the value observed in Figure 2b, it must be assigned to desorption of the lowest layer. This is because binding to a hydrophilic acid (AlZ0,) or hydrophobic (Au) support may be very different. On the other hand, the support is irrelevant considering thicker films (greater than two monolayers). Hence we may compare these data obtained from a Au support with those from hydrophilic SiOz supports. Included in Figure 3 is therefore a desorption spectrum calculated from the optical measurement cited below. Comparing the optical and the spectroscopic measurement in Figure 3c and d one realizes agreement of essential features as positions of maximum and shoulder. Yet one observes a larger line width in the case of mass spectroscopic detection. This may be due to inhomogeneities of temperature distribution over the broader detection area in the latter case. The increase in desorption temperatures with film thickness is qualitatively understandable as it takes a longer time and hence higher temperature to desorb a thicker film. A model to quantitatively understand this behavior and then to extract physical parameters as reaction order, binding energy, and frequency factor will be presented in a separate publication.1° Figure 4a shows the change in deposit thickness on linearly increasing the temperature. The ordinate could be transformed in absolute thickness values4 by knowing the refractive index (n = 1.52)l' and the thickness of one monolayer (d = 26.7 A).12 The fact that the coverage (in monolayers) a t low temperatures agrees with the coverage (10) Laxhuber, L. A.; MBhwald, H. Surf. Sci, in press. (11)den Engelsen, D. J. Opt. Soc. Am. 1971, 61, 1460. (12) Rabe, J.; Knoll, W. Opt. Commun. 1986, 57, 189.
840 Langmuir, Vol. 3, No. 5, 1987
expected from the number of dippings indicates that the transfer was nearly complete, that there is little influence of contamination, and that the transformation from reflectivity into thickness is ~ o r r e c t . ~ Obviously desorption does again not result from one single component but from several ones with different binding energies. Principally this would also be expected if there were sample damage. However, the probability of damage is presumably not very large as the sample is typically kept only for 200 s a t a temperature of 100-200 OC. This demonstrates that the monolayer is not in a unique binding state and that the preparation will have to be improved to obtain a f i i that is uniform considering the microscopic interactions underlaying the spectra. Nevertheless, one realizes that the multilayer mainly desorbs between 100 and 200 "C with a small component extending to much higher temperatures. This is in qualitative agreement with the mass spectroscopic result of Figure 3 indicating that thermodesorption under vacuum and in atmosphere occurs by similar mechanisms and that the optical signal is indeed due to desorption and not to any structural change. On the other hand, comparing the spectra of Figures 3 and 4, one has to consider that we had to use higher heating rates to obtain a sufficient signal to noise ratio in the mass spectroscopic measurement. In the optical measurement, we could vary /3 over a broad range (0.13-5 K/s) and thus conclude on the reaction order, but we generally used lower heating rates to better resolve the structure in the spectra. Knowing the interdependence between heating rate and desorption temperature (see eq 2) and forming the derivative we can then extrapolate the spectrum of Figure 3c into one (dotted line) that can be compared to the multilayer spectra in Figure 2. The same procedure can be applied when comparing the monolayer spectrum of Figure 2b with the monolayer results for different heating rates in Figure 4a. However, a qualitative comparison already reveals considerble differences. While for heating rates (5 K/s) similar to that of Figure 2b desorption occurs mainly between 140 and 240 "C in Figure 4a, it appears between 80 and 200 "C according to the mass spectroscopic detection. This difference stresses the influence of the support that existed of SiOz in the optical and of A1,0, in the pressure measurement. We presently do not study the support influence in detail, because we do not possess the surface analytical tools to microscopically control and characterize the inorganic support. Beyond that additional experiments will be needed to clarify if the support investigated in a UHV chamber is identical with that onto which the LE film is transferred. The experiments presented above have demonstrated prime advantages of the optical technique: being applicable under vacuum as well as in atmosphere, being less sensitive to contamination, and being able to study a small surface area sensitively and fast. These advantages made us concentrate on further developing the reflection technique. (An excellent as well as comprehensive review on the application of optical techniques to study monolayers has been given by Drexhage.13) But we should state that this detection does not replace the more elaborate mass spectroscopy analysis mainly for the following reasons: (1) Optically, one measures changes of one parameter which may also be due to processes different from desorption. Examples of this, observed by us, were light scattering due to phase transitions and refractive index and (13) Drexhage, K. H. In Progress in Optics; Wolf, E., Ed.; North Holland: Amsterdam, 1974; Vol. VII, p 165ff.
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Thermodesorption Spectroscopy of LB Films A theoretical desorption description for the model compound arachidic acid has not yet been performed, but the head group influence on the pressure/area isotherm is easy to observe. Figure 5a shows these isotherms for a buffer-free subphase a t pH 5.5 for various concentrations of Cd ions in the subphase. Increasing the Cd concentration, one realizes the almost complete disappearance of the phase of medium compressibility. This phase, extending from about 24 to 21 A2, was previously called liquidle but may also be called crystalline tilted according to recent electron diffraction data." In any case, addition of CdC1, causes a condensation of the monolayer. This may be assumed due to Cd2+ions linking two head groups, and the latter seems to be a condition favorable for film transfer. In fact most of the transfers by the LB technique are performed a t pH 5.5 and a t CdC1, concentrations between lo4 and M. The selection of this pH is, however, not a physically intelligible condition, because in the absence of other ions the monolayer is partly charged a t pH 5.5.18 Yet it appears that Cd2+ binding is strong compared to the proton binding effecting the change in the isotherm. On the other hand we will show below that even in the presence of excess Cd2+ions the microstructure still depends on pH, although the isotherm does not. If the subphase contains an ion with an apparently weaker binding constant like Mg2+,the isotherm also depends on pH (see Figure 5b): a t low pH Mg2+does not seem to affect the isotherm whereas the condensing effect is obvious a t high pH. Hence the dissociation of protons is followed by binding of the divalent ion, abolishing the intermediate crystalline tilted phase. While generally the transfer onto the support occurred from the crystalline phase a t a pressure of 30 mN/m, we also studied the influence of pressure during transfer on thermostability. Depositing the LB films at pressures and conditions indicated by the crosses in Figure 5a, we could not detect any differences in the thermodesorption spectra of monolayers. Concerning the influence of counterions and pH on stability, we have to qualitatively distinguish mono- from multilayers. In the latter case bridging of head groups by divalent ions may be very specific whereas in the former case the ion binding to the head group on the water surface may be replaced by the metal ion from the support. In accordance with the latter assumption Auger spectra of monolayer^'^ had shown that the counterion is not transferred with the first monolayer during deposition. Consequently we do not expect any drastic influence of counterion or pH on the desorption spectra of monolayers as presented in Figure 6. In all cases considered the main desorption peaks are around 100 "C (for 0 = 0.5 K/s); in absence of the strongly binding Cd2+ion there is a component with low binding energy, presumably due to nonelectrostatically bound Ara molecules. Increasing the CdClz concentration produces a tail extending toward higher desorption temperatures. This may be called a stabilizing effect due to Cd binding, but it also is a broadening of the spectra meaning that there is a distribution of binding states a t the SiO, surface. This broad distribution may be also due to the fact that CdCl, is not completely dissociating to yield Cd2+ions and that there (15)L6sche, M.; Helm, C.; Mattes, H. D.; Mbhwald, H. Thin Solid F i l m 1985,133,51. (16)Gaines, G. Insoluble Monolayers at the Liquid-Gas Interface; Interscience: New York, 1966. (17)Kjaer, K.; Als-Nielsen, J.; Helm, C. A.; Laxhuber, L. A.; M6hwald, H. Phys. Rev. Lett. 1987,58,2224. (18)Petrov, J. G.; Kuleff, I.; Platikanov, D. J . Colloid Interface Sci 1982. 8.29. (19)'Barraud, A.; Rosilio, C.; Ruaudel-Teixier, A. Thin Solid Films 1968,68,7.
Langmuir, Vol. 3, No. 5, 1987 841
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842 Langmuir, Vol. 3, No. 5, 1987
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