Langmuir 1991, 7, 2287-2292
2287
Thermodesorption of Langmuir-Blodgett Films Studied by Mass Spectrometry M. Schreck,'l+ H. Schier,l and W. Gopelf Institut fur Physikalische und Theoretische Chemie, Universitat Tiibingen, Auf der Morgenstelle 8, D - 7400 Tiibingen, F.R.G., and Max-Planck-Institut fur Festkorperforschung, Heisenbergstrasse 1, D- 7000 Stuttgart 80, F.R.G. Received February 14,1991. In Final Form: April 19,1991 Thermodesorption spectra (TDS) are reported for various types of Langmuir-Blodgett (LB) multilayers consisting either of arachidic acid or of the corresponding Cd and Mg salts. Several distinct desorption mechanisms were identified to be dominant within characteristic temperature ranges. Phase transitions in the films and thermal decomposition of molecules, both of which led to pronounced TDS features, depend strongly on the head group forces and, hence, on the metal ions. These forces have been characterized quantitatively. On the basis of the spectra, the applicability of TDS is discussed for the purpose of quantifyingthe stability and quality of LB films. Our results demonstrate the unique advantages of mass spectrometry for studying the desorption behavior of organic films.
I. Introduction The increasing interest in Langmuir-Blodgett films is based upon their properties as well-defined model systems and their possible technical applications.' In addition to the difficulties of preparing metal contacts free of short circuits on LB filmsF4 a main restriction for their application is their often limited thermal stability. Thermal stabilities of thin films of carboxylic acids have been studied in earlier thermodesorption experiments. Pimbley and MacQueen, for example, used ellipsometry to investigate the desorption of stearic acid from different metal surfaces under isothermal conditions near room temp e r a t ~ r e .On ~ the basis of a simple model for multilayer desorption, they determined binding energies and compared these with vaporization and sublimation energies of the same substance, in order to decide which of the theoretically discussed adsorption types was valid. In a series of experiments Laxhuber et al. investigated the desorption of Langmuir-Blodgett mono- and multilayers by monitoring the film thickness optically in a similar way.@ They used films of different metal salts from a variety of carboxylic acids. In contrast to the isothermal experiments of Pimbley and MacQueen, Laxhuber et al. raised the temperature of the sample linearly. They extended the isothermal model for this more practical, but also much more complicated, case.8 In both models the assumption was made that the molecules were bound to the neighboring layers and that the binding within the layers was negligible.5 Laxhuber et al. obtained complex desorption spectra, which they attributed to different binding states. By + Institut fiir Physikalische und Theoretische Chemie, Universitat Tiibingen. t Max-Planck-Institut fiir Festk6rperforschung. (1)Roberta, G. G. In Langmuir-Blodgett Films; Roberta, G . G., Ed.; Plenum Press: New York, 1990. (2) Couch, N. R.; Montgomery, C. M.; Jones, R. Thin Solid Films 1986,135,173. (3) Geddes, N. J.; Sambles, J. R.; Jarvis, D.J. Thin Solid Films 1988, 167, 261. (4) Schreck,M.; Schier, H.; Schmeisser, D.; Gbpel, W.; Habermeier, H. U.; Roth, S.; Dulog, L. Thin Solid Films 1989, 175, 95. (5) Pimbley, W. T.; MacQueen, H. R. J. Phys. Chem. 1964,68,1101.
(6) Laxhuber, L. A.; Rothenhider, B.; Schneider, G.; Mbhwald, H. Appl. Phys. A 1986, A39, 173. (7) Laxhuber, L. A. Ph.D. Thesis, TU Munich, 1986. (8)Laxhuber, L. A.; Mbhwald, H. Surf. Sci. 1987, 186, 1. (9)Laxhuber, L. A.; MBhwald, H. Langmuir 1987, 3,837.
defining a mean desorption temperature, they quantified the stability of the films. They showed the influence of the head group forces, i.e. the binding of the metal ion and of the pH value of the subphase during film preparation, on the stability. On the basis of their model, they deduced activation energies for the desorption of Mg and Cd salts of arachidic acid. Independent of the theoretical model, the ellipsometric method has some disadvantages for studying the desorption of such complicated systems. First, it is an integrating method in the sense that different processes which might contribute to the desorption cannot be distinguished. It is not even possible to identify a single desorbing species. Second, the linearity between the measuring signal and the amount of desorbed material is only given for smooth films. In this paper we are presenting the results of a unique mass spectrometer thermodesorption study, which overwmes these difficulties and, hence, leads to more detailed information about different desorption processes and the corresponding activation energies. After the description of the experimentalsetup in section II., temperature-dependent mass spectra will be presented and evaluated in section 111.1. The spectra form a basis for the desorption experiments described in section 111.2. In part 111.3, experiments are discussed in which the number of monolayers and the type of the metal ion were varied. The spectra show a dominant influence of the head group forces. We present a simple model which allows for quantifying their strength by the method of varying the rate of heating (111.4). A discussion is also given in part 111.5 of the possibilities and the limits of a quantitative characterization of film properties by means of thermodesorption studies. An outlook on further experiments in part 111.6 sho*s the great potential of the method. 11. Experimental Section The LB films were prepared with a Lauda film balance in a class-100 clean room. Arachidic acid (Merck) and CdClr2H~0 (Fluka) were purified by repeated recrystallization. For magnesium arachidate films, MgC12.6Hz0(Aldrich)was used. Chloroform was of Merck Uvasol quality. Water was purified in a Milli-Q-System (Millipore) to 18 MQ cm. For the substrates, a good thermal conductivity was required. We therefore, used silicon Si(lll),which was either etched in 30% HF prior to the film preparation or coated with 200 A of
0743-7463/91/2407-2287~02.50/0 0 1991 American Chemical Society
Schreck et al.
2288 Langmuir, Vol. 7, No. 10, 1991 chromium and 2000 A of gold. The subphase in the trough contained 5 X 10-9 M CdCl2, or 5 X 10-9 M MgClz, respectively. It was buffered with KOH to pH 7.5 at a temperature of 19 "C. The films were spread from a 5 X lW3 M chloroform solution and kept at rest for 5 min before compression. The monolayers were compressed at a constant rate of 1.14 cmz/s up to the transfer pressure of 30 mN/m, corresponding to 20 AZ/molecule. The dipping speed was 0.83 mm/s for all cycles. The transfer ratio was better than 0.98. Even-sequenced multilayers between 2 and 20 monolayers were prepared. The samples were contacted with Ga/In to the sample carrier in the ultrahigh vacuum (UHV) chamber, as described below. In order to avoid faleifying contributions in the desorption experiment from the backside of the substrate, only one side was coated with the LB film; two strips of Si substrates were pressed against one another and dipped together. For subsequent desorption experiments only that sample was used which had been oriented toward the moving barrier. Ionization mode was E1 with an electron energy of 70 eV. Both the mass spectraand the desorption spectra were recorded with a quadrupole mass spectrometer QMG 511 (Balzers) with a mass range of 1-lo00 m u . The spectrometer was controlled via a 16-bit digital-to-analog converter by a personal computer. The intensity was recorded with a pulse counting module in the PC. The program provided the two options of recording conventional mass spectra and of operating in a single ion monitoring (SIM) mode for eight masses. In the SIM mode, the temperature of the sample was recorded simultaneously. The UHV chamber for the desorption studies was equipped with a turbomolecular pump and a liquid nitrogen cold trap. The base pressure of the chamber was 1 X 10-lo mbar. The design of the chamber was optimized to achieve a high pumping speed for the desorbing molecules. As a consequence we could assume a proportionality between the signal of a species and ita desorption rate, which allowed for a simple analysis of the spectra. Sample and analyzer of the mass spectrometer were arranged in a cross configuration, so that the desorbing molecules flowed directly into the cold trap, a fraction of them passing the ionization volume and being recorded. Contributions from moleculeswhich were reflected from the walls could be neglected. A fast entry sample lock enabled samples to be exchanged quickly. The transfer rod allowed cooling of the sample to liquid nitrogen temperature. Before the transfer the sample was cooled to 10 "C. In the measuring position of the UHV chamber the temperature program was started after cooling the sample to a few degrees below 0 "C at a base pressure of 1 X 10-8 mbar. Each TDS run, including the transfer of the sample into the chamber, lasted approximately 1 h. The sample holder was heated by direct current flow. The temperature was measured by a Ni/NiCr thermocouple fixed directly to the sample holder. To produce a good thermal contact between sample holder and the sample, we used an eutectic mixture of Ga/In. The accuracy in the determination of absolute temperatures (f2O) was checked by a second thermocouple which was contacted directly to the sample surface with Ga/In. The homogeneity, as estimated from the phase transition peak in the desorption spectra, was better than 1-2 "C (see 111.2).
111. Results and Discussion 1. Temperature-Dependent Mass Spectra. Typical examples of temperature-dependent mass spectra of cadmi.um arachidate films are shown in Figure 1. Evidently different mechanisms govern the desorption process in characteristic temperature ranges: In t h e low temperature range (
