Evaporated films of stearic acid studied by x-ray diffraction

Evaporated films of stearic acid studied by x-ray diffraction

L31 Thin Solid Films, 33 (1976) L31-L35 @ Elsevier Sequoia S.A., Lausanne - Printed in Switzerland Letter Evaporated films of stearic acid studied ...

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Thin Solid Films, 33 (1976) L31-L35 @ Elsevier Sequoia S.A., Lausanne - Printed in Switzerland

Letter

Evaporated films of stearic acid studied by X-ray diffraction V. K. AGARWAL,

Y. IGASAKI and H. MITSUHASHI

Research Institute

of Electronics,

(Received February 3,1976;

Shizuoka

University,

Hamamatsu

432 (Japan)

accepted February 13,1976)

The long chain fatty acids CHs(CHa),- zCOOH, e.g. stearic acid, are to form monolayer and multilayer films on a substrate when the latter is dipped and raised through a water surface covered by a fatty acid monolayer. This technique was proposed by Langmuir and Blodgett, and the films so obtained, referred to as Langmuir films, have been used for various types of electrical, optical and other studies (reviewed elsewhere’*‘) for about four decades. Unfortunately, however, only two attempts, to the knowledge of the authors, have been made to obtain fatty acid films by thermal evaporation in vacuum. Baker3 reported that films of stearic and melissic acids were obtained by thermal evaporation. Recently, Thomas4 has used evaporated films of stearic acid and other similar materials in his studies on fingerprint thin films. Both of these workers have reported that the films could be obtained as thin as one monolayer (~30A), but neither of them made a structural analysis of the films nor mentioned the essential parameters of the evaporation method used. The reason why only little work has been done on obtaining films of organic salts by thermal evaporation appears to be that thermal decomposition of high molecular weight materials was anticipated. In fact, such a possibility was first ruled out in the work of Baker3 and now we have obtained quantitative proof by studying evaporated films of stearic acid using X-ray diffraction. The aim of the present studies was to seek information about the film structure and to ascertain whether thermal decomposition of the acid took place during the evaporation process. Such investigations are also of significance because X-ray diffraction studies have been mainly confined to inorganic compounds and their films; in the case of crystalline organic compounds, the amount of work is relatively very small and, in particular, none has been done for evaporated organic films. In the present investigations, stearic acid in its bulk form as well as in film form has been studied by X-ray diffraction. However, here we shall only present a typical diffraction pattern for evaporated films of stearic acid as data on the bulk are already available in ASTM cards5. It should be pointed out that the peaks exhibited by powder samples in our studies were in close agreement with the cr-stearic acid data in ASTM cards (9-618) and hence we have compared our film data only with those. known

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A conventional type of vacuum coating unit coupled with a quartz crystal thickness monitor was used in the present studies. A crucible type tantalum boat with a very small opening in its top was used to avoid decomposition of the salt and to minimize the scattering of its vapour. The distance between the source and substrate was kept at about 10 cm. The glass slides, thoroughly cleaned by well-known methods, were heated to a high temperature for degassing and then cooled to ambient temperature before the films were evaporated. It is very important to let the substrate cool to a temperature far below the melting point of stearic acid (z69 “C) to avoid the possibility of re-evaporation of the vapour before final condensation. The acid was also allowed to degas before the shutter was removed to let the vapour condense on the slide. During evaporation the temperature of the boat was held practically constant at about 85 “C and the pressure inside the vacuum chamber was approximately 3-4 X 10P6 Torr. In these studies, a slow controlled evaporation rate (lo-12 a s- ‘) was used to obtain uniform and relatively thick films (2800 A). However, for very thin films it is desirable to maintain a very slow rate of evaporation (about 3 a ss’) by using a resistance-heated boat. For X-ray diffraction studies, a Geiger counter spectrometer was used and the range of scanning was from 28 = 1” to 28 = 30”; no peaks have been observed beyond this value in the case of evaporated films. First, the finely ground powder sample of stearic acid was uniformly packed into the rectangular depression of a glass slide and the surface of the powder was smoothed by exerting a to-and-fro motion with the plane surface of an ordinary microscope slide. To hold the powder more securely, an organic solvent (silicone) was used and it was allowed to evaporate before the data were obtained. The range of scanning for the powder sample was up to 20 = 40” as we were interested to make sure that all the peaks corresponding to ASTM data were obtainable. Experimentally, almost all the peaks were exhibited in the case of the powder samples. Films directly evaporated ontc the glass slides were then studied using a copper target in the X-ray source and a Ni filter was introduced to filter out Cu K, peaks in the observed diffraction pattern. The requirement of this method that films be sufficient ly thick with uniform surfaces was met to a large extent in each set of measurements. The area of the slide used for film evaporation was 20 mm X 10 mm and the central portion was selected for the X-ray studies because it was assumed that this part would have maximum uniformity. Twelve samples of the evaporated films were studied during these investigations and the peaks observed were found to be reproducible, within experimental limits, with regard to their position and relative intensity. A typical diffraction pattern is shown in Fig. 1; the results were compared with the ASTM data for a-stearic acid in powder form. The film data for peak positions and a visual estimate of their relative intensities I/Ii are tabulated in Table 1, together with similar data for bulk samples from the ASTM card. Clearly the initial six peaks exhibited by the evaporated films of stearic acid closely resemble those of the bulk. The situation with respec to their relative intensities, however, is quite tlifferent and is discussed belo-

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Fig. 1. A typical diffraction pattern of an evaporated film of stearic acid, obtained with a modern X-ray spectrometer at 20 kV and 10 mA. The inset shows the diffraction pattern for the region 28 = 19.00 to 26 = 23.00, which was taken at an applied voltage of 30 kV and at a current of 15 mA. TABLE 1 X-ray diffraction data for bulk samples of u-stearic acid and for films obtained by thermal evaporation. Data on bulk stearic acid powder* hkl 28 (talc.) Z/Z1

Data on evaporated 28 (obs.)

films Intensity

001 002 003 004 005 007 012 013

100 05 15 01 05 03 01 07

2.23 4.46 6.69 8.92 11.10 15.68 18.45 19.17

2.2 4.4 6.6 8.9 11.1 15.6 -

very high low high very low low very low -

204 112 110 016

01 25 100 01

19.91 20.37 21.62 22.34

19.0-20.0** 20.3-20.4** 21.5-21.6** 22.2-22.3**

very low very low high very low

(visual estimation)

*Adapted from ASTM card no. 9-618; the peaks beyond 26 = 22.34 are not given here because the films do not exhibit any of them. **These peaks are clearly visible in the inset of Fig. 1 which was taken at 30 kV and 15 mA.

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According to ASTM data, the three main peaks of stearic acid are at 28 = 2.23 (OOl), 21.62 (110) and 20.37 (112), whereas in the case of films the first three peaks in terms of decreasing intensity are at 28 = 2.21, 6.6 and 4.4. Obviously, the first peak (28 = 2.23) exhibits the highest intensity both in the film and in the bulk. The other two characteristic peaks of the powder sample, however, do not correspond to the characteristic ones of the film. This is understandable because the initial seven peaks (up to 28 = 15.6) exhibited by the film samples lie within the 001 group, showing that the film has a preferred orientation parallel to the basal plane of the monoclinic structure possessed by them. Nevertheless, it is of interest that the tw peaks 20 = 21.62 and 28 = 20.37 present in the bulk are not completely missing in the film pattern. The peaks corresponding to 28 = 19.9, 20.3 and 22.2 are shown by the traces (in the inset of Fig. 1) which were observed when the supply voltage and current (30 kV and 15 mA) were increased from the initial values of 20 kV and 10 mA. A small shift in the peak positions is attributable mainly to the experimental errors. In the given diffraction pattern, these peaks are not visible simply because of their low intensities; they appear to have been obliterated by noise. Since no additional peaks other than those exhibited by the stearic acid in bulk are observed, it is inferred that the evaporated films are free from metal impurities. This is expected because the temperature required for evaporation of stearic acid is too low (g85 “C) to initiate melting of metals. The fact that the peaks occupy positions corresponding to those given in the ASTM card is an indication that no thermal decomposition of the organic salt has taken place, since in that case the composition of the salt and hence the peak positions would change. Even though our experimental data suggest that the film structure is crystalline, we have no firm evidence as there might be relatively large crystalline areas within an amorphous region as has been observed in evaporated polymers. With regard to other foreign impurities such as Os, Na which might be adsorbed during thermal evaporation under vacuum, we believe that such possibilities are remote because the pressure inside the chamber was sufficiently low and also the stearic acid was degassed adequately before its vapour was allowed to condense onto the slide. A few of the film samples were aged for one month or more and X-ray diffraction studies were repeated. Such aging under atmospheric conditions did not affect the appearance of the peaks either in their position or intensity, and the diffraction patterns were reproducible within experimental limits. This shows the stability of stearic acid films for all experimental studies. The X-ray diffraction studies have shown that stearic acid films obtained by thermal evaporation under vacuum posses a monoclinic structure, in contrast to films of the same substance deposited by the LangmuirBlodgett technique which are known to have an h.c.p. structure2. These evaporated films are characterized by the 001 group, having a preferred orientation parallel to the basal plane of their monoclinic structure. The films are free from metal or other foreign impurities. In addition to their

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practical application in the field of tribology for lubrication3 and in the useful investigations of fingerprint thin films*, we are of the opinion that they are also suitable for various basic studies. Evaporated films of organic salts such as long chain fatty acids thus open up a new field of investigation for basic research. One of the authors (VKA) acknowledges with thanks the Japan Society for the Promotion of Science (JSPS) for granting him a research fellowship in Japan. 1 V. K. Agarwal, Electrocomponent Sci. Technol., 2 (1975) 1-31 (Part I); 75-107 II). 2 V. K. Srivastava, Phys. Thin Films, 7 (1974) 311. 3 M. A. Baker, Thin Solid Films, 8 (1971) R13. 4 G. L. Thomas, Thin Solid Films, 24 (1974) S52. 5 Vand, J. Appl. Phys., 19 (1948) 852 (ASTM card No. 9-618).

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