4170
Langmuir 1995,11, 4170-4172
Structural Characterization of Long-chain Hydrocarbon Thin Films Using Ultra-Soft Polarized Near-Edge X-ray Absorption Spectroscopy
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Gerard P. Hastie and Kevin J. Roberts*,'
Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 IXL, Scotland, UK Received December 19, 1994. I n Final Form: June 13, 1995
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(a) Figure 1. Schematic illustrating the use of plane polarization in determining the orientation ofcertain bonds. The maximum
intensity of transition is achieved when the electric field vector Understanding the molecular science underpinning the is parallel to the respective bonding orbital; thus, when the crystallization of long-chain hydrocarbons is important beam is normal, (a)the dominant transition will be the C-H*, for a wide-range of industrial products and processes and at glancing angle, geometry (b) the major transition will be the C-C u*. including surfactants, food products, electronic materials [Langmuir-Blodgett (LB) films], membrane technology, and colloid science. A variety of techniques have been reversed a t glancing angles of incidence where the latter used to characterize the structure of long-chain hydrointeractions are highlighted and become more prominent carbons including infrared transmission and reflection(see Figure 1).In this way, by examining both the position absorption spectroscopies,'S2 atomic force m i c r o ~ c o p y , ~ ~ ~and intensity of the resonance features resulting from guided wave,5 X-ray diffra~tion.~@-ll molecular dynamic specific interatomidintermolecular interactions it is possimulations,12 and atom-atom potential calculation^.^^ sible to discriminate between the same atoms involved in From both a fundamental and industrial point of view it different bonding configurations. Recent work has seen the application of NEXAFS to more representative is important to clarify the structural parameters, such as condensed systems such as LB films,16parafins,17J8and chain lenth and head group nature, which influence lube oils.lg In this paper we further demonstrate the utility whether condensed hydrocarbons form supramolecular of the technique through the examination of the sustructures or not. Near-edge X-ray absorption fine pramolecular organization adopted by condensed thin structure (NEXAFS)spectroscopy15is particularly useful films of a number of representative long-chain hydrocarin this respect and has been extensively applied by the bons with varying head groups. In particular we examined surface science community to determine the molecular hexacosane (C26H54),eicosanol (ClgH390H),araichidic acid structure and orientation of chemisorbed molecules and (ClgH&OOH), and sodium stearate (C17H35COO.Na) monolayers. The features appearing in NEXAFS spectra prepared as thin films deposited on Si (111)substrates. can be attributed to transitions to localized electronic states which are strongly dependent on the bonding orbitals involved. By using the polarization of synchrotron Experimental Section radiation and by varying the angle of the incident photon The samples were all obtained from Aldrich Chemical Ltd.: beam to the substrate, the molecular orientation with the purities were stated as being '98%. Thin films were prepared respect to the substrate can be examined. For example, by vacuum evaporation onto Si (111)substrates. The silicon when a long-chain hydrocarbon, aligned along the subwafers (400pm thick), produced by Micro-Image Technology Ltd., strate normal, is examined a t normal incidence the electric were cleaned in chromic acid, rinsed in deionized water, and dried in air; thus, a native oxide layer is assumed to have been field vector is strongly sensitive to the C-H interaction formed. The evaporation was undertaken in a vacuum of 3 x and less so to the C-C o bonds. The situation becomes
* Author to whom correspondence
should be addressed. CLRL Daresbury Laboratory, Warrington WA4 4AD, UK. (1)Marshbanks, T. L.; Jugduth, H. K.; Delgass, W. N.; Franses, E. +
I. Thin Solid Films 1993,232,126. (2) Fujimoto, Y.; Ozaki, Y.; Kato, T.; Matsumoto, J. A. N.; Iriyama, K. Chem. Phvs. Lett. 1992.196.347. (3) Schwa&, D. K.; Gakaes, J.; Viswanathan, R.; Zasadzinski, J. A. N.; Science, 1992,257,508. (4) Leuthe, A,; Chi, L. F.; Riegler, H. Thin Solid Films 1994,243, 351.
( 5 ) Martin, A. S.; Sambles, J. R. Surf: Sei. 1990,225,390. (6) Sasuanuma, Y.; Kitano, Y.; Ishitani, A,; Nakahara, H.; Fukuda,
K. Thin Solid Films 1991,199,359. (7) Lin, B.; Peng, J. B.; Ketterson, J. B.; Dutta, P. Thin Solid Films 1988,159,111.
( 8 ) Barberka, T. A,; Hohne,U.; Pietsch, U.; Metzger,T. H. Thin Solid Films 1994,244,1061. (9) Yase, K.; Yamanaka, M.; Mimura, K.; Ueno, S.; Sato, K. Thin Solid Films 1994.243.389. (10)Yamanaka, M.: Mimura, K.; Yase, K.; Sato, K.; Inaoka, K. J. Cryst. Growth 1993,128,113. (ll)Yamanaka,M.;Sato,K.;Inaoka,K.;Yase,K. Jpn. J.Appl. Phys. Lett. 1992,31, 1632. (12) Karaborni, S. Langmuir 1993,9,1334. (13) Alfimov, M. V.; Bagaturyants, A. A.; Burshtein, K. Y. Thin Solid Films 1991,200,165. (14)Rabe, J. P.; Swalen, J. D.; Stohr, J.; Outka, D. A. Thin Solid Films 1988,159,275. (15)Stohr, J. NEXAFS Spectroscopy, Springer-Verlag: New York, 1992.
0743-7463/95/2411-4170$09.00/0
mbar a t an evaporation rate =2 nm s-l (monitored by a quartz crystal oscillator); thin films x200 nm thick were deposited. Following evaporation the films were annealed for 6 h in order to obtain maximum orientation.20 The NEXAFS spectra were recorded in the electron yield mode using a Channeltron detector over the spectral region 280-320 eV (carbon K-edge x284 eV) on beamline U1A of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory using an extended range grasshopper (ERG) monochromator.21 The experimental chamber provides for measurements to be made using a sample chamberz2which is separated from the main ultrahigh vacuum (UHV) beamline by a differentially pumped aluminum window (typically0.1-0.4pm thick). Thus the samples can be maintained a t moderate pressures (ca.
(16)Outka, D. A.; Stohr, J.;Rabe, J. P.; Swalen, J. D.; Rottermund, H. H. Phys. Rev. Lett. 1987,59,1321. (17) Stevens, P. A.; Martella, D. J. Mat. Res. SOC.Symp. Proc. 1993, 307,107. (18)Hastie, G.P.; Johnstone, J.;Roberts, K. J.;Fischer, D. submitted to J . Chem. SOC.Faraday Trans. (19) Hastie, G. P.; Roberts, K. J.;Adams, D.; Fischer, D.; Meitzner, G. Jpn. J . Appl. Phys. 1993,32-2,407. (20) Fukao, K.; Kawamoto, H.; Horiuchi,T.; Matshige,K. Thin Solid Films 1990,197,157. (21) Sansone, M.; Hewitt, R.; Eberhardt, W.; Sondericker,D. Nucl. Instrum. Methods, Phys. Res. 1988,A266, 422. (22) Fischer, D. A.; Colbert, J.; Gland, J. L. Rev. Sei. Instrum. 1989, 60, 1596.
0 1995 American Chemical Society
Langmuir, Vol. 11, No. 10, 1995 4171
Notes
LO
d0
Orientation laegrees
Figure 3. The incident angle dependence of the intensity of the C-Ca* resonance peak with respect to the normalized C K-edge NEXAFS spectra recorded for the sodium stearate sample.
4
Table 1. Results of the Molecular Tilt Angle ( r ) Determination Taken from the C K-Edge N E W S Data material tilt angle (t)( N E W S ) hexacosane (C26Hjq) 28 i 2" eicosanol (CIgHsgOH) 18 2" araichidic acid (C~QH~QCOOH) 30 i 2" sodium stearate(C17H3jCOO.Na) 30 If 2"
*
t
-a0
280
290
300
L
, - i 0
310: 280
290
300
310
Photon E n e r g y l e V
Figure 2. Experimental electron yield NEXAFS data: (a) hexacosane,(b)eicosanol, (c) araichidicacid, (d)sodium stearate. From top to bottom the angles of incidence for all spectra are go", 70", 50°,30" and lo", respectively. 1x mbar) and thus the measurements are suitable for the examination of organic thin films. The spectra for each sample were recorded at angles with respect to the incident beam at 10" intervals from 90" to 10". Over this angular range the electron yield penetration depth estimated using the CSDA methodz3varied between 100 and 35 A, respectively. This is negligible with respect to the much larger sample thickness. The spectra were all normalized with respect to the initial (at 280 eV) and final (at 320 eV)points15using the plotting/analysis program PLOTEK (Daresbury Laboratory). The tilt angle (t) with respect to the substrate surface normal was estimated by examining the spectra as a function of incident angle and by making a plot of the intensity of a particular transition (e.g.C-C a*) versus the incident angle.
Results and Discussion The recorded carbon K-edge N E W S spectra, following normalization, are summarized in Figure 2. It can be seen that the spectra are dominated by resonance features a t 289 eV (A), 293 eV (B) and 303 eV (C) which are characteristic, respectively, of the C-H*, C-Ca* and C-Co* contributions to the molecular orbitals. From the strong angular dependence of these resonance features, it is clear that all the samples show welldeveloped supramolecular order. The sharpness of the C-H* resonance (A) a t the lower glancing angles (top) reveals the long chain molecular axis to be aligned close to the sample normal. In Figure 3 this effect is more clearly demonstrated for the sodium stearate sample. Here ~~
~
~
(23) Ashley, J. C.; Tung, C. J.; Richie, R. H.IEEE Trans. Nucl. Sci. 1978,NS-26, 1566.
the plot of the intensity ofthe C-C 8 (B)resonance feature versus the incident angle of the beam with respect to the substrate reveals a tilt angle of 30". The results of the tilt angle calculations for all four samples are given in Table 1. Unfortunately there is a distinct lack of crystallographic data for the compounds examined here. In the case of hexacosane we can make the comparison with the close homologue t e t r a c ~ s a n ewhich , ~ ~ has a tilt angle of 17.8". On this basis, our data reveal that the thin films pack with a much larger tilt angle. However, some caution is needed in making this comparison, because there is some debate30-32as to the exact crystallographic structure adopted by hexacosane, as it lies close to-the changover point between the triclinic and monoclinic n-alkane (n = even) polymorphs. The reverse trend is seen in the case of eicosanol in comparison to the published structurez5for the shorter alcohol hexadecanol, which has a larger tilt angle of 33". The N E W S data (t = 30") for araichidic acid (C19H3&OOH), reveals a good match for close homologue stearic acid (C17H35COOH),although the precision (k2") of our measurements is not sufficiently good to be able to comment on the consistency of our data with the three known p, y , E polymorphs for stearic acid,26-28which have slightly different tilt anges of 27.2", 31.7", and 29.4", respectively. For anhydrous sodium stearate (C17H35COO*Na) soap, we have only the hydrated form ofthe shorter chain soap potassium palmitate (C15H31C00*K.Hz0),z8which has a tilt angle of 38.0",to compare with. The lower tilt angle for vacuum-prepared anhydrous film presumably reflects the more efficient packing of the (24) Gerson, A. R.; Nyburg, S . C. Acta Crystallogr. 1992,48, 737. (25) Abrahamsson, S.; Larsson, G.; von Sydow, E. Acta Crystallogr. 1960,13, 770. (26) Larsson, G.; von Sydow, E. Acta Chem. Scand. 1966,20, 1203. (27) Malta, V.; Gelotti, G.; Zarvetti, R.; Martelli, A. F.J . Chem. SOC. B 1971,548. (28) Kaneko, F.;Kobayashi, M.; Kiagawa, Y.; Matsaura, Y. Acta Crystallogr. C 1990,46, 1490. (29) Dumbelton, J. H.; Lomer, T. R. Acta Crystallogr. 1966,19,301. (30) Gerson, A. R.; Roberts, K. J.; Sherwood, J . N. Acta Crystallogr. 1991,B47,280. (31) Gerson, A. R.; Roberts, K. J.; Sherwood, J. N. Acta Crystallogr. 1992,B48, 746. (32) Craig, S. R.; Hastie, G. P.;Roberts, K. J.;Shemood, J. N. J M a t . Chem. 1994,4,977.
4172 Langmuir, Vol. 11, No. 10, 1995 alkyl chains brought about by the lack of hydration water molecules in the interlaminae region. In this paper we have demonstrated the utility of NEXAFS spectroscopy to the structural characterization, under non-UHV conditions, of condensed thin films prepared from a number of representative long-chain hydrocarbon systems. Further work in this area can be anticipated, notably with a view to assessing, in a more complete manner, the effect of chain length, head group, and preparative method on the structural properties of such systems. Applications associated with the design of surface active molecules specifically tailored for a given substrate material are also likely.
Notes
Acknowledgment. We gratefully acknowledge the Exxon PRT a t the National Synchrotron Light Source a t Brookhaven National Laboratory for use of beamline U1A. In particular we wish to thank Brian DeVries and Paul Stevens of Exxon and Dan Fischer of NIST for their support of this study and for helpful discussions. We also gratefully acknowledge SERCLEPSRC for travel funds (research grant G W 1 3 3 3 2 ) and for the financial support of a research studentship (G.H.) and a senior fellowship (K.J.R.). LA9410218
