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Molecular Orientation in thin Monolayer Films by Infrared Spectroscopy
Journal of Electron Spectroscopy and Related Phenomena, 30 (1983) 29-34 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
29
MOLECULAR ORIENTATION IN THIN MONOLAYER FILMS BY INFRARED SPECTROSCOPY
J. F. Rabolt, F. C. Burns, N. E. Schlotter and J. D. Swalen IBM Research Laboratory, San Jose, California 95193
ABSTRACT
Fourier transform infrared spectroscopic measurements have been made on monolayer samples of cadmium arachidate in order to determine orientation and molecular packing on the surface. This was accomplished by using both grazing angle reflection methods, where the polarization of the infrared radiation is very close to being perpendicular to the surface, and transmission methods, where the incident optical electric field is polarized parallel to the surface. Hence these two methods are sensitive to molecular vibrations whose change in dipole moment lies along different directions. Our results showed that independent of the substrate, silver for the reflection experiments and silver bromide for the transmission experiments, the chains of the fatty acid salt (no evidence for any free acid was found) are oriented within a few degrees of the normal to the surface of the substrate. From a detailed analysis of the observed vibrational bands in the two orientations, combined with the known literature values and assignments, we were able to make a "complete assignment" of the observed bands. Our experimental results and conclusions will be presented.
INTRODUCTION
Infrared spectroscopy has been used to investigate the orientation of the aliphatic chains of fatty acid monolayers and their salts relative to the substrate surface. Francis and Ellison (Ref. I) employed a multiple reflection technique in which the monolayers were deposited directly onto two silvered mirrors to improve the signal to noise ratio from the few surface molecules.
Takenaka et al. (Ref. 2) used attenuated total reflection (ATR) to obtain
polarized IR spectra of 33 layers of stearic acid deposited on a germanium ATR plate and they estimated a tilt angle of 25 to 35 0 between the stearic acid tail and the surface normal, but found no evidence of in-plane anisotropy.
Chollet's studies (Ref. 3) of behenic acid
monolayers and its calcium salt by both reflection and variable angle transmission measurements indicated that the acid was inclined at 25 0 ± 4 0 and the salt was inclined at 8 0 ± 50. This conclusion for salts was supported by Allara and Swalen (Ref. 4) where grazing incidence IR at an angle of 86 0 (Greenler reflection) was used to investigate 1-10 monolayers of cadmium arachidate on silver. 0368-2048/83/0000-0000/$03.00 © 1983 Elsevier Scientific Publishing Company
30 Our work reported here (see Refs. 5 and 6 for other details) combines grazing incidence infrared (GIIR) spectroscopy with IR transmission measurements to accurately determine and characterize molecular orientation in a cadmium arachidate monolayer film.
EXPERIMENTAL All infrared measurements were made with an IBM IR98 FTIR interferometer equipped with a room temperature DTGS detector. Spectra of the monolayers were recorded at 2 cm- l resolution with the addition of corresponding data points from 1024 scans.
The grazing
incidence IR technique, described by Greenler (Ref. 7), selects the component of the electric field perpendicular to the surface and use its enhanced magnitude at the surface to obtain the polarized absorption spectra of thin films. Langmuir-Blodgett monolayers of fatty acid salts were prepared by methods previously described in detail (Ref. 8). These monolayers were deposited from a water trough onto silver coated microscope slides for reflection studies and on silver bromide substrates for transmission measurements. The trough contained, in addition to water and buffer (to provide a pH=6.3), a small amount of cadmium chloride to form the cadmium arachidate on the surface.
The arachidic acid (CH 3(CH2)lSCOOH) used in this work was obtained from ANALAB, Inc. from which the cadmium arachidate was synthesized in this laboratory. Bulk samples were prepared in KBr under high pressure.
RESULTS AND DISCUSSION Arachidic acid contains a long (-CH 2-)n sequence which exists in a trans planar structure in the solid state similar to the n-alkanes, Snyder and Schachtschneider (Ref. 9) have systematically studied the IR spectra of n-alkanes and made complete vibrational assignments.
In addition to the localized modes observed in the CH stretching region (2800-3000 cm- l) a number of band progressions involving a mixture of -CH 2- wagging, twisting and rocking vibrations appear in the 1150-1450 cm- l region; these modes can be clearly seen in the accompanying figure. Snyder (Ref. 10) has shown that these progressions, found in the spectrum of an oligomer, can be understood by consideration of the spectroscopic activity allowed through the breakdown in the optical selection rules for an infinite polyethylene chain. Since, as mentioned, recent IR studies (Refs. 3,4) on fatty acid salt monolayers indicate that after deposition on a metal surface, the hydrocarbon tail is oriented_approximately normal to the surface, a combination of GIIR, to obtain the IR spectrum.!"ith E parallel to the chain axis, and transmission measurements, to obtain the spectrum with E perpendicular to the chain axis, was used to assign the observed bands. In the CH stretching region (2800-3000 cm- l) there are a number of bands attributable to methyl (-CH 3) and methylene (-CH 2-) stretching vibrations.
As shown in the
31 accompanying figure, five bands of similar intensity are observed when E is perpendicular (E J.) to the substrate (or parallel to the hydrocarbon tail) while only three_bands of sizeable intensity, with perhaps a weak shoulder, are present in the spectrum with E pa.:allel (En) to the substrate (or perpendicular to the hydrocarbon tail).
The spectrum with E unpolarized
from a random sample is shown in the bottom spectrum. This should be a combination of one perpendicular or two parallel spectra. Visually this can be seen to be approximately correct, that is, combining the two top spectra in the right ratio should give the bottom spectrum In monolayers with the aliphatic tail truly perpendicular to the substrate it would be expected that the -CH 2- stretching vibrations would occur with sizable intensity only in the Ell spectrum and hence the bands at 2919 and 2850 cm- 1 are assigned respectively to the asymmetric (l'a(CH 2» and symmetric (l's(CH 2» stretching vibrations. The methyl CH stretching bands, found at 2874, 2962 and 2954 em-I, are assigned to l's(CH 3) and two components of the l'a(CH 3), respectively. The latter two bands exhibit different symmetry and polarization since the asymmetric methyl stretching modes can have a change in dipole moment either perpendicular or parallel to the skeletal plane. Thus, the 2962 cm- 1 mode, associated with the dipole moment change in the plane of the backbone, appears in the E.L spectrum while the 2954 cm- 1 band is observed in the E~ spectrum since its change in dipole moment is perpendicular to the skeletal plane.
When E is perpendicular to the plane of the surface, a medium band found in the vicinity of l'a(CH 2) at 2931 cm- 1 has recently (Ref. 11) been assigned to a second component of the l's(CH3) fundamental at 2874 cm- 1 split by Fermi resonance and is consistent with the assignment rendered previously by Spiker and Levin. 12 When E is parallel to the surface (the middle spectrum b) in addition to the 2962 cm- 1 mode, there is a shoulder at 2895 em-Ion the strong 2919 cm- 1 band. This could either be the l's mode of the methyl group, approximately at the average of the two symmetric methyl modes when E is perpendicular but shifted to lower energy as the asymmetric mode is in going from the E perpendicular to parallel orientation. Or it could be a Fermi resonance between the l'a(CH2) at 2919 cm- 1 and a combination of two 8(CH2) modes at the Brillouin zone center and in the middle of the zone. The l's(CH 3) would then probably be under the 2919 cm- 1 band. In the region below 1600 cm- 1 are found several strong bands whose intensities show a strong polarization dependence and which are attributable to the asymmetric and symmetric CO 2 stretching vibrations of the carboxylate group.
As seen in the figure in an isotropic sample, the asymmetric stretch, l'a(C0 2), is found at 1541 cm- 1 and is considerably more intense than the symmetric stretch, l's(C0 2), located at 1433 em-I. Since in the l's(C0 2) mode the change in dipole moment is perpendicular to the substrate surface while in the l'a(C0 2) mode it is approximately parallel to the surface, a significant polarization effect of an
32
0.25 3200
2800
2400
2000
Wavenumbers cm- 1
INFRARED SPECTRA OF CADMIUM ARACHIDATE
a) six monolayers on silver with E normal to substrate; b) eighteen monolayers on silver bromide with E parallel to substrate and c) isotropic bulk sample in KBr pellet with E unpolarized where {J is the CH 2 bending vibrational mode, y is the CH 2 twisting vibrational mode, w is the CH 2 wagging vibrational mode and a refers to the a carbon atom adjacent to the carboxylate group
33
oriented sample is expected. As seen in the figure, this is, in fact, observed. This supports earlier work (Refs. 3,4) that monolayers of fatty acid salts are oriented approximately perpendicular to the substrate. In a series of transmission experiments on an eighteen layered multilayer film at various positions of an IR polarizer, orientation differences within the monolayer plane were investigated. Within experimental error no relative intensity changes or frequency shifts were observed, indicating that in this case no preferential deposition direction from the monolayer trough had occurred.
After subsequent measurements of the crystal field splitting in the
-CH 2- bending and rocking region, orthorhombic subcell packing with the molecules nearly perpendicular to the substrate (orthorhombic modification) was identified as the crystal packing.
Two hydrocarbon chains in an orthorhombic unit cell have their molecular planes
oriented at an angle of 90° with respect to one another.
In sampling a large area with the
polarized IR beam, it would be expected that on the average, both orientations contribute and no apparent anisotropy should be detected. CONCLUSIONS Spectroscopic investigation of Langmuir-Blodgett mono layers of cadmium arachidate by grazing incidence and transmission measurements indicate that the deposited layers exist as the fatty acid salt and not as the free acid.
The measurements at these two orthogonal
polarizations led to an almost complete assignment from which it can be concluded that the hydrocarbon tails are, within a few degrees, oriented normal to the substrate surface whether they are deposited, in monolayer form, on either clean silver (for GIIR measurements) or on silver bromide crystals (for transmission studies). In-plane polarized transmission studies to determine the extent of orientation in the plane of the monolayers did not detect any anisotropy. However, this was not surprising since the perpendicular orientation of the molecular planes of the two molecules in the orthorhombic unit cell would lead to an effective averaging of the molecular contributions to the vibrational spectrum which in this case prevents differentiation between uniaxial or biaxial orientation. ACKNOWLEDGMENT We would like to thank M. Jurich for his preparation of the monolayer samples on silver bromide. REFERENCES 1. 2. 3. 4.
S. A. Francis and A. H. Ellison, J. Opt. Soc. Amer., 49 (1950) 131. T. Takenaka, K. Nogami, H. Gotoh and R. Gotoh, J. ColI. and Interf. Sci., 35 (1971) 395 and 40 (1971) 409. P. A. Chollet, Thin Solid Films, 52 (1978) 343. D. L. Allara and J. D. Swalen, J. Phys. Chern., 86 (1972) 2700.
34
5. 6. 7. 8. 9. 10. 11. 12.
F. A. Bums, N. E. Schlotter, J. F. Rabolt and J. D. Swalen, IBM Instruments, Inc., Application Note No.1, (1981). J. F. Rabolt, F. C. Burns, N. E. Schlotter and J. D. Swalen, J. Chern. Phys. (submitted) R. G. Greenler, J. Chern. Phys., 44 (1966) 310. G. L. Gaines, "Insoluble Monolayers at Liquid-Gas Interfaces," (Interscience, New York, 1966). R. G. Snyder and J. H. Schachtschneider, Spectrochim. Acta, 19 (1960) 85. R. G. Snyder, J. Mol. Spectrosc., 4 (1960) 411. R. G. Snyder, S. L. Hsu and S. Krimm, Spectrochim. Acta, 34A (1978) 395. R. C. Spiker and I. W. Levin, Biochim. Biophys. Acta, 388 (1975) 361.
29
MOLECULAR ORIENTATION IN THIN MONOLAYER FILMS BY INFRARED SPECTROSCOPY
J. F. Rabolt, F. C. Burns, N. E. Schlotter and J. D. Swalen IBM Research Laboratory, San Jose, California 95193
ABSTRACT
Fourier transform infrared spectroscopic measurements have been made on monolayer samples of cadmium arachidate in order to determine orientation and molecular packing on the surface. This was accomplished by using both grazing angle reflection methods, where the polarization of the infrared radiation is very close to being perpendicular to the surface, and transmission methods, where the incident optical electric field is polarized parallel to the surface. Hence these two methods are sensitive to molecular vibrations whose change in dipole moment lies along different directions. Our results showed that independent of the substrate, silver for the reflection experiments and silver bromide for the transmission experiments, the chains of the fatty acid salt (no evidence for any free acid was found) are oriented within a few degrees of the normal to the surface of the substrate. From a detailed analysis of the observed vibrational bands in the two orientations, combined with the known literature values and assignments, we were able to make a "complete assignment" of the observed bands. Our experimental results and conclusions will be presented.
INTRODUCTION
Infrared spectroscopy has been used to investigate the orientation of the aliphatic chains of fatty acid monolayers and their salts relative to the substrate surface. Francis and Ellison (Ref. I) employed a multiple reflection technique in which the monolayers were deposited directly onto two silvered mirrors to improve the signal to noise ratio from the few surface molecules.
Takenaka et al. (Ref. 2) used attenuated total reflection (ATR) to obtain
polarized IR spectra of 33 layers of stearic acid deposited on a germanium ATR plate and they estimated a tilt angle of 25 to 35 0 between the stearic acid tail and the surface normal, but found no evidence of in-plane anisotropy.
Chollet's studies (Ref. 3) of behenic acid
monolayers and its calcium salt by both reflection and variable angle transmission measurements indicated that the acid was inclined at 25 0 ± 4 0 and the salt was inclined at 8 0 ± 50. This conclusion for salts was supported by Allara and Swalen (Ref. 4) where grazing incidence IR at an angle of 86 0 (Greenler reflection) was used to investigate 1-10 monolayers of cadmium arachidate on silver. 0368-2048/83/0000-0000/$03.00 © 1983 Elsevier Scientific Publishing Company
30 Our work reported here (see Refs. 5 and 6 for other details) combines grazing incidence infrared (GIIR) spectroscopy with IR transmission measurements to accurately determine and characterize molecular orientation in a cadmium arachidate monolayer film.
EXPERIMENTAL All infrared measurements were made with an IBM IR98 FTIR interferometer equipped with a room temperature DTGS detector. Spectra of the monolayers were recorded at 2 cm- l resolution with the addition of corresponding data points from 1024 scans.
The grazing
incidence IR technique, described by Greenler (Ref. 7), selects the component of the electric field perpendicular to the surface and use its enhanced magnitude at the surface to obtain the polarized absorption spectra of thin films. Langmuir-Blodgett monolayers of fatty acid salts were prepared by methods previously described in detail (Ref. 8). These monolayers were deposited from a water trough onto silver coated microscope slides for reflection studies and on silver bromide substrates for transmission measurements. The trough contained, in addition to water and buffer (to provide a pH=6.3), a small amount of cadmium chloride to form the cadmium arachidate on the surface.
The arachidic acid (CH 3(CH2)lSCOOH) used in this work was obtained from ANALAB, Inc. from which the cadmium arachidate was synthesized in this laboratory. Bulk samples were prepared in KBr under high pressure.
RESULTS AND DISCUSSION Arachidic acid contains a long (-CH 2-)n sequence which exists in a trans planar structure in the solid state similar to the n-alkanes, Snyder and Schachtschneider (Ref. 9) have systematically studied the IR spectra of n-alkanes and made complete vibrational assignments.
In addition to the localized modes observed in the CH stretching region (2800-3000 cm- l) a number of band progressions involving a mixture of -CH 2- wagging, twisting and rocking vibrations appear in the 1150-1450 cm- l region; these modes can be clearly seen in the accompanying figure. Snyder (Ref. 10) has shown that these progressions, found in the spectrum of an oligomer, can be understood by consideration of the spectroscopic activity allowed through the breakdown in the optical selection rules for an infinite polyethylene chain. Since, as mentioned, recent IR studies (Refs. 3,4) on fatty acid salt monolayers indicate that after deposition on a metal surface, the hydrocarbon tail is oriented_approximately normal to the surface, a combination of GIIR, to obtain the IR spectrum.!"ith E parallel to the chain axis, and transmission measurements, to obtain the spectrum with E perpendicular to the chain axis, was used to assign the observed bands. In the CH stretching region (2800-3000 cm- l) there are a number of bands attributable to methyl (-CH 3) and methylene (-CH 2-) stretching vibrations.
As shown in the
31 accompanying figure, five bands of similar intensity are observed when E is perpendicular (E J.) to the substrate (or parallel to the hydrocarbon tail) while only three_bands of sizeable intensity, with perhaps a weak shoulder, are present in the spectrum with E pa.:allel (En) to the substrate (or perpendicular to the hydrocarbon tail).
The spectrum with E unpolarized
from a random sample is shown in the bottom spectrum. This should be a combination of one perpendicular or two parallel spectra. Visually this can be seen to be approximately correct, that is, combining the two top spectra in the right ratio should give the bottom spectrum In monolayers with the aliphatic tail truly perpendicular to the substrate it would be expected that the -CH 2- stretching vibrations would occur with sizable intensity only in the Ell spectrum and hence the bands at 2919 and 2850 cm- 1 are assigned respectively to the asymmetric (l'a(CH 2» and symmetric (l's(CH 2» stretching vibrations. The methyl CH stretching bands, found at 2874, 2962 and 2954 em-I, are assigned to l's(CH 3) and two components of the l'a(CH 3), respectively. The latter two bands exhibit different symmetry and polarization since the asymmetric methyl stretching modes can have a change in dipole moment either perpendicular or parallel to the skeletal plane. Thus, the 2962 cm- 1 mode, associated with the dipole moment change in the plane of the backbone, appears in the E.L spectrum while the 2954 cm- 1 band is observed in the E~ spectrum since its change in dipole moment is perpendicular to the skeletal plane.
When E is perpendicular to the plane of the surface, a medium band found in the vicinity of l'a(CH 2) at 2931 cm- 1 has recently (Ref. 11) been assigned to a second component of the l's(CH3) fundamental at 2874 cm- 1 split by Fermi resonance and is consistent with the assignment rendered previously by Spiker and Levin. 12 When E is parallel to the surface (the middle spectrum b) in addition to the 2962 cm- 1 mode, there is a shoulder at 2895 em-Ion the strong 2919 cm- 1 band. This could either be the l's mode of the methyl group, approximately at the average of the two symmetric methyl modes when E is perpendicular but shifted to lower energy as the asymmetric mode is in going from the E perpendicular to parallel orientation. Or it could be a Fermi resonance between the l'a(CH2) at 2919 cm- 1 and a combination of two 8(CH2) modes at the Brillouin zone center and in the middle of the zone. The l's(CH 3) would then probably be under the 2919 cm- 1 band. In the region below 1600 cm- 1 are found several strong bands whose intensities show a strong polarization dependence and which are attributable to the asymmetric and symmetric CO 2 stretching vibrations of the carboxylate group.
As seen in the figure in an isotropic sample, the asymmetric stretch, l'a(C0 2), is found at 1541 cm- 1 and is considerably more intense than the symmetric stretch, l's(C0 2), located at 1433 em-I. Since in the l's(C0 2) mode the change in dipole moment is perpendicular to the substrate surface while in the l'a(C0 2) mode it is approximately parallel to the surface, a significant polarization effect of an
32
0.25 3200
2800
2400
2000
Wavenumbers cm- 1
INFRARED SPECTRA OF CADMIUM ARACHIDATE
a) six monolayers on silver with E normal to substrate; b) eighteen monolayers on silver bromide with E parallel to substrate and c) isotropic bulk sample in KBr pellet with E unpolarized where {J is the CH 2 bending vibrational mode, y is the CH 2 twisting vibrational mode, w is the CH 2 wagging vibrational mode and a refers to the a carbon atom adjacent to the carboxylate group
33
oriented sample is expected. As seen in the figure, this is, in fact, observed. This supports earlier work (Refs. 3,4) that monolayers of fatty acid salts are oriented approximately perpendicular to the substrate. In a series of transmission experiments on an eighteen layered multilayer film at various positions of an IR polarizer, orientation differences within the monolayer plane were investigated. Within experimental error no relative intensity changes or frequency shifts were observed, indicating that in this case no preferential deposition direction from the monolayer trough had occurred.
After subsequent measurements of the crystal field splitting in the
-CH 2- bending and rocking region, orthorhombic subcell packing with the molecules nearly perpendicular to the substrate (orthorhombic modification) was identified as the crystal packing.
Two hydrocarbon chains in an orthorhombic unit cell have their molecular planes
oriented at an angle of 90° with respect to one another.
In sampling a large area with the
polarized IR beam, it would be expected that on the average, both orientations contribute and no apparent anisotropy should be detected. CONCLUSIONS Spectroscopic investigation of Langmuir-Blodgett mono layers of cadmium arachidate by grazing incidence and transmission measurements indicate that the deposited layers exist as the fatty acid salt and not as the free acid.
The measurements at these two orthogonal
polarizations led to an almost complete assignment from which it can be concluded that the hydrocarbon tails are, within a few degrees, oriented normal to the substrate surface whether they are deposited, in monolayer form, on either clean silver (for GIIR measurements) or on silver bromide crystals (for transmission studies). In-plane polarized transmission studies to determine the extent of orientation in the plane of the monolayers did not detect any anisotropy. However, this was not surprising since the perpendicular orientation of the molecular planes of the two molecules in the orthorhombic unit cell would lead to an effective averaging of the molecular contributions to the vibrational spectrum which in this case prevents differentiation between uniaxial or biaxial orientation. ACKNOWLEDGMENT We would like to thank M. Jurich for his preparation of the monolayer samples on silver bromide. REFERENCES 1. 2. 3. 4.
S. A. Francis and A. H. Ellison, J. Opt. Soc. Amer., 49 (1950) 131. T. Takenaka, K. Nogami, H. Gotoh and R. Gotoh, J. ColI. and Interf. Sci., 35 (1971) 395 and 40 (1971) 409. P. A. Chollet, Thin Solid Films, 52 (1978) 343. D. L. Allara and J. D. Swalen, J. Phys. Chern., 86 (1972) 2700.
34
5. 6. 7. 8. 9. 10. 11. 12.
F. A. Bums, N. E. Schlotter, J. F. Rabolt and J. D. Swalen, IBM Instruments, Inc., Application Note No.1, (1981). J. F. Rabolt, F. C. Burns, N. E. Schlotter and J. D. Swalen, J. Chern. Phys. (submitted) R. G. Greenler, J. Chern. Phys., 44 (1966) 310. G. L. Gaines, "Insoluble Monolayers at Liquid-Gas Interfaces," (Interscience, New York, 1966). R. G. Snyder and J. H. Schachtschneider, Spectrochim. Acta, 19 (1960) 85. R. G. Snyder, J. Mol. Spectrosc., 4 (1960) 411. R. G. Snyder, S. L. Hsu and S. Krimm, Spectrochim. Acta, 34A (1978) 395. R. C. Spiker and I. W. Levin, Biochim. Biophys. Acta, 388 (1975) 361.