J. Phys. Chem. 1993,97, 9009-9012
9009
Infrared External Reflection Study of Molecular Orientation in Thin Langmuir-Blodgett Films Takeshi Hasegawa, Junzo Umemura, and Tohru Takenaka' Institute for Chemical Research, Kyoto University, Uji, Kyoto-Fu 61 1 , Japan Received: March 9, 1993; In Final Form: June 13, 1993
FT-IR external reflection spectra of l-19-monolayer Langmuir-Blodgett (LB) films of cadmium stearate deposited on gallium arsenide substrates were measured for s- and p-polarized beams at various angles of incidence from 2 5 O to 80°. For the 9-monolayer LB film, the reflection absorbances for both polarized beams were also analyzed theoretically as a function of the angle of incidence. In the case of p-polarization measurements, the vibration bands with transition moments parallel to the film surface give rise to negative absorbances at incident angles smaller than the Brewster angle (73O), while those with transition moments perpendicular to the film surface give rise to positive absorbances at the same incident angles. However, the signs of these absorbances arecompletely reversed at incident angles larger than the Brewster angle. In the case of s-polarization measurements, on the other hand, all the vibration bands give negative absorbances throughout the whole angle of incidence, and their absolute values decrease with the increase in the incident angle. It was concluded from minute comparisons between the theoretical and experimental results that the molecules of cadmium stearate were oriented almost perpendicularly to the film surface. The absorbance values for the symmetric CH2 stretching bands were found to show excellent linearities against the number of monolayers. This reveals that themolecular orientation of cadmium stearate is kept unchanged even when the number of monolayers is changed.
Introduction Recently, molecular structure and orientation in LangmuirBlodgett (LB) films have been studied extensively because of their highly oriented structure, exactly controllable thickness, and good possibilities for functionalized molecular assemblies. Infrared spectroscopy has been considered to be one of the most useful tools for this purpose. Recent development of Fourier transform infrared (FT-IR) spectroscopy has increased its sensitivityup to a sufficient level for observationof reliable spectra of ultrathin LB films of one and a few monolayers. Among the various infrared reflection techniques,the reflection-absorption (RA) method has increased its importance for the study of molecular orientation in organic films deposited on metal substrates because of its orientational surface selection property. On the other hand, the infrared external reflection method for the same films on nonmetallic substrates with absorption coefficients less than unity has also been considered to be the most promising technique. In this case, the absorption bands provide not only positive but also negative absorbancesdepending on the direction of the transition moment, the polarization of the infrared beam, and the angle of incidence. Udagawa, Matsui, and Tanaka' measured external reflection spectra of a stearic acid film 100 nm thick casted on a glass or indium tin oxide substrate and discussed the optimal measurement conditions for polarization of the incident beam and the angle of incidence on the basis of the calculated results for a model of a three-phase system consisting of air, an organic film, and an inorganic substrate. Ishino and Ishidaz studied the optically anisotropic nature of LB films in the infrared region by the Kramers-Kronig analysis and spectroscopic ellipsometry. Based on ordinary and extraordinary optical constants thus obtained, they simulated positive and negative peaks in infrared external reflection spectra of cadmium arachidate LB films on silicon and poly(methy1 methacrylate) plates and found that the observed spectra were identical with the simulated results. Mielczarski and Yoon3s4 used the infrared external reflection techniqueto study molecular orientation in spontaneously adsorbed layers of ethyl xanthate (CzHsOCSz- ion) on a low-absorption substrate, cuprous sulfide (CuzS). They calculated the changes in peak intensities as a
* Author to whom correspondence should be sent.
function of the angle of incidence for the p-polarized beam (with the parallel and perpendicular components to the film surface) as well as for the s-polarized beam and obtained good agreements of the tendency with the experimental results. Although they discussed the molecular orientation in the adsorbed layers on the basis of these studies, there were discrepancies between the calculated and measured absorbancevalues. They ascribedthese differencesto the anisotropiccharacter of the adsorbed layer and also to differences between real and model systems. Recently, Parikh and Allara have also studiedthe external reflection spectra of thin LB films of cadmium icosanoate on carbon and silica glasss They simulated the spectra by adopting the 4 X 4 transfer matrix method developed by Yeha,' and discussed the C-H stretching region to evaluate the tilt angle of the alkyl chain axis. In this study, the infrared external reflection spectra of 1-19monolayer LB films of cadmium stearate deposited on gallium arsenide substrates were accurately measured under various conditions of polarization of the infrared beam and the angle of incidence. The observed intensities of the major absorption bands were compared quantitatively with the theoretical values calculated for our experimentalsystem by using analytical formulas for optics of thin multilayer films,3.4.* and then the molecular orientation in LB films was discussed. The thicknessdependence of the band intensities was also examined.
Experimental Section Pure stearic acid used in this study was the sameas that reported previously.9JO Distilled water was obtained by a modified Mitamura Riken Model PLS-DFR automatic lab still consisting of a reverse osmosis module, an ion-exchange column, and a double distiller. Cadmium stearate monolayers were prepared by spreading a 1.O mg/mL benzene solution of stearic acid on water containing 3 X 10-4 M CdClz and buffered with 3 X 10-4 M NaHCO3 to pH 6.2. After being left for 10 min to allow the solvent to be fully evaporated, the monolayers were transferred onto finely polished gallium arsenide circular wafers with the (100) crystalline plane by the conventional LB methodll at a surface pressure of 30 mN/m. The wafers had been cleaned in advance by successive ultrasonication in ethanol, acetone, and dichloromethane for 10 min each at 20 OC. Dipping and
0022-3654/93/2097-9009$04.00/0 0 1993 American Chemical Society
Hasegawa et al.
9010 The Journal of Physical Chemistry, Vol. 97,No. 35, 199'3 P
A
T
I
I
Y
I/
GaAs
/
Y
I ;
Y Figure 1. Schematicdiagram of the infrared external reflection system.
320T3000 2800 2 i O O "1700 1500 1300
Wavenumber I cm"
s-Polarizedinfraredexternalreflectionspectra of a 9-monolayer LB film of cadmium stearate on GaAs at angles of incidenceof 25', 50°, Figure3.
60°, and 80'.
In order to understand the above observations, the reflection absorbances for s- and p-polarized beams, A, and A,, were calculated for our experimental system shown in Figure 1 by using optics for thin multilayer films.3~4-8In the case of the p-polarization measurements, the electric field is divided into two components. One is the x-component which is parallel to the film surface, and the other is the z-component which is perpendicular to the film surface (Figure 1). Therefore, we calculated the reflection absorbances for these x- and z-components, A, and A,, and that for the s-polarized beam, A,, by3*438 "
I
,
1
I
3100 3dOO 2800 2600 1700 1500 1300
Wavenumber I cm-'
Figure2. p-Polarizedinfrared externalreflectionspectraof a 9-monolayer LB film of cadmium stearate on GaAs at angles of incidence of 25', 50°, 60°, and 80'.
withdrawingspeedsof the wafers were 5 mm/min, and the transfer ratio was 0.97 f 0.05 throughout the experiments. External reflection spectra were recorded on a Nicolet Model 7 10 Fourier transform infrared spectrophotometer equipped with an MCT detector with a resolution of 4 cm-l. A Harrick Model VRA versatile reflection attachment with a retromirror and a focus transfer accessory was used for p- and s-polarized beams obtained through a Hitachi wire-grid polarizer. The angle of incidence 0 varies from 25O to 80° (Figure 1). The p- and s-polarized infrared spectra of the same substrates without LB films were measured as references. The number of interferogram accumulations was 5000 in all the measurements.
under the condition that both air and gallium arsenide are transparent in this infrared region. Here, nl, n2, and n3 are the refractive indices for air, an LB film, and a gallium arsenide substrate, respectively, a2 and k2, ( i = x, y, and z)are components of the absorption and extinction coefficients of the LB film, respectively, and h2 is its thickness. Further, C Y Z and ~ €3 are given by CYzi
Results and Discussion Figure 2 shows the p-polarized infrared external reflection spectra of a 9-monolayerLB film of cadmium stearate on a gallium arsenide substrate. Angles of incidence are 25O, 50°, 60°, and 80°. The observed bands at 2917,2850, 1543, and 1468 cm-l assigned to the antisymmetric and symmetric CHI stretching (va(CH2) and vs(CH2)), antisymmetric COO stretching tua(COO)), and CH2 scissoring (6(CH2)) vibrations, respectively, give negative absorbances at smaller angles of incidence of 25', 50°, and 60°. However, the symmetric COO stretching (us(COO)) band at 1433 cm-1 and the progression bands from 1400 to 1200 cm-1 due to the CH2 wagging (w(CH2)) vibrations give positive absorbances at the same angles of incidence. Further, all the bands increase their intensity with an increase in the angle of incidence. When the angle of incidence is increased to 80°, however, drastic changes of the spectra happen. All the negative bands change to positive ones, and the positive bands change to negative ones. The s-polarizedinfrared external reflection spectra of the same LB film are shown in Figure 3. All the observed bands give negative absorbances, and the peak intensities decrease with the angle of incidence.
= 4Tk2,Y
(4)
and
t3= (n;
- nl' sin'
(5) where Y is the wavenumber of the infrared beam in vacuum. The results of the calculation for the u,(CH2) band at 2850 cm-l of the perpendicularly oriented alkyl chain axis to the film surface are shown in Figure 4. Optical constants used in this casewerenl= 1.0,n2=1 . 5 , k z , = k 2 ~ = 0 . 3 , k ~ , = O , ~ ~ n 3 = 3 . 2 8 , ~ ~ and h2 = 22.5 nm. The component A, shown by the solid line in Figure 4 gives a negative value at smaller angles of incidence below the Brewster angle Bg = tan-l(n3/nl) = 73O,and increases its absolute value with increasing angle of incidence. When the angle of incidence is beyond Os, A , suddenly changes its sign from negative to positive and quickly decreases in value with the increase in the angle of incidence. The component A , is zero irrespective of the angle of incidence because the extinction coefficient k , = 0 (eq 3).12 In the case of the s-polarization measurements, the reflection absorbance A, shown by the dotted line in Figure 4 is always negative and decreases the absolute value with increasing angle of incidence. The same analysiswas also performed for the normal orientation of the transition moments of the v,(COO) band at 1433 cm-l by
Molecular Orientation in Langmuir-Blodgett Films 0.04
The Journal of Physical Chemistry, Vol. 97, No. 35, 1993 9011 Number of
-
0
p-polarization
s-polarization (a)
s-polarization
mono'ayers
(b)
'
r
-
9
'
5
-0.04
Angle of incidence / degree
Figure 4. Reflection absorbance curve calculated for the vs(CH2) band at 2850 cm-l of the perpendicularly oriented alkyl chain in a 9-monolayer cadmium stearate LB film on GaAsby p-and s-polarizationmeasurements (Apand As). The former is divided into two components A, and A,. The open and solid circles are the observed values for p- and s-polarized beams, respectively.
l oOB, : , - ;
'
3LzAoo
3 00 Wavenumber / cm-' Wavenumber / cm.' Figure 6. Infrared externalreflection spectra in the CH2 stretchingregion of 1-19-monolayer LB films of cadmium stearate on GaAs observed for (a) a p-polarized beam at B = 60°, and (b) an s-polarized beam at B =
c4
30°.
-0.01 -0.02 a 2 -0.03 -0.02 -0-04 -0.05
t
u
0 10 20 30 40 50 60 70 80 90
Angle of incidence / degree
Figure 5. Reflection absorbancecurve calculated for the v,(COO)band at 1433 cm-' of the normal orientation of its transition moment in a 9-monolayercadmium stearate LB film on GaAs by p-and s-polarization measurements (APand A,). The former is divided into two components A, and A., The open and solid circles are the observed values for p-
and s-polarized beams, respectively.
using the optical constants nl = 1.0, n2 = 1.5, k k = kzy = 0, kb = 0.33,'4 n3 = 3.20," and h2 = 22.5 nm. The results are shown in Figure 5 . The component A,, shown by the solid line in Figure 5 gives a positive value below OB and increases its value with increasing angle of incidence. Above $, it changes its sign from positiveto negative and quickly increases in value with increasing angle of incidence. The components A , and A, are always zero because kk = kzY= 0 (eqs 1 and 2).14 Observed phenomena for the vn(CH2),v,(CH2), v,(COO), and 6(CH2) bands in Figure 2 agree qualitatively well with the theoretical results for A , and A, in Figure 4, and those for the v,(COO) band and the band progression due to the w(CH2) vibrations in Figure 2 agree with the tendencies of A , and A, in Figure 5 . Since the transition moments of the va(CH2), v,(CHz), v,(COO),and 6(CHz) vibrations are nearly perpendicular to the molecular axis of the all-trans alkyl chain and those of the us(COO) andw(CH2) vibrationsare nearly parallel to themolecular axis, we can qualitatively conclude from the above results that cadmium stearate with the straight alkyl chain is oriented perpendicularly to the film surface. In order to discuss this problem quantitatively, the absorbance values of the v,(CH2) band at 2850 cm-1 in p- and s-polarization measurementsare plotted with open and solid circles, respectively, in Figure 4, and the same plot for the v,(COO)band at 1433 cm-1 was also made in Figure 5. In all cases, agreements between the experimental values and theoretical curves are fairly good. Discrepancies between the theoretical A , curve and experimental values near the Brewster angle in Figure 4 and those between the theoretical A , curve and experimental values near the Brewster angle in Figure 5 may be partly due to experimental errors and partly to a slight inclination of the alkyl chain axis from the surface n0rma1.l~ An attempt to establish a new method for quantitative evaluation of the orientation angles of the molecular
axis by this infrared external reflection techniqueis now in progress by using a new refraction law for anisotropic optical systems.16 All the experiments described so far were carried out for LB films deposited on the very smooth (mirror-like) surface of the gallium arsenide wafer as mentioned above. When the same LB film was deposited on a nonpolished rough surface, however, the experimental results were quite different from those shown in Figures 4 and 5 in both points, the absolute intensity and its sign. This means that the surface roughness causes the diffused reflection of the infrared beams. Therefore, smoothness of the surface is an important factor for obtaining reliable external reflection data. Finally, the effect of the LB film thickness on the band intensity of the external reflection spectra was studied. Figure 6a and b shows the spectra in the CH2 stretching region of 1-19-monolayer LB films of cadmium stearate observed for the p-polarized beam at an angle of incidence of 60° and for the s-polarized beam at an angle of incidence of 30". It is to be noted that the highquality spectra could be obtained by this method even for the 1-monolayer LB film. Although the poor signal-to-noiseratio of external reflection spectroscopy on nonmetallic surfaces has been pointed out in previous papers,1P2 we can say that it is so good as to be able to obtain high-quality spectra of extremely thin films at the monolayer level. The reflection absorbances observed for the vs(CH2) band at 2850 cm-l were plotted in Figure 7 as a function of the number of monolayers. In both polarization measurements, excellent linearities were obtained. The same relation is also expected from the formulas (eqs 1-3). Therefore, it can be said that the molecular orientation of cadmium stearate is kept unchanged at all the LB film thicknesses examined in the present study.
Conclusion FT-IR external reflection spectra of 1-19-monolayer LB films of cadmium stearate on the smooth surface of a gallium arsenide wafer were measured for p- and s-polarized beams at various angles of incidence from 25O to 80°. At the same time, the wavenumber dependences of the reflection absorbances A,, A,, and A, were calculated for our three-phase plane-bounded system consisting of air, an LB film, and a gallium arsenide substrate by using Hansen's form~las.3~4~~ It was found that the peak intensities of the v,(CH2) bands obtainedfor the p- and s-polarized beams agreed quantitatively well with the theoretical A , and A, curves, respectively, calculated at 2850 cm-1, and those of the
9012 The Journal of Physical Chemistry, Vol. 97,No. 35, 1993 0
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Hasegawa et al. wafers used in the present study. This work was partly supported by the Grant-in-Aid for Special Project Research No. 04205083 and the Grant-in-Aid for Scientific Research No. 03640406from the Ministry of Education, Science and Culture, Japan, to whom the author’s thanks are due.
References and Notes
0
2
4
6
8
10 12 1 4 16 18 20
Number of monolayers Figure 7. Observed absorbancevalues of the u,(CHz) band a t 2850 cm-1 for (0) a p-polarized beam a t 0 = 60° and ( 0 )an s-polarized beam a t 0 = 30° as a function of the number of monolayers.
v,(COO) bands agreed well with the theoretical A,, and A, curves calculated at 1433 cm-l. From these results, we concluded that the molecular axis of cadmium stearate with the all-trans alkyl chain is almost perpendicular to the film surface. Furthermore, the peak intensities of the v,(CH*) band are plotted against the number of monolayers and we obtained excellent linearities, indicating that the molecular orientation is kept unchanged at all the LB film thicknesses examined. Finally, this method was found to be a powerful and sensitive tool to measure reliable spectra of organic ultrathin films such as single or a few monolayer LB films and to obtain useful information about molecular structure and orientation in the films.
Acknowledgment. The authors thank Dr. Koji Tada, Sumitomo Electric Industries, Ltd., for his kind gift of the gallium arsenide
(1) Udagawa, A.;Matsui, T.; Tanaka, S. Appl. Specrrosc. 1986,40,794. (2) Ishino, Y.; Ishida, H. Lungmuir 1988,4, 1341. (3) Mielczarski, J. A.; Yoon, R. H. J. Phys. Chem. 1989,93,2034. (4) Mielczarski, J. A.;Yoon, R. H. Lungmuir 1991,7 , 101. (5) Parikh, A. N.;Allara, D. L. J. Chem. Phys. 1992,96,927. (6) Yeh, P. J. Opr. Soc. Am. 1979,69,742. (7) Yeh, P. Surf. Sci. 1980,96,41. (8) Hansen, W. N.Symp. Faraday Soc. 1970,4,27. (9) Kimura, F.; Umemura, J.; Takenaka, T.Lungmuir 1986,2, 96. (10)Kamata, T.; Kato, A.; Umemura, J.; Takenaka, T. Lungmuir 1987, 3, 1150. (11) Blodgett, K. B. J. Am. Chem. Soc. 1934,56,495. (12) Popenoe, D. D.; Stole, S. M.;Porter, M.D. Appl. Specrrosc. 1992, 46, 79. The isotropic k value of 0.2 was given for the u1(CH2) band of CH,(CH2)&H in this work. T h i isotropickvalue is related to theanisotropic k,, ky,and k, values by the equation 3k = k, + ky+ k,. For the perfect normal orientation of the alkyl chain axis where the transition moments of the us(CH2) band are uniaxially oriented parallel to the film surface, 3k = 2k, since k, = ky,and k, = 0. (1 3) Kudo, K. Kiso Bussei Zuhyo (Tables for fundamental solid stare physics); Kyoritsu Shuppan: Tokyo, 1972;p 454. (14) We measured the transmission spectrum of a KBr pellet of the bulk crystallinestate cadmium stearate. From this spectrum, a value of k = 0.1 1 was obtained for the v,(COO) band by comparing its intensity to the intensity of the u,(CH2) band for which a value of k = 0.2was given by Popenoe et al. (see ref 12). In this case, 3k = k, since k, = ky = 0. (15) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990,94,62. (16) Hasegawa, T.; Umemura, J.; Takenaka, T.Appl. Specrrosc. 1993, 47, 338.
