1150
Langmuir 1987, 3, 1150-1154
Sciences, Fundamental Interactions Branch, is also gratefully acknowledged. The use of the equipment at the Center for Fast Kinetics Research, a facility supported by the National Institutes of Health and by the University
of Texas at Austin, is greatly appreciated. Registry No. MV2+,4685-14-7; SPVO, 77951-49-6; PAA, 9003-01-4;PDA, 26062-79-3; PEI, 9002-986; PVA, 9002-89-5;CdS, 1306-23-6;Ti02, 13463-67-7.
Intensity Enhancement of Infrared Attenuated Total Reflection Spectra of Stearic Acid Langmuir-Blodgett Monolayers with Evaporated Silver Island Films Toshihide Kamata, Akira Kato, Junzo Umemura, and Tohru Takenaka" Institute for Chemical Research, Kyoto University, Uji, Kyoto-Fu 611, Japan Received May 28, 1987. In Final Form: August 31, 1987 Band intensity enhancement of infrared spectra of stearic acid Langmuir-Blodgett (LB) monolayers with evaporated Ag islands was studied by using the attenuated total reflection (ATR) technique. In the case of a one-monolayer LB film, all stearic acids react with Ag forming silver stearate. The symmetric COO-stretching band, the transition moment of which is perpendicular to the Ag surface, is enhanced most. Polarization measurements of the ATR spectra and examination of the grazing incidence reflection-absorption spectra and the normal incidence transmission spectra of the stearic acid monolayers reveal that only the in-plane component of the electric field in the Ag film can enhance infrared absorptions of the monolayer. This fact is consistent with the intensity enhancement due to the high ele,ctromagnetic field due to collective electron resonances associated with the island nature of the thin metal films. In the case of multilayer LB films, the intensity enhancement decreases very rapidly with an increasing number of monolayers. This fact indicates that the effect is most prominent for the vibration bands of stearate ions directly attached to the Ag surface and that it has a long range nature of at least one molecular length, showing the enhancement is electromagnetic in nature.
Introduction In a previous work,l we studied Fourier transform infrared (FTIR) attenuated total reflection (ATR) spectra of Langmuir-Blodgett (LB) films of stearic acid with one to nine monolayers. It was found that the lower limit of detection by the FTIR-ATR technique was a t a level of the band intensity from the single monolayer films. Therefore, an improvement of the sensitivity of this technique is necessary for obtaining more reliable infrared spectra of these thin films. In recent years, considerable attention has been paid to the high electromagnetic field of surface plasmon polariton (SPP)in relation to the mechanism of surface enhanced Raman scattering (SERS).2 Recently, Hartstein et al.3 and Hatta et al.4-7reported that the infrared absorption from organic molecules was 1 or 2 orders of magnitude enhanced by the presence of thin metal overlayers or underlayers in the ATR technique. This effect was interpreted by an electric field enhancement due to SPP or collective electron resonances associated with the island nature of the thin metal films.8 Furthermore, Hatta et al.4 observed intensity enhancement of ATR spectra of a stearic acid monolayer due to a 5-nm Ag underlayer. Sigarev and Yakovle$ studied an effect of thin Ag overlayers on the ATR band intensity of mixed LB films of stearic acid and barium stearate. However, their observations were limited in the CH stretching region. In order to have more detailed information of this phenomenon, we observed the intensity enhancement of ATR spectra in the wider frequency range from 4000 to 800 cm-' of the stearic *Author to whom all correspondence should be addressed.
0743-7463/87/2403-1150$01.50/0
acid LB monolayers with evaporated thin Ag films.
Experimental Section The stearic acid used in this study is the same as that reported on previously.' Water was purified with a modified Mitamura Riken Model PLS-DFR automatic lab still consisting of a reverse osmosis module, an ion-exchange column, and a double distiller. The stearic acid monolayers were transferred from the water surface onto a Ge ATR plate (Figure 1)by the LB methodloin the same way as that described previously.' The surface pressure of the monolayer transfer was 20 mN m-l. The Ge plate was pulled down and up through the monolayer at a rate of 11mm mi&. The transfer ratios were 0.97 f 0.03 throughout the experiments. Prior to the monolayer transfer, the Ge plate was carefully polished by using diamond pastea with particles of 1-and 0.25rm diameters and then successively ultrasonicated in ethanol, benzene, chloroform, and distilled water for 10 min each. Finally, it was subjected to air plasma cleaning (a Harrick Model PDC-23G) at 0.1 Torr for 10 min. The Ag films with average thickness from 0.5 to 5 nm were deposited on one large face of the Ge plate with stearic acid (1) Kimura, F.; Umemura, J.; Takenaka, T. Langmuir 1986, 2, 96. (2) Chang, R. K.; Furtak, T. E. Surface Enhanced Raman Scattering;
Plenum: New York, 1982. ( 3 ) Hartstein,A; Kirtley, J. R.; Tsang, J. C. Phys. Reo. Lett. 1980,45, -A.
LUI.
(4)Hatta, A.; Ohshima, T.; Suetaka, W. Appl. Phys. A 1982, A29,71. (5) Hatta, A.; Suzuki, Y.; Suetaka, W. Appl. Phys. A 1984, A35,135. (6) Hatta, A.; Chiba, Y.; Suetaka, W. Surf. Sci. 1985, 158, 616. (7) Osawa, M.; Kuramitsu, M.; Hatta, A.; Suetaka, W.; Seki, H. Surf. Sci. 1986, 175, L787. (8) Moskovits, M. J. Chem. Phys. 1978,69,4159. (9) Sigarev, A. A.; Yakovlev, V. A. Opt. Spectrosc. (Engl. Transl.) 1984, 56, 336. (10)Blodgett, K. B.; Langmuir, I. Phys. Reo. 1937,51,964.
0 1987 American Chemical Society
IR ATR Spectra of Stearic Acid LB Monolayers
S
Langmuir, Vol. 3, No.6, 1987 1151
I
Ge PLate
LL Y e
WAVENUMBER I cm"
Figure 3. Dependenm of i n f ' r d reflectivity of Ge plates (with stearic acid monolayer) on Ag film thickness.
1 l l l l l 1 Ag Evaporotion
Figure 1. Ge ATR plate and its coordinate system.
4"
Figure 2. Scanning electron miemgraph of 4.5-nm Ag f h evaporated on stearic acid monolayer on Ce plate. monolayem (Figure 1) in an Ulvac Model EBH-6vacuum evap orator at 3 X lod Torr.The rate of evaporation wlls 6 nm min-'. The average thickness of the Ag fh wan evaluated by an Ulvac Model CRTM deposition monitor equipped with an oseillati quartz crystal. All infrared measurements were made on a Nicolet Model Goooc FllR s-photometer equipped with an MCT detector. A Perkin-Elmer multiple intemal reflection attachment wau used for A T R measurements. The angle of incidence was 45O and the number of internal reflections determined by the length of transferred monolayers along the I axis of the ATR plate (Figure 1) was 23. Polarized spectra were obtained with the aid of a wire.grid polarizer with the AgBr substrate. Five thousand interferograms collected with the maximum optical retardation of 0.25 cm were madded. apodized with the Happ-Cenzel function, and Fourier transformed with one level of zero filling to yield spectra of high signal-bnoise ratio with a resolution of 4 cm-l. Since the stearic acid monolayers were deposited on both large faces of the Ge plate (A and Bin Figure 1) hut the Ag film was evaporated only on one large face (A), all spectra reponed here were results of subtraction of the spectra obtained for the same Ce plate with the same stearic acid monolayers only on the opposite large face (B). For reference, the influence of thin Ag films on the infrared reflectionahrption and traMmisnion spectra of the stearic acid monolayer was examined. The reflectiowabsorption spectra were measured by using a Harrick reflection-absorption attachment with a standard system of [monolayer/thick (100 nm) Ag layer/slide glass] overmated with a thin (4.5 nm)Ag film. 7he angle of incidence was 85". and the accumulation number of the in-
WAVENUMBER I
sn*
Pienre 4. ppolarizad infrared A T R spectra of stearic acid monolayer with various Ag film thicknesses.
terferogrm
wan 3000. The normal incidence transmienion
megst"ts were d e ofthe atearic acid monolayer transferred on a Ge plate overcoated with a 4.5-nm Ag film. The interfew grams were accumulated 3000 times.
Scanning electron micrographs were observed with a JEOL
Model JEM-2000 FX electron micrmcope at 200 kV. Results and Discussion Intensity Enhancsment of Stearic Acid Monolayer. Figure 2 is a scanning electron micrograph of the 4 . 5 m Ag film evaporated on the stearic acid monolayer. A p parently, the Ag fh consists of closely packed islands with the uniform diameter ahout 30 nm. Figure 3 shows the dependence of infrared reflectivity of the Ge plate on the Ag film thickness. Since the reflectivity was markedly decreased from the high frequency side as a consequence of the nonselective absorption of Ag, we used Ag films thinner than 5 nm in this study. Figure 4 represents the ATR spectra of the stearic acid monolayer with various Ag film thicknesses measured for ppolarized radiation which has the electric vector parallel to the plane of incidence. The same spectra and intensity enhancement due to the Ag film were obtained for s-polarized radiation which has the electric vector perpendicular to the plane of incidence. The absorption bands at 2960,2920, and 2850 a d are undoubtedly assigned to the asymmetric CH, stretching and the antisymmetric and symmetric CH, stretching vibrations, respectively. The
1152 Langmuir, Vol. 3, No. 6,1987
Kamata et al.
15x10'' 25
g
-
20-
I-
O
a
LL
t
6
15-
3O
I
//?\(GIB)
I
10
2975
2450
2425 2400 2675 WAVENUMBER / cm-'
2650
2625
2 00
Figure 6. Reflection-absorption spectra in the CH stretching region of stearic acid monolayer before and after evaporation of 4.5-nm Ag film. 1
2
3
4
5
I
Ag FILM THICKNESS / nm
Figure 5. Intensity enhancement factors for major absorption bands as a function of Ag film thickness. Open and solid symbols refer to p- and s-polarized radiations, respectively. bands a t 1467 and 1120 cm-' are due to the CH2 scissoring and C-C stretching vibrations, respectively. Before Ag evaporation these bands are very weak, but their intensities are largely enhanced with increasing Ag film thickness. In a previous study,' we found that stearic acid in the onemonolayer LB f ilm was present as pure acid before the Ag evaporation. However, there is no sign of the C=O stretching band around 1700 cm-l even a t the Ag film thickness of 4.7 nm in the present investigation. Instead, a markedly enhanced band is seen at 1395 cm-l which is ascribed to the symmetric COO- stretching vibration of silver stearate." This fact reveals that the evaporated Ag islands are not on top of the hydrocarbon chains of the stearic acid monolayer but are between the polar groups of the monolayer and the Ge plate14 and react with stearic acid, forming silver stearate. It is to be noted that, even in this situation, the antisymmetric COO- stretching band cannot be seen in the expected frequency region around 1520 cm-l.12 The intensity enhancement factors for major absorption bands are shown in Figure 5 as a function of Ag-film thickness as derived from the data shown in Figure 4. For the asymmetric CH3 stretching and the antisymmetric and symmetric CH2stretching bands, the enhancement factors were calculated on the basis of the corresponding band intensities obtained without Ag. For the symmetric COOstretching band, however, it was based on the intensity obtained with the 0.5-nm Ag film, since no signal was observed without Ag. It is clear from Figure 5 that the effect of the Ag evaporation is not saturating, even with the 5-nm Ag film. Obviously, the intensity enhancement is the largest for the symmetric COO- stretching band. If silver stearate is oriented with ita long axis perpendicular (11) The frequency of bulk silver stearate is 1418 cm-l.I2 T h e lower frequency observed here indicates that stearate ions are the adsorbed species on the Ag surface, having interaction with the image dipolea.ls (12) Gotoh, R.;Takenaka, T. Nippon Kagaku Zasshi 1963,84, 392. (13) Hollins, P.; Pritchard,J. VibrutionulSpectroscopy of Adsorbates; Willis, R. F., Ed.; Springer-Verlag: Berlin, 1980; p 126. (14) This is partly due to the hydrophilic nature of Ag islands.
to the Ag surface, the transition moment of this band is perpendicular to the Ag surface. The largest enhancement is by a fador of 26 with the 5-nmAg film.The asymmetric CH3 stretching band has the second largest enhancement. The transition moment of this band is slightly inclined from the surface normal. The antisymmetric and symmetric CH2 stretching bands, the transition moments of which are largely inclined from the surface normal, are the last. Further, the antisymmetric COO- stretching band, the transition moment of which is parallel to the Ag surface, cannot be seen as mentioned above. Therefore, we can conclude that the enhancement factor of the bands decreases with increasing angle B between the transition moment and the surface normal (inset in Figure 5). In Figure 5, open and solid symbols refer to the p- and s-polarized radiations, respectively. Obviously, the corresponding open and solid data points are within the experimental error, indicating that the intensity enhancement is the same for the p and s-polarization. The former gives rise to both x and z components (Figure 1) of the evanescent electric field for total reflection a t the Ge surface, but the latter gives rise to the y component only. Recently, Osawa et al.' calculated three components of the electric field in a 5-nm Au film evaporated onto the Ge surface as a function of the angle of incidence. The results show that the z component is negligible and therefore that the electric field is essentially parallel to the Ge surface (the xy plane). Furthermore, the x and y components are about the same above the critical angle. These results are consistent with the above-mentioned fact that the p- and s-polarized radiations give rise to the same intensity enhancement of the stearic acid monolayer. Thus, we can conclude that the intensity enhancement is generated only by the electric field in the Ag film polarized parallel to the Ge surface. This conclusion was further confirmed by infrared reflection-absorption and transmission measurements of the stearic acid monolayers with the evaporated Ag island films. Figure 6 is the reflection-absorption spectra in the CH stretching region of the stearic acid monolayer before and after evaporation of the 4.5-nm Ag film. Intensity enhancement due to Ag is hardly seen. However, the transmission spectra of stearic acid monolayer shown in Figure 7 demonstrate that the band intensities are considerably enhanced upon introduction of the 4.5-nm Ag fh. The enhancement factors (11 for the asymmetric CH, stretching band and 6 for the antisymmetric and sym-
Langmuir, Vol. 3, No. 6, 1987 1153
IR ATR Spectra of Stearic Acid LB Monolayers
*:-7 LAYERS
W u
z a
m E
0
m m
a
10
2475
2450
2625
2600
2875
2850
2825
2 00
WAVENUMBER / cm"
Figure 7. Transmission spectra in the CH stretching region of stearic acid monolayer before and after evaporation of 4.5-nm Ag film.
I
:
Stearate Ion
3000
2800
tleoo
1800
WAVENUMBER
/
1400
1200
cm-'
Figure 9. ppolarized infrared ATR spectra of one-, three-, and seven-monolayer LB f i i before and after evaporation of 4.5-nm
Ag film.
Ge Figure 8. Schematic drawing of one-monolayer LB film incor-
porated with evaporated Ag islands.
Ge
metric CH2 stretching bands) were substantially the same as the corresponding values of the ATR measurements (Figure 5). The grazing incidence reflection-absorption geometry has the electric field component perpendicular to the substrate surface while the normal incidence transmission geometry has the electric field component in the plane of the monolayer. Therefore, the above facts indicate that only the in-plane component of the electric field in the Ag island film can enhance the infrared absorptions of the monolayer. We consider, on the basis of these discussions, the mechanism for the intensity enhancement due to the evaporated Ag film. Figure 8 is a schematic drawing of the one-monolayer LB film incorporated with the evaporated Ag islands. The Ag islands come between the polar groups of the stearic acid monolayer and the Ge plate and react with stearic acid, forming silver stearate. The intensity enhancement is generated only by the electric field in the Ag film polarized parallel to the Ge surface as mentioned previously. This may be due to a great enhancement of the local electromagnetic field a t the gap between adjacent Ag islands presumably through the excitation of the transverse collective electron resonances of the small metal islands8by irradiation of the incident light. Therefore, stearate ions located on the hills of the Ag islands make little contribution to intensity enhancement, but those located a t the edges of the Ag islands make a large contribution. Since stearate ions located at the edges of the islands orient almost parallel to the Ge surface, the absorption bands such as the symmetric COO- stretching band, the transition moments of which are parallel to the molecular axis and therefore perpendicular to the Ag surface, are largely enhanced by the strong electromagnetic field polarized parallel to the Ge surface.
Figure 10. Schematicdrawing of multilayer LB film incorporated with evaporated Ag islands.
Intensity Enhancement of t h e Stearic Acid Multilayer. So far, we dealt with the effect of Ag films on the ATR spectra of the one-monolayer LB film of stearic acid. Now let us discuss the effect of Ag films on the ATR spectra of the multilayer LB films. Figure 9 shows the p-polarized ATR spectra of the one-, three-, and seven-monolayer LB films before and after evaporation of the 4.5-nm Ag film. The same results are also obtained for the e-polarized radiation. As described above, in the case of the one-monolayer film, the marked intensity enhancement is obtained for the CH stretching, symmetric COO- stretching, and CH2 scissoring bands. However, in the case of the seven-monolayer film, almost no enhancement is observed for the CH stretching and CH2 scissoring bands. Interestingly, the C 4 stretching band due to stearic acid is slightly decreased in intensity upon Ag evaporation, while the symmetric COO- stretching band due to silver stearate is enhanced. These facts are interpreted as follows. In multilayer LB films, the evaporated Ag islands may randomly get between the polar groups of the succeeding two monolayers as shown in Figure 10. A part of the stearic acid comes into contact with Ag islands and reacts with Ag, forming silver stearate. The symmetric COO- stretching band of these stearate ions is enhanced by Ag. However, the other part of stearic acid is out of contact with Ag islands, and the C=O stretching band of these stearic acids is not enhanced. Since the proportion of stearic acids in the LB f i i decreases on Ag evaporation, the intensity of the C=O stretching band decreases. The three-monolayer f i i shows intermediate spectral features between those of the one- and seven-monolayer films
1154 Langmuir, Vol. 3, No. 6, 1987
Kamata et al. a
6
F 102
w
-
\
0
\
$ 4
z m a d
0 Ln
m 4
2
3 5 7 NUMBER OF LAYERS
9
Figure 11. Intensity enhancement factors for the CH stretching bands as a function of number of monolayer. Open and solid symbols refer to p- and s-polarized radiations, respectively. (Figure 9). Figure 11 shows the intensity enhancement factor for the three C H stretching bands as a function of the number of monolayers. The enhancement factor was calculated on the basis of the intensity of each band obtained without Ag. Apparently, the enhancement factors decrease very rapidly with increasing the number of monolayers. In Figure 12, the intensity of the symmetric COO-stretching band is plotted against the number of monolayers. The reason we adopt the intensity itself instead of the enhancement fador for this band is that no signal is observed here before Ag evaporation. Apparently, the band intensity also decreases with the number of monolayers. But the intensity decrease is milder for this band than for the C H stretching bands. The rapid decrease in the enhancement factors for the C H stretching bands can be understood by considering the following three factors. (1)The intensity enhancement is most prominent for the vibration bands of stearate ions directly attached to the Ag surface. (2) With an increasing number of monolayers, the proportion of stearic acid molecules that are out of contact with the Ag surface increases. Because the observed C H stretching bands are the superposition of the unenhanced bands due to these stearic acids and the enhanced bands due to stearate ions directly attached to the Ag surface, the increase in the proportion of stearic acid causes a decrease in the observed C H stretching intensities. (3)With an increasing number of monolayers, separations between adjacent Ag islands are statistically increased. Thus, the electromagnetic field
0
1
3 5 7 NUMBER OF LAYERS
9
Figure 12. Intensity of the symmetric COO- stretching band as a function of number of monolayers. Open and solid symbols refer to p- and s-polarized radiations, respectively. intensity a t the gap between the Ag islands is largely decreased and consequently the intensity enhancement is reduced. For the symmetric COO- stretching bands, however, factor 2 is not true, because the number of stearate ions is constant if the number of Ag islands is constant, and the observed symmetric COO-stretching band comes from these stearate ions. This may be responsible for the lower rate of decrease in the intensity of this band. With regard to factor 1, the asymmetric CH, stretching band of stearate ions that are in contact with the Ag islands is enhanced, as seen in Figures 4,5, and 11. Therefore the intensity enhancement has a long range nature of at least one molecular length and is consequently electromagnetic. This is consistent with the electric field enhancement due to collective electron resonances associated with the small metal islands.* Finally, it can be said that the intensity enhancement due to the Ag islands is significant, and this technique is useful for analysis and for obtaining structural information of very thin molecular films.
Acknowledgment. The authors wish to thank Drs. R. Nemori and M. Arai of Ashigara Research Laboratory, Fuji Photo Film Co.,La., for observations of scanning electron micrographs of the Ag films. The authors’ thanks are also due to Professors Y. Bando and M. Takano and Dr. T. Terashima of this institute for their valuable suggestions rendered at the initial stage of this work.
