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Layer-by-layer characterization of cellulose Langmuir–Blodgett monolayer films
Applied Surface Science 142 Ž1999. 585–590
Layer-by-layer characterization of cellulose Langmuir–Blodgett monolayer films Shin-ichi Kimura b
a,b,)
, Hiroyuki Kusano a , Masahiko Kitagawa b, Hiroshi Kobayashi
b
a Research Institute of Technology, Tottori Prefecture, Akisato, Tottori 680-0902, Japan Department of Electrical and Electronic Engineering, Tottori UniÕ., Koyama, Tottori 680-8552, Japan
Abstract We have investigated the preparation of palmitoyl cellulose Langmuir–Blodgett monolayer films on the indium tin oxide ŽITO. film layer-by-layer via Fourier Transform-Infrared ŽFT-IR. spectroscopy and Scanning probe microscopy ŽSPM.. The dependence of infrared absorption on mono-layer number has been analyzed. The absorption intensity has been shown to increase exponentially with the layer number in the LB film. P-polarized electric field in the FT-IR reflection absorption spectroscopy ŽRAS. revealed that the C–H stretch mode is stronger than the C s O stretch mode on the palmitoyl unit in the LB film, which is in contrast to a randomly condensed cast film. The mono-layer thickness determined from FT-IR structure analysis is in close agreement with the result obtained by SPM. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Cellulose; Langmuir–Blodgett films; Fourier Transform-Infrared spectroscopy; Scanning probe microscopy
1. Introduction Structure-controlled LB thin films can be used in organic devices such as functional sensors w1–3x and electronic devices w4x. In LB films, molecules have a specific orientation and alignment of both the main chain and the side chain. Microstructure of Cellulose LB films have been studied by Kawaguchi et al. w5x, Matsumoto et al. w6x and Itoh et al. w7x. The film thickness in nanometer scale can easily and definitely be defined in a self-aligned way. Nevertheless, cellulose based LB films are versatile
)
Corresponding author. Research Institute of Technology, Tottori Prefecture, Akisato, Tottori 680-0902, Japan
in flexibility of orientation and alignment in both the main chain and side chain. Therefore, the controllability or perfection of the so-called ‘monolayer’ stacked film has diversified categories, even if the film type of X, Y or Z is specified. It is mainly formed by the mono-layer formation through straightforward condensation of islands on the water surface without strong binding in the lateral direction. Hence, further layer-by-layer characterization is required in the course of and after the solid-like film is made, especially for use as functional electronic devices in which area-defects must be avoided. Although organic LB thin films offer an attractive method of designing advanced molecular materials for various applications as described, the properties of the films arise not only from the specific characteristics of the molecules incorporated but also from
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 9 2 3 - 4
S.-i. Kimura et al.r Applied Surface Science 142 (1999) 585–590
586
their macroscopic and microscopic arrangement within the film. Polarized infrared spectroscopy has been used to obtain information about microscopic molecular organization w8,9x. Since the infrared ŽIR. absorption depends on the relative orientation of the incident electric field and the dipole transition moment belonging to the molecular vibration of alkene, alkane, ketone and ether, it is possible to obtain information on the orientation of these molecules even in the mono-layer and also in layer-by-layer with respect to the substrate surface by using polarized infrared radiation. In this study, we have investigated the palmitoyl cellulose LB film on the indium tin oxide ŽITO. film by FT-IR and SPM for the first time. Layer-by-layer dependence of monolayer perfection including grain size, size distribution, mis-orientation and aggregation are presented and discussed.
2. Experimental Method for sample preparation has been described in detail elsewhere w1x. Fig. 1 shows the molecular structure of the palmitoyl cellulose. A monolayer of stearic acid was deposited at first w10x on the indium tin oxide ŽITO. substrate surface to enhance the
Fig. 1. Molecular structure of the palmitoyl cellulose.
Fig. 2. Surface pressure-area isotherm for palmitoyl cellulose at 290 K.
contact and then well-defined X-type layers of palmitoyl cellulose LB film were built up by using the horizontal lifting method at a trough temperature of 290 K. Fig. 2 shows surface pressure-area isotherm for palmitoyl cellulose on the surface of double-distilled water at 290 K. The limiting molecular area for the condensed region at the surface pressure from 5 to 30 mNrm of palmitoyl cellulose obtained from ˚ 2 per glucose unit Fig. 2 was determined to be 60 A and the molecular area for condensation was about ˚ 2 at 20 mNrm, which confirms the formation of 44 A the well-defined solid-like film with the area of ˚ 2 . The value is less than that of cellulose about 50 A ˚ 2 per glucose unit as reported by esters, 54 to 66 A Kawaguchi et al. w5x. Therefore, it is emphasized that stable palmitoyl cellulose Langmuir-monolayers were obtained by adjusting the optimum surface pressure during deposition at 20 mNrm. The infrared spectra were recorded on a JASCO Model FT-IR 7000. For RAS measurement, a JASCO Model PR-500 reflection attachment was used at the angle of incidence of 858. As in most studies incorporating this approach, the reflectance spectra of a film lying on a substrate has been recorded for the electric field parallel ŽS. and perpendicular ŽP. to the incident plane. The reflectance is defined as the ratio of the reflected intensity of the film-covered substrate to that of the
S.-i. Kimura et al.r Applied Surface Science 142 (1999) 585–590
587
Fig. 3. FT-IR reflection–absorption spectra of palmitoyl cellulose LB film.
bare substrate, to get the absorbance of the LB film. The surface profiles were recorded by a DIGITAL
INSTRUMENT Model Nano Scope IIIa. All traces were obtained in the non-contact a.c. Žtapping. mode. 3. Results and discussion Fig. 3 shows layer-by-layer dependence of the infrared spectra by successive FT-IR RAS measure-
Fig. 4. Layer number dependence of the absorption intensities of the IR modes in the palmitoyl cellulose LB films.
Fig. 5. IR absorptions between 1500 cmy1 and 3000 cmy1 for the palmitoyl cellulose films Ža. LB film and Žb. cast film.
588
S.-i. Kimura et al.r Applied Surface Science 142 (1999) 585–590
ment during deposition of up to five mono-layers of palmitoyl cellulose LB films. The spectrum exhibited the palmitoyl cellulose absorption bands at 2962 cmy1 :C–H asymmetric stretching vibration mode in CH 3 ; 2926 cmy1 :C–H asymmetric stretching mode in CH 2 ; 2872 cmy1 :C–H symmetric stretching vibration mode in CH 3 ; 2853 cmy1 :C–H symmetric stretching vibration mode in CH 2 ; 1751 cmy1 :C5O stretching vibration mode in COO–R Žglucose.; 1171
cmy1 :C–O stretching vibration mode in C–O–C. Absorption peak intensities have showed an increase with the increase of the mono-layer numbers in the LB films. Fig. 4 shows mono-layer number dependence of the integrated absorption intensities of each vibration mode described in Fig. 3. Integrated absorption intensity was defined as the area under the respective peak. The bold curves were drawn by fitting the
Fig. 6. The SPM of palmitoyl cellulose 5 layered LB film. Ža. SPM image and Žb. SPM section analysis.
S.-i. Kimura et al.r Applied Surface Science 142 (1999) 585–590
absorption intensities using a test exponential function such as Ii expŽyai n., where i denote the ith infrared mode i, n indicates the layer-number in the LB film, Ii is the intensity for i mode and a i is the absorption coefficient of the ith mode. The dependence of the intensity of all absorption peaks on the number of layers was well reproduced by assuming the exponential form as a function of the layer number, n. Therefore, the intensity of the infrared absorbance can be explained as a function of monolayer number in the cellulose LB film. The result, in turn, indicates that the present LB film is made of constant and definite number of glucose units per mono-layer which forms the multi-layered LB film by the n-times of successive condensation. Fig. 5Ža. and Žb. show expanded infrared spectra for palmitoyl cellulose between 1500 cmy1 and 3000 cmy1 , for LB films and cast films. In the FT-IR RAS measurement, p-polarized electric field was used, where the electric field vector of the incident light excites selectively the IR mode which has the molecular vibration perpendicular to the film surface, i.e., to the substrate surface. Hence, in the case of the well-defined palmitoyl cellulose LB film, the CH 2 asymmetric mode which has a stretching direction perpendicular to the substrate can be excited selectively compared with the CO mode which has a stretching direction parallel to the substrate. On the other hand, on film that has been cast, each palmitoyl cellulose takes a random orientation with respect to the substrate surface. As a result, the orientation of CH 2 bonds and CO bonds are random. Therefore, intensities of CO vibration modes decrease in the LB films compared with the cast films. In this case, the ratio Im rI k of LB film was 0.45, where Im is the intensity for the CH 2 asymmetric mode and I k is the intensity for the C5O mode. The ratio for the cast film was 0.20. These results confirm the formation of the mono-layered LB film. Fig. 6 shows the SPM profile of the palmitoyl cellulose 5 layers LB film. SPM profile showed grains with average size of 200 nm in diameter. The observed height of the grain was about 11 nm for the five mono-layered LB film. Thus, the mono-layer ˚ was estimated, which is consistent thickness of 22 A ˚ w1x. with the reported values between 20 and 26 A The results obtained by TEM and capacitance measurements are consistent with the SPM results.
589
4. Conclusion We have characterized the structure of the palmitoyl cellulose LB films on the indium tin oxide ŽITO. film, layer-by-layer, for the first time. The dependence of the IR absorption intensities of the selected IR modes have been described and interpreted for the palmitoyl cellulose LB films based on the stacked mono-layer numbers. FT-IR RAS measurements have revealed that in the LB film, the palmitoyl side chain has definite regular orientation with respect to the substrate, unlike cast film. The IR absorption in the LB film was explained by the exponential absorption function well, with the mono-layer number as the variable, which also conclusively suggests that well-defined mono-layer stacked palmitoyl cellulose LB films have been condensed on the ITO film. The mono-layer thickness of ˚ determined by SPM was consistent with the 22 A reported values.
Acknowledgements We are much indebted to Prof. S. Tanaka of Tottori University and Prof. Y. Yamashita of Okayama University for valuable advice, and H. Yokogawa, A. Kaneda, K. Sato, A. Nishio, Y. Suzuki of Research Institute of Technology for advice and support of this work. We would also like to express gratitude to Dr.Akagami M.D.B.Sc. Neurosurgery resident of Vancouver B.C Canada for assistance in proof reading this manuscript.
References w1x H. Kusano, S. Kimura, M. Kitagawa, H. Kobayashi, Thin Solid Films 295 Ž1997. 53. w2x M. Sugi, in: F.L. Carter, R.E. Siatkowski, H. Wohltjen ŽEds.., Molecular Electronic Devices, Elsevier, Amsterdam, 1988 p. 441. w3x S. Paddeu, F. Antolini, T. Dubrovsky, C. Nicolini, Thin Solid Films 268 Ž1995. 108. w4x T. Nakamoto, T. Moriizumi, in: B.R. McAvoy ŽEd.., Proc. IEEE Ultrason. Symp., IEEE, Chicago, 1988, p. 613. w5x T. Kawaguchi, H. Nakahara, K. Fukuda, Thin Solid Films 133 Ž1985. 29.
590
S.-i. Kimura et al.r Applied Surface Science 142 (1999) 585–590
w6x M. Matsumoto, T. Itoh, T. Miyamoto, in: H. Inagaki, G.O. Phillips ŽEds.., Cellulose and Its Utilization, Elsevier, Amsterdam, 1989, p. 151. w7x T. Itoh, H. Suzuki, M. Matsumoto, T.Miyamoto, in: J.F. Kennedy, G.O. Phillips, P.A. Williams ŽEds.., Functionalization of Cellulose, Ellis Horwood, Chichester, 1989, p. 409.
w8x C. Fiol, S. Alexandre, N. Dubreuil, J.M. Valleton, Thin Solid Films 261 Ž1995. 287. w9x H. Hui-Litwin, L. Servant, M.J. Digmann, M. Moskovits, Langmuir 13 Ž1997. 7211. w10x M. Fukuda, M. Miyagawa, H. Kawai, N. Yagi, O. Kimura, T. Ohta, Polym. J. 19 Ž1987. 785.
Layer-by-layer characterization of cellulose Langmuir–Blodgett monolayer films Shin-ichi Kimura b
a,b,)
, Hiroyuki Kusano a , Masahiko Kitagawa b, Hiroshi Kobayashi
b
a Research Institute of Technology, Tottori Prefecture, Akisato, Tottori 680-0902, Japan Department of Electrical and Electronic Engineering, Tottori UniÕ., Koyama, Tottori 680-8552, Japan
Abstract We have investigated the preparation of palmitoyl cellulose Langmuir–Blodgett monolayer films on the indium tin oxide ŽITO. film layer-by-layer via Fourier Transform-Infrared ŽFT-IR. spectroscopy and Scanning probe microscopy ŽSPM.. The dependence of infrared absorption on mono-layer number has been analyzed. The absorption intensity has been shown to increase exponentially with the layer number in the LB film. P-polarized electric field in the FT-IR reflection absorption spectroscopy ŽRAS. revealed that the C–H stretch mode is stronger than the C s O stretch mode on the palmitoyl unit in the LB film, which is in contrast to a randomly condensed cast film. The mono-layer thickness determined from FT-IR structure analysis is in close agreement with the result obtained by SPM. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Cellulose; Langmuir–Blodgett films; Fourier Transform-Infrared spectroscopy; Scanning probe microscopy
1. Introduction Structure-controlled LB thin films can be used in organic devices such as functional sensors w1–3x and electronic devices w4x. In LB films, molecules have a specific orientation and alignment of both the main chain and the side chain. Microstructure of Cellulose LB films have been studied by Kawaguchi et al. w5x, Matsumoto et al. w6x and Itoh et al. w7x. The film thickness in nanometer scale can easily and definitely be defined in a self-aligned way. Nevertheless, cellulose based LB films are versatile
)
Corresponding author. Research Institute of Technology, Tottori Prefecture, Akisato, Tottori 680-0902, Japan
in flexibility of orientation and alignment in both the main chain and side chain. Therefore, the controllability or perfection of the so-called ‘monolayer’ stacked film has diversified categories, even if the film type of X, Y or Z is specified. It is mainly formed by the mono-layer formation through straightforward condensation of islands on the water surface without strong binding in the lateral direction. Hence, further layer-by-layer characterization is required in the course of and after the solid-like film is made, especially for use as functional electronic devices in which area-defects must be avoided. Although organic LB thin films offer an attractive method of designing advanced molecular materials for various applications as described, the properties of the films arise not only from the specific characteristics of the molecules incorporated but also from
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 9 2 3 - 4
S.-i. Kimura et al.r Applied Surface Science 142 (1999) 585–590
586
their macroscopic and microscopic arrangement within the film. Polarized infrared spectroscopy has been used to obtain information about microscopic molecular organization w8,9x. Since the infrared ŽIR. absorption depends on the relative orientation of the incident electric field and the dipole transition moment belonging to the molecular vibration of alkene, alkane, ketone and ether, it is possible to obtain information on the orientation of these molecules even in the mono-layer and also in layer-by-layer with respect to the substrate surface by using polarized infrared radiation. In this study, we have investigated the palmitoyl cellulose LB film on the indium tin oxide ŽITO. film by FT-IR and SPM for the first time. Layer-by-layer dependence of monolayer perfection including grain size, size distribution, mis-orientation and aggregation are presented and discussed.
2. Experimental Method for sample preparation has been described in detail elsewhere w1x. Fig. 1 shows the molecular structure of the palmitoyl cellulose. A monolayer of stearic acid was deposited at first w10x on the indium tin oxide ŽITO. substrate surface to enhance the
Fig. 1. Molecular structure of the palmitoyl cellulose.
Fig. 2. Surface pressure-area isotherm for palmitoyl cellulose at 290 K.
contact and then well-defined X-type layers of palmitoyl cellulose LB film were built up by using the horizontal lifting method at a trough temperature of 290 K. Fig. 2 shows surface pressure-area isotherm for palmitoyl cellulose on the surface of double-distilled water at 290 K. The limiting molecular area for the condensed region at the surface pressure from 5 to 30 mNrm of palmitoyl cellulose obtained from ˚ 2 per glucose unit Fig. 2 was determined to be 60 A and the molecular area for condensation was about ˚ 2 at 20 mNrm, which confirms the formation of 44 A the well-defined solid-like film with the area of ˚ 2 . The value is less than that of cellulose about 50 A ˚ 2 per glucose unit as reported by esters, 54 to 66 A Kawaguchi et al. w5x. Therefore, it is emphasized that stable palmitoyl cellulose Langmuir-monolayers were obtained by adjusting the optimum surface pressure during deposition at 20 mNrm. The infrared spectra were recorded on a JASCO Model FT-IR 7000. For RAS measurement, a JASCO Model PR-500 reflection attachment was used at the angle of incidence of 858. As in most studies incorporating this approach, the reflectance spectra of a film lying on a substrate has been recorded for the electric field parallel ŽS. and perpendicular ŽP. to the incident plane. The reflectance is defined as the ratio of the reflected intensity of the film-covered substrate to that of the
S.-i. Kimura et al.r Applied Surface Science 142 (1999) 585–590
587
Fig. 3. FT-IR reflection–absorption spectra of palmitoyl cellulose LB film.
bare substrate, to get the absorbance of the LB film. The surface profiles were recorded by a DIGITAL
INSTRUMENT Model Nano Scope IIIa. All traces were obtained in the non-contact a.c. Žtapping. mode. 3. Results and discussion Fig. 3 shows layer-by-layer dependence of the infrared spectra by successive FT-IR RAS measure-
Fig. 4. Layer number dependence of the absorption intensities of the IR modes in the palmitoyl cellulose LB films.
Fig. 5. IR absorptions between 1500 cmy1 and 3000 cmy1 for the palmitoyl cellulose films Ža. LB film and Žb. cast film.
588
S.-i. Kimura et al.r Applied Surface Science 142 (1999) 585–590
ment during deposition of up to five mono-layers of palmitoyl cellulose LB films. The spectrum exhibited the palmitoyl cellulose absorption bands at 2962 cmy1 :C–H asymmetric stretching vibration mode in CH 3 ; 2926 cmy1 :C–H asymmetric stretching mode in CH 2 ; 2872 cmy1 :C–H symmetric stretching vibration mode in CH 3 ; 2853 cmy1 :C–H symmetric stretching vibration mode in CH 2 ; 1751 cmy1 :C5O stretching vibration mode in COO–R Žglucose.; 1171
cmy1 :C–O stretching vibration mode in C–O–C. Absorption peak intensities have showed an increase with the increase of the mono-layer numbers in the LB films. Fig. 4 shows mono-layer number dependence of the integrated absorption intensities of each vibration mode described in Fig. 3. Integrated absorption intensity was defined as the area under the respective peak. The bold curves were drawn by fitting the
Fig. 6. The SPM of palmitoyl cellulose 5 layered LB film. Ža. SPM image and Žb. SPM section analysis.
S.-i. Kimura et al.r Applied Surface Science 142 (1999) 585–590
absorption intensities using a test exponential function such as Ii expŽyai n., where i denote the ith infrared mode i, n indicates the layer-number in the LB film, Ii is the intensity for i mode and a i is the absorption coefficient of the ith mode. The dependence of the intensity of all absorption peaks on the number of layers was well reproduced by assuming the exponential form as a function of the layer number, n. Therefore, the intensity of the infrared absorbance can be explained as a function of monolayer number in the cellulose LB film. The result, in turn, indicates that the present LB film is made of constant and definite number of glucose units per mono-layer which forms the multi-layered LB film by the n-times of successive condensation. Fig. 5Ža. and Žb. show expanded infrared spectra for palmitoyl cellulose between 1500 cmy1 and 3000 cmy1 , for LB films and cast films. In the FT-IR RAS measurement, p-polarized electric field was used, where the electric field vector of the incident light excites selectively the IR mode which has the molecular vibration perpendicular to the film surface, i.e., to the substrate surface. Hence, in the case of the well-defined palmitoyl cellulose LB film, the CH 2 asymmetric mode which has a stretching direction perpendicular to the substrate can be excited selectively compared with the CO mode which has a stretching direction parallel to the substrate. On the other hand, on film that has been cast, each palmitoyl cellulose takes a random orientation with respect to the substrate surface. As a result, the orientation of CH 2 bonds and CO bonds are random. Therefore, intensities of CO vibration modes decrease in the LB films compared with the cast films. In this case, the ratio Im rI k of LB film was 0.45, where Im is the intensity for the CH 2 asymmetric mode and I k is the intensity for the C5O mode. The ratio for the cast film was 0.20. These results confirm the formation of the mono-layered LB film. Fig. 6 shows the SPM profile of the palmitoyl cellulose 5 layers LB film. SPM profile showed grains with average size of 200 nm in diameter. The observed height of the grain was about 11 nm for the five mono-layered LB film. Thus, the mono-layer ˚ was estimated, which is consistent thickness of 22 A ˚ w1x. with the reported values between 20 and 26 A The results obtained by TEM and capacitance measurements are consistent with the SPM results.
589
4. Conclusion We have characterized the structure of the palmitoyl cellulose LB films on the indium tin oxide ŽITO. film, layer-by-layer, for the first time. The dependence of the IR absorption intensities of the selected IR modes have been described and interpreted for the palmitoyl cellulose LB films based on the stacked mono-layer numbers. FT-IR RAS measurements have revealed that in the LB film, the palmitoyl side chain has definite regular orientation with respect to the substrate, unlike cast film. The IR absorption in the LB film was explained by the exponential absorption function well, with the mono-layer number as the variable, which also conclusively suggests that well-defined mono-layer stacked palmitoyl cellulose LB films have been condensed on the ITO film. The mono-layer thickness of ˚ determined by SPM was consistent with the 22 A reported values.
Acknowledgements We are much indebted to Prof. S. Tanaka of Tottori University and Prof. Y. Yamashita of Okayama University for valuable advice, and H. Yokogawa, A. Kaneda, K. Sato, A. Nishio, Y. Suzuki of Research Institute of Technology for advice and support of this work. We would also like to express gratitude to Dr.Akagami M.D.B.Sc. Neurosurgery resident of Vancouver B.C Canada for assistance in proof reading this manuscript.
References w1x H. Kusano, S. Kimura, M. Kitagawa, H. Kobayashi, Thin Solid Films 295 Ž1997. 53. w2x M. Sugi, in: F.L. Carter, R.E. Siatkowski, H. Wohltjen ŽEds.., Molecular Electronic Devices, Elsevier, Amsterdam, 1988 p. 441. w3x S. Paddeu, F. Antolini, T. Dubrovsky, C. Nicolini, Thin Solid Films 268 Ž1995. 108. w4x T. Nakamoto, T. Moriizumi, in: B.R. McAvoy ŽEd.., Proc. IEEE Ultrason. Symp., IEEE, Chicago, 1988, p. 613. w5x T. Kawaguchi, H. Nakahara, K. Fukuda, Thin Solid Films 133 Ž1985. 29.
590
S.-i. Kimura et al.r Applied Surface Science 142 (1999) 585–590
w6x M. Matsumoto, T. Itoh, T. Miyamoto, in: H. Inagaki, G.O. Phillips ŽEds.., Cellulose and Its Utilization, Elsevier, Amsterdam, 1989, p. 151. w7x T. Itoh, H. Suzuki, M. Matsumoto, T.Miyamoto, in: J.F. Kennedy, G.O. Phillips, P.A. Williams ŽEds.., Functionalization of Cellulose, Ellis Horwood, Chichester, 1989, p. 409.
w8x C. Fiol, S. Alexandre, N. Dubreuil, J.M. Valleton, Thin Solid Films 261 Ž1995. 287. w9x H. Hui-Litwin, L. Servant, M.J. Digmann, M. Moskovits, Langmuir 13 Ž1997. 7211. w10x M. Fukuda, M. Miyagawa, H. Kawai, N. Yagi, O. Kimura, T. Ohta, Polym. J. 19 Ž1987. 785.