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Optical properties and transmission features of J-aggregate in the merocyanine dye LB films by SNOAM
Optical Materials 21 (2002) 401–406 www.elsevier.com/locate/optmat
Optical properties and transmission features of J-aggregate in the merocyanine dye LB films by SNOAM H.-K. Shin, Y.-S. Kwon
*
Department of Electrical Engineering and CIIPMS, Dong-A University, 840, Hadan-2dong, Saha-gu, Busan 604-714, South Korea
Abstract The changes of the absorption spectra by UV irradiation were measured with the optical spectrum of the J-band and the monomer band. We illustrated the topographical structure and optical structure of the merocyanine dye Langmuir– Blodgett (LB) films obtained by the scanning near-field optical/atomic force microscopy. The topographic image of the mutually mixed LB films shows more rough surface structure than expected and grains with average size of about 50 nm. In the optical transmission image, the elevated area means higher transmission than lower parts. In the continuous measurement of these dyes and the mutually mixed films, the appearance of near-field optical transmission images showed a certain dependence on the kinds of dyes with the shift and increase of transmission and the mutual mixing ratios of dyes. These experimental results suggest that there is a certain kind of interaction between these two dyes. Ó 2002 Published by Elsevier Science B.V. PACS: 68.18.)g Keywords: Merocyanine dye; Aggregate; Topography; Transmission image; SNOAM; LB Film
1. Introduction J-aggregate formation of dye molecules has been investigated extensively in the Langmuir–Blodgett (LB) films [1,2]. Typical features of J-aggregates are the red-shift of absorption band with a small bandwidth and strong photoluminescence with a small Stokes shift. J-aggregate formation of dyes has been induced by the photoisomerization of azobenzene molecules existing in the same LB film [3,4]. Drastic morphological changes of the films
*
Corresponding author. Tel.: +82-51-200-5549/7738; fax: +82-51-200-5523/5550. E-mail address: [email protected] (Y.-S. Kwon).
were also noted. Much work has been aimed at determining the molecular structure of J-aggregates and clarifying the structure-spectroscopic relation [5,6]. Merocyanine dyes, originally developed as photosensitizers for silver halide photography, have renewed interest as a class of organic photoconductor, which may be useful for high efficiency photovoltaic device [7]. On the studies of photoconductivity, the formation of J-aggregates is one of the important subjects for molecular arrangement of dye because the existence of J-aggregation can improve the optical applications [8,9]. Even though, various reports were faced on the optical properties of merocyanines, there were no reports on the optical images of these dyes.
0925-3467/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 9 2 5 - 3 4 6 7 ( 0 2 ) 0 0 1 6 7 - 2
402
H.-K. Shin, Y.-S. Kwon / Optical Materials 21 (2002) 401–406
On the other hand, the research on the ultrathin film at atomic and molecular level has been rapidly expanded due to the development and improvement of scanning probe microscopes (SPMs) [10,11]. The atomic force microscopy (AFM) is a new tool for surface imaging which was introduced as one application of scanning tunneling microscopy (STM) [12]. Among many applications, due to its extreme resolution AFM has been used to image organic samples including LB films [13]. As compared with STM, the AFM can provide important advantages since no surface conductivity on the substrate is need to get an AFM image and it is thus independent of the charge transfer between the sample and the tip [14]. Here, the scanning near-field optical/atomic force microscopy (SNOAM) is a new tool for surface imaging which was introduced as one application of the AFM. Operated with non-contact forces between the optical fiber and sample as well as equipped with the piezoscanners, the instrument reports on surface topology without damaging or modifying the surface for measure of optical features in the thin films. In this report, we present the optica1 properties of merocyanine LB films in the far and near-field based on various film deposition conditions such as different kinds of merocyanine and mutual mixing ratios of the two kinds of merocyanine. Furthermore, we have illustrated the SNOAM image in obtaining the merocyanine dye LB films as well as the optical transmission image.
2. Experimental 2.1. SNOAM Setup Fig. 1 shows the schematic diagram of the SNOAM in tile transmission mode. A SNOAM instrument is based on a conventional AFM unit (SPI 3700, Seiko Instruments Inc.), which contains a dynamic-mode AFM function. A sharpened and bent optical fiber probe is mounted on a bimorph [14]. The distance between the tip of probe and sample surface is controlled by a laser-beam-deflecting
Fig. 1. Schematic diagram of SNOAM instrument is based on a conventional AFM unit that contains a dynamic-mode AFM function.
AFM technique, in which the cantilever vibrates vertically at the resonant frequency. The distance is controlled with a piezoelectric device module. The amplitude of the cantilever vibration decreases with a decrease in the distance. This AFM function allows that the light is to be easily illuminated from the optical fiber probe in an optically near-field region. Light of 488 nm from an Ar ion laser with the output power of 50 mW is coupled the end of probe and illuminated from the tip of the probe to sample surface. The light of argon ion laser is modulated by an acousto-optic(AO) modulator for improving S=N ratio of optical signal and controlling irradiation rage in the probe vibration. After transmission through both the aperture of optical fiber probe and sample, the remaining light is detected with a photomultiplier through a collimation lens, a mirror and an optical fiber. The signal from the photodetector is demodulated and amplified by a lock-in amplifier and noise in the optical signal is reduced. With this configuration, the SNOAM can provide topographical and optical images simultaneously with high resolution. The near-field probe was made of a single-mode optical fiber, whose core and cladding are 3.2 and 125 lm in diameter, respectively. In other to taper the probe and sharpen the tip to less than 100 nm diameter, the optical fiber was pulled and bent by a CO2 laser beam. The ridge of the probe was ground and polished into a minor for an optical lever of AFM function. The aperture at apex of the probe was formed by a 100 nm thick of aluminum film.
H.-K. Shin, Y.-S. Kwon / Optical Materials 21 (2002) 401–406
2.2. Ultra-thin film preparation The samples for the optical absorption, the optical and topographical images measurements were prepared by the standard procedure of LB film deposition with monolayer thickness of about 3.32 nm using glass substrates by y-type. 5-[2-(3otadecyl-2(3H)-benzothiazolylidene)ethylidene]-3arboxylmethyl-2-thioxo-4-thiazdidi-none, merocyanine dye (DO) and there derivatives 6Me-DS (6-methyl-5-[2-(3-otadecyl-2(3H)-benzothiazolylidene)ethylidene]-3-arboxylmethyl-2-thioxo-4-thiazdidi-none) and Dse(5-[2-(3-otadecyl-2(3H)-benzoselenazolylidene)ethylidene]-3-arboxylmethyl-2-thioxo-4-thiazdidi-none) were used as surface-active materials at the air-water interface [6]. The dyes weres mixed with arachidic acid (C20 ) as to molar ratio of 1:2. The NL-501-MHW (NLE Co.) moving-wall type trough was used to obtain LB film and CdCl2 (4 104 M) and KHCO3 (2 105 M) were used as subphase materials. The LB films
403
were deposited in vertical dipping method. Temperature was 19 °C, pH was 6:0 0:1. Temperature of 20 1 °C was controlled with a thermostat trough and target pressure was 25 mN/m. The subphase was hydrophilic slide glasses deposited five layers arachidates.
3. Results and discussion 3.1. Spectrum changes by far-field irradiation The changes of the absorption spectra by irradiation of UV light (185, 254, 366 nm) were measured. Fig. 2 shows the absorption spectra of multilayer (10 layers) LB films. The spectrum is taken just after the deposition and is taken after the irradiation of UV light for 3 min interval. From the Fig. 2, we found that relatively little structure changes occurred by the irradiation. It can be explained that the spectrum shift change is
Fig. 2. Spectral changes of the films caused by UV irradiation for far-field optical modification, but not absorption peak shifts. (a) Spectra for various numbers of irradiation cycles (interval 3 min) of 6Me-DS film; (b) DSe film; (c) DO film. By increasing the irradiation time, the J-band (sharp peak at 590 nm) and monomer band (broad peak at 500 nm) peaks such as solution absorption peak of dyes are gradually decreased (a), (b) and (c) films.
404
H.-K. Shin, Y.-S. Kwon / Optical Materials 21 (2002) 401–406
difficult to occur because the deposited LB film has an intermediate structure of the dye single aggregation. This is intermediate state that can easily be certified by itÕs color change. To obtain the J-band spectrum the structure of the LB film must be changed from the intermediate state to the welldefined J-aggregates. Fig. 2(a) shows the spectral changes by the irradiation of UV light to the multilayer LB film which was changed to J-band or -aggregates as shown at Fig. 2(b). We can know the fact that the LB films at the state of J-aggregates can easily change to anisotropic aggregates as the same manner with solution. Fig. 2(c) shows the result of the case that the UV light was irradiated to the monomer band film, which was changed to monomer band by UV light. The peaks of absorption at 500 and 590 nm were decreased. It shows that the LB films on the state of aggregates are changed to absorption intensity, but not band shift, by irradiation of UV light. From these results we can summarize the structural changes of the merocyanine multilayer LB films. To obtain the anisotropy, LB film must be changed to the absorption band by using the optical spectrum property such as well-defined Jaggregates. 3.2. Optical property modification In the continuous measurement on these dyes, near-field optical transmission images showed a certain dependence on the near-field irradiation laser beam. In the optical transmission image, the dark orange color parts correspond to the dye surface of weak transmission and the yellow and white color parts to the dyes depend on strong transmission because the all parts are transparent. It is clear from the comparison between these continuous four images with shift and increase of transmission areas of Fig. 3 that the lower light intensity areas in Fig. 3(b) correspond to the areas in Fig. 3(a) where aggregated dyes seems to cover the glass substrate. This agrees with the above observation on the sample without the aggregated dye film that the intensity of the evanescent wave behind the non-aggregated dye film is weak. The screening of the excitation beam by the dye aggre-
Fig. 3. The optical transmission images show shift and increase of optical transmission areas in DSe film. A 488 nm line of Ar ion laser supported SNOAM is used as a continuous scans. Next scan areas are presented as circular area, and continuous scans are repeated for two times in the area of 500 nm2 . The distance between optical fiber end and the sample is maintained as 50 nm during the continuous scans. The scan size is 2:5 2:5 lm2 .
gated film also gave clear contrast in the transmission intensity mapping of Fig. 3(c) and (d), where the excitation beam picked up by the optical fiber was cut by the filter in front of the SNOAM system. For the use of the present SNOAM, a light irradiation mode through the optical fiber tip is preferable to avoid appreciable photodamage and bleaching of the dye and is now under investigation. 3.3. Local optical property modification Fig. 4 shows continuous SNOAM images of the mixed monolayer of arachidic acid and merocyanine dye with long alkyl chains (molar ratios). Addition of a small amount of the dye affected little on the phase behavior of the mutual mixture. It is reasonable to assume that the long alkylated merocyanine dye is much more soluble in the aggregates than the monomer phase. In practice, much stronger emission of the merocyanine dye was observed in higher than in the lower of molar ratio in the mixed monolayer phase. This observation together with that of the previous well-defined aggregate
H.-K. Shin, Y.-S. Kwon / Optical Materials 21 (2002) 401–406
405
Fig. 6. Surface topography (a) and optical transmission image (b) of merocyanine dye LB film (DO) in the area of 5 5 m2 .
Fig. 4. The changes of optical transmission image show by continuous scans of 6Me-DS film. The scan areas of 2:5 2:5 lm2 are presented as rectangular areas of (a) and (b): (a) near-field transmission image of the first scan, (b) change of near-field transmission image after 50 scans on the rectangular area of (a) and (c) the change of near-field transmission image after 50 scans on the rectangular area of (b) with dynamic force mode.
during the experiment, and scan speed held 0.127 Hz with 256 scan lines. In the topographic image of Fig. 7, the mixed LB film, such as (a) 6Me-DS:DO ¼ 8:2 and (c) 6Me-DS:DO ¼ 2:8, shows more rough surface structure than that of our expectation, and is grained with average size about 50 nm. The grain size is influenced by the chemical and deposition condition of film. In Fig. 7(a) and (c), the bright part corresponds to the high sample surface, which means that bright areas show. The high and low parts are understandable as the gathered region of
systems agrees with the chemical structure of the higher which consist of the aggregates amphiphile. SNOM/AFM relative system of the mixed monolayer is now under investigation. 3.4. Images of mutual mixing effect Figs. 5 and 6 show the dynamic mode AFM image (a) and near-field optica1 transmission (b) of merocyanine dye LB film. The distance between cantilever and sample maintained about 50 nm
Fig. 5. Surface topography (a) and optical transmission image (b) of merocyanine dye LB film (6Me-DS) in 2:5 2:5 lm2 .
Fig. 7. Mixing effect of two kinds of dye merocyanine on LB structure, i.e. the topographic structure (a) and (c) and nearfield optical structure (c) and (d); (a) and (b) are the molar mixing ratio of 6Me-DS:DO ¼ 8:2, (c) and (d) are the molar mixing ratio of 6Me-DS:DO ¼2:8. The scan size is 2:5 2:5 lm2 .
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activated and inactivated photocarrier from welldefined aggregate size, respectively. In the optical transmission images, the elevated area means higher transmission than lower parts. These experimental results suggest that there is a certain kind of interaction between these different dyes. In the continuous measurement on these mixed dyes such as Figs. 3 and 4, the appearance of near-field optical transmission images showed a certain dependence on the mixing effect of kinds of dyes and the mutual mixing ratios of different dyes.
certain kind of interaction between these two dyes. The dual information, their optical structure and surface morphology, afforded by SNOAM system is valuable for LB film research. This suggests also that the J-aggregate formation and change are affected by the domain shape and excitation laser.
4. Summary
References
The typical structural changes of the aggregates of the merocyanine molecules were measured in the state of multilayer ultra-thin LB films. The chemical assignment of the morphology and transmission response was confirmed by the comparison between the AFM and optical transmission images of the same mixed monolayers of an arachidic acid and merocyanine dyes with various mixing ratios. As-deposited LB films show a little structural change by irradiation of UV lights because it was at the intermediate state of the J-band and not peak shift. Each J-band and -aggregates structure was maintained until the light of other wavelength was irradiated. In the continuous measurement on these dyes, the appearance of near-field optical transmission images showed a certain dependence on the kinds of dyes and the mutual mixing ratios of dyes. These experimental results suggest that there is a
[1] E.E. Jelley, Nature 138 (1936) 1009. [2] H. Kuhn et al., in: Techniques of Chemistry, Part III B, vol. 1, Wiley, New York, 1973, p. 577. [3] M. Matsumoto et al., J. Appl. Phys. 71 (1995) 437. [4] H. Tachibana et al., Thin Solid Films 327–329 (1998) 813. [5] S. Terrettaz, H. Tachibana, M. Matsumoto, Langmuir 14 (1998) 7511. [6] H. Tachibana, Y. Yamanaka, H. Sakai, M. Abe, M. Matsumoto, J. Luminescence 87–89 (2000) 800. [7] K. Ikegami, C. Mingtaud, M. Lan, J. Phys. Chem. B 103 (1999) 11261. [8] T. Kobayashi, in: J-Aggregates, World Scientific, Singapore, 1996, p. 25. [9] K. Murata, H.K. Shin, S. Kuroda, K. Saito, Mol. Cryst. Liq. Cryst. 294 (1997) 113. [10] E. Meyer et al., Nature 349 (1991) 298. [11] M. Fujihira et al., Ultramicroscopy 57 (1995) 118. [12] R. Wiesendanger, H.-J. Guntherodt, Scanning Tunneling Microscopy II, Springer-Verlag, Berlin, 1992. [13] J.A. De Rose, R.M. Leblanc, Sur. Sci. Rep. 22 (3) (1995) 73. [14] H. Muramatsu, N. Chiba, T. Ataka, S. Iwabuchi, N. Nagatani, E. Tamiya, M. Fujihira, Opt. Rev. 3 (6B) (1996) 470.
Acknowledgement This work was supported by the KOSEF through the CIIPMS at Dong-A University.
Optical properties and transmission features of J-aggregate in the merocyanine dye LB films by SNOAM H.-K. Shin, Y.-S. Kwon
*
Department of Electrical Engineering and CIIPMS, Dong-A University, 840, Hadan-2dong, Saha-gu, Busan 604-714, South Korea
Abstract The changes of the absorption spectra by UV irradiation were measured with the optical spectrum of the J-band and the monomer band. We illustrated the topographical structure and optical structure of the merocyanine dye Langmuir– Blodgett (LB) films obtained by the scanning near-field optical/atomic force microscopy. The topographic image of the mutually mixed LB films shows more rough surface structure than expected and grains with average size of about 50 nm. In the optical transmission image, the elevated area means higher transmission than lower parts. In the continuous measurement of these dyes and the mutually mixed films, the appearance of near-field optical transmission images showed a certain dependence on the kinds of dyes with the shift and increase of transmission and the mutual mixing ratios of dyes. These experimental results suggest that there is a certain kind of interaction between these two dyes. Ó 2002 Published by Elsevier Science B.V. PACS: 68.18.)g Keywords: Merocyanine dye; Aggregate; Topography; Transmission image; SNOAM; LB Film
1. Introduction J-aggregate formation of dye molecules has been investigated extensively in the Langmuir–Blodgett (LB) films [1,2]. Typical features of J-aggregates are the red-shift of absorption band with a small bandwidth and strong photoluminescence with a small Stokes shift. J-aggregate formation of dyes has been induced by the photoisomerization of azobenzene molecules existing in the same LB film [3,4]. Drastic morphological changes of the films
*
Corresponding author. Tel.: +82-51-200-5549/7738; fax: +82-51-200-5523/5550. E-mail address: [email protected] (Y.-S. Kwon).
were also noted. Much work has been aimed at determining the molecular structure of J-aggregates and clarifying the structure-spectroscopic relation [5,6]. Merocyanine dyes, originally developed as photosensitizers for silver halide photography, have renewed interest as a class of organic photoconductor, which may be useful for high efficiency photovoltaic device [7]. On the studies of photoconductivity, the formation of J-aggregates is one of the important subjects for molecular arrangement of dye because the existence of J-aggregation can improve the optical applications [8,9]. Even though, various reports were faced on the optical properties of merocyanines, there were no reports on the optical images of these dyes.
0925-3467/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 9 2 5 - 3 4 6 7 ( 0 2 ) 0 0 1 6 7 - 2
402
H.-K. Shin, Y.-S. Kwon / Optical Materials 21 (2002) 401–406
On the other hand, the research on the ultrathin film at atomic and molecular level has been rapidly expanded due to the development and improvement of scanning probe microscopes (SPMs) [10,11]. The atomic force microscopy (AFM) is a new tool for surface imaging which was introduced as one application of scanning tunneling microscopy (STM) [12]. Among many applications, due to its extreme resolution AFM has been used to image organic samples including LB films [13]. As compared with STM, the AFM can provide important advantages since no surface conductivity on the substrate is need to get an AFM image and it is thus independent of the charge transfer between the sample and the tip [14]. Here, the scanning near-field optical/atomic force microscopy (SNOAM) is a new tool for surface imaging which was introduced as one application of the AFM. Operated with non-contact forces between the optical fiber and sample as well as equipped with the piezoscanners, the instrument reports on surface topology without damaging or modifying the surface for measure of optical features in the thin films. In this report, we present the optica1 properties of merocyanine LB films in the far and near-field based on various film deposition conditions such as different kinds of merocyanine and mutual mixing ratios of the two kinds of merocyanine. Furthermore, we have illustrated the SNOAM image in obtaining the merocyanine dye LB films as well as the optical transmission image.
2. Experimental 2.1. SNOAM Setup Fig. 1 shows the schematic diagram of the SNOAM in tile transmission mode. A SNOAM instrument is based on a conventional AFM unit (SPI 3700, Seiko Instruments Inc.), which contains a dynamic-mode AFM function. A sharpened and bent optical fiber probe is mounted on a bimorph [14]. The distance between the tip of probe and sample surface is controlled by a laser-beam-deflecting
Fig. 1. Schematic diagram of SNOAM instrument is based on a conventional AFM unit that contains a dynamic-mode AFM function.
AFM technique, in which the cantilever vibrates vertically at the resonant frequency. The distance is controlled with a piezoelectric device module. The amplitude of the cantilever vibration decreases with a decrease in the distance. This AFM function allows that the light is to be easily illuminated from the optical fiber probe in an optically near-field region. Light of 488 nm from an Ar ion laser with the output power of 50 mW is coupled the end of probe and illuminated from the tip of the probe to sample surface. The light of argon ion laser is modulated by an acousto-optic(AO) modulator for improving S=N ratio of optical signal and controlling irradiation rage in the probe vibration. After transmission through both the aperture of optical fiber probe and sample, the remaining light is detected with a photomultiplier through a collimation lens, a mirror and an optical fiber. The signal from the photodetector is demodulated and amplified by a lock-in amplifier and noise in the optical signal is reduced. With this configuration, the SNOAM can provide topographical and optical images simultaneously with high resolution. The near-field probe was made of a single-mode optical fiber, whose core and cladding are 3.2 and 125 lm in diameter, respectively. In other to taper the probe and sharpen the tip to less than 100 nm diameter, the optical fiber was pulled and bent by a CO2 laser beam. The ridge of the probe was ground and polished into a minor for an optical lever of AFM function. The aperture at apex of the probe was formed by a 100 nm thick of aluminum film.
H.-K. Shin, Y.-S. Kwon / Optical Materials 21 (2002) 401–406
2.2. Ultra-thin film preparation The samples for the optical absorption, the optical and topographical images measurements were prepared by the standard procedure of LB film deposition with monolayer thickness of about 3.32 nm using glass substrates by y-type. 5-[2-(3otadecyl-2(3H)-benzothiazolylidene)ethylidene]-3arboxylmethyl-2-thioxo-4-thiazdidi-none, merocyanine dye (DO) and there derivatives 6Me-DS (6-methyl-5-[2-(3-otadecyl-2(3H)-benzothiazolylidene)ethylidene]-3-arboxylmethyl-2-thioxo-4-thiazdidi-none) and Dse(5-[2-(3-otadecyl-2(3H)-benzoselenazolylidene)ethylidene]-3-arboxylmethyl-2-thioxo-4-thiazdidi-none) were used as surface-active materials at the air-water interface [6]. The dyes weres mixed with arachidic acid (C20 ) as to molar ratio of 1:2. The NL-501-MHW (NLE Co.) moving-wall type trough was used to obtain LB film and CdCl2 (4 104 M) and KHCO3 (2 105 M) were used as subphase materials. The LB films
403
were deposited in vertical dipping method. Temperature was 19 °C, pH was 6:0 0:1. Temperature of 20 1 °C was controlled with a thermostat trough and target pressure was 25 mN/m. The subphase was hydrophilic slide glasses deposited five layers arachidates.
3. Results and discussion 3.1. Spectrum changes by far-field irradiation The changes of the absorption spectra by irradiation of UV light (185, 254, 366 nm) were measured. Fig. 2 shows the absorption spectra of multilayer (10 layers) LB films. The spectrum is taken just after the deposition and is taken after the irradiation of UV light for 3 min interval. From the Fig. 2, we found that relatively little structure changes occurred by the irradiation. It can be explained that the spectrum shift change is
Fig. 2. Spectral changes of the films caused by UV irradiation for far-field optical modification, but not absorption peak shifts. (a) Spectra for various numbers of irradiation cycles (interval 3 min) of 6Me-DS film; (b) DSe film; (c) DO film. By increasing the irradiation time, the J-band (sharp peak at 590 nm) and monomer band (broad peak at 500 nm) peaks such as solution absorption peak of dyes are gradually decreased (a), (b) and (c) films.
404
H.-K. Shin, Y.-S. Kwon / Optical Materials 21 (2002) 401–406
difficult to occur because the deposited LB film has an intermediate structure of the dye single aggregation. This is intermediate state that can easily be certified by itÕs color change. To obtain the J-band spectrum the structure of the LB film must be changed from the intermediate state to the welldefined J-aggregates. Fig. 2(a) shows the spectral changes by the irradiation of UV light to the multilayer LB film which was changed to J-band or -aggregates as shown at Fig. 2(b). We can know the fact that the LB films at the state of J-aggregates can easily change to anisotropic aggregates as the same manner with solution. Fig. 2(c) shows the result of the case that the UV light was irradiated to the monomer band film, which was changed to monomer band by UV light. The peaks of absorption at 500 and 590 nm were decreased. It shows that the LB films on the state of aggregates are changed to absorption intensity, but not band shift, by irradiation of UV light. From these results we can summarize the structural changes of the merocyanine multilayer LB films. To obtain the anisotropy, LB film must be changed to the absorption band by using the optical spectrum property such as well-defined Jaggregates. 3.2. Optical property modification In the continuous measurement on these dyes, near-field optical transmission images showed a certain dependence on the near-field irradiation laser beam. In the optical transmission image, the dark orange color parts correspond to the dye surface of weak transmission and the yellow and white color parts to the dyes depend on strong transmission because the all parts are transparent. It is clear from the comparison between these continuous four images with shift and increase of transmission areas of Fig. 3 that the lower light intensity areas in Fig. 3(b) correspond to the areas in Fig. 3(a) where aggregated dyes seems to cover the glass substrate. This agrees with the above observation on the sample without the aggregated dye film that the intensity of the evanescent wave behind the non-aggregated dye film is weak. The screening of the excitation beam by the dye aggre-
Fig. 3. The optical transmission images show shift and increase of optical transmission areas in DSe film. A 488 nm line of Ar ion laser supported SNOAM is used as a continuous scans. Next scan areas are presented as circular area, and continuous scans are repeated for two times in the area of 500 nm2 . The distance between optical fiber end and the sample is maintained as 50 nm during the continuous scans. The scan size is 2:5 2:5 lm2 .
gated film also gave clear contrast in the transmission intensity mapping of Fig. 3(c) and (d), where the excitation beam picked up by the optical fiber was cut by the filter in front of the SNOAM system. For the use of the present SNOAM, a light irradiation mode through the optical fiber tip is preferable to avoid appreciable photodamage and bleaching of the dye and is now under investigation. 3.3. Local optical property modification Fig. 4 shows continuous SNOAM images of the mixed monolayer of arachidic acid and merocyanine dye with long alkyl chains (molar ratios). Addition of a small amount of the dye affected little on the phase behavior of the mutual mixture. It is reasonable to assume that the long alkylated merocyanine dye is much more soluble in the aggregates than the monomer phase. In practice, much stronger emission of the merocyanine dye was observed in higher than in the lower of molar ratio in the mixed monolayer phase. This observation together with that of the previous well-defined aggregate
H.-K. Shin, Y.-S. Kwon / Optical Materials 21 (2002) 401–406
405
Fig. 6. Surface topography (a) and optical transmission image (b) of merocyanine dye LB film (DO) in the area of 5 5 m2 .
Fig. 4. The changes of optical transmission image show by continuous scans of 6Me-DS film. The scan areas of 2:5 2:5 lm2 are presented as rectangular areas of (a) and (b): (a) near-field transmission image of the first scan, (b) change of near-field transmission image after 50 scans on the rectangular area of (a) and (c) the change of near-field transmission image after 50 scans on the rectangular area of (b) with dynamic force mode.
during the experiment, and scan speed held 0.127 Hz with 256 scan lines. In the topographic image of Fig. 7, the mixed LB film, such as (a) 6Me-DS:DO ¼ 8:2 and (c) 6Me-DS:DO ¼ 2:8, shows more rough surface structure than that of our expectation, and is grained with average size about 50 nm. The grain size is influenced by the chemical and deposition condition of film. In Fig. 7(a) and (c), the bright part corresponds to the high sample surface, which means that bright areas show. The high and low parts are understandable as the gathered region of
systems agrees with the chemical structure of the higher which consist of the aggregates amphiphile. SNOM/AFM relative system of the mixed monolayer is now under investigation. 3.4. Images of mutual mixing effect Figs. 5 and 6 show the dynamic mode AFM image (a) and near-field optica1 transmission (b) of merocyanine dye LB film. The distance between cantilever and sample maintained about 50 nm
Fig. 5. Surface topography (a) and optical transmission image (b) of merocyanine dye LB film (6Me-DS) in 2:5 2:5 lm2 .
Fig. 7. Mixing effect of two kinds of dye merocyanine on LB structure, i.e. the topographic structure (a) and (c) and nearfield optical structure (c) and (d); (a) and (b) are the molar mixing ratio of 6Me-DS:DO ¼ 8:2, (c) and (d) are the molar mixing ratio of 6Me-DS:DO ¼2:8. The scan size is 2:5 2:5 lm2 .
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H.-K. Shin, Y.-S. Kwon / Optical Materials 21 (2002) 401–406
activated and inactivated photocarrier from welldefined aggregate size, respectively. In the optical transmission images, the elevated area means higher transmission than lower parts. These experimental results suggest that there is a certain kind of interaction between these different dyes. In the continuous measurement on these mixed dyes such as Figs. 3 and 4, the appearance of near-field optical transmission images showed a certain dependence on the mixing effect of kinds of dyes and the mutual mixing ratios of different dyes.
certain kind of interaction between these two dyes. The dual information, their optical structure and surface morphology, afforded by SNOAM system is valuable for LB film research. This suggests also that the J-aggregate formation and change are affected by the domain shape and excitation laser.
4. Summary
References
The typical structural changes of the aggregates of the merocyanine molecules were measured in the state of multilayer ultra-thin LB films. The chemical assignment of the morphology and transmission response was confirmed by the comparison between the AFM and optical transmission images of the same mixed monolayers of an arachidic acid and merocyanine dyes with various mixing ratios. As-deposited LB films show a little structural change by irradiation of UV lights because it was at the intermediate state of the J-band and not peak shift. Each J-band and -aggregates structure was maintained until the light of other wavelength was irradiated. In the continuous measurement on these dyes, the appearance of near-field optical transmission images showed a certain dependence on the kinds of dyes and the mutual mixing ratios of dyes. These experimental results suggest that there is a
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Acknowledgement This work was supported by the KOSEF through the CIIPMS at Dong-A University.