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Investigation of morphologies and optical features in ultra-thin organic films by SNOAM
Synthetic Metals 117 (2001) 199±201
Investigation of morphologies and optical features in ultra-thin organic ®lms by SNOAM H.K. Shina,*, J.M. Kimb, S.M. Changb, Y.S. Kwona a
Department of Electrical Engineering, Dong-A University, Pusan 604-714, South Korea Department of Chemical Engineering, Dong-A University, Pusan 604-714, South Korea
b
Abstract The thin ®lms have been studied by scanning near-®eld optical/atomic force microscopy (SNOAM). The typical structural changes of the aggregates of the merocyanine molecules were measured in the state of ultra-thin LB ®lms by SNOAM. In the continuous measurement on the dye, the appearance of near-®eld optical transmission images showed dependence on the near-®eld irradiation laser beam. the topographical image indicate that this LB ®lm has more rough surface morphology than that of normal expectation and LB ®lm is grained with average grain size about 50 nm. The near-®eld transmission image shows highly separated structure of transmission and independent image on the surface topography. The dual information, their optical structure and surface morphology, afforded by SNOAM system is valuable for thin ®lm research. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Scanning near-®eld optical/atomic force microscopy; Topography; Optical transmission image
1. Introduction During the last years, a rather new technique is being developed that combines the high resolution of scanning probe microscopes (SPMs) with speci®c contrast of conventional optical microscopy, which is named as scanning near-®eld optical microscopy (SNOM) [1]. In SNOM, a small aperture is scanned close to a surface (near-®eld) acting as a subwavelength-sized source. Very recently, we developed scanning near-®eld optical/atomic force microscopy (SNOAM) in which the distance control between cantilever and sample is based on the atomic force microscopy (AFM) method [2,3]. Thus, SNOAM can provide us simultaneous topographical and optical images of merocyanine dyes using a sharp and bent optical ®bber as a near-®eld optical probe with contact (AFM mode) or dynamic mode (constant force mode) [4] (Fig. 1). 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 ef®ciency photovoltalic device [5]. Various contributions on the optical and photoelectric characteristics of merocyanine thin ®lms have been investigated extensively in systems fabricated using vacuum evaporations, casting or Langmuir±Blodgett (LB) ®lms [6]. On the studies of photo*
Corresponding author.
conductivity, 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 and variously reported [7±9]. Even though, various reports were faced on the optical properties of merocyanines, there were no reports on the optical structures of these dyes. In this paper, we have illustrated the surface structure and optical properties in obtaining the merocyanine dye LB ®lms as well as the optical image by SNOAM. 2. Experimental A merocyanine dye such as 5-[2-(3-otadecyl-2(3H)-benzooxazolylidene)ethylidene]-3-carboxylmethyl-2-thioxo-4thiazdidinone (DO) has been used to deposit LB ®lms. The DO was 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 ®lm and CdCl2 (4 10ÿ4 M) and KHCO3 (2 10ÿ5 M) was used as subphase materials. The LB ®lms were deposited in vertical dipping method. Temperature was 198C, pH was 6.1±6.2 and target pressure was 30 mN/m. The subphase was hydrophilic slide glasses deposited 5 layers arachidate. The sample for SNOAM were prepared by the standard procedure of LB ®lm deposition with 10 layers using glass substrates by y-type [9].
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 5 0 0 - 2
200
H.K. Shin et al. / Synthetic Metals 117 (2001) 199±201
Fig. 1. Schematic diagram of SNOAM instrument is based on a conventional AFM unit that contains a dynamic mode AFM function.
The distance between the tip of the probe and sample surface is controlled by a laser-beam-de¯ecting AFM technique in which the cantilever vibrates vertically at the resonant frequency (Model SPI3800, Seiko Instruments). SNOAM measurements were performed applying approximately 3 mW of 488 nm line of Ar ion laser in coupled with the untapped end of the ®ber probe with dynamic (noncontact mode) and transmission mode operation. The distance between cantilever and sample maintained about 50 nm during the experiment and scan rate held 0.127 Hz with 256 scan lines. 3. Results and discussion The merocyanine dye LB ®lms has been intensively studied, because their can be investigate aggregate states of molecular level in ultra-thin organic ®lms of functional molecules. As has been reported previously, the merocyanine dye LB ®lms show anisotropic properties and for some dyes, characteristic properties J-like bands appear when dye molecules aggregate. Also, their anisotropy and the J-aggregates of dye molecules have been con®rmed by optical spectrum [8] and the aggregates size calculated about 10 nm diameter by extended-dipole modeling. It could be ascribed to enhancement of a highly ordering of molecular in J-aggregates [9]. Fig. 2 shows the dynamic mode AFM image (a) and near®eld optical transmission image (b) of merocyanine dye LB
Fig. 2. Surface topography (a) and optical transmission image (b) of merocyanine dye LB film (DO) in the area of 5 mm 5 mm.
Fig. 3. A 488 nm line of Ar ion laser supported SNOAM is used as a continuous scans. Next scan areas are presented as rectangles and continuous scans are repeated for 2 times in the area of 500 nm2. The distance between optical fiber end and the sample maintained as 50 nm during the continuous scans with dynamic force mode operation. The scan size is 2:5 mm 2:5 mm.
®lm (DO). In Fig. 2a, the topographical image indicate that this LB ®lm has more rough surface morphology than that of normal expectation and LB ®lm is grained with average grain size about 50 nm. In the topographic image, elevated parts correspond to dyes (DO) and the lower parts to the glass surface. In Fig. 2b, the near-®eld transmission image shows highly separated structure of transmission and independent image on the surface topography. In the optical transmission image, the dark orange color parts correspond to the dye surface of weak optical responses and the yellow and white color parts to the dyes depend on strong optical responses because the all parts are transparent. It is clear from the comparison between these continuous four images of Fig. 3 that the lower light intensity areas in Fig. 3b correspond to the areas in Fig. 3a where aggregated dyes seems to cover the glass substrate. This agrees with the above observation on the sample without the aggregated dye ®lm that the intensity of the evanescent wave behind the non-aggregated dye ®lm is weak. The screening of the excitation beam by the dye aggregated ®lm also gave clear contrast in the transmission intensity mapping of Fig. 3c and d, where the excitation beam picked up by the optical ®ber was cut by the ®lter in front of the SNOAM system. For the use of the present SNOAM, a light irradiation mode through the optical ®ber tip is preferable to avoid appreciable photodamage and bleaching of the dye and is now under investigation. In the continuous measurement on these dyes, near-®eld optical transmission images showed a certain dependence on the near-®eld irradiation laser beam by the optical ®ber. In
H.K. Shin et al. / Synthetic Metals 117 (2001) 199±201
conclusion, the complementary optical information afforded by SNOAM system is valuable for organic thin ®lm research, their optical responses and surface morphology. References [1] U.T. DuÈrig, D.W. Pohl, F. Rohrer, J. App. Phys. 59 (1991) 3318. [2] H. Muramatsu, N. Chiba, T. Ataka, H. Monobe, M. Fujihira, Ultramicroscopy 57 (1995) 141.
201
[3] N. Chiba, H. Muramatsu, T. Ataka, M. Hujihira, Jpn. J. App. Phys. 34 (1995) 321. [4] H. Muramatsu, N. Chiba, T. Ataka, S. Iwabuchi, N. Nagatani, E. Tamiya, M. Fujihira, Opt. Rev. 3 (6B) (1996) 470. [5] E.E. Jelley, Nature 138 (1936) 1009. [6] H. Kuhn, D. MoÈbius, H. BuÈcher, in: Techniques of Chemistry Part III B, Vol. 1, Wiley, New York, 1973, p. 577. [7] Y. Yonezawa, D. MoÈbius, H. Kuhn, J. Appl. Phys. 62 (1987) 2016. [8] H. K Shin, Y.S. Kwon, Synth. Met. 102 (1999) 1514. [9] K. Murata, H.K. Shin, K. Saito, S. Kuroda, Thin Solid Films 446 (1998) 327±329.
Investigation of morphologies and optical features in ultra-thin organic ®lms by SNOAM H.K. Shina,*, J.M. Kimb, S.M. Changb, Y.S. Kwona a
Department of Electrical Engineering, Dong-A University, Pusan 604-714, South Korea Department of Chemical Engineering, Dong-A University, Pusan 604-714, South Korea
b
Abstract The thin ®lms have been studied by scanning near-®eld optical/atomic force microscopy (SNOAM). The typical structural changes of the aggregates of the merocyanine molecules were measured in the state of ultra-thin LB ®lms by SNOAM. In the continuous measurement on the dye, the appearance of near-®eld optical transmission images showed dependence on the near-®eld irradiation laser beam. the topographical image indicate that this LB ®lm has more rough surface morphology than that of normal expectation and LB ®lm is grained with average grain size about 50 nm. The near-®eld transmission image shows highly separated structure of transmission and independent image on the surface topography. The dual information, their optical structure and surface morphology, afforded by SNOAM system is valuable for thin ®lm research. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Scanning near-®eld optical/atomic force microscopy; Topography; Optical transmission image
1. Introduction During the last years, a rather new technique is being developed that combines the high resolution of scanning probe microscopes (SPMs) with speci®c contrast of conventional optical microscopy, which is named as scanning near-®eld optical microscopy (SNOM) [1]. In SNOM, a small aperture is scanned close to a surface (near-®eld) acting as a subwavelength-sized source. Very recently, we developed scanning near-®eld optical/atomic force microscopy (SNOAM) in which the distance control between cantilever and sample is based on the atomic force microscopy (AFM) method [2,3]. Thus, SNOAM can provide us simultaneous topographical and optical images of merocyanine dyes using a sharp and bent optical ®bber as a near-®eld optical probe with contact (AFM mode) or dynamic mode (constant force mode) [4] (Fig. 1). 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 ef®ciency photovoltalic device [5]. Various contributions on the optical and photoelectric characteristics of merocyanine thin ®lms have been investigated extensively in systems fabricated using vacuum evaporations, casting or Langmuir±Blodgett (LB) ®lms [6]. On the studies of photo*
Corresponding author.
conductivity, 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 and variously reported [7±9]. Even though, various reports were faced on the optical properties of merocyanines, there were no reports on the optical structures of these dyes. In this paper, we have illustrated the surface structure and optical properties in obtaining the merocyanine dye LB ®lms as well as the optical image by SNOAM. 2. Experimental A merocyanine dye such as 5-[2-(3-otadecyl-2(3H)-benzooxazolylidene)ethylidene]-3-carboxylmethyl-2-thioxo-4thiazdidinone (DO) has been used to deposit LB ®lms. The DO was 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 ®lm and CdCl2 (4 10ÿ4 M) and KHCO3 (2 10ÿ5 M) was used as subphase materials. The LB ®lms were deposited in vertical dipping method. Temperature was 198C, pH was 6.1±6.2 and target pressure was 30 mN/m. The subphase was hydrophilic slide glasses deposited 5 layers arachidate. The sample for SNOAM were prepared by the standard procedure of LB ®lm deposition with 10 layers using glass substrates by y-type [9].
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 5 0 0 - 2
200
H.K. Shin et al. / Synthetic Metals 117 (2001) 199±201
Fig. 1. Schematic diagram of SNOAM instrument is based on a conventional AFM unit that contains a dynamic mode AFM function.
The distance between the tip of the probe and sample surface is controlled by a laser-beam-de¯ecting AFM technique in which the cantilever vibrates vertically at the resonant frequency (Model SPI3800, Seiko Instruments). SNOAM measurements were performed applying approximately 3 mW of 488 nm line of Ar ion laser in coupled with the untapped end of the ®ber probe with dynamic (noncontact mode) and transmission mode operation. The distance between cantilever and sample maintained about 50 nm during the experiment and scan rate held 0.127 Hz with 256 scan lines. 3. Results and discussion The merocyanine dye LB ®lms has been intensively studied, because their can be investigate aggregate states of molecular level in ultra-thin organic ®lms of functional molecules. As has been reported previously, the merocyanine dye LB ®lms show anisotropic properties and for some dyes, characteristic properties J-like bands appear when dye molecules aggregate. Also, their anisotropy and the J-aggregates of dye molecules have been con®rmed by optical spectrum [8] and the aggregates size calculated about 10 nm diameter by extended-dipole modeling. It could be ascribed to enhancement of a highly ordering of molecular in J-aggregates [9]. Fig. 2 shows the dynamic mode AFM image (a) and near®eld optical transmission image (b) of merocyanine dye LB
Fig. 2. Surface topography (a) and optical transmission image (b) of merocyanine dye LB film (DO) in the area of 5 mm 5 mm.
Fig. 3. A 488 nm line of Ar ion laser supported SNOAM is used as a continuous scans. Next scan areas are presented as rectangles and continuous scans are repeated for 2 times in the area of 500 nm2. The distance between optical fiber end and the sample maintained as 50 nm during the continuous scans with dynamic force mode operation. The scan size is 2:5 mm 2:5 mm.
®lm (DO). In Fig. 2a, the topographical image indicate that this LB ®lm has more rough surface morphology than that of normal expectation and LB ®lm is grained with average grain size about 50 nm. In the topographic image, elevated parts correspond to dyes (DO) and the lower parts to the glass surface. In Fig. 2b, the near-®eld transmission image shows highly separated structure of transmission and independent image on the surface topography. In the optical transmission image, the dark orange color parts correspond to the dye surface of weak optical responses and the yellow and white color parts to the dyes depend on strong optical responses because the all parts are transparent. It is clear from the comparison between these continuous four images of Fig. 3 that the lower light intensity areas in Fig. 3b correspond to the areas in Fig. 3a where aggregated dyes seems to cover the glass substrate. This agrees with the above observation on the sample without the aggregated dye ®lm that the intensity of the evanescent wave behind the non-aggregated dye ®lm is weak. The screening of the excitation beam by the dye aggregated ®lm also gave clear contrast in the transmission intensity mapping of Fig. 3c and d, where the excitation beam picked up by the optical ®ber was cut by the ®lter in front of the SNOAM system. For the use of the present SNOAM, a light irradiation mode through the optical ®ber tip is preferable to avoid appreciable photodamage and bleaching of the dye and is now under investigation. In the continuous measurement on these dyes, near-®eld optical transmission images showed a certain dependence on the near-®eld irradiation laser beam by the optical ®ber. In
H.K. Shin et al. / Synthetic Metals 117 (2001) 199±201
conclusion, the complementary optical information afforded by SNOAM system is valuable for organic thin ®lm research, their optical responses and surface morphology. References [1] U.T. DuÈrig, D.W. Pohl, F. Rohrer, J. App. Phys. 59 (1991) 3318. [2] H. Muramatsu, N. Chiba, T. Ataka, H. Monobe, M. Fujihira, Ultramicroscopy 57 (1995) 141.
201
[3] N. Chiba, H. Muramatsu, T. Ataka, M. Hujihira, Jpn. J. App. Phys. 34 (1995) 321. [4] H. Muramatsu, N. Chiba, T. Ataka, S. Iwabuchi, N. Nagatani, E. Tamiya, M. Fujihira, Opt. Rev. 3 (6B) (1996) 470. [5] E.E. Jelley, Nature 138 (1936) 1009. [6] H. Kuhn, D. MoÈbius, H. BuÈcher, in: Techniques of Chemistry Part III B, Vol. 1, Wiley, New York, 1973, p. 577. [7] Y. Yonezawa, D. MoÈbius, H. Kuhn, J. Appl. Phys. 62 (1987) 2016. [8] H. K Shin, Y.S. Kwon, Synth. Met. 102 (1999) 1514. [9] K. Murata, H.K. Shin, K. Saito, S. Kuroda, Thin Solid Films 446 (1998) 327±329.