Journal of Luminescence 98 (2002) 35–40
Optical properties of individual nanostructures of molecular J-aggregates Martin Vachaa,*, Masaaki Saekib, Makoto Furukic, Lyong Sun Puc, Ken-ichi Hashizumeb, Toshiro Tanib b
a Department of Chemistry, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan Department of Applied Physics, Tokyo University of Agriculture and Technology, 2-24-16 Naka-machi, Koganei, Tokyo 184-8588, Japan c Corporate Research Laboratories, Fuji Xerox Co. Ltd., 430 Sakai, Nakaimachi, Kanagawa 259-0157, Japan
Abstract Simultaneous atomic force microscope and reflection micro-spectroscopic study of nanostructures of pseudoisocyanine J-aggregates shows morphology related variety of optical properties in the form of exciton- and polariton-like reflectance spectra. Modulation of the polarization state of incident light is used to study locally the absolute orientations of exciton transition dipole moments. The lowest exciton state assumes a wide range of directions with respect to the long axis of the fibers, from parallel to perpendicular. This behavior is correlated with the onset of polariton-like character of the local reflectance spectra. r 2002 Published by Elsevier Science B.V. PACS: 73.22.f; 78.40.Me; 78.67.n; 68.37.d Keywords: Microscopy; AFM; Reflection spectroscopy; Polarization; J-aggregate; Pseudoisocyanine; Exciton–polariton
1. Introduction Molecular aggregates are ordered assemblies of organic molecules that have specific optical properties. Because of them, aggregates play crucial roles in many biological processes, such as photosynthesis, and have great potential in technological applications as linear and non-linear nano-optical materials. Especially, cyanine dye molecules [1,2] readily form large aggregated structures that consist of hundreds of molecules. *Corresponding author. Tel.: +81-3-3986-0221 ext. 6421; fax: +81-3-5992-1029. E-mail address:
[email protected] (M. Vacha).
These aggregates are also interesting from the viewpoint of basic physical concepts, as they represent linear molecular chains where Frenkel excitons of large coherent length [3–5] and the associated phenomena of motional line narrowing [4,5] and superradiance [6] have been observed. Linear 1-dimensional (1D) aggregates of pseudoisocyanine (PIC) molecules assemble at high dye concentrations or upon addition of concentrated NaCl to dilute aqueous solutions [1,2]. When mixed with certain polyanions in dilute aqueous solutions 1D J-aggregates of PIC can form by electrostatic interaction of the PIC cationic sites with anionic groups of the polymers [7]. It has
0022-2313/02/$ - see front matter r 2002 Published by Elsevier Science B.V. PII: S 0 0 2 2 - 2 3 1 3 ( 0 2 ) 0 0 2 4 8 - X
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been found that the spacing of the negative groups on the polymer chain is important as not all polyanions initiate J-aggregate formation. The structure of the linear 1D chain has been a subject of continuous debates. Its spectroscopic properties (extent and polarity of the spectral shift) are best modeled by the so-called brick-stone arrangement [8,9] where individual molecules are aligned in an alternating fashion with their long axes approximately parallel to each other and with large lateral displacements of the adjacent molecular centers. This model implies strong anisotropy of the exciton transition dipole moment. It has been confirmed by numerous experiments [10–13] on oriented PIC aggregates that the absorption band at 572 nm is polarized along the orientation direction. However, in all these experiments the evidence that the J-band transition dipole is oriented along the 1D molecular chain is only inferential. Recently, PIC aggregates have been studied with an optical imaging technique, the near-field scanning optical microscope [12,14]. Instead of a homogeneous distribution of 1D aggregates the images have shown self-assembled structures that resemble flexible fibers. The similarity of a local fluorescence spectrum with the bulk fluorescence suggested that the excitonic structure within the fibers is due the 1D J-aggregate Frenkel excitons. Later, reflection micro-spectroscopic study of a closely related system [15] revealed large inhomogeneity of local optical properties of the fiber-like aggregate nanostructures. The optical properties have been correlated with the aggregate morphology observed by an atomic force microscope (AFM) [16] and with the local polarization characteristics of the samples [17]. These experiments have posed questions on the nature of the nanostructures observed, on the role of the polymer in their assembly, and on the character of the PIC J-aggregates themselves. Here, we attempt to address some of these questions by preparing aggregate nanostructures in different polymer matrices and by measuring the topography (with AFM), local reflection spectra and polarization properties (with reflection microscope) of the same areas of the sample.
2. Experimental J-aggregates of 1,10 -diethyl-2,20 -cyanine chloride (PIC-Cl, Nippon Kankoh Shikiso Kenkyusho) were prepared in thin films of two different polymers, polyvinyl sulfate (PVS) and polyvinyl alcohol (PVA). Preparation of the PIC/PVS system (based on the method of Ref. [14]) involves addition of 10 mM methanol solution of PIC-Cl to hot stirring aqueous solution of 7.5 mg/ml of potassium PVS and immediate spin-coating of the mixture at 3000 rpm on a cover glass. The PIC/ PVA system was prepared according to the method of Ref. [18]. 40–60 mg of PVA were mixed with 2 ml of water and heated up to 901C. After complete dissolution of the polymer, 5 mg of PICCl were added to the stirring solution and the mixture was spin-coated at 3000 rpm on a cover glass. The method of reflection micro-spectroscopy has been described previously [15,17]. Briefly, light from a tungsten lamp and a 1.5 m monochromator is introduced into a large core optical fiber and brought to an inverted reflection mode microscope (100 oil immersion lens, NA 1.25). Typical light intensity at the sample is 1.2 mW cm2 at the J-band maximum. Reflection images at specified wavelengths are detected with a cooled CCD camera (Princeton Instruments PentaMAX) and local reflectance spectra are reconstructed from a series of images by plotting the reflected light intensity at certain location against the wavelength. The polarization of the incident light is modified by rotating a sheet polarizer placed behind the microscope collimator. The extinction ratio at the sample position is typically 70:1. Polarized reflectance data at specified wavelengths are obtained by rotating the polarizer in increments of 51 and recording a reflection image with the CCD camera at each step. Local polarization dependence is reconstructed by plotting the reflected light intensity at certain location against the polarizer angle. The data have been corrected for the spectral and polarization characteristics of the light source, optical fiber, microscope and CCD camera. The AFM studies were performed with a Digital Instruments Nanoscope MultiMode SPM system with a silicon probe in a non-contact
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tapping mode. To locate the same 10 10 mm area in the 10 10 mm sample in both instruments, a ‘‘finger-print’’ mark in the polymer film has been scratched with sharp tweezers. All experiments are done at room temperature.
3. Results and discussion 3.1. Correlation of the topographic, spectral and polarization properties Typical results on AFM and reflection microscopy of a PIC-Cl/PVS J-aggregate sample are shown in Fig. 1. The AFM topological image (Fig. 1a) reveals morphological features of the
Fig. 1. Atomic force and reflection microscopy of PIC-Cl/PVS sample. a—AFM image in non-contact tapping mode; b—reflection image of the same location taken with unpolarized light at the wavelength of 572 nm; line bars indicate orientation of exciton transition dipole moments at 572 nm; c—detailed cross-sections of the AFM image taken at points 1–3; d—series of reflection spectra taken from the corresponding area of the reflection image at points 1–3; e—simulated reflectance spectra according to Ref. [15] as described in the text; thickness is taken as the thickness above the film (Fig. 1c) plus the thickness of the film of 40 nm. The thickness values are 40, 90 and 105 nm for spectra 1–3, respectively.
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sample similar to those reported before [12]. The features include fiber-like structures with length on the order of microns, diameter between 80 and 300 nm (measured at half-height above the film) and thickness above the film of a few to about 70 nm. Other samples prepared by this method show thickness of more than 100 nm. The polymer film itself has a thickness of 40 nm. The corresponding reflection image taken at the wavelength of the absorption J-band maximum is shown in Fig. 1b. Apart from the resolution limited by farfield optics (about 280 nm at the wavelength used) it shows the same pattern as the AFM image. It is evident that only those aggregate structures that protrude above the film reflect light efficiently, and that the thickness distribution and reflectivity correspond to one another (thicker structures reflect more). Cross-sections of a selected fiber at three typical locations are shown in Fig. 1c. For this particular fiber one can see that on the length scale of a few hundred nm the size of the fiber can change dramatically: its thickness from 21 to 68 nm, width from 150 to 300 nm. The change in morphology is accompanied by a similar change in the reflectance spectra (Fig. 1d) where a narrow peak of 360 cm1 linewidth and 8% reflectivity gradually broadens to 925 cm1 and its reflectivity increases to 20%. The spectra of the thickest parts of the fiber (above and only above 55 nm) show further a narrow dip near their maxima (spectrum 3 of Fig. 1d). Evolution of this mode structure, typically observed in reflectance spectra of thin organic crystals [19], has been ascribed to an onset of exciton–polariton character due to increased exciton–photon coupling [15]. The reflectance dips (virtual polariton modes [20]) are due to standing waves inside the fiber nanostructures that start coupling above certain thickness of the fiber. Fig. 1e shows simulation of the reflectance spectra lineshapes following the procedure used in Ref. [15]. The simulation uses a single Lorentzian oscillator for the J-band exciton in a general theory of optical properties of a dielectric slab with arbitrary strength of electron–photon coupling [20]. Individual fibers are regarded as thin dielectric layers of varying thickness and oscillator strength. Since the thickness is measured in AFM imaging the only adjustable parameter in the
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simulation is the oscillator strength. The effective oscillator strength must be increased 7 times (going from spectrum 1 to 3 in Fig. 1e) to obtain rough qualitative agreement between Figs. 1d and e. This result confirms previous conclusions [15,16] that the change in size of the aggregate must be accompanied by a qualitative change in its structure to account for the polariton-like features of the reflectance spectra. This finding is further demonstrated in the local polarization properties. Line bars in the reflection image (Fig. 1b) indicate absolute orientations of the exciton transition dipole moments at 572 nm. The orientations were obtained by fitting the polarization dependence data at each location with an a þ b cos2 ðcg þ dÞ function. The resulting phase d has been taken as the exciton dipole angle projection in the image plane. One can see that the dipoles are not necessarily oriented parallel with the fiber long axis. For this particular sample the deviation from parallel orientation at locations 2 and 3 is about 251. Other samples prepared by slightly modified methods show dipole orientations that range from parallel to perpendicular [17] with respect to the fibers. Based on a statistical ensemble of 42 locations of three different samples we observed a correlation between the magnitude of the deviation (between the exciton dipole direction and the long axis of the fiber structures) and the local reflectivity, as plotted in Fig. 2. With increasing reflectivity, the 572 nm exciton dipole moment gradually changes from direction parallel to the fiber structure to direction perpendicular to
Fig. 2. Dependence of the angle between the 572 nm exciton transition dipole moment and the direction of the fiber long axis on the reflectivity of J-band.
it. The typical observation for most of the PIC/ PVS samples studied so far is that the fraction of locations that show large deviations from the parallel direction varies from sample to sample, but it has never been observed that the dipole moment would be aligned solely parallel with the fiber direction throughout any of the samples studied. This is in contradiction with most of the results of polarization absorption experiments on oriented aggregates reported [11–13]. Our experiments provide evidence that the fiber structures are not simply aligned 1D threads of J-aggregates and that together with the growth of the fibers there is a qualitative structural change that demonstrates itself, apart from the dipole angle change, by increased polariton character of the reflectance spectra. 3.2. Influence of the polymer matrix To study the influence of the polymer matrix in the formation and optical properties of the aggregate nanostructures we prepare PIC-Cl aggregates in two different polymer matrices: PVS and PVA. The mechanisms of J-aggregate formation in each matrix are different. In case of PVS, aggregates are formed by electrostatic interactions of the PIC cationic sites with the PVS anionic groups of SO 3 [7,21]. In PVA, aggregates selfassemble by cooling in the concentrated aqueous/ polymer solution [1]. Morphology and optical properties of the PIC/ PVS samples have been described above. Similarly, the features of PIC aggregates in thin film PVA strongly depend on the sample preparation procedure. As an example, Figs. 3a and c show reflection images of samples prepared with different PVA concentrations. In the case of 2 wt% PVA sample (Fig. 3a) the image consists of strongly reflecting aggregate film background, on top of which an oriented network of fiber-like structures is formed. In contrast to PIC/PVS samples, the reflectivity ratio between the fibers and the background, e.g., between the strongly reflecting location 1 and the background location 4 of Fig. 3a, is very low (only 1.3 for the above example). The fiber structures themselves resemble those of the PIC/PVS samples. An example of a
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Fig. 3. Reflection micro-spectroscopy of PIC-Cl/PVA samples. a—reflection image of 2 wt% PVA film taken with unpolarized light at 572 nm; b—local reflectance spectra taken at locations 1–4 of Fig. 3a; c—reflection image of 3 wt% PVA film taken with unpolarized light at 572 nm; d—local reflectance spectra taken at locations 1–3 of Fig. 3c; line bars indicate orientation of exciton transition dipole moments at 572 nm.
3 wt% PVA sample is shown in Fig. 3c. The fibers appear more rigid and randomly oriented than those of the 2 wt% PVA sample, and the background between the fiber structures has very low reflectivity. Figs. 3b and d show local reflectance spectra taken at different locations of the images. In case of the 2 wt% sample, all four spectra consist of a single narrow peak with a maximum at 571.5 nm and halfwidth of 9–10 nm. The only difference between the spectra of the fiber structures and those of the background is the total reflectivity. The shapes of the spectra resemble the exciton-like reflectance peaks observed in low reflectivity structures of the PIC/PVS samples [15]. Polarization experiments show that the exciton dipole moments within the background film are oriented isotropically. The dipole moments of the fiber structures show definite orientation but their extinction ratio (ratio of the maximum to minimum polarized reflectivity) is rather low due to the overlap with the strong isotropic background. In contrast, local reflectance spectra of the 3 wt% sample shown in Fig. 3d are strongly locationsensitive. For example, locations 1 and 2 along the
straight part of the strong fiber feature a broadened main J-band, a shoulder around 540 nm and pronounced mode structure. Signs of mode structure appear also in the narrower band of the spectra at location 3. The exciton transition dipole at all three locations is oriented almost perpendicular to the fiber directions, with the extinction ratios on the order of 9:1 for the data shown. All these characteristics are very similar to those observed in many strongly reflecting structures in PIC/PVS samples [15–17]. The above results show that, in spite of the different J-aggregate formation mechanisms, the films consist of similar fiber-like nanostructures with comparable optical properties. This indicates that the polymer itself might not be structurally involved in the growth of the nanostructures and that molecular arrangement of the PIC and polymer molecules inside the nanostructures is definitely different from 1D aggregate chain as well as from bulk crystal state. We note that recently reported results on glassy PIC films show similar morphological and optical properties [22] and might be structurally close to the fiber nanostructures observed here.
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Acknowledgements The authors acknowledge O. Isobe, S. Takei, Y. Yamaguchi and H. Matsumoto for their part in the experimental work. The research was partly supported by grant-in-aid no.11490008 of the Ministry of Education of Japan, NSG Foundation, Iketani Foundation and Hitachi CRL.
References [1] [2] [3] [4] [5] [6]
E.E. Jelly, Nature 138 (1936) 1009. G. Scheibe, Angew. Chem. 50 (1937) 212. P.O.J. Scherer, S.F. Fischer, Chem. Phys. 86 (1984) 269. E.W. Knapp, Chem. Phys. 85 (1984) 73. J. Knoster, J. Chem. Phys. 99 (1993) 8466. J. Grad, G. Hernandez, S. Mukamel, Phys. Rev. A 37 (1988) 3835. [7] W. Appel, G. Scheibe, Z. Naturforschg. 13b (1958) 359. [8] V. Czikkely, H.D. Foersterling, H. Kuhn, Chem. Phys. Lett. 6 (1970) 11. [9] V. Czikkely, H.D. Foersterling, H. Kuhn, Chem. Phys. Lett. 6 (1970) 207.
[10] G. Scheibe, L. Kandler, Naturwissenschaften 26 (1938) 412. [11] K. Misawa, H. Ono, K. Minoshima, T. Kobayashi, Appl. Phys. Lett. 63 (1993) 577. [12] D.A. Higgins, P.J. Reid, P.F. Barbara, J. Phys. Chem. 100 (1996) 1174. [13] H. von Berlepsch, C. Bloettcher, L. Daehne, J. Phys. Chem. B 104 (2000) 8792. [14] D.A. Higgins, P.F. Barbara, J. Phys. Chem. 99 (1995) 3. [15] M. Vacha, S. Takei, K. Hashizume, Y. Sakakibara, T. Tani, Chem. Phys. Lett. 331 (2000) 387. [16] M. Vacha, M. Furuki, L.S. Pu, K. Hashizume, T. Tani, J. Phys. Chem. B 105 (2001) 12226. [17] M. Vacha, M. Seaki, O. Isobe, K. Hashizume, T. Tani, J. Chem. Phys. 115 (2001) 4973. [18] K. Misawa, H. Ono, K. Minoshima, T. Kobayashi, Appl. Phys. Lett. 63 (1993) 577. [19] M. Orrit, P. Kottis, Adv. Chem. Phys. 74 (1988) 1. [20] R. Fuchs, K.L. Kliewer, W.J. Pardee, Phys. Rev. 150 (1966) 589. [21] D.F. Bradley, M.K. Wolf, Proc. Natl. Acad. Sci. USA 45 (1959) 944. [22] H. von Berlepsch, S. Moeller, L. Daehne, J. Phys. Chem. B 105 (2001) 5689.
