Influence of the Deposition Method on the Topography and

Uroš Ralević , Goran Isić , Dragana Vasić Anicijević , Bojana Laban , Una Bogdanović , Vladimir M. Lazović , Vesna Vodnik , Radoš Gajić. Applied Surfa...
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Langmuir 2002, 18, 8407-8417

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Influence of the Deposition Method on the Topography and Spectroscopy of J-Aggregates of a Thiacarbocyanine Dye Adsorbed to a Langmuir Film Nadia Vranken, Philippe Foubert, Fabian Ko¨hn, Roel Gronheid, Ivan Scheblykin, Mark Van der Auweraer,* and Frans C. De Schryver Laboratory for Photochemistry and Spectroscopy, Katholieke Universiteit Leuven, Department of Chemistry, Celestijnenlaan 200F, 3001 Heverlee, Belgium Received March 7, 2002. In Final Form: August 6, 2002 The fluorescence characteristics and the topography of domains of J-aggregates formed by the thiacarbocyanine dye THIATS (3,3′-disulfopropyl-5,5′-dichloro-9-ethylthiacarbocyanine) adsorbed onto a dioctadecyldimethylammonium bromide (DODAB) layer and deposited on a solid substrate were studied using confocal fluorescence microscopy and atomic force microscopy (AFM). The fluorescence and topography of the films reveal a one-to-one correlation for the domains formed by the adsorbed cyanine dyes. The packing of the dye molecules in the bulk of the domains remains unchanged after deposition. The thickness of the dye layer, determined by atomic force microscopy, confirms that only a single dye layer, wherein the dye molecules have an edge-on orientation, is deposited in these domains. The influence of the deposition process (Langmuir-Blodgett or Langmuir-Schaefer) is mostly limited to the amphiphile regions between the two-dimensional dye crystals and to the rim of the two-dimensional dye crystals. For the film deposited using the Langmuir-Blodgett technique, the aggregation was disturbed at the rim of the quasi-circular domains. Dye material was however deposited as indicated by the monomer fluorescence. Polarization experiments revealed a random orientation for the dye molecules at the rim, in contrast with the polarization effect observed for the bulk of the domains. The film deposited using the Langmuir-Schaefer technique seems to remain structured at the rim of the domains. For these films, deposited on a hydrophobic substrate, however, the packing of the DODAB material between the domains undergoes drastic changes as shown by the AFM experiments. Due to the decreased stability outside the domains, collapse of the DODAB film occurs. Fluorescence spectra could be collected at different positions of the domains, revealing whether the dye molecules were present as monomer or J-aggregates.

Introduction Upon incorporation of cyanine dyes in mono- or multilayers, the photophysical properties of these dyes undergo drastic changes due to aggregate formation.1-3 When J-aggregates are formed, they reveal a narrow, intense, and red-shifted absorption and emission band, with only a small Stokes shift.4,5 In the past, a number of techniques were used to visualize the morphology and spectral properties of J-aggregates formed upon adsorption of dyes to a substrate or incorporation in Langmuir films. Electron microscopy,6,7 atomic force microscopy,8 and scanning tunneling microscopy9 gave morphological information, without the possibility however to correlate this information directly with the spectral properties of the film. Fluorescence microscopy has been used successfully to investigate film * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +32(0)16 32 79 90. (1) Yonezawa, Y.; Mo¨bius, D.; Kuhn, H. Phys. Chem. 1986, 90, 1183. (2) Lehmann, U. Thin Solid Films 1988, 160, 257. (3) Kirstein, S.; Mo¨hwald, H.; Bliznyuk, V. N. J. Phys. Chem. 1993, 97, 569. (4) Jelley, E. E. Nature 1936, 138, 1006. (5) Scheibe, G. Angew. Chem. 1939, 52, 633. (6) Fisher, A.; Lo¨she, M.; Mo¨hwald, H.; Sackmann, E. J. Phys. Lett. 1984, 45, L785. (7) Duschl, C.; Kemper, D.; Frey, W.; Meller, P.; Ringsdorf, H.; Knoll, W. J. Phys. Chem. 1989, 93, 4587. (8) Wolthaus, L.; Schaper, A.; Mo¨bius, D. Chem Phys. Lett. 1994, 225, 322. (9) Janssens, G.; Touhari, F.; Gerritsen, J. W.; van Kempen, H.; Callant, P.; Deroover, G.; Vandenbroucke, D. Chem. Phys. Lett. 2001, 344, 1.

morphologies, based on the fluorescence properties of the dye molecules in the film.7,10,11 Kirstein and Mo¨hwald provided electron diffraction data and fluorescence microscopy data together with limited topography data on domains of aggregated dye molecules adsorbed onto an oppositely charged Langmuir film.12-15 However, this approach did not allow the simultaneous observation of structural and spectroscopic data. In the present contribution, we report on the simultaneous observation of topography and spectral data to investigate the organization in the domains of J-aggregates formed by a thiacarbocyanine dye adsorbed onto an oppositely charged Langmuir film. These experiments have been conducted using atomic force microscopy (AFM) and confocal fluorescence microscopy of the same site. The higher spatial resolution of the confocal and atomic force microscopy furthermore was able to give, after deposition, more detailed information on the organization of the domains that were already observed at the airwater interface.16 Those experiments determined to what extent the molecular organization obtained at the airwater surface is maintained in the films. To get a better (10) Knobler, C. M. Science 1990, 249, 870. (11) Chi, L. F.; Johnston, R. R.; Ringsdorf, H. Langmuir 1991, 7, 2323. (12) Kirstein, S.; Mo¨hwald, H.; Shimomura, M. Chem. Phys. Lett. 1989, 154, 303. (13) Kirstein, S.; Mo¨hwald, H. Chem. Phys. Lett. 1992, 189, 408. (14) Kirstein, S.; Bliznyuk, K.; Mo¨hwald, H. Physica A 1993, 200, 759. (15) Kirstein, S.; Steitz, R.; Grabella, R.; Mo¨hwald, H. J. Chem. Phys. 1995, 103, 818. (16) Vranken, N.; Lavoie, H.; Van der Auweraer, M.; Salesse, C.; De Schryver, F. C. Langmuir 2002, 18, 1641.

10.1021/la020230m CCC: $22.00 © 2002 American Chemical Society Published on Web 10/05/2002

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understanding of the influence of the deposition process, 3,3′-disulfopropyl-5,5′-dichloro-9-ethylthiacarbocyanine (THIATS)/dioctadecyldimethylammonium bromide (DODAB) films obtained with the Langmuir-Blodgett (LB) method were compared to those obtained by the horizontal lifting or Langmuir-Schaefer (LS) technique. Experimental Section The dye THIATS was a gift from Agfa. The amphiphile DODAB was purchased from Sigma-Aldrich (purity 95%) and was used as received. The deposited films were prepared on a commercially available LB trough (KSV Instruments Ltd.) A 10-6 M solution of THIATS in pure water was used as the subphase. The ultrapure water (Milli-Q) used for all experiments and all cleaning steps was obtained by using a filter system with several filtration steps (Millipore, catalog no. CFOF 012 05) to remove ions, organic materials, and small particles. The resistivity of the water was eventually 18 MΩ. DODAB was spread on this subphase from a 10-3 M CHCl3 solution. After evaporation of the CHCl3, the DODAB layer was compressed to a surface pressure of 30 mN/m. The area per DODAB molecule was approximately 100 Å2, indicating that the film was still in the liquid-expanded phase.10,16 The temperature of the subphase was around 20 °C. The surface pressure was measured with a Wilhelmy type balance. In the center of the trough, a rectangular basin allowed us to move the substrate, clamped in a computer-controlled mechanical arm, vertically through the Langmuir film at a constant dipping speed (typically 5 mm/min). After 60 min of adsorption time, a monolayer is deposited onto the hydrophilic substrate (Langmuir-Blodgett film) by moving it upward through the THIATS/ DODAB layer. The deposition ratio was typically between 0.9 and 1.1. To deposit the Langmuir-Schaefer film, a hydrophobic glass substrate was mounted on a T-shaped metal plate held by the clamp of the mechanical arm, to position the substrate horizontally. When the position of the arm was moved close to the air-water interface, a part of the film could be lifted and deposited onto the hydrophobic substrate. Glass substrates were cleaned by keeping them for 20 min in a “piranha” mixture (a 1/1 mixture of concentrated H2SO4 and a 30% H2O2 solution). Afterward, the substrates were thoroughly rinsed with water and kept under water until depositions were made. AFM revealed that the root mean square (rms) of the glass surface was 1.0 nm. To obtain the hydrophobic substrates, they were dried and afterward immersed for 10 min in a 1% dichlorodimethylsilane solution in chloroform. Consequently, a Zeonex film with a thickness of 10-20 nm was spin-coated to obtain a homogeneously flat film. AFM showed that the rms of the height amounted to 0.1 nm. Steady-state absorption spectra were recorded on a doublebeam UV/vis spectrophotometer of the Lambda40 series (PerkinElmer Instruments). Fluorescence spectra were recorded with a fluorimeter (Spex Fluorolog model 1691) in front face conditions (26°) to eliminate as much scattered light as possible. To record the wide field fluorescence micrographs, a photo camera (Nikon FX-35 WA, Agfa 400C film) was coupled to an optical microscope (Nikon OPTIPHOT-2, objectives, 20× and 40×) with an epifluorescence extension (Nikon EFD-3). The light source was a standard Hg lamp of which a band at 543 nm was selected with a band-pass filter (width of 10 nm) and a dichroic mirror (DM575, Nikon). A long pass filter of 580 nm (BA580, Nikon) was used for the detection of the sample fluorescence. The size of the illuminated area could be adjusted with a diaphragm that functions as a pinhole and can be opened or closed manually. Fluorescence spectra (with a spectral resolution of 1 nm) could be recorded making use of an IRRAD2000 Miniature Fiber Optics spectrophotometer (Ocean Optics Inc.) and a CCD camera (Sony, ILX511) that was computer controlled. The Lumina microscope (Thermomicroscopes, Sunnyvale CA) combines the possibility to perform topography and optical fluorescence measurements of the same position in the sample. The setup has been described in detail elsewhere.17,18 Two different He-Ne lasers (of 543 and 633 nm) were used to excite the samples. Emission of the samples was collected with two

Vranken et al. APDs after passing through a 50/50-beam splitter. The first APD (M-detector) collects monomer fluorescence (M-emission) using a 600 nm short-pass filter. The second APD (J-detector) collects aggregate fluorescence (J-emission) using a 650 nm band-pass filter (with a bandwidth of 25 nm). This setup allows splitting the fluorescence from the sample into two beams of equal intensity and, using the proper optics, collecting the M-emission and the J-emission. Topography images were collected using the intermittent contact mode with Si tips (1660, Thermomicroscopes). According to the manufacturer, the tip radius amounts to 20 nm. This value must be considered as a lower limit as during the experiments pickup of contamination can increase the effective tip radius. The cantilevers had a specified force constant of 48 N/m and a resonance frequency (in air) of around 190 kHz. Measurements were done at a constant amplitude amounting to 60% of the free amplitude. Processing and presentation of the data were done with the spmlab 5.0 control software (Thermomicroscopes). Height differences were determined using unprocessed data. While the topography images that are shown are line-leveled and contrast enhanced, the fluorescence images are not processed. The setup allows us to record fluorescence micrographs, using two different detection wavelengths and monochromatic, polarized (linear or circular) excitation light, and to record fluorescence spectra at specific locations in the micrograph, with a spatial resolution near the diffraction limit (g300 nm). Furthermore, it is possible to record topography images of the same position of the sample allowing a correlation of the fluorescence and topography characteristics of the film. For the high-resolution AFM imaging, a PicoSPM (Molecular Imaging, Phoenix, AZ) was used. This microscope is operated by a spm 100 control unit (RHK, Troy, MI). An intermittent contact mode was implemented (AAC mode, Molecular Imaging) in this setup. All images were recorded using sharp Si tips (d-levers, Thermomicroscopes). According to the manufacturer, the tip radius amounts to 20 nm. This value must be considered as a lower limit as during the experiments pickup of contamination can increase the effective tip radius. The cantilevers had a specified force constant of 2.2 N/m and a resonance frequency (in air) of 80-100 kHz. Measurements were done at a constant amplitude that was typical 95% of the free amplitude just above the sample surface. Processing and presentation of the data were done with the spm32 control software (RHK). Height differences were determined using unprocessed data. The images that are shown are line-leveled and contrast enhanced.

Results and Discussion Steady-State Absorption and Emission Spectra of the LB Films and LS Films. The steady-state absorption spectrum of the LB monolayer, shown in Figure 1, reveals a narrow J-band with a maximum at 650 nm and a bandwidth at 2/3 of the maximum of 340 cm-1. Fluorescence spectra were recorded with the excitation wavelengths of 625 and 543 nm. Both spectra reveal a J-band with a very small Stokes shift (30 cm-1) and a bandwidth at 2/3 of the maximum of 240 and 280 cm-1, respectively. The fluorescence spectrum (Figure 1) recorded with the excitation at the shorter wavelength (543 nm) reveals, besides the J-band, a band with a maximum at 585 nm and a bandwidth at 2/3 of the maximum of 620 cm-1 which is most likely resulting from monomer fluorescence. The small red shift of this band observed in the LB film compared to the maximum of THIATS observed in dilute aqueous solution19 (10 nm - 300 cm-1) is due to a different polarizability of the environment of the dye molecules.20,21 The bandwidth of the emission of (17) Hofkens, J.; Schroeyers, W.; Loos, D.; Cotlet, M.; Ko¨hn, F.; Vosch, T.; Maus, M.; Herrmann, A.; Mu¨llen, K.; Gensch, T.; De Schryver, F. C. Spectrochim. Acta, Part A 2001, 57, 2093. (18) Foubert, P.; Vanoppen, P.; Martin, M.; Gensch, T.; Hofkens, J.; Helser, A.; Seeger, A.; Taylor, R. M.; Rowan, A. E.; Nolte, R. J. M.; De Schryver, F. C. Nanotechnology 2000, 11, 16. (19) Vranken, N. Ph.D. Thesis, K.U.Leuven, Heverlee, Belgium, 2001.

J-Aggregates of a Thiacarbocyanine Dye

Figure 1. Absorption and emission spectra of the THIATS/ DODAB film deposited using the LB technique. Emission spectra were recorded at two different excitation wavelengths: 543 and 625 nm.

the monomeric dye molecules in the LB film is also somewhat smaller (approximately 100 cm-1) than that in solution.19 The absorption and emission spectra of the LS films (not shown) are within the experimental errors identical with those of the LB films. Fluorescence Micrographs of the LB and LS Films. Fluorescence micrographs of the LB film of THIATS adsorbed onto a monolayer of DODAB were recorded using a photo camera mounted on an epifluorescence microscope. Upon excitation at 543 nm, all fluorescence above 580 nm was collected. The micrograph on the left of Figure 2 (objective 40×, total magnification 267×) reveals a high density of quasi-circular, fluorescent domains, which correspond to the domains that were observed at the airwater interface.16 The quasi-circular domains show a very homogeneous red fluorescence. In the micrograph on the right of Figure 2, obtained using a larger magnification of the negative (objective 20×, total magnification 666×), the orange color of the fluorescence at the rim of the domains indicates that in this environment the fluorescence occurs at least partially at shorter wavelengths. Fluorescence spectra recorded from different positions of the LB film using the fiber optics of the Ocean Optics fluorimeter reveal a J-band at 650 nm and a bandwidth of 250 cm-1. The bandwidth did not narrow when zooming in on a single domain. This indicates that interdomain inhomogeneities do not lead to broadening of the fluorescence spectrum. Due to the limited spatial resolution of the setup, it was not possible to record spectra from selected parts of the domain. To obtain a better spatial and spectral resolution, the LB film was studied using a confocal fluorescence microscope, which by being coupled to an atomic force microscope allowed simultaneous acquisition of optical and topography information. Figure 3 shows a few fluorescence micrographs of the LB film of THIATS/ DODAB recorded with the M-detector (right micrographs) and the J-detector (left micrographs) upon excitation at 543 nm with linear-polarized light. The micrographs in the lower part of the figure were obtained by zooming in on one domain. These micrographs clearly reveal fluorescent domains with very irregular borders and a diameter which varies between 5 and 10 µm. The occurrence of deformations at the rim of the domains is even clearer in the micrographs recorded with the (20) Kuhn, H.; Mo¨bius, D.; Bu¨cher, N. Physical Methods in Chemistry; Weissberger & Rossiter, Eds.; Wiley: New York, 1972; Vol. I (3B), p 577. (21) Van der Auweraer, M.; Willig, F. Isr. J. Chem. 1985, 25, 274.

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M-detector. The rim of the domains definitely reveals M-fluorescence. Inside the domain, no fluorescence could be observed with the M-detector. The width of the rim that shows M-fluorescence is between 500 nm and 1 µm. The spatial distribution of J- and M-fluorescence and hence the shape and size of the domains could be confirmed using circular-polarized excitation at 543 nm (Figure 4 and Supporting Information). As fluorescence microscopy of the same monolayer at the air-water interface indicates the presence of circular domains,16 this deformation could be induced by the deposition process.22,23 It is also possible that irregular boundaries were already present at the air-water interface, as the resolution of the microscope used to detect the fluorescence of the film16 was insufficient to observe these irregular edges of the domains. In analogy to the fluorescence micrographs of a Langmuir film on a water surface,16 a more intense fluorescence is observed from two opposing quadrants of the domains using linear-polarized excitation at 543 nm and the J-detector (Figure 3). Furthermore, fluorescence micrographs of the LB-deposited films of THIATS/DODAB excited with linear-polarized light of 633 nm (Supporting Information) and obtained with the same J-filter reveal the same characteristics as the micrographs recorded with the J-detector upon excitation at 543 nm. Upon excitation at 633 nm, of course, no M-fluorescence could be obtained. On the other hand, fluorescence micrographs that were recorded with circular-polarized excitation light of 543 nm (Figure 4 and Supporting Information) using the J-detector no longer show the alternating dark and light quadrants. This indicates that the observation of these quadrants using linear-polarized excitation is not an artifact of the detection system. These results prove that after deposition of the film on a solid substrate using the LB technique the major fluorescence properties of the domains, and hence the packing of the dye molecules in the bulk of the domains, remains unchanged, except for the rim of the domains. The fluorescence micrographs recorded with the M-detector upon excitation with linearpolarized light at 543 nm show no fluorescence polarization. This indicates that the isolated dye molecules (that are not part of the aggregate) always had a random packing or obtained a random packing upon deposition of the film. Even if the randomly oriented monomers were already present in the film at the air-water interface, it was impossible to observe their fluorescence as their fluorescence quantum yield was reduced by the higher local mobility.24-26 Figure 5 shows a number of fluorescence micrographs of the deposited LS film of THIATS/DODAB, recorded with linear-polarized light of 543 nm. The detection of the fluorescence was done with the same M- and J-detectors as before. The fluorescence micrographs recorded with the J-detector are very similar to the ones that were recorded for the LB film. The domains seem however to have a more regular shape at the rim. Indeed, the fluorescence micrographs recorded with the M-detector show, in contrast to the results of the LB film, no M-fluorescence from the positions where the domains of (22) Petty, M. C. In Langmuir Blodgett Films - An Introduction; Cambridge University Press: Cambridge, 1996; p 60. (23) Mo¨hwald, H. J. Mol. Electron. 1988, 4, 47. (24) Van der Auweraer, M.; Van den Zegel, M.; De Schryver, F. C.; Boens, N.; Willig, F. J. Phys. Chem. 1986, 90, 1169. (25) Pevenage, D.; Corens, D.; Van der Auweraer, M.; De Schryver, F. C. Bull. Chem. Soc. Belg. 1997, 106, 565. (26) Oster, G.; Nishijima, Y. J. Am. Chem. Soc. 1956, 78, 1581.

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Figure 2. Fluorescence micrographs of the LB film of THIATS/DODAB recorded with the epifluorescence microscope. To record the right micrograph, the diaphragm was closed until only one domain was illuminated. The micrographs show real colors, recorded with a normal photo camera.

Figure 3. Fluorescence micrographs recorded of the LB film of THIATS/DODAB with linear-polarized light of 543 nm, emission recorded with the M-detector (micrographs on the right) and J-detector (micrographs on the left).

J-aggregates are present, not even at the rim of the domains. The only M-fluorescence observed in these micrographs results from the background of the film between the domains. This fluorescence however is very weak. These results indicate that the LS technique generates less distortion when the films are deposited. Fluorescence micrographs of the LS film recorded with linear-polarized light of 633 nm (Supporting Information) have the same characteristics as the ones recorded with excitation at 543 nm. The micrographs shown at the bottom of Figure 5 were obtained by zooming in on two domains. In the center of one of these two domains, fluorescence can be observed with the M-detector. This M-fluorescence can be seen even more clearly in fluorescence micrographs (Figure 6 and

Supporting Information) that were recorded using circularpolarized light of 543 nm. In the matching positions of the image recorded with the J-detector, those micrographs show that some domains do not yield J-fluorescence in the center. Both observations indicate that there is adsorbed dye material which is however not aggregated. From the model that was proposed earlier16 for the orientation of the dye molecules, it is easily understood why the dye molecules in the center of the domains have a smaller tendency to form wellorganized aggregates. At this point, the size of the molecules is not negligible with respect to the distance over which the orientation of the molecules changes to obtain the spatial distribution of transition dipoles suggested by the linear-polarized fluorescence of a Langmuir

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Figure 4. Fluorescence micrographs of the deposited LB film of THIATS/DODAB recorded with the J-detector (top left micrograph) and the M-detector (top right micrograph); emission spectra recorded at the four different positions indicated in the micrographs. The excitation light beam of 543 nm was circular polarized.

film of DODAB on a 10-6 M THIATS subphase.16 The resolution of the epifluorescence microscope was insufficient to reveal the different fluorescence behavior in the center of the domains. Atomic Force Microscopy of LB and LS Films. Using the AFM mode of the Lumina confocal setup, we tried to record topography images of the LB film of THIATS/DODAB in order to directly correlate the topography with the fluorescence properties of the film. Unfortunately, imaging was nearly impossible due to a very low signal-to-noise ratio. Imaging the topography of the deposited LB film of THIATS/DODAB on the PicoSPM microscope was however no problem. The topography images were recorded using the intermittent contact mode to avoid damage by minimization of the shear forces between the tip and the surface of the film. The first image, in the upper left corner of Figure 7, gives the typical appearance of the topography of the background of the deposition. The background is homogeneously flat with an rms of 0.50 nm. It is interrupted only by a number of small holes and small circular structures. These circular structures have an average height of 3.5 ( 1 nm and a diameter of 30 ( 10 nm. The very regular shape of these structures is most probably caused by convolution of the tip. This means that there are indeed small structures with an average height of 3.5 nm, but the diameter is reflecting the tip-sample

convolution rather than the effective diameter of the structures. These structures could be the result of local instabilities in the film, maybe due to the local roughness of the substrate (bare cover slip) or impurities causing collapse of the film on a very small scale. The holes in the film have a diameter of 20 ( 5 nm and a measured depth of 1.5 ( 0.5 nm. Due to the dimensions of the tip (estimated from the circular structures), it can most probably not enter the holes down to the substrate level, underestimating in this way the depth of the holes. Hence the homogeneous layer corresponds to a well-ordered monolayer of DODAB, that reveals the typical defects of an amphiphile film on a solid substrate.27,28 The rms of the height of the layer is smaller than that of an uncovered glass slide where it amounts to 1.0 nm. The second image (top right) of Figure 7 shows the structure of a domain of which the dimensions agree with those of the domains observed with the confocal microscope. The height of the domain, relative to the monolayer, was determined at 1.4 nm with an rms of 0.61 nm. This could correspond to the thickness of a monolayer of THIATS molecules (cfr. infra). Tsukruk et al.29 found the same value (1.5 ( 0.2 nm) for the height of a monolayer (27) Hansma, H. G.; Gould, S. A. C.; Hansma, P. K.; Gaub, H. E.; Longo, M. L.; Zasadzinski, J. A. N. Langmuir 1991, 7, 1051. (28) Viswanathan, R.; Schwartz, D. K.; Garnaes, J.; Zasadzinski, J. A. N. Langmuir 1992, 8, 1603.

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Figure 5. Fluorescence micrographs recorded of the LS film of THIATS/DODAB with linear-polarized light of 543 nm; emission was recorded with an M-detector (micrographs on the right) and a J-detector (micrographs on the left).

of the similar dye 3,3′-sulfopropyl-9-methylthiacarbocyanine, using AFM in contact mode. This value is larger than the corresponding lattice parameter (1.05 nm) in single crystals of 3,3′,9-triethyl-5,5′-dichloro-thiacarbocyanine.30 This can partially be due to the longer nitrogen substituents. Those data indicate that the domains consist of a single layer of THIATS molecules in an edge-on orientation with the long axis parallel to the substrate and the short axis more or less perpendicular to the substrate. This is also the orientation which would yield maximum π-interaction between neighboring THIATS molecules and justify the observed spectral shift and exciton coupling.31 The third image (bottom left image) is a zoom into the edges of the domain. The roughness of the edges is very clear in this image. The ruptures observed at the rim of the domains are attributed to the strain exerted on the domains during the deposition of the film. Furthermore, some substructure is observed on top of the domains. Zooming in (bottom right) shows indeed a high density of circular structures similar to those observed for the background of the film. Again, the lateral dimensions of the structures are determined by convolution with the tip, since the distance between these structures is so small they cannot be detected as separate structures using the confocal microscope. Furthermore, the fluorescence of the underlying aggregate domains would most probably (29) Tsukruk, V. V.; Reneker, D. H.; Bliznyuk, V. N.; Kirstein, S.; Mo¨hwald, H. Thin Solid Films 1994, 244, 763. (30) Asanuma, H.; Ogawa, K.; Fukunaga, H.; Tani, T.; Tanaka, J. Proceedings of the ICPS ’98 International Congress on Imaging Science; University of Antwerp (UIA): Antwerp, Belgium, 1998; Vol. 1, p 178. (31) Van der Auweraer, M.; Scheblykin, I. Chem. Phys. 2001, 275, 285.

be too intense to be able to distinguish fluorescence coming specifically from these structures. Without this information, it is not so straightforward to assign these structures to either dye (monomer or aggregate) or “inert” amphiphile material. As for the LB film, we also tried to obtain information on the topography of the LS film using the AFM mode of the Lumina setup in order to correlate the topography directly to the fluorescence characteristics of the film. In contrast with the LB film, it was possible for the LS film to get topography images with a good signal-to-noise ratio. Figure 8 shows the correlation between the J-type fluorescence (excited by linear-polarized light) and the topography for the LS film. The domains that reveal aggregate fluorescence show up in the topography images as relatively flat (cfr. infra) domains of the same size and equal height. Furthermore, these topography images reveal a large surface roughness between the domains where only a very weak M-type luminescence was observed. At this point, it is not possible to determine whether the roughness of these structures is resulting from discrete levels or not (cfr. infra). Since it is very weakly fluorescent, this material is most probably amphiphile material. To characterize the domains and the roughness of the background, we zoomed in on one domain (Figure 9). From this new topography image, the rough edges of the domains are very clear. This could not be visualized using fluorescence microscopy due to the limited spatial resolution. Furthermore, the domain shows holes in the center. At this position, no aggregate fluorescence is observed (Figure 6 and cfr. infra) which is not surprising since the packing of the dye molecules at this position is

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Figure 6. Fluorescence micrographs of the deposited LS film of THIATS/DODAB recorded with the J-detector (top left micrograph) and the M-detector (top right micrograph); emission spectra recorded at the four different positions indicated in the micrographs. The excitation light beam of 543 nm was circular polarized.

subject to defects or no dye is present at all. The height profile on the right in Figure 9 shows the height differences of the domain and the nonfluorescent material. The nonfluorescent structures show a height, with respect to the substrate level, of 6 and 9 nm. These values correspond with the thickness of a double and a triple layer of DODAB. The height of the domain itself is close to 4 nm. This corresponds to the thickness of one DODAB layer (3 nm) plus the thickness of a monolayer of edge-on oriented THIATS molecules (1.4 ( 0.3 nm29 or 1.05 nm30). Since the AFM images recorded with the Lumina setup are still subject to quite some noise, topography images were also recorded independent of the fluorescence microscopy experiments for the LS film using the PicoSPM. The features of the images obtained with the latter instrument (Figure 10) resemble closely those obtained with the Lumina. The height of the quasi-circular domains is very homogeneous and was found to be 4.5 nm with an rms of 0.76 nm. This height corresponds very well to that found from the experiments described above for the LB layer and to the sum of the thicknesses of a DODAB and a THIATS layer.29,30 Contrary to the LB films, the domains are decorated only marginally with small substructures (compare Figure 7, upper right or lower part, and Figure 10, left). Between the domains, irregular structures, that show no fluorescence, are visualized. When zooming on those

structures, we observed other, much smaller circular domains with a diameter of about 50 nm and a height of 2 ( 0.5 nm. These small domains are much more abundant than the small circular structures that were found between the domains in the LB films. Although the large and the small circular domains are very uniform in height, this is not true for the irregular structures in between. Distinct height levels can be distinguished. This was not possible for the topography images obtained with the Lumina setup. The majority of these structures have a height of 6 ( 0.3 nm. Subsequently, a number of levels that are 2.5 ( 0.3 nm higher were also found. Furthermore, there are a few levels that are yet another step of 2.5 ( 0.3 nm higher. The total height, with respect to the background, of this last level is 11 ( 0.5 nm. The domains to which dye molecules have adsorbed and aggregated are stabilized and are thus not influenced by this process. The large difference between the LB and LS films is due to the fact that in the former strong Coulomb forces bind the DODAB layer to the surface while in the latter this binding is limited to van der Waals interactions between the Zeonex and terminal methyl groups of the DODAB. Hence a more extensive reorganization of the DODAB film can occur after deposition to yield more stable double, triple, and quadruple layers. This reorganization into multilayers can be considered as a first step in the

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Figure 7. Topography images of the LB film of THIATS/DODAB recorded in the intermittent contact mode with the PicoSPM. The image in the left upper corner gives the topography of the background of the deposited film. The gray scale covers a range from 0 to 2 nm. The other images reveal the topographic features of a domain of J-aggregates. The lower images, from left to right, are further zooms of the upper right image. In the graph in the lower left corner, the gray scale runs from 0 to 4 nm. In the two graphs on the right, the gray scale runs from 0 to 3.5 nm.

Figure 8. Direct correlation between the fluorescence (left), obtained with linear-polarized excitation, and the topography (right) of the LS film.

reorganization to the thermodynamically more stable crystalline bulk phase. A possible explanation for the mechanism of the growth of these structures can be a kind of dewetting phenomenon. During the deposition of the LS film (a horizontal lifting), inevitably a small amount of water is left on the sample, since the hydrophilic side is oriented away from the hydrophobic substrate. This water will slowly evaporate which causes the water layer to become thinner and less stable. Since at some places the hydrophobic substrate is not perfectly covered, due to small defects in the deposited film, the film of water will burst. This results in a meniscus that withdraws due to the surface tension of the water.

This process will exert capillary forces on the deposited layer, which might cause the DODAB film to collapse. The overall result is a bare substrate coexisting with a collapsed film with a two-, three-, or even four-layer thickness of DODAB. Leuthe and co-workers32 found very similar structures during their study of the influence of temperature on the structure of multilayers of long-chain fatty acids. After heating (to 65 °C) a three-layer deposition of behenic acid, they observed the formation of terraces and double layers (32) Leuthe, A.; Chi, L. F.; Riegler, H. Thin Solid Films 1994, 243, 351.

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Figure 9. Topography image (left) of the LS film of THIATS/DODAB, recorded by zooming on one domain of Figure 8, and linescan (right) recorded at the position of the vertical line.

Figure 10. Topography images recorded from the LS film of THIATS/DODAB using intermittent contact AFM. To record the topography image on the right, we zoomed in on the structures between the domains. In the figure on the left, the gray scale runs from 0 to 15 nm. In the figure on the right, the gray scale runs from 0 to 18 nm.

which they also related without further discussion to dewetting. Also Schwartz and co-workers33 studied the topography of multilayers of long-chain acids, such as CdAA2 films (cadmium arachidate). They observed a pronounced reorganization of the films in the presence of water. After deposition of two layers of CdAA2 on a hydrophilic substrate, the film is eventually under water. Waiting for a few minutes before lifting the substrate out of the subphase (using the LB technique) to deposit a third layer resulted in terraces of double-layer thickness. Schwartz and co-workers33 ascribed this to the peeling of a piece of the double layer (dewetting) that flips over to an area of the substrate that is already covered with a film. Spatially Resolved Fluorescence Spectra of LB and LS Films. To record fluorescence spectra with an excitation wavelength of 543 nm, the circular-polarized excitation light beam is positioned above certain distinct spots of the micrograph. Figure 4 shows the micrographs and four fluorescence spectra that were each recorded at a single domain of the LB film of THIATS/DODAB. The positions where the excitation light beam was placed to record the spectra are indicated in the fluorescence micrographs (at the top of Figure 4). The emission spectrum recorded from position 1 in Figure 4 reveals a broad band with a maximum around 590 nm. This indicates the presence of M-fluorescence at site 1. The shoulder at longer wavelengths could be due to red-emitting dimers, or it could also be due to vibrational (33) Schwartz, D. K.; Viswanathan, R.; Zasadzinski, J. A. N. J. Phys. Chem. 1992, 96, 10444.

progression. The bandwidth at 2/3 of the maximum is 1100 cm-1 which is much broader than was observed for the dye molecules in dilute solution (approximately 700 cm-1).19 The fact that only M-fluorescence and no Jfluorescence could be detected indicates that adsorption of the dye molecules was not completely prevented but J-aggregation did not take place at site 1 or was lost during the deposition of the film. This domain is indeed much smaller (less than a micrometer in diameter) than the domains that reveal J-fluorescence. It is not clear to which extent those domains can be correlated with the small circular structures found between the domains using AFM (compare Figure 4, top right, with Figure 7, top right). Position 2 in Figure 4 is situated at the rim of the highly fluorescent domain. Although this rim shows distinct M-fluorescence (detected with the M-detector) and no J-fluorescence (detected with the J-detector), the emission spectrum is completely dominated by an intense J-band although a less intense M-fluorescence band is present. The resolution of the intensity scale used for the Jfluorescence of the fluorescence micrograph is probably insufficient to reveal the J-fluorescence at the rim of the domain, where it is in absolute count rates well below that in the bulk of the domain. Indeed, when an emission spectrum is recorded in position 3, in the bulk of the domain, the intensity of the J-band is more than 100 times that of the J-band in the spectrum recorded at position 2. It is not known whether this is due to a decreased density of J-aggregates at the rim or to a decreased fluorescence quantum yield of the J-aggregates at the rim. The bandwidth of the J-band is 260 cm-1, which is in good agreement with the bulk spectrum of the LB film and is

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within experimental error identical to the bandwidth of the spectrum recorded at the air-water interface (230 cm-1). The spectrum recorded in position 4 reveals no fluorescence, neither of monomer dye molecules nor of aggregated molecules. The signal is the result of background noise. This indicates that there is no residual fluorescence present in the deposited film between the domains. Also for the LS film, we determined fluorescence spectra of several characteristic sites, with an excitation at 543 nm with circular-polarized light. The spectra are given in Figure 6, together with two fluorescence micrographs to indicate the positions where the spectra were recorded. At position 1 of the micrographs, fluorescence is observed with the M-detector but no fluorescence can be seen with the J-detector. Due to the small size of this domain, it is reasonable to assume that if dye molecules in the domain were aggregated at the air-water interface any aggregation is lost upon deposition of the film. The fluorescence spectrum at this site has a maximum at 590 nm and a shoulder at longer wavelengths, probably due to the vibrational transition. The bandwidth at 2/3 of the maximum is 500 cm-1 which is 200 cm-1 narrower than that in the spectrum of THIATS in a dilute solution.19 This smaller bandwidth suggests that most adsorbed dye molecules are present as a pure all-trans isomer or that the dye molecules are in a more homogeneous environment when incorporated in a film. At this position, there is no indication of J-aggregate fluorescence. The spectrum recorded in position 2 gives the background noise of the LS film. It is not clear to which extent sites 1 and 2 (Figure 6, top right) can be correlated with a height of respectively zero and 6 or 9 nm (Figures 8-10). The third spectrum was recorded at the center of the large domain, where there is apparently no J-fluorescence (see top left micrograph) but only M-fluorescence (see top right fluorescence micrograph). The spectrum reveals an M-band as well as a J-band. The M-band is very similar to the one recorded in position 1, although the intensity is somewhat higher. The J-band is almost 4 times less intense than the one that is observed in position 4, where the J-fluorescence is most intense. The bandwidth of the J-band is 280 cm-1, well in agreement with the previous results.16 Conclusions The ordered circular domains of J-aggregates formed by adsorption of a dye to a Langmuir film of DODAB are maintained upon deposition of the Langmuir film on a solid substrate, using both the Langmuir-Blodgett and the Langmuir-Schaefer techniques. In the bulk of the domains, the molecular packing and orientation, probed by the spectral features and the polarization of the fluorescence, are kept intact. Each deposition technique preserves however the integrity of the domain boundaries to a different extent. The fluorescence micrographs recorded for the LB film reveal M-fluorescence at the rim of the domains, indicating a loss of aggregate formation. Furthermore, the Mfluorescence observed at the rim does not reveal the fluorescence polarization found for the bulk of the domains. It is not clear from previous experiments whether this random orientation of the dye molecules at the rim of the domains was already present at the air-water interface, as the resolution of the latter experiments was much lower.16 However, the LS film does not show any Mfluorescence at the rim of the domains, which might

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indicate that the M-defects are introduced by the LB deposition process. Both types of films reveal ruptures and deviations from the circular domains observed at the air-water interface. It might be that this was not observed at the air-water interface due to the limited spatial resolution, or the deposition processes might induce these defects. Furthermore, the setup allowed recording topography images from the same position of the films where the fluorescence micrographs were recorded. There was a clear correlation between both images. The height of the domains was 4.5 nm with an rms of 0.76 nm for the LS film, which corresponds very well with a monolayer thickness of DODAB plus a monolayer of THIATS molecules in an edge-on orientation with the long axis parallel to the substratum. In the LB film, the domains emerged by 1.4 nm (with an rms of 0.61 nm) from the layer. Since the background of the film revealed holes with a depth corresponding to a monolayer of DODAB, this step corresponds to the presence of a monolayer of adsorbed THIATS molecules below the DODAB layer. In contrast to the LB films, the background of the LS films (between the domains) is very rough and seems to be made up of plateau regions with different heights, corresponding to a double-layer and sometimes even to a four-layer thickness of DODAB. Between the plateaus, the bare substrate (the Zeonex film) can be seen. These structures can be assigned to a dewetting process, as described by other authors33 for multilayers of amphiphilic compounds. Fluorescence spectra were recorded from the different features observed in the fluorescence micrographs. For the LB films, the fluorescence between the domains reveals an M-band, with a maximum at 590 nm and a broad shoulder at longer wavelengths. The bandwidth is larger than that observed for THIATS in a dilute aqueous solution, which might indicate that the shoulder is the result of residual dimers or aggregates with a smaller effective coherence length than the J-aggregates. Spectra recorded in the bulk of the domains revealed an intense J-band, with a bandwidth similar to that observed at the air-water interface, indicating no loss of aggregation upon deposition. The LS films sometimes revealed M-fluorescence in the center of the domains or from domains with a very small diameter. Fluorescence spectra recorded at these positions revealed M-bands with a maximum at 590 nm. The bandwidths however were much narrower than the ones observed for the LB films. They were even narrower than for the spectra of THIATS in a dilute aqueous solution. This might indicate that the surroundings for the dye molecules in the film are much more homogeneous or that just one type of isomer34 (the alltrans) of the dye molecules has adsorbed. Acknowledgment. N. Vranken and P. Foubert thank the “Vlaams instituut voor de bevordering van het wetenschappelijk en technologisch onderzoek” (IWT). I. Scheblykin thanks the “Fonds voor Wetenschappelijk Onderzoek Vlaanderen” (F.W.O.) and the Research Council of the K.U.Leuven for a postdoctoral Fellowship. F. Ko¨hn and R. Gronheid thank the DWTC (Belgium) through I.U.A.P. IV-11 and the research Council of the K.U.Leuven through GOA 2001/2 for financial support. The authors gratefully acknowledge the continuing sup(34) Vranken, N.; Jordens, S.; De Belder, G.; Lor, M.; Rousseau, E.; Schweitzer, G.; Toppet, S.; Van der Auweraer, M.; De Schryver, F. C. J. Phys. Chem. A 2001, 105, 10196.

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port from DWTC (Belgium) through Grant IUAP-IV-11 and I.U.A.P. V-03, the F.W.O.-Vlaanderen and the Nationale Loterij, the Research Council of the K.U.Leuven through GOA 96/1 and 2001/2, and the EU through COSTD14. The authors are grateful to Agfa N.V. for the gift of the dye THIATS.

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Supporting Information Available: Further fluorescence micrographs of the LB and LS films using circular-polarized excitation at 543 nm and linear-polarized excitation at 632 nm. This material is available free of charge via the Internet at http://pubs.acs.org. LA020230M