J. Phys. Chem. 1995, 99, 3-7
3
Excitonic Transitions in J-Aggregates Probed by Near-Field Scanning Optical Microscopy Daniel A. Higgins and Paul F. Barbara* Department of Chemistry, University of Minnesota, 207 Pleasant St. SE, Minneapolis, Minnesota 55455 Received: September 26, 1994; In Final Form: October 21, 1994@
Near-field scanning optical microscopy (NSOM) is used to probe the excitonic transitions in J-aggregates of l,l’-diethyl-2,2’-cyanineiodide grown in poly(viny1 sulfate) thin films. Near-field images of the fluorescence from the excitonic state of the aggregates are obtained with good signal-to-noise and spatial resolution better than 100 nm. Fluorescence spectra recorded as a function of the NSOM tip position along individual aggregates show only slight variations and are very similar to the bulk aggregate spectrum. The absence of spectral broadening due to static inhomogeneities is interpreted as direct evidence for a uniform, well-ordered molecular structure within the aggregates. The excitonic transitions in the aggregates are locally photobleached by the light from the NSOM tip. The spatial extent of photobleaching observed here is limited by the resolution of the instrument; these results are used to place an upper limit of %SO nm on the physical extent of exciton migration along these aggregates.
I. Introduction Extensive theoretical and experimental research has been directed toward a better understanding of aggregated molecular systems and their interesting optical and electronic properties.*s2 The vast majority of these works have employed bulk spectroscopic methods to probe their electronic structures, while others have utilized inherently nonspectroscopictechniques like SEM3 and AFM4to characterize their microscopic physical structures. A few works have employed conventional optical microscopy to obtain information on a localized scale;s however, the optical resolution in these studies was limited to d l 2 by the wellknown Rayleigh criterion. While many of the microscopic features of these aggregates have been inferred from these previous results, numerous questions remain pertaining to the local optical and electronic properties of aggregates on a nanometer dimensional scale. NSOM is a new optical microscopic t e c h n i q ~ e ~which - ~ ~ provides spatial resolution on this scale in imaging applications as well as for spectroscopy.16,18 In this Letter we report the results of experiments in which NSOM was used to acquire fluorescence images of molecular aggregates with 100 nm spatial resolution. In addition, remaining questions about the amount of inhomogeneous broadening in their spectra are addressed, and an upper limit is placed on the distance over which optically-produced excited states (excitons) can migrate within the aggregates. The aggregates studied in this Letter are the J-aggregates of l,l’-diethy1-2,2’-cyanideiodide, commonly known as pseudoisocyanine or PIC, grown in poly(viny1 sulfate) (PVS) thin filmsz4 J-aggregates are distinguished from other types of aggregates by the characteristic red-shifting and narrowing of their absorption and fluorescence spectra from that of the monomeric dye specie^.^^^^^ The spectral shift which occurs upon aggregation arises from the formation of a delocalized excitonic state through the electronic coupling of the tightlypacked dye molecules.27 On a microscopic scale, these aggregates take on a range of structures, the most common being that of needles or rods with lengths of several microns and widths in the submicron However, spectroscopic measurements on these aggregates indicate that the excitonic states are only delocalized over ~ 1 0 molecules 0 at most28-33 @
Abstract published in Advance ACS Abstracts, December 1, 1994.
due to strong coupling to phonons.34 The excitons are believed to migrate on a larger distance scale through an incoherent energy transfer m e c h a n i ~ m , 2 ~which % ~ ~ is restricted by the presence of aggregate edges, structural defects, grain boundaries, and traps. The high spatial resolution in the optical images obtained with NSOM is achieved by employing a light source that is on the order of a few tens of nanometers in diameter. This light source is typically a tapered, aluminum-coated, single-mode optical fiber which has an aperture of a few tens of nanometers at one end.6 Laser light is coupled into the opposite, cleaved end. The sample is placed on an X,Y,Z piezostage and is held within the “near field” of the fiber optic tip by a shear-force feedback mechanism36incorporating the standard electronics used in STM and AFM. Optical images are obtained via several different contrast mechanisms3’ by collecting the light from the sample as the sample is raster scanned under the tip. The high sensitivity of NSOM has recently been demonstrated in a number of papers which have reported the imaging and static and timeresolved spectroscopy of single dye molecules.15J7~18~20~21 11. Experimental Section Samples were prepared by dissolving 30 mg of PVS (potassium salt) in 4 mL of deionized water. The solution was heated to 70 “C, and 0.4 mL of 10 mM PIC in methanol was added to the stirred s o l ~ t i o n . The ~ ~ ,hot ~ ~solution was spin-coated onto a glass microscope slip cover at 2000 rpm. This procedure yielded samples of J-aggregates incorporated in PVS films of ~ 3 nm 0 average thickness as determined by ellipsometry. Figure 1 shows the UV-vis absorption spectrum of the monomeric form of PIC in dilute methanol solution and the spectrum obtained for the PIC J-aggregates in aqueous PVS solution. The fluorescence spectrum of the aggregates in PVS solution is shown in Figure 2 (dashed line). Both PVS and PIC were obtained from Aldrich and were used as received. The NSOM instrument was purchased from Topometrix (Aurora model). The shear-force feedback mechanism36is used for control of the tip-sample separation in the Aurora instrument. The Al-coated fiber optic tips were dithered on resonance with 10 nm amplitude. The dither was detected with light from a diode laser operating at 670 nm which was focused onto the tip, reflected off the sample surface, and detected by a split
0022-365419512099-0003$09.00/00 1995 American Chemical Society
Letters
4 J. Phys. Chem., Vol. 99, No. I , 1995
Wavelength (nm) Figure 1. Absorbance spectra for the monomeric form of PIC in dilute
methanolic solution (-) and the J-aggregates grown in an aqueous PVS soltion (-). The narrow, red-shifted absorption band in the aggregate spectrum is due to an electronic transition to a delocalized excitonic state created upon aggregation of the PIC molecules. 1.0
In the experiments presented here, 0.5-1 mW of 514 nm light from a CW argon ion laser was coupled into the cleaved end of the fiber, giving x3 x lo9 photonsls from the tip (detected in the far field). The polarization of the light from the tip was determined to be slightly elliptical. An oil immersion objective (Zeiss Model 440280, 100 X, 1.25 NA) was used to collect the light from the sample. Two short-pass filters (CVI Model SWP-650) were used to remove residual diode laser light. Two long-pass filters (Schott OG-55Oj were used to block the 514 nm argon ion line. A single-photon-counting avalanche photodiode (SiAPD, EG&G Model SPCM-203-PQ) with 7 dark countsk was used to detect the fluorescence from the aggregates. An additional band-pass filter was placed before the SiAPD to remove spurious broad-band emission (at *630 nm) from the diode laser. In the dispersed fluorescence experiments, the light was diverted to a 0.25 m polychromator (ISA Model HR320) which was coupled to a Princeton Instruments liquid nitrogencooled CCD detector. The spectral resolution was limited by binning of the CCD pixels to 1.2 nm. No evidence of Raman scattering from the fiber was observed within our signal bandwidth (540-640 nmj.
111. Results and Discussion W
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Wavelength (nm) Figure 2. Fluorescence spectra obtained from an aqueous bulk PVS solution of PIC I-aggregates (-) and that obtained in the near field for the aggregates in a PVS film on a glass substrate (-). The near-field fluorescence spectrum was recorded by positioning the near-field probe over an aggregate and integrating the dispersed fluorescence for 10 s on a CCD detector. The spectral resolution was limited by pixel binning and was x1.2 nm. The similar positions and widths for the bulk and near-field spectra demonstrate that their is little inhomogeneous broadening in this system; the monomer species forming the aggregate must therefore be packed in a well-ordered geometry. The near-field spectrum has been corrected to remove the effects of the structure in the transmission spectrum of the short-pass filter and also to remove a peak due to broad-band emission from the diode laser. The vibronic band does not appear in the near-field spectrum due to difficulties in background subtraction. The inset shows the fiist-order decay of the fluorescence from the excitonic state due to the photobleaching of the aggregates. photodiode. A Stanford Research digital lock-in amplifier (Model SR850) was used to measure the dither signal from the photodiode. The X-output of the lock-in was used in the Topometrix digital feedback loop. The NSOM instrument was operated on a vibrationally-isolated TMC table. A tip-sample separation of x 1 0 nm with 1 nm of noise was obtained with this system. NSOM tips were produced in-house from 3M single-mode optical fiber (Thorlabs FS-SN-3224). The tips were pulled in a laser-based fiber puller (Sutter Model P2000) with parameters that consistently produced ~ 1 0 nm 0 diameter tips (verified by SEMj which were then coated with 75-100 nm of aluminum (verified by SEM).
Figure 3 shows two separate fluorescence NSOM images and one topographic image obtained for PIC J-aggregates embedded in PVS films. The fluorescence from these aggregates is due to emission from the excitonic ~ t a t e . * The ~ , ~ NSOM ~ images clearly show the long, needlelike structure of these aggregate^;^-^ and features of ~ 1 0 nm 0 fwhm are observed with good signalto-noise (see line scan in Figure 3a), demonstrating spatial resolution of better than A b . Maximum signal levels are ~ 1 2 0 photons in the 20 ms pixel time of these images. About 20 counts of background are observed in this same time period, giving a shot-noise-limited signal-to-noise ratio of 10. Low signal levels are expected from these aggregates as the fluorescence quantum yield is *0.02,24 and the excitation wavelength is far from the absorption maximum (see Figure 1). Reduction in signal due to exciton a n n i h i l a t i ~ ncan ~ ~ be excluded as a concern in these experiments due to the low excitation rate. The topographic image in Figure 3b (left) shows only a few features corresponding to those in the fluorescence image (Figure 3b, right). Such an effect could be a result of the embedding of the aggregates in the film or from the growth of two-dimensional aggregates on top of the film. In general, the aggregates that show little or no topography yielded lower fluorescence signals. The observed variations in fluorescence intensity could thus be due to structural differences in these aggregates which may be manifested in the local spectroscopic characteristics of the excitonic transition. Alternatively, if these aggregates are embedded in the film, the aggregate must necessarily be further from the NSOM tip. Such an effect would lead to lower excitation intensities due to the exponential decrease in the evanescent field strength as a function of distance from the Intensity variations due to tip-aggregate distance variations could also arise from the effects of energy transfer to the tip, as has recently been shown to be a factor in near-field fluorescence measurements on single molecule^.^^^^^ However, the inherent nonradiative decay rate of excitons in J-aggregates is much faster than for single molecules; thus, in the worst case, the rate of energy transfer to the tip and the inherent decay rate of the excitons are equal.15~20~24~29 Future time-resolved fluorescence measurements will directly address this issue. In addition, differences in orientation of the molecules within the aggregate can also lead to variations in
J. Phys. Chem., Vol. 99, No. I , 1995 5
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Figure 3. (a) Fluorescence near-field image (6.6 x 6.6 pm2) of PIC J-aggregates grown in a PVS film of ~ 3 nm 0 average thickness spin-coated onto a glass substrate. Features as small as xl00 nm fwhm are observed in this figure with good signal-to-noise, demonstrating resolution better than A/5. A line scan from the image is shown in the graph to the right of the image. No features corresponding to these aggregates are observed in the topographic image, indicating that the aggregates are embedded within the polymer film. The polymer film itself showed maximum peakto-valley topography of -50 nm. (b) Topography (left) and fluorescence near-field (right) images (10 x 10 pm2) of J-aggregates embedded in a thinner PVS film. Corresponding features are observed in the fluorescence and topographic images which indicate that the widths of the aggregates in the near-field image are a good measure of their actual widths. The largest features in the topographic image are a100 nm above the base line.
the observed intensity of fluore~cence.~Future polarization studies will address this issue. However, variations in the concentration of traps and defects across a given aggregate as
well as simple variations in the concentration (arising from thickness differences) of the aggregate are the most likely causes of the observed variations in the fluorescence intensity.
Letters
6 J. Phys. Chem., Vol. 99, No. I , 1995 TABLE 1: Spectral Characteristics of the Fluorescence from Localized Points along a Single PIC J-Aggregate Grown in a PVS Film" spot Am (nm) fwhm (nm) intensity (countsh) 1 2 3 4
5 6
574b 575 575 580 574 575
15b 13 16 18 16 15
100" 210 220 320 140 300
"In these experiments, the NSOM tip was placed over a specific spot on an aggregate (determined with the topographic image) with no light coupled into the fiber. Immediately after 2 mW of 514 nm light was coupled into the fiber, a 10 s integration was started on the CCD detector. Following the acquisition of a spectrum, the laser was blocked, a new spot along the same aggregate was selected, and a new spectrum was recorded. The spectra have a resolution limited to 1.2 nm by the binning of pixels on the CCD detector. The error in the wavelength measurement is f 0 . 2 nm. The intensity reported here is that of the peak of the fluorescence spectrum. The collection and detection efficiency is ~ 5 % .As discussed in the text, the intensity variations are correlated with the apparent thickness of the aggregate at each point (as determined from the topography).
Further information on the localized electronic structure of individual aggregates can be probed by wavelength-dispersing the fluorescence obtained from the aggregates excited by the near-field light. l 6 9 l 8 Comparison of the individual spatiallyresolved fluorescence spectra from different sites on the same aggregate with those from different aggregates and that of the bulk sample gives direct information on the amount of spectral broadening due to static inhomogeneities that are present in the aggregates. The fluorescence spectrum acquired by positioning the NSOM tip over a single spot on an aggregate is shown in Figure 2 along with the fluorescence spectrum obtained for the bulk sample. Very similar results are obtained for the nearfield fluorescence spectrum (which is localized to the size of the NSOM tip) and for the bulk sample. The spectral shift (or the spectral narrowing) upon aggregation can be used to obtain a delocalization length for the excitons and yields a value of about 25 monomers for the PIC J-aggregates studied here.28 Slight variations in the near-field fluorescence spectra are observed when the tip is positioned at several different points along one aggregate and when it is positioned over different aggregates. The spectral characteristics for several points along one aggregate are listed in Table 1. As seen in the table, the intensity of the fluorescence varies dramatically across a single aggregate (discussed above). The positions of the fluorescence maxima and their widths, however, are seen to vary by only 3-5 nm. While these variations are not large, they are well outside the spectral resolution of 1.2 nm. The similarities between the bulk and localized fluorescence spectra and the lack of large variations in the site-to-site spectra demonstrate that there is very little inhomogeneous broadening on a spatial scale down to a few tens of nanometers. These results are a direct indication of the long-range structural order present in these aggregates and verify the conclusions presented in previous bulk studies.41 As noted above, the excitonic states in J-aggregates are delocalized over a number of monomer units. It has also been suggested that these excitons may migrate throughout a fairly large volume of the aggregate in an incoherent fashion via an energy transfer m e ~ h a n i s m . ~However, ~ . ~ ~ the extent of this migration has not yet been directly measured to our knowledge, although it has been proposed that this volume could be as large as 5 x cm3.29 We have utilized the high spatial resolution of the NSOM probe to locally create excitons in the aggregates. These excitons then migrate along the aggregates and decay by
1 /
Figure 4. Near-field fluorescence images (2.5 x 2.5 pm2) of unbleached J-aggregates (top) and the same region following photobleaching of a stripe (bottom) across the image. The arrows in the bottom image indicate the photobleached region. Photobleaching was accomplished by irradiating the aggregates over a 300 nm wide region across the center of the image with 10 times the intensity used for imaging. If the mechanism for photobleaching occurs through the excitonic state, then the photobleached image provides a measurement of the distance over which the excitons migrate within their lifetime.29 These results demonstrate that the observed photobleaching pattern is limited by the resolution of the near-field probe used here ( ~ 1 0 nm 0 diameter) and indicate that the excitons do not migrate much more than ~ 5 nm 0 at most.
several mechanisms, one of which is a photobleaching process. The kinetics of photobleaching of the aggregates were found to be first order with respect to the irradiation time (see Figure 2 inset); both the excitonic absorption and fluorescence peaks were destroyed in the process. The spatial characteristics of
Letters locally-photobleachedimages of aggregates should give a direct measurement of the exciton migration distance, if several assumptions Figure 4 shows two fluorescence images of a pair of J-aggregates before (Figure 4a) and after (Figure 4b) photobleaching a stripe across the aggregates. Following collection of the initial image, the same area was rescanned with the argon ion laser blocked for most of the image except for a 300 nm wide region through the center of the image. In this region, 10 times the intensity of light used for the original image was used to photobleach the aggregates. The final image shown in Figure 4b was subsequently recorded using the same intensity as the initial image. The dimensions of the visibly photobleached rkgion in Figure 4b are approximately that of the irradiated area. If the excitons could migrate large distances, the area of photobleaching would be expected to be much larger than the irradiated area. The observed result therefore suggests that the distance over which the majority of the excitons are migrating in this system is smaller than the spatial resolution obtained in these near-field images. An upper limit of m50 nm can therefore be assumed for the exciton migration distance in these aggregates. Heating of the aggregates may also contribute to the photodestruction of the aggregates. However, the photobleaching experiment was repeated on a bulk aggregate sample in the far field with an intensity a20 mW/cm2. In this experiment in which no sample heating should occur, a photodestruction time of 2 h was measured. Comparison of the near-field and farfield experiments therefore shows an approximately linear dependence of the rate of photodestruction on illumination intensity. If sample heating was contributing to the destruction of the exciton band in the near-field experiment, a higher-order nonlinear dependence of the photobleaching rate on intensity would be expected. Future work will be directed toward a more complete characterization of the exciton migration distance and directionality.
IV. Conclusions We have employed NSOM to obtain important new information on the electronic properties of the PIC J-aggregates and their relationship to the physical structure of these aggregates. First, we have shown that fluorescence images of the Jaggregates grown in PVS films can be obtained with high spatial resolution on the order of 100 nm. The dispersed fluorescence from the excitonic state of the aggregates was recorded as a function of the NSOM tip position along individual aggregates a d showed only slight variations from point to point; these spectra were also found to be very similar to the bulk aggregate spectrum. These results demonstrate that there is minimal inhomogeneous broadening of the spectra on a nanometer dimensional scale, leading to the conclusion that the molecules within the aggregate are highly ordered. It was also demonstrated that the excitonic transition in the aggregates could be locally modified as a result of the photodestruction of the aggregate. The localized photodestruction of the aggregates was used as a measure of the physical extent of exciton migration along the aggregate. The results of these experiments indicate that the majority of the excitons in these aggregates do not survive long enough to migrate more than m50 nm from the tip. Acknowledgment. The authors thank Jay Trautman of AT&T Bell Labs, Sunney Xie of Pacific Northwest Labs, and Patrick Moyer of Topometrix for their helpful suggestions on tip making and on setting up and testing the NSOM apparatus. Josef Kerimo and Andrey Kosterin, both of the University of Minnesota, are acknowledged for their help in tip making and
J. Phys. Chem., Vol. 99, No. 1, 1995 7 in the measurement of PVS film thickness, respectively. Philip Reid and Eric Olson, also of the University of Minnesota, are acknowledged for their help in collecting the aggregate fluorescence spectra. This work was supported by the Office of Naval Research and the University of Minnesota. D.H. gratefully acknowledges the National Science Foundation Postdoctoral Fellowship Program for his support. References and Notes (1) Herz, A. H. Adv. Colloid Sci. 1977, 8, 237. (2) Bohn, P. W. Annu. Rev. Phys. Chem. 1993,44, 37. (3) Emerson, E. S.; Conlin, M. A.; Rosenoff, A. E.; Norland, K. S.; Rodriguez, H.; Chin, D.; Bird, G. R. J . Phys. Chem. 1967, 71, 2396. (4) Wolthaus, L.; Schaper, A.; Mobius, D. Chem. Phys. Lett. 1994, 225, 322. (5) Maskasky, J. E. Langmuir 1991, 7, 407. (6) Betzig, E.; Trautman, J. K.; Harris, T. D.; Weiner, J. S.; Kostelak, R. L. Science 1991, 251, 1468. (7) Betzig, E.; Trautman, J. K. Science 1992, 257, 189. (8) Pohl, D. W.; Denk, W.; Lanz, M. Appl. Phys. Lett. 1984,44, 651. (9) Diirig, U.; Pohl, D. W.; Rohner, F. J. Appl. Phys. 1986, 59, 3318. (10) Harootunian, A.; Betzig, E.; Isaacson, M.; Lewis, A. Appl. Phys. Lett. 1986, 49, 674. (11) Lieberman, K.; Harush, S.; Lewis, A,; Kopelman, R. Science 1990, 247, 59. (12) Courjon, D.; Vigoureux, J.; Spajer, M.; Sarayeddine, K.; Leblanc, S. Appl. Opt. 1990, 29, 3734. (13) Vigoureux, J. M.; Depasse, F.; Girard, C. Appl. Opt. 1992,31,3036. (14) Dunn, R. C.; Holtom, G. R.; Mets, L.; Xie, X. S. J. Phys. Chem. 1994, 98, 3094. (15) Xie, X . S.; Dunn, R. C. Science 1994, 265, 361. (16) Bimbaum, D.; Kook, S.-K.; Kopelman, R. J . Phys. Chem. 1993, 97, 3091. (17) Betzig, E.; Chichester, R. J. Science 1993, 262, 1422. (18) Trautman, J. K.; Macklin, J. J.; Brus, L. E.; Betzig, E. Nature 1994, 369, 40. (19) Grober, R. D.; Harris, T. D.; Trautman, J. K.; Betzig, E.; Wegscheider, W.; Pfeiffer, L.; West, K. Appl. Phys. Lett. 1994, 64, 1421. (20) Ambrose, W. P.; Goodwin, P. M.; Martin, J. C.; Keller, R. A. Science 1994, 265, 364. (21) Ambrose, W. P.; Goodwin, P. M.; Martin, J. C.; Keller, R. A. Phys. Rev. Lett. 1994, 72, 160. (22) Vaez-Iravani, M.; Toledo-Crow, R. Appl. Phys. Len. 1993,62, 1044. (23) Toledo-Crow. R.: Yang. P. C.: Chen. Y.: Vaez-Iravani, M. A D ~ . Phys. Lett. 1992, 60, 2957. 124) Homg. M.-L.: Ouitevis. E. L.J . Phvs. Chem. 1993. 97. 12408. (25) Scheze, G. Aniew. Chem. 1936, 49, 563. (26) Jelley, E. E. Nature 1936, 138, 1009. (27) Czikkely, V.; Forsterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 11. (28) Knapp, E. W. Chem. Phys. 1984, 85, 73. (29) Sundstrom, V.; Gillbro, T.; Gadonas, R. A,; Piskarskas, A. J . Chem. Phys. 1988, 89, 2754. (30) Takahashi, K.; Obi, K.; Tanaka, I.; Tani, T. Chem. Phys. Lett. 1989, 154, 223. (31) De Boer, S.; Wiersma, D. A. Chem. Phys. Lett. 1990, 165, 45. (32) Kemnitz, K.; Yoshihara, K.; Tani, T. J . Phys. Chem. 1990,94,3099. (33) Fidder, H.; Knoester, J.; Wiersma, D. A. J . Chem. Phys. 1993, 98, 6564. (34) Spano, F. C.; Kuklinski, J. R.; Mukamel, S. J. Chem. Phys. 1991, 94, 7534. (35) Mobius, D.; Kuhn, H. J . Appl. Phys. 1988, 64, 5138. (36) Betzig, E.; Finn, P. L.; Weiner, J. S. Appl. Phys. Lett. 1992, 60, 2484. (37) Trautman, J. K.; Betzig, E.; Weiner, J. S.; DiGiovanni, D. J.; Harris, T. D.; Hellman, F.; Gyorgy, E. M. J. Appl. Phys. 1992, 71, 4659. (38) Misawa, K.; Ono, H.; Minishima, K.; Kobayashi, T. Appl. Phys. Lett. 1993, 63, 577. (39) Bethe, H. A. Phys. Rev. 1944, 66, 163. (40) Bouwkamp, C. J. Phillips Res. Rep. 1950, 5, 321. (41) Durrant, J. R.; Knoester, J.; Wiersma, D. A. Chem. Phys. Len. 1994, 222, 450. (42) The determination of exciton migration distances from spatial photobleaching measurements requires that the following assumptions are valid: that the photobleaching process proceeds through the excitonic state via a singlet mechanism, that energy transfer to the tip does not significantly constrain the excitons to the region directly under the tip (see text for a discussion), that the lifetimes of the excitons are not significantly altered by photobleaching, and that heating of the aggregates due to the presence of the tip does not contribute significantly to aggregate destruction as discussed in the text.
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