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J. Phys. Chem. B 2000, 104, 9378-9387
Near-Field Scanning Optical Microscopy (NSOM) Studies of Nanoscale Polymer Ordering in Pristine Films of Poly(9,9-dialkylfluorene) Julie Teetsov and David A. Vanden Bout* Department of Chemistry and Biochemistry & Texas Materials Institute, UniVersity of Texas at Austin, Texas 78712 ReceiVed: April 3, 2000; In Final Form: July 6, 2000
Near-field scanning optical microscopy (NSOM) is used to characterize nanoscale topographic and fluorescence features of pristine films of the stiff-chain polymer polyfluorene. The pristine films of polyfluorene bearing two hexyl (1) or dodecyl (2) groups at the 9 position were studied. Pristine films appear isotropic using conventional polarized light microscopy. NSOM images show two distinct types of film morphology, the first of which is characterized by 50-150 nm clusters in the topography. These clusters seen in films of 1 and 2 correlate directly with regions of lower fluorescence. The cluster correlated fluorescence (CCF) is unpolarized and has a slightly greater percent contribution from low energy emission than the rest of the polymer film. The larger and more frequent regions of CCF in films of 1 versus films of 2 indicate they are formed due to poor solubility. NSOM images show a second type of morphology that is characterized by 50-500 nm polarized domains denoted as long range order (LRO). These domains are distributed uniformly in pristine films of 1 and 2. Unlike CCF, the topography and fluorescence of LRO do not correlate. A method of NSOM image math is introduced to quantify film anisotropy from simultaneously collected fluorescence images at orthogonal polarizations. The anisotropy of 2 is found to be significantly larger than 1. NSOM images collected at 440 & 600 nm show that intra- and interpolymer emitting species are distributed evenly throughout the film’s LRO. Additional polarization images show that intra- and interpolymer fluorescence are polarized along the same axis. The dominance of LRO in 2 and the large interpolymer emission in 2 (as measured in previous studies) implies that the LRO, not the clusters, is responsible for most of the interpolymer emission in these polymer films.
Introduction Since the original demonstrations of conjugated polymer lightemitting diodes (LEDs) by Partridge1 and Burroughes et al.,2 there continues to be great interest in achieving polymer device performance that rivals that of inorganic or small molecule organic devices. In addition to LEDs, polymers have been demonstrated as the active material in a variety of electrooptic devices such as light-emitting electrochemical cells,3 photodiodes,4 integrated circuits,5 lasers,6 and polarizing filters in LCD displays.7,8 A major advantage of using polymers is that they can be processed into thin films of any size and shape via spin coating the appropriate substrate. Inorganics and small molecules on the other hand, are grown as highly ordered crystalline films through epitaxial and high vacuum evaporative methods, respectively.9-11 While this limits the number of applications for which they can be used in comparison to polymers, it produces highly ordered and relatively homogeneous material, free from defects which lead to greater fluorescence quantum efficiencies and charge carrier mobilities.12 Conversely, the advantage of easy processability of polymers introduces the disadvantage of a wide variety of intrapolymer conformations and interpolymer interactions in polymer thin films. Because these heterogeneities have been shown to reduce the fluorescence efficiency as well as alter the energy of fluorescence in devices,13-16 a better understanding of their origins and the * Corresponding author.
mechanisms that lead to fluorescence quenching are of great importance. Polyfluorene was first introduced by Yoshino et al.17,18 and there has been increased interest in poly(9,9-dialkylfluorene)s for use in light-emitting electrooptic devices.19-27 Polyfluorene belongs to a class of “stiff-chain” polymers whose structure is rod like and whose substituents serve not only to solubalize the backbone but also to impart a nematic liquid crystalline state.28 Interest has renewed recently in these polymers due to their potential applications in polarizing filters where a high thin film anisotropy can be achieved through alignment of the polymer rods via their liquid crystalline state.7,22 Poly(9,9dialkylfluorene)s are of particular interest because of their excellent chemical and thermal stability21 and high fluorescence quantum yield in solution.26 In addition, the facile nature of the 9 position provides a means to change the nature of the alkyl substituent and in turn a means to control the polymer’s physical and chemical properties. In dilute solutions where the polymer is able to adopt any number of conformations, the substituent does not affect the polymer’s electronic properties. In the solid state, however, the alkyl substituents can have a dramatic affect on the polymer’s electronic properties by directing intrapolymer conformational changes and interpolymer aggregation. These changes in the solid state are physiochemical; they do not involve a further chemical change to the polymer backbone. Furthermore, changing the linear alkyl substituent from six to 12 carbons in length dramatically
10.1021/jp0012799 CCC: $19.00 © 2000 American Chemical Society Published on Web 09/20/2000
NSOM Studies of Poly(9,9-diakylfluorene) increases the polymer’s solubility in a variety of organic solvents and decreases its liquid crystalline phase transition temperature. These changes in solubility and packing have also been shown to significantly alter the fluorescence properties of thin films made from a series of polyfluorenes with alkyl chains varying from six to 12 carbons in length.29 In an effort to unravel the complexities of interpolymer interactions, a number of studies have been done on the spectral properties, fluorescence lifetimes, and fluorescence quantum yields of conjugated polymer films.13-16,19,20,30-34 There are three possible causes for change in the fluorescence behavior in going from dilute solutions to polymer films: (1) changes in the conformation of the polymer backbone, (2) presence of groundstate aggregate species,15,20 and (3) the ability to form an excitedstate aggregate, known as excimer.13,14,19,30 In going from solution to solid state, it is unclear how greater interpolymer interaction may affect the extended conformation. The driving force for both aggregate and excimer formation is the creation of a lower energy complex due to effective π-π electronic overlap.35 In addition to the close proximity of polymers in the solid state, a favorable coplanar geometry can be enhanced through polymer packing which will enhance the π-π electronic overlap. While ground state aggregate is excited directly and may involve numerous polymer molecules, excimer involves a two molecule complexsone in the ground and one in the excited stateswhich dissociates in the ground state. Both aggregate and excimer formation have been identified as long-lived fluorescence species with significantly red-shifted fluorescence. These complexes often result in dramatic fluorescence quenching, because an increase in lifetime and the rotational degrees of freedom of a loosely bound complex increases the likelihood and the number of pathways for nonradiative decay. For polyfluorene, this is not always the case. Bradley et al. have induced specific varieties of aggregation in films of poly(dioctylfluorene) which significantly increase the fluorescence quantum yield in comparison to nonaggregated films.21,22 Other studies on pristine and annealed films of dihexylpolyfluorene suggest that polymer aggregation forms with repeated film excitation as a result of excimer formation.19,24 In these studies, excimer, not ground-state, aggregate absorption, resulted in a definite decrease in the fluorescence quantum yield. Studies on both pristine and annealed films of a series of dialkylpolyfluorenes suggest that fluorescence quenching occurs as a result of both excimer and aggregate formation as a function of the length of the alkyl substituent.29 Assuming similar molecular weight polymer samples, the disparities among these studies may be attributed to differences in the length of the alkyl substituents, and the sample processing procedures. Both will affect the polymer’s solubility and degree of packing in the solid state. Indeed disparities exist for studies of the fluorescence properties of other conjugated polymers as well due in large part to the important role that sample processing plays in determining these results.32 It has been suggested16 that highly crystalline polymers with the appropriate inter polymer spacing may dramatically increase the fluorescence quantum yield from what is currently achieved with randomly distributed and aggregated polymers. Therefore, sample processing and alkyl chain length can be used to induce favorable interpolymer interactions that enhance fluorescence quantum yields. A better understanding of the effect of solubility issues and long-range order associated with these interactions is necessary to achieve improved fluorescence efficiencies. In addition to structural and processing parameters, the interdependent nature of intra- and interpolymer interactions contribute to the com-
J. Phys. Chem. B, Vol. 104, No. 40, 2000 9379 plexity in identifying the mechanisms involved in polymer films fluorescence. For example, do longer alkyl substituents which improve solubility in processing also lead to more interpolymer interaction in the solid state? Clearly, one cannot give an intuitive answer to this question because of its multifaceted nature. In this study we further examine the interdependence of polymer solubility and polymer packing as a function of the length of the alkyl substituent in pristine films made with poly(9,9-dihexylfluorene) (1) and poly(9,9-didodecylfluorene) (2). Prior to annealing films to induce liquid crystalline order, it is has been assumed that pristine films are largely amorphous because unlike annealed films, they appear isotropic with conventional polarized light microscopy.36 Recent studies have shown that the morphology of pristine films can have a dramatic effect on the emission seen from films of poly(2-methoxy-5(2′-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV). Yang et al. have observed that manipulation of the solvent conditions and spin casting speeds can yield films with varying contents of two interpolymer emitting species: one with a low quantum yield and one with a high quantum yield.37 Schwartz et al. have shown that solvent, polymer concentration, and annealing can be used to control polymer aggregation and improve device performance.38,39However, there are have been relatively few studies that have microscopically characterized conjugated polymer film morphology.38,40-42 We use Near-field scanning optical microscopy (NSOM) to resolve nanostructures in pristine films which cannot be resolved with conventional light microscopy. By correlating our NSOM results with previous bulk fluorescence data,29 we identify the nanostructures which may be responsible for the interpolymer emitting species observed in the films. NSOM belongs to the family of scanning probe microscopies which measure the topography of the surface through scanning with a force feedback mechanism. NSOM is unique in that it uses light to probe the sample surface while simultaneously collecting topographic data.43,44 This is achieved by directing the excitation light onto a sample through a fiber optic probe with a subwavelength aperture. In a region very close to the aperture there is an evanescent electric field whose lateral extent is confined by the size of the aperture. By keeping the sample and probe separation constant and within this near-field region (∼7 nm), an image of the sample is formed by scanning the aperture over the sample. The resolution of the optical NSOM image is determined by the size of the aperture of the NSOM probe, which is typically on the order of 50-100 nm. Images provide approximately an order of magnitude better resolution than can be achieved with conventional microscopy whose resolution is limited by the diffraction of light. NSOM has been used to probe a variety of organic, inorganic, and biological systems.45-48 In our study we keep sample preparation constant and compare pristine polymer films of 9,9-dialkylpolyfluorenes 1 and 2. We resolve two specific types of nanoscale interpolymer heterogeneities. These are small polymer clusters caused by insoluble clumps of polymer in solution, and regions of partially ordered polymer caused by attraction between polymers during spin casting. Using NSOM to quantify the fluorescence anisotropy in these films, we can see how these nanoscale heterogeneities form as a function of the length of the polymer’s alkyl chain substituents whose presence affects the polymer’s solubility, conformation, and long range ordering in the solid state. We then directly correlate the nano-resolved fluorescence from specific features in the film with the film’s bulk properties.
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Figure 1. Diagram of the NSOM instrument showing the two schemes used for dual image collection. Scheme 1: two images are collected simultaneously at orthogonal polarizations. Scheme 2: two images are collected simultaneously at two wavelengths. BS, LP, and BP refer to beam splitter, low-pass, and band-pass filters.
Experimental Section Instrumental. All near-field and topographic images presented here were recorded with a Topometrix Aurora NSOM that has been modified for high sensitivity fluorescence. A diagram of the instrument and the optical paths employed are shown in Figure 1. A Spectra Physics Tsunami Ti:sapphire Laser was focused through a doubling crystal (BBO) to achieve excitation wavelengths at 390 and 437 nm. The beam was coupled into the cleaved end of an approximately 0.5 m long single-mode optical fiber (Thorlabs FS-SN-3224) whose other end was tapered and coated with aluminum to form the nearfield probe.49 Polarization of the excitation light was controlled using a combination of half- and quarter-wave plates and a fiber paddle (Fiber Control Industries). The sample was mounted beneath the NSOM probe on a piezo-electrically driven sample stage with X, Y, and Z motions. The sample-probe distance was maintained to within 7 ( 0.25 nm by either an optical50 or tuning fork shear-force mechanism.51,52 The fluorescence excited by the NSOM probe was collected with a 0.6 numerical aperture objective and imaged onto the active areas of two single-photoncounting avalanche photodiodes APDs (EG&G Canada). Two images were taken simultaneously by splitting the fluorescence onto the two detectors with a polarizing or a nonpolarizing beam splitter, represented by “Schemes 1 and 2” in Figure 1, respectively. Spectral imaging was accomplished by using a combination of 440 and 600 nm band-pass filters ((10 nm) or 485 short pass and 530 nm long-pass filters. Filters were placed either before the beam splitter during simultaneous polarization imaging, or immediately before the APDs in the case of simultaneous two wavelength imaging (Schemes 1 and 2, respectively). Emission spectra were obtained at specified topographic features by directing the fluorescence to a spectrometer (Acton) equipped with a CCD camera (Princeton Instruments, LN-400EB). Light levels reaching the sample were approximately 1-3 nW with 1-5 mW coupled into the fiber.
Teetsov and Vanden Bout Scattered laser light was removed by various combinations of low fluorescing long-pass colored glass filters (Schott KV-type) or by using band-pass filters. Fluorescence and topographic images were all recorded simultaneously as the sample was raster scanned over 2 and 5 um distances at rates of 1 and 2.5 µm/s respectively with resolution set at 200 by 200 pixels. This gave a pixel time in all images of 10 ms and a pixel size of 10 and 25 nm for 2 and 5 µm scans, respectively. Far-field absorbance spectra were recorded on a Shimadzu spectrometer; fluorescence measurements were made with an SLM Aminco SPF 500 fluorimeter. Thin Film Preparation. Thin films were prepared with a Specialty Coating Systems Inc. Spin Coater (model P6204-A). Polymer solutions (3 wt %, clear with light purple color) were prepared in toluene and placed on 1-in. square coverslips centrally scribed with a 1 mm marker and cleaned with a soapy water followed by an acid and solvent rinse. The substrate was rotated at 3000 rpm for 30 s, producing 250 ( 30 nm thick films, as determined by a Tencor Instruments Alpha Step 100 profilometer, and confirmed by topography measurements. Films were dried overnight in atmosphere and used as spun. The history of the samples used in this study varies from 1 day to one month after preparation. Samples were stored in air and did not appear to change significantly over time. NSOM Simultaneous Collection Schemes, Image Math, and Error Analysis. Using Scheme 1 of Figure 1, addition and anisotropy images are calculated from two normalized fluorescence NSOM images (X and Y) collected simultaneously at orthogonal polarizations using the relationship: X - Y/X + Y. The anisotropy images then have a scale from +100% to -100% where +100% indicates all fluorescence was in the X direction, and -100% indicates that all fluorescence was in the Y direction. A new color scale is given for these images, with red representing X and blue representing Y. The extent of anisotropy is characterized by the standard deviation of all of the pixel values in one image. Because the domains in the films are randomly ordered, the standard deviation of the distribution of values is necessarily smaller than the maximum and minimum anisotropy values in any given image. However, the standard deviation is a better reflection of the overall anisotropy since it includes all regions of the image. Division by the total fluorescence removes variations due to laser drift and also serves to better “resolve” changes in lower levels of fluorescence. In relation to the corresponding X and Y images, addition images show (1) regions of enhanced contrast for nonpolarized features in the film along with photobleaching, laser drift and other instrument-related deviations and (2) regions of lost contrast for polarized features. Anisotropy images show the opposite effect. We consider both random instrumental and systematic sample preparation error in our NSOM anisotropy calculations. The standard deviation of the anisotropy values for each anisotropy image exceeds the shot noise except for pristine films of 1 where there is little or no nanoscale order. Error propagation using the shot noise for the two images in an anisotropy calculation gives between (0.5 and (2% relative standard deviation (RSD) for all calculations. The systematic errors related to sample preparation are addressed in two ways. First, anisotropy data points were collected per sample within a 1 mm region located in the center of each sample, to minimize sample preparation error related to spin coating. These anisotropy values were averaged to give an approximately 13% RSD for systematic error due to variations within each sample. A second source of sample related error comes from comparing measurements
NSOM Studies of Poly(9,9-diakylfluorene)
Figure 2. Excitation and fluorescence spectra of 1 (solid gray) and 2 (solid black) as 250 nm pristine films (λcollect ) 440 nm); Fluorescence spectra of films at λexc. ) 390 nm, both normalized to the 2nd vibronic peak. Excitation and fluorescence spectra of a solution of 1 (dotted). Note the greater contribution of low energy emission from 2 than from 1 and the increase in low energy emission from the film compared to the solution. R ) C6H13 (1), C12H25 (2).
taken from several different samples. We saw the same trends in fluorescence when measuring different films made from the same material. We report the average of measurements made at different regions of the same sample and on different samples for our anisotropy values. A median filter (3 × 3) was applied to Figure 3a,c,d to remove high-frequency noise. Results and Discussion Previous and New Bulk Fluorescence Results. To better understand how changes in the polymer morphology can affect the film spectroscopy, it is important to look at the difference between the emission observed in solution and the films. Upon going from solution to film, the largest changes that are seen are an increase in low energy emission and a dramatic drop in the quantum yield for fluorescence. A brief summary of previously measured fluorescence properties29 of dilute solutions and films of 1 and 2 is given in Table 1 and discussed below. The fluorescence spectra of 1 and 2 are identical in dilute solutions of a good solvent such as toluene, with distinct vibrational structure near 420 and 440 nm, and very little fluorescence past 550 nm (Figure 2). In contrast to fluorescence, the absorption spectra is featureless with a maxima at 387 nm. The deviation from the standard mirror image rule reflects a change in conformation between ground and excited state.53,54 Excitation spectra in solution are also similar for 1 and 2 and reflect the absorption spectra at all emission wavelengths. The similarity between absorption and fluorescence spectra of 1 and 2 implies that the alkyl substituents do not affect the electronic properties of the polymer in solution.29 Thin film absorbance spectra for 1 and 2 are slightly broader than in solution. Excitation spectra are reported here for 1 and 2 for the first time in Figure 2 and are independent of collection wavelength; the spectra is structured and has greater emission from a species whose absorbance is at a higher energy than the absorption maxima. This inconsistency between film absorbance and excitation spectra supports the presence of a new excitation path at high energy, different from the one present in solution, that provides more favorable emission. In contrast to solution, films of 1 and 2 show red shifted and increased lower energy fluorescence as seen in Figure 2. The red shift is more pronounced in films of 2 than 1 which could be the result of greater effective conjugation in the excited state as a result of polymer packing. The additional band at 550 nm is the result of excimer formation19 and is larger for 2 than 1. From these results, we believe that both excimer and ground state aggregate
J. Phys. Chem. B, Vol. 104, No. 40, 2000 9381 are present in our polyfluorene films, and that both occur more readily as a function of greater interpolymer interactions. Time-resolved fluorescence studies of 1 and 2 in dilute solution show a single emitting species with a 500 ps lifetime. We refer to this species as an intrapolymer species since solution fluorescence is exclusively from single polymer chains. In contrast, pristine films of 1 and 2 show more complicated emission. At 440 nm there continues to be an intrapolymer emitting species whose emission is quenched to between 200 and 300 ps. There is significant new emission at low energies and two new lifetimes associated with this emission which we believe arise from interpolymer emitting species. Both species have longer lifetimes; the first has a 1-3 nano-second lifetime and the second has a 10-12 ns lifetime as measured at 550 nm. Films of 1 have a small component of the 1-3 ns species (5%) and almost none of the longer lifetime species (1%). Films of 2 show similar amounts of the 1-3 ns species (6%) and significantly more of the 10-12 ns species (6%) as compared to 1. Overall, there is a 6% contribution from interpolymer 600 nm emission in 1 versus 12% in 2. This would suggest that a longer alkyl chain may contribute to an increase in the percent contribution of interpolymer emission. Previous fluorescence quantum yield measurements show that in solution 1 and 2 have identical fluorescence quantum yields. Thin films of 1 and 2 also have similar fluorescence quantum yields, although much lower than in solution. With twice the contribution from interpolymer emission in films of 2 than 1, it is curious that these films have similar fluorescence quantum yields. Interpolymer interactions are thought to lower the fluorescence quantum yield. In the following sections we provide explanations for these inconsistencies in bulk fluorescence lifetimes and fluorescence quantum yields in films of 1 and 2. Insoluble Clusters. The most striking feature seen in the topographic NSOM images of pristine films of 1 and 2 are randomly dispersed polymer clusters which are 50-150 nm wide and 10-150 nm in the z direction on an otherwise relatively flat film. Clusters are significantly larger and more frequent in films of 1 than 2. Figures 3a and 4a show the topography of films of 1 and 2 respectively (taller features are represented by lighter colors). NSOM fluorescence images of 1, Figures 3c,d show that the topographic clusters correlate directly with regions of lower fluorescence. Figure 3b shows topographic and fluorescence lines scans of the lines in Figure 3a,c. Clearly, there is a correlation between topographic clusters and regions of lower fluorescence which we refer to as clustercorrelated fluorescence (CCF). Polarization studies show the clusters in 1 and 2 to be amorphous. Images taken simultaneously at two orthogonal polarizations (Figure 3c,d) both show identical regions of CCF (with approximately 20% less fluorescence than the background). Had the clusters been strongly polarized, one would expect the intensity of the fluorescence from the cluster regions to vary in the two images. In comparison to 1, the smaller clusters in 2 tend to be pushed around because 2 has a lower softening point than 1 (leading to the streaking seen in Figure 4a). This generally did not affect the NSOM. Clusters in 1 and 2 most likely occur because of insolubility of the polymer in the spinning solvent prior to spinning. Past studies have shown that 1 is less soluble than 2 in the spinning solvent29 which correlates with the larger and more numerous clusters which occur in films of 1 than 2. The CCF has a similar spectrum to the surrounding polymer film, which rules out the existence of a separate chemical species in these regions of the film. However, subtle differences in the fluorescence intensities
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Figure 3. NSOM images of a 2 × 2 µm scan of a 250 nm thick film of 1 prepared by spin coating from a 3 wt % toluene solution: (a) topography, (b) line scans of lines shown in Figures 3a and 3c, (c and d) NSOM fluorescence images collected simultaneously at orthogonal polarizations at all wavelengths, and (e and f) anisotropy and addition images respectively; calculated from Figure 3c,d.
of intra- and interpolymer species can be seen. For films of 1, regions of CCF were examined more quantitatively using simultaneous dual wavelength imaging (Scheme 2 of Figure 1). By comparing the 440 and 600 nm emission from CCF versus overall fluorescence, an approximately 10% greater contribution from 600 nm emitting species in regions of CCF was determined. An average of 20 larger regions of CCF gave a value of 0.92 for the ratio of 400/600 nm emission vs a value of 1 for the rest of the film. This suggests that greater interpoly-
mer interactions in clusters are responsible for their lower fluorescence. The direct correlation of the drop in fluorescence with topography suggests that loss of fluorescence may be due to scanning artifacts. A small portion of the drop may be the result of the aperture moving away from the surface. However, if the drop were all due to scanning over the clusters, one would expect larger effects at the edges of the objects. Large clusters clearly show the lowest fluorescence in the middle of the cluster.
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TABLE 1: Bulk Fluorescence Data29 fluorescence lifetime (ns) λcollect ) 550 nm λmax/nm of % long quantum 2nd vibronic λcollect ) % short peak emission 440 nm (1.5 ns) yield 1 solution 1 film 2 solution 2 film
439 447 439 451
0.56 0.27 0.55 0.32
100 94 100 88
0 6 0 12
0.64 0.15 0.65 0.15
In addition, the spectral differences observed between the clusters and the films indicate that the features are not artifactual or merely dust particles. Previous studies in poly(p-phenylenevinylene) (PPV)41 and MEH-PPV38 have observed similar features with NSOM and atomic force microscopy. In both cases they have postulated that the clumps are entangled polymer from solution. In addition, these studies have suggested that the clumps would contain a substantially higher content of low quantum yield interpolymer emitting species. Our polarization results show these clusters to be amorphous supporting their assignment to entangled polymer clumps from solution. However, the spectra from the clusters show they do not contain significantly more interchain emission than the rest of the film. In addition, the fact that bulk time-resolved studies show twice as much fluorescence from interpolymer species in 2 than in 1 suggests that the regions of CCF are not responsible for the majority of the film’s interpolymer emission since the CCF is more prominent in films of 1. Long-Range Order. NSOM images reveal a second type of interpolymer fluorescence that is not localized in particular areas of the film but distributed evenly throughout. These 50-500 nm domains are resolved in films of 2 but not in films of 1, and are referred to as long-range order (LRO). Polarized NSOM images of 2 show dramatic contrast. Figure 4c,d imaged simultaneously at orthogonal polarizations reveal domains of order within the polymer film. Dark regions of less fluorescence in Figure 4c correspond directly to light regions of greater fluorescence in Figure 4d and visa versa indicating that all the contrast is due to polarization. In films of 2 LRO is also seen in the topography. The film shows undulations of 5-10 nm in the z direction (Figure 4a) with a root-mean-square roughness of 1.6 nm. Topographic LRO may be present in films of 1, but it is difficult to detect due to frequent 50-150 nm topographic clusters which inhibit the tip’s ability to resolve the subtle LRO as measured in smoother films of 2. While the overall pattern is similar, there is no correlation between topographic LRO and the orientation and spatial resolution of fluorescence LRO. The pattern of fluorescence LRO resembles Schleiren texture which is characteristic of films made with nematic liquid crystalline materials.55,56 Schleiren texture has been reported in annealed films of 1 and 2 in past studies using conventional light microscopy29 with a lower limit of resolution of 0.5 to 1 µm. Using NSOM, we resolve features as small as 50 nm which cannot be observed in pristine films using far-field light microscopy. Because 1 and 2 have similar degrees of polymerization, we can conclude that the length of the alkyl substituent rather than molecular weight controls the formation of LRO. The fact that the nanoscale LRO of pristine films of 2 resembles the microscale Schleiren texture observed once these films are annealed is interesting. This means that the LRO begins to form during the spin-coating procedure, prior to heating to induce liquid crystalline order, and that longer alkyl substituents facilitate this behavior. The fact that there is LRO in films of 2 and not in films of 1 also suggests that the length of the alkyl
substituent strongly influences the polymer ability to pack as a pristine film. The following bulk photophysical properties which are more pronounced in films of 2 than 1 can be attributed to LRO: red-shifted fluorescence spectra and enhanced low energy emission with a significant contribution from long-lived fluorescence species. A more detailed quantification of the LRO follows below. Addition and Anisotropy Images. Two NSOM procedures illustrated in Figure 1 provide unique qualitative and quantitative image interpretation as a result of dual detection capability. The first is simultaneous imaging at two orthogonal polarizations (Scheme 1 of Figure 1) and the second is simultaneous imaging at 440 and 600 nm (Scheme 2 of Figure 2). Fluorescence anisotropy can be used to describe the magnitude of a material’s ordering. We probe direction-dependent fluorescence by exciting polymer films with circularly polarized light and collecting at orthogonal polarizations (X and Y). This provides a qualitative understanding of the relative orientations of polarized regions within the film. The regions that appear dark are actually highly fluorescent with the majority of their fluorescence being polarized in the other direction. For example, features that appear dark in X and light in Y are polarized in the Y direction. By scanning over 2 × 2 or 5 × 5 um areas, we assume a random distribution of polymers where some will line up perfectly with the two collection polarizations. The degree of anisotropy can be assessed by performing image math on the dual polarization images. Addition of X and Y polarized images provides useful qualitative information. For example, Figure 4b is an image of the sum of the X and Y polarized images (Figures 4c,d) and it shows that there is a “washing-out” effect. Addition of pixels of opposite intensities results in a loss of any LRO features as originally viewed in either polarization images. The fact that the contrast is lost in the total (addition) fluorescence image indicates that all the contrast is the result of polarization differences. The only contrast in Figure 4b is due to photobleaching and excitation laser drift. Conversely, structures whose contrast increases as a result of addition of the X and Y polarized images is evidence that these structures are not polarized. For example, in the addition of X and Y polarized images of 1 shown in Figure 3f the contrast is enhanced for CCF, indicating that these regions are not polarized. In contrast to addition, subtraction of X and Y polarized images provides greater contrast for polarized regions and loss of contrast for nonpolarized features. We calculate anisotropy by taking a subtracted image and dividing it by the total fluorescence. Dividing by the total fluorescence reduces variations in the image from laser drift and other fluctuations. Figure 4e shows a typical anisotropy image for a film of 2 calculated from Figure 4b,c,d. The color scale reflects the ordering. Red corresponds to regions oriented parallel to the scan direction (X) and blue corresponds to regions oriented perpendicular to the scan direction (Y). The dark color in the middle is the result of regions oriented at 45° to the collection polarizations. There are no regions in the film without LRO. The orientation smoothly varies throughout the film. None of the regions are completely polarized in one direction (anisotropy ) 1.00 denoted further as 100%). Instead they show a maximum value of 8 ( 1% polarized in both X and Y. This could be due to poor ordering of the polymers in these LRO regions or, more likely, that the true domains are slightly smaller than our resolution. The anisotropy from small domains has been simulated and is shown in Figure 4f. The images shows the anisotropy that would be observed if the film were composed
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Figure 4. NSOM images of a 2 × 2 µm scan of a 250 nm thick film of 2 prepared by spin-coating from a 3 wt % toluene solution: (a) topography, (b) total fluorescence calculated from sum of the two polarized images, (c and d) NSOM fluorescence collected simultaneously at orthogonal polarizations, (e) anisotropy image calculated from Figure 4c,d, and (f) simulated anisotropy image assuming the material is perfectly ordered in 15 nm cubic domains.
of 15 nm domains of perfectly ordered material. The simulation was generated by convolving the signal that would be observed from a film composed of perfectly ordered domains, 15 nm on a side, with the electric field from the NSOM tip. The field was approximated using the model of Bethe and Bouwkamp57,58 The remarkable similarly between the two images show that the smaller domains would give rise to the LRO observed in polymer 2. The 15 nm size is a lower limit on the domain size as the polymer are more likely to be less ordered but in larger domains. Figure 3e shows a typical anisotropy image for a film
of 1 with little to no contrast. This further supports the absence of LRO as seen in the X and Y polarized images and the nonpolarized nature of the CCF. A histogram of all the pixel values in an anisotropy image gives the distribution of anisotropy present. The standard deviation of this distribution is used to quantify the degree of polarization of LRO in the film. Highly ordered films will have broad distributions from large positive and negative anisotropy values, while films without order will show distributions that reflect only random fluctuations. The standard deviations of our
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Figure 5. Histograms of anisotropy values calculated from images of (a) shot noise determined experimentally with a constant external light source, (b) anisotropy of a film of 1, and (c) anisotropy of a film of 2. (d) Histogram of simulated anisotropy image of film 2 in Figure 4f. Note that the spread of the anisotropy values is greatest for 2. The spread of the anisotropy values of 1 is only slightly greater than that of the shot noise.
TABLE 2: Fluorescence Percent Anisotropy NSOM Image Calculationa standard deviation of anisotropy images (%) collection wavelengths polymer
all λs (400 to 650 nm)
440 ( 5 nm
600 ( 5 nm
1 2
0.53 ( 0.44 2.04 ( 0.07
3.27 ( 1.74
3.27 ( 1.74
a
Percent anisotropy is calculated from two fluorescence NSOM images collected at orthogonal polarizations (X and Y) using the expression: X - Y/X + Y. The standard deviation of the anisotropy is used to represent the spread of the anisotropy values from all the pixels in the image. Tabulated data are the average of the spread of anisotropy values based on numerous measurements taken on more than one sample. Band-pass filters were used to collect wavelengths of interest and to block out 390 nm excitation wavelength. Anisotropy values were measured on scans that had between 20 and 50 K counts where both the experimentally determined noise and the calculated shot noise is < 0.7%. Error is due to systematic errors in looking at different samples and different regions of the same sample. See Experimental Section for details concerning the anisotropy calculation and random versus systematic error.
anisotropy images are shown in Table 2 as determined from multiple measurements on numerous samples. These results show that films of 2 are approximately four times more ordered than films of 1. The anisotropy values for 1 are similar to the distribution from random fluctuations dominated by photon counting shot noise. Figure 5 shows the histograms of anisotropy values for an image of 1 and 2, as well as a distribution that represents the shot noise. It is clear that the film of 2 shows significantly more order than 1. The distribution of 1 is slightly larger than the shot noise, but not significantly. The “shot noise” histogram was generated by taking an image of a constant light source with a similar intensity to that of the fluorescence images. It is only slightly larger that the standard deviation one would calculate for the shot noise at these count rates. The histogram of the simulation in Figure 4f is shown as a dotted overlay on the anisotropy of film of polymer 2. The size of the domains in the simulation was chosen to reproduce the spread in anisotropy values observed for polymer 2. The simulation does a reasonable job of reproducing the true anisotropy image and the histogram of its anisotropy values. Addition and Anisotropy Images With Spectral Filtering. Any differences in the ordering in intra- and interpolymer species can be assessed by placing a band-pass filter before the polarizing beam splitter as illustrated in Scheme 1 of Figure 1 and by calculating the anisotropy at a particular wavelength. We refer to this as “spectral filtering.” In the case of polyfluo-
rene this is particularly effective, since the intra versus interpolymer fluorescence can be partially separated by collecting at 440 nm versus 600 nm, respectively. These experiments were only performed on 2 because there was minimal LRO observed in films of 1. In polarized images of 2, similar order was seen at both 440 and 600 nm. In addition, the individual wavelengths showed roughly 40% greater order than images collected at all wavelengths. Intuitively, one might expect that the 600 nm interpolymer emitting species would be more highly ordered than the 440 nm intrapolymer emitting species. This is clearly not the case. These are several explanations for these results. The first is that the 600 nm species may be more ordered, but because it is not excited directly (its emission results from energy migration), its order is washed out. This washing out would occur if there were small domains of polymer (beyond the current resolution) that gave off predominately emission at 600 nm. Energy transfer to these domains in a larger spatial area would dilute the polarization as more domains were averaged. This was tested by excitation of the film at 437 nm where more interpolymer emission may be excited directly.20 The anisotropy found was identical to that of the 390 nm excitation. The similar results with excitation at 390 and 437 nm suggest that the 600 nm regions are not significantly more ordered than the 440 nm. The second explanation is the limited resolution of the instrument. The fact that anisotropy at all wavelengths is lower than at 440 or 600 nm implies that the true highly ordered domains are beyond the resolution of our microscope. With the current resolution all regions of the film appear to have the same contribution from intra- and interpolymer species. However, it is reasonable to assume that on a smaller length scale, there are domains beyond the current resolution that have varying contributions from 440 and 600 nm emitting species. Selecting a smaller wavelength range would increase the anisotropy because a more limited subset of the total domains would be imaged. These smaller subsets have fewer possible orientations and would show higher order. The existence of smaller domains beyond the current resolution is supported by the fact that the anisotropy values for the LRO are approximately 8-10% for 2 rather than 100%. A third explanation is that the 440 and 600 nm emit light along perpendicular directions to one another so as to cancel out the anisotropy when they are measured together. It is possible that interpolymer emission may be polarized orthogonal to the polymer axis or the dipole of absorption.59,60 Experimentally we use several methods to show that this last possibility is not the case. The fluorescence dipole of the 440 in relation to the 600 nm species was probed by measuring the anisotropy of the same region of the film scanned first at 440 nm and then at 600 nm. If the 440 and 600 nm species were perpendicular to each other, one would expect the two images to be anticorrelated. That is, regions oriented in X at 440 nm should be oriented in Y at 600 nm. No difference can be seen when comparing these two sets of X and Y images. While there may be slight differences in the angle of emission of the 440 and 600 nm species, it does not appear to be orthogonal. This is in line with the work of Bradley et al.20 who propose an aggregate species with a dipole parallel to the intrapolymer emission. To resolve differences in regions of 440 and 600 nm emission, images were collected simultaneously at 440 and 600 nm using a 50/50 beam splitter and band-pass filters (Scheme 2 of Figure 1). The 440 nm image displays only regions of intrapolymer fluorescence and the 600 nm image displays only interpolymer emitting species. Qualitatively, the images at the two wavelengths appear identical. By placing a polarizing filter at one
9386 J. Phys. Chem. B, Vol. 104, No. 40, 2000 random orientation before the beam splitter, we can increase the contrast of the LRO to help resolve differences. Addition of the two wavelength images creates even greater contrast than either individual image. This implies a direct correlation between fluorescence features in the 440 and 600 nm images. This is further evidence that both species emit the same polarization and are formed in the same relative abundance throughout the film. Summary and Conclusions Using NSOM, we can image two distinct types of interpolymer fluorescence in pristine films of 1 and 2 which cannot be resolved with conventional PLM. The first is clusters (50-150 nm) which correlate with regions of lower fluorescence efficiency (CCF) in films of both 1 and 2. These regions of CCF are nonpolarized and have a larger percent contribution from interpolymer 600 nm emission than noncluster regions. These clusters form as a result of insolubility of the polymer in the solvent used for spin coating. The shorter dihexyl substituents are less solubalizing than the didodecyl substituents and therefore films of 1 have larger and more frequently occurring clusters than films of 2. Clusters formation can be controlled by the length of the alkyl substituent and also by processing conditions such as choice of solvent and polymer concentration during spin coating. The second type of interpolymer fluorescence is 50-500 nm long-range order (LRO) which is imaged in pristine films of 2. The LRO resembles the Schleiren texture seen using PLM in annealed films but on an order of magnitude smaller scale. The qualitative observation of LRO in films is quantified by the standard deviation of the distribution of the calculated anisotropy values. Films of 2 have a 2% standard deviation versus 0.5% for films of 1. The anisotropy for 1 is equal to the percent deviation based on shot noise. This suggests that attraction between the polymer chains with long alkyl substituents induces ordering during the spin-coating procedure in pristine films of dialkylpolyfluorene. Anisotropy values show that increasing the linear alkyl chain appended to the polyfluorene backbone from six to 12 carbons increases the degree of LRO by over 50%. We believe that this increase in LRO is responsible for the 50% increase in the percent contribution of long-lived lower energy fluorescent species as measured in previous bulk experiments for films of 2 versus films of 1. Simulations of the NSOM anisotropy show the film’s LRO can be reproduced by modeling the film and are composed of perfectly ordered domains that are at least 15 nm in size. The domain size could be even larger if the polymers in the domain were less well ordered. Collecting at a specific polarization, we do not resolve wavelength dependent heterogeneities in fluorescence from LRO. This suggests that regions of intra- and interpolymer fluorescent species are evenly distributed within the resolution of our NSOM and have similar emission dipole orientations. The preference for CCF in films of 1 and LRO in films of 2 suggests that there may be two separate interpolymer related nonradiative decay pathways responsible for fluorescence quenching in dialkylpolyfluorene thin films. This would help to explain the similar fluorescence quantum yields of 1 and 2 as measured in the bulk. In addition, we have shown how simultaneous NSOM scanning to detect either two orthogonal polarizations or two wavelengths of fluorescence can be used to quantify NSOM measurements. A new method for calculating NSOM fluorescence anisotropy provides both qualitative and quantitative information on nanoscale ordering in heterogeneous materials. Future experiments will use this new method of anisotropy
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