Near-Field Scanning Optical Microscopy Studies of Nanoscale Order

Langmuir 2002, 18, 897-903

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Near-Field Scanning Optical Microscopy Studies of Nanoscale Order in Thermally Annealed Films of Poly(9,9-diakylfluorene) Julie Teetsov† and David A. Vanden Bout* Department of Chemistry & Biochemistry, Center for Nano- and Molecular Science and Technology, Texas Materials Institute, University of Texas, Austin, Texas 78712 Received August 15, 2001. In Final Form: October 10, 2001 Near-field scanning optical microscopy (NSOM) is used to characterize nanoscale topographic and fluorescence features in thermally annealed films of the conjugated polymer polyfluorene. Thin films of polyfluorenes with either two hexyl (1), octyl (2), or dodecyl (3) alkyl groups at the 9 position were studied. Annealed films were made by holding the films above their respective liquid crystalline phase transition temperature and then rapidly cooling the films. Upon annealing, the films show large spectroscopic and morphological changes. The emission spectra films of 1 show a large increase in emission at wavelengths greater than 500, while films of 2 and 3 show small or no change in the long wavelength emission. Polarized NSOM images of all three films show that the films organize into highly ordered nanoscale domains. The order in the films is found to be largest in the polymer with the shortest alkyl chains growing progressively less ordered with increasing chain length. Films of 3 have domains on the order of 15 nm, while films of 1 and 2 have domains 25-30 nm in size. The domains in films of 1 have additional translational order as they align into larger ribbonlike polymer structures. NSOM imaging at two wavelengths reveals that intra- and interpolymer emitting species are found nearly uniformly throughout all three films. Small insoluble clusters that remain in the annealed films show no contrast in the polarization or wavelength images. The spectroscopy and NSOM together show that close packing of polymer chains in films of 1 can provide highly ordered films but only at the expense of increased excimer emission. The dioctyl polymer (2) has an ideal alkyl chain length to be able to achieve high molecular order while maintaining a minimum of interpolymer interactions. Films of 3 with the longest alkyl substituent show poor polymer order while maintaining a substantial component of interpolymer emission.

Introduction There is a great interest in structure-property relationships of stiff-chain fluorescent polymers, because their stiff backbone affords liquid crystalline properties and processing advantages for ordering polymers over large areas in thin films for use in a variety of electro-optic devices.1 Several studies have used stiff-chain polymers to demonstrate polarized light-emitting films,2-4 molecularly ordered nanoribbons,5 and aggregated polymer species with enhanced fluorescence yields.6,7 An advantage of using polymers, instead of the inorganic materials currently used in most light-emitting devices, is that polymers can be processed into thin films of any size and shape via spin-coating the appropriate substrate from common organic solvents. Creating highly ordered polymer systems similar to crystalline inorganic materials presents a number of challenges such as controlling the degree of interpolymer interaction and characterizing film hetero* To whom correspondence should be addressed. E-mail: [email protected]. † Current address: General Electric Corporation Research and Development, Materials Characterization Laboratory, Bldg. K11C10, P.O. Box 8, Schenectady, NY 12301. (1) Wegner, G. Acta Mater. 2000, 48, 253-262. (2) Palmans, A. R. A.; Eglin, M.; Montali, A.; Weder, C.; Smith, P. Chem. Mater. 2000, 12, 472-480. (3) Miteva, T.; Meisel, A.; Grell, M.; Hothofer, H.-G.; Lupo, D.; Yasuda, A.; Knoll, W.; Kloppenburg, L.; Bunz, U. H. F.; Scherf, U.; Neher, D. Synth. Met. 2000, 111-112, 173-176. (4) Grell, M.; Knoll, W.; Lupo, D.; Meisel, A.; Miteva, T.; Neher, D.; Nothofer, H.-G.; Scherf, U.; Yasuda, A. Adv. Mater. 1999, 11, 671-674. (5) Samori, P.; Francke, V.; Mullen, K.; Rabe, J. P. Chem.sEur. J. 1999, 5, 2312-2317. (6) Jenekhe, S. A.; Chen, X. L. Science 1998, 279, 1903-1907. (7) Grell, M.; Bradley, D. D. C.; Long, X.; Chaberlain, T.; Inbasekaran, M.; Woo, E. P.; Soliman, M. Acta Polym. 1998, 49, 439-444.

geneities. Interpolymer interactions can have a dramatic effect on the film absorbance and fluorescence spectra and the fluorescence quantum yield. As a result of the multivariant nature of the formation of interpolymer species and the mechanisms responsible for the fluorescence of these species, the ability to control spectral properties and maximize quantum yields in polymer films remains a great challenge for materials scientists.8 Historically, polymer liquid crystalline ordering has led to interpolymer interactions that lower the fluorescence quantum yield.9,10 Recently, a number of studies have suggested that this may not always be the case and that interpolymer interactions may in some cases enhance the fluorescence quantum yield.6,7 It has been suggested that interpolymer packing can be optimized through either polymer design or processing protocols to produce a high degree of order without loss of fluorescence efficiency.11 In particular, the fluorescence quantum yield of films of dioctylpolyfluorene has been shown to increase as a result of liquid crystalline ordering.7,12-14 The opposite effect has been observed for dihexylpolyfluorene films where the formation of an excited excimer leads to a decrease in fluorescence quantum yields.14-16 (8) Conwell, E. Trends Polym. Sci. 1997, 5, 218-222. (9) Jenekhe, S. A.; Osaheni, J. A. Science 1994, 265, 765-768. (10) Jackubiak, R.; Collison, C. J.; Wan, W. C.; Rothberg, L. J. J. Phys. Chem. A 1999, 103, 2394-2398. (11) Yan, M.; Rothberg, L. J.; Kwock, E. W.; Miller, T. M. Phys. Rev. Lett. 1995, 75, 1992-1995. (12) Bradley, D. D. C.; Grell, M.; Long, X.; Mellor, H.; Grice, A.; Inbasekaran, M.; Woo, E. P. In Proceedings SPIE-Int. Soc. Opt. Eng.: Optical Probes of Conjugated Polymers; SPIE: Bellingham, WA, 1997; Vol. 3145, pp 254-259. (13) Grell, M.; Bradley, D. D. C.; Ungar, G.; Hill, J.; Whitehead, K. S. Macromolecules 1999, 32, 5810-5817. (14) Teetsov, J.; Fox, M. A. J. Mater. Chem. 1999, 9, 2117-2122.

10.1021/la011302x CCC: $22.00 © 2002 American Chemical Society Published on Web 12/06/2001

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Microscopic characterization of conjugated polymer film morphology and fluorescence heterogeneities can provide a better understanding of interpolymer fluorescence. A number of near-field scanning optical microscopy (NSOM) studies of conjugated polymer thin films have been reported.17-25 Polarization studies have shown nanoscale order with polymers oriented in domains smaller than 100 nm in size.17,18,20 Spectroscopic studies have revealed in some cases strong heterogeneities in the emission from the films,23 while others have shown the films to be quite homogeneous in their emission.17,24,25 In all cases, variation in the polymer and solvent used for casting the films has a dramatic effect on the end morphology.26,27 In this study, annealed polymer films have been studied that have less dependence on the solubility and are controllable based on thermal processing conditions. Dialkylpolyfluorene is a blue light-emitting and liquid crystalline polymer.28,29 Polyfluorene’s alkyl substituents are of particular interest. They solubilize the polymer’s stiff backbone in a variety of organic solvents as well as in the solid state by providing a low melting aliphatic environment for an otherwise intractable and highly insoluble material. The alkyl substituents also affect polyfluorene’s nematic liquid crystalline phase transition temperature and the polymer packing in both pristine and annealed films.14 Shorter alkyl substituents lead to higher liquid crystalline phase transition temperatures than longer alkyl substituted polyfluorenes. The length of the substituents can be altered to change polymer solubility and interpolymer interaction without strongly affecting the π-conjugated backbone. A significant advantage of dialkylpolyfluorene is that the degree of interpolymer interaction can be monitored spectroscopically.13,14,16 Interpolymer fluorescence in dialkylpolyfluorene thin films occurs at lower energies and has a longer lifetime than the respective intrapolymer fluorescence.12,14 By studying the relative fluorescence at high and low energies, one can determine the contribution of intraversus interpolymer fluorescence. However, the use of bulk spectroscopic data to characterize interpolymer interactions cannot assign specific morphology features responsible for interpolymer fluorescence, nor can it characterize the degree of ordering on a nanoscale. NSOM is a scanning probe technique that combines the high-resolution topographical data with simulta(15) Lee, J.; Klaerner, G.; Miller, R. D. Chem. Mater. 1999, 11, 10831088. (16) Bliznyuk, V. N.; Carter, S. A.; Scott, J. C.; Kla¨rner, G.; Miller, R. D.; Miller, D. C. Macromolecules 1999, 32, 361-369. (17) Teetsov, J.; Vanden Bout, D. A. J. Phys. Chem. B 2000, 104, 9378-9387. (18) Teetsov, J.; Vanden Bout, D. A. Macromol. Symp. 2001, 167, 153-166. (19) Teetsov, J. A.; Vanden Bout, D. A. J. Am. Chem. Soc. 2001, 123, 3605-3606. (20) Blatchford, J. W.; Gustafson, T. L.; Epstein, A. J.; Vanden Bout, D. A.; Kerimo, J.; Higgins, D. A.; Barbara, P. F.; Fu, D.-K.; Swager, T. M.; MacDiarmid, A. G. Phys. Rev. B 1996, 54, R3683-R3686. (21) DeAro, J. A.; Weston, K. D.; Buratto, S. K.; Lemmer, U. Chem. Phys. Lett. 1997, 277, 532-538. (22) DeAro, J. A.; Moses, D.; Buratto, S. K. Appl. Phys. Lett. 1999, 75, 3814-3816. (23) Nguyen, T.-Q.; Schwartz, B. J.; Schaller, R. D.; Johnson, J. C.; Lee, F. L.; Haber, L. H.; Saykally, R. J. J. Phys. Chem. B 2001, 105, 5153-5160. (24) Huser, T.; Yan, M. Synth. Met. 2001, 116, 333-337. (25) Kwak, E.-S.; Kang, T. J.; Vanden Bout, D. A. Anal. Chem. 2001, 73, 3257-3262. (26) Nguyen, T.; Martini, I. B.; Liu, J.; Schwartz, B. J. J. Phys. Chem. B 2000, 104, 237-255. (27) Nguyen, T.-Q.; Kwong, R. C.; Thompson, M. E.; Schwartz, B. J. Appl. Phys. Lett. 2000, 76, 2454-2456. (28) Fukuda, M.; Sawda, K.; Yoshino, K. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 2465-2471.

Teetsov and Vanden Bout

neously collected fluorescence.30,31 The fluorescent nature of the polymer allows for molecular order in the films to be characterized by polarized fluorescence imaging, while inter- and intrapolymer emission species can be mapped out using emission wavelength as a contrast mechanism. A variety of processing parameters can affect the degree of packing in polymer films via spin-coating: choice of solvent, polymer concentration, and spinning speed used in spin-coating, along with the time and temperature protocol of the annealing process. In this study, all of the above parameters are kept constant and the effect of alkyl substituent length on nanoscale ordering is determined with NSOM imaging of films with all three alkyl chain lengths. Experimental Section Films were prepared from 2.0 wt % toluene solutions by spincoating onto glass cover slips at 3000 rpm for 30 s, producing films of 170 nm. Film thickness was verified by NSOM topographic measurements of films that had been scratched to reveal the substrate. Films of 1-3 were annealed for 2 h at (or slightly above) their liquid crystalline phase transition temperature (250, 160, and 160 °C, respectively).14 Films of 1 were also annealed for 12 h. Films were stored in air and did not appear to change significantly within the 3 week interval of their characterization, after which there was a slow change in the size and frequency of insoluble clusters in films of 1 and 2. Absorbance spectra were recorded on a Milton Roy Spectronic 3000 array spectrophotometer, and fluorescence spectra were collected with a 90° geometry in a Photon Technology Instruments Fluorimeter Quantum Master C-60/2000. All near-field and topographic images were recorded on a modified Topometrix Aurora NSOM that has been previously described.17-19 Briefly, the system uses NSOM probes constructed in-house by coating tapered single mode optical fiber with aluminum to form 75-100 nm apertures at the end of the tips. The size of the apertures was evaluated by scanning electron micrographs of the ends of a few of the probes as well as the resolution of the optical images. The NSOM tips were held at a constant distance of approximately 7 nm from the sample using a piezo-electric tuning fork detected shear-force.32-34 Frequency doubled light from a Ti:Sapphire laser was used as an excitation source at 390 nm. The polarization of the excitation light was controlled with a combination of quarter- and half-wave plates before coupling the light into the NSOM probe. The polarization of the excitation light was adjusted to be circular so as to give equal excitation in the two polarizations of the measured fluorescence. The sample was scanned beneath the tip, and a 0.6 N.A. extra-long working distance objective held below the sample collected the transmitted light and generated fluorescence. The collected light was filtered with a long-pass filter and then sent to two single photon counting avalanche photodiodes. For polarization imaging, the light was split into two orthogonal polarization components that were imaged simultaneously by the two detectors. Spectral imaging was accomplished by splitting the emitted light with a 50/50 nonpolarizing beam splitter cube and filtering with either a 440 or 600 nm band-pass ((10 nm) filter at each detector. Simultaneous imaging at two polarizations or wavelengths allows for direct pixel by pixel comparison of the signal in the two images. Anisotropy images were calculated by taking the difference in the signal at the two polarizations and dividing by the total fluorescence. Spectral variation in the images was evaluated based on the ratio of the two images at different wavelengths. All scans were 2 × 2 µm in size with 200 by 200 (29) Grell, M.; Bradley, D. D. C.; Inbasekaran, M.; Woo, E. P. Adv. Mater. 1997, 9, 798-802. (30) Dunn, R. C. Chem. Rev. 1999, 99, 2891-2927. (31) Vanden Bout, D. A.; Kerimo, J.; Higgins, D. A.; Barbara, P. F. Acc. Chem. Res. 1996, 30, 204-212. (32) Betzig, E.; Finn, P. L.; Weiner, J. S. Appl. Phys. Lett. 1992, 60, 2484-2486. (33) Ruiter, A. G. T.; Veerman, J. A.; van der Werf, K. O.; van Hulst, N. F. Appl. Phys. Lett. 1997, 71, 28-30. (34) Karrai, K.; Grober, R. D. Ultramicroscopy 1995, 61, 197-205.

NSOM Studies of Annealed Poly(9,9-dialkylfluorene)

Figure 1. Excitation (λem ) 450 nm) and emission (λex ) 385 nm) fluorescence spectra of annealed films of (solid) 1, (gray) 2, and (dashed) 3. The inset shows the structure of poly(dialkylfluorene). pixel resolution. Photons were counted with 10 ms bin time per pixel, and the scan rate was kept constant at 2.5 µm/s on the return scan during which time no images were collected.

Results The fluorescence and excitation spectra for 170 nm annealed films of 1-3 are shown in Figure 1. The major difference in going from pristine to annealed fluorescence is a decrease in the band near 485 nm and an increase in fluorescence between 500 and 600 nm.14,18,19 The increased emission between 500 and 600 nm has been assigned to interpolymer excimer emission.16,35 The relative ratio of the second vibronic peak (448 nm) to the low energy emission at 550 nm is used to compare the magnitude of low energy emission in films of 1-3. The actual intensity of emission at 550 nm cannot be compared directly due to variations in film thickness and optical density as well as crystallinity which results in highly angle dependent fluorescence. The second vibrational peak is used for normalization, because it is far enough to the red to be unaffected by self-absorption. While the quantitative analysis is difficult, the qualitative trends are extremely reproducible. Films of 1 exhibit the largest changes from the pristine upon annealing. Films annealed for a few hours show an increase in the long wavelength band. Further annealing causes a dramatic drop in the intensity of the short wavelength emission.19 Films of 2 show little or no increase in long wavelength emission with the largest change from the pristine spectrum being a sharpening of the vibronic structure observed in the emission. Films of 3 show little or no change in the emission spectrum upon annealing. The excitation spectra of the annealed films all show a new shoulder (415 nm) relative to the solutionlike polymer absorption (390 nm) observed in the pristine films. This feature is most prominent in the films of 1. The excitation spectra resemble the absorbance spectra with a slight increase in the size of the red shoulder apparent in the excitation spectra. The excitation spectra all have a flat region near the peak of the absorption in contrast to the absorbance spectra. Excitation and emission spectra were taken on films of the same thickness as those used in the imaging experiments since the film morphology changes in very thin samples. This creates samples of high optical density (OD). Since the excitation spectrum is essential a transmission spectrum, the features at high OD are reduced and yield a “flattened” spectrum. (35) Prieto, I.; Teetsov, J.; Vanden Bout, D. A.; Bard, A. J. J. Chem. Phys. A 2001, 105, 520-523.

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Typical NSOM images of the films are shown in Figure 2. The images show the topography, polarized fluorescence, total fluorescence, and fluorescence anisotropy for a 2 × 2 µm area of an annealed film of polymer 2. It is clear from the images that the films are composed of innumerable small highly oriented regions. The topography shows a large number of small