NANO LETTERS
Optical Emission of Conjugated Polymers Adsorbed to Nanoporous Alumina
2003 Vol. 3, No. 9 1265-1268
Dongfeng Qi,† Keri Kwong,‡ Katja Rademacher,‡ Michael O. Wolf,*,‡ and Jeff F. Young*,† Department of Physics & Astronomy and Department of Chemistry, UniVersity of British Columbia, VancouVer, BC, V6T 1Z1 Received February 5, 2003; Revised Manuscript Received June 30, 2003
ABSTRACT Composites of conjugated polymers poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV) and poly(2,3-diphenyl)phenylene vinylene) (DP-PPV) adsorbed to porous alumina (PA) membranes have been prepared. Time-integrated and time-resolved optical emission properties of these materials at 77 K have been studied and compared with those of bulk thin films of these polymers. Observed blue shifts of the PL spectra (0.034−0.183 eV) for the PA-adsorbed polymers as compared to the bulk are attributed to the isolation of the polymer chains on the PA surface. The energy dependences of the fast decay rates measured on the blue side of the no-phonon emission peak in bulk and PA-adsorbed DP-PPV are very similar to each other. These results suggest that the intrachain diffusion of electron−hole excitations, either in the bulk or adsorbed to porous alumina, is very similar.
Conjugated polymers have been the subject of intensive research during the last two decades mainly because of their semiconductor-like optical and electronic properties. These organic materials can be used in optoelectronic devices such as organic light-emitting diodes (OLEDs), light-emitting electrochemical cells, photovoltaics, image sensors, waveguides, and lasers.1 In comparison to conventional inorganic semiconductors such as GaAs, conjugated polymers can be readily processed on a variety of substrates, and the color of their emission can be tuned over a wide range by controlling their chemical structure. Recently, full-color OLED-based products such as cellular phone displays have become commercially available.2 The electronic properties of conjugated polymers may be altered by preparing nanocomposites of them with inorganic materials.3 Others have mixed conjugated polymers with nanoparticles such as C60, carbon nanotubes, CdSe, and TiO2 to form polymer/nanoparticle composites.4 Conjugated polymers have also been adsorbed to porous silica3,5 and porous GaP6 to modify their optical properties. In the present work, we use nanoporous alumina (PA) membranes as a host material. These membranes are composed of γ-alumina, are rigid and stable, and have a low absorbance in the visible region of the electromagnetic spectrum.7 Porous alumina has been considered for use in a * Corresponding authors. E-mail:
[email protected] and young@ physics.ubc.ca. † Department of Physics & Astronomy. ‡ Department of Chemistry. 10.1021/nl034070q CCC: $25.00 Published on Web 07/22/2003
© 2003 American Chemical Society
Figure 1. Structures of PPV and derivatives.
variety of applications including photonic crystals, memory devices, and polarizers.8 It has been used as a host or template for the preparation of carbon nanotubes, conductive polymers, semiconductors, metals, and rare earth complexes.9 Here, we report the time-integrated and time-resolved optical properties of two poly(phenylenevinylene) derivatives, poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV) and poly(2,3-diphenyl)phenylene vinylene) (DPPPV) (Figure 1), adsorbed to nanoporous alumina. MEH-PPV10 and DP-PPV precursor polymer11 were synthesized according to literature procedures. Bulk thin films of MEH-PPV and DP-PPV precursor polymer were cast from THF solutions; for DP-PPV, the cast films were heated to 280 °C for 2 h under vacuum to convert them to
Figure 2. Three-Gaussian fits for (a) bulk MEH-PPV, (b) bulk DP-PPV, (c) PA-adsorbed MEH-PPV, and (d) PA-adsorbed DP-PPV. In all cases, the dashed lines show the individual Gaussians, and the solid lines show the sum of the Gaussians. The dots are the experimental results.
the fully conjugated form. Porous alumina membranes (Anodisc 13, VWR Whatman) consisting of an ∼57-µm layer of 200-nm pores on one side with an ∼2-µm-thick layer of 20-nm pores on the other side were used.12 The pore density is 1 × 109 cm-2. The samples were prepared by first placing the membranes under vacuum (0.5 mmHg) for 1 day to remove any weakly adsorbed water and oxygen from the pores. The films were then immersed in a solution of MEHPPV or DP-PPV precursor polymer (0.03% w/w) in tetrahydrofuran (THF) for 2 days at room temperature in the dark. After 2 days, the films were removed and immersed in a vial containing THF to remove polymer that was not adsorbed. This rinse was repeated four times with fresh THF until no further color was extracted from the films. The samples were then dried under nitrogen in the dark. For the DP-PPV samples, conversion to the conjugated form of the polymer was accomplished by transferring the samples after the above procedure to a Schlenk flask and heating them to 280 °C for 2 h under vacuum. The optical properties of the conjugated polymers and the hybrid materials were studied using both time-integrated and time-resolved photoluminescence techniques. The excitation and optical gating pulses were derived from a Ti:sapphire oscillator operating at 82 MHz and a photon energy of 1.55 eV. The relative delay of the 100-fs gate and excitation beams was controlled by using a translation stage with ∼1 µm resolution. The excitation pulses were frequency doubled to 3.10 eV in a 0.5-mm-thick BBO crystal and subsequently focused to form a spot of ∼100 µm diameter on the sample surface. The average power of this excitation beam was kept below 0.1 mW to minimize sample degradation. For the timeresolved experiments, the photoluminescence from the sample was collimated and combined with the delayed gate pulses in a 0.5-mm-thick BBO crystal. The up-converted signal was spectrally dispersed in a monochromator and detected with either a photomultiplier tube or a silicon CCD 1266
operating at 180 K. The temperature of the samples was maintained at 77 K in an optical cryostat. The absorption spectrum of the PA-adsorbed MEH-PPV samples (not shown) peaks at 500 nm at room temperature, similar to what has been observed in solution13 and thin films.14 Polarized absorption spectra taken with the sample held at an angle of 45° relative to the incident light show little difference in peak intensity, indicating that the polymer is isotropic in the pores. From the absolute absorption, the known absorption cross section per monomer unit of MEHPPV,15 and the average pore size and density, the average surface coverage of polymer is ∼9200 nm2/polymer chain. Confocal microscopy experiments show a uniform distribution of MEH-PPV throughout the alumina membrane, hence the polymer in the PA is dilute, with little possibility for interchain interactions regardless of the conformation of the polymer. Several types of interactions between the conjugated polymers and the alumina surface are possible. The surface of the alumina consists of both Lewis acid Al centers as well as Brønsted acid surface hydroxyl groups. Both polymers contain Lewis base aromatic rings, which can interact with the Al centers on the pore walls via electron donation, as observed for benzene on alumina, as well as with the surface hydroxyl groups via a hydrogen-bonding-type interaction.16 Furusawa has shown that polystyrene adsorbs to γ-alumina via Lewis acid-base interactions involving the phenyl groups.17 In addition, the ether groups in MEH-PPV are Lewis bases and may interact with the alumina. The time-integrated photoluminescence spectra from bulk and PA-adsorbed MEH-PPV and DP-PPV at 77 K (Figure 2) reveal substantial effects due to the different environments experienced by the polymer chains in these samples. Three Gaussian peaks are fit to each spectrum, and for a more direct comparison, the normalized PL spectra of each polymer are plotted together in Figure 3. The emission spectra from both Nano Lett., Vol. 3, No. 9, 2003
Figure 4. Comparison of population lifetimes of bulk and PAadsorbed DP-PPV. Time constants of PA-adsorbed DP-PPV have been shifted -0.068 eV, the blue-shift energy of the 0-0 transition extracted from the time-integrated spectra of bulk and PA-adsorbed DP-PPV, to align the DOS peaks for bulk DP-PPV (- - -) and PAadsorbed DP-PPV (- ‚ -).
Figure 3. Normalized 77 K PL spectra of MEH-PPV and DPPPV for bulk (s) and PA-adsorbed (- - -) samples. Excitation energy was 3.10 eV. Table 1. Effect of Adsorbing Conjugated Polymer to Porous Alumina on Luminescence Spectra
PA-adsorbed MEH-PPV PA-adsorbed DP-PPV bulk MEH-PPV bulk DP-PPV
blue shift of 0-0 peak for PA-adsorbed sample (eV)
broadening of 0-0 peak for PA-adsorbed sample (%)
relative intensity of 0-1 peak with respect to 0-0 peak (%)
+0.183
+100
81
+0.068
+35
58 25 35
PA-adsorbed polymers exhibit similar qualitative changes in comparison to the corresponding bulk spectra. Specifically, there is a significant but polymer-specific blue shift (0.0340.183 eV), the relative intensity of the 0-1 transition peak (with respect to the 0-0 transition peak) increases, and the peaks in the spectra of the PA-adsorbed samples are wider. Table 1 provides a quantitative summary of these effects. The third Gaussian peak is consistently present in all of the spectra and may arise from the presence of multiple stable polymer conformations, as recently observed in singlemolecule experiments by Barbara18 and in polymer blends.19 The observed blue shift is remarkable, and although a detailed structural picture is not yet available, it is possible to speculate about the origin of this effect using the available data. The polymer chains adsorbed to the alumina are isolated from each other and interact to some degree with the alumina surface. This environment is quite different from one in which the polymer chains are surrounded either by solvent or by other polymer chains. The red shift that is observed in thin films of conjugated polymers relative to solution20 has been attributed to the “gas-to-crystal” effect. For example, in a frozen Me-THF glass at 77 K, the 0-0 transition in the PL spectrum appears at 2.18 eV,21 whereas in the film at 77 K this transition occurs at 2.06 eV. Others have seen large Nano Lett., Vol. 3, No. 9, 2003
(0.3 eV) blue shifts in the emission from nanostructured PPV composites, and these were attributed to the isolation of the conjugated polymer chains.22 Ab initio structural calculations of PPV oligomers suggest that the lower-energy, more conjugated planar conformation is more difficult to achieve in the gas phase than in solution.23 In our experiments, at 77 K, the nonplanar conformation is likely favored, contributing at least in part to the observed blue shift. The dielectric environments also vary considerably between the bulk thin film and adsorbed samples. Again, ab initio calculations suggest that the energy levels of oligomers are substantially affected by the dielectric constant of the surrounding solvent.23 Atomic force microscopy images of the porous alumina indicate that the pore walls are very rough, on an ∼20-nm length scale, so the dielectric environment experienced even by different chains in the same sample will also vary. This may explain the strong correlation of the blue shift and line-broadening results in the two different polymer samples. Time-resolved experiments were performed on bulk DPPPV and on PA-adsorbed DP-PPV using the same technique that we used previously to study the decay dynamics of unsubstituted PPV.24 In the bulk PPV study, the decay profiles measured at different probe energies at and above the 0-0 peak energy, under fixed excitation conditions, were fit to a biexponential function, and fast and slow time constants were extracted. All of the time constants so extracted were shown to be associated with a single inhomogeneously broadened density of electron-hole states (DOS), manifest via a superposition of 0-0, 0-1, and 0-2 transitions. All of the data scaled to the underlying DOS when the time constants associated with the 0-1 and 0-2 transitions were translated by one or two phonon energies, respectively.24 Figure 4 compares the population lifetime dependence on energy obtained in the same manner for both bulk and PA forms of DP-PPV. The time constants for PAadsorbed DP-PPV have been shifted by 0.068 eV, the blue shift of the respective DOS peaks deduced from the timeintegrated spectra. It is clear that the influence of adsorption on the fast decay dynamics of the optical excitation is 1267
negligible, even though the time-integrated spectra are dramatically different. The energy dependence of the time-resolved data is remarkably similar for all bulk and PA-adsorbed samples when considered with respect to the principal peak in the respective DOS. All samples studied with the time-resolved system exhibit lifetimes that decrease monotonically and nearly exponentially with excess energy above the peak in the DOS. This is qualitatively consistent with the theory of Kersting et al.,25 who describe fast decay dynamics in terms of an intrachain diffusion mechanism of the photoexcited electron-hole pairs. The present results suggest that the interactions with the alumina pore walls do not affect these intrachain dynamics on subnanosecond time scales The observed changes in the time-integrated photoluminescence spectra of both MEH-PPV and DP-PPV are very encouraging and suggest that there is significant potential for modifying the optical emission properties of PPV-based polymers by isolating the polymers on alumina surfaces. The substantial blue shifts observed here in the nanoporous materials, up to 0.183 eV, may prove useful in light-emitting device applications if methods are developed to improve the emission efficiency and to reduce photodegradation. The significance of the third distinct, broader Gaussian peak in the time-integrated data from the bulk and PA-adsorbed samples is not yet understood and will be explored further but may be related to the presence of multiple conformations or defects in the polymer. Further optical experiments, including an analysis of polarization dependences and the influence of magnetic fields at temperatures from 4 K to room temperature, are also warranted. Acknowledgment. We thank the Natural Sciences and Engineering Research Council of Canada and the Canadian Institute for Advanced Research for supporting this research. We also thank Iva Cheung for providing atomic force microscopy images of the samples. References (1) (a) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539-541. (b) Pei, Q. B.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A. J. Science 1995, 269, 1086-1088. (c) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 17891791. (d) Yu, G.; Srdanov, G.; Wang, J.; Wang, H.; Cao, Y.; Heeger, A. J. Synth. Met. 2000, 111, 133-137. (e) Gabler, T.; Waldhausl, R.; Brauer, A.; Bartuch, U.; Stockmann, R.; Horhold, H. H. Opt. Commun. 1997, 137, 31-36. (f) Hide, F.; DiazGarcia, M. A.; Schwartz, B. J.; Andersson, M. R.; Pei, Q. B.; Heeger, A. J. Science 1996, 273, 1833-1836. (2) Tullo, A. Chem. Eng. News 2001, 79, 7. (3) Nguyen, T. Q.; Wu, J. J.; Doan, V.; Schwartz, B. J.; Tolbert, S. H. Science 2000, 288, 652-656. (4) (a) Smilowitz, L.; Sariciftci, N. S.; Wu, R.; Gettinger, C.; Heeger, A. J.; Wudl, F. Phys. ReV. B: Condens. Matter 1993, 47, 1383513842. (b) Ago, H.; Petritsch, K.; Shaffer, M. S. P.; Windle, A. H.; Friend, R. H. AdV. Mater. 1999, 11, 1281. (c) Gao, M. Y.; Richter,
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NL034070Q
Nano Lett., Vol. 3, No. 9, 2003
