pubs.acs.org/Langmuir © 2009 American Chemical Society
Multilevel Organization in Hybrid Thin Films for Optoelectronic Applications Varun Vohra,*,†,‡ Alberto Bolognesi,†,‡ Gion Calzaferri,§ and Chiara Botta†,‡ †
Istituto per lo Studio delle Macromolecole (ISMac-CNR), Via Bassini, 15, Milan 20133, Italy, ‡Polo Scientifico Tecnologico, CNR, Via Fantoli 16, Milan 20133, Italy, and §Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland Received August 27, 2009. Revised Manuscript Received September 17, 2009
In this work we report two simple approaches to prepare hybrid thin films displaying a high concentration of zeolite crystals that could be used as active layers in optoelectronic devices. In the first approach, in order to organize nanodimensional zeolite crystals of 40 nm diameter in an electroactive environment, we chemically modify their external surface and play on the hydrophilic/hydrophobic forces. We obtain inorganic nanocrystals that self-organize in honeycomb electroluminescent polymer structures obtained by breath figure formation. The different functionalizations of the zeolite surface result in different organizations inside the cavities of the polymeric structure. The second approach involving soft-litography techniques allows one to arrange single dye-loaded zeolite L crystals of 800 nm of length by mechanical loading into the nanocavities of a conjugated polymer. Both techniques result in the formation of thin hybrid films displaying three levels of organization: organization of the dye molecules inside the zeolite nanochannels, organization of the zeolite crystals inside the polymer cavities, and micro- or nanostructuration of the polymer.
Introduction As nature and, more specifically, plants teach us, well organized and assembled structures can lead to highly efficient optoelectronic systems. Supramolecular organization in zeolite L crystals has shown peculiar optical properties and dye stabilization and could therefore be the basis for fabricating highly efficient optoelectronic devices. The concept of using materials of different chemical nature and assembling them to obtain a new one that exhibits enhanced properties can be applied to many different fields.1 Although, to obtain interesting properties, a simple mixture of organic and inorganic materials is not enough, one has to organize them in a specific way. Some of the most significative examples of such hybrid systems can be found in nature and more specifically in the human body. If we take a closer look at our bones, we understand that nature has been able to design a material with extremely high mechanical properties by combining organic and inorganic materials arranged in the appropriate architecture.2 The enhanced mechanical properties of this hybrid material are not only a result of the combination of the intrinsic properties of both materials, but the dimensions and arrangement of the different species are also key factors.3 In the past years, a lot of attention has been given to the design of molecules that can self-assemble to give rise to interesting nano or microstructures using low cost preparation methods.4,5 *Corresponding author. E-mail:
[email protected]. (1) Sanchez, C. J. Mater. Chem. 2005, 15, 3557–3558. (2) Reilly, D. T.; Burstein, A. H. J. Bone Jt. Surg. Am. 1974, 56, 1001–1022. (3) Van der Linden, J. C.; Birkenh€ager-Frenkel, D. H.; Verhaar, J. A.; Weinans, H. J. Biomech. 2001, 34, 1573–1580. (4) Kotch, F. W.; Raines, R. T. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 3031– 3036. (5) Goswami, D. K.; Satpati, B.; Satyam, P. V.; Dev, B. N. Curr. Sci. 2003, 84, 903–910. (6) Kawasaki, T.; Tokuhiro, M.; Kimizuka, N.; Kunitake, T. J. Am. Chem. Soc. 2001, 123, 6792–6800. (7) Nicolosi, V.; Nellist, P. D.; Sanvito, S.; Cosgriff, E. C.; Blau, W J.; Krishnamurthy, S.; Green, M. L. H.; Vengust, D.; Dvorsek, D.; Mihailovic, D.; Compagnini, G.; Sloan, J.; Stolojan, V.; Carey, J. D.; Pennycook, S. J.; Coleman, J. N. Adv. Mater. 2007, 19, 543–547.
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Self-assembly is usually driven by interactions such as hydrogen bonds6 and van der Waals forces.7 A simple approach to obtain hexagonal arrangements at the micrometer scale is known as breath figure (BF) formation.8-10 By casting the appropriate polymeric solution in a water-vapor-rich environment, water droplets condense and arrange on the evaporating solution. The water droplets consequently evaporate, leaving their imprints in the formed polymeric thin film. BF formation presents many advantages and can be combined with soft lithography to obtain functional films.11 Using soft lithography, the selectivity of the BF technique with respect to the polymer can be overcome.12 The process of formation is complex, and different theories have been suggested in order to understand it.13-15 Some important parameters have been identified throughout the studies of this phenomenon. One of those parameters is the nature of the polymer used, as it should stabilize the droplets in order to avoid coalescence. Adding some hydrophilic species to the polymer solution can lead to a further organization of these molecules at the polymer-water interface during the BF formation process.16 Studies of light harvesting systems in plants revealed that the concept of enhancement through hybrid organization can also be applied to opto electronics.17 Many examples in literature are (8) Erdogan, B.; Song, L.; Wilson, J. N.; Park, J. O.; Srinivasarao, M.; Bunz, U. H. F. J. Am. Chem. Soc. 2004, 126, 3678–3679. (9) Song, L.; Bly, R. K.; Wilson, J. N.; Bakbak, S.; Park, J. O.; Srinivasarao, M.; Bunz, U. H. F. Adv. Mater. 2004, 16, 115–118. (10) Yabu, H.; Tanaka, M.; Ijiro, K.; Shimomura, M. Langmuir 2003, 19, 6297– 6300. (11) Bolognesi, A.; Botta, C.; Yunus, S. Thin Solid Films 2005, 492, 307–312. (12) Vohra, V.; Yunus, S.; Attout, A.; Giovanella, U.; Scavia, G.; Tubino, R.; Botta, C.; Bolognesi, A. Soft Matter 2009, 5, 1656–1661. (13) Srinivasarao, M.; Collings, D.; Philips, A.; Patel, S. Science 2001, 292, 79– 83. (14) Peng, J.; Han, Y.; Fu, J.; Yang, Y.; Li, B. Macromol. Chem. Phys. 2003, 204, 125–130. (15) Maruyama, N.; Koito, T.; Nishida, J.; Sawadaishi, T.; Ceiren, X.; Kjiro, K.; Karthaus, O.; Shimomura, M. Thin Solid Films 1998, 327, 854–856. (16) Sun, W.; Ji, J.; Shen, J. Langmuir 2008, 24, 11338–11341. (17) Gfeller, N.; Megelski, S.; Calzaferri, G. J. Phys. Chem. B 1998, 102, 928– 932.
Published on Web 09/21/2009
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related to hybrid solar cells with increased efficiencies18 or hybrid light emitting devices based on an organic matrix embedding inorganic nanoparticles for color tunning.19 Zeolite L crystals are interesting inorganic materials that can be used as a host for organic molecules.20,21 The crystals provide hexagonally ordered one-dimensional nanochannels where the organic molecules of appropriate size and shape enter one by one through the 7.1 A˚ wide channel entrances. Once in the channels, the molecules stay separated one from another. Aggregation is one of the main issues when it comes to fluorescent organic dyes.22 By including cationic dyes through ion exchange in the zeolite L framework, the aggregation of the dye molecules is avoided, which further leads to an increase of the emission quantum yield. Proper organization of these highly emissive systems into polymeric materials23,24 is an important issue in view of optoelectronic applications. The methods here reported allow the formation of thin hybrid films presenting three simultaneous levels of organization: organization of organic dyes in the inorganic crystals, organization of the polymer film in hexagonal arrangement, and organization of the host guest organic-inorganic compounds in the hexagonal template.
Experimental Section Materials. Fumaric acid (A-CdC-A) was bought from Sigma-Aldrich. Random poly[(9,9-dioctylfluorenyl-2,7-diyl)co-(1,4-benzo-{2,10 ,3}-thiadiazole)] (PF8BT) and alternating poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(1,4-benzo-{2,10 ,3}-thiadiazole)] (F8BT) copolymers were bought from the American Dye Source. Aminopropyltriethoxy silane (APTES) and fluorescamine was bought from Sigma-Aldrich. Polydimethylsiloxane (PDMS) inorganic elastomer base and curing agent were bought from Dow Corning (Silgard 184). Amphiphilic polystyrene (aPS)25 and the oxonine molecules (Ox)26 were synthesized as described previously. Zeolite L crystals of about 40 nm size were used if not otherwise stated. Zeolite L has been synthesized and characterized as described previously.27 Optical Characterization. Confocal microscopy images were collected with a Nikon Eclipse TE2000-U inverted confocal microscope with a long working distance and using a Plan Apo VC objective (magnification 100, N.A. 1.4). The measurements were done with three simultaneous excitations using a DAPI diode laser at 407 nm, an Arþ-ion laser at 488 nm, and an HeNe laser at 543 nm. Preparation of Oxonine-Loaded Zeolite Crystals (OxZLs). The Ox loading was done as follows: A solution of
Ox in H2O (c = 1 10-4 mol 3 L-1) was prepared. The aimed loading for the samples was 20% of the total loading possible for Ox in the zeolite channels. The required amount of the solution (18) Beek, W. J. E.; Wienk, M. M.; Janssen, R. A. J. Adv. Mater. 2004, 16, 1009– 1013. (19) Coe-Sullivan, S.; Woo, W.-K.; Steckel, J. S.; Bawendi, M.; Bulovic, V. Org. Electron. 2003, 4, 123–130. (20) Calzaferri, G.; Lutkouskaya, K. Photochem. Photobiol. Sci. 2008, 7, 879– 910. (21) Schultz-Ekloff, G.; W€ohrle, D.; van Duffel, B.; Schoonheydt, R. A. Microporous Mesoporous Mater. 2002, 51, 91–138. (22) Calzaferri, G.; Br€uhwiler, D.; Megelski, S.; Pfenniger, M.; Pauchard, M.; Hennessy, B.; Maas, H.; Devaux, A.; Graft, U. Solid State Sci. 2000, 2, 421–447. (23) Yunus, S.; Spano, F.; Patrinoiu, G.; Bolognesi, A.; Botta, C.; Br€uhwiler, D.; Ruiz, A. Z.; Calzaferri, G. Adv. Funct. Mater. 2006, 16, 2213–2217. (24) Vohra, V.; Devaux, A.; Dieu, L.-Q.; Scavia, G.; Catellani, M.; Calzaferri, G.; Botta, C. Adv. Mater. 2009, 21, 1146–1150. (25) Galeotti, F.; Chiusa, I.; Morello, L.; Giani, S.; Breviario, D.; Damin, F.; Chiari, M.; Bolognesi, A. Eur. Polym. J., in press. (26) Maas, H.; Khatyr, A.; Calzaferri, G. Microporous Mesoporous Mater. 2003, 65, 233–242. (27) Zabala Ruiz, A.; Br€uhwiler, D.; Dieu, L.-Q.; Calzaferri, G. In Material Synthesis, A Practical Guide; Schubert, U., N. H€using, R. Laine, Eds.; Springer Wien: New York, 2008.
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was consequently added to a suspension of zeolites in water, which was then stirred at 80 °C overnight. To collect the loaded zeolites, the suspension was then centrifuged at 5000 rpm for 20 min. Once the solvent was removed, the zeolites were washed twice with 5 mL of distilled water to remove the Ox molecules adsorbed on the zeolite’s external surface and finally dried in an oven at 75 °C.
Preparation of Amine-Functionalized Oxonine-Loaded Zeolite Crystals (OxZLNH2). Typically, 25 mg of OxZL was suspended in 5 mL of toluene in a polymer tube to which 500 μL of APTES was added. The mixture was then sonicated at room temperature for 20 min and stirred overnight at 60 °C. To collect the zeolites, the suspension was then centrifuged at 5000 rpm for 20 min. Once the solvent was removed, the zeolites were washed at least three times with 5 mL of toluene to remove the excess of APTES molecules in the solution and finally dried in an oven at 75 °C.
Preparation of Carboxylic Acid-Functionalized OxonineLoaded Zeolite Crystals (OxZLCOOH). Typically, 25 mg of OxZLNH2 was suspended in 5 mL of toluene. An excess of A-CdC-A was added to the suspension in order to avoid the formation of bridges between two zeolite crystals. The mixture was then sonicated at room temperature for 20 min and stirred overnight at 60 °C. To collect the zeolites, the suspension was then centrifuged at 5000 rpm for 20 min. Once the solvent was removed, the zeolites were washed at least three times with 5 mL of toluene to remove the excess of molecules in the solution and finally dried in an oven at 75 °C. Spectroscopic Study of the Surface Modification. Both the zeolites OxZLNH2 and OxZLCOOH have been studied using Raman spectroscopy (Supporting Information). Raman spectra of powder samples were measured on a Bruker IF66RFRA 106 Fourier transform infrared spectrometer using the 1064 nm excitation line. The spectrum obtained for OxZLNH2 displays a broadband at about 2900 cm-1, which was previously assigned to the formation of bonds between the zeolite and the APTES.28 The spectrum of OxZLCOOH exhibits a strong peak around 1650 cm-1, which can be coattributed to the CdC stretch of the fumaric acid derivative as well as the CdO stretch of the formed amide. The other peaks can be attributed to the different vibrational modes of the carboxylic acid group as well as the substituted CdC double bond. Formation of BF Arrays and BF Replica on F8BT. To obtain the BF arrays on the polymer films, solutions of PF8BT and aPS in carbon disulfide (CS2) were prepared. A few drops of each of the solutions were placed on a glass substrate and left under moist air flow until complete evaporation of the solvent and water droplets. Typically, 5 mg of polymer was diluted in 1 mL of CS2, and the humidity of the moist air flow was kept above 60%. The films were kept under air flow at room temperature until their solidification and complete drying. The PDMS monomer, mixed with the curing agent (10:1 in weight), was placed on top of the holey film. The sample was then placed in an oven at 75 °C for more than 3 h. After the curing step, the stamp was peeled off from the template. A 120 nm, a thin film of F8BT was obtained by spin coating a polymer solution on a glass substrate. Consequently, the PDMS stamp was pressed against this thin film at 160 °C for 30 min in an inert atmosphere.
Results and Discussion Ox is a red-emitting organic dye that can be introduced into the zeolite L channels through cationic exchange. It has been demonstrated that, once inserted in the anionic framework, the dye exhibits optical properties that are similar to the ones in dilute solution, as the quenching of luminescence due to aggregation is (28) Zhan, B.-Z.; White, M. A.; Lumsden, M. Langmuir 2003, 19, 4205–4210.
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Figure 1. Schematic representation of the different zeolite L crystals and molecules.
avoided.17 Zeolite crystals are aluminosilicates, and therefore modification of the outer surface of the crystals can be done using the reaction pathways that are applied to silicon nanoparticles functionalization. Pickering emulsions use inorganic particles to stabilize water/ oil interfaces.29 Such systems can be used to form BF decorated with nanoparticles.16 In the case of disk-shaped zeolite L nanocrystals, such decoration is not obtained by simply adding the crystals in the dropcasted solutions. Further modifications of the zeolites are needed in order to give a more hydrophilic or hydrophobic character to the crystals. Ox-loaded zeolite L crystals’ external surface was chemically modified, as shown in Figure 1, in order to give hydrophilic properties to the inclusion complex. The silanol groups on the external surface easily react with APTES to give rise to surface amine-functionalized zeolite crystals (OxZLNH2). The resulting amine groups present on the external surface of the inorganic host are hydrophilic. OxZLNH2 can be further modified using fumaric acid. One of the acidic functions reacts with the amino groups of OxZLNH2 to form an amide, while the second gives rise to carboxylic acid groups present on the outer surface of the zeolites connected to the crystals through a bridge containing a CdC double bond. This second type of modified crystal is designated as OxZLCOOH and presents hydrophilic properties as well as hydrophobic ones. The formation of BF arrays on a polymeric thin film depends on many parameters. One of the major parameters is the nature of the molecules present in the cast solution. Ambipolar polymers are the best candidates for the process, as they easily form films and stabilize the amphiphilic interface between the hydrophobic film and the water droplets. Some conjugated polymers, such as PF8BT, when dropcast from a low concentration solution in CS2 under moist airflow, induce the formation of BF. Such films can also be obtained using an aPS. The dimensions of the pores resulting from the presence of the water droplets vary between 400 nm and 5 μm depending on the nature of the polymer used: the better the polymer can stabilize the water droplets, the smaller the pore diameter. Other parameters such as the moist air flow and the concentration of the solution have also to be taken in account to be able to obtain nice and regular arrays. Amino-functionalized zeolites (5 w% with respect to the weight of the polymer) are added to the polymer solutions. (29) Pickering, S. U. J. Chem. Soc. 1907, 91, 2001–2021.
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The hydrophilic zeolites are expected to diffuse close to the water droplets during the film formation and stay at the polymer air interface in the cavities as the water droplet evaporates (Figure 2, top). Figure 3 presents the thin film obtained from OxZLNH2 dispersed in an aPS solution to which a blue pigment (fluorescamine) was added (2 wt % with respect to the weight of the polymer) to give a blue-green emission from the polymeric film. The confocal image clearly shows that the OxZLNH2 crystals are present only where the water droplet was before its complete evaporation. The other information we get from these images is that the presence of OxZLNH2 actually disturbs the formation of BF. aPS forms regular pores of a mean diameter of 600 nm when used without those zeolites. The film obtained in the presence of OxZLNH2 is the result of two very complicated phenomena: BF formation and pickering emulsion stabilization. It has been previously demonstrated that amino-functionalized spherical nanoparticles of silica of a mean diameter of 100 nm can be used to stabilize the water droplets during the BF formation.16 Pickering emulsion stabilization depends on properties such as hydrophobicity, shape, and size of the particle, and can have an effect on the stability of the emulsion. Although OxZLNH2 particles present the same hydrophobicity as the functionalized spherical silica particles, they do not have the same wettability as a result of their size and shape. As can be seen in Figure 3, their stabilization ability seems to be insufficient to avoid the coalescence of the water droplets, and therefore irregular arrays are obtained, which display larger pore diameters. Pickering emulsions using amphiphilic particles can lead to a further stabilization of the emulsion.30 In order to study the influence of such amphiphilic properties, a new system based on OxZLCOOH was introduced. OxZLCOOH, unlike OxZLNH2, has amphiphilic properties. The presence of the CdC double bond and the functional groups in the diacid molecule grafted on the crystals induce compatibility of OxZLCOOH with both conjugated polymers and water. Particles that are partially hydrophobic are better stabilizers because they are partially wettable by both the water and the polymer solution and therefore bind better to the surface of the droplets. Figure 4 shows a confocal image of BF arrays obtained from a solution of PF8BT containing OxZLCOOH (5 w% with respect to the weight of the polymer). The image clearly shows two different emissions from the borders of the cavities and from the rest of the film. The array obtained in this case is regular with dimensions of the cavities in the same range as the ones formed with conjugated polymers when no zeolites are present. The amphiphilic loaded crystals move toward the water droplet to stabilize it but can be mixed with the polymer, which also stabilizes the droplet. Unlike the previous case, here the zeolites and the polymer are not incompatible, and therefore, during the film formation, the zeolites are entrapped in the polymer close to the water-polymer interface. Using such a system, three levels of organization are obtained: a regular hexagonal array of microcavities in a polymeric film, a selective positioning of the inclusion compounds in the polymeric micropatterned film, and a molecular arrangement of the dye molecules within the hexagonal channels provided by the inorganic host. A similar arrangement can be obtained through a different approach. This second method also aims to suppress the chemical reaction steps and to give more versatility toward the polymeric materials that can be used. The idea of using soft lithography to (30) Glaser, N.; Adams, D. J.; B€oker, A.; Krausch, G. Langmuir 2006, 22, 5227– 5229.
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Figure 2. Schematic representation of (top) direct formation of zeolite L-decorated honeycomb structures on micrometer thick polymer films and (bottom) honeycomb structured 120 nm thin polymer films filled with zeolite L crystals.
Figure 3. Confocal images of aPS (top) and OxZLNH2 in aPS (bottom) films formed under moist air flow.
reproduce the BF array on polymers that do not form BF arrays with the conventional process was introduced in previous works.12 This method consists in creating a negative PDMS film of the BF array. The PDMS film is consequently used as a stamp for creating replica BFs on a different polymer thin layer. Direct BF formation leads to film thicknesses on the micrometer scale. The reproduction of BFs on another polymer thin layer allows us to obtain the BF arrangement on a thin film of desired thickness (e.g., around 120 nm to use it as an active layer for a light emitting device). The cavities present in such polymeric films are consequently filled with OxZL by simply casting a dispersion of crystals in water on top of the film (Figure 2, bottom). After water evaporation, the zeolites cover the whole polymeric surface and fill the cavities. In order to obtain zeolite crystals organized in a honeycomb arrangement, the excess of OxZL present on the surface is removed by pressing a flat PDMS block on top of the hybrid film. The zeolite crystals that are not in the cavities are therefore transferred on the inorganic elastomere. The only zeolites that remain in the film are the ones in the hexagonally arranged cavities. Depending on the size of the zeolites used, the cavities will contain one or more zeolite crystals. This second 12022 DOI: 10.1021/la9032089
Figure 4. Confocal images of BF formation on PF8BT (top) and self-assembled OxZLCOOH at the polymer-air interface in the cavities of a BF PF8BT (bottom).
Figure 5. Confocal images of OxZL in F8BT embossed using PDMS replicas of BF on aPS.
method therefore gives more versatility toward the polymer material used as well as the quantity of zeolite crystals present in one cavity. Although dye-loaded zeolites’ optical properties are not affected by the aggregation in solid state, the good dispersion of the crystals is one of the major issues to obtain good functional Langmuir 2009, 25(20), 12019–12023
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thin films that could be used for applications such as zeolite-based organic light emitting devices. Figure 5 displays a confocal image of a thin film of F8BT loaded with OxZL of a mean diameter of 600 nm and a mean length of 800 nm. Unlike the nanocrystals (40 nm 40 nm) used for the method based on hydrophilic interactions, only one crystal will be present in the cavities due to spatial and geometrical restrictions. The confocal images show a bicolor fluorescent film. The green emission corresponds to the photoluminescence of F8BT, whereas the red emission comes from the Ox molecules present in the zeolite crystals. The crystals are trapped in the cavities within the film, which leads to this bicolor microstructured emission.
Conclusions Zeolite-loaded BF cavities in polymeric films are obtained using two different approaches. The first approach consists in the chemical modification of the zeolites’ external surface, inducing enhanced hydrophilic/hydrophobic properties of the crystal. Playing on these hydrophilic/hydrophobic interactions, zeolites can self-assemble at the polymer-air interface of the BF cavities. Hydrophilic crystals do not have the ability to completely stabilize the BF formation process, leading to a loss of the structure regularity in the polymer film, while ambipolar crystals with increased stabilizing properties lead to the formation of regular arrays. The hydrophobic properties of the double bond present on the surface of the amphiphilic zeolites also increase their miscibility in conjugated polymers, allowing one to obtain highly organized functional hybrid films. The second method to assemble the zeolites in an hexagonal framework of a conjugated polymer is based on a mechanical approach. It uses the same concept as the loading of zeolite crystals: size and geometrical
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restrictions coupled with the use of the right dimension crystals leads to the inclusion of a single crystal in each cavity. The organization obtained in such a way is a double hybrid organic/ inorganic host-guest system: the inorganic host is loaded with an organic dye, and this inorganic host then becomes an inorganic guest for the organic host template. Thin films of electro-optically active polymers containing dye-loaded crystals confined in a welldefined microstructure can be produced using cost-effective techniques. The concentration of dye-loaded crystals in such hybrid thin films is very high, and therefore such thin films with novel architectures based on electroluminescent polymers open new challenging perspectives in fields such as hybrid electronics or sensing using zeolite L crystal based devices. Recent studies about dye-loaded zeolite crystals have demonstrated that energy can be efficiently injected into the zeolite nanochannels from an electroluminescent polymer.24 Combining such energy injection and the highly concentrated hybrid material obtained with the BF soft lithographic technique, we can obtain active layers for hybrid light-emitting devices using the polymer as a charge transport material and the dye-loaded zeolites as the emitting materials. Acknowledgment. The work was supported by the European Commission through the Human Potential Program (MarieCurie RTN “Nanomatch” Contract No. MRTN-CT-2006035884; Website: www.nanomatch.eu). We thank the group of Dominik Br€uhwiler (University of Z€urich) for providing us with the zeolite crystals. Supporting Information Available: Raman spectra of OxZLNH2 and OXZLCOOH. This material is available free of charge via the Internet at http://pubs.acs.org.
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