Polyaniline Nanocomposites

Jul 29, 2008 - Joanne D. Kehlbeck,*,† Michael E. Hagerman,*,† Brian D. Cohen,‡ Jennifer Eliseo,†. Melissa Fox,† William Hoek,† David Karli...
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Langmuir 2008, 24, 9727-9738

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Directed Self-Assembly in Laponite/CdSe/Polyaniline Nanocomposites Joanne D. Kehlbeck,*,† Michael E. Hagerman,*,† Brian D. Cohen,‡ Jennifer Eliseo,† Melissa Fox,† William Hoek,† David Karlin,† Evan Leibner,† Emily Nagle,† Michael Nolan,† Ian Schaefer,† Alexandra Toney,† Michael Topka,† Richard Uluski,† and Charles Wood† Department of Chemistry, Union College, Schenectady, New York 12308, and Department of Biological Sciences, Union College, Schenectady, New York 12308 ReceiVed March 26, 2008. ReVised Manuscript ReceiVed June 5, 2008 Laponite films provide versatile inorganic scaffolds with materials architectures that direct the self-assembly of CdSe quantum dots (QDs or EviTags) and catalytic surfaces that promote the in situ polymerization of polyaniline (PANI) to yield novel nanocomposites for light emitting diodes (LEDs) and solar cell applications. Water-soluble CdSe EviTags with varying, overlapping emission wavelengths in the visible spectrum were incorporated using soft chemistry routes within Na-Laponite host film platforms to achieve broadband emission in the visible spectrum. QD concentrations, composition and synthesis approach were varied to optimize photophysical properties of the films and to mediate self-assembly, optical cascading and energy transfer. In addition, aniline tetramers coupled to CdSe (QD-AT) surfaces using a dithioate linker were embedded within Cu-Laponite nanoscaffolds and electronically coupled to PANI via vapor phase exposure. Nanotethering and specific host-guest and guest-guest interactions that mediate nanocomposite photophysical behavior were probed using electronic absorption and fluorescence spectroscopies, optical microscopy, AFM, SEM, powder XRD, NMR and ATR-FTIR. Morphology studies indicated that Lap/QD-AT films synthesized using mixed solvent, layer by layer (LbL) methods exhibited anisotropic supramolecular structures with unique mesoscopic ordering that affords bifunctional networks to optimize charge transport.

Introduction Many of the strategies proposed in nanoscience to bridge topdown and bottom-up methods of nanomaterials syntheses hinge on an understanding of self-assembly to realize macroscopic structures. Applying concepts of supramolecular chemistry, structural information encoded in the nanobuilding blocks can be translated into the macrostructure of self-assembled nanomaterials.1,2 Self-assembly has been described as the autonomous organization of components into patterns or structures without human intervention.3 Self-assembly has gained much attention as it affords routes to fabricate novel organic-inorganic nanocomposites4–10 that have found practical applications as conductive assemblies,11,12 high temperature and flame resistant plastics,13,14 and gas sensors.15 Key insights on self-assembly * To whom correspondence should be addressed. E-mail: kehlbeck@ union.edu (J.D.K.); [email protected] (M.E.H.). † Department of Chemistry. ‡ Department of Biological Sciences. (1) Giraldo, O.; Brock, S. L.; Willis, W. S.; Marquez, M.; Suib, S. L.; Ching, S. J. Am. Chem. Soc. 2000, 122, 9330–9331. (2) Gardner, G. B.; Venkataraman, D.; Moore, J. S.; Lee, S. Nature 1995, 374, 792–795. (3) Whitesides, G.; Grzybowski, B. Science 2002, 295, 2418–2421. (4) Walt, D. R. Acc. Chem. Res. 1998, 31, 267–278. (5) Colorado, R.; Villazana, R. J.; Lee, T. R. Langmuir 1998, 14, 6337–6340. (6) Thompson, R. B.; Rasmussen, K. O.; Lookman, T. Nano Lett. 2004, 4, 2455–2459. (7) Joo, J.; Na, H. B.; Yu, T.; Yu, J. H.; Kim, Y. W.; Wu, F.; Zhang, J. Z.; Hyeon, T. J. Am. Chem. Soc. 2003, 125, 11100–11105. (8) Ozin, G. A. AdV. Mater. 1992, 4, 612–649. (9) Vaia, R. A.; Ishii, H.; Giannelis, E. P. Chem. Mater. 1993, 5, 1694–1696. (10) Mehrotra, V.; Giannelis, E. P. Solid State Ionics 1992, 51, 115–122. (11) Liu, Y.-J.; Schindler, J. L.; DeGroot, D. C.; Kannewurf, C. R.; Hirpo, W.; Kanatzidis, M. G. Chem. Mater. 1996, 8, 525–534. (12) Kanatzidis, M. G.; Wu, C.-G. J. Am. Chem. Soc. 1989, 111, 4139–4141. (13) Gilman, J. W.; Jackson, C. L.; Morgan, A. B.; Harris, R.; Manias, E.; Giannelis, E. P.; Wuthenow, M.; Hilton, D.; Phillips, S. H. Chem. Mater. 2000, 12, 1866–1873. (14) Phillips, S.; Blankski, R.; Srejda, S. A.; Haddad, T. S.; Lee, A.; Lichtenhan, J. O.; Feher, F.; Mather, P.; Hsiao, B. Mater. Res. Soc. Symp. Proc. 2001, 628, CC4.6.1CC4.6.10.

processes in nanomaterials and host-guest interactions and nanomorphology are essential to inform soft chemistry synthetic strategies to realize functional nanoarchitectures through facile, inexpensive syntheses that do not require high vacuum and expensive substrates. Our study focuses on the inclusion of quantum dot guests into Laponite and Laponite/polyaniline (PANI) host assemblies; these nanocomposites hold great promise for the development of lightemitting diodes and photovoltaics. The guests in our system are CdSe semiconductor nanoparticles (quantum dots, or QDs) (see Figure 1A). QDs are ideal for LED and solar cell applications owing to their tunable absorption and emission behavior, stability against photobleaching, flexible molecular coupling, multiple exciton generation, and high quantum yields.16 For LED applications the CdSe core is capped with a ZnS shell to protect the material from photooxidation and to ensure confinement of the exciton. For photovoltaic applications, this protective shell is removed to enable charge separation and transfer. Tuning the self-assembly of QD nanoparticles and controlling nanoparticle aggregation remain paramount to achieve photofunctional devices. Semiconductor nanocrystals have been entrapped in various matrices, including sol-gel derived silica spheres17 and polymer assemblies.18–21 Our nanohybrid assembly uses the synthetic clay, Laponite. Laponite is comprised of relatively uniform disk(15) Porter, T. L.; Hagerman, M. E.; Eastman, M. P. Science 1997, 1, 1–17, Surface and Intergallery Polymerization Reactions of Organic Monomers on Layered Silicate Hosts. In Recent Research Developments in Polymer. (16) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226–13239. (17) Mokari, T.; Sertchook, H.; Aharoni, A.; Ebenstein, Y.; Avnir, D.; and Banin, U. Chem. Mater. 2005, 17, 258–263. (18) Skaff, H.; Sill, K.; Emrick, T. J. Am. Chem. Soc. 2004, 126, 11322– 11325. (19) Odoi, M. Y.; Hammer, N. I.; Sill, K.; Emrick, T.; Barnes, M. D. J. Am. Chem. Soc. 2006, 128, 3506–3507. (20) Potapova, I.; Mruk, R.; Prehl, S.; Zentel, R.; Basche, T.; Mews, A. J. Am. Chem. Soc. 2003, 125, 320–321. (21) Gaponik, N. P.; Talapin, D. V.; Rogach, A. L.; Eychmuller, A. J. Mater. Chem. 2000, 10, 2163–2166.

10.1021/la800953w CCC: $40.75  2008 American Chemical Society Published on Web 07/29/2008

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Figure 1. Structures of material components for self-assembled nanocomposites: (A) Inclusion guests: CdSe nanocrystals with varying diameters, Fort Orange (FO), Hops Yellow (HY), Adirondack Green (AG), Lake Placid Blue (LPB); Copper(II) cations; Tris(2,2′-bipyridine) ruthenium(II) cations (Rubpy). Water-soluble type II EviTags (Evident Technology) have lipid bilayer shells (not shown). FO diamater is approximately 4.1 nm without lipid bilayer. (B) Laponite nanoparticle and smectite 2:1 layer silicate structure (C) polyaniline (AT) and QD with aniline tetramer nanotether (D) self-assembled Laponite nanoparticles with varying nanoarchitectures including house of cards, lamellar ordered and tactoid aggregated phases with four microenvironments for quest inclusion: (1) surface (2) interlamellar (3) edge (4) mesopore.

shaped nanoparticles approximately 40 nm in diameter and 3.5 nm thick22 (see Figure 1B). These nanoparticles self-assemble to form tunable nanoarchitectures that have distinct microenvironments for guest entrapment (Figure 1D). Additionally, the excellent optical transparency of Laponite enhances its versatility as a material for photophysical applications. Laponite films can be coated on various substrates through facile self-assembly from the aqueous phase.23–25 The architecture of the smectite-type clays consist of a 2:1 ratio of tetrahedral to octahedral oxide layers and an interlamellar region between each tetrahedraloctahedral-tetrahedral (TOT) layer.26 Owing to monovalent lithium ion substitution on the divalent magnesium framework cation site, nonframework sodium cations are present in the matrix for charge balance. The sodium cations can be exchanged (cation exchange capacity (CEC) of 72mmol/100 g for Laponite)27 with other cations including: ruthenium polypyridine such as tris(2,2′bipyridine) ruthenium(II) cations (Rubpy) and surfactants to control nanoparticle self-assembly and photophysical behavior,28 (22) Saunders, J. M.; Goodwin, J. W.; Richardson, R. M.; Vincent, B. J. Phys. Chem. B 1999, 103, 9211–9218. (23) Wheeler, P. A.; Wang, J.; Baker, J.; Mathias, L. J. Chem. Mater. 2005, 17, 3012–3018. (24) Vaia, R. A.; Ishii, H.; Giannelis, E. P. Chem. Mater. 1993, 5, 1694–1696. (25) Lezhnina, M.; Benavente, E.; Bentlage, M.; Echevarria, Y.; Klumpp, E.; Kynast, U. Chem. Mater. 2007, 19, 1098–1102. (26) Shriver, D. F.; Atkins, P. W. ; Overton, T. L.; Rourke, J. P.; Weller, M. T.; Armstrong, F. A. Inorganic Chemistry, 4th ed.; W. H. Freeman: New York, 2006; p 628. (27) Laponite Technical Directory, Laporte Industries.

rhodamines for sensing and imaging,29 and lanthanum30 and copper cations31 for catalytic applications. Ion exchanged copper cations play an important role in promoting the in situ polymerization of aniline monomers to realize polymer-clay nanocomposites. Polyaniline (PANI) (see Figure 1C) has attracted much attention as it offers high conductivity, environmental stability, and flexible processibility,32,33 coupled with versatility for design and fabrication of nanostructures materials including nanotubes,34 nanoscale conducting cylinders,35 and hybrid nanocomposites.36 We have previously reported that aniline vapor readily polymerizes on the surfaces of copper(II)-exchanged hectorite films, leaving them rigid and black.37,38 Intergallery transition (28) Hagerman, M. E.; Salamone, S. J.; Herbst, R. W.; Payeur, A. L. Chem. Mater. 2003, 15, 443–450. (29) Martinez, V. M.; Arbeloa, F. L.; Prieto, J. B.; Lopez, T. A.; and Arbeloa, I. L. Langmuir 2004, 20, 5709–5717. (30) Frey, S. T.; Hutchins, B. M.; Anderson, B. J.; Schreiber, T. K.; Hagerman, M. E. Langmuir 2003, 19, 2188–2192. (31) Porter, T. L.; Eastman, M. P.; Zhang, D. Y.; Hagerman, M. E. J. Phys. Chem. B 1997, 101, 11106–11111. (32) Stejskal, J.; Sapurina, I.; Trchova, M.; Prokes, J.; Krivka, I.; Tobolkova, E. Macromolecules 1998, 31, 2218–2222. (33) MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1994, 65, 103–116. (34) Qiu, H.; Wan, M.; Matthews, B.; Dai, L. Macromolecules 2001, 34, 675–677. (35) Kosonen, H.; Ruokolainen, J.; Knaapila, M.; Torkkeli, M.; Jokela, K.; Serimaa, R.; ten Brinke, G.; Bras, W.; Monkman, A.; Ikkala, O. Macromolecules 2000, 33, 8671–8675. (36) Mehrotra, V.; Giannelis, E. P. Solid State Commun. 1991, 77, 155–158. (37) Eastman, M. P.; Hagerman, M. E.; Attuso, J. L.; Bain, E. D.; Porter, T. L. Clay Clay Miner. 1996, 44, 769–773.

Directed Self-Assembly in Nanocomposites

metal cations catalyze the polymerization of aniline through their reduction and concomitant oxidation of aniline monomers to primary radical cations that promote free radical polymerization to form PANI.39 Selective control of the extent of polymerization, polymer oxidation state, and chain length within these nanocomposites through the choice of intergallery cation affords advanced materials design strategies for chemical sensing and conductive composite applications. PANI nanocomposites have also been fabricated through the insertion or encapsulation of aniline (ANI) or polyaniline (PANI) within inorganic host frameworks including mica-type silicates,40 vanadia xerogels,41 zeolites,42 layered metal phosphates,43 ruthenium chloride,44 maghemite,45 colloidal silica46 and silica sol gels.47 Polyaniline’s conductivity relies upon both its oxidation state and acid doping levels.48,49 Bulk polyaniline is found in three oxidation states (see Figure 1C): the nonconducting (totally reduced) leucoemeraldine form (y ) 1), the metal-like (semioxidized) emeraldine form (y ) 0.5), and the semiconducting (totally oxidized) pernigraniline form (y ) 0).50 Each form has distinct, active infrared bands.51–54 Our interests in QD/PANI/Lap nanomaterials lie in the integration of functionalized nanoparticles into a fully electronically coupled macrostructure. The development of alternative semiconductor nanocrystal surface ligands55,56,18 has opened the door to various targeted commercial applications for semiconductor nanocrystals including nanoelectronics,57 biolabels and biosensors,56,58,59 photovoltaic and solar cells,60 and light-emitting diodes.56 Querner and co-workers60 have developed nanocrystal surface ligands containing a carbodithioate (-C(S)S-) functional group, which serves as an anchor to the quantum dot shell surface. After the carbodithioate is coupled to the QD surface, the ligand is then covalently attached to an aniline tetramer (see Figure (38) Porter, T. L.; Thompson, D.; Bradley, M.; Eastman, M. P.; Hagerman, M. E.; Attuso, J. L.; Votava, A. E.; Bain, E. D. J. Vac. Sci. Technol. A 1997, 15, 500–504. (39) Porter, T. L.; Eastman, M. E.; Zhang, D. Y.; Hagerman, M. E. J. Phys. Chem. B 1997, 101, 11106–11111. (40) Kellicut, M. J.; Suzuki, I. S.; Burr, C. R.; Suzuki, M. Phys. ReV. B 1993, 47, 13664–13673. (41) Wu, C.-G.; DeGroot, D. C.; Marcy, H. O.; Schindler, J. L.; Kannewurf, C. R.; Liu, Y.-J.; Hirpo, W.; Kanatzidis, M. G. Chem. Mater. 1996, 8, 1992–2004. (42) Bein, T.; Enzel, P. Synth. Met. 1989, 29, E163–E168. (43) Liu, Y.-J.; Kanatzidis, M. G. Chem. Mater. 1995, 7, 1525–1533. (44) Wang, L.; Brazis, P.; Rocci, M.; Kannewurf, C. R.; Kanatzidis, M. G. Chem. Mater. 1998, 10, 3298–3300. (45) Tang, B. Z.; Geng, Y.; Lam, J. W. Y.; Li, B.; Jing, X.; Wang, X.; Wang, F.; Pakhomov, A. B.; Zhang, X. X. Chem. Mater. 1999, 11, 1581–1589. (46) Riede, A.; Helmstedt, M.; Riede, V.; Zemek, J.; Stejskal, J. Langmuir 2000, 16, 6240–6244. (47) Verghese, M. M.; Ramanthan, K.; Ashraf, S. M.; Kamalasanan, M. N.; Malhotra, B. D. Chem. Mater. 1996, 8, 822–824. (48) Hua, M.-Y.; Hwang, G.-W.; Chuang, Y.-H.; Chen, S.-A.; Tsai, R.-Y. Macromolecules 2000, 33, 6235–6238. (49) Glomm, W. R.; Volden, S.; Sjoblom, J.; Lindgren, M. Chem. Mater. 2005, 17, 5512–5520. (50) Gruger, A.; Novak, A.; Regis, A.; Columban, Ph. J. Mol. Struct. 1994, 328, 153–167. (51) Boyer, M. I.; Quillard, S.; Louarn, G.; Froyer, G.; Lefrant, S. J. Phys. Chem. B 2000, 104, 8952–8961. (52) Kostic, R.; Rakovic, D.; Davidova, I. E.; Gribov, L. A. Phys. ReV. B 1992, 45, 728–733. (53) Shacklette, L. W.; Wolf, J. F.; Gould, S.; Baughman, R. H. J. Chem. Phys. 1988, 88, 3955–3961. (54) Bloor, D.; Monkman, A. Synth. Met. 1987, 21, 175–179. (55) Kosonen, H.; Ruokolainen, J.; Knaapila, M.; Torkkeli, M.; Jokela, K.; Serimaa, R.; ten Brinke, G.; Bras, W.; Monkman, A.; Ikkala, O. Macromolecules 2000, 33, 8671–8675. (56) Wu, C.-G.; DeGroot, D. C.; Marcy, H. O.; Schindler, J. L.; Kannewurf, C. R.; Liu, Y.-J.; Hirpo, W.; Kanatzidis, M. G. Chem. Mater. 1996, 8, 1992–2004. (57) Osterloh, F. E.; Martino, J. S.; Hiramatsu, H.; and Hewitt, D. P. NanoLett. 2003, 3, 125–129. (58) Wang, D.; Rogach, A. L.; and Caruso, F. NanoLett. 2002, 2, 857–861. (59) Wolcott, A.; Gerionb, D.; Visconte, M.; Sun, J.; Schwartzberg, A.; Chen, S.; Zhang, J. Z. J. Phys. Chem. B 2006, 110, 5779–5789. (60) Querner, C.; Reiss, P.; Bleuse, J.; and Pron, A. J. Am. Chem. Soc. 2004, 126, 11574–11582.

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1C). These aniline tethers enable charge transfer through their conjugated pi systems for applications in LEDs and photovoltaics.61 In order for nanomaterials to be commercially competitive with crystalline silicon or multicrystalline silicon solar cells, significant increases in photoefficiences and advances in facile and inexpensive fabrication methods must be achieved. Several key players have emerged to compete with traditional silicon based solar cells including Graetzel cells or dye sensitized solar cells, 62 QD based solar cells,63,64 and conductive polymer organic solar cells.65 QD based solar cells offer remarkable photoefficiencies, tunable surfaces to optimize charge transfer, multiple exciton generation, and broadband absorption of solar spectrum. Different QD sensitizers could be used including CdS, PbS, Bi2S3, CdSe, and InP to optimize solar energy absorption.66 In many of these photovoltaics, TiO2 is the preferred semiconductor oxide film that lies in contact with the sensitizer owing to its mesoporosity, high surface area, and matchable band gap with other conductive materials. Watson and colleagues have illustrated site selective deposition of QDs onto TiO2 surfaces through photocatalytic patterning.67 Robel and colleagues68 have achieved success with QD based solar cells that build on the Graetzel design. In this system, the large surface area of colloidal TiO2 nanoparticles allows for the grafting of many smaller QDs to be linked to it by a sulfide coupler. This covalent tethering promotes the transfer of electrons between the QD and the TiO2. The colloidal TiO2 nanoparticles exhibit island growth on the indium tin oxide (ITO) surface; these close packed TiO2 colloids contain spaces in between their close packed spheres. Improved epitaxial interface with the electrode as achieved in conductive polymer photovoltaics is desired for improved efficiency. In addition, charge buildup at the nonepitaxial interface leads to charge rectification and eventual cell failure upon solar cycling. Charge recombination and scattering at the CdSe/TiO2 heterointerface and grain boundaries also limit photoconversion. The reported heterogeneity in charge transfer rate constants for various light harvesting nanoassemblies needs to be better understood through direct links to particle dispersity, morphology, and studies of electronic coupling between the sensitizer and the semiconductor; new emphases should focus on irregular/ nonperiodic architectures with nanoscale disorder amidst ordered bulk.69 The control of device function using nanoparticles in polymer nanocomposites relies not only on tuning nanoparticle dispersion but incorporating functionality directly at the interface to use the expansive interfacial surface areas of the nanomaterials to realize novel properties.70 Building anisotropy into these interfaces will broaden the utility of these nanocomposites for (61) Kieffel, Y.; Travers, J. P.; Ermolieff, A.; and Rouchon, D. J. Appl. Polym. Sci. 2002, 86, 395–404. (62) Gra¨tzel, M. Nature 2001, 414, 338–344. (63) Luther, J. M.; Beard, M. C.; Song, Q.; Law, M.; Ellingson, R. J.; Nozik, A. J. Nano Lett. 2007, 7, 1779–1784. (64) Nozik, A. J. Inorg. Chem. 2005, 44, 6893–6899. (65) Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834–2860. (66) LoCascio, M. Application of Semiconductor Nanocrystals to PhotoVoltaic Energy ConVersion DeVices; Technical Report, Evident Technologies; August 2002, 1-12. (67) Dibbell, R. S.; Soja, G. R.; Hoth, R. M.; Watson, D. F. Langmuir 2007, 23, 3432–3439. (68) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2006, 128, 2385–2393. (69) Adams, D. M.; Brus, L.; Chidsey, C. E. D.; Creager, S.; Creutz, C.; Kagan, C. R.; Kamat, P. V.; Lieberman, M.; Lindsay, S.; Marcus, R. A.; Metzger, R. M.; Michel-Beyerle, M. E.; Miller, J. R.; Newton, M. D.; Rolison, D. R.; Sankey, O.; Schanze, K. S.; Yardley, J.; Zhu, X. J. Phys. Chem. B 2003, 107, 6668–6697. (70) Vaia, R. A.; Maguire, J. F. Chem. Mater. 2007, 19, 2736–2751.

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Figure 2. Device design schematics: (A) Laponite/EviTag nanomaterials for white light LED applications (B) solar cell with Laponite/QD-AT/PANI active layer, ITO cathode and Al anode.

use in active devices such as nanoelectromechanical systems (NEMS), sensors, and electrochromic displays.71 We report herein specific soft chemistry routes to synthesize new nanocomposites that offer strategies to tune CdSe/host heterointerfaces and to control CdSe aggregation. More specifically we have investigated the self-assembly of robust thin film platforms containing Laponite nanoparticles, CdSe nanocrystals and conductive polymer assemblies. Two targeted applications for these nanocomposites are white light emitting diodes and solar cells. Figure 2 provides schema for our device designs for LED and photovoltaic applications based on Laponite/QD and Laponite/ QD/PANI architectures. Achermann and co-workers72 have explored a variety of QLEDs achieving color conversion efficiencies as high as 10%. These devices are comprised of thin-film LEDs that contain an active layer of II-VI semiconductor nanocrystals sandwiched between metal electrodes. Photons emitted for the traditional LED encounter the discrete energy bands specific to the nanocrystal. Because the bandgap of a quantum dot can be finely tuned by precise fabrication methods that exquisitely control their size, QLEDs can be customized to a predetermined fixed emission wavelength ranging the whole visible spectrum. A monolayer of trioctylphosphine oxide (TOPO)-covered CdSe quantum dots/PMMA is spin coated on top of a InGaN/GaN quantum well. Electron-hole pairs recombine in the quantum well and produce blue light emission that pumps the QDs. The nanocrystals are excited by the energy transfer and emit a specific wavelength determined by their crystal size. In our tailored design (see Figure 2A), we plan to replace the single QD/polymer composite architectures with multiple QD/Laponite assemblies cast from aqueous phase self-assembly to achieve white light emission. The design of our Lap/QD-AT/PANI solar cell is shown in Figure 2B. The active layer is comprised of a Cu Lap films with QD islands grown from layer by layer (LbL) methods netted together by PANI grown by vapor phase polymerization. This active layer is sandwiched between ITO and Al electrodes. This cell design addresses the previously mentioned problems with the dye sensitized solar cell design. The colloidal TiO2 has been replaced with a Lap/PANI matrix. PANI has a semiconductive bandgap of 1.1 to 1.3 eV73 that overlaps the CdSe band edge and serves as the key interface for electron transport to the electrode. (71) Farrell, J. R.; Lavoie, D. P.; Pennell, R. T.; Cetin, A.; Shaw, J. L.; Ziegler, C. J. Inorg. Chem. 2007, 46, 6840–6842. (72) Achermann, M.; Petruska, M. A.; Kos, S.; Smith, D. L.; Koleske, D. D.; Klimov, V. I. Nature 2004, 429, 642–646. (73) Kwon, O.; McKee, M. L. J. Phys. Chem. B 2000, 104, 1686–1694.

Quantum dots that contain aniline tetramers attached to them serve as the sensitizer. Three distinct advantages of the aniline nanotether include:60 excellent overlap of the ligand electronic energy levels with CdSe; increased stability of the dithioate linkage compared to the typical (TOPO) ligand; and optimization of charge transport chain to connect to conductive polymer networks. A key advantage of this design is the better interfacial contact of PANI with the electrode (see Figure 2B). Cu-Laponite serves as the catalytically active host for in situ polymerization and as a key templating agent to mediate charge transport. Nonframework metal cations and the bipolaron charge transport chain in PANI serve as mediators to reset holes in the QD sensitizers eliminating the need for liquid electrolytes and secondary mediators. While FO QD was chosen for this study, the design offers the flexibility to use PbS and other QDs to extend the absorption band for increased photoefficiencies. In addition, PANI has a tunable band gap73 that can be altered to match the energy of the various tethered QD surfaces.

Experimental Section Material and Methods. Sodium Laponite RD was distributed by Southern Clay Products in the USA and donated by Laporte Industries, UK. Water-soluble type II EviTags were purchased from Evident Technologies. Fort Orange type I EviDots used for nanotether studies were donated by Evident Technology. Reagents for nanotether syntheses and ion exchange of Laponite nanoparticles in aqueous solution were purchased from Aldrich Co. The synthesis was accomplished essentially by the method of Querner.60 All synthesized precursors and ligands were assessed by 1H and 13C NMR spectroscopy and conformed to published literature characterization. ATR FTIR analyses were completed with a Thermo Nicolet 380 FTIR spectrometer with a multipass Smart ARK Accessory. Each film was characterized with 256 collections, gain ) 2, resolution ) 2 cm-1, FTIR mulitpass tension setting ) 4.5, background corrected for H2O and CO2 and collected from 3500 cm-1 to 650 cm-1. Electronic absorption and fluorescence responses were characterized with a Hewlett-Packard 8452A Diode Array UV/ Visible spectrophotometer (wavelength range 200-800 nm) and Photon Technology Quantamaster Fluorometer. Powder XRD was completed using a Philips PW1840 powder X-Ray diffractometer system operated at a constant 35 W, 100 A, and 1.788Å Co KR radiation. AFM studies were completed using a VEECO Dimension V system. Tapping mode phosphorus (n) doped silicon probes were used. SEM analyses were completed with a Zeiss EVO W filament system at Union College and a Zeiss FEG system at UAlbany. Synthesis of the Nanotether. 2-(4-Bromophenyl)-5,5-dimethyl1,3-dioxane (1). 4-Bromobenzaldehyde (3.7216 g, 20 mmol) and 2,2-dimethyl-1,3-propanediol (20.9220 g, 200 mmol) were dissolved in 120 mL of toluene in a round bottomed flask under inert atmosphere

Directed Self-Assembly in Nanocomposites and trifluoroacetic acid (0.461 mL, 6 mmol) was added. The mixture was heated to 90 °C for 17 h. The reaction mixture was then washed with saturated potassium carbonate (2 × 50 mL) and water (2 × 50 mL). The organic layer was concentrated and the product recrystallized in water to give 5.0 g (92% yield) of the white solid 2-(4bromophenyl)-5,5-dimethyl-1,3-dioxane (1). 4-(5,5-Dimethyl-1,3-dioxan-2-yl)-dithiobenzoic acid (2). Dried Mg turnings (2.24 g, 92.18 mmol) were covered with 20 mL of anhydrous THF in a round bottomed flask under inert atmosphere. Five grams (18.44 mmol) of 1 was dissolved in 40 mL of anhydrous THF and added dropwise to the Mg turnings. The mixture was stirred vigorously under reflux for 2.5 h becoming brown in color. The solution was then transferred via canula to a flask containing 3.34 mL (55.32 mmol) carbon disulfide in 50 mL anhydrous THF previously cooled to -5 to 0 °C. The mixture was allowed to warm to room temperature while stirring overnight. The product was then hydrolyzed with 200 mL of 1:1 diethyl ether: water mixture. The aqueous layer was acidified with 200 mL 0.2 M HCl and extracted with diethyl ether. The combined organic layers were washed two times with water, dried over magnesium sulfate, and concentrated to form a dark purple-red oil. The product was then crystallized in methanol to give 0.1563 g (3.2%) of 4-(5,5-dimethyl-1,3-dioxan2-yl)-dithiobenzoic acid (2). Potassium 4-Formyldithiobenzoate (3). Water (0.5 mL) and concentrated TFA (5 mL) were added to a solution of 2 (100.8 mg, 0.373 mmol) in 1 mL of chloroform. The solution was stirred at room temperature for 4 h. The reaction mixture was diluted with 5 mL chloroform and neutralized with saturated potassium carbonate solution to form potassium 4-formyldithiobenzoate (3) (11.5 mg, 14%). 1H NMR (acetone-d6, 200 MHz): δ 10.08 (s, 1H), 8.14 (d, 2H), 7.99 (d, 2H). Aniline Tetramer74,75 The free-base of N-phenyl-1,4-phenylenediamine (0.9627 g, 5.2 mmol) was dissolved in 100 mL of 1.0 M HCl with vigorous stirring and heating. The solution was cooled to 0 °C in an ice water bath. Ferric chloride hexahydrate (2.924 g, 10.4 mmol) was dissolved in 15 mL of 0.1 M HCl at room temperature. This yellow solution was then cooled to 0 °C and quickly added to the solution of dianiline hydrochloride. The blue mixture became pasty and was stirred mechanically for 4 h. The product was washed in a Buchner funnel 20x with 50 mL portions of 0.1 M HCl and dried overnight. The product was then redispersed in deionized water for 2 h and subsequently deprotonated with 400 mL of 0.1 M ammonium hydroxide (48 h). The product was separated by filtration and washed 14x with 40 mL portions of 0.1 M ammonium hydroxide. The product was dried in a vacuum desiccator to give 0.4023 g (21%) of aniline tetramer (4). Preparation of Tailored Quantum Dots. Ligand Exchange on Surface of Nanocrystal. Trioctylphosphine oxide ligands were exchanged for potassium 4-formyldithiobenzoate forming a dithioate bridge between the nanocrystal and the ligand. Potassium 4-formyldithiobenzoate (3.7 mg, 0.0167 mmol) was placed under inert atmosphere in a round bottomed flask and dissolved in 5 mL of ethanol. One milliliter (0.815 nmol) of Fort Orange CdSe/ZnS Quantum Dots (Evident Technologies) (60.31 µg/mL, Concentration: 8.15 × 10-7 M, MW of core ) 74 µg/nmol) in chloroform was added and allowed to stir at room temperature for 2 h. The reaction mixture was purified by precipitation and washing with methanol. The product 5 was then redissolved in ethanol. UV-vis (ethanol): Broadband absorption, λ ) 584 nm. Fluorescence Spectroscopy (ethanol), exc. at 360 nm: λmax ) 480 nm. Grafting of Aniline Tetramer Tether. The solution of 5 was diluted with 4 mL ethanol in an inert atmosphere. Aniline tetramer 4 (10.2 mg, 0.0280 mmol) was dissolved in 3 mL of ethanol. The tetramer solution was added dropwise to the functionalized quantum dots and allowed to reflux overnight. The black precipitate was filtered and washed twice with methanol and then twice with chloroform. The product 6 (4.2 mg, Concentration ) 1.56 × 10-6 M, MW ) 1.25 (74) Dufour, B.; Rannou, P.; Travers, J. P.; and Pron, A. Macromolecules 2002, 35, 6112–6120. (75) Feng, J.; Zhang, W.; MacDiarmid, A. G.; and Epstein, A. J. Annu. Technol. Conf. 1997, 2, 1373–1377.

Langmuir, Vol. 24, No. 17, 2008 9731 × 10-7 mg/mmol) was allowed to dry and redissolved in 5 mL ethanol (0.84 mg/mL). (Note: The calculation of molecular weight of the final complex assuming that 100% ligand exchange took place on each quantum dot and no free quantum dots or free ligands remained in solution). Thin Film Preparation. Preparation of Copper Laponite Films. 0.5 g of Na-Laponite RD synthetic clay (Na0.7[Li0.3Mg5.5Si8O22(OH)4]) (Southern Clay Products) was dissolved in 100 mL of 0.1 M CuSO4(aq). The mixture was left to stir and monodisperse overnight. Portions of the Laponite solution were centrifuged for 30 min at 10,000 rpm five times (washing with 25 mL deionized water between runs). After the final run, samples were redissolved in 25 mL deionized water and all samples were allowed to stir overnight. The Cu2+ exchanged Laponite solution had a final concentration of 5 mg/mL. Thin films were cast from the Cu/Lap solution by transferring 1.0 mL of the solution to a standard quartz slide and allowing to dry over 48-72 h. Synthesis of Cu Laponite/QD-AT Films. 1.0 mL of QD-AT ethanol solutions at varying concentrations were cast on top of preformed Cu Lap films using LbL approach leading to QD/Lap island growth. Vapor Phase Loading of Aniline within Cu Laponite and Cu Laponite/QD-AT Films. In order to synthesize the PANI nanocomposites, the Cu/Lap and Cu/Lap/QD-AT films were exposed to aniline in a vacuum desiccator. 10 mL of aniline in a separate small beaker was placed within the desiccator along with the films on quartz and exposed for 48 h. Synthesis of Laponite/EViTag Nanocomposites. 0.05 g of NaLaponite clay (Na0.7[Li0.3Mg5.5Si8O22(OH)4]) (Southern Clay Products) was dissolved in 10 mL of deionized H2O. This solution with a concentration of 5 mg/1 mL was then shaken and vortexed to ensure monodispersion. Adirondack Green (AG, Concentration ) 11.7 nmol/mL), Hops Yellow (HY, Concentration ) 6.8 nmol/mL), Fort Orange (FO, Concentration ) 10 nmol/mL), and Lake Placid Blue (LPB, Concentration ) 15.2 nmol/mL) QD amine functionalized EviTags in water were acquired from Evident Technology. For our initial studies, 500 µL of 5 mg/mL of Laponite, 500 µL of deionized H2O, and 500 µL of equal volumes of QDs were combined in Eppendorf microfuge tubes, shaken by hand and vortexed. In order to correct for differences in molar absorptivities, alternative volumes of 495 µL, 510 µL, and 15 µL for Adirondack Green, Hops Yellow, and Fort Orange, were used respectively. The Laponite volume was kept constant at 500 µL and concentration of 5 mg/ 1 mL for every film made. Solutions were made and then cast onto clean microscope slides and allowed to completely dry before characterization. Aqueous phase direct combination (APDC) and LbL methods were used to cast films. APDC consisted of dispersing Laponite, deionized water, and the targeted QDs and then casting a film on fused quartz. The LbL method entailed making trilayers of APDC single QD films. Each monolayer was allowed to dry completely for 48 h before placing the next layer over it. For example, one of the films made had AG as the first layer, HY as the second layer, and FO as the third layer. Another film was made using the opposite ordering of layers to investigate energy transfer and effects of synthetic method on photophysical responses.

Results and Discussion Laponite/EviTag Films for LEDs. In order to establish the viability of our approach for multiple QD chromophore entrapment, we began with studies of the entrapment of one type of EviTag, Lake Placid Blue (LPB), into Laponite assemblies. Aqueous phase direct combination was used to make a single LPB QD film. An intense emission peak centered at 500 nm and a single exciton peak centered at 475 nm were observed indicating that water-soluble LPB EviTags were successfully embedded in the Laponite film and that the host was clearly not restricting the QD photophysical response (see Figure 3). The narrow emission peak with fwhm of approximately 40 nm indicated that the lipid shell coating on the EviTag remained intact. Similar individual studies of AG, FO and HY QD exchanged Laponite films were

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Figure 3. Electronic absorption spectrum and fluorescence emission spectrum of water-soluble Lake Placid Blue (LPB) EviTag/Laponite film. Excitation wavelength was 400 nm.

also completed and showed narrow, bright emission bands. In order to achieve white light emission, several EviTags with varying emission wavelengths were incorporated into the same Laponite film assembly. Direct aqueous phase self-assembly from solutions with Laponite and AG, HY, and FY EviTags led to bright composites with clear optical transparency. Upon excitation with 400 nm light, three intense emission peaks for AG, HY, and FO centered at 520 nm, 560 nm, and 600 nm, respectively, were observed (see Figure 4A). Bright, white light emission is certainly viable in these systems. By changing the concentrations of the QDs within the film assembly it is also possible to tailor the emission response to be centered in the red (Figure 4B) or blue (Figure 4C) region of the visible spectrum. Gatta´s-Asfura and colleagues76 determined the molar absorptivities of many QDs and observed that molar absorptivity was particle size-dependent and proportional to the QD surface area. For equimolar concentrations of QDs exchanged within Laponite films, FO QD exhibited the brightest emission under 400 nm excitation. This data is consistent with the fact that FO has the highest molar absorptivity among the QDs studied herein. In addition, photostability and longevity of the EviTag photoresponse may be enhanced by entrapment within the inorganic matrix as Laponite nanocrystal surfaces and lamellar ordering inhibit oxidation. To date, no loss or quenching of luminescence signal has been observed with films aged for twelve months. The excitation wavelength was also varied to tune photophysical responses of these nanomaterials. Changing the excitation wavelength leads to marked differences in the relative intensities of the respective emissions. The differences in photophysical response and distinct emission traces observed for varying input wavelengths opens the door to sensing and photonics applications and may prove important for optical gating and telecommunications to detect varying input wavelengths at logic gates and switching devices.77–79 (76) Gatta´s-Asfura, K.; Constantine, C.; Lynn, M.; Thimann, D.; Ji, X.; Leblanc, R. J. Am. Chem. Soc. 2005, 127, 14640–14646. (77) Samuel, I. D. W.; Turnbull, G. A. Chem. ReV. 2007, 107, 1272–1295.

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Aqueous phase direct combination offers facile nanocomposite synthesis with minimal QD aggregation and high monodispersity of both Laponite and CdSe nanoparticles. In contrast, Fogg and colleagues have reported that incorporation of QDs in various polymer matrices resulted in either diminished fluorescence or complete quenching of emission.80 Zhao also found that it was not possible to resolve the emission of individual QDs owing to strong polymer matrix interactions and significant aggregation within the polymer hosts.81 Zinoni and co-workers concluded that low volume fractions of the chromophore were required to limit aggregation that compromised efficiencies for LED emission.82 Compared to these QD/polymer systems, our Laponite host system offers higher capacity for QD loading with limited evidence of chromophore aggregation. The order of the layers deposited and LbL can also be used for optimizing diverse functional properties of layered nanostructured materials.83,84 Alternative syntheses using LbL strategies were employed to examine energy transfer between QD/ Laponite multilayers. Schematics of each layer of the QD/Lap nanocomposite films synthesized using LbL methods are shown in Figure 5 along with their corresponding emission spectra. Each layer was comprised of an individual QD/Lap film. QD concentrations were held constant in both systems; only the order of film deposition was varied. Films were placed at 45° angle from the excitation source and the fluorescence detector and fluorescence was detected from the front face. In both cases (FO/HY/AG or AG/HY/FO), FO emission dominates the emission traces. However, the differences in emission intensities of AG compared to FO can be used to track energy transfer and optical cascading. The emissions of AG and HY QDs feed FO absorption resulting in a stronger signal and emission from the FO QD. This result is interesting as it has practical application for optical waveguiding where signal amplification and processing relies on energy transfer between QD nanocrystals.85 When aligning a single laser of energy equivalent to separation between the first exciton state to the input edge of the waveguide, photons generated by stimulated emission may cascade through the LED creating an amplified output. Models of energy transfer efficiency in CdSe systems have been reported by Dayal and colleagues.86 Laponite/QD/PANI Films for Solar Cells. An important limitation encountered in polymer/QD blends synthesized by LbL techniques has been increased intrinsic roughness as the number of layers was increased.85 Preliminary AFM studies indicate smooth surfaces with less than 50 nm step heights for both aqueous phase direct exchange and LbL Lap/QD films indicating minimal surface roughness and high optical clarity. Work is in progress to assemble Laponite/QD films upon quantum well structures to fabricate white light LEDs. Similar to natural hectorite clay, the synthetic clay Cu(II) exchanged Laponite films exhibit unique two-dimensional ordering and catalytically active surfaces that promote polymerization. SEM studies of these Laponite films indicate lamellar stacking of nanoparticles (see Figure 6). This 2D ordering of (78) Raymo, F. M.; Giordani, S. J. Am. Chem. Soc. 2002, 124, 2004–2007. (79) Matsui, J.; Mitsuishi, M.; Aoki, A.; Miyashita, T. J. Am. Chem. Soc. 2004, 126, 3708–3709. (80) Fogg, D.; Radzilowski, L.; Dabbousi, B.; Schrock, R.; Thomas, E.; Bawendi, M. Macromolecules 1997, 30, 8433–8439. (81) Zhao, J.; Zhang, J.; Jiang, C.; Bohnenberger, J.; Basche´, T.; Mews, A. J. Appl. Phys. 2004, 96, 3206–3210. (82) Zinoni, C.; Alloing, C.; Paranthoen, C.; Fiore, A. Appl. Phys. Lett. 2004, 85, 2178–2180. (83) Mamedov, A.; Belov, A.; Giersig, M.; Mamedova, N.; Kotov, N. J. Am. Chem. Soc. 2001, 123, 7738–7739. (84) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065– 13069. (85) Wang, C.; Huang, L.; Parviz, B.; Lin, L. NanoLett. 2006, 6, 2549–2553. (86) Dayal, S.; Burda, C. J. Am. Chem. Soc. 2007, 129, 7977–7981.

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Figure 4. Fluorescence emission spectra of Laponite/QD nanocomposite films synthesized with aqueous phase direct exchange with water-soluble FO, HY, and AG EviTags at varying concentrations: (A) corrected for differences in molar absorptivity (excitation wavelength of 400 nm) (B) higher FO concentration (C) higher AG (D) Same as (A) with excitation wavelength of 450 nm. Color inset shows monodispersed water-soluble EviTags within Laponite host matrix.

laponite nanoparticles is clearly evident in the strata and layered edge effects seen in the SEM data. Moreover, this lamellar stacking is maintained after in situ polymerization of PANI. The ideal architecture to enhance charge carrier transport in photovoltaics is an interpenetrating network of both electron and hole conducting constituents. One way to achieve this architecture is to increase mesoscopic order and to control crystallinity in these polymer systems.87 Laponite can be used to template polymer assemblies facilitating intriguing mesoporous connections with embedded sensitizers to achieve charge transport. In addition, varying microenvironments for guest entrapment can be used to build in anisotropy within the hybrid structures. UV/vis spectroscopy, powder XRD, and ATR FTIR spectroscopy were used to verify polymerization and to probe host-guest and guest-guest interactions in these polymer/clay nanocomposites. UV/vis data for Cu Lap/PANI films at varying times of vapor phase aniline exposure (Figure 7A) indicated that

PANI is in the bipolaron, emeraldine salt form after 4 h. Peaks at 320 nm are assigned to π-π* transitions and the band at 750 nm results from CT exciton formed by the transition of the benzenoid to an adjacent quinoid group.88 This result is significant as the bipolaron conduction pathway of PANI may afford both electron and hole transport and support charge separation to enhance photoefficiences. Figure 7B shows the absorption spectra of the aniline tetramer in ethanol, the Quantum Dot/aniline tetramer (QD/AT) in ethanol, and the Quantum Dot/aniline tetramer in a Cu(II) exchanged Laponite host film (QD/AT/Lap). The aniline tetramer absorption band was centered at 600 nm. This peak at 600 nm and additional peaks evident in the QD/AT solution centered at 440 nm (weak) and 350 nm (intense) indicated that the aniline tetramer was successfully grafted to the FO QD surface. The intense UV peak as well as the weaker shoulder that overlaps with the first exciton band indicates charge transfer and a mixing of the energy bands

(87) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. ReV. 2007, 107, 1324– 1338.

(88) Polk, B. J.; Potje-Kamloth, K.; Josowicz, M.; Janata, J. J. Phys. Chem. B 2002, 106, 11457–11462.

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Figure 5. Fluorescence emission spectra of Laponite/QD nanocomposite films synthesized using LbL with varying order of QD films: (A) FO/ HY/AG/glass substrate and (B) AG/HY/FO/glass substrate. Films were excited with front face alignment with an excitation wavelength of 400 nm.

between the chromophore and the CdSe surface.60 With the inclusion of the QD/AT into the Laponite film, this major peak previously centered at 600 nm has now blue-shifted to 560 nm. This blue shift may be attributed to rigidochromism as seen with molecular chromophores such as Rubpy.89 Within the solid/thin film nanocomposite the chromophore (QD/AT), has decreased rotational freedom, because it is entrapped in the Laponite host. The decrease in the rotational freedom is a consequence of direct noncovalent interactions with the host surface, most likely owning to H-bonding sites at Si-O interfaces. This destabilizes the excited-state of the electron causing a lower wavelength to be absorbed and a blue shift to occur. This blue shift has been observed in other nanocomposites and provides direct evidence of successful nanocomposite synthesis and strong host-guest interactions.28 Spectral blue shifts and red shifts have also been reported for cross-linked CdTe molecules.90 In addition, new shallow bands emerge at 700 and 800 nm that are likely the result of intra and intermolecular guest-guest interactions and delocalization of charge within the quinoid and benzene rings as a result of aggregation. The band above 800 nm may indicate an increase in the number of π-polaron transitions.88 (89) Innocenzi, P.; Kozuka, H.; Yoko, T. J. Phys. Chem. B 1997, 101, 2285– 2291. (90) Koole, R.; Liljeroth, P.; de Mello Donega, C.; Vanmaekelbergh, D.; Meijerink, A. J. Am. Chem. Soc. 2006, 128, 10436–10441.

Figure 8A shows powder XRD spectra of Cu Lap/QD-AT films before and after aniline polymerization. The family of (00l) Bragg diffraction peaks observed is indicative of lamellar ordering of clay nanoparticles.28 There are no substantial changes to these peaks indicating that the PANI growth does not disrupt the 2D tiling of Laponite nanoparticles. Figure 8B shows ATR FTIR spectra of Cu Lap/QD-AT films before and after aniline polymerization. N-H stretching at 3100 cm-1, C-H stretches at 2900 and 2850 cm-1 and the quinoid and benzene ring modes at 1600 cm-1 and 1500 cm-1, respectively, are evident along with an obvious C-N-C band at approximately 1290 cm-1. Evidence of C-N-C stretching shifted to 1230 cm1 indicates hydrogen bonding between the nitrogen of aniline and silicate hydroxyl or oxide groups on Laponite. The intensity ratio of peaks at approximately 1600 cm-1/1500 cm-1 is a qualitative measure of polyaniline oxidation state.91 The oxidation state of the polymer increases as this ratio increases. The ratio of bands at 1600/1500 cm-1 in Figure 8B (right) shows that the PANI is in its partially oxidized emeraldine form within the film; this data is consistent with the previously described electronic absorption data. Three major peaks were also observed at 1050, 950 and 860 cm-1, which correspond to SiO stretches that are distinct to the (91) Asturias, G. E.; MacDiarmid, A. G. Synth. Met. 1989, 29, E157–E162.

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Figure 6. SEM micrographs of: (A) Laponite film (inset shows clear evidence of lamellar stacking of Laponite nanoparticles) (B) Laponite/PANI film showing stacking edges at film corner.

Figure 7. UV-vis spectra of: (A) Laponite/PANI films with varying VPL times and (B) AT(bottom), QD-AT chromophore (middle) in ethanol solution, and Laponite/QD-AT film (top).

varying microenvironments in the Laponite structure. Si-O bonds exist at the surface, at the edge, and at sites internal to the framework. We observed significant spectral broadening of the SiO bands in the QD exchanged LbL films indicating strong host-guest interactions.92–95 Key host-guest interactions gov(92) Kumaraswamy, G.; Deshmukh, Y.; Agrawal, V. V.; Rajmohanan, P. J. Phys. Chem B 2005, 109, 16034–16039. (93) Breen, C.; Zahoor, F. D.; Madejova, J.; Komadel, P. J. Phys. Chem. B 1997, 101, 5324–5331. (94) Potapova, I.; Mruk, R.; Prehl, S.; Zentel, R.; Basche, T.; and Mews, A. J. Am. Chem. Soc. 2003, 125, 320–321.

erning nanocomposite stability involve hydrogen bonding between the aniline subunits and hydroxyl and oxide groups on the clay surfaces and edges and ion-dipole interactions with interlamellar cations. Morphology Studies of Self-Assembed Lap/QD-AT Aggregates and Lap/QD-AT/PANI Films. Figure 9 shows key results of morphology studies in the 2-4 µm range for Laponite/ QD-AT nanocomposites synthesized using LbL methods. The (95) Talapin, D. V.; Rogach, A. L.; Kornowski, A.; Haase, M.; and Weller, H. NanoLett. 2001, 1, 207–211.

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Figure 8. Powder XRD spectra (A) and ATR-FTIR spectra (B) of Laponite/QD-AT films before (bottom traces) and after (top traces) vapor phase loading and in situ polymerization of polyaniline.

Figure 9. Nanomorphology studies of Laponite/QD-AT nanocomposites synthesized using LbL: (A) SEM micrograph showing micron and submicron supramolecular structures of Laponite/QD-AT aggregates (B) 3D topography map of aggregates at 2 µm × 2 µm and corresponding (C) tapping mode amplitude image and (D) tapping mode phase image (phase angle from 0-45°).

SEM micrograph delineates micron and submicron disordered supramolecular structures of Laponite/QD-AT aggregates with unique mesoporous morphologies among lamellar ordered bulk Laponite. During the LbL synthesis, Laponite nanoparticles from the Cu/Lap were redispersed into the solvent (ethanol) phase that contained the QD-AT species and upon drying formed supramolecular structures with anisotropic, mesoporous networks. Tapping mode AFM topography and amplitude images confirm micron to submicron-sized aggregates with step heights ranging from a few nanometers to 500 nm. The tapping mode phase

image (D) indicates clustering of CdSe nanoparticles near Laponite scaffold edges at the bright field (circled in image). This clustering is significant as it indicates that the QD-AT moieties are near active Cu(II) centers that anchor polymerization. Our previous AFM studies have indicated that intergallery cations at clay edges and step faults on smectite surfaces are the foci for aniline monomer attachment and subsequent polymerization.39 This result is important as it provides direct evidence of interfacial contact between QD-AT and PANI, necessary for electron transport and solar cell function.

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Figure 10. Nanomorphology studies (submicron range) of Laponite/QD-AT nanocomposites synthesized using LbL: (A) optical microscopy image of supramolecular structures of Laponite/QD-AT aggregates (B) 3D topography map of aggregates at 500 nm × 500 nm (C) tapping mode amplitude image at 500 × 500 nm (D) corresponding tapping mode phase image (phase angle from 0-45°).

Figure 10 shows nanomorphology data in the submicron range for Laponite/QD-AT nanocomposites synthesized using LbL methods. An optical microscopy image of supramolecular structures of Laponite/QD-AT aggregates is also shown. The blue and dark purple hue of copper(II) Laponite and QD-AT chromophores, respectively, confirm film composition and provide direct evidence of self-assembly. The worm-like and branched architectures evident in the optical microscopy images have been reported in triblock polymer/QD systems where solvent plays a critical role in micelle and supermicelle formation.96 The 3D topography map (500 nm × 500 nm) and amplitude AFM images show lamellar stacking of disk-shaped nanoparticles with clear mesoscopic morphology that facilitates the interpenetration and connectivity between QD-AT nanoparticles and PANI through open pores in the matrix. Figure 11 shows nanomorphology studies of Laponite/QDAT/PANI nanocomposites synthesized using LbL and vapor phase loading methods. The shift from blue to brown/black and darkening of the optical microscopy image provides clear evidence of in situ polymerization of PANI. Tapping mode amplitude (B) and phase (C) images at low density PANI coverage show that at low coverage (light brown region of the OM picture) on the flat surface of the nanocomposite, polymerization appears homogeneous with minimal step heights that track amplitudes on nonexposed Cu Lap films. Tapping mode amplitude (D) and phase (E), (F) at high density PANI coverage (dark brown region of OM picture) show clear stacking faults and large step boundaries. Phase images show three distinct regions with clear (96) Duxin, N.; Liu, F.; Vali, H.; Eisenberg, A. J. Am. Chem. Soc. 2005, 127, 10063–10069.

evidence of voluminous polymer growth covering the nanoparticle supramolecular assemblies. AFM 3D topography map (G) of step edge boundary shows terraced lamellar stacking region and retention of 2D ordering within the nanocomposite. Mallouk and Hata have reported similar AFM studies that illustrated successful templating of PDDA polymers within synthetic micas for anionic exchange applications.97 These images of Lap/QD/PANI heterointerfaces coupled with optical characterization described previously suggest strong host-guest and guest-guest interactions important for optimized photofunctional responses.

Conclusions Using varying EviTags within the same Laponite host film and both aqueous phase direct combination and LbL methods, tunable and bright broadband emission was achieved with very high monodispersity of QDs within the Laponite films. Successful nanotethering of aniline tetramer to QD surfaces encapsulated within Laponite/PANI host platforms was also achieved. Solar cells based on QD/AT/PANI/Lap films offer the prospect of inexpensive fabrication through soft chemistry routes together with enhanced flexibility and improved photoefficiences. Nanomorphology studies indicated self-assembled QD/Laponite supramolecular architectures with strong host-guest, guest-guest and surface interactions. Incident photon-to-current efficiency (IPCE) studies are currently being completed to confirm solar cell viability. (97) Hata, H.; Kobayashi, Y.; Mallouk, T. E. Chem. Mater. 2007, 19, 79–87.

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Figure 11. Nanomorphology studies of Laponite/QD-AT/PANI nanocomposites synthesized using LbL and VPL: (A) optical microscopy image; tapping mode amplitude (B) and phase (C) images at low density PANI coverage; tapping mode amplitude (D) and phase (E), (F) at high density PANI coverage; (G) AFM 3D topography map of step edge boundary.

In their studies with CdSe nanorods for solar cell applications, Alivisatos and colleagues98have shown that the ability to synthesize and study novel anisotropic nanostructures remains crucial in order to optimize photoefficiencies, control charge separation and limit electron-hole recombination. To this end, our ongoing studies have involved using advanced sensitizers (Rubpy/QD)99 to achieve charge separation and incorporating hole-conducting polymers such as pEDOT and P3HT copolymerized with electron-conducting polymers such as PANI.87 The mixing of QD and Ru polypyridine complexes results in the formation of electronically coupled assemblies in which the hole is localized to the Ru center and the electron remains on the QD. Through control of chromophore aggregations and in situ (98) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425– 2427. (99) Sykora, M.; Petruska, M. A.; Alstrum-Acevedo, J.; Bezel, I.; Meyer, T. J.; Klimov, V. I. J. Am. Chem. Soc. 2006, 128, 9984–9985.

polymerization afforded by Laponite nanoparticle interactions, it may be possible to build anisotropy at the interface to optimize charge transport. We are also exploring covalent grafting of polymers to the Laponite frameworks to optimize charge transport and expand design strategies.100 Acknowledgment. M.E.H. thanks Ron Bucinell (UC), Mary Carroll (UC), Doug Klein (UC) and Michael Carpenter (CNSE at UAlbany) for materials characterization support and Evident Technologies, IBM, NSF, Dreyfus Foundation, and Semiconductor Research Corporation for materials support and consultation. J.D.K. thanks Research Corporation and Union College for funding. LA800953W (100) Wheeler, P. A.; Wang, J.; Baker, J.; Mathias, L. J. Chem. Mater. 2005, 17, 3012–3018.