Self-Assembled Structures of Semiconductor Nanocrystals and

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Self-Assembled Structures of Semiconductor Nanocrystals and Polymers for Photovoltaics. 1. CdSe Nanocrystal-Polymer Multilayers. Optical, Electrochemical, Photoelectrochemical and Photoconductive Properties G. Zotti* and B. Vercelli Istituto CNR per l’ Energetica e le Interfasi, c.o Stati Uniti 4, 35127 PadoVa, Italy

A. Berlin* Istituto CNR di Scienze e Tecnologie Molecolari, Via C. Golgi 19, 20133 Milano, Italy

P. T. K. Chin Molecular Materials and Nanosystems, EindhoVen UniVersity of Technology, P.O. Box 513, NL-5600 MB EindhoVen, The Netherlands

U. Giovanella Istituto CNR per lo Studio delle Macromolecole, Via E. Bassini 15, 20133 Milano, Italy ReceiVed February 16, 2009. ReVised Manuscript ReceiVed April 6, 2009

Hybrid materials of polymers and cadmium selenide nanocrystals (CdSe-NCs) were produced through a layer-by-layer deposition technique. Polymer series comprises sulfonic, phosphonic, and carboxylic acids; pyridine- and amine-based polymers; and some related bipolar amphiphiles, including a dithiol. Nonaqueous dispersions of amine- and oleate-capped CdSe-NCs with 3, 4, and 5 nm diameters were used. Polymers and CdSe-NCs are alternately deposited on indium-tin-oxide/glass surfaces. CdSe-NCs layers undergo a sharp and irreversible electroxidation process in acetonitrile corresponding to the passage of two electrons per CdSe unit. The multilayer build-up, monitored by UV-vis spectroscopy and cyclic voltammetry, shows an increase in the film absorbance and oxidation stripping charge proportional with the number of adsorbed CdSe layers and with the particle diameter. Layer formation rate by acids follows their acidity, with thiols being much slower than sulfonic, phosphonic, or carboxylic acids. The semiconductor properties of the multilayers were evidenced by photoluminescence (PL), photoelectrolchemical, and photoconduction analysis. Good PL was recorded in pyridine-, amine-, and carboxylatebased multilayers, PL quenching in sulfonate-, phosphonate-, and thiolate-based multilayers. PL quenching is accompanied by higher photoelectrochemical and photoconductivity responses.

1. Introduction Inorganic nanocrystals (NCs) such as cadmium selenide (CdSe-NCs) constitute a very interesting class of lowdimensional semiconducting materials.1 Semiconductor nanocrystals, also referred to as quantum dots, show a distinctive size dependence of the absorption and emission energy due to the quantum confinement effect, giving rise to properties between those of the molecule and the bulk.2 The integration of inorganic semiconductor nanoparticles and organic polymers leads to composite materials with interesting new * Corresponding author. Tel.: (39)049-829-5868. Fax: (39)049-829-5853. E-mail [email protected].

(1) Low-Dimensional Semiconductor Structures: Fundamentals and DeVice Applications; Barnham, K.; Vvedensky, D., Eds.; Cambridge University Press: Cambridge, U.K., 2001. (2) (a) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (b) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (c) Nirmal, M.; Brus, L. Acc. Chem. Res. 1999, 32, 407. (d) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545.

physical properties and important potential applications, in particular for CdSe-NCs the production of solar cells.3 To exploit the electronic and optical properties of hybrid CdSe-NCs and polymers, the selection of appropriate molecular assemblies is necessary. CdSe-NCs are previously coated to prevent their aggregation so that the selected coordinating ligand present in the polymer must substitute the capping ligand. Thiolate strongly binds to group IIB metals ions on the surface of nanocrystals so that it can effectively replace original weak ligands such as amines and phosphines on CdSe-NCs. However, thiol groups significantly quench the photoluminescence of the nanocrystals,4 which may be a drawback in several applications, and are subject to photooxidation.5 In addition, the starting thiolcontaining compounds are not pleasant to work with and are (3) (a) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (b) Kamat, P. V. J. Phys. Chem. C 2008, 112, 18737. (4) (a) Guo, W.; Li, J. J.; Wang, Y. A.; Peng, X. Chem. Mater. 2003, 15, 3125. (b) Guo, W.; Li, J. J.; Wang, Y. A.; Peng, X. J. Am. Chem. Soc. 2003, 125, 3901.

10.1021/cm900460j CCC: $40.75  2009 American Chemical Society Published on Web 04/27/2009

CdSe Nanocrystal-Polymer Multilayers

often considered to be quite toxic. Better alternatives, easily identified in the literature, may be anions such as carboxylate, sulfonate, or phosphonate and, with a lower strength, nucleophiles such as pyridines and amines. The interaction between organic surfactant molecules and the surfaces of CdSe semiconductor nanoparticles has been recently calculated for phosphonic and carboxylic acids and amine ligands to a range of CdSe nanoparticle facets.6 The dominant binding interaction is between oxygen atoms in the acidic ligands and cadmium atoms on the nanoparticle surfaces.6 Anions (such as, for example, carboxylate) are the most strongly bound whereas amines are weakly bound.6 The direct interaction of CdSe-NCs and carboxylatebearing molecules is well documented, as in the case of CdSe/CdS core-shell particles made water-soluble by dendronized hydroxyl ends7 or of stable carboxylate-capped CdS-NCs within polyacrylate copolymers produced via chemical synthesis from the corresponding Cd2+ salts.8 The latter procedures are used in place of the direct addition of the semiconductor particles to the polymer because conventionally prepared CdS-NCs tend to undergo aggregation in common polyacrylate polymers. With a chemical route similar with that used for polyacrylates, CdS-NCs have been stabilized within telechelic polymers carrying carboxylate and sulfonate groups.9 In fact, sulfonate has also been employed as a stabilizing function for CdS- and CdSe-NCs in polystyrenesulfonates10 and in Nafion.11 Finally, CdSe semiconductor nanoparticles were stabilized by an amphiphilic diblock copolymer of polystyrene and poly-4vinylpyridine in a nonaqueous medium.12 The CdSe-NCs in this latter case are held in the polymeric matrix via the lone pairs of electrons in the pyridine rings of the block copolymer. Layer-by-layer (LBL) multilayers of CdSe- or CdS-NCs and polymers or organic linkers are in any case sparsely described. We mention the use of carboxylic-acid ended CdSe and polyvinylpyridine,13 chitosan,14 polyammonium salts15 or polyamines16 to produce regular multilayers via electrostatic alternation. 1,6-Hexanedithiol was used with CdS17 and 1,12-diaminododecane with CdSe.18 LBL alterna(5) Aldana, J.; Wang, Y. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 8844. (6) Puzder, A.; Williamson, A. J.; Zaitseva, N.; Galli, G.; Manna, L.; Alivisatos, A. P. Nano Lett. 2004, 4, 2361. (7) Liu, Y.; Kim, M.; Wang, Y.; Wang, Y. A.; Peng, X. Langmuir 2006, 22, 6341. (8) Wang, C. W.; Moffitt, M. G. Langmuir 2004, 20, 11784, and ref. therein. . (9) Kim, J.; Kim, S. S.; Kim, K. H.; Jin, Y. H.; Hong, S. M.; Hwang, S. S.; Cho, B. G.; Shin, D. Y.; Im, S. S. Polymer 2004, 45, 3527. (10) (a) Du, H.; Xu, G. Q.; Chin, W. S.; Huang, L.; Ji, W. Chem. Mater. 2002, 14, 4473. (b) Gatsouli, K. D.; Pispas, S.; Kamitsos, E. I. J.Phys.Chem.C 2007, 111, 15201, and ref. therein. (11) Smotkin, E. S.; Rabenberg, L. K.; Salomon, K.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.; Webber, S. E.; White, J. M. J. Phys. Chem. 1990, 94, 7543. (12) Fahmi, A. W.; Braun, H. G.; Stamm, M. AdV. Mater. 2003, 15, 1201. (13) Hao, E.; Lian, T. Langmuir 2000, 16, 7879. (14) Constantine, C. A.; Gatta’s-Asfura, K. M.; Mello, S. V.; Crespo, G.; Rastogi, V.; Cheng, T. C.; DeFrank, J. J.; Leblanc, R. M. J. Phys. Chem. B 2003, 107, 13762. (15) Zhang, S.; Chen, J.; Li, X. Nanotechnology 2004, 15, 477. (16) Zucolotto, V.; Gatta’s-Asfura, K. M.; Tumolo, T.; Perinotto, A. C.; Antunes, P. A.; Constantino, C. J. L.; Baptista, M. S.; Leblanc, R. M.; Oliveira, O. N., Jr. Appl. Surf. Sci. 2005, 246, 397. (17) Hu, K.; Brust, M.; Bard, A. J. Chem. Mater. 1998, 10, 1160.

Chem. Mater., Vol. 21, No. 11, 2009 2259 Chart 1

tion of CdSe with a polysiloxane19 has been performed using thiol-end modification of the polymer layers. A combination of LBL polyelectrolytes and Langmuir CdSe layers has also been reported.20 LBL multilayers of CdS-NCs capped with thiols bearing undecanol21a or nucleotide21b ends were formed via carbamate- or H-bonding, respectively. Polyaniline sulfonate has formed LBL multilayers with CdS-NCs.22 Polyacrylate-capped CdS-NCs were electrostatically self-assembled with poly(diallyldimethylammonium chloride) on different surfaces.23 Composite LBL assembly with inorganic nanoparticles has been quite recently reviewed.24 In spite of this amount of research, it results that the formation of LBL multilayers from the commonly used weakly capped CdSe-NCs and metal-coordinative polymers is not described in the literature. For this reason and for a preliminary approach to structures of conjugated organics connecting semiconductor particles for photovoltaic applications, we have undertaken the direct preparation of LBL structures of CdSe-NCs and polymers. In our study, we have used colloidal, monodisperse, amine- and oleate-capped CdSe quantum dots of different size (Chart 1) produced by the hot-injection route as a well-established approach to generating nanoparticles in an organic solvent.25,26 Polymers and molecules used as linkers, comprising acid-based (sulfonic-, phosphonic-, carboxylic-acid), pyridine- and amine-based polymers, and related bipolar amphiphiles including a dithiol, are shown in Charts 2 and 3. Such functional groups are expected to efficiently substitute the amine and carboxylate ligands capping the starting nanoparticles. A pictorial view of a typical monolayer and of multilayer formation is shown (18) Lee, J.; Mathai, M.; Jain, F.; Papadimitrakopoulos, F. J. Nanosci. Nanotechnol. 2001, 1, 59. (19) Ishii, S.; Ueji, R.; Nakanishi, S.; Yoshida, Y.; Nagata, H.; Itoh, T.; Ishikawa, M.; Biju, V. J. Photochem. Photobiol., A 2006, 183, 285. (20) Lowman, G. M.; Nelson, S. L.; Graves, S. M.; Strouse, G. F.; Buratto, S. K. Langmuir 2004, 20, 2057. (21) (a) Tsuruoka, T.; Akamatsu, K.; Nawafune, H. Langmuir 2004, 20, 11169. (b) Xu, J. P.; Weizmann, Y.; Krikhely, N.; Baron, R.; Willner, I. Small 2006, 2, 1178. (22) Ma, X.; Lu, G.; Yang, B. Appl. Surf. Sci. 2002, 187, 235. (23) Halaoui, L. I. Langmuir 2001, 17, 7130. (24) Srivastava, S.; Kotov, N. A. Acc. Chem. Res. 2008, 41, 1831. (25) (a) Yu, W. W.; Peng, X. G. Angew. Chem. 2002, 114, 2474. (b) Yu, W. W.; Peng, X. G. Angew.Chem.Int.Ed. 2002, 41, 2368. (c) Asokan, S.; Krueger, K. M.; Alkhawaldeh, A.; Carreon, A. R.; Mu, Z. Z.; Colvin, V. L.; Mantzaris, N. V.; Wong, M. S. Nanotechnology 2005, 16, 2000. (26) Boatman, E. M.; Lisensky, G. C.; Nordell, K. J. J. Chem. Educ. 2005, 82, 1697.

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Chem. Mater., Vol. 21, No. 11, 2009 Chart 2

in Scheme 1. The layers were characterized by UV-vis spectroscopy and cyclic voltammetry, and their semiconductor properties were determined by photoluminescence and photoelectrolchemical and photoconduction analysis. 2. Experimental Section Materials, substrates and multilayer film formation, nanocrystal synthesis and purification, and procedures can be found in detail in the Supporting Information. 2.1. Materials. 2.1.1. CdSe-NCs. Soluble CdSe-NCs with the surface capped with trioctylphosphineoxide (TOPO) hexadecylamine (HDA) and stearic acid (SA) were produced as in the Supporting Information and named amine-capped CdSe-NCs or AC550, AC580 and AC610 (core with 3.0 ( 0.2,4.0 ( 0.2 and 5.0 ( 0.2 nm diameter, determined by TEM). Their absorption spectrum in CHCl3, shown in Figure S1a of the Supporting Information for AC580, shows their maximum at 550, 580, and 610 nm respectively, corresponding to an average size of 3.0, 4.0, and 5.0 nm,27 in agreement with TEM analysis. The corresponding PL spectra (see, for example, Figure S1a in the Supporting Information) are narrow as usually found for defect-free surfaces in NCs. Amine-capped core-shell CdSe/CdS NPs, with a threelayer shell of CdS, are also produced as described in the Supporting Information. Soluble CdSe-NCs with the surface capped by oleate and named oleate-capped CdSe-NCs or OC550 are also produced as described in the Supporting Information. On the basis of TEM analysis, the average particle size of the CdSe clusters is 3.0 nm. The spectrum in CHCl3, shown in Figure S1b in the Supporting Information, shows its maximum at 545-550 nm (depending on the preparative batch), corresponding to an average size of 3.0 nm.27 The corresponding PL spectrum (see Figure S1b in the Supporting Information) is broad with a tail extending into the red due to surface defect emission. CdSe-NC dispersions in CHCl3 used for layering were 1 × 10-3 M and 1 × 10-2 M (in CdSe units) for amine-capped and oleatecapped samples, respectively. CdSe-NC dispersions were stored in the dark under nitrogen.28 (27) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 2854. (28) Bowen-Katari, J. E.; Colvin, V. L.; Alivisatos, A. P. J. Phys. Chem. 1994, 98, 4109.

Zotti et al. 2.1.2. Polymers and Bipolar Amphiphiles. Polymers, shown in Chart 2, were poly(p-styrenesulfonic acid) (PSSH), poly(vinylsulfonic acid) (PVSH), poly(vinylphosphonic acid) (PVPH), poly(acrylic) acid (PAAH), poly(vinylbenzoic) acid (PVBH), poly(4vinylpyridine) (PVPY), branched polyethylenimine (PEI), and poly(allylamine) base (PAB). Bipolar amphiphiles (Chart 3) were 1,6-hexanedithiol (HDT), methylene diphosphonic acid (MDPH), methylene disulfonic acid (MDSH), carboxy-ethylenephosphonic acid (CEPH), linear dicarboxylic acids such as oxalic acid (CA2) and adipic acid (CAC4CA), 4,4′-dipyridine (DIPY), and alkane-bridged homologues such as 1,2-bis(4-pyridyl)ethane (PYC2PY) and 1,3-bis(4-pyridyl)propane (PYC3PY). PAB, PEI, PVPH, and PVSH were used in 1 × 10-3 M concentration in 1:1 EtOH:water. PSSH, PAAH, PVBH, PVPY, and bipolar amphiphiles were used in 1 × 10-3 M in EtOH solution. 2.2. Substrates and Multilayer Film Formation. Transparent thiol-modified surfaces were prepared from glass sheets or indiumtin-oxide (ITO)/glass electrodes by treatment with 3-mercaptopropyl-trimethoxysilane (MTS),29 which provides on both sides a surface bearing free thiol groups. Nafion monolayers with a surface mass of 1.1 ( 0.1 µg cm-2 are formed on ITO-glass according to a published procedure.30 The ITO electrodes were also modified with the different bipolar amphiphiles or polymers used in this investigation using an exposure time of 5 min. The subsequent build-up of multilayers was performed according to the methodology introduced by Decher,31 i.e., by dipping the electrodes alternatively into the solutions of the two components. 2.3. Apparatus and Procedure. Optical Spectroscopy and Electrochemistry. UV-vis electronic absorption spectra of layers on ITO-glass electrodes were run on a Perkin-Elmer Lambda 15 spectrometer. Absorbance data are given as measured, i.e., for the sum of the ITO and glass sides, the same level of functionalization being obtained at both sides of the electrode. Electrochemistry was performed in acetonitrile containing 0.1 M tetrabutylammonium perchlorate (Bu4NClO4) as the supporting electrolyte, at room temperature under nitrogen in three electrode cells. The counter electrode was platinum; unless differently stated, the reference electrode was a silver/0.1 M silver perchlorate in acetonitrile (0.34 V vs SCE). The voltammetric apparatus was AMEL, Italy. The working electrode for cyclic voltammetry (CV) of cast films was a glassy carbon minidisc electrode (0.2 cm2). CV of multilayers was performed on 1 × 4 cm2 ITO-glass samples (ca. 20 ohm/square resistance) with an exposed area of 1 cm2. CV was performed at a scan rate of 0.02 V s-1. Photoluminescence. PL experiments were carried out at room temperature with a monochromator equipped with a CCD detector, by exciting with a monochromated Xenon lamp. Measurements of solution PL quantum yields (QY) were obtained by comparing the integrated emission from rhodamine 640 in ethanol and that of CdSe-NCs dispersed in CHCl3. CHCl3 was used also because of its higher efficiency in PL generation.32,33 Concentrations of the solutions were adjusted to provide optical densities of ∼0.10 at an (29) Goss, C. A.; Charych, D. H.; Majda, M. Anal. Chem. 1991, 63, 85. (30) Vercelli, B.; Zotti, G.; Berlin, A.; Grimoldi, S. Chem. Mater. 2006, 18, 3754. (31) (a) Decher, G.; Hong, J. Makromol. Chem. Macromol. Symp. 1991, 46, 321. (b) Decher, G.; Hong, J. Ber. Bunsenges. Phys. Chem. 1991, 95, 1430. (c) Lvov, Y.; Decher, G.; Mohwald, H. Langmuir 1993, 9, 481. (d) Decher, G. Science 1997, 277, 1232. (32) Biju, V.; Kanemoto, R.; Matsumoto, Y.; Ishii, S.; Nakanishi, S.; Itoh, T.; Baba, Y.; Ishikawa, M. J. Phys. Chem. C 2007, 111, 7924. (33) Myung, N.; Bae, Y.; Bard, A. J. Nano Lett. 2003, 3, 747.


Chem. Mater., Vol. 21, No. 11, 2009 2261 Chart 3

Scheme 1

sample, the glass side facing down the excitation source (same as for photoelectrochemistry, see above). A clean mercury drop was injected in the center of the ring. The size of the circular junction (0.023 cm2) was therefore constant from sample to sample. A Pt wire, clipped to a rigid cable mounted on a micromanipulator, contacted the mercury pool in the ring, and a pin in the sample holder contacted a square of exposed ITO on the sample. The test area could be changed simply sliding with care the ring along the surface.

3. Results and Discussion

excitation wavelength of 488 nm in 10 mm optical path quartz cuvettes. IRRAS. FTIR spectra were taken in reflection-absorption mode on a Perkin-Elmer 2000 FTIR spectrometer. Infrared Reflection Absorption Spectroscopy (IRRAS) spectra of the layers were taken with a grazing incidence reflection unit (Specac). All spectra were recorded with 2 cm-1 resolution at an angle of incidence of 80° relative to the surface normal. Ten cycles were run for each spectrum and weighted subtraction of the background at the end of the series of measurements was applied. No gas purging of the chamber was necessary. AFM and Prophilometry. Atomic force microscopy (AFM) was performed in noncontact mode in air at room temperature using a NT-MDT NTEGRA apparatus equipped with noncontact mode silicon tips. Multilayer thicknesses were determined with an Alphasstep IQ prophilometer from KLA Tencor. Photoelectrochemistry. Photoelectrochemical experiments were performed in O2-saturated aqueous 0.1 M NaClO4 solution using a conventional three-electrode cell with a saturated calomel electrode (SCE) as a reference and a platinum counter electrode. All potentials and energies are given with respect to SCE, which is 4.43 eV below the vacuum level. The working electrode was illuminated on the solution side with a water-filtered 100 W halogen lamp, spotted over ca. 10 cm2. The resulting light power, calibrated with a Si photodiode, was ca. 100 mW cm-2. Light was chopped with a manually driven shutter. PhotoconductiVity. Measurements of multilayers were obtained with a special arrangement. We constructed an electrode in which mercury filled the hole in a rubber O-ring, analogously with the technique described by Whitesides for In-Ga alloy electrodes.34 The ring was collocated on the multilayer-covered ITO side of the

3.1. Modifications of ITO Surface. For this investigation, the surface of ITO electrodes was modified (besides with MTS29 and Nafion30) with different ligands (bipolar amphiphiles or polymers) in order to produce uniform monolayer-covered electrode surfaces for CdSe-NCs adsorption. The cleaned ITO-glass electrodes were treated with the specific ligand in the appropriate solvent (see Experimental Section), washed, and dried. 3.1.1. Carboxylic- and Phosphonic-Acid-Primed ITO. Carboxylic-acid modified ITO-glass surfaces were prepared by treatment with the bifunctional molecule carboxy-ethylenephosphonic acid (CEPH) or with a carboxylic polymer (PAAH). For CEPH, indication has been previously given35 that the phosphonic acid head is more strongly bound to the ITO surface than is carboxylic acid and therefore its adsorbs preferentially. Moreover it was previously reported35 that under the used conditions, the phosphonic acid head coordinates strongly to the surface producing a dense layer. For PAAH the literature36a reports that under the used conditions a monolayer ca. 1.3 nm thick is formed on titanium oxide surfaces and that a similar monolayer is also formed on porous alumina.36b The extent of active coverage has been measured by ferrocene modification of the acid-treated surface as follows. Immersion of the ITO/CEPH or ITO/PAAH electrode in a 1 × 10-3 M solution in CH2Cl2 of N,N-dimethylaminomethyl-ferrocene (DMA-Fc) and subsequent washing with the same solvent (where the formed ammonium salt does not dissolve) makes the electrode show in CH2Cl2 + 0.1 M But4NClO4 the CV reversible response of adsorbed ferrocene. The response, centered at a redox potential E0 ) 0.30-0.35 V, has the same features of the analogous monolayer of hexylferrocene phosphonic acid previously reported.35 The 0.10-0.15 V positive shift of the E0 value from that of DMA-

(34) Weiss, E. A.; Porter, V. J.; Chiechi, R. C.; Geyer, S. M.; Bell, D. C.; Bawendi, M. G.; Whitesides, G. M. J. Am. Chem. Soc. 2008, 130, 83.

(35) Vercelli, B.; Zotti, G.; Schiavon, G.; Zecchin, S.; Berlin, A. Langmuir 2003, 19, 9351.

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Fc in solution may be explained by the action of the carboxylic group of the surface-bound acid, which forms the more electronwithdrawing ammonium salt of DMA-Fc. The redox charge for ferrocene oxidation corresponds to a coverage of 1.0 × 10-10 mol cm-2 (0.7 × 10-10 for PAAH), i.e., it is lower than that expected for a full monolayer coverage (4 × 10-10 mol cm-237) but is in any case appreciably high. Phosphonic-acid modified ITO-glass surfaces were prepared by treatment with methylene diphosphonic acid (MDPH) or polyvinylphosphonic-acid (PVPH). MDPH and PVPH form a strongly bound monolayer on ITO as in the case of hexylferrocene phosphonic acid.35 In the case of MDPH, presumably only one head is used for grafting (no “hairpin” production) as generally found in such molecules.38 The phosphonic-acid modified surface shows essentially the same behavior of carboxy-modified ITO, with the same CV response of the ferrocene-modified surface (active coverage of 1.0 × 10-10 mol cm-2). 3.1.2. Pyridine- and Amine-Primed ITO. Priming of glass or oxide surfaces with polyvinylpyridine (PVPY) was previously reported.39 Different amounts of PVPY are adsorbed on the substrate from solutions with different polymer concentrations, which can be explained by differences in conformations that polymer molecules adopt on the surface. Anyway polymer molecules adsorbed from dilute solutions (1 × 10-3 M or lower) acquire a flat conformation to maximize their interaction with the surface and the coverage becomes independent from concentration. We could perform an analysis of the PVPY-coverage by electrochemical oxidation of the pyridine moieties. CV of the PVPY-treated ITO in acetonitrile + 0.1 M Bu4NClO4 has in fact shown a clear and reproducible irreversible oxidative process at Ep ) 1.3 V (see Figure S2 in the Supporting Information), in which a total charge of 80 µC cm-2 is involved. Because the oxidation of pyridine in this medium occurs at a very close potential (1.4 V) with the passage of one electron per pyridine moiety,40 the charge corresponds to a coverage of 8 × 10-10 mol cm-2. To compare the coordinating ability of this surface with the others, we have also performed in this case a ferrocenefunctionalization using ferrocene carboxylic acid (FcCOOH) as the probe. The ITO-glass/PVPY surface was thus exposed to FcCOOH in EtOH solution, washed, and dried. The CV response for ferrocene oxidation was positive also in this case, and the redox charge at the redox potential E0 ) 0.2 V corresponds to an active coverage of 1.2 × 10-10 mol cm-2. This value is 15% of the total amount of pyridine units (the others are either engaged in surface grafting or not accessible) but is in any case essentially the same as the other modified

surfaces. Moreover, analogous coverages have been previously measured for different Fc compounds on ITO surfaces.41 Priming of ITO surfaces with branched polyethylenimine (PEI) is commonly found in the literature.42 We have primed the ITO surface with the same procedure used for PVPY and checked the resulting active coverage by CV analysis of FcCOOH uptake. The result is that the coverage is 0.8 × 10-10 mol cm-2; namely, it is comparable with that of PVPY. 3.2. CdSe Monolayers. The formation of CdSe monolayers has been routinely investigated by UV-visible spectroscopy with 3 nm CdSe-NCs (AC550 and OC550). Because the number n of CdSe units in a spherical cluster of Wurtzite (d ) 5.81 g cm-3) of diameter d (in nm) is given by n ) 9.4 d3, in our case, n ) ca. 250 units. A spherical cluster is in fact generally obtained under the selected synthetic conditions.43 For a full ferrocene-like coverage of the surface (4 × 10-10 mol cm-2 36b), we estimate that ca. 70 molecules may be at most adsorbed on a single nanocrystal. The concentration of CdSe-NC dispersions in CHCl3 used for layering depended on the capping type of the NCs. Thus with amine-capped samples, a 1 × 10-3 M concentration (as CdSe units) was sufficient, whereas a higher concentration (1 × 10-2 M) was required for layering with oleate-capped samples. This result may be attributed to the higher lability of amine compared with oleate caps. CdSe monolayers are obtained at saturation level after different exposure times depending on the surface primer. Thus 60 min are required on thiol-primed ITO/MTS, whereas 5 min are more than enough for sulfonic-acid-primed ITO/ Nafion. Layer formation appears to follow the anion availability. In fact, if we compare their acidity in water, thiols (pKa ) 10) are much less acidic than sulfonic acids (pKa < -3, -14 for perfluoroalkyl-sulfonic acids such as Nafion) and even carboxylic acids (pKa ) 5). Recent reports44 are in agreement with this suggestion. Thus longer times are required for saturation from thiol (1 h) compared with all the others (5 min), which discourages the use of thiols for layering. The limiting two-side absorbance of the monolayer, A0, measured at 550 nm and summarized in Table 1, is different in dependence of the surface primer. The absorbance is thus 2 × 10-3 and 5 × 10-3 for ITO/MTS and ITO/Nafion, respectively. Given a molar extinction coefficient of 1 × 105 M-1cm-1 for this size of the nanocrystals,27 the coverage on ITO/MTS is 1 × 10-11 nanocrystal mol cm-2 (0.6 × 1013 particles cm-2). The latter value, compared with the theoretical one for a full geometrical coverage (1.1 × 1013 particles cm-2) indicates that a dense coverage is obtained. The higher value for ITO/Nafion electrodes (1.5 × 1013 particles cm-2) is

(36) (a) Ichinose, I.; Kawakami, T.; Kunitake, T. AdV. Mater. 1998, 10, 535. (b) Xiao, K. P.; Harris, J. J.; Park, A.; Martin, C. M.; Pradeep, V.; Bruening, M. L. Langmuir 2001, 17, 8236. (37) Gui, J. Y.; Stern, D. A.; Lu, F.; Hubbard, A. T. J. Electroanal. Chem. 1991, 305, 37. (38) Danahy, M. P.; Avaltroni, M. J.; Midwood, K. S.; Schwarzbauer, J. E.; Schwartz, J. Langmuir 2004, 20, 5333. (39) Malynych, S.; Luzinov, I.; Chumanov, G. J. Phys. Chem. B 2002, 106, 1280. (40) Turner, W. R.; Elving, P. J. Anal. Chem. 1965, 37, 467.

(41) Zotti, G.; Schiavon, G.; Zecchin, S.; Berlin, A.; Pagani, G. Langmuir 1998, 14, 1728. (42) see e.g. (a) Wang, X.; Gershman, Z.; Kharitonov, A. B.; Katz, E.; Willner, I. Langmuir 2003, 19, 5413. (b) Kim, Y. H.; Lee, S. H.; Noh, J.; Han, S. H. Thin Solid Films 2006, 510, 305. (43) Al-Salim, N.; Young, A. G.; Tilley, R. D.; McQuillan, A. J.; Xia, J. Chem. Mater. 2007, 19, 5185. (44) (a) Aldana, J.; Lavelle, N.; Wang, Y. J.; Peng, X. G. J. Am. Chem. Soc. 2005, 127, 2496. (b) Owen, J. S.; Park, J.; Trudeau, P. E.; Alivisatos, A. P. J. Am. Chem. Soc. 2008, 130, 12279.


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Table 1. Two-Side Exciton-Band Absorbance of First Layer (A0) and Differential Absorbance of Multilayers (∆A) on ITO for 3 nm CdSe-NCs Using Different Primers and Spacers ∆A × 103 (au layer-1)

primer

A0 × 103 (au)

Nafion Nafion Nafion Nafion PVPH MDPH

5.0 5.0 5.0 5.0 2.0 2.0

Sulfonate-Spaced Nafion PSSH PVSH MDSH PSSH PSSH

5.0 5.0 0.7 0.7 2.6 2.0

PVPH MDPH

2.0 2.0

Phosphonate-Spaced PVPH MDPH

0.8 2.0

PAAH CEPH PVBH

1.2 2.0 1.5

Carboxylate-Spaced PAAH PAAH PVBH

1.2 1.2 1.5

PVPY MTS PEI

2.0 2.0 2.0

Amine-Spaced PVPY PAB PEI

2.0 1.2 2.0

2.0

Sulfide-Spaced HDT

1.4

MTS

spacer

accounted for by the higher surface concentration of sulfonic acid functional groups in a rodlike structure (6 nm thick) overriding the molecular chain diameter.30 Also, priming the ITO surface with carboxylic-acid (e.g., CEPH) or phosphonic-acid (e.g., MDPH) moieties makes it able to strongly coordinate the CdSe-NCs. Similarly, a polypyridine (e.g., PVPY) or polyamine (e.g., PEI) treatment makes the ITO surface able to efficiently form monolayers of CdSe-NCs, with the same limiting two-side absorbance (see Table 1). In all these cases, the limiting adsorption was achieved within 5 min exposition. Colloidal CdSe quantum dots form aggregates on surfaces. In a recently described self-assembled monolayer formation of covalently bound carboxyl-terminated CdSe to aminoprimed gold,45 the 5 nm diameter particles form a submonolayer of max 50% fractional coverage. The bound nanoparticles have been shown to group together into islands at the surface.45 On this basis, an analogous clustering on the surface could in principle occur also in our case. AFM analysis of a CdSe monolayer on, for example, MTS-primed glass does not show any difference from the untreated MTSprimed glass, which indicates that aggregation, if any, does not produce agglomerates larger than ca. 10 nm. 3.3. Nafion/CdSe Multilayers. Given the high level of monolayer coverage on Nafion-primed ITO and the corresponding higher thickness of the Nafion layer itself, we have chosen the Nafion/CdSe system as the prototype for multilayering CdSe-NCs and polymers. 3.3.1. UV-Visible Analysis. UV-visible spectroscopy was used to monitor the assembling process of CdSe LBL films. The absorption spectra of Nafion/CdSe multilayer films prepared on ITO-glass (5 min exposure times) with different numbers of layers is shown in Figure 1 for 3 nm CdSe-NCs. The absorption spectra of CdSe in films exhibit well-resolved bands. The band at 550 nm is the same as the first exciton band of CdSe colloidal particles in solution shown in Figure (45) Cameron, P. J.; Zhong, X.; Knoll, W. J. Phys. Chem. C 2007, 111, 10313.

Figure 1. (a) UV-vis spectra of ITO/(Nafion/CdSe)n multilayers (AC550) and (b) relevant plot of absorbance at 550 nm vs n. Spectra are backgroundcorrected.

Figure 2. FTIR spectrum of (a) oleate-capped and (b) amine-capped CdSeNCs cast film on platinum.

S1b of the Supporting Information. This showed that the CdSe-NCs were assembled successfully into thin film without forming aggregates with directly contacting nanoparticles. In addition, the observed linear increase of CdSe absorbance vs the number of layers (Figure 1b) indicated a stepwise and uniform assembly process. The two-side exciton-band absorbance increase per layer (differential absorbance ∆A) is 5 × 10-3 layer-1, i.e., the same value for the monolayer. 3.3.2. IRRAS Analysis. For a clearer analysis, we will first consider the vibrational markers of the organic-capped CdSe component and then those of the Nafion linker, both involved in the buildup of the multilayers. The FTIR spectrum of an oleate-capped CdSe-NCs film cast on platinum (Figure 2a) is quite similar to that reported previously.46 It shows a medium absorption band at 1540 cm-1 and a weak one at 1380 cm-1 due to the (COO) asymmetrical and symmetrical stretching modes of the oleate ligand capping the surface of the nanocrystals,47 which indicate the presence of bidentate carboxylate bonding.46 Strong bands at 2930 and 2850 (stretching), 1470 (scissoring), and 720 (rocking) cm-1 are due to the methylene

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Zotti et al. Table 2. Two-Side Exciton-Band Differential Absorbance (∆A) of 5 nm CdSe-NCs Multilayers on ITO for Different Primer Polymers and Bifunctional Spacers

Figure 3. (a) IRRAS spectra of gold/(Nafion/CdSe)n multilayers (AC550) and (b) relevant plot of absorbance at 1250 cm-1 vs n.

moieties in the alkyl chain of the same oleate ligand. In any case the presence of some surface-bound TOP (surface Se atoms bonded as trialkylphosphine-selenide), which also displays the latter set of bands, cannot be excluded. The spectrum of an amine-capped CdSe-NCs film (Figure 2b) shows two additional strong bands at 3310 and 1630 cm-1 due to the (NH) stretching and bending, respectively, of the surface-capping HDA amine ligand. This ligand is clearly present along with the stearate cap on the NC surface. The IRRAS spectrum of a Nafion monolayer displays the strong band at 1250 cm-1 due to the antisymmetrical stretching mode of the -CF2- moieties. A CdSe monolayer formed on this Nafion monolayer, keeps the strong antisymmetrical -CH2- stretching band at 2930 cm-1 of the original capping molecules but its intensity is much lower (ca. 10-20 times) than expected from retention of the capping layer. The absence of a significant amount of original caps in the surface-bound CdSe-NCs indicates the occurrence of direct coordination of the sulfonate ends of Nafion with cadmium dangling bonds on the CdSe-NCs surface, i.e., the sulfonic acid ends effectively substitute the carboxylate- or aminecapping molecules. IRRAS analysis of multilayer formation on a gold surface has confirmed the regular growth also of the polymer (Nafion) component, as illustrated in Figure 3, showing the linearity of the response at 1250 cm-1 with the number of Nafion sublayers. All these results indicate that the layers are mainly composed by CdSe and Nafion, the CdSe-NCs being connected by the polymer functional ends with the loss of most of the originally bound caps, and that the NC and polymer components alternate in the multilayer structure. 3.4. Other CdSe Multilayers. Other CdSe multilayers have been produced with different linkers (polymers and (46) Young, A. G.; Al-Salim, N.; Green, D. P.; McQuillan, A. J. Langmuir 2008, 24, 3841. (47) Shukla, N.; Liu, C.; Jones, P. M.; Weller, D. J. Magn. Magn. Mater. 2003, 266, 178.

primer

spacer

∆A × 103 (au layer-1)

PVPY

DIPY PYC2PY PYC3PY PVPY

1.0 1.2 1.1 3.5

PAAH

CA2 CAC4CA PAAH

1.1 1.4 1.9

bipolar amphiphiles). The results, summarized in Tables 1 and 2, are reported below by functional ends. 3.4.1. Sulfonic, Phosphonic, Carboxylic Acids, and Dithiols. PSSH/CdSe multilayers are produced similarly with Nafion/CdSe from a Nafion monolayer and present a high absorbance increase, comparable with Nafion. In contrast with the polymer, the bifunctional MDSH does not form multilayers; on the contrary, it dissolves a CdSe monolayer preformed on a ITO/Nafion surface. Dense PSSH/CdSe multilayers are in any case produced without the help of a relatively thick Nafion primer layer if PVPH-primed ITO is used (see below). PVSH/CdSe multilayers may be obtained from a ITO/ Nafion/CdSe surface but have a scarce nanoparticle content (7-8 times lower, see Table 1), which may be due to the short distance of the sulfonate moiety from the polymer chain, compared with the aromatic PSSH. Short polymer arms may in fact find a higher difficulty in encountering surface handles on the CdSe nanoparticle. In the case of phosphonic acids, though both PVPH and MDPH form a dense monolayer on ITO, linker/CdSe multilayer structures are formed at a high rate with MDPH but at a reduced rate, comparable with that of PVSH, with PVPH (see Table 1). The constraints of the polymer chain and of the short link of the phosphonic head to it may once more account for the lower performance of the polymer. The carboxylic moiety as promoter and component of CdSe multilayering has been considered both in the polymeric (PAAH and PVBH) and the bifunctional (bicarboxylic) forms. Polymer/CdSe multilayers are produced on ITO/ PAAH and ITO/PVBH with a good differential absorbance (see Table 1). Multilayering with bipolar amphiphiles (see Table 2) does occur but the increase is lower than for the polymers. Dithiol-based CdSe multilayers are built successfully from ITO/MTS electrodes alternating CdSe and the dithiol HDT with 60 min exposure for both. The exciton-band differential absorbance is appreciable and is the same as the monolayer absorbance, but considerably longer times are once more required for multilayering, as for monolayering, which makes this type of functionalization less practical. 3.4.2. Pyridines and Amines. PVPY- and PAB-based CdSe multilayers were produced starting from ITO/PVPY and ITO/ MTS surfaces, respectively, and present a differential absorbance comparable with those of the acids. Once more, the short distance of the amine moiety from the polymer chain in PAB is responsible for the lower CdSe content per layer. This has been confirmed by the use of PEI, which is


Chem. Mater., Vol. 21, No. 11, 2009 2265

Table 3. Total (d) and Differential (d0) Thickness of Some Multilayers for CdSe-NCs of Various Diameters (L) and Different Polymers multilayer

CdSeL (nm)

d (nm)

d0 (nm layer-1)

(Nafion/CdSe)5 (PSSH/CdSe)15 (PVPY/CdSe)20 (PVPY/CdSe)20

3.0 3.0 3.0 5.0

42 ( 2 39 ( 5 50 ( 5 82 ( 2

8.5 2.6 2.5 4.0

branched and therefore extends its arms from the surface. In fact, multilayers obtained with PEI starting from ITO/ PEI are more dense, with a high differential absorbance comparable with that of PVPY. We have tested the ability to multilayer formation of bifunctional 4-pyridines with 0, 2, and 3 methylene spacers between the pyridine heads. As a result (see Table 2), multilayers form at a much lower rate (ca. 3-times lower) than for the polymer PVPY; this rate is the same for the different lengths of the spacer. These results parallel those obtained with bicarboxylic acids (see above). The higher ability of the polymer is assigned to entropy factors, namely, to the availability of more heads on the same molecule for coordination to the CdSe crystal surface. 3.4.3. Mechanical Properties of Multilayers. The efficiency by which multilayers are produced is given by the differential absorbance data in Table 1. The formulation of these structures is hereafter also abbreviated as (linker/CdSe)n. All the structures are very uniform under AFM examination and robust enough to stand a standard sticky-tape test with no appreciable loss. Thickness analysis (see Table 3) has given results compatible with well-packed structures. The (Nafion/CdSe)n structure is the thickest one, with a differential thickness d0 of 8.5 nm layer-1 compatible with the Nafion thickness (6 nm) and the CdSe crystal size (3 nm). In agreement with this result, (PSSH/CdSe)n and (PVPY/CdSe)n multilayers produced from the same CdSe crystals display a differential thickness of 2.5 nm layer-1. The (PVPY/CdSe)n multilayer from 5 nm CdSe has given a differential thickness of 4 nm layer-1 in fact compatible with the increased diameter of the CdSe crystal. It is noticeable that the ratio of differential thickness and NP diameter (Nafion apart) for all the structures is 0.8, i.e., very close to the ideal value (√2/3) for a regular 3D hexagonal packing of NCs. 3.4.4. Core-Shell CdSe Multilayers. Regular multilayers have also been obtained from core-shell CdSe/CdS nanoparticles. We used amine-capped CdSe-NPs with a threelayer shell of CdS to decrease lattice strain, characterized by a maximum at 587 nm corresponding to a diameter of 4 nm for the CdSe core. Multilayers with Nafion or PVPY are built with linearity and high rates (differential absorbances) of 6.5 × 10-3 and 3 × 10-3 au layer-1, respectively. 3.5. Nanoparticle Size Modulation in CdSe Multilayers. The roughly 40% increased differential absorbance of core-shell CdSe/CdS nanoparticles compared with AC550 or OC550 layers (see above) is attributable to the higher size (4 vs 3 nm) of the nanocrystal diameter. We here show that, in general, the increase is in fact proportional with the diameter of the nanoparticle, which is a nontrivial outcome of high relevance for the matter.

Figure 4. Differential absorbance ∆A of (∆) ITO/(PVPY/CdSe)n and (O) ITO/(PAAH/CdSe)n multilayers vs NC diameter.

A linear increase in nanoparticle monolayer full coverage, expressed as mass per surface unit M0 (in g cm-2), for spherical nanoparticles of radius r is accounted for by simple geometry. The surface mass M0 is given by: Μ0 ) N0ΜNP ) N0F(4/3)πr3

(1)

where N0 is the surface density of nanoparticles, MNP the mass of the nanoparticle and F the material density. If ANP is the projected area occupied by the nanoparticle in a closepacked 2D hexagonal disposition, then N0 ) 1/ANP ) (1/2√3)r-2

(2)

and from eqs 1-2 Μ0 = Fr

(3)

which expresses the proportionality between surface mass and particle radius. Such proportionality has been confirmed using AC samples with 3 and 5 nm nanoparticle diameter and PAAH or PVPY as primer and linker. The differential absorbance of multilayer growth ∆A has been used as a measure of the surface mass M0. The plot of ∆A vs NC diameter, shown in Figure 4, shows the proportionality between them. Further evidence of the validity of this relationship has been obtained from the electrochemical oxidation of nanoparticle layers, reported in the following section. 3.6. Electrochemistry. 3.6.1. CdSe-NC Cast Films. The literature reports that CV measurements on TOP/TOPOcapped CdSe-NC films show a reversible reduction process.48 The process, which is assigned to electron injection into the LUMO 1Se and 1Pe shells of the quantum dots, is reversible using anhydrous and low-temperature conditions.48b In contrast, electrochemistry of CdS-NCs in solution49 has shown voltammetric responses to be irregular and irreversible because of decomposition of the particles on charge transfer, leading to multiple electron-transfer reactions. For our electrochemical analysis of cast or multilayer films, we have used acetonitrile + 0.1 M Bu4NClO4 at room temperature without special techniques to make the medium water-free. Under analogous conditions the literature reports for (TOP/TOPO)-capped CdSe-NCs cast films irreversible (48) (a) Yu, D.; Wang, C.; Guyot-Sionnest, P. Science 2003, 300, 1277. (b) Guyot-Sionnest, P.; Wang, C. J. Phys. Chem. B 2003, 107, 7355. (49) Haram, S. K.; Quinn, B. M.; Bard, A. J. J. Am. Chem. Soc. 2001, 123, 8860.

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Figure 6. Cyclic voltammograms of ITO/PAAH/CdSe layers (AC550) in acetonitrile + 0.1 M Bu4NClO4. Scan rate: 0.02 V s-1. Figure 5. Cyclic voltammogram of (- - -) thick and (s) thin CdSe-NC layers (OC550) cast on GC in acetonitrile + 0.1 M Bu4NClO4. Scan rate: 0.02 V s-1.

reduction (at Ep ) -0.7 V vs Ag/Ag+) and oxidation (at Ep ) 1.5 V) processes.50 In contrast, Bard et al.51 found that for analogous CdSe-NCs in solution oxidation is evidenced but not reduction. Still different is the response obtained recently for (TOP/TOPO)-capped tetrapodal CdSe-NC films,52 which have shown both oxidation and reduction peaks (at Ep ) 1.2 and -1.2 V), though in fact the oxidation is much more evident than the reduction. Therefore, it appears that although oxidation is clearly detected, reduction is not so definitely evident. We have in fact found that both amine- and oleate-capped samples cast on GC electrodes show only the oxidation process (at Ep ) ca. 1.2 V, as for the reported tetrapods,52 Figure 5), whereas no sign of reduction is detected down to -2.0 V, neither for these films nor for the multilayers described below. It must here be stressed that the oxidation is a massive and not a surface-confined process, involving the whole CdSe content of the NCs, as detailed in the following section. The agreement with previous results51,52 indicates that the irreversible oxidation is favored over the reduction, which may be accounted for by the absence of significant demolition processes (with multielectron transfer) in reduction compared with those in oxidation. Attempts to favor multielectron reduction via coordination of the eventually released selenide anions, such as, for example, the use of concentrated (1 M) NaClO4 in acetonitrile as electrolyte, failed. 3.6.2. CdSe-NC Layers. PAAH/CdSe. CV analysis of the ITO/PAAH/CdSe monolayer with 3 nm CdSe-NCs (amineor oleate-capped samples) has shown a sharp irreversible oxidative process (Figure 6) at Ep ) ca. 1.30 V, in which a net total charge of 0.3 mC cm-2 is involved. Because the PAAH sublayer does not give any contribution to the oxidation charge, the latter is totally due to the CdSe units. Considering that the CdSe coverage, evaluated from the optical response, is ca. 1.5 × 10-9 CdSe mol cm-2, the charge corresponds to the passage of two electrons per CdSe unit, (50) Kucur, E.; Riegler, J.; Urban, G. A.; Nann, T. J. Chem. Phys. 2003, 119, 2333. (51) Myung, N.; Ding, Z.; Bard, A. J. Nano Lett. 2002, 2, 1315. (52) Li, Y.; Zhong, H.; Li, R.; Zhou, Y.; Yang, C.; Li, Y. AdV. Funct. Mater. 2006, 16, 1705.

Figure 7. (a) Cyclic voltammograms and (b) relevant oxidative charges of ITO/(PAAH/CdSe)n multilayers (AC550, n ) 1-5) in acetonitrile + 0.1 M Bu4NClO4. Scan rate: 0.02 V s-1.

i.e., to the total demolition of the CdSe-NCs constituting the monolayer, according to the equation CdSe - 2e f 1/n(Se)n + Cd2+

(4)

ITO/(PAAH/CdSe)n multilayers (Figure 7a) show a progressive positive shift of the oxidation peak potential and a linear increase of the oxidation charge with the number of layers n (Figure 7b). From Figure 7a, it may be observed that as n increases, the CV evidence a lower process (A) at ca. 1.2 V followed by the main one (B) at progressively higher potentials. We assign these processes to the oxidation of the outer and inner CdSe units in the nanocrystal, respectively, the former being stabilized (and therefore cathodically shifted) by coordination of the produced Cd2+ ions to the surrounding PAAH molecules. Similarly, in thin cast films of CdSe-NCs, where the oxidation is distinguished into two peaks (Figure 5), stabilization of the oxidized surface CdSe units is provided by the capping oleate anions. Anodic stripping may be performed also on thicker multilayers (n > 5) but electrolyte permeation becomes increasingly difficult so that CV analysis is no longer practical. In any case, the amount of CdSe-NCs can be evaluated by potentiostatic oxidation at 2.0 V. In a typical


Chem. Mater., Vol. 21, No. 11, 2009 2267

The suggestion that peak A is due to oxidation of the surface CdSe units of the 5 nm particles, already put forward in the case of PAAH (see above), has been confirmed by deconvolutive analysis of the CV (Figure 8b). After subtraction of the bare ITO background, the CV has been analyzed by means of a fitting model consisting of the sum of two isoelectronic surface-confined processes A and B with Nernstian shape. At room temperature, the relationship is therefore i ) IAexp[-3.5(E - EpA)/WA]/(1 + exp[-3.5(E EpA)/WA])2 + IBexp[-3.5(E - EpB)/WB]/(1 + exp[-3.5(E - EpB)/WB])2 (5)

Figure 8. (a) Cyclic voltammogram of ITO/MDPH/CdSe layerl (AC610) in acetonitrile + 0.1 M Bu4NClO4. Scan rate: 0.02 V s-1; (b) curve fitting (X) with eq 5. χ2 ) 0.00027; R2 ) 0.99699.

25-layer sample, we have obtained a total stripping charge of 7.5 mC cm-2, corresponding once more to 0.3 mC cm-2 per layer. We have also tested samples in which CdSe is 5 nm diameter. The response is the same apart from the stripping charge, which clearly follows the higher CdSe coverage (linear with radius) compared with the 3 nm NCs. Accordingly, the charge is 0.5 mC cm-2 per layer. All these results show that the stripping analysis is independent of the CdSe size and preparation protocol. PVPY/CdSe. CV analysis of the ITO/PVPY/CdSe monolayer from the same 3 nm CdSe-NCs has also shown in this case an irreversible oxidative process (at Ep ) 1.40 V) in which a higher total charge (0.6 mC cm-2) is involved. After subtraction of the oxidation charge of the PVPY sublayer (ca. 0.1 mC cm-2, see before) and considering the optically evaluated CdSe coverage, the measured charge confirms the passage in this case of two electrons per CdSe unit. Similarly, CV analysis of ITO/(PVPY/CdSe)n multilayers has evidenced a progressive positive shift of the oxidation potential and a linear increase of the oxidation charge with n, as expected from a complete CdSe-NC oxidative stripping. Nafion/CdSe and MDPH/CdSe. As for carboxylate, also in the cases of CdSe-supporting sulfonate (e.g., Nafion) or phosphonate (e.g., DMP) monolayer, no contribution to the CdSe oxidation (stripping) charge is provided. CV of the ITO/Nafion/CdSe monolayer shows the twoelectron irreversible oxidative process of CdSe units at Ep ) 1.7 V. The peak potential is anodically shifted from those of the PAAH- and PVPY-supported layers by the ohmic drop expected from the higher polymer film thickness. In the case of the ITO/MDPH/CdSe monolayer, for which we have used AC610 samples, CV shows a main irreversible oxidative process (B) at Ep ) 1.40 V, as for the other cases, after a lower process (A) peaking at Ep ) 0.70 V (Figure 8a). The analysis has shown that a charge of two electrons per CdSe unit is involved in the sum of the two processes.

with three fitting parameters (I, W, and Ep) for each component. As can be seen in Figure 8b, the model fits the experimental data nicely. From fitting analysis, the half-height width W is 0.5 and 0.3 V for A and B, respectively, which may suggest a stronger mutual interaction among CdSe units at the surface compared with those in the bulk. From the combination of I and W, the fitting gives a distribution of 20 and 80% to processes A and B, respectively. The result has been compared with the theoretical one, calculated on a geometrical basis as follows. The Cd-Se bond length in CdSeNCs is 0.36 nm so that the unit area is 0.26 nm2 and the coverage of the 5 nm diameter NC is 300 units. On the other hand, the total content of CdSe units of the NC is 1200, so that the theoretical percent of A turns out to be 25%. The result is in very good agreement with the measured one. The strongly negative shift of the A oxidation potential in the MDPH phosphonate heads in comparison with the PAAH carboxylate caps (see above) is accounted for by the stronger coordination of Cd2+ ions with phosphonate. This suggestion corresponds to the observation that phosphonic acid binds more strongly to the indium and tin dangling bonds of an ITO surface than carboxylic acid does.35 3.7. Photoluminescence. 3.7.1 Solution PL. The roomtemperature absorption and PL spectra of amine-capped CdSe crystallites in CHCl3 solution is shown in Figure S1a of the Supporting Information. The PL spectrum shows a narrow (30 nm wide) exciton response red-shifted by only 20 nm from the absorption maximum. The PL-QY is appreciably high (10%), in line with those normally found in such samples.2c,53 Figure S1b of the Supporting Information shows the analogous spectra of oleate-capped CdSe crystallites. The PL-QY is comparable, but the PL spectrum displays an emission tail at lower energies. The sharp emission band we observed at the absorption onset, i.e., with a negligible Stokes shift, in the amine-capped samples, is due to the radiative recombination of free charge carriers (band-edge PL). The PL spectrum of the oleatecapped CdSe-NCs has an additional red-shifted PL tail arising from the radiative recombination of trapped charge carriers (surface-trap PL). 3.7.2. Multilayer PL. The PL of the multilayer films has been investigated with the following results. (53) Hoheisel, W.; Colvin, V. L.; Johnson, C. S.; Alivisatos, A. P. J. Chem. Phys. 1994, 101, 8455.

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Figure 9. PL and absorption spectra (normalized) of (a) ITO/(PVPY/CdSe)n multilayer (AC610) and (b) ITO/PVPY/(CdSe/PAB)n multilayer (OC550).

No PL has been recorded for sulfonate-, phosphonate- or thiolate-CdSe multilayers. On the contrary, a strong PL (for this family of CdSe nanocrystals), with the same features of the CdSe-NCs in dispersion (Figure 9), has been recorded for (PVPY/CdSe)n. A still appreciable response is given by (PEI/CdSe)n, (PAB/CdSe)n, and (PAAH/CdSe)n structures, ca. 4- and 6-times weaker for the amine and carboxylate systems, respectively. The effects of Lewis bases on radiative recombination in CdSe quantum dots54b has in fact shown that long-chain primary amines are the most effective capping agents for luminescent CdSe QDs in nonpolar solvents. In particular for (TOP/TOPO)-capped CdSe QDs the PL-QY of 5-15% increases to over 50% upon addition of hexadecylamine (HDA).55 On the contrary, alkanethiols generally quench the PL of CdSe nanocrystals.54 CdSe-NC surface passivation is accomplished by capping the particle surface with a shell of a larger band gap semiconductor (e.g., ZnS) or an organic coordinating molecule. It appears that the nucleophilic power of the coordinating functionality on the polymer is the key factor for PL enhancement. Only strongly coordinating ends, such as amine, pyridine, and carboxylate, can block the recombination centers, whereas scarcely nucleophilic anions such as phosphonate or sulfonate are weakly coordinated and allow recombination. We recall here that treatment with hydroxide, a very small and strongly basic molecule, is known to increase strongly the PL-QY in CdS-NCs.56 3.7.3. Aggregation-PL Correlation by AFM Analysis. Aggregating colloids often fuse together and form larger units with bulk properties. Systems in which the closely connected nanoparticles maintain their original size and individuality are of great current interest for the design of nanostructured (54) (a) Kalyuzhny, G.; Murray, R. W. J. Phys. Chem. B 2005, 109, 7012. (b) Bullen, C.; Mulvaney, P. Langmuir 2006, 22, 3007. (c) Munro, A. M.; Plante, I. J. L.; Ng, M. S.; Ginger, D. S. J. Phys. Chem. C 2007, 111, 6220. (d) Wuister, S. F.; Donega, C. D.; Meijerink, A. J. Phys. Chem. B 2004, 108, 17393. (55) (a) Qu, L.; Peng, X. J. Am. Chem. Soc. 2002, 124, 2049. (b) de Mello Donega’, C.; Hickey, S. G.; Wuister, S. F.; Vanmaekelbergh, D.; Meijerink, A. J. Phys. Chem. B 2003, 107, 489. (56) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 5649, and refs therein.

Zotti et al.

materials. PL and UV-vis spectroscopy combined with AFM analysis are powerful tools to show the presence of nanoscopic cross-linked nanostructures possibly precipitated from solution. AFM analysis of both fluorescent glass/MTS/(CdSe/ PVPY)20 and PL-quenched glass/MTS/(CdSe/HDT)10 multilayers, showing the same UV-vis spectra of the original solution, has also shown similar domains ca. 50 nm wide with the same roughness of ca. 3 nm. As a result, PL quenching in the multilayers cannot be merely attributed to aggregation but must be assigned to the type and strength of bonding with the intervening capping polymer, as pointed out above. 3.8. Photoelectrochemistry. Photoelectrochemistry is a powerful tool for the investigation of the charge transfer ability of semiconductor (including CdSe-NCs) layers. Thus, for instance, photoelectrochemistry of a film cast from a hybrid C60-CdSe conjugate has shown a significantly enhanced photocurrent compared with the film of CdSe-NCs as well as that of C60 alone.57 The photovoltaic properties of our hybrid multilayers have been investigated in a liquid-junction arrangement, i.e., immersed in aqueous solution containing oxygen as a photoelectron acceptor.58 Under illumination, charge separation is measured as a photocurrent in the external circuit. Light of energy greater than the band gap in the nanoparticle creates an excited state, which can decay via intraparticle recombination of the electron and hole or via transfer of the electron to the electrode. If an electron acceptor (or “electron scavenger”, oxygen in our case) is present in the solution, the excited electron can be transferred to the solution, generating a photocurrent in the external circuit. Though the electrochemical junction presents the advantage of an easy realization, it is well-known that semiconductor NCs undergo photoanodic dissolution in the presence of oxygen. The relevant mechanism has previously been clarified for the case of CdS.56 Oxygen acts as an acceptor of the electrons that are generated by light absorption and also as a scavenger of the positive holes that remain. It is this double action of oxygen that makes the dissolution rather efficient in the presence of this solute. On the same basis, CdSe-NCs will dissolve in aerated solutions upon illumination (eq 6). hν

CdSe + 2O2 98 Cd2 + SeO24

(6)

We have in fact observed that the absorption spectrum of CdSe-NCs changes upon illumination. As reported previously,56,59 the absorption maximum is shifted toward shorter wavelengths while absorbance decreases. In practice, the colloidal particles become smaller during the photoanodic dissolution showing the typical size quantization effects. In search of a better system, we discarded another commonly used electron acceptor such as methylviologen, (57) Liu, D.; Wu, W.; Qiu, Y.; Lu, J.; Yang, S. J. Phys. Chem. C 2007, 111, 17713. (58) Zotti, G.; Schiavon, G.; Mengoli, G. Mol. Cryst. Liq. Cryst. 1985, 121, 341. (59) Sharma, H.; Sharma, S. N.; Singh, G.; Shivaprasad, S. M. J. Nanosci. Nanotechnol. 2007, 7, 1953.


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-1

Figure 10. Linear sweep voltammogram (V ) 0.02 V s ) under chopped light for ITO/(PVPY/CdSe)5 multilayer (from AC610) in O2-saturated 0.1 M NaClO4.

because although it does not promote the photoanodic dissolution in the absence of oxygen, it accelerates the reaction substantially in its presence. Another practical possibility, namely that of using sulfide as hole scavenger, as for example in photoelectrochemistry of CdSe-sensitized titania,60 was discarded because our CdSe-NCs layers are not stable in 0.1 M Na2S solution. In fact, after a few minutes, the spectrum shows a bathochromic shift from, for example, 560 to 580 nm, indicating the occurrence of aggregation of NCs by the sulfide. For these reasons, we still used oxygen as acceptor but care was taken to minimize degradation, such as using low light power and short exposure times. Optical controls before and after analysis assured that degradation was in fact minimal. As a typical example, Figure 10 shows the current-potential plot of an ITO/(PVPY/CdSe)5 (from AC610) electrode in a O2-saturated aqueous 0.1 M NaClO4 solution. The electrode was illuminated on the solution side with chopped light from a halogen lamp. Light-induced reduction currents on a low background current were observed between -0.6 and 0.3 V vs SCE, with no dependence on stirring, i.e., with no mass transfer control, and no appreciable degradation during short time examination (some pulses of some seconds duration). The applied negative bias, with its charge-compensating action, helped keeping the material stable. Positive photovoltages (ca. 0.2-0.3 V) are also recorded, as expected for a photoreducing electrode. Moreover, no photoresponse was recorded under nitrogen. At potentials more positive than 0.3 V, anodic photocurrent was observed, corresponding possibly to anodic dissolution of the CdSe quantum dots. For this reason, investigations were performed in reduction, typically at a -0.2 V applied potential. 3.8.1. CdSe-NC Multilayers. Photocurrents at different multilayers are such that OC or AC samples display similar responses in the range 1-5 µA cm-2. In contrast with the multilayers, directly cast layers of CdSe-NC samples, with comparable optical density (e.g., 4 × 10-3 au), prepared by (60) (a) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2006, 128, 2385. (b) Kongkanand, A.; Tvrdy, K.; Takechi, K.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2008, 130, 4007.

Figure 11. Photocurrent of (O) ITO/PVPH/CdSe/(PSSH/CdSe)n-1 and (∆) ITO/(PVPY/CdSe)n multilayers vs n for (a) OC550 and (b) AC580 in O2saturated 0.1 M NaClO4.

Figure 12. Bilogarithmic plot of photocurrent of ITO/PVPH/CdSe/(PSSH/ CdSe)5 (OC550) vs incident light power W.

dip-coating from CHCl3 dispersion, display photocurrents of 0.1 µA cm-2 or lower, an indication of a slower photoelectron transfer, likely due to the higher distance among NPs. To investigate the dependence of photocurrent on the layer sequence as an approach to evaluate the photoconductive properties of the multilayers, we tested two series of multilayers, namely the (PVPY/CdSe)n series with the most efficient PL and the (PSSH/CdSe)n series, i.e., its counterpart with no PL. The optical density per layer for the two series is the same so that no correction for differences in power absorption was needed. In (PVPY/CdSe)n (from AC610 and OC550) multilayers, the photocurrent is the same (3 and 0.2 µA cm-2, respectively) for n from 1 to 5. The absence of an increase of photoresponse with the amount of absorbing material appears to evidence localization of photoelectron transfer at the film/ water interface, as a consequence of a slow exchange between adjacent layers. On the contrary, the (PSSH/CdSe)n multilayers built from OC550 display the same photocurrent for the first layer (0.2 µA cm-2), which increases linearly with n (up to at least n ) 5) by 0.8 µA cm-2 layer-1 (Figure 11a). The photocurrent increases with light power W (in the range 1-100 mW cm-2) with a linear log-log plot sloping unity (Figure 12). This result, which indicates that there is no appreciable recombination of photocarriers,61 supports the idea of free diffusion of carriers between layers. (61) Ito, H.; Niimi, Y.; Suzuki, A.; Marumoto, K.; Kuroda, S. Thin Solid Films 2008, 516, 2743.

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Figure 13. Photocurrent transients (normalized) of ITO/PVPH/CdSe/(PSSH/ CdSe)n-1 multilayers (AC580) in O2-saturated 0.1 M NaClO4 for n ) (a) 1 and (b) 5.

The (PSSH/CdSe)n multilayers built from AC580 display a photocurrent response much higher than for the OC550 case (3.5 µA cm-2 for the first layer), which increases to 7 µA cm-2 layer-1 at n ) 2. Anyway, the response decreases impressively for subsequent layers (Figure 11b). Also in this case, the photocurrent increases with light power W with a linear log-log plot sloping unity but the photocurrent transient is fast (

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