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Fabrication of Organic-Inorganic Semiconductor Composites Utilizing the Different Aggregation States of a Single Amphiphilic Dendrimer Jack J. J. M. Donners,† Richard Hoogenboom,† Albert P. H. J. Schenning,† Paul A. van Hal,† Roeland J. M. Nolte,†,‡ E. W. Meijer,† and Nico A. J. M. Sommerdijk*,† Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands, and Laboratory of Organic Chemistry, University of Nijmegen, Nijmegen, The Netherlands Received June 29, 2001. In Final Form: November 5, 2001 A third generation poly(propylene imine) dendrimer functionalized with oligo(phenylene vinylene) groups is used as a stabilizing agent in the fabrication of CdS nanoparticles. The solvent-dependent conformational behavior of this electroactive dendrimer is used to generate various organic-inorganic semiconductor composites. In chloroform solution, CdS spherical nanoparticles are formed, whereas in a chloroform/ methanol mixture (6:1, v/v) platelike structures evolve. At the air-water interface, nanoparticulate films with a leaf-nerve-like organization of nanoparticles are formed, whereas vesicles generated in aqueous media become overgrown with CdS.
Introduction The production of semiconductor nanoparticles and their organization are of great interest for the fabrication of nanoelectronic devices.1,2 Tailoring of the photophysical properties of these nanoparticles is required to optimize the matching between their energy levels and those of the electroactive polymers in such devices. The electronic and optical properties of the nanoparticles are related to their size and morphology, and consequently, control over the crystallization process is an important issue in their fabrication.3 Many examples are known, for example, in the field of biomineralization, in which surfactants act as control agents in mineralization processes through the formation of complexes with inorganic ions.4 Also, during nanoparticle synthesis surfactants have been used as surface capping agents in order to stabilize the nanoparticles and to prevent coagulation. For the preparation of these particles in solution, zeolites,5 micelles,6 membranes,7 lyotropic phases,8 and polymers9 were utilized. Cadmium sulfide is one of the best-characterized semiconducting quantum dot materials. However, only a few examples in which cadmium sulfide nanoparticles are combined with conjugated polymers have been reported.10 Here, we report the use of an amphiphilic dendrimer comprising a dendritic poly(propylene imine) core functionalized with oligo(phenylene vinylene) (OPV) groups † ‡
Eindhoven University of Technology. University of Nijmegen.
(1) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (2) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (3) (a) Reactions and Synthesis in Surfactant Systems; Texter, J., Ed.; Marcel Dekker: New York, 2001. (b) Handbook of Microemulsion Science and Technology; Kumar, P., Mittal, K., Eds.; Marcel Dekker: New York, 1999. (4) Mann, S. Nature 1993, 365, 499. (5) Herron, N.; Wang, Y.; Eddy, M. M.; Stucky, G. D.; Cox, D.; Moller, K.; Bein, T. J. Am. Chem. Soc. 1989, 11, 350. (6) Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990, 23, 183. (7) Zhao, X. K.; Baral, S.; Rolandi, R.; Fendler, J. H. J. Am. Chem. Soc. 1988, 110, 1012. (8) Braun, P. V.; Osenar, P.; Stupp, S. I. Nature 1996, 380, 325. (9) Yuan, Y.; Fendler, J. H.; Cabasso, I. Chem. Mater. 1992, 4, 312.
(1, see Chart 1) in the generation of CdS nanoparticles. Like dendrimers modified with alkyl groups, these electroactive macromolecules consist of a polar core and an apolar periphery and consequently also behave as macromolecular surfactants showing very similar aggregation behavior.11,12 When dissolved in an organic solvent, the molecules behave as unimolecular inverted micelles. In water, the molecules adopt a geometry in which the hydrophilic core is in contact with water while the alkyl chains are oriented away from the aqueous phase, resulting in the formation of vesicular aggregates. Utilizing the amphiphilic nature of this electroactive dendrimer, we set out to generate composites of organic and inorganic semiconductor materials and to study the interaction between the component parts of the resulting hybrid materials. Experimental Section Cadmium chloride hydrate was purchased from Riedel-De Haen. Hydrogen sulfide 2.8 was purchased from Hoekloos. Sodium sulfide nonahydrate was purchased from Acros. Chloroform p.a., methanol p.a., and THF p.a. were purchased from Biosolve and were used as received. Ultrapure water was generated using a Barnstead Easypure LF water purification system. UV/vis spectra were recorded on a Perkin-Elmer Lambda 40 instrument. Fluorescence spectra were recorded on a PerkinElmer LS 50 B instrument. Transmission electron micrographs of unstained samples were recorded on a JEOL JEM 2000 FX at 120 kV. CdS Particle Formation in Chloroform Using the H2S Route. An amount of dendrimer corresponding to a concentration of 1.2 mM in amine groups was dissolved in 30.0 mL of chloroform. An equimolar amount of cadmium chloride hydrate was dissolved (10) (a) Greenham, N. C.; Peng, X.; Alivisatos, A. P. Phys. Rev. B 1996, 54, 17628. (b) Kumar, N. D.; Joshi, M. P.; Friend, C. S.; Prasad, P. N.; Burzynski, R. Appl. Phys. Lett. 1997, 71, 1388. (c) Narayan, K. S.; Manoj, A. G.; Nanda, J.; Sarma, D. D. Appl. Phys. Lett. 1999, 74, 871. (d) Winiarz, J. G.; Zhang, L.; Lal, M.; Friend, C. S.; Prasad, P. N. J. Am. Chem. Soc. 1999, 121, 5287. (11) Schenning, A. P. H. J.; Elissen-Roman, C.; Weener, J.-W.; Baars, M. W. P. L.; van der Gaast, S. J.; Meijer, E. W. J. Am. Chem. Soc. 1998, 120, 8199. (12) Schenning, A. P. H. J.; Peeters, E.; Meijer, E. W. J. Am. Chem. Soc. 2000, 122, 4489.
10.1021/la010997z CCC: $22.00 © 2002 American Chemical Society Published on Web 01/23/2002
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in a minimal amount of methanol and added to the dendrimer solution. The resulting solution was placed in a closed chamber, purged with nitrogen, and equilibrated for 1/2 h. Subsequently, 500 µL of hydrogen sulfide was added by slow diffusion from a syringe, and the particles were allowed to form overnight. The solution was purged with argon to remove the residual hydrogen sulfide. Transmission electron microscopy (TEM) grids were prepared by placing one drop of solution on a carbon-covered copper grid followed by immediate drainage. CdS Particle Formation in Chloroform by the Na2S Route. An amount of dendrimer corresponding to a concentration of 1.2 mM in amine groups was dissolved in 30.0 mL of chloroform and maintained under an argon atmosphere. Stock solutions of cadmium chloride hydrate and sodium sulfide nonahydrate in methanol were prepared. Subsequently, 500 µL aliquots of both stock solutions were added at 2 min intervals. In total, five additions were made. TEM grids were prepared by placing one drop of solution on a carbon-covered copper grid followed by immediate drainage. CdS Particle Formation under Langmuir Monolayers. An amount of 1.1 mg of dendrimer was dissolved in 50 mL of chloroform. On a subphase of 11 mM cadmium chloride, 100 µL of the dendrimer solution was spread. After 1 h of equilibration and purging with nitrogen, the system was exposed to 200 µL of H2S. Subsequently, the brittle yellow film was transferred to quartz slides (for photophysical characterization) or carboncovered copper grids (for TEM) by Langmuir-Schaeffer dipping. CdS Particle Formation in Water. An amount of dendrimer containing 13 µmol of amine groups was dissolved in 200 µL of THF and injected into 3.0 mL of water. The dispersion was heated in order to remove the THF and vortexed. Subsequently, the dispersion was added to 27.0 mL of 0.5 mM cadmium chloride solution. The mixture was placed in a closed chamber, purged with nitrogen, and equilibrated for 1 h. Subsequently, 500 µL of hydrogen sulfide was added by slow diffusion from a syringe. After 18 h, the chamber was purged with argon to remove residual hydrogen sulfide. TEM grids were prepared by placing a drop of the dispersion on a carbon-covered copper grid followed by immediate drainage. 1H NMR Experiments. An amount of 1.5 mg of dendrimer 2 was dissolved in 1.5 mL of CDCl3. To this solution, 100 µL of a saturated CdCl2 solution in CD3OD was added. 1H NMR spectra were subsequently recorded on a Varian Gemini 300 MHz spectrometer and compared to spectra of solutions containing only dendrimer and an identical amount of CD3OD.
Langmuir-Blodgett Experiments. Monolayer experiments were performed using a themostatted double-barrier R&K trough (6 × 25 cm). The surface pressure of the monolayers was measured using a Wilhelmy plate, which was calibrated with octadecanol. On a subphase of 11 mM cadmium chloride in water, 50 µL of a dendrimer solution (1.2 mM in nitrogen) in chloroform was spread. After 1 h of equilibration, the monolayer was compressed at a speed of 8.8 cm2/min.
Results and Discussion CdS Particle Formation in Chloroform Solution. Chloroform solutions of the dendrimers were equilibrated with cadmium ions equimolar with respect to the amine groups in the dendritic core. Upon addition of cadmium ions, no shifts were observed in the 1H NMR spectrum of the dendrimer, suggesting that the dendrimer core acts as a solubilizing environment for the cadmium ions rather than as a selective complexation agent. The precipitation of nanoparticles was established by the exposure of the equilibrated mixture to hydrogen sulfide. TEM studies revealed the formation of CdS clusters with sizes ranging from 5 to 20 nm for both types of dendrimer (Figure 1a). UV/vis characterization of the G3-OPV3/CdS was hampered by the OPV3 chromophore absorbance at 420 nm ( ∼ 1 × 105), which dominates the cadmium sulfide absorption ( ∼ 1000). Therefore, the third-generation palmitoyl-functionalized dendrimer (2) was used to generate a model system for the 1/CdS nanocomposites in which the photophysical properties can be studied. The UV/vis spectrum of the yellow 2/CdS composite showed an adsorption edge at 435 nm (Figure 1b). From this value, a band gap of 2.86 eV was calculated, which according to Henglein corresponds to a CdS particle size of 3.0 nm.13 The dimensions of the CdS nanoparticles exceed the hydrodynamic diameter determined for third-generation poly(propylene imine) dendrimers (2.36 nm),14 implying that the growth of the inorganic particles is not confined (13) Henglein, A. Chem. Rev. 1989, 89, 1861. (14) Scherrenberg, R.; Coussens, B.; van Vliet, P.; Edouard, G.; Brackman, J.; de Brabander, E. Macromolecules 1998, 31, 456.
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Figure 1. (a) TEM micrograph of 1/CdS composites in chloroform. (b) UV/vis spectra of 1/CdS (dot) and 2/CdS (solid). (c) Fluorescence emission spectra of 1 (solid), 1/Cd2+ (dash), and 1/CdS (dot-dash) (excitation at 420 nm). (d) Schematic representation of the proposed composite formation process in chloroform solution.
to the dendritic core. Furthermore, no changes in size or in band gap were observed using different generations of dendrimer, indicating that in this system the dendrimers do not act as defined container molecules in which the CdS precipitates. This is in contrast to literature reports in which the arrested precipitation of inorganic nanoparticles inside the dendrimers was described.15 Control experiments using N,N-dimethyldodecylamine led to the formation of particles with a wide size distribution, indicating that the use of dendrimers allows for a higher degree of control over nanoparticle synthesis than an individual component. The composite of alkylated dendrimer 2 and CdS displayed a broad weak emission spectrum upon excitation at 350 nm with maxima at 504 and 590 nm. For the nanoparticles grown in the presence of 1, the fluorescence spectrum was dominated by the emission of the OPV3 groups; that is, upon excitation at wavelengths between 350 and 420 nm in all cases a fluorescence maximum at 530 nm was observed (Figure 1c). Interestingly, before exposure to H2S the 1/Cd2+ mixture displayed a fluorescence maximum at 505 nm. The red shift of the fluorescence maximum of the OPV3 chromophores during the mineralization process is indicative of an increase in the degree of stacking of the chromophores.12 This suggests that during the growth of the particles the dendrimers change their geometry from the inverted micellar conformation to the “cometlike” conformation they adopt in aqueous media (Figure 1d). TEM studies on 1/Cd2+ demonstrated that no inverted micellar structures are formed prior to the mineralization process. Importantly, no significant quenching of fluorescence of the OPV3 chromophores by CdS was observed, which indicates that no electronic interaction between the OPV3 chromophores and the nanoparticles occurs in the organic medium. The slight decrease in fluorescence intensity compared to a CHCl3 solution of the electroactive dendrimer was attributed to self-quenching by aggregated (15) Arrested precipitation in dendrimer cores was successful using PAMAM dendrimers: (a) Sooklal, K.; Hanus, L. H.; Ploehn, H. J.; Murphy, C. J. Adv. Mater. 1998, 10, 1083. (b) Huang, J.; Sooklal, K.; Murphy, C. J. Chem. Mater. 1999, 11, 3595. (c) Hanus, L. H.; Sooklal, K.; Murphy, C. J.; Ploehn, H. J. Langmuir 2000, 16, 2621.
OPV3 chromophores. Hence, dendritic surfactants appear to simply act as stabilizing macromolecular capping agents for the nanoparticles, although at this stage we cannot exclude that the distance between the OPV3 and the CdS is such that there is only a very weak interaction making electron-transfer inefficient. CdS Nanoparticle Formation Using the Na2S Route. From aggregation studies in water, it is known that upon protonation of the dendritic core a conformational rearrangement to the cometlike conformation takes place, leading to the formation of vesicles.11,12 Since in the described synthetic route protons are released by H2S, the CdS synthesis was repeated using Na2S as the sulfide source. A typical procedure involved the dissolution of the dendrimer in 30 mL of chloroform whereafter 500 µL aliquots of Cd2+ and S2- in methanol were added with 2 min intermissions. In total, five additions were made, amounting to 1 equiv of both Cd2+ and S2- (with respect to the core nitrogens).16 TEM revealed the formation of platelets with an average length of 200 nm and widths ranging from 30 to 150 nm for both dendrimers (Figure 2a). From the onset of absorption of the UV/vis spectrum of the composite based on the alkylated dendrimer 2, a band gap of 3.37 eV was estimated (Figure 2b). The corresponding particle diameter as estimated by the relationship of Henglein is 2.0 nm. Synthesis of CdS using H2S under similar conditions led to the formation of 3.0 nm particles identical to those observed in pure chloroform. The photoluminescence (PL) spectrum exhibits a maximum at 485 nm (Figure 2c). The UV/vis and fluorescence emission spectra of 1/CdS were identical before and after nanoparticle synthesis, and both indicate a dendrimer conformation in which the OPV3 chromophores are stacked (Figure 2c). TEM studies on 1 in methanol/chloroform demonstrated that also in the absence of Cd2+ platelike aggregates are formed (Figure 2d). Moreover, the addition of methanol to a chloroform solution of the OPV3-modified dendrimer induced a red shift in the PL spectrum, pointing to a solvent-induced (16) In this synthetic route, a larger quantity of methanol is used (i.e., 5 mL instead of 0.2 mL) resulting in a solvent mixture of chloroform/ methanol 6:1 (v/v).
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Figure 2. (a) TEM micrograph of 1/CdS platelets formed in chloroform/methanol and selected area electron diffraction of 1/CdS (inset). (b) UV/vis spectra of 1/CdS (dot) and 2/CdS (solid) platelets. (c) Fluorescence emission spectra of 1 platelets (solid), 1/CdS (dot), and 2/CdS (dash). (d) TEM micrograph of 1/Cd2+ aggregates formed in chloroform/methanol. (e) Schematic representation of the proposed composite formation process in chloroform/methanol.
conformational rearrangement of the dendrimer to the comet conformation. Fluorescence titration experiments indeed indicated that 4% of methanol is required to form the platelike aggregates, whereas methanol contents below 2% do not influence the conformation of the dendrimer. Apparently, upon addition of sulfide ions CdS particles are formed within the already existing aggregates. Electron diffraction of the platelets showed a simple spot pattern, which can be interpreted to originate from wurtzite-type CdS crystals in different orientations (inset of Figure 2a). The formation of such a spot pattern rather than a ring-type pattern typical of a polycrystalline sample suggests that a limited number of highly ordered domains of nanoparticles exist within the platelike structure. Unfortunately, TEM did not resolve the individual particles. We propose that in this case, the dendrimer aggregate forms a restricted reaction environment for the arrested precipitation of CdS. Moreover, the individual particles are organized in a well-defined fashion. Although the formation of an organized array of quantum dots is the most plausible explanation for the observed results, it cannot be excluded that a large single CdS crystal is formed (Figure 2e). However, the estimated band gap is lower than the bulk band gap that would be expected in such a case. CdS Formation under Langmuir Films. It has been demonstrated that both dendrimers can form close-packed Langmuir films at the air-water interface, and these may act as suitable templates for the formation of CdS crystals.17 Surface pressure-surface area isotherms were recorded on an 11 mM cadmium chloride subphase in order
to investigate the influence of cadmium ions on the behavior of the dendritic core in aqueous media. Molecular areas observed for 1 and 2 were 13.75 and 4.50 nm2, respectively (Figure 3a). These values both are in good agreement with values obtained for these compounds on a water subphase,11,12 again indicating that also in this configuration no specific interaction between the dendrimer core and the cadmium ions is observed. The difference in headgroup size between the two compounds is due to the fact that the molecular area is determined by the volume of the apolar part and not by the dendritic core. Hence, though the templating core is identical, packing of the dendrimers in the monolayer may be different. Upon exposure to H2S, a brittle nanoparticulate film was formed for both dendrimers which was transferred to quartz slides and TEM grids by LangmuirSchaeffer dipping. UV/vis spectroscopy on transferred composite films showed an absorption edge at 515 nm for both 1 and 2, indicating the formation of bulk CdS (Figure 3b). Upon excitation at 420 nm, weak bands at 450 and 540 nm were observed for 1/CdS whereas the PL spectrum of 2/CdS exhibited a maximum at 430 nm (Figure 3c). TEM revealed the formation of distorted spherical particles with dimensions of 20-60 nm, which were organized in a structure that resembles the nerves of leaves (Figure 3d). Apparently, the absence of specific interactions (17) (a) Yang, J.; Meldrum, F. C.; Fendler, J. H. J. Phys. Chem. 1995, 99, 5500. (b) Zhao, X. K.; Xu, S.; Fendler, J. H. Langmuir 1991, 7, 520. (c) Guo, S.; Konopny, L.; Popovitz-Biro, R.; Cohen, H.; Porteanu, H.; Lifshitz, E.; Lahav, M. J. Am. Chem. Soc., 1999, 121, 9589. (d) Bekele, H.; Fendler, J. H.; Kelly, J. W. J. Am. Chem. Soc. 1999, 121, 7266.
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Figure 3. (a) Langmuir isotherms of 1 (dot) and 2 (solid) on a 11 mM Cd2+ subphase. (b) UV/vis spectra of films of 1/CdS (dot) and 2/CdS (solid). (c) Fluorescence emission spectra of films of 1/CdS (solid) and 2/CdS (dash). (d) TEM micrograph of transferred nanoparticulate film of 1/CdS; inset: low magnification view showing the nervelike structure.
Figure 4. (a) TEM micrograph of 1/CdS composites in water. (b) UV/vis spectra of 1/CdS (dash) and 2/CdS (solid). (c) Fluorescence emission spectra of 1 (solid), 1/Cd2+ (dash), 1/CdS (dot-dash), and 1/CdS (exposure to excess H2S) (dot).
between the dendrimers and the cadmium ions leads to the formation of bulk material in which the particle size is limited only by the extent the stabilizing effect of the dendrimer exerts into the subphase. The origin of the
organization of these particles is unknown, but it may be an expression of the monolayer structure. Attempts to elucidate this relationship have been unsuccessful so far. CdS Particles Grown on Vesicles. To further explore the versatile aggregation behavior of these dendrimers in the fabrication of CdS, 1 and 2 were dispersed in water. In analogy to the alkylated dendrimer 2, the OPV3modified dendrimer 1 also formed vesicles with diameters of 100-160 nm (not shown).11 When CdS nanoparticles were grown on these aggregates, TEM revealed that these vesicles became overgrown with cadmium sulfide (Figure 4a). Scattering due to the presence of the vesicles hampered the UV/vis characterization of these particles. After solubilization of the organic component by the addition of 2-propanol, UV/vis spectroscopy revealed the presence of a weak, broad band with an onset of 515 nm, corresponding to a band gap of 2.42 eV, typical for bulk CdS (Figure 4b). The UV/vis spectrum of 1/CdS (Figure 3b) was again dominated by the absorption band (λmax ) 400 nm) of the OPV3 units. No fluorescence was observed for 2/CdS upon excitation at wavelengths ranging from 300 to 400 nm. This phenomenon has been observed previously and was attributed to nonradiative recombination of charge carriers immobilized in traps at the surface.13 The fluorescence spectra of 1, 1/Cd2+, and 1/CdS (excitation at 400 nm) yielded an emission band of the OPV3 chromophores at 530 nm in each case (Figure 4c). However, the signal intensity was invariant upon addition of Cd2+, while partial quenching of the OPV3 fluorescence was observed upon formation of CdS nanoparticles. This observation is indicative of an electronic interaction between the OPV3 chromophores and the semiconducting nanoparticles. Composites comprising bulk cadmium sulfide with a higher number of sulfide cross-links were prepared by using an excess of hydrogen sulfide resulting in an even more efficient fluorescence quenching, that is, a residual fluorescence intensity of 20% compared to the original signal. Conclusion Although in the described case the dendrimers do not act as molecular containers, the fact that they can be
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regarded as headgroup polymerized surfactants allows for the generation of different dendrimer/CdS composites from a single template. In chloroform, the restricted environment formed by the dendrimer assembly leads to arrested precipitation of the cadmium sulfide in which the dendrimer acts as stabilizing agent. The nanoparticle formation proceeds in a controlled way, despite the absence of specific interactions between cadmium ions and the dendrimer core prior to precipitation. The particles that are formed have a controlled size of 3.0 nm independent of the dendrimer generation, suggesting that the minimization of surface energy and the packing of the dendrimers on the particle surface determine this size. In a chloroform/methanol mixture, the dendrimer forms platelike aggregates which subsequently become mineralized. Electron diffraction suggests that the platelets consist of highly organized domains of nanoparticles. The use of monolayers leads to the formation of bulk CdS particles with diameters of 20-60 nm organized into a
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leaf-nerve pattern, which is possibly an expression of the monolayer structure. The use of vesicular templates leads to the formation of a cadmium sulfide overgrowth with bulk characteristics on the aggregate surface. Whereas no electronic communication between organic and inorganic components was observed in organic media, for the CdS overgrown vesicles it was observed that the presence of cadmium sulfide led to quenching of the fluorescence of the OPV3 chromophores, which was tentatively attributed to stabilization of the triplet-triplet transition of 1. Acknowledgment. The authors thank B. de Waal (Laboratory for Macromolecular and Organic Chemistry, Eindhoven) for providing dendrimer 2 and R. A. J. Jansen (Laboratory for Macromolecular and Organic Chemistry, Eindhoven) for the helpful discussions. LA010997Z
