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Chemical synthesis of polyphenylacetylene nanospheres with controlled dimensions for photonic crystals
Materials Science and Engineering C 23 (2003) 861 – 865 www.elsevier.com/locate/msec
Chemical synthesis of polyphenylacetylene nanospheres with controlled dimensions for photonic crystals R. D’Amato a,*, L. Medei a, I. Venditti a, M.V. Russo a, M. Falconieri b a b
Department of Chemistry, University of Rome ‘‘La Sapienza’’, p.le A. Moro 5, 00185 Rome, Italy UTS Materiali e Nuove Tecnologie, ENEA, C. R. Casaccia, v. Anguillarese 301, 00060 Rome, Italy
Abstract Nano-beads of controlled size of a conjugated polymer, polyphenylacetylene (PPA), were prepared for the first time by using a catalytic emulsion polymerisation technique. Reaction conditions, such as concentration of monomer, co-solvent, and reaction time, were varied in order to study the feasibility of tuning particle size and their self-assembling properties. Besides routine chemical analysis, the new prepared materials were characterised by Scanning Electron Microscopy (SEM) in order to determine the diameters of the beads and their size dispersion. The beads have regular round shape, with diameters ranging from 300 to 1000 nm depending on the reaction conditions, and when a low degree of polydispersion is achieved, a short range regular array occur. D 2003 Elsevier B.V. All rights reserved. Keywords: Photonic crystals; Polyphenylacetylene; Emulsion polymerisation
1. Introduction The phrases photonic crystal and photonic band gap are now used widely by research workers in the field of optoelectronics. For people concerned with the possibility of applying new physical concepts to technological developments, the first challenge was to identify possible routes to the fabrication of structures with a significant fraction of the properties of the ideal photonic band gap structure, as initially set out by Yablonovitch [1] and John [2]. Photonic band structures occur if light travels through a three dimensional dielectric lattice with a refractive index that varies periodically on length scales comparable to the wavelength and are analogous to electronic band structures in atomic crystals. If, for some frequency range, a photonic crystal reflects light of any polarisation totally at any incident angle, the crystal is said to have an absolute photonic band gap (PBG). In such a photonic crystal, no electromagnetic wave can propagate if the frequency is within that PBG range [1,2]. To create photonic crystals operating at optical wavelengths, the smallest feature sizes must be of the order of 100 nm, clearly in the realm of nanotechnology. State-ofthe-art semiconductor technology has recently been used to * Corresponding author. Tel.: +39-649913347; fax: +39-6490324. E-mail address: [email protected] (R. D’Amato). 0928-4931/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2003.09.104
fabricate three-dimensional photonic-bandgap crystals at 1.5- and 10-Am wavelengths, using stacks of microfabricated polysilicon bars [3]. Further development of lithography techniques (such as two-photon methods) at smaller length scales will be needed to extend this technology to visible wavelengths. There has been rapid progress recently in the development of alternative approaches to fabricate photonic crystals by using self-assembled systems that order spontaneously [4] at nanoscale dimensions. Inorganic spheres as SiO2, TiO2, semiconductors, as well as organic or polymeric spheres have been used as the framework material. Further development in self-assembly techniques should allow assembling these structures into integrated device systems at low cost [5]. Great advantages are expected from these structures, including the control of radiative properties and lifetimes that in turn can control chemical reactions and catalysis. A novel phenomenon is the localisation of light in a disordered photonic crystal, in analogy to the localisation of electrons in disordered systems [6]. Such photonic crystal structures have immense potential for a large variety of optoelectronic devices. Another remarkable recent development with interesting potential applications is the demonstration of organic photonic crystal lasers [7]. Conjugated polymers are widely studied for application in optoelectronics, but they have never been synthesised in
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R. D’Amato et al. / Materials Science and Engineering C 23 (2003) 861–865
subsequent drying at room temperature and normal pressure or by spinning. The obtained materials were characterised by means of conventional techniques as IR, 1H NMR spectroscopies, Gel Permeation Chromatography (GPC). The diameters of the beads were determined directly from SEM image. Scanning Electron Microscopy was carried out using a SEM-LEO1450VP on metalled samples. Fig. 1. Reaction scheme for the polymerisation of phenylacetylene.
the form suitable for photonic crystals, i.e. in the form of nano-beads of controlled and monodisperse dimension, so that they can give origin to a three-dimensional close packed array. Nanostructures of poly( p-phenylene vinylene) (PPV) were fabricated by direct scanning near-field lithography of its soluble precursor [8]. Nanoparticles from poly( p-phenylene), polyfluorene and polycyclopentadithiophenes derivatives were produced via the miniemulsion process of the already formed polymers in aqueous dispersion [9]. In this work, we report for the first time the synthesis of nanospheres of polyphenylacetylene (PPA), a monosubstituted polyacetylene widely studied for applications in electronics and photonics as well as in optical devices. The synthesis is performed by using a modified emulsion polymerisation technique and the control of diameters of beads is obtained by varying reaction conditions. Together with the spectroscopic characterisation of the obtained materials, Scanning Electron Microscopy (SEM) investigation gives notice about dimension, polydispersion and organisation of PPA beads.
2. Experimental All solvents and materials were reagent grade (Carlo Erba); phenylacetylene (PA) (Aldrich) was distilled under reduced pressure before use; the catalyst [Rh(cod)Cl]2tmeda (COD = cis, cis-1,5-cyclooctadiene, tmeda = N,N,N V,N Vtetramethylethylendiamine) was synthesised according to a published method [10]. In a typical procedure, 50 ml of deionized water was degassed under a moderate flow of Ar, for 30 min; then the calculated amounts of phenylacetylene and toluene (if appropriate) were added and the mixture was stirred at 80 jC for 2 h in order to make a homogeneous suspension. Solution of [Rh(cod)Cl]2tmeda ([catalyst] = 2.0 10 4 M, satured liquid) was added under Ar flow and the reaction was refluxed under vehement stirring in Ar atmosphere. The polymerisation was stopped with a cold ice bath and the emulsion of polymer nanospheres was separated from unreacted monomer and toluene by settling and from catalyst and bulk polymer (side reaction products always present) by filtration, centrifugation and washing with deionized water. Films of PPA were prepared by dropping a small amount of the emulsion onto glass substrates and
3. Results and discussion Polyphenylacetylene was synthesised by using a catalytic polymerisation in aqueous dispersion for producing nanobeads of polymer (see Fig. 1). The catalyst [Rh(cod)Cl]2tmeda was used because Rh(I) complexes were very active in catalytic polymerisation of phenylacetylene and for its solubility in water. Monomer, insoluble in water, is mainly contained in droplets, even if some monomer molecules are still present in water. The reaction starts in the aqueous phase immediately after the addition of the catalyst, then the oligomers form and when they are large enough, they tend to segregate, resulting in the formation of colloidal particles. Toluene was added to the reaction batch in order to help in forming the emulsion and to vary the size of the droplets. The uniformity of size of the particles produced by this method depends on the physicochemical conditions of the reaction and on the concentration of the reactants. Low polydispersion is the most important factor for obtaining a long range ordered packed materials, above all for noninteracting particles, as PPA beads are. Moreover, it is important that the diameters of beads are smaller than 800 nm for application in optical devices. Polymerisations were carried out by using different conditions (see Table 1), i.e. we varied the concentration of the monomer (H2O/PA ratio from 10:1 to 50:1) and the Table 1 Recipe details for the synthesis of nano-beads of PPA, their dimensions and dispersion No
Toluene/PA (ml/ml)
H2O/PA (ml/ml)
Time (min)
Diameters (nm)
Dispersion (dmax dmin)/d¯
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
0:1 0:1 0:1 1:1 1:1 1.6:1 2:1 4:1 4:1 4:1 4:1 4:1 4:1 4:1 8:1 8:1 10:1
10:1 20:1 25:1 10:1 10:1 20:1 20:1 20:1 20:1 20:1 40:1 40:1 40:1 40:1 20:1 40:1 –
60 60 30 60 30 60 60 60 30 10 120 60 30 10 60 60 24 h
180 – 500 450 – 900 400 – 1000 900 – 1500 200 – 750 400 – 600 560 – 800 430 – 560 500 – 2500 250 – 2000 550 – 1200 380 – 750 280 – 650 400 – 2500 300 – 2500 350 – 3600 –
0.94 0.67 0.86 0.50 1.16 0.40 0.35 0.20 1.33 1.59 0.74 0.65 0.80 1.45 1.57 1.65 –
R. D’Amato et al. / Materials Science and Engineering C 23 (2003) 861–865
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Fig. 2. SEM image of a film of PPA nano-beads synthesised with toluene/PA ratio 8:1 (sample No. 15).
reaction time (from 10 to 120 min); in particular, we investigated the influence of the co-solvent performing reactions without toluene and with different toluene/PA ratio, from 1:1 to 8:1. Polyphenylacetylene was also synthesised in toluene, without water, in an homogeneous catalytic polymerisation, in order to notice likenesses and
differences with emulsion technique: it did not give rise to spheres, but to polymer in bulk. All the materials obtained have the polyphenylacetylene structure as we proved by different spectroscopic investigations, together with elemental analysis and determination of molecular weights.
Fig. 3. SEM image of a film of PPA nano-beads synthesised with toluene/PA ratio 4:1 and reaction time 30 min (sample No. 13).
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R. D’Amato et al. / Materials Science and Engineering C 23 (2003) 861–865
Fig. 4. SEM image of a film of PPA nano-beads synthesised with toluene/PA ratio 1.6:1 (sample No. 6).
SEM investigation shows films consisting of layers of regular round shape beads; diameters of the spheres and their dispersion were determined directly from these images (see Table 1). The concentration of phenylacetylene in water was found to have a great influence both on dispersion and on beads diameters, while reaction time is of importance above all for the control of dimensions. However, if the reaction time is very short, i.e. 10 min, it gives rise to very high polydispersion: this is probably due to a low conversion of monomer, so that when the reaction is stopped, PA is still present in the reaction mixture together with the catalyst, and the polymerisation still continues in cold ice bath, and during the purification of materials; and no control of emulsion homogeneity can be performed in these conditions. To confirm this hypothesis, we can note that polydispersion is still high when longer time was used, i.e. 30 min, with high concentration of phenylacetylene, for example, H2O/PA = 10:1. High polydispersion was also found when toluene/PA ratio is high (8:1), (see Fig. 2) probably because it is more difficult to preserve the uniformity of droplets size. We widely investigated the reactions with toluene/PA = 4:1, and we note that the reaction time has a great influence on the diameters of spheres, in particular, longer time means bigger size and fairly narrow dispersion (see Fig. 3). When toluene/PA ratios decreased, at the end without toluene (0:1), we often obtained dispersion d < 1, but there is not yet a clear trend correlating the diameters of spheres with the reactions conditions. The best results were obtained for reaction Nos. 6, 7, 8 (see Table 1): the sizes
of the beads cover a good range for photonic crystals application; they are quite monodisperse and in addition give rise to well-ordered films within an area of 5 5 Am2 (see Fig. 4).
4. Conclusions Nano-beads of a conjugated polymer, i.e. polyphenylacetylene, were prepared for the first time by catalytic emulsion polymerisation technique. The diameters of the beads are in the range 400– 800 nm, suitable for achievement of photonic crystals in the optical regime. By chemical methods, we have been able to obtain controlled particle size and low dispersion, that allow to prepare ordered layers of spheres even if in a short range. Further investigations will be devoted to better tune the dimensions and to broaden the crystalline domains.
References [1] E. Yablonovitch, Phys. Rev. Lett. 58 (1987) 2059. [2] S. John, Phys. Rev. Lett. 58 (1987) 2486. [3] S.-Y. Lin, J.G. Fleming, D.L. Hetherington, B.K. Smith, R. Biswas, K.M. Ho, M.M. Sigalas, W. Zubrzycki, S.R. Kurtz, J. Bur, Nature 394 (1998) 251. [4] C.E. Reese, C.D. Guerrero, J.M. Weissman, K. Lee, S.A. Asher, J. Colloid Interface Sci. 232 (2000) 76. [5] Y.N. Xia, G.M. Whitesides, Angew. Chem. 37 (1998) 551. [6] Y.A. Vlasov, M.A. Kaliteevski, V.V. Nikolaev, Phys. Rev., B 60 (1999) 1555.
R. D’Amato et al. / Materials Science and Engineering C 23 (2003) 861–865 [7] M. Meier, A. Dodabalapur, J.A. Rogers, R.E. Slusher, A. Mekis, A. Timko, C.A. Murray, R. Ruel, O. Nalamasu, J. Appl. Phys. 86 (1999) 3502. [8] R. Riehn, A. Charas, J. Morgado, F. Cacialli, Appl. Phys. Lett. 82 (2003) 526.
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[9] K. Landfester, R. Montenegro, U. Scherf, R. Gu¨ntner, U. Asawapirom, S. Patil, D. Neher, T. Kietzke, Adv. Mater. 14 (2000) 651. [10] A. Furlani, C. Napoletano, M.V. Russo, A. Camus, N. Marsich, J. Polym. Sci., A, Polym. Chem. 27 (1989) 75.
Chemical synthesis of polyphenylacetylene nanospheres with controlled dimensions for photonic crystals R. D’Amato a,*, L. Medei a, I. Venditti a, M.V. Russo a, M. Falconieri b a b
Department of Chemistry, University of Rome ‘‘La Sapienza’’, p.le A. Moro 5, 00185 Rome, Italy UTS Materiali e Nuove Tecnologie, ENEA, C. R. Casaccia, v. Anguillarese 301, 00060 Rome, Italy
Abstract Nano-beads of controlled size of a conjugated polymer, polyphenylacetylene (PPA), were prepared for the first time by using a catalytic emulsion polymerisation technique. Reaction conditions, such as concentration of monomer, co-solvent, and reaction time, were varied in order to study the feasibility of tuning particle size and their self-assembling properties. Besides routine chemical analysis, the new prepared materials were characterised by Scanning Electron Microscopy (SEM) in order to determine the diameters of the beads and their size dispersion. The beads have regular round shape, with diameters ranging from 300 to 1000 nm depending on the reaction conditions, and when a low degree of polydispersion is achieved, a short range regular array occur. D 2003 Elsevier B.V. All rights reserved. Keywords: Photonic crystals; Polyphenylacetylene; Emulsion polymerisation
1. Introduction The phrases photonic crystal and photonic band gap are now used widely by research workers in the field of optoelectronics. For people concerned with the possibility of applying new physical concepts to technological developments, the first challenge was to identify possible routes to the fabrication of structures with a significant fraction of the properties of the ideal photonic band gap structure, as initially set out by Yablonovitch [1] and John [2]. Photonic band structures occur if light travels through a three dimensional dielectric lattice with a refractive index that varies periodically on length scales comparable to the wavelength and are analogous to electronic band structures in atomic crystals. If, for some frequency range, a photonic crystal reflects light of any polarisation totally at any incident angle, the crystal is said to have an absolute photonic band gap (PBG). In such a photonic crystal, no electromagnetic wave can propagate if the frequency is within that PBG range [1,2]. To create photonic crystals operating at optical wavelengths, the smallest feature sizes must be of the order of 100 nm, clearly in the realm of nanotechnology. State-ofthe-art semiconductor technology has recently been used to * Corresponding author. Tel.: +39-649913347; fax: +39-6490324. E-mail address: [email protected] (R. D’Amato). 0928-4931/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2003.09.104
fabricate three-dimensional photonic-bandgap crystals at 1.5- and 10-Am wavelengths, using stacks of microfabricated polysilicon bars [3]. Further development of lithography techniques (such as two-photon methods) at smaller length scales will be needed to extend this technology to visible wavelengths. There has been rapid progress recently in the development of alternative approaches to fabricate photonic crystals by using self-assembled systems that order spontaneously [4] at nanoscale dimensions. Inorganic spheres as SiO2, TiO2, semiconductors, as well as organic or polymeric spheres have been used as the framework material. Further development in self-assembly techniques should allow assembling these structures into integrated device systems at low cost [5]. Great advantages are expected from these structures, including the control of radiative properties and lifetimes that in turn can control chemical reactions and catalysis. A novel phenomenon is the localisation of light in a disordered photonic crystal, in analogy to the localisation of electrons in disordered systems [6]. Such photonic crystal structures have immense potential for a large variety of optoelectronic devices. Another remarkable recent development with interesting potential applications is the demonstration of organic photonic crystal lasers [7]. Conjugated polymers are widely studied for application in optoelectronics, but they have never been synthesised in
862
R. D’Amato et al. / Materials Science and Engineering C 23 (2003) 861–865
subsequent drying at room temperature and normal pressure or by spinning. The obtained materials were characterised by means of conventional techniques as IR, 1H NMR spectroscopies, Gel Permeation Chromatography (GPC). The diameters of the beads were determined directly from SEM image. Scanning Electron Microscopy was carried out using a SEM-LEO1450VP on metalled samples. Fig. 1. Reaction scheme for the polymerisation of phenylacetylene.
the form suitable for photonic crystals, i.e. in the form of nano-beads of controlled and monodisperse dimension, so that they can give origin to a three-dimensional close packed array. Nanostructures of poly( p-phenylene vinylene) (PPV) were fabricated by direct scanning near-field lithography of its soluble precursor [8]. Nanoparticles from poly( p-phenylene), polyfluorene and polycyclopentadithiophenes derivatives were produced via the miniemulsion process of the already formed polymers in aqueous dispersion [9]. In this work, we report for the first time the synthesis of nanospheres of polyphenylacetylene (PPA), a monosubstituted polyacetylene widely studied for applications in electronics and photonics as well as in optical devices. The synthesis is performed by using a modified emulsion polymerisation technique and the control of diameters of beads is obtained by varying reaction conditions. Together with the spectroscopic characterisation of the obtained materials, Scanning Electron Microscopy (SEM) investigation gives notice about dimension, polydispersion and organisation of PPA beads.
2. Experimental All solvents and materials were reagent grade (Carlo Erba); phenylacetylene (PA) (Aldrich) was distilled under reduced pressure before use; the catalyst [Rh(cod)Cl]2tmeda (COD = cis, cis-1,5-cyclooctadiene, tmeda = N,N,N V,N Vtetramethylethylendiamine) was synthesised according to a published method [10]. In a typical procedure, 50 ml of deionized water was degassed under a moderate flow of Ar, for 30 min; then the calculated amounts of phenylacetylene and toluene (if appropriate) were added and the mixture was stirred at 80 jC for 2 h in order to make a homogeneous suspension. Solution of [Rh(cod)Cl]2tmeda ([catalyst] = 2.0 10 4 M, satured liquid) was added under Ar flow and the reaction was refluxed under vehement stirring in Ar atmosphere. The polymerisation was stopped with a cold ice bath and the emulsion of polymer nanospheres was separated from unreacted monomer and toluene by settling and from catalyst and bulk polymer (side reaction products always present) by filtration, centrifugation and washing with deionized water. Films of PPA were prepared by dropping a small amount of the emulsion onto glass substrates and
3. Results and discussion Polyphenylacetylene was synthesised by using a catalytic polymerisation in aqueous dispersion for producing nanobeads of polymer (see Fig. 1). The catalyst [Rh(cod)Cl]2tmeda was used because Rh(I) complexes were very active in catalytic polymerisation of phenylacetylene and for its solubility in water. Monomer, insoluble in water, is mainly contained in droplets, even if some monomer molecules are still present in water. The reaction starts in the aqueous phase immediately after the addition of the catalyst, then the oligomers form and when they are large enough, they tend to segregate, resulting in the formation of colloidal particles. Toluene was added to the reaction batch in order to help in forming the emulsion and to vary the size of the droplets. The uniformity of size of the particles produced by this method depends on the physicochemical conditions of the reaction and on the concentration of the reactants. Low polydispersion is the most important factor for obtaining a long range ordered packed materials, above all for noninteracting particles, as PPA beads are. Moreover, it is important that the diameters of beads are smaller than 800 nm for application in optical devices. Polymerisations were carried out by using different conditions (see Table 1), i.e. we varied the concentration of the monomer (H2O/PA ratio from 10:1 to 50:1) and the Table 1 Recipe details for the synthesis of nano-beads of PPA, their dimensions and dispersion No
Toluene/PA (ml/ml)
H2O/PA (ml/ml)
Time (min)
Diameters (nm)
Dispersion (dmax dmin)/d¯
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
0:1 0:1 0:1 1:1 1:1 1.6:1 2:1 4:1 4:1 4:1 4:1 4:1 4:1 4:1 8:1 8:1 10:1
10:1 20:1 25:1 10:1 10:1 20:1 20:1 20:1 20:1 20:1 40:1 40:1 40:1 40:1 20:1 40:1 –
60 60 30 60 30 60 60 60 30 10 120 60 30 10 60 60 24 h
180 – 500 450 – 900 400 – 1000 900 – 1500 200 – 750 400 – 600 560 – 800 430 – 560 500 – 2500 250 – 2000 550 – 1200 380 – 750 280 – 650 400 – 2500 300 – 2500 350 – 3600 –
0.94 0.67 0.86 0.50 1.16 0.40 0.35 0.20 1.33 1.59 0.74 0.65 0.80 1.45 1.57 1.65 –
R. D’Amato et al. / Materials Science and Engineering C 23 (2003) 861–865
863
Fig. 2. SEM image of a film of PPA nano-beads synthesised with toluene/PA ratio 8:1 (sample No. 15).
reaction time (from 10 to 120 min); in particular, we investigated the influence of the co-solvent performing reactions without toluene and with different toluene/PA ratio, from 1:1 to 8:1. Polyphenylacetylene was also synthesised in toluene, without water, in an homogeneous catalytic polymerisation, in order to notice likenesses and
differences with emulsion technique: it did not give rise to spheres, but to polymer in bulk. All the materials obtained have the polyphenylacetylene structure as we proved by different spectroscopic investigations, together with elemental analysis and determination of molecular weights.
Fig. 3. SEM image of a film of PPA nano-beads synthesised with toluene/PA ratio 4:1 and reaction time 30 min (sample No. 13).
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R. D’Amato et al. / Materials Science and Engineering C 23 (2003) 861–865
Fig. 4. SEM image of a film of PPA nano-beads synthesised with toluene/PA ratio 1.6:1 (sample No. 6).
SEM investigation shows films consisting of layers of regular round shape beads; diameters of the spheres and their dispersion were determined directly from these images (see Table 1). The concentration of phenylacetylene in water was found to have a great influence both on dispersion and on beads diameters, while reaction time is of importance above all for the control of dimensions. However, if the reaction time is very short, i.e. 10 min, it gives rise to very high polydispersion: this is probably due to a low conversion of monomer, so that when the reaction is stopped, PA is still present in the reaction mixture together with the catalyst, and the polymerisation still continues in cold ice bath, and during the purification of materials; and no control of emulsion homogeneity can be performed in these conditions. To confirm this hypothesis, we can note that polydispersion is still high when longer time was used, i.e. 30 min, with high concentration of phenylacetylene, for example, H2O/PA = 10:1. High polydispersion was also found when toluene/PA ratio is high (8:1), (see Fig. 2) probably because it is more difficult to preserve the uniformity of droplets size. We widely investigated the reactions with toluene/PA = 4:1, and we note that the reaction time has a great influence on the diameters of spheres, in particular, longer time means bigger size and fairly narrow dispersion (see Fig. 3). When toluene/PA ratios decreased, at the end without toluene (0:1), we often obtained dispersion d < 1, but there is not yet a clear trend correlating the diameters of spheres with the reactions conditions. The best results were obtained for reaction Nos. 6, 7, 8 (see Table 1): the sizes
of the beads cover a good range for photonic crystals application; they are quite monodisperse and in addition give rise to well-ordered films within an area of 5 5 Am2 (see Fig. 4).
4. Conclusions Nano-beads of a conjugated polymer, i.e. polyphenylacetylene, were prepared for the first time by catalytic emulsion polymerisation technique. The diameters of the beads are in the range 400– 800 nm, suitable for achievement of photonic crystals in the optical regime. By chemical methods, we have been able to obtain controlled particle size and low dispersion, that allow to prepare ordered layers of spheres even if in a short range. Further investigations will be devoted to better tune the dimensions and to broaden the crystalline domains.
References [1] E. Yablonovitch, Phys. Rev. Lett. 58 (1987) 2059. [2] S. John, Phys. Rev. Lett. 58 (1987) 2486. [3] S.-Y. Lin, J.G. Fleming, D.L. Hetherington, B.K. Smith, R. Biswas, K.M. Ho, M.M. Sigalas, W. Zubrzycki, S.R. Kurtz, J. Bur, Nature 394 (1998) 251. [4] C.E. Reese, C.D. Guerrero, J.M. Weissman, K. Lee, S.A. Asher, J. Colloid Interface Sci. 232 (2000) 76. [5] Y.N. Xia, G.M. Whitesides, Angew. Chem. 37 (1998) 551. [6] Y.A. Vlasov, M.A. Kaliteevski, V.V. Nikolaev, Phys. Rev., B 60 (1999) 1555.
R. D’Amato et al. / Materials Science and Engineering C 23 (2003) 861–865 [7] M. Meier, A. Dodabalapur, J.A. Rogers, R.E. Slusher, A. Mekis, A. Timko, C.A. Murray, R. Ruel, O. Nalamasu, J. Appl. Phys. 86 (1999) 3502. [8] R. Riehn, A. Charas, J. Morgado, F. Cacialli, Appl. Phys. Lett. 82 (2003) 526.
865
[9] K. Landfester, R. Montenegro, U. Scherf, R. Gu¨ntner, U. Asawapirom, S. Patil, D. Neher, T. Kietzke, Adv. Mater. 14 (2000) 651. [10] A. Furlani, C. Napoletano, M.V. Russo, A. Camus, N. Marsich, J. Polym. Sci., A, Polym. Chem. 27 (1989) 75.