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Fullerene non-linear excited state absorption induced by gold nanoparticles light harvesting
Synthetic Metals 155 (2005) 283–286
Fullerene non-linear excited state absorption induced by gold nanoparticles light harvesting V. Amendola a , G. Mattei b , C. Cusan c , M. Prato c , M. Meneghetti a,∗ a
Department of Chemical Sciences, University of Padova, Via Marzolo 1, Padova, Italy b Department of Physics, University of Padova, Via Marzolo 8, Padova, Italy c Department of Pharmaceutical Sciences, University of Trieste, Piazzale Europa 1, Trieste, Italy Received 5 January 2005; accepted 5 January 2005 Available online 27 October 2005
Abstract Au nanoparticles can be synthesized in solution by a laser ablation methodology which allows to obtain funtionalized metal nanoparticles with a disulfide fullerene derivative in a simple one step process. The supramolecular system is shown to be an efficient non-linear absorbers of 532 nm nanosecond laser pulses. The mechanism of the non-linear absorption is shown to proceed through a light harvesting step by the metal nanoparticles and an efficient energy transfer to the fullerene moieties which absorb in a non-linear regime through their triplet states. © 2005 Elsevier B.V. All rights reserved. Keywords: Non-linear optical methods; Fullerenes and derivatives; Models of non-linear phenomena
1. Introduction Gold nanoparticles have been used since the ancient time for many purposes and particularly for their singular red colour [1]. It is now well known that the colour of nanoparticles of the order of tens of nanometers, and down to some nanometers, derives from a surface plasmon resonance with an absorption peak at about 520 nm [2]. This optical resonance can be useful not only for linear spectroscopy, namely for applications based on the colour of the nanoparticles, but also for non-linear optical responses since the nanoparticles can be easily excited and become the source of a strong local field which can be useful for non-linear optic. Surface Enhanced Raman Spectroscopy (SERS), usually obtained with silver nanoparticles, can be seen as an example of such a behaviour. Here we want to show how it is possible to use functionalised gold nanoparticles for obtaining a stronger reverse saturable absorption response of fullerene and particularly of C60 . We linked a disulfide derivative of C60 to nanometer size gold nanoparticles and excited the supramolecular system at 532 nm with nanosecond laser pulses. We observed a large excited state
∗
Corresponding author. Tel.: +39 049 8275127; fax: +39 049 8275239. E-mail address: [email protected] (M. Meneghetti).
0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2005.01.032
non-linear absorption of fullerene that we interpret as due to a transfer of energy from the excited gold nanoparticles to the fullerenes moieties which, in their excited triplet state, activate a reverse saturable absorption behaviour. The importance of being able to populate excited states for obtaining strong non-linear optical responses, is a very important approach to non-linear optics [3] since excited states are much more polarizable than ground states and therefore their nonlinear optical responses are expected to be larger than responses of molecules in their ground state. The transfer of energy from excited metal nanoparticles is one of the possibilities of populating excited states and it will be shown how it works for enhancing fullerene reverse saturable absorption. 2. Results Synthesis of metal nanoparticles has been possible by using a recent laser ablation methodology of bulk metals in solution [4]. Until now, water was mainly used as a solvent medium for obtaining gold nanoparticles, but solubility of fullerene, or of their derivatives, in such a solvent is very poor. Therefore, we have chosen dimethylsulfoxide (DMSO) as another biocompatible solvent, like water, to produce gold nanoparticles since the fullerene derivative used for the functionalization of the gold nanoparticles showed a sufficient solubility in such a solvent.
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V. Amendola et al. / Synthetic Metals 155 (2005) 283–286
Fig. 1. The fullerene derivative (FT) used for functionalizing the gold nanoparticles.
We found that the production of gold nanoparticles in DMSO gave stable solutions over periods of weeks without the need of using surfactants or stabilising agents like thiols as in the case of the synthesis of gold nanoparticles by reduction of gold ions in solution. This was an important step in the synthesis of the supramolecular system of gold nanoparticles with the disulfide fullerene derivative since usually the synthesis of such a systems are obtained by exchange of the stabilizing agent with the molecules of interest, namely with a two step process which poses more problems than a direct process like in our case. The fullerene derivative we used is called FT and is shown in Fig. 1. It was prepared by condensation of the aminofunctionalized fullerene derivative and thiotic acid. The compound was purified by chromatography and kept under nitrogen to avoid oxidation. FT has a long solubilizing chain terminating with a disulfide which can open in the presence of gold allowing for the formation of two S Au bonds. Fig. 2 shows the UV–vis spectrum of the DMSO solution of the supramolecular system of Au nanoparticles linked to FT (AuNP-FT). The synthesis can be obtained with a solution containing FT and by focusing 1064 nm nanosecond pulses of a Nd:YAG laser with a fluence of the order of some J cm−2 on a bulk metal plate. Transmission electron microscopy shows that we obtained nanoparticles with a mean diameter of 4 nm. Fig. 3 shows an HR-TEM image of the synthesized AuNP-FT.
Fig. 2. DMSO solution of gold nanoparticles functionalised with FT (dotted line) and FT in DMSO (dashed dotted line). Solid line shows the difference spectrum of the two previous ones and the dashed line a fitting with a Mie-Gans model.
Fig. 3. AuNP-FT observed by HR-TEM.
The AuNP-FT linear spectrum can be understood as the sum of those of its moieties. The spectrum of FT in DMSO is reported in Fig. 2 and subtracting this spectrum from that of the supramolecular assembly one finds a characteristic surface resonance plasmon band of the AuNP. The interpretation of this spectrum on the basis of a Mie-Gans model [5] makes it possible to have a description of the nanoparticles. We found that a fitting of the spectrum can be obtained with a distribution of nanoparticles represented by 92% of spheres with a diameter of 4 nm and 8% of spheroids with the two diameters of 5.6 nm and 2.4 nm. The result is consistent with that obtained by the HR-TEM images. 2.1. Non-linear transmission measurements We measured the non-linear optical transmission at 532 nm with a set up based on the control of the energy of nine nanosecond pulses of a duplicated Nd:YAG laser (Quantel YG980E) at 2 Hz and in an open aperture configuration. Two-millimeter glass cells containing DMSO solution of AuNP-FT were used for the measurements. In Fig. 4 are reported the results we obtained. One can see a decreasing non-linear transmission for the AuNP-FT solution (stars) which is characteristic of a reverse saturable absorption of C60 [6]. We also measured the excited state spectrum by pumping the solution at 532 nm with the same pulses used for the non-linear transmission measurements. We found the typical spectrum of the triplet of a derivative of a fulleropirrolidine with a maximum at 700 nm [7]. This shows that the non-linear transmission derives from the excited state absorption of the fullerene moiety of the supramolecular assembly. Confirmation that this is the origin of the non-linear optical behaviour can be also obtained by looking at the non-linear transmission measurements for a solution of AuNP with the same linear transmission (open triangles) of the AuNP-FT solution. This curve shows that AuNP have first a saturable behaviour and then, at higher intensities, a small decreasing transmission which is usually considered to derive from a non-linear scattering behaviour of the nanoparticles. Therefore, one finds that the AuNP do not contribute to the non-linear transmission of
V. Amendola et al. / Synthetic Metals 155 (2005) 283–286
Fig. 4. Non-linear transmission measurements at 532 nm of DMSO solutions of AuNP-FT (stars), FT (open circles) and AuNP (open triangles) with a linear T = 75% and of FT with T = 86% (circles).
the AuNP-FT since, in particular at high intensities, fullerene becomes a efficient passive optical filter. In Fig. 4 are also reported other two curves related to two DMSO solutions of FT, the first with the same linear transmission of the AuNP-FT solution (open circles) and the second with the concentration of the FT present in the AuNP-FT solution (closed circles). This last concentration was determined by fitting the UV–vis spectrum of AuNP-FT with the spectrum of FT, as shown in Fig. 2. These two non-linear transmission curves are references for interpreting the behaviour of AuNP-FT. One can see that the AuNP-FT solution (stars) has a larger non-linear absorption than that of the solution with the same content of FT (closed circles) showing that a new phenomenon is occurring. On the other hand, the FT solution with the same linear transmission of that of AuNP-FT (open circles) shows a larger non-linear absorption. This behaviour can be consistently understood by considering that the excited Au nanoparticles transfer energy to FT which, in the excited state, absorbs more than the ground state of the supramolecular structure. Since pump and probe measurements show that the long lived excited state is the triplet state of fullerene the overall mechanism can be understood as a light harvesting by the Au nanoparticles, a transfer of energy to the fullerene moieties which in the excited triplet state absorbs more than the ground state of the supramolecular structure. A more quantitative result can be obtained by fitting the nonlinear transmission data with a model for the dynamic of the excited states [8]. One finds that the experimental data of the FT solutions can be reproduced [9] by considering that fullerenes are excited to the first excited singlet state, then, following an intersystem crossing to the triplet state, the triplet to triplet transition becomes active with a larger cross section than the ground state. At higher intensities one also finds that the second higher excited triplet state has a very small population and a second transition in the triplet manifold is possible. With only one set
285
Fig. 5. Fitting of the non-linear transmittance of the concentrated FT solution with a model for the dynamic of the excited states (see the text).
of parameters, but the concentration of fullerene, one is able to reproduce the non-linear behaviour both of the FT solution with higher concentration (NFT,H = 4.0 × 1017 cm−3 ) (see Fig. 5), and that with lower concentration (NFT,L = 2.1 × 1017 cm−3 ). Moreover one can use the same parameters also to reproduce the data of the AuNP-FT solution with an effective fullerene concentration NAuNP-FT = 3.2 × 1017 cm−3 . This fullerene concentration derives from the indirect excitations of fullerenes through the Au nanoparticles. One can use this concentration to obtain the efficiency of the energy transfer (ΦET ) from Au nanoparticles to FT as: ΦET =
NAuNP-FT − NFT,L NFT,H − NFT,L
The result shows that an energy transfer between Au nanoparticles and fullerene occurs with an efficiency of 61%. In conclusion we have shown how to synthesize Au nanoparticles in solution by a laser ablation methodology and to functionalise them with a fullerene derivative. Non-linear optical transmission measurements have shown that the supramolecular system is an efficient non-linear absorber of nanosecond laser pulses at 532 nm and that this happens through the light harvesting of metal nanoparticles and an efficient energy transfer to the fullerene moieties. The found microscopic mechanism is an example of how excited states can be used for obtaining strong optical non-linear responses of molecular systems. Acknowledgements Authors would like to thank G. Marcolongo and S. Crivellaro for technical help. We thank MIUR (cofin prot. 2004035502) for financial support. References [1] M.C. Daniel, et al., Chem. Rev. 104 (2004) 293.
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V. Amendola et al. / Synthetic Metals 155 (2005) 283–286
[2] U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer, Berlin Heidelberg, 1995. [3] Q.L. Zhou, J.R. Heflin, K.Y. Zamani-Khamiri, A. Garito, Phys. Rev. A 43 (1991) 1673; ˚ F. Gel’mukanov, A. Baev, P. Mac´ak, Y. Luo, H. Agren, J. Opt. Soc. Am. B 19 (2002) 937. [4] F. Mafun`e, J-Y. Kohno, Y. Takeda, T. Kondo, J. Phys. Chem. B 105 (2001), 5114, 9050. [5] G. Mie, Ann. Phys. 25 (1908) 377;
[6] [7] [8] [9]
J.A. Stratton, Electromagnetic Theory, McGraw-Hill, 1941; S. Link, et al., J. Phys. Chem. B 103 (1999) 8410. L.W. Tutt, A. Kost, Nature 356 (1992) 225. K. Kordatos, T. Da Ros, M. Prato, S. Leach, E.J. Land, R.V. Bensasson, Chem. Phys. Lett. 334 (2001) 221. R. Menzel, Photonics—Linear and Nonlinear Interactions of Laser Light and Matter, Springer Verlag, Berlin, Heidelberg, 2000. V. Amendola, M. Meneghetti, in preparation.
Fullerene non-linear excited state absorption induced by gold nanoparticles light harvesting V. Amendola a , G. Mattei b , C. Cusan c , M. Prato c , M. Meneghetti a,∗ a
Department of Chemical Sciences, University of Padova, Via Marzolo 1, Padova, Italy b Department of Physics, University of Padova, Via Marzolo 8, Padova, Italy c Department of Pharmaceutical Sciences, University of Trieste, Piazzale Europa 1, Trieste, Italy Received 5 January 2005; accepted 5 January 2005 Available online 27 October 2005
Abstract Au nanoparticles can be synthesized in solution by a laser ablation methodology which allows to obtain funtionalized metal nanoparticles with a disulfide fullerene derivative in a simple one step process. The supramolecular system is shown to be an efficient non-linear absorbers of 532 nm nanosecond laser pulses. The mechanism of the non-linear absorption is shown to proceed through a light harvesting step by the metal nanoparticles and an efficient energy transfer to the fullerene moieties which absorb in a non-linear regime through their triplet states. © 2005 Elsevier B.V. All rights reserved. Keywords: Non-linear optical methods; Fullerenes and derivatives; Models of non-linear phenomena
1. Introduction Gold nanoparticles have been used since the ancient time for many purposes and particularly for their singular red colour [1]. It is now well known that the colour of nanoparticles of the order of tens of nanometers, and down to some nanometers, derives from a surface plasmon resonance with an absorption peak at about 520 nm [2]. This optical resonance can be useful not only for linear spectroscopy, namely for applications based on the colour of the nanoparticles, but also for non-linear optical responses since the nanoparticles can be easily excited and become the source of a strong local field which can be useful for non-linear optic. Surface Enhanced Raman Spectroscopy (SERS), usually obtained with silver nanoparticles, can be seen as an example of such a behaviour. Here we want to show how it is possible to use functionalised gold nanoparticles for obtaining a stronger reverse saturable absorption response of fullerene and particularly of C60 . We linked a disulfide derivative of C60 to nanometer size gold nanoparticles and excited the supramolecular system at 532 nm with nanosecond laser pulses. We observed a large excited state
∗
Corresponding author. Tel.: +39 049 8275127; fax: +39 049 8275239. E-mail address: [email protected] (M. Meneghetti).
0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2005.01.032
non-linear absorption of fullerene that we interpret as due to a transfer of energy from the excited gold nanoparticles to the fullerenes moieties which, in their excited triplet state, activate a reverse saturable absorption behaviour. The importance of being able to populate excited states for obtaining strong non-linear optical responses, is a very important approach to non-linear optics [3] since excited states are much more polarizable than ground states and therefore their nonlinear optical responses are expected to be larger than responses of molecules in their ground state. The transfer of energy from excited metal nanoparticles is one of the possibilities of populating excited states and it will be shown how it works for enhancing fullerene reverse saturable absorption. 2. Results Synthesis of metal nanoparticles has been possible by using a recent laser ablation methodology of bulk metals in solution [4]. Until now, water was mainly used as a solvent medium for obtaining gold nanoparticles, but solubility of fullerene, or of their derivatives, in such a solvent is very poor. Therefore, we have chosen dimethylsulfoxide (DMSO) as another biocompatible solvent, like water, to produce gold nanoparticles since the fullerene derivative used for the functionalization of the gold nanoparticles showed a sufficient solubility in such a solvent.
284
V. Amendola et al. / Synthetic Metals 155 (2005) 283–286
Fig. 1. The fullerene derivative (FT) used for functionalizing the gold nanoparticles.
We found that the production of gold nanoparticles in DMSO gave stable solutions over periods of weeks without the need of using surfactants or stabilising agents like thiols as in the case of the synthesis of gold nanoparticles by reduction of gold ions in solution. This was an important step in the synthesis of the supramolecular system of gold nanoparticles with the disulfide fullerene derivative since usually the synthesis of such a systems are obtained by exchange of the stabilizing agent with the molecules of interest, namely with a two step process which poses more problems than a direct process like in our case. The fullerene derivative we used is called FT and is shown in Fig. 1. It was prepared by condensation of the aminofunctionalized fullerene derivative and thiotic acid. The compound was purified by chromatography and kept under nitrogen to avoid oxidation. FT has a long solubilizing chain terminating with a disulfide which can open in the presence of gold allowing for the formation of two S Au bonds. Fig. 2 shows the UV–vis spectrum of the DMSO solution of the supramolecular system of Au nanoparticles linked to FT (AuNP-FT). The synthesis can be obtained with a solution containing FT and by focusing 1064 nm nanosecond pulses of a Nd:YAG laser with a fluence of the order of some J cm−2 on a bulk metal plate. Transmission electron microscopy shows that we obtained nanoparticles with a mean diameter of 4 nm. Fig. 3 shows an HR-TEM image of the synthesized AuNP-FT.
Fig. 2. DMSO solution of gold nanoparticles functionalised with FT (dotted line) and FT in DMSO (dashed dotted line). Solid line shows the difference spectrum of the two previous ones and the dashed line a fitting with a Mie-Gans model.
Fig. 3. AuNP-FT observed by HR-TEM.
The AuNP-FT linear spectrum can be understood as the sum of those of its moieties. The spectrum of FT in DMSO is reported in Fig. 2 and subtracting this spectrum from that of the supramolecular assembly one finds a characteristic surface resonance plasmon band of the AuNP. The interpretation of this spectrum on the basis of a Mie-Gans model [5] makes it possible to have a description of the nanoparticles. We found that a fitting of the spectrum can be obtained with a distribution of nanoparticles represented by 92% of spheres with a diameter of 4 nm and 8% of spheroids with the two diameters of 5.6 nm and 2.4 nm. The result is consistent with that obtained by the HR-TEM images. 2.1. Non-linear transmission measurements We measured the non-linear optical transmission at 532 nm with a set up based on the control of the energy of nine nanosecond pulses of a duplicated Nd:YAG laser (Quantel YG980E) at 2 Hz and in an open aperture configuration. Two-millimeter glass cells containing DMSO solution of AuNP-FT were used for the measurements. In Fig. 4 are reported the results we obtained. One can see a decreasing non-linear transmission for the AuNP-FT solution (stars) which is characteristic of a reverse saturable absorption of C60 [6]. We also measured the excited state spectrum by pumping the solution at 532 nm with the same pulses used for the non-linear transmission measurements. We found the typical spectrum of the triplet of a derivative of a fulleropirrolidine with a maximum at 700 nm [7]. This shows that the non-linear transmission derives from the excited state absorption of the fullerene moiety of the supramolecular assembly. Confirmation that this is the origin of the non-linear optical behaviour can be also obtained by looking at the non-linear transmission measurements for a solution of AuNP with the same linear transmission (open triangles) of the AuNP-FT solution. This curve shows that AuNP have first a saturable behaviour and then, at higher intensities, a small decreasing transmission which is usually considered to derive from a non-linear scattering behaviour of the nanoparticles. Therefore, one finds that the AuNP do not contribute to the non-linear transmission of
V. Amendola et al. / Synthetic Metals 155 (2005) 283–286
Fig. 4. Non-linear transmission measurements at 532 nm of DMSO solutions of AuNP-FT (stars), FT (open circles) and AuNP (open triangles) with a linear T = 75% and of FT with T = 86% (circles).
the AuNP-FT since, in particular at high intensities, fullerene becomes a efficient passive optical filter. In Fig. 4 are also reported other two curves related to two DMSO solutions of FT, the first with the same linear transmission of the AuNP-FT solution (open circles) and the second with the concentration of the FT present in the AuNP-FT solution (closed circles). This last concentration was determined by fitting the UV–vis spectrum of AuNP-FT with the spectrum of FT, as shown in Fig. 2. These two non-linear transmission curves are references for interpreting the behaviour of AuNP-FT. One can see that the AuNP-FT solution (stars) has a larger non-linear absorption than that of the solution with the same content of FT (closed circles) showing that a new phenomenon is occurring. On the other hand, the FT solution with the same linear transmission of that of AuNP-FT (open circles) shows a larger non-linear absorption. This behaviour can be consistently understood by considering that the excited Au nanoparticles transfer energy to FT which, in the excited state, absorbs more than the ground state of the supramolecular structure. Since pump and probe measurements show that the long lived excited state is the triplet state of fullerene the overall mechanism can be understood as a light harvesting by the Au nanoparticles, a transfer of energy to the fullerene moieties which in the excited triplet state absorbs more than the ground state of the supramolecular structure. A more quantitative result can be obtained by fitting the nonlinear transmission data with a model for the dynamic of the excited states [8]. One finds that the experimental data of the FT solutions can be reproduced [9] by considering that fullerenes are excited to the first excited singlet state, then, following an intersystem crossing to the triplet state, the triplet to triplet transition becomes active with a larger cross section than the ground state. At higher intensities one also finds that the second higher excited triplet state has a very small population and a second transition in the triplet manifold is possible. With only one set
285
Fig. 5. Fitting of the non-linear transmittance of the concentrated FT solution with a model for the dynamic of the excited states (see the text).
of parameters, but the concentration of fullerene, one is able to reproduce the non-linear behaviour both of the FT solution with higher concentration (NFT,H = 4.0 × 1017 cm−3 ) (see Fig. 5), and that with lower concentration (NFT,L = 2.1 × 1017 cm−3 ). Moreover one can use the same parameters also to reproduce the data of the AuNP-FT solution with an effective fullerene concentration NAuNP-FT = 3.2 × 1017 cm−3 . This fullerene concentration derives from the indirect excitations of fullerenes through the Au nanoparticles. One can use this concentration to obtain the efficiency of the energy transfer (ΦET ) from Au nanoparticles to FT as: ΦET =
NAuNP-FT − NFT,L NFT,H − NFT,L
The result shows that an energy transfer between Au nanoparticles and fullerene occurs with an efficiency of 61%. In conclusion we have shown how to synthesize Au nanoparticles in solution by a laser ablation methodology and to functionalise them with a fullerene derivative. Non-linear optical transmission measurements have shown that the supramolecular system is an efficient non-linear absorber of nanosecond laser pulses at 532 nm and that this happens through the light harvesting of metal nanoparticles and an efficient energy transfer to the fullerene moieties. The found microscopic mechanism is an example of how excited states can be used for obtaining strong optical non-linear responses of molecular systems. Acknowledgements Authors would like to thank G. Marcolongo and S. Crivellaro for technical help. We thank MIUR (cofin prot. 2004035502) for financial support. References [1] M.C. Daniel, et al., Chem. Rev. 104 (2004) 293.
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V. Amendola et al. / Synthetic Metals 155 (2005) 283–286
[2] U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer, Berlin Heidelberg, 1995. [3] Q.L. Zhou, J.R. Heflin, K.Y. Zamani-Khamiri, A. Garito, Phys. Rev. A 43 (1991) 1673; ˚ F. Gel’mukanov, A. Baev, P. Mac´ak, Y. Luo, H. Agren, J. Opt. Soc. Am. B 19 (2002) 937. [4] F. Mafun`e, J-Y. Kohno, Y. Takeda, T. Kondo, J. Phys. Chem. B 105 (2001), 5114, 9050. [5] G. Mie, Ann. Phys. 25 (1908) 377;
[6] [7] [8] [9]
J.A. Stratton, Electromagnetic Theory, McGraw-Hill, 1941; S. Link, et al., J. Phys. Chem. B 103 (1999) 8410. L.W. Tutt, A. Kost, Nature 356 (1992) 225. K. Kordatos, T. Da Ros, M. Prato, S. Leach, E.J. Land, R.V. Bensasson, Chem. Phys. Lett. 334 (2001) 221. R. Menzel, Photonics—Linear and Nonlinear Interactions of Laser Light and Matter, Springer Verlag, Berlin, Heidelberg, 2000. V. Amendola, M. Meneghetti, in preparation.