Optical Limitation induced by Gold Clusters. 1. Size Effect - The

Jun 10, 2000 - The limitation effect is attributed to large light-scattering centers induced by the pulse around the initial particles. View: PDF | PD...
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J. Phys. Chem. B 2000, 104, 6133-6137

6133

Optical Limitation induced by Gold Clusters. 1. Size Effect Lionel Franc¸ ois, Mehran Mostafavi,* and Jacqueline Belloni Laboratoire de Chimie Physique, UMR 8610 CNRS, UniVersite´ Paris-Sud, 91405 Orsay, Cedex, France

Jean-Franc¸ ois Delouis and Jacques Delaire Laboratoire de Photophysique et Photochimie Supramole´ culaires et Macromole´ culaires, UMR 8531, Ecole Normale Supe´ rieure de Cachan, 94235 Cachan Cedex, France

Patrick Feneyrou Laboratoire Central de Recherches, Thomson-CSF, Domaine de CorbeVille, 91404 Orsay Cedex, France ReceiVed: December 20, 1999; In Final Form: March 8, 2000

The nonlinear optical response of gold particles (with 2.5, 9, or 15 nm radius) prepared by γ-radiolysis in water solution and stabilized by poly(vinyl alcohol) is size-dependent. The 2.5 nm clusters do not limit light transmission even at very high fluence of nanosecond laser pulses, while the larger clusters strongly limit the laser beam transmission at 530 nm. The threshold of limitation and the amplitude depend on the size of the particles. The rise of the optical limiting effect is measured by picosecond laser spectroscopy. For the largest particles, it lasts 1 ns. The limitation effect is attributed to large light-scattering centers induced by the pulse around the initial particles.

Introduction There is an increasing interest to synthesize new materials possessing nonlinear optical properties. In particular, optical limiters in the visible domain with a broad nonlinear optical spectrum and with short response time are required for eye protection and switching applications.1 As photoactive materials, the reverse saturable absorbers presenting an intense absorption in their excited states were found to be very efficient. However, their light transmission at low fluence is usually not satisfactory. Carbon black suspensions (CBSs) have also been extensively investigated as optical limiters.2,3 These particles present a broad absorption spectrum without any particular band in the visible, and their transmission is high at low fluence, while the transmission of a nanosecond laser pulse of high fluence is limited. They are quite stable but the mechanism of optical limiting by CBSs is not yet clear. A recent study confirming previous works showed that, after the absorption of light, the CBS particles heat up and then give rise to a growing microplasma that scatters the light. The influence of the solvent for light transmission limitation is significant only at longer times. This suggests that the first step of the process is restricted to the particle itself and does not include the environment.4 Recently, semiconductor and metal clusters, owing to their strong stability, were also studied as optical limiters. CdS (r ) 5 nm) and PbS (r ) 6.6 nm) nanoparticles were found to have particularly poor optical limiting performance, but it was shown that, when coated with Ag2S, they present an enhancement of the optical nonlinear absorption.5 Silver and nickel particles were also investigated for their nonlinear optical response. A strong optical limiting effect of silver nanocrystallites (r ) 5.6 nm) in stable suspensions was found, while Ni particles (r ) 5.8 nm) display a very weak optical limiting response. This work * To whom correspondence should be addressed.

demonstrates that the optical limiting response depends strongly on the metal constituting the clusters.5 Kamat and co-workers6 reported that silver colloids of particle radius 80-120 nm display a broad transient absorption in the visible when subjected to laser pulse excitation. They observed that silver clusters break up into smaller clusters (10-40 nm). Logunov et al.7 studied the subpicosecond transient absorption of passivated gold particles with different sizes ranging from 1.9 to 3.2 nm. They observed a bleaching of the plasmon band which depends on the particle size. The size-dependent spectral change was attributed to the reduction of the density of states for small nanoparticles. More recent studies showed that the intense laser heating of gold particles induces a shape and size transformation, which depends on the laser energy and pulse width. It was found that the shape change and size reduction occur through melting and vaporization of the gold particles.8-10 However, as concerns its optical limiting response at high fluence, the influence of the particle size for a given material was not yet investigated. In the present work, we describe the dependence of the efficiency for nonlinear optical response of gold clusters on their size. Then, we study the picosecond dynamics of the optical limiting response of these particles. Experimental Section All reagents were pure chemicals: gold salt, KAu(Cl)4, was from Degussa. 2-Propanol was from Prolabo, and methyl viologen chloride and poly(vinyl alcohol) (PVA; hydrolyzed at 99% with average Mw 85000-146000 g mol-1) were from Aldrich. Solutions are thoroughly deaerated by flushing N2 gas. The γ-irradiation source is a 60Co γ-facility of 7000 Ci with a maximum dose rate of 7 kGy h-1. The concentrations of AuIII (5 × 10-4 M) and alcohol (0.2 M) are the same for all the samples. The PVA concentration and irradiation dose are variable.

10.1021/jp9944482 CCC: $19.00 © 2000 American Chemical Society Published on Web 06/10/2000

Franc¸ ois et al.

6134 J. Phys. Chem. B, Vol. 104, No. 26, 2000 Gold ions AuIIICl4- are reduced by eaq- and alcohol radicals into AuII. The following step is the fast disproportionation of AuII into AuI and AuIII. Then, accumulated aurous ions AuI may also compete for the scavenging of eaq- (and (CH3)2C•OH), so that they are reduced during the second part of irradiation into the atoms Au0, which are the precursors of clusters:11

AuICl2- + eaq- (or (CH3)2C•OH) f Au0Cl22- (or + (CH3)2C•OH + H+) (1) The atoms are formed with a homogeneous distribution throughout the solution. Then atoms dimerize when encountered by pairs or associated with excess ions, and these species progressively coalesce into clusters by a multistep process. The coalescence, however, may be limited provided a polymeric molecule, such as PVA acting as a cluster stabilizer, is added. The higher the polymer concentration (10-3-10-1 mol L-1), the smaller the final cluster radius.12 It is also possible to combine the radiolytical reduction with chemical reduction to favor the cluster growth relative to nucleation, as in a developing photographic process.13,14 In this study the electron donor used as the developer is the reduced form of methyl viologen, MV•+, which is itself formed from MV2+ ions by irradiation, in parallel with Au0:

MV2+ + eaq- [or (CH3)2C•OH] f MV+• [+ (CH3)2CO] (2) The solutions containing AuIIICl4- and MV2+ are quite stable before irradiation. No band is detected except those of AuIIICl4and MV2+ around 200-300 nm. The reduced radical, MV•+, is unable to directly reduce AuI into Au0. However, the radical MV•+ acts as a delayed electron relay. MV•+ achieves the reduction of the rest of the ions after they have been adsorbed on the radiation-induced clusters acting as nuclei of autocatalytic reduction and growth (reaction 3). In fact, the redox potential of Aun+xx+ is now more positive than that of MV2+/MV•+:

Aun+xx+ + xMV‚+ f Aun+x + xMV2+

(3)

Therefore, for the same total equivalent of reduction distributed on MV•+ and Au0, the smaller the number of nuclei, the higher the number of atoms reduced per nuclei, and the size of the final developed clusters becomes much larger than in the absence of methyl viologen. The surface plasmon band spectra of gold clusters induced by irradiation were recorded in suprasil cells with a HewlettPackard 8453 spectrophotometer. Transmission electron microscopy (TEM) images were obtained using a 100 kV transmission electron microscope. The optical limitation effect is measured by focusing the second harmonic of a Nd:YAG nanosecond laser beam (fwhm ) 14 ns) at 532 nm (1 Hz) onto a quartz cell of 2 mm containing the subcolloidal solution of gold clusters. The focusing optics is f/125. The presence of a diaphragm located between the detector and the cell allows 100% of the energy to be collected at low fluence while the scattered light at high fluence is mostly stopped. The value of the waist is measured with a CCD camera; it is around 50 µm. The variation of the input energy is obtained by using different neutral densities and wave plates with a polarizer. The energy detector is a Laser Precision instrument (RJP765) calibrated by an RJ7620 energy radiometer. The dynamics measurements are performed with a single 30 ps pulse, selected in the pulse train delivered by a mode-locked Nd:YAG laser. At the output of the second amplifier, the energy

was about 70 mJ in one single pulse at 1064 nm (8 Hz). The pump beam (532 nm, with an energy from 100 µJ to 2 mJ) is focused near to the cell (l ) 1 mm) containing the gold particles by a lens of 250 mm focal length (f/25). The value of the waist carefully measured by the knife edge technique is 450 µm. To create a probe continuum, a part of the fundamental flash of the laser was focused onto a tungsten electrode in a glass cell filled with xenon at a pressure of 2 bar. The laser creates a plasma, whose emission lifetime is 50 ns with a rise time within the pulse width and an emission plateau after 10 ns delay. This light source has an emission spectrum very similar to the emission of a xenon arc. Both pump and probe beams were nearly collinear inside the cell. The probe waist is smaller than the pump waist, so the probe could test a uniformly excited area. At the output of the cell, the beam is collected by a lens (f ) 10 cm, diameter 4 cm) located 30 cm from the cell. This beam is then focused on the entrance slit of a monochromator. At low fluence the analyzing beam covers all the lens aperture surface, so the optical arrangement is such that both transient absorption and light scattering induced in the cell will lead to a decrease in transmission. A streak camera was used in a singleshot mode and was triggered by the laser pulse itself. Each absorbance change at a given wavelength was obtained by averaging the signal of both the probe light without and with excitation over the same number of laser shots (50-150). If not otherwise stated, the solution is refreshed from pulse to pulse by a circulation system. Results and Discussion Three different types of 5 × 10-4 M gold solutions, I, II, and III, were irradiated. The concentration of the PVA is 0.1 M for samples I and III and 10-3 M for sample II. Only solution III contains MV2+ (10-3 M), which does not absorb visible light. After irradiation, all the solutions display the specific surface plasmon band of gold clusters with a maximum around 520 nm (OD ≈ 0.22-0.28 for 2 mm), but the detailed band shape depends on the cluster size (Figure 1). The shape and the intensity of these radiation-induced nanocolloid spectra are quite reproducible. Solution I presents a large band with a maximum around 515 nm. The absorption spectrum of solution II presents a maximum at 531 nm with a shoulder around 600 nm, and the spectrum of III, besides a maximum at 527 nm, displays a broad band in the red (Figure 1). According to the maximum position and width of the surface plasmon band of solution I, the clusters are of very small size. In accordance with the spectrum of sample I, the TEM image shows indeed the presence of nonagglomerated spherical particles with an average radius of 2.5 nm. The maximum position at 531 nm and the presence of a shoulder around 600 nm in the spectrum of II indicate the formation of gold clusters larger than in sample I. This is confirmed by a TEM image showing the presence of particles with an average radius of 9 nm. Finally, the intense light scattering in the red for the spectrum of sample III bears witness to the presence of still larger gold clusters. The TEM measurement confirms indeed that individual particles have an average radius of 15 nm in sample III (Figure 1 bottom). Note that, despite the presence of a high concentration of PVA, these particles developed by MV•+ are generally agglomerated and that they are nonspherical. Limiting Response. Figure 2 shows the observed optical limiting curves using the nanosecond laser at 532 nm at increasing fluence and at room temperature for samples I-III. The response of the nonirradiated solution used as a reference which does not contain metal ions is also reported in Figure 2.

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Figure 3. Relative transmission of samples II and III versus the number of nanosecond laser pulses at 532 nm (with fluence around 5 J/cm2) without solution shaking. l ) 2 mm.

Figure 1. (top) Absorption spectra (l ) 2 mm) of γ-induced gold clusters from solutions containing 5 × 10-4 mol L-1 AuIII and 0.2 mol L-1 2-propanol. The dose is 180 krad. The PVA concentration is not the same for the three solutions, and the third solution also contains MV2+. (I) 0.1 mol L-1 PVA. (II) 10-3 mol L-1 PVA. (III) 0.1 mol L-1 PVA. 10-3 mol L-1 MV2+. (bottom) TEM micrographs of samples I -III. The bar represents 100 nm.

Figure 2. Transmission obtained at 532 nm versus the fluence for samples I (b), II (O), and III (4) and a solution containing PVA and MV2+ without gold clusters (0). Solutions are refreshed by shaking. l ) 2 mm.

It turns out that, at 532 nm, the optical densities at low fluence of samples I-III are comparable. The response is plotted as the ratio of the transmitted energy to input energy (E/E0), versus the fluence of the laser beam. Note that the reference sample without clusters displays a constant transmission even at high fluence (Figure 2). At low fluence, the ratio of transmitted energy is independent of the fluence and the values are almost superimposed up to 0.2 J/cm2 for the three types of clusters (Figure 2). For sample I (r ) 2.5 nm), the output energy through the small gold particles keeps its linear transmission (E/E0 constant) even at very high input fluence (up to 5 J/cm2). For sample II (r ) 9 nm), the optical limiting effect appears at 0.5 J/cm2 input energy, and above this threshold there is a strong linear decrease of the transmission versus the fluence (double logarithmic scale). For sample III (r ) 15 nm), the optical limiting effect is observed from a lower threshold of input fluence (0.2 J/cm2), and then

Figure 4. Time profiles of induced optical density (∆OD) signals at 600 nm after a 30 ps pulse at the fluence (0.6 J/cm2) is focused onto samples I-III. l ) 1 mm.

the transmission falls drastically at increasing input fluence with the same law as for sample II but shifted by a factor of 5 less in fluence. The fact that the behavior is different for the three samples clearly demonstrates the size effect on the optical limiting response at the same nominal gold concentration. When the laser beam at a fluence around 5 J/cm2 is focused onto gold clusters of types III and II, and when the solutions are not refreshed during exposure, we observe with repetitive pulses a better stability of sample III than of sample II. As shown in Figure 3, only a slight increase of relative transmission (that is, a slight decrease of the nonlinear response) occurs for sample III, but an important increase of the relative transmission (that is, a decrease of the nonlinear response) occurs for sample II. For sample III, there is a loss of limitation efficiency for the first few pulses, but after 20 pulses the sample response becomes stable and the light limitation effect is constant (2 times lower than for the first pulse). On the contrary, the nonlinear response of sample II is completely degraded after 10 pulses, and the sample is even slightly bleached. The absorption spectra of the solutions obtained after a few hundreds of laser pulses at high fluence show in both cases that they absorb less in the red, and therefore that the clusters become smaller, indicating a fragmentation of the clusters under the laser fluence. The fragmentation is more efficient for sample II than for sample III. Dynamics. The experimental results (Figure 2) demonstrate that, for samples II and III, the nonlinear process occurs within the pulse length, which means that it develops in a time shorter than 10 ns. Therefore, to study the dynamics of the nonlinear optical response, we observed the kinetics of the absorbance increase induced at 600 nm by a picosecond laser for the three types of samples, as reported in Figure 4. Again, sample I does not limit the pulse intensity in contrast with samples II and III, and the response for sample III is stronger than for sample II. The process of limitation is over within 2 ns. All these results are consistent with those reported in Figure 2. After the

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Figure 6. Induced Is/I0 spectrum obtained after the laser pulse (with fluence around 0.25 J/cm2) for sample III, at different delays: (O) just after the pulse, (b) at 0.45 ns, (() at 2.5 ns. l ) 1 mm. The calculated curves correspond to a fitting according to the Rayleigh law.

Figure 5. (Top) Maximum induced optical density at 2.5 ns versus the fluence of the picosecond laser beam for sample III. l ) 1 mm. (Bottom) Kinetics at 600 nm of the nonlinear response of the sample III for different input energies. l ) 1 mm.

development of the effect, the decay of the process responsible for the light limitation lasts for a few tens of nanoseconds. In the case of silver particles (diameter 40-60 nm), reported by Kamat et al.,6 it was concluded that the primary event following the laser excitation is the photoexcitation of electrons, causing the plasmon absorption band to bleach. After the bleaching an induced optical density at 3 ns was also observed. It was shown that the induced optical density presented a quadratic dependence on the laser energy at 600 nm. We checked the fluence effect of the excitation laser on the limitation at 600 nm in the case of gold sample III. As shown in Figure 5, the light limitation response, after a threshold, increases linearly on a logarithmic scale in intensity with the fluence. Moreover, in the present work, we do not observe any bleaching between 350 and 720 nm, and the induced optical density at 2.5 ns increases with the intensity of the laser beam (Figure 5, top), in agreement with the results obtained with the nanosecond laser irradiation. We do not observe any bleaching or increase just after the pulse at 0.25 J/cm2 for sample III (Figure 6). But if we consider the intensity of the light transmitted at different delays after the excitation, an increase of the light extinction relative to the initial absorption spectrum of the gold particles develops within 2.5 ns. Different hypotheses, such as absorption of an excited state, thermal nonlinear refraction effect, and light-scattering centers, could explain the limiting effect. However, the spectrum shape does not display any absorption band at a specific wavelength: the extinction increases monotonically at decreasing wavelength to the UV, and the shape changes only slightly with the time evolution. This suggests that the extinction is not due to the light absorption. A thermal nonlinear refraction effect (thermal lens) can be produced in absorbing solutions through the variation of the nonlinear refractive index. Such a variation of the nonlinear refractive index has already been reported by Durand et al.4 at longer time (few tens of microseconds) than our time measurements. Besides, the size effect on optical

limitation cannot be explained by the nonlinear refraction effect. Indeed, for the samples containing the smallest clusters, which absorb at 532 nm, the optical limitation effect was not observed. The shape of the optical limiting curves with a very sharp threshold is also typical of nonlinear light scattering. Figure 6 shows the wavelength dependence of the fraction of light nontransmitted and considered as scattered (Is) on the light input intensity (I0) induced by the laser pulse (λ ) 532 nm). It is clear that the shape of the Is/I0 spectra does not display any particular band. The shape of the spectra can be fitted pretty well with the well-known Rayleigh law, giving the wavelength dependence of the scattered light intensity Is/I0 ) C/λn (where C is a constant for a given size and the n index decreases with increasing size of the scattering particle). We note that the value of n is about 4 at 0.45 ns and decreases to about 1 at 2.5 ns when the limiting effect is fully developed. The decrease of the n value implies that the size of the scattering centers increases with time. As concerns the nature of the light-scattering centers, whose size must be much larger than that of the initial particles, two types can be rapidly produced by the laser pulse: expanded clusters in a microplasma state or large clouds constituted by the vaporization of initial clusters. We exclude that the observed effect in the subnanosecond range would be due to the formation of bubbles formed from the solvent evaporation by heat transfer from the metal clusters. Indeed, the heat transfer from the metal to the water and the development of the bubbles around the particles take usually longer than a few nanoseconds.4 Therefore, we propose to assign the phenomenon of the induced pseudoabsorbance to the vaporization or the fragmentation of the metal particles inducing a large light-scattering center around the initial particles. Indeed, such a vaporization or fragmentation of the gold clusters induced by a thermal effect has been recently reported by several groups.8-10 Because the extinction coefficient of the plasmon band per atom does not depend much on the cluster size at the laser wavelength 532 nm, the absorbed energy per particle in the case of large particles is more important than in the case of smaller ones. Therefore, the energy concentrated in each particle and available to form the scattering centers is more intense and causes the improvement of the optical limiting effect. The size of the induced centers increases with time to reach a maximum, yielding a strong limiting effect. It is possible that the initial agglomeration of clusters in sample III favors the creation of much larger scattering clusters. The subsequent decay which is faster at increasing input fluence (Figure 5) could be due to the fact that high fluence produces more numerous fragments confined in the same scattering center and that their encounter

Optical Limitation by Gold Clusters. 1. probability and recombination are increased. This may also be due to the increase of the contact area with the solvent and to a more efficient cooling of the scattering centers by the solvent. Conclusion The results show clearly that the gold particles present a sizedependent optical limiting effect. The fluence threshold of the limiting effect decreases and its amplitude increases with increasing particle size. The origin of the effect is assigned to the formation of strongly-light-scattering centers due to the vaporization of the initial particles induced by the laser pulse. The scattering centers are developed in a few nanoseconds and relax mostly reversibly with a partial degradation into smaller particles after repetitive pulses. The degradation is more important for the small gold particles. The gold clusters surrounded by a surfactant in solution are efficient systems for optical power limiting under high laser fluences. Acknowledgment. This work, which is supported by the De´le´gation Ge´ne´rale pour l′Armement (Defense Ministry of France), is a part of the thesis of L. F. We acknowledge DGA with gratitude for this support.

J. Phys. Chem. B, Vol. 104, No. 26, 2000 6137 References and Notes (1) Proceedings of the First International Workshop on Optical Power Limiting, Cannes, July 1998; Kajzar, F., Ed.; Gordon and Breach Science Publishers: Langhorne, PA, 1999; Vol. 21, pp 1-552. (2) Mansour, K.; Soileau, M. J.; Van Stryland, E. W. J. Opt. Soc. Am. B 1992, 9, 1100. (3) Nashold, K. M.; Walter, D. P. J. Opt. Soc. Am. B. 1995, 12, 1228. (4) Durand, O.; Grolier-Mazza, V.; Frey, R. J. Opt. Soc. Am. B 1999, 16, 1431. (5) Sun, Y. P.; Riggs, J. E.; Rollins, H. W.; Guduru, R. J. Phys. Chem. B. 1999, 103, 77. (6) Kamat, P. V.; Flumiani, M.; Hartland, G. V. J. Phys. Chem. B. 1998, 102, 3123. (7) Logunov, S. L.; Ahmadi, T. S.; El-Sayed, A.; Khourt, J. T.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3713. (8) Link, S.; El-Sayed, M. A. J. Phys. Chem. B, 1999, 103, 8410. (9) Takami, A.; Kurita, H.; Koda, S. J. Phys. Chem. B, 1999, 103, 1226. (10) Fujiwara, H. Yanagida, S. Kamat, P. V. J. Phys. Chem. B, 1999, 103, 2589. (11) Gachard, E.; Remita, H.; Khatouri, J.; Keita, B.; Nadjo, L.; Belloni, J. New J. Chem. 1998, 1257. (12) Belloni, J.; Mostafavi, M.; Remita, H.; Marignier, J. L.; Delcourt, M. O. New J. Chem. 1998, 1239. (13) Mostafavi, M.; Marignier, J. L.; Amblard, J.; Belloni, J. Radiat. Phys. Chem. 1989, 34, 605. (14) Henglein, A.; Meisel, D. Langmuir 1998, 14, 7392.