8706
J. Phys. Chem. B 1999, 103, 8706-8710
Enhancement of the Second Harmonic Response by Adsorbates on Gold Colloids: The Effect of Aggregation P. Galletto, P. F. Brevet, and H. H. Girault Laboratoire d’Electrochimie, Ecole Polytechnique Fe´ de´ rale de Lausanne, CH-1015 Lausanne, Switzerland
R. Antoine* and M. Broyer Laboratoire de Spectrome´ trie Ionique et Mole´ culaire, UMR n°5579 CNRS-UniVersite´ Claude Bernard Lyon I, 43 Bd du 11 NoVembre 1918, F-69622 Villeurbanne Cedex, France ReceiVed: June 11, 1999; In Final Form: August 3, 1999
The extinction profiles as well as the hyper Rayleigh scattering signal of a highly monodisperse 22 nm diameter gold colloidal sol in the presence of pyridine have been measured. A strong enhancement of the HRS signal is observed in the presence of small amounts of pyridine. The relationship between the extinction spectra, the distribution of aggregated particles, and the second harmonic response has been investigated by evaluating the enhancement factor of the electromagnetic field in the colloids as they slowly aggregate. It is also shown that at larger pyridine concentrations, the hyper Rayleigh scattering signal levels off owing to the formation of large aggregates.
I. Introduction Since the observation of a surface enhanced Raman scattering signal (SERS) from pyridine adsorbed on roughened silver electrodes, SERS has attracted and continues to attract a lot of attention.1 The origin of the SERS effect stems from both the localization of the surface plasmon resonance and the enhanced polarizability of the adsorbates associated to the charge transfer with the metal substrate.2 In most cases, the former contribution dominates and therefore several types of structured metallic surfaces have been investigated,3,4 among which colloidal suspensions received special attention. Hence, SERS from adsorbed pyridine on colloidal particles has also been reported, and the significance of the shape of the particles for the surface plasmon resonance localization has been recognized.5-8 However, it was also observed that aggregation of the colloidal particles in the presence of pyridine was occurring. By UV/ visible absorption and TEM analysis, it was noted that the morphology of the aggregates was mainly lines of particles rather than compact aggregates. Further enhancing of the SERS signal was obtained in these conditions, though it was emphasized that the SERS effect occurred only when the excitation wavelength was set to lie within the new band formed by the aggregation process on the red side of the surface plasmon resonance. Since this electromagnetic effect of localization is known to be the dominant one in these studies, it is thus of interest to use other sensitive techniques to investigate this system of gold colloids in the presence of pyridine. One of these techniques is second harmonic generation (SHG), which has been reported to be highly sensitive to pyridine adsorption on electrochemically roughened silver electrodes.9 Reports of SHG of small metal colloidal particles are rather scarce, but we have reported in the past the SH signal from gold particles, either supported at an air/liquid interface10 or embedded in a thin alumina film.11 More recently, we have reported the first hyperpolarizability dependence of the particles as a function of their diameter.12 In
this latter experiment, the technique of hyper Rayleigh scattering (HRS) was used. With the same technique, the adsorption of pyridine on the gold particles has also been reported and shown to yield large enhancement of the SH signal. The present work reports a complementary study of the linear and nonlinear optical response of gold colloids as a function of the pyridine concentration. The large enhancement of the HRS signal is in particular described within the framework introduced to describe the SERS effect in a similar system. II. Experimental Section The experimental arrangement for the HRS setup has already been described elsewhere.12 The light source consisted of a Q-switched Nd:YAG laser providing pulses of 5 ns and 4 mJ at a repetition rate of 10 Hz and a frequency of 1064 nm. The fundamental light was focused into a 1 × 1 cm wide spectrophotometric quartz cell by means of a symmetric convex lens with a focal length of 50 mm. The experiments were performed well below the threshold for any other undesirable nonlinear effects such as stimulated Brillouin and Raman scattering. The HRS signal at 532 nm was collected at an angle of 90° from the incident direction. To efficiently collect the second order scattered light over a wide solid angle, a condenser system consisting of a flat mirror behind the cell and a symmetric convex lens with a focal length of 150 mm were used. The detection was performed with a photomultiplier tube and the signal was integrated with a boxcar averager. Spectral discrimination of the second harmonic light from the background light was performed by two low-pass near-IR filters and a 3 nm bandwidth interference filter peaked at 532 nm. The quadratic dependence of the HRS signal from the gold colloid solutions with the incident light intensity was always checked beforehand, and the experimental conditions were first assessed with p-nitroaniline (pNA) in methanol.12 The preparation of the gold colloids followed the same procedure as originally proposed by Turkevich et al.13 A solution
10.1021/jp991937t CCC: $18.00 © 1999 American Chemical Society Published on Web 09/23/1999
Second Harmonic Response
Figure 1. Experimental extinction spectra of gold colloidal solutions for different pyridine concentrations.
containing 20 mg of HAuCl4-3H2O (corresponding to about 10 mg of gold) in 190 mL of water was heated to its boiling point, while continuously stirring: 10 mL of 1% sodium citrate was then added to the boiling solution. After citrate addition and constant stirring, the color turned to wine red in about 10 min. Further boiling did not induce any further color variation. A water flow was used to cool the solution vapor thus avoiding any loss of volume: this precaution was required to ensure that the colloid concentration was correctly calculated. Pyridine was of analytical grade. TEM photographs of the colloidal preparation were taken, and the average particle diameter along with the standard deviation were determined from at least 200 particles. The gold colloids had a diameter of 22 nm with a standard deviation of 1.7 nm. III. Results The extinction spectra of the freshly prepared colloidal solution presented a strong extinction band with a maximum at 520 nm, characteristic of the collective excitation of the free conduction band electrons of the particles known as the surface plasmon resonance. On adding 5 µL, 10 µL, and 50 µL aliquots of pure pyridine solution to a 4 mL solution of the gold colloidal solution, a significant change in the extinction spectra was observed as shown in Figure 1. A new broad band appeared on the red side of the spectrum, moving off toward longer wavelengths, from 600 to 700 nm, with the increasing pyridine concentration. Concurrently, the surface plasmon resonance band at 520 nm was damped and almost completely disappeared at a pyridine concentration of 0.155 M. These results have already been observed in the past and have been shown to stem from the aggregation of the colloidal particles upon the addition of pyridine. The morphology of the aggregates has been observed by TEM and recognized to be of a linear shape rather than of a compact cluster of particles. The new long wavelength band is then associated with the longitudinal mode of the electronic plasma oscillation along the long axis of the linear colloidal aggregates.5,6 The transverse mode retains a location similar to the sphere resonance mode. Parallel to the linear optical properties, the nonlinear optical properties of aggregating colloids have been reported only on very few occasions and only for HRS experiments.14,15 Figure 2 presents the HRS intensity data obtained for a 22 nm diameter gold colloidal solution as a function of the addition of pyridine. Since the plasmon resonance band was modified by addition of pyridine (see Figure 1), the HRS intensity data have been
J. Phys. Chem. B, Vol. 103, No. 41, 1999 8707
Figure 2. Hyper Rayleigh scattering intensity as a function of the pyridine concentration.
corrected by a factor 10-A (where A is the absorbance at 532 nm). This factor accounts for the losses due to the linear absorption of the scattered light by the aggregated colloidal solutions at 532 nm and was determined from the corresponding UV-visible spectrum. As soon as pyridine is added, the HRS intensity is enhanced by a 5-fold factor as compared to the HRS signal of the pure gold solution. With further additions of pyridine, the HRS intensity enhancement steadily increases up to a 10-fold factor as compared to the pure solution but then slightly decreases to finally saturate at high pyridine concentrations above 0.020 M. IV. Discussion Extinction Profiles and Distribution of Aggregates. Since the first reports on the observation of the intense Raman spectrum of pyridine adsorbed on gold or silver particles, a lot of experimental methods have been devoted to the characterization of the aggregated colloids. Notably, TEM measurements have clearly demonstrated that the aggregates predominantly formed strings of particles rather than compact clusters and that the aggregation process was rather slow. Furthermore, the aggregates were shown to remain smaller than the wavelength of light at low concentrations of pyridine.6 A simple model where the aggregated clusters appear as objects of elongated shape has been devised to quantify the UV-visible evolution spectra.16 Aggregates can be taken as spheroids of various ratios r ) a/b of the semimajor axis a to the semiminor axis b. In these systems, the dipolar plasma modes within the aggregates are split into longitudinal and transverse components as observed experimentally in the extinction profiles. It is indeed known that the modes are highly sensitive to the aspect ratio of the elongated structures. At low gold solution concentrations, the aggregates may be assumed to be far from each other and therefore to scatter light in an independent way. As a result, the extinction cross-section Cext of the solution is the adequately weighted sum of the various extinction cross-sections Cext(i) of the different aggregates
Cext )
∑i wiCext(i) ) ∑i wi(Cabs(i) + Csca(i))
(1)
where Cabs(i) and Csca(i) are, respectively, the absorption and the scattering cross-section of the aggregate. The weights wi define the distribution of the aspect ratio of the aggregated colloidal solution. Since at low concentrations of pyridine, aggregates remain smaller than the incident wavelength, the
8708 J. Phys. Chem. B, Vol. 103, No. 41, 1999
Galletto et al.
electric dipole approximation can be used to determine the absorption and the scattering cross-sections, namely17
Csca )
128π5 (|Rl|2 + 2|Rt|2) 9λ4
(2)
8π2 Im(Rl + 2Rt) 3λ
(3)
Cabs )
where λ is the wavelength of the incident radiation and Rl and Rt are, respectively, the longitudinal and the transverse static polarizability tensor components of the aggregate. These two parameters are given by
Rl,t(ω) )
V((ω) - 1) 4π + ((ω) - 1)Pl,t
(4)
where V ) 4/3 πrb3 is the volume of the spheroid, (ω) is the dielectric constant of gold with respect to the one of water at the corresponding wavelength, and Pl,t is a shape factor given by17
Pl )
4π - Pl 4π ) 2 2 r -1
(x
r
r -1 2
ln(r + xr2 - 1) - 1
)
(5)
The dielectric constant of gold is taken from the experimental data of P. B. Johnson and R. W. Christy,18 and r is the aspect ratio of the aggregate. From a normalized distribution of weight wi, we can model the normalized extinction profile, obtaining the corresponding weights for each extinction coefficient Cext(i) by a recursive adjustment to get the best possible agreement between experimental and simulated curves. The models for the extinction band profile of these aggregated colloids for 0.016 and 0.031 M pyridine concentration are compared to the experimental profiles in Figure 3. The agreement is rather good and this model correctly explains the features of the UV-visible spectra as already noted previously. For a pure nonaggregated colloidal solution, colloid particles are spherical with an aspect ratio of 1 and a radius of 11 nm. For the extinction profiles of aggregated colloidal solutions with 0.016 and 0.031 M pyridine concentration, the semiminor axis b has been fixed to the mean radius of the individual particles, that is 11 nm, and we have varied r from 1.00001 to 10. The distribution of the weights, shown in Figure 4, is constituted of two parts: a narrow distribution centered around r ≈ 1.3-1.4 which is associated with the nonaggregated particles, and a much broader distribution ranging over larger r values arising from the particles at various stages of aggregation. With the low concentration of pyridine, that is 0.016 M, the aggregation is rather low and the total amount of clusters ranging up to r ≈ 3 remains below 35% (see Figure 4a). When the concentration of pyridine is increased up to 0.031 M, the aggregation is more important and the amount of aggregates with large r values is about 60%, the distribution extending to aspect ratios of about 4.5. From this model it appears that the extinction spectra of the gold colloidal aggregates in the presence of pyridine are well described by a collection of strings of particles and Mie theory applied at the electric dipole level. The distribution of the aggregated particles with respect to the aspect ratio can also be determined. Second Harmonic Response of the Aggregates. The second harmonic (SH) response from centrosymmetric metallic media has two origins. The first one is a surface contribution arising from the breaking of the centrosymmetry at the metal particle
Figure 3. Theoretical extinction cross-sections per unit particle (O) and experimental extinction profiles for the gold solution for (a) 0.016 M and (b) 0.031 M pyridine concentrations.
surface. Indeed, in the electric dipole approximation, SHG is forbidden in centrosymmetric media and can arise only from interfaces. However, since the particle diameter is much smaller than the wavelength of light in the problem at hand, this contribution vanishes for symmetry reasons: It is always possible to find a surface element opposite to the surface element of consideration with an inverted orientation of its nonlinear polarization. Note though, that in the presence of a substrate or for deformed ellipsoids, this assumption may not be valid anymore. The second contribution is the so-called bulk contribution arising from the electromagnetic field gradients due to the presence of the interface. This contribution extends over several nanometers away from the interface. For small particles of a few nanometers in diameter, the bulk origin is the dominant contribution to the SH response. The SH response from small size particles, as described above, is enhanced as compared to infinite surfaces through the same mechanism responsible for the field enhancement in Raman spectroscopy, SERS for instance. Owing to the confinement of the electron plasma in asperities at roughened surfaces or in the small volume of nanoparticles, charge repulsion forces are acting at the small volume wall with the net result of a large field enhancement there. In nonlinear optics, this effect is active on both the incident fundamental field and the harmonic field and thus may be responsible for large intensity enhancements. The magnitude of this enhancement is determined by the ratio of the modulus of the field amplitude inside the particle and the modulus of the incident field. The difficulty arises from the evaluation of this field enhancement factor, though an expression
Second Harmonic Response
J. Phys. Chem. B, Vol. 103, No. 41, 1999 8709 TABLE 1: Calculated Electric Field Enhancement Factors and Normalized HRS Enhancement η for Gold Spherical Colloids and Aggregates Obtained for Pyridine Concentrations of 0.016 and 0.031 Ma pyridine concn (M)
f532 nm
f1064 nm
η
exptl HRS enhancement
0 0.016 0.031
3.14 3.94 3.55
0.011 0.018 0.040
1 11.7 222.6
1 10.6 9.4
a The experimental HRS enhancements of the aggregates normalized to those of the pure gold colloids are also given.
Figure 4. Aggregate aspect ratio distributions for the gold solution for (a) 0.016 M and (b) 0.031 M pyridine concentrations.
may be derived for a distribution of particles. Using eq 4 giving the polarizability of a single particle, the field enhancement of a distribution of particles may be taken as the weighted distribution of the field enhancement factors, introducing the ellipsoidal nature of the particles
f(ω) )
4π
Im(Rli(ω) + 2Rti(ω))
V
Im((ω))
∑i wi
(6a)
and
f(2ω) )
4π
Im(Rli(2ω) + 2Rti(2ω))
V
Im((2ω))
∑i wi
(6b)
Hence, the enhancement of the HRS signal for aggregated particles normalized to the HRS signal for spherical particles is simply
η)
|fagg(ω)2fagg(2ω)|2 |fsph(ω)2fsph(2ω)|2
(7)
Experimentally, η is just the ratio of the intensities provided the background signal from the solvent has been removed. The field enhancement factors for the spherical and the aggregated particle solutions at 1064 and 532 nm are given in Table 1. The two pyridine concentrations are the ones for which the corresponding weighted distributions were determined from the UV-visible spectra. A similar operation could be performed
for each concentration of pyridine. It is also to be noted that these distributions are adjusted to correctly reproduce the UVvisible spectra but are used as is for the HRS enhancement calculations. It is observed that the field factor fagg(2ω) is increased by a mere 25% from the pure gold colloidal solution, or even less for the 0.031 M pyridine concentration, for both aggregated distributions. If this effect contributes to the overall increase of the enhancement, the real contribution, however, must be attributed to the field factor fagg(ω) since the latter factor evolves by a factor of 2 and furthermore contributes to the power of 4 to the total ratio. The field enhancement factor at the harmonic frequency is a delicate balance between the volume effect, the aspect ratio effect, and the evolution of the weighted distribution, as seen from eqs 1 to 5. The volume effect, corresponding to the increase of the aggregate volume with the pyridine concentration, is damping the field enhancement factor because the condition for the confinement of the electron plasma is relaxed at larger volumes. However, this effect is counteracted by the increase of the polarizability of the aggregates with the volume, and the field enhancement at the harmonic frequency is overall slightly increased. This latter effect is actually amplified by the simultaneous shift of the weighted distribution toward larger aggregated particles with larger volumes and therefore larger polarizability. Thus from these calculations, it appears that the field enhancement factor at the fundamental frequency is the dominant one. This is clearly the result of the appearance of the absorption band on the red side of the spectrum and the resulting increase of the imaginary part of the polarizability of the particles. The calculated HRS enhancement η thus increases with the concentration of pyridine in the gold colloidal solution. As seen from Table 1, for the low concentration of pyridine, namely 0.016 M, the calculated HRS enhancement of 11.7 compares rather well with the experimental HRS signal enhancement of 10.6. For the higher concentration of pyridine of 0.031 M, the calculated HRS enhancement increases up to 222.6, a value dramatically overestimated by more than 1 order of magnitude as compared to the experimental one of 9.4. At such high concentrations, the aggregates’ HRS response cannot be described by the model at proposed here. In fact, in the model presented, the evolution of the HRS signal with the pyridine concentration is described on the basis of individual aggregates of a linear shape and a size smaller than the wavelength of light. The response of a single aggregate may thus be obtained within the usual description for centrosymmetric species whereby the HRS response originates from a surface and a bulk contribution, the former vanishing for symmetry reasons. Such a description can, however, be correct only at sufficiently small pyridine concentrations where the aggregates retain sizes much smaller that the wavelength of light. At higher concentrations, and this is certainly true already at 0.020 M, it is indeed known that fractal aggregates are formed of sizes of the order of or even larger that of the wavelength of
8710 J. Phys. Chem. B, Vol. 103, No. 41, 1999 light. In this regime, the description presented here is no longer valid. Furthermore, the Mie theory cannot be applied any longer at the electric dipole level. This is experimentally observed by the limiting value of the hyperpolarizability tensor magnitude at concentrations of pyridine larger than 0.020 M, whereas calculations suggest an increase in the hyperpolarizability of the aggregates. Furthermore, at larger aggregated sizes, since absorption at 1064 nm becomes nonnegligible, we cannot exclude some laser-induced phenomena which modify the size and shape of colloids.19 Surprisingly, it is still possible to describe the linear optical properties of the aggregated solution with this model, as seen from Figure 3. This may suggest a higher sensitivity of HRS to the morphology of the aggregates. Indeed, at small aggregate sizes, the HRS signal depends on both the overall nonlinear optical activity of a single aggregate and the number density of the aggregates in the solution. At larger aggregate sizes, breaking of the centrosymmetry rule occurs and the HRS signal is dominated by the surface contribution from the aggregates. As a matter of fact, at constant amount of gold in the solution, it appears that there is an optimum size for the aggregates to yield the maximum HRS signal, neither monodispersed as pure nanoparticles nor as large aggregates. V. Conclusion The present study correlates the linear and nonlinear optical properties of aggregated gold colloids obtained by addition of pyridine in a solution of gold colloids. The UV-visible extinction profiles exhibit the classical band on the red side of the spectra characteristic of aggregated particles. These profiles are well simulated by considering the assembly of aggregates as a collection of elongated spheroidal clusters. From the UV-visible extinction profiles, it is also possible to extract the weighted aspect ratio distribution. Furthermore, the nonlinear optical data exhibit a strong enhancement in the HRS signal in the presence of pyridine, with a saturation effect at higher concentrations, namely above 0.020 M. For low concentrations of pyridine, that is below 0.020 M, the HRS signal enhancement may be described through field enhancement factors at both the fundamental and harmonic wavelengths. The dominant contribution arises from the fun-
Galletto et al. damental field factor, in correlation with the appearance of the red band in the linear optical spectrum. At higher pyridine concentrations, where the HRS signal levels off, the model breaks down as a result of the formation of fractal aggregates the size of which is likely to exceed the wavelength of light. The approximation of centrosymmetric particles is therefore no longer valid. Acknowledgment. P.G., P.F.B., and H.H.G. kindly acknowledge the Fonds National Suisse under the Grant 2100055762.98/1 for its support. References and Notes (1) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26, 163. (2) Moskovits, M. ReV. Mod. Phys. 1985, 57, 783. (3) Bilmes, S. A. Chem. Phys. Lett. 1990, 171, 141. (4) Bilmes, S. A.; Rubim, J. C.; Otto, A. Chem. Phys. Lett. 1989, 159, 89. (5) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. II 1979, 75, 790. (6) Blatchford, C. G.; Campbell, J. R.; Creighton, J. A. Surf. Sci. 1982, 120, 435. (7) Freeman, R. G.; Hommer, M. B.; Grabar, K. C.; Jackson, M. A.; Natan, M. J. J. Phys. Chem. 1996, 100, 718. (8) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629. (9) Chen, C. K.; Heinz, T. F.; Ricard, D.; Shen, Y. R. Phys. ReV. Lett. 1981, 46, 1010. (10) Antoine, R.; Brevet, P.-F.; Girault, H. H.; Bethell, D.; Schiffrin, D. Chem. Commun. 1997, 1901. (11) Antoine, R.; Pellarin, M.; Palpant, B.; Broyer, M.; Pre´vel, B.; Galletto, P.; Brevet, P.-F.; Girault, H. H. J. Appl. Phys. 1998, 84, 4532. (12) Galletto, P.; Brevet, P. F.; Girault, H. H.; Antoine, R.; Broyer, M. Chem. Commun. 1999, 581. (13) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55. (14) Vance, F. W.; Lemon, B. I.; Hupp, J. T. J. Phys. Chem. B 1998, 102, 10091. (15) Clays, K.; Hendrickx, E.; Triest, M.; Persoons, A. J. Mol. Liq. 1995, 67, 133. (16) Fe´lidj, N.; Le´vy, G.; Pantigny, J.; Aubard, J. New J. Chem. 1998, 725. (17) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; Wiley: New York, 1983. (18) Johnson, P. B.; Christy, R. W. Phys. ReV. B 1972, 3, 4370. (19) Takami, A.; Kurita, H.; Koda, S. J. Phys. Chem. B 1999, 103, 1226.
