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J. Phys. Chem. 1996, 100, 5166-5168
Unusual Extinction Spectra of Nanometer-Sized Silver Particles Arranged in Two-Dimensional Arrays George Chumanov, Konstantin Sokolov, and Therese M. Cotton* Ames Laboratory and the Department of Chemistry, Iowa State UniVersity, Ames, Iowa 50014 ReceiVed: December 11, 1995X
Two-dimensional arrays of 1000 Å silver particles were prepared by adsorption from colloidal suspensions onto glass slides derivatized with (3-mercaptopropyl)trimethoxysilane. By variation of the concentration of the colloidal suspensions, slides with different interparticle distances were obtained, and their extinction spectra were measured in water. A blue shift (up to 90 nm) and sharpening of the peak corresponding to the plasmon resonance was observed as the distance between particles decreased to a value comparable to, or less than, their diameter. The angular dependence of the extinction spectra was studied for s- and p-polarized light. The data are interpreted to result from coherent coupling of the plasmon resonances in closely spaced silver particles upon irradiation with light.
Introduction Colloidal suspensions of nanometer-sized silver particles are a bright yellow-greenish color due to the intense bands around 400-500 nm in their extinction spectra. These bands correspond to the excitation of surface plasmon resonances.1 The frequency of the surface plasmon depends on the size of the particles and its dielectric environment. For sizes comparable to the wavelength of light, retardation effects play an important role. These result from differences in the phase of the incident light within a single particle and, as a consequence, the surfacecharge oscillations (surface plasmons) will have multipolar character. On the basis of phenomenological description by Mie,2 the multipolar charge oscillations in a sphere can be presented as a series of spherical harmonics of different orders.3 As the size of the metal particle increases, the contribution of the higher harmonics becomes more important. For closely spaced metal particles which exhibit plasmon resonances, coupling of the oscillating multipoles might be expected. For strong coupling, changes in the extinction spectrum of an ensemble of closely spaced particles should be observed relative to that of a system in which the particles are far apart. In addition, if the particles are arranged in a regular array, coherent effects may be present in both excitation and scattering. The concept of interaction of plasmon resonances between metal particles was first introduced by Yamaguichi4 and the theory was developed in his subsequent publications5 to explain the extinction spectra of silver island films. Later, Wokaun,6 using a similar approach, calculated the Raman enhancement for molecules near the surface of silver particle arrays produced by a lithographic technique. Both researchers used the local field concept in their calculations in which the field experienced by an individual particle is the sum of the field of the incident radiation and fields produced by the other particles. The plasmon resonance is approximated as a dipole and interaction between dipoles is considered in the far field regime. Retardation effects are also included. In essence, their calculations predict a red-shift and broadening of the plasmon resonance when dipole-dipole interactions are included. Gersten and Nitzan7 have considered the effect of the close proximity of * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, March 1, 1996.
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two metal spheres on the plasmon resonance. In their calculations the electrostatic interaction of two dipoles was taken into account. As a result the resonance frequency of this system was also red-shifted relative to that of an isolated sphere. Laor and Schatz8 calculated the effect of cluster size of contacting silver hemispheroids on the red-shift of the plasmon resonance. A large body of experimental work exists regarding the study of the optical properties of silver and gold island films (see, for example, refs 9-11). Activity in this area was greatly stimulated in recent years by the observation of the surfaceenhanced Raman effect.12,13 Temperature annealing of island films of different mass thickness produces irregular arrays of oblate spheroids with a size ranging from 275 to 4000 Å for silver and 160 to 740 Å for gold with a fairly large size distribution.11 Regular arrays of silver and gold particles were produced using microlithography.14,15 The silver particles were approximately 1300 Å with a periodicity of 3000 Å. The plasmon maximum was 530 nm, and no unusual optical features were observed. In the case of the gold arrays, the particles were 200-350 Å with a 500 Å interparticle spacing. The broad peak at 540 nm in the extinction spectrum was assigned to a localized plasmon resonance of the small gold particles. The authors stated that its position is affected by the shape of the particles, the medium in which they are embedded, dipole-dipole interactions between particles, including retardation effects, and interaction of the particle dipole with its image dipole in the substrate. In this letter, experimental observations are presented which are believed to reflect a new phenomenon: coherent coupling of the plasmon resonances of closely spaced silver particles arranged in semiregular, two-dimensional arrays called colloidal metal films (CMFs). This phenomenon is manifest by a dramatic blue shift (ca. 90 nm) and narrowing of the extinction maximum of the film relative to that of a suspension of noninteracting particles. Experimental Section Colloid metal films have been introduced as new substrates for enhanced spectroscopies.16,17 They are prepared by adsorption of metal particles onto substrates derivatized with functional groups that have a high affinity for metals. In this study, CMFs were prepared on glass slides derivatized with (3-mercaptopropyl)trimethoxysilane (MPS). The slides were exposed to a © 1996 American Chemical Society
Letters
Figure 1. Extinction spectra of silver colloidal films. Numbers 1-4 denote spectra of films with different particle densities on the surface and correspond to the images shown in Figure 2. The extinction spectrum of colloidal suspension from which all films were prepared and an enlarged spectrum 1 are shown in the insert.
colloidal suspension of silver particles with an average diameter of ca. 1000 Å for a period of time, resulting in irreversible adsorption of the metal to the surface due to the formation of silver-sulfur bonds. Because of the electrostatic interaction between individual particles, a semiregular two-dimensional array is produced. The concentration of the colloidal suspension was varied, thereby producing films with different particle densities. Exposure to concentrated (O.D. g3) colloids for several days resulted in highly dense films in which the mean distance between particles is on the order of, or smaller than, their diameter. Extinction spectra were measured on a Lambda 6 (PerkinElmer) UV-vis spectrophotometer. When appropriate, a bare glass slide was used as a reference. All spectra were obtained from the CMF immersed in water. Electron microscopy was performed in the reflection mode using a JEOL 1200EX scanning transmission electron microscope. A platinum/palladium (80:20) film of approximately 100 Å thickness was deposited on the specimens using a Polaron E5100 sputter coater. Results and Discussion The extinction spectra of CMFs of different particle densities are presented in Figure 1, and the corresponding electron micrographs in Figure 2. The insert in Figure 1 depicts the extinction spectrum of the colloidal suspension used for the preparation of the films, together with that of the low particle density CMF in which the mean distance between particles is significantly greater than their diameter. Because the interparticle spacing in the colloidal suspension of optical density 1.0 is approximately 5 µm, the extinction spectrum represents the plasmon resonances of noninteracting particles. It should be noted that in order to achieve interparticle spacing in colloidal suspensions comparable to that of a highly dense CMF, an optical density of ca. 104 is required. This is impossible because of aggregation at high particle concentrations. The structures shown in the electron micrographs in Figure 2 are representative of the entire film, as determined from analysis of more than 10 different spots across the CMF. The occasional dimer or rod-shaped particle constitute no more than a few percent of the surface. In many fields none of these were observed. Therefore, it can be concluded that the structures in Figure 2 are uniformly distributed within the area probed by the light beam in the extinction measurements. Because the electron microscopy of the CMF was performed using thin-
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Figure 2. Electron micrographs of the silver colloidal films with different particle densities. The numbers 1-4 correspond to the spectra shown in Figure 1. Prior to imaging the films were overcoated with ca. 100 Å metal film.
film metal overcoating, information about the true shape and size of the silver particles was obtained from electron micrographs of the colloidal suspension from which films were prepared. Electron diffraction reveals that greater than 80% of all particles are twins with a polyhedral shape. Less than 5% are single crystals with a pyramidal or rodlike (2-3 major/minor axis ratio) shape. Also, multiple-twin particles are present in a small amount. It should be noted that all displayed highly regular shapes with clearly defined edges and facets. No roughness on the scale up to 2 nm was observed. In view of the fact that most silver particles are polyhedral, the CMFs can be considered as an array of spherically symmetric particles. Several distinct features are observed in the extinction spectrum of colloidal suspension: two maxima near 510 and 425 nm and two shoulders around 380 and 350 nm. All of these features can be attributed to excitation of different spherical harmonics of the plasmon resonance for a single particle.18 These correspond closely to the results of numerical calculations for 1000 Å silver spheres based on Mie theory by Kreibig and Zacharias.19 The maxima at 510 and 425 nm are assigned to dipole-like and quadrupole-like oscillations, respectively, whereas the shoulders at 380 and 350 nm are probably due to excitation of higher multipoles. All bands in the extinction spectrum are also somewhat broadened due to the particle size distribution. As the density of the particles in the CMF increases, a dramatic change is observed in the extinction spectrum. The lowest energy maximum appears to shift to the blue spectral region and finally, collapses to a single sharp peak at ca. 431 nm. It is not clear if this sharp maximum is a blue-shifted peak corresponding to dipole oscillation or if it is an “enhanced” peak, corresponding to the quadrupole oscillation of single particles. The observed phenomenon occurs when the mean distance between particles is less than, or equal to, their diameter. In addition, our preliminary data show that changes in extinction spectrum occur when the particle diameter exceeds 600-700 Å. At this size the multipole character of the plasmon oscillations become important. However, it seems unlikely that coupling through quadrupole oscillations will dominate that of the dipole because the latter interaction is stronger and extends over a longer range than the former. Yet, each particle has several neighbors, and light-induced plasmon oscillation in the neighboring particles must be coherent. This suggests that symmetry conditions may still favor coupling through multipoles rather than dipoles.
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Letters different from those required for coupling of components parallel to the surface. This is manifested in the two maxima in the extinction spectra, the relative intensities of which vary with tilt angle. Thus, the complex behavior of the high-density CMF extinction spectra with tilt angle for p-polarized light results from the differences in coupling of the normal and parallel components of the plasmon resonances, both of which are also affected by dephasing. Conclusions
Figure 3. Dependence of the extinction spectra of a high density silver colloidal film on the tilt angle for s- and p-polarized light. The arrows indicate the direction of the changes in the spectra with increasing tilt angle.
The spectra of the CMFs in Figure 1 were obtained at normal incidence. The dependence of the extinction spectrum of the high density CMF on the tilt angle for s- and p-polarized light is presented in Figure 3. The fact that the extinction spectrum is dependent upon the tilt angle of the film is itself compelling evidence for a collective phenomenon. Otherwise (in the absence of coupling between particles) there is no reason for such a dependence because the silver particles have spherical symmetry. Indeed, with low particle density CMFs, no unusual angular dependence of the extinction spectra was observed. The effect of the underlying substrate is believed to be minimal and primarily causes a small red-shift in the plasmon resonance.9 Although a rigorous interpretation of the angular dependence requires theoretical modeling, tentative explanations can be suggested. In the case of s-polarized light, the broadening and decrease in the intensity of the extinction band as the tilt angle is increased (normal incident is considered 0°) result from dephasing of the plasmon oscillation in neighboring silver particles. With p-polarized light, the dependence of the extinction spectrum is more complex. As the tilt angle of the CMF increases the main maximum in the spectrum (431 nm) first decreases in intensity, slightly broadens, and shifts to the red. It reaches a minimum in intensity around a 30° angle. Then it continues to shift to the red, increases in intensity, and reaches a maximum around 55° at the 471 nm position. A decrease in intensity and broadening is observed with even further increase of tilt angle (not shown in Figure 3). These changes result from a dephasing phenomenon. The question is whether there are actually two independent maxima, 431 and 471 nm, contributions of which to the overall extinction spectrum vary with tilt angle, or if there is a gradual evolution of a single maximum and 431 nm and 471 nm are its extreme positions. The shoulder at 431 nm which remains at high tilt angles supports the former assignment. At normal incidence multipolar oscillations are in a plane parallel to the surface of the CMF. With tilting the component normal to the surface becomes excited. Obviously, the conditions for coupling of the normal components are
We observed a new phenomenon which suggests strong coupling of plasmon resonances in semiregular arrays of silver particles. To our knowledge, this is the first observation in which coherent interaction (as manifest by a blue shift of the plasmon resonance) occurs upon irradiation of a system composed of nanometer-sized metal particles. Although we have offered a tentative explanation for the experimental observations, a comprehensive theoretical analysis is needed. This is especially critical in view of the fact that currently available theoretical treatments predict a red-shift and broadening of the corresponding peak upon interaction between plasmon resonances in different particles. In addition to the points discussed above, this analysis should include possible interference effects in the scattered light. Because of nonradiative coupling between particles, the highly dense CMFs may be considered as an efficient energy-transfer system. As such these films may have both fundamental and practical importance in the future. Currently, a detailed study is underway of the optical and energy-transfer properties of the CMFs as a function of the silver particle size and their density on the surface of the film. Acknowledgment. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. W-7405-Eng-82. This article was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences. References and Notes (1) Kerker, M. The Scattering of Light and Other Electromagnetic Radiation; Academic Press: New York, 1969. (2) Mie, G. Ann. Phys. 1908, 25, 377. (3) Born, M.; Wolf, E. Principles of Optics; Pergamon Press: New York, 1993. (4) Yamaguchi, S. J. Phys. Soc. Jpn. 1960, 15, 1577-1581. (5) Yamaguchi, T.; Yoshida, S.; Kinbara, A. Thin Solid Films 1973, 18, 63-67. (6) Wokaun, A. Solid State Phys. 1984, 38, 223-294. (7) Gersten, J.; Nitzan, A. Surf. Sci. 1985, 158, 165-189. (8) Laor, U.; Schatz, G. C. Chem. Phys. Lett. 1981, 82, 566. (9) Royer, P.; Goudonnet, J. P.; Warmack, R. J.; Ferrell, T. L. Phys. ReV. B 1987, 35, 3753. (10) Kennerly, S. W.; Little, J. W.; Warmack, R. J.; Ferrell, T. L. Phys. ReV. B 1984, 29, 2926. (11) Allen, E. A.; Scott, G. D.; Thompson, K. T.; Vess, F. J. Opt. Soc. Am. 1974, 64, 1190. (12) Zeman, E. J.; Carron, K. T.; Schatz, G. C.; Van Duyne, R. P. J. Chem. Phys. 1987, 87, 4189. (13) Brandt, E. S.; Cotton, T. M. In Physical Methods In Organic Chemistry; Rossiter, B. W., Baetzold, R. C., Eds.; John Wiley & Sons: New York, 1993; Vol. IXB, pp 633-718 and references therein. (14) Liao, P. F.; Bergman, J. G.; Chemla, D. S.; Wokaun, A.; Melngailis, A. M.; Hawryluk, A. M.; Economou, N. P. Chem. Phys. Lett. 1981, 82, 355. (15) Craighead, H. G.; Niklasson, G. A. Appl. Phys. Lett. 1984, 44, 1134. (16) Chumanov, G.; Sokolov, K.; Gregory, B.; Cotton, T. M. J. Phys. Chem. 1995, 99, 9466-9471. (17) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735. (18) Chu, L.; Wang, S. J. Opt. Soc. Am. B 1985, 2, 950. (19) Kreibig, U.; Zacharias, P. Z. Phys. 1970, 231, 128-143.
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