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Surface-Enhanced Raman Spectroscopy Hot-Spots on Ostwald Ripened Silver Nanoparticles Prepared by Galvanic Displacement Przemyszaw R. Brejna,† Uttara Sahaym,‡ M. Grant Norton,‡ and Peter R. Griffiths*,† † ‡
Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343, United States School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington 99164-2920, United States ABSTRACT: Silver particles grown on silicon in a galvanic displacement process undergo Ostwald ripening in which small particles merge into bigger ones. In this process the particles are shown by scanning electron microscopy to become smoother. Even though the surface area of the nanoparticles is halved on ripening, the Raman enhancement provided by them is at least five times higher than of the particles that did not undergo the ripening. In several regions, transmission electron microscopy (TEM) images show that the ripened particles resemble agglomerated colloids. In these regions the signal due to surface-enhanced Raman scattering (SERS) is greatly enhanced, suggesting that they are the SERS “hot spots”. The importance of boundaries formed between silver particles to the increased Raman signal is suggested.
’ INTRODUCTION The increased signal that is generated from surface-enhanced Raman spectroscopy (SERS) substrates stems mainly from the electromagnetic (EM) enhancement produced by silver or gold particles upon laser irradiation. To achieve conditions suitable for the enhanced EM field, the electrons in the silver particles must be confined either in the surface features of roughened gold or silver or in nanometer-size particles of these metals, so that the plasmons (the collective motion of electrons) are brought into resonance with the frequency of the laser. For this purpose, the particles or the roughness features must be smaller than 100 nm.1 There are many methods for the preparation of small silver or gold structures, and structural features leading to high enhancement have been identified.2-6 Aggregated colloids are one example of such structures, since they are known to provide significantly higher enhancement than separated particles of the same diameter.7-10 Single-molecule SERS has been reported on such assemblages when the adsorbate has an absorption band at or near the laser wavelength (i.e., the spectrum is both surfaceenhanced and resonance-enhanced).11-13 The aggregation of small particles is said to be a necessary factor for such high enhancement.14 The major part of the enhancement arises from the increase of the EM field between particles. For large clusters the enhancement is independent of the cluster size.15,16 However, the aggregates are not always “hot”.8 The reason might be that, for agglomerates of particles, protrusions smaller than the wavelength of visible light are important for high enhancement.17 Szteinbuch has found that a monolayer aggregate of gold particles must possess planar protrusions that can support surface plasmons to provide strong enhancement.18 Without such protrusions the aggregate enhances only twice as much as isolated r 2011 American Chemical Society
particles. Furthermore, as reported by Pignataro et al., the stacking of particles in the z direction contributes significantly to the SERS enhancement.19 Xu et al. calculated that spherical particles located very close to each other may contribute to an enhancement of the EM field by 108 that increases to 1011 when protrusions are introduced.20 The same group calculated that the enhancement found at crevices in round merged particles contributes an enhancement factor of about 107 compared to isolated particles. The factor depends strongly on the diameter of the two spheres. A favorable condition for creating such clusters may also be achieved by electroless deposition which leads to the formation of fractal structures that possess many protrusions.4,21-23 Those were found to localize hot spots in small volumes.4,24,25 According to García-Vidal and Pendry, localized plasmons created by EM coupling between touching metal particles give the largest contribution to surface enhancement of Raman spectra.26 It has also been known that the capacitive form of conduction prevails over ohmic conduction for small particles in visible light frequencies.27,28 This leads to multipole coupling that may change the magnitude of the enhancement factor, as suggested by Stockman et al.27 In addition to the spatial organization of the silver particles, the importance of the environment in which they are located with has been acknowledged.29 It has been calculated that silver particles that are coated with a dielectric layer and are touching may provide 1014 enhancement on the symmetry axis and as much as 5 1013, a distance of 1.5 nm away from the axis.30 Received: August 11, 2010 Revised: December 17, 2010 Published: January 12, 2011 1444
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Figure 1. SEM images of silver particles grown on silicon from 10 mM AgNO3 þ 0.24% HF for 12 min; (a) as prepared, and after immersion in water for (b) 1 h and (c) 6 h.
Interestingly silica beads coated with a layer of gold exhibit higher enhancement than gold particles alone, although they were not highly enhancing when aggregated.9 A higher enhancement has also been observed by layering dieletric material between silver nanoparticles. Leverette et al. showed that two layers of silver particles with a silver oxide layer sandwiched between them may enhance the Raman signal more efficiently than similar particles without the oxide layer.31 Similarly a 10-fold signal increase was reported by Li and Cullum for dual or multiple coating of silver prepared by physical vapor deposition as compared to a single layer of silver.32 The enhancement was reported on similar structures made of gold.33 Fan and Brolo used sol-gel substrates in which several layers of silver have been deposited.34 It was found that the highest enhancement from the substrates arose after seven depositions of the sol-gel. It might be possible that similar coupling to the one proposed by Kinnan and Chumanov extends beyond the sandwich structures.35 In the system we have investigated, the SERS response of adsorbates on silver particles that are separated from each other by boundaries formed between the particles is shown to be increased as compared to the fractal structures from which they are derived. Silver particles were grown on a silicon substrate by the galvanic displacement process. When subsequently exposed to water, the particles detach from the substrate and form agglomerates or attach themselves to larger dendritic structures during which process the coarse dendritic structures become smoother. Even though the fractal particles that are formed originally appear to become smoother upon ripening, the ripened particles give rise to a significantly increased enhancement.36
’ EXPERIMENTAL METHODS Silver nanoparticles were produced as described in ref 36. Briefly, silicon disks (Lattice Materials LLC, Bozeman, MT) were immersed in a 0.24% (w/v) HF (48% ACS reagent grade, SigmaAldrich, St. Louis, MO) solution that was 10 mM in AgNO3 (High-Purity Chemical, Inc., Portland, OR) for a few minutes. Deionized (DI) water was used as a solvent. After about 10 min, the disk becomes coated with small particles as well as with dendritic structures. The silver-coated silicon disk was transferred to DI water for 1 h to allow Ostwald ripening and then to a 0.36 mM solution of benzenethiol (99% purity, Acros Organics, Morris Plains, NJ) in methanol (HPLC grade, EMD Chemicals Inc., Gibbstown, NJ) to form a monolayer of the thiol molecules on the surface of the silver. The particles were finally transferred onto a transmission electron microscopy (TEM) grid that was coated with Formvar and carbon. A few drops of the same solution of benzenethiol were applied to ensure that the particles were completely coated with the thiol after they were transferred to the TEM grid, and the grid was rinsed with methanol. Raman spectra and maps were measured on a WITec (WITec Instruments
Corp., Ulm, Germany) alpha300 spectrometer equipped with a 532 nm and a 785 nm laser. Maps were acquired with the 532 nm laser of 22 μW power through 100 objective; the integration time was 0.1 s. The spatial resolution was approximately equal to the wavelength. TEM analysis of the as-prepared and ripened Ag particles was conducted on a JEOL 1200EX II and a Philips CM200. Scanning electron microscope (SEM) images were taken on Zeiss Supra 35 instrument. The surface area of the particles was measured using a Micromeritics FlowSorb II 2300.
’ RESULTS AND DISCUSSION We have shown previously that silver particles grown on silicon disks undergo Ostwald ripening when the particles are exposed to water, and they lose their fractal structure and surface roughness; that is, the smaller particles disappear, and the bigger ones grow larger and smoother.36 When some of the small particles detach from the silicon substrate, they may merge into the large ones or form agglomerates. (This ripening process was also observed in water/methanol mixtures but was not observed when the silver particles were left in 100% methanol.) It has been welldocumented that particles may change shape upon exposure to certain chemicals.37,38 When germanium was used as a substrate, however, ripening did not take place. The observation may be explained based on the paper of Peng et al. in which it was shown that, during the growth of silver particles, silicon dioxide is recovered below them.39 Hence it is possible that this phenomenon could contribute to the detachment of the silver particles grown on silicon but not on germanium. As can be seen in Figure 1, compared to the ripened particles, the unripened particles possess more junctions and higher surface roughness and hence would be expected to have more hot spots as there are more places for close contact of silver features as was observed by Fang et al. for a flower-like pattern.40 To determine the change in surface area on ripening, several milligrams of the particles before and after ripening were grown and transferred to a Brunauer-Emmett-Teller (BET) apparatus. The surface area of the unripened particles was around 10 m2/g, and that of the ripened ones was ∼4.6 m2/g. Surprisingly, however, the Raman signal from those particles that underwent ripening was enhanced by at least a factor of 5 compared with the signal from those particles prepared under the same conditions but that were not ripened; see Figure 2. The extent of the enhancement of benzenethiol adsorbed on the ripened particles over the asprepared particles was approximately the same irrespective of the laser wavelength used (532 and 785 nm), which is congruent with previous reports concerning fractal structures.24,41 The structure of both types of particles, as-prepared and ripened, was examined by TEM. Figure 3 shows a montage of electron diffraction patterns (DPs) and a corresponding bright field image of an as-prepared Ag particle. The DPs, which were 1445
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The Journal of Physical Chemistry C taken from different regions of the particle, indicate that originally the particles grow as a single crystal. However, during ripening the particle morphology changes, and small particles are attached to the periphery of the larger transformed particle, as shown in Figure 4. Figure 5a shows an optical image of a ripened particle that had been transferred onto a TEM grid. Figure 5b shows superimposed Rayleigh (blue) and Raman (red) maps of the region marked in Figure 5a. One long particle was found in this region along with two smaller ones nearby. The regions of these particles that did not provide Raman enhancement are shown in blue, while the locations where the Raman signal was enhanced are shown in red. Certain regions on the larger long particle yielded an enhanced SERS signal, as did one of the spots (marked
Figure 2. SERS spectra of a monolayer of benzenethiol on substrates as prepared (below) and on substrates left in water for 1 h (above) before the application of the analyte. The spectra were acquired with a 532 nm laser through a 10 objective with 10 accumulations and an integration time of 1 s; the power at the sample was 43 μW. The morphologies of the substrates are shown in Figure 1a,b.
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#2), while the other (marked #1) did not. Figure 6 shows spectra measured in each of these two regions. A bright field TEM image of the enhancing parts of the long particle is shown in Figure 7. Small protrusions and/or clusters of particles were found to be present in the enhancing region. Bright field TEM images of the nonenhancing (#1) and enhancing (#2) particles from the Raman map in Figure 5b are also shown in Figure 8. The nonenhancing particle appears to be a single particle, whereas the enhancing particle was found to be an agglomerated pair. Figure 9 shows analogous data from clusters of nanoparticles. Figure 9a shows a map of the Rayleigh line scattering of silver particles on the TEM grid. Figure 9b shows a Raman map of benzenethiol on the same particles, and Figure 9c shows a TEM image of an enhancing cluster marked in Figure 9b. The morphology of the particle shown in Figure 8a somewhat resembles that of the smooth rod-like particle seen in Figure 7; such particles lead to much lower enhancement than agglomerated particles. Particles that provide SERS enhancement are shown in Figures 8b and 9c. The makeup of these particles resembles that of aggregated colloids; that is, they are clusters of connected individual single crystals. There may be several reasons for the higher enhancement provided by the ripened particles. In light of previous reports, most single molecule SERS spectra have been observed on clustered particles;12,14,42 thus, it is not surprising that the ripened particles that form clusters provide higher enhancement than the ones that grow separately. The agglomerates are known to possess a fractal structure. One of the properties of fractals is that high local fields exist that may exceed the external field. Such fluctuations contribute to the enormous enhancement of the Raman scattering.28,43 Agglomerated particles also contain many protrusions in planar and vertical directions which are necessary for providing high enhancement as suggested by Szteinbuch and Pignataro et al.18,19 The dendritic particles do not seem to contribute to the enhancement unless small particles are merged into them, creating
Figure 3. Montage showing (c) a bright field TEM image of part of a Ag particle that did not undergo ripening and (a, b, and d) diffraction patterns recorded from regions marked with circles in the bright field image. These patterns show that each region is monocrystalline. 1446
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Figure 4. Bright field TEM image (center) of a particle that underwent ripening and selected area diffraction patterns taken from regions marked in image. The upper right image shows an example of an attached nanoparticle. Only in this region is the particle polycrystalline.
Figure 5. (a) Optical image of the TEM grid containing silver particles transferred from a silicon substrate and (b) superimposed Rayleigh and Raman images of the area shown in the black square on a; the red area shows where the 1573 cm-1 benzenethiol peak is enhanced, and the blue regions show where the Rayleigh signal exceeded a certain threshold. It can be seen that most of the long particle does not enhance the SERS signal and that particle 1 is also not SERS active.
Figure 6. Spectra of benzenethiol measured in the red (above) and blue (below) regions of Figure 5b. The measurement time for each spectrum was 0.1 s.
protrusions. The particles that detach from the silicon substrate may fuse with each other, creating interfaces that possess a more disordered structure due to crystal misorientation between the two particles and create a metal-insulator composite.43 The interfaces will therefore act to isolate the electrons that are confined in
Figure 7. TEM images of the enhancing part of the long particle shown in Figure 5. The width of the protrusion is around 50 nm, and the height is about 30 nm.
the crystalline parts of the agglomerates in a setup that resembles metal-insulator-metal (MIM) waveguides. In this type of configuration the electric field creates a standing wave between metal 1447
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Figure 8. (a) Nonenhancing (#1) and (b) enhancing (#2) particles from the Raman map shown in Figure 5b.
Figure 9. (a) Rayleigh line map of silver particles on TEM grid, (b) Raman map of benzenethiol on the same particles, and (c) TEM image of an enhancing cluster marked on b.
layers and is strongly confined at the ends of the insulator layer.44,45 Choi et al. treated such a setup as an electric-circuit model and calculated an enhancement factor of the Raman signal as high as 1010.46 The influence of the electrical properties of isolator-conductor setup on the EM field cannot be neglected, as reported by Qian et al.47 That group studied the Raman response of silver particles deposited onto various substrates that were selected based on their conductive properties. It was suggested that the more conductive substrates may weaken the localization and coupling between surface plasmons. It was also observed that silver particles deposited on an isolating surface led to a higher SERS response than the particles that were in contact with a conductive surface. Qi et al. observed a higher than expected SERS response from silver particles deposited on ZnO.48 As in the case of the grain shown on Figure 7, the size of particles investigated by Qi et al. was in the range 20-30 nm, and a similar laser excitation wavelength to the one employed by us was used. The extended electronic coupling may take place through the crystal defects and boundaries. Such behavior has already been proposed by several groups, who suggested that electronic coupling of surface plasmons may extend beyond two silver particles separated by adsorbed molecules.35,49,50 A similar enhancement may be taking place in the substrates reported in refs 21 -24 which may be approximated as MIM configurations. One other possible cause of the higher enhancement was suggested by Prokes et al. who proposed that the dielectric-metal system may lead to multiple reflections leading to further enhancement.51 It may therefore be concluded that, since the interfacial region (whether it is a grain boundary or simply the contact region between two touching/adjacent particles) separating the monocrystalline
silver particles possesses a different dielectric constant than the bulk metal, this layer helps to confine the electrons in the individual particles and to bring them into resonance. For example, it is well-known that grain boundaries in polycrystalline materials impact the electrical conductivity (e.g., ref 52). In this respect, it is worth noting that Reiss et al. found that the (dc) conductivity decays exponentially with increasing number of boundary layers per mean free path.53 In addition to this type of coupling or abnormal optical absorption,54 grooves in the touching particles provide strong enhancement that is comparable to the enhancement arising from the junction between two closely spaced particles.55 A similar observation was published by McMahon et al.56 Glembocki et al. also suggested that the EM field from silver particle is focused in the region between nanoparticles and a dielectric substrate and that the particle-substrate interactions must be taken into account in model calculations.57
’ CONCLUSION The paper provides a further insight into the nature of hot spots on SERS-active substrates. Silver structures grown by galvanic displacement on a silicon substrate were found to yield a higher enhancement factor when they are allowed to undergo Ostwald ripening. During this process silver particles are attached to the main central structure creating additional particle-particle interfaces. It is suggested that the boundaries between these particles play an important role in helping confine electrons within the individual silver particles. This effect increases the EM field at the surface of the particles as well as between them and therefore further increases the SERS signal. Thus in the design of SERS 1448
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The Journal of Physical Chemistry C substrates the inclusion of a dielectric layer around the silver particles should be considered for best enhancement.
’ AUTHOR INFORMATION Corresponding Author
*E-mail: pgriff@uidaho.edu; phone: 1-208-885-5807; fax: 1-208885-6173.
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