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Reduction of HAuCl4 by Na2S Revisited: The Case for Au Nanoparticle Aggregates and Against Au2S/Au Core/Shell Particles† A. M. Schwartzberg,‡,§ C. D. Grant,| T. van Buuren,| and J. Z. Zhang*,‡ Department of Chemistry and Biochemistry, UniVersity of California, Santa Cruz, California 95064, and Lawrence LiVermore National Laboratory, 7000 East AVenue, LiVermore, California 94550 ReceiVed: NoVember 20, 2006; In Final Form: January 24, 2007
The reaction of sodium sulfide with chloroauric acid has been surrounded by a controversy over the structure of the resulting product. The original report proposed a Au2S/Au core/shell structure based on strong near-IR resonance and limited transmission electron microscopy. Subsequent reports used the same model without further attempts to determine the structure of the products. With a significant body of experimental work compiled over a period of several years, we have shown that the major product of this reaction is aggregated spherical nanoparticles of gold with a minority component consisting of triangular and rod-like structures. This is in contradiction to the core/shell structures as originally proposed. Recently, there have been additional reports that again suggest a Au2S/Au core/shell structure or irregularly shaped Au nanoparticles as an explanation for the near-IR resonance. To help resolve this issue, we have carried out further experiments to determine how the reaction products may depend on experimental conditions such as concentration and aging of the reactants, particularly Na2S. It has been determined that sodium thiosulfate is the likely product from Na2S aging. In addition, persistent spectral hole burning experiments have been conducted on gold nanoparticle aggregate (GNA) samples at excitation intensities that are lower than that required to melt the nanostructures. We have observed a decrease in optical absorption on resonance with the excitation laser wavelength, with simultaneous increases in absorption to the blue and red of this wavelength region. However, in the presence of the stabilizer poly(vinyl pyrrolidone) (PVP), no increase in absorbance was observed but rather a blue shifting and decrease in intensity of the near-IR plasmon resonance. These results imply that the non-stabilized GNAs are able to break apart and reform into off resonant aggregate structures. In contrast, this behavior is suppressed in PVP stabilized GNAs because of the presence of polymer which quickly passivates the individual nanoparticles that comprise the GNAs after they are disrupted by laser irradiation. These results would be very difficult to explain if the nanostructures were core/shell. Therefore, these new results again support the model of GNAs as the best possible explanation for the product of the HAuCl4 and Na2S reaction.
1. Introduction Given their long lineage, it is impressive that metal nanoparticles continue to be a considerable topic of scientific interest. The fascination with the striking red and yellow coloration of gold and silver colloids, which are responsible for the color in many stained glass windows, has intrigued scientists for centuries.1 Today the interest in metal nanoparticles stems mainly from their interesting physical and chemical properties as well as their many potential applications such as the trace chemical detection of toxic, biological, or explosive materials.2-5 The main reason for their striking color is due to the surface plasmon resonance (SPR). The SPR is the collective oscillation of conduction band electrons induced by a resonant electromagnetic field.6 By careful manipulation of nanoparticle size, shape, and structure, it is possible to tune the SPR and therefore the scattering and absorption properties.7-10 †
Part of the special issue “Kenneth B. Eisenthal Festschrift”. * Corresponding author. E-mail:
[email protected]. Phone: (831) 459-3776. Fax: (831) 459-2935. ‡ University of California. § Present address: Lawrence Berkeley National Laboratory, Cyclotron Road Mail Stop 2-300, Berkeley, CA 94720. | Lawrence Livermore National Laboratory.
The iconic example of SPR tunability is the gold nanorod.11-15 By elongating a spherical particle and thereby forming a rod, the normal single resonance is split into two, the transverse, a resonance along the short axis of the rod, and the longitudinal, which resonates along the long axis. While the transverse resonance in gold is located at approximately 520 nm, the longitudinal band is red-shifted ranging from 600 nm to greater than 1 µm depending on length. In this way, it is possible to tune the optical resonances of the structures across most of the visible and near-IR. However, gold nanorods are not the only nanostructures where the SPR may be tuned. For example, core/shell particles consisting of an SiO2 core with a Au shell,16-18 hollow nanospheres of gold,19-21 and the controlled aggregation of spherical gold nanoparticles,22-24 to name just a few, all produce a resonance in the red portion of the visible spectrum or into the near-IR. All of these different types of structures have been exploited for surface enhanced Raman scattering (SERS), a powerful technique for detection and imaging.25-28 This is especially interesting for biological applications, such as noninvasive glucose detection and monitoring, since living tissue has low near-IR absorptivity. All of these potential uses have further driven the development of these structures.29-31
10.1021/jp067697g CCC: $37.00 © 2007 American Chemical Society Published on Web 03/10/2007
Reduction of HAuCl4 by Na2S An interesting early attempt to design optically controllable nanostructures came from Zhou et al.32 and later by Halas et al.33 in their reports of a Au2S/Au core/shell nanostructure starting with the reactants HAuCl4 and Na2S. This reaction produced products with the optical signatures of a transverse resonance centered approximately at 520 nm as well as a broad near-IR band similar to gold nanorods. It was claimed that core/ shell structures were formed in a one pot process. This simple reaction was exciting because the formation of core/shell particles generally takes place via a multistep process, such as the creation of a core with the subsequent growth of a shell, which can be difficult and time-consuming.16-18,34 This result was also surprising in that it is well-known that aqueous gold(III) salts will not react directly with sulfides like Na2S or H2S at room temperature to produce gold(I) sulfide but rather Au0, and the direct reaction between elemental gold and sulfur is unfavorable.35 There have been some recent reports of the synthesis of bare Au2S nanoparticles, but only from Au(I) precursors.36,37 This evidence, in addition to the general lack of direct experimental characterization, led us to an in-depth study of this reaction to fully understand the underlying mechanism and the products of this reaction. Because of the perceived disconnect between what had been published on the reaction of sodium sulfide with chloroauric acid and what seemed a plausible result of this reaction, our first work in this area was a study of the structure and optical properties of the products.38,39 Synthetically, we were readily able to reproduce the optical properties of what had been previously published; however, transmission electron microscopy (TEM) showed no evidence of core/shell particles. Further, with gold and sulfur electron energy loss spectroscopy (EELS), only a thin layer of sulfur is visible on the surface of the nanoparticles and is not confined mainly to the core as would be expected in a Au2S/Au core/shell system of this nature. In fact, the majority of what was observed from our TEM studies was a mix of small and large nanoparticles (5-50 nm) seemingly aggregated together along with a minor component that included rod and triangular shaped particles.39 With these results, we proposed that this reaction produced gold nanoparticle aggregates (GNAs) rather than core/shell particles. The strongest argument that previous works in the literature presented for a core/shell system involved the presence of a near-IR band in the optical absorption and theoretical modeling that reproduced the UV-vis and near-IR extinction spectrum. Because a thin shell of gold will confine the surface plasmon to a longer path length than a solid gold particle, there will generally be a red shift in the SPR.8,19,40 It was initially reported that this reaction produced a combination of solid particles (responsible for the transverse band at ∼520 nm) and Au2S/Au core/shell particles (responsible for the longitudinal band in the near-IR); however, other structures are capable of producing similar absorption spectra.22,41,42 For instance, upon aggregation, the normal plasmon of gold particles will couple, producing a two band SPR consisting of a transverse band representing the still present plasmon mode of individual nanoparticles and the near-IR band, which we have termed the extended plasmon band (EPB) from the extended aggregate structure. This indicates that while a core/shell system could potentially produce such an optical signature, it is not the only possible explanation. To further our understanding of the optical properties of the GNAs, we used a common time-resolved technique known as ultrafast transient absorption spectroscopy. This pump-probe method allows us to directly examine dynamic processes, such
J. Phys. Chem. C, Vol. 111, No. 25, 2007 8893 as electronic relaxation, that occur on the femto- and picosecond time scales. Various metal nanoparticle systems of different structures and composition have been studied in this manner,14,43-57 and in this study of GNAs, the initial electronic relaxation time (electron-phonon coupling) was found to be similar to that of isolated nonaggregated gold particles as well as bulk Au. Interestingly, we observed the presence of periodic oscillations in the transient absorption of these samples.58 This phenomenon has been observed in many different metal particle systems, as well as extended structures such as ellipsoids, nanorods, nanoprismatic monolayers, nanotriangles, and nanocages.59-71 The key to observing this phenomenon, however, has always lied in the monodispersity of the sample although there is a recent article reporting the observation of coherently excited vibrational motion in a polydisperse Ag nanoparticle system that contained many different particle shapes.72 Nevertheless, typically in polydisperse spherical nanoparticle systems, variations in oscillation periods due to particles of various sizes will average out in the observed transient, smearing the undulations into a flat decay curve.73 The discovery of oscillations from GNAs is surprising because aggregates tend, by their very nature, to have very large polydispersity in size and shape. The observation of oscillations in such aggregates strongly suggests that the near-IR EPB is inhomogeneously broadened by different sizes and/or structures. Depending on probe wavelength, different individual subsets of GNAs are interrogated and must possess similar oscillatory frequencies. Indeed, as the probe wavelength was tuned from 720 to 850 nm, the period of the oscillations became longer, varying from 37 to 55 ps, which was consistent with the above conjecture. While these behaviors are potentially possible in a core/shell system, the observed oscillation periods are longer than would be expected for a solid particle based on an elastic sphere model and are likely due to large, extended structures.58 Preliminary persistent spectral hole burning (PSHB) experiments were conducted to test and further confirm the proposal that the near-IR band is inhomogeneously broadened by different sized or structured GNAs. It was observed that an optical “hole” was clearly burned into the spectrum of the GNAs with ∼800 nm laser excitation and, thus, supports the above broadening model. Additional support also comes from earlier experimental and theoretical studies of GNAs that also suggested the nearIR band consists of sub-bands due to different GNA sizes, structures, or both.6,74,75 To further confirm our GNA model, small angle and ultrasmall angle X-ray scattering (SAXS and USAXS) experiments were performed to correlate the structure of the GNAs with the EPB.76 It was shown that both the radius of gyration and the fractal dimension derived from the SAXS and USAXS data supported the proposal that the product was GNAs rather than spherical core/shell particles.77 It was also shown that the change in the position of the near-IR resonance as the reaction progressed was due to a change in size or shape of the GNAs. These results further strengthen our proposal that the product of the reaction of chloroauric acid with sodium sulfide is GNAs and not a Au2S/Au core/shell system as was previously proposed. While our previous works provided characterization of the GNA system, they were not application driven. Given the excellent results obtained with aggregated particles in previous SERS work,78-80 we determined that perhaps this system might be promising for this application.81 The first set of experiments attempted were with the dye molecule rhodamine 6G (R6G) as a model system that has been extensively studied previously in
8894 J. Phys. Chem. C, Vol. 111, No. 25, 2007 other works in the literature.82-84 In general, R6G does not adsorb well to gold surfaces, which is requisite for good SERS enhancement given the requirement that the analyte molecule must be within the enhanced electromagnetic field at the surface of the particle, typically less than 5 nm.32,85 R6G is often used with silver SERS substrates because of favorable adsorption to silver but is not as common with gold. Generally, SERS enhancements are lower in gold systems than in silver systems because of the greater electron scattering rate in gold;28 however, this novel GNA system produces results close to those of silver substrates of similar dimensions despite the typically lower enhancements of gold systems. By making a direct comparison between the Raman intensity of a solution of R6G with and that without GNAs, an enhancement factor of 107 was calculated, a remarkable number for a gold nanoparticle SERS substrate. To determine the compatibility of this GNA system with molecules that are biologically relevant, several amino acids and other interesting biomolecules were chosen and readily produced a SERS signal.81 Many of these biomolecules have never before been observed on gold nanoparticle substrates via the SERS technique. All of the analytes chosen yielded excellent signal and thus demonstrated the truly flexible nature of the GNA surface chemistry. Despite this significant body of work on this single subject, there are still a number of articles that continue to claim that core/shell86 or other structures are responsible for the EPB without significant evidence. For instance, it was claimed in a publication that an unexpected product of the Na2S + HAuCl4 reaction was the formation of gold tabular particles.87 It was pointed out by Norman et al.39 as well as earlier in this article that many different particle shapes are observed as a product from the reaction including triangular and tabular particles but that spherical particles are predominant. The presence of tabular particles does not guarantee that they are the sole source of the EPB. To the extent possible, we have responded to these claims.88 There has also been a study using femtosecond transient absorption dynamics claiming to be the first to observe coherent vibrational motion supposedly from Au2S/Au core/ shell particles made from the reduction of HAuCl4 with Na2S.89 This was a very surprising claim since we had already published findings in a 2003 Journal of the American Chemical Society article on our own time-resolved femtosecond transient absorption study showing the same coherent vibrational motion from exactly the same reaction which we attributed to the GNAs.58 As we have already discussed in some detail above and supported by many pieces of data from different physical techniques, the major product from the reaction of HAuCl4 and Na2S is that of aggregated gold particles and not a core/shell system. Guillon et al. make no further attempt to confirm the presence of core/shell particles.89 Interestingly, the results reported mirror our own previous work in the transient absorption dynamics of aggregates produced by the exact same synthesis. It is clear that further work is necessary to convince some that these claims cannot be made without significant characterization. In this article, we will present further evidence to show that in general GNAs are formed from the reaction of sodium sulfide and chloroauric acid. Extensive chemical analysis of the reagents will be shown, and the nature of aging in aqueous sodium sulfide solutions is elucidated in an effort to improve reaction consistency and tunability. As a result of this improved synthetic control, we can conclude that despite changing many reaction parameters, no core/shell particles were ever observed. Further,
Schwartzberg et al. PSHB at laser intensities that fall below that required for melting the nanostructures will show that despite breaking apart and depleting the absorption profile of the GNAs, new GNAs are created which absorb to the blue and red of the resonant beam. This observation is unlikely with core/shell particles at the power densities employed in this study. To reinforce this, a stabilizing polymer was added to the solution with a completely different behavior observed. Particles no longer reform and absorb off resonance, but rather the band is merely depleted and blueshifted with no increase in absorption implying that the stabilizing polymer blocks the reformation of aggregates. Again, this behavior is not likely in a core/shell system. 2. Materials and Methods The synthesis of GNAs has been reported previously.39,58,81 Briefly, 100 µL of a 0.1 M HAuCl4 stock solution was diluted to 5 × 10-4 M with Milli-Q water. To this is added 40-60 µL of a 0.1 M aqueous solution of sodium sulfide, sodium thiosulfate, or sodium dithionate. Both aged and fresh solutions of sodium sulfide were used. Sodium sulfide aging consisted of storage in scintillation vials at 2 °C with occasional exposure to air. Upon the addition of reductant, the color of the solution changed immediately from straw yellow to brown and continued to change over 60-120 min. The final solution color was red or purple depending on reagent concentrations and choice of reductant. Near-edge X-ray absorption fine structure (NEXAFS) experiments were conducted at beamline 8.0.1 of the Advanced Light Source. The sulfur L-edge NEXAFS spectra was acquired in total electron yield mode with an estimated resolution of 0.1 eV. Liquid samples were allowed to evaporate on a clean Ta substrate before entering the analysis chamber. PSHB experiments were carried out with a regeneratively amplified femtosecond laser system.51 The samples of GNAs with and without poly(vinyl pyrrolidone) (PVP) were placed in a 1 cm quartz cuvette. A sample volume of 3 mL was used for every hole burning experiment. The sample was placed at the output of the amplified femtosecond laser system and was slowly stirred at approximately 100 rpm to ensure proper mixing during the PSHB experiment. Laser power was ∼200 mW which corresponded to a power density of 0.2 mJ/cm2. The laser intensity was controlled through the use of absorptive neutral density filters. UV-vis absorption measurements were performed periodically on the GNA samples at various intervals throughout the hole burning process. 3. Results and Discussion 3.1. Chemical State of Sodium Sulfide. The reduction of HAuCl4 with Na2S appears, on the surface, to be a simple reaction, and when properly performed, the absorption profile will be similar to Figure 1a and will consist of structures like that of Figure 1b. These two absorption bands in Figure 1a yield an optical profile which results in a purple solution. Ideally, this color should be observed at every attempted synthesis when combining these two reagents in their correct proportions. However, it became evident early on in our investigations that reactions performed with fresh aqueous solutions of sodium sulfide produced poor quality samples, while aged sodium sulfide solutions would produce spectra closer to what was expected and reported in the literature. Unbeknownst to us, and unreported in the literature before our first works, in order to produce “good” samples, the sodium sulfide solution requires an aging period. The aging time and reaction conditions required for this process were poorly defined and appeared to be a
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Figure 1. (a) Typical UV-visible spectrum of GNA solution. (b) TEM image of the GNAs corresponding to the UV-vis in (a).
seemingly shifting parameter of an otherwise straightforward synthesis. For example, at the exact same concentrations, volumes, and reaction times, the two absorption spectra in Figure 2a were obtained by merely using a fresh solution of sodium sulfide (red) and one aged for several months (blue). While the general shapes of these two spectra are similar, there is a drastic increase in near-IR absorption as well as a blue shift in solutions produced with aged sodium sulfide. This is thought to be due to an increase in the concentration of GNAs produced. However, it should be noted that on the basis of similar optical densities (ODs) of the transverse resonance centered at ∼520 nm, the total concentration of gold particles stays relatively constant independent of aging, aggregated or not. TEM data support the formation of similarly sized and shaped gold nanoparticles; however, because of aggregation during TEM sample preparations, there is little difference in the images of GNAs made with fresh Na2S than in what is shown in Figure 1b. The nature of the aging process and how it affects the resultant solution must be understood to determine a robust, consistent mechanism and synthesis for this reaction. Because these sodium sulfide solutions were stored and “aged” in a refrigerator, it was thought that the dehumidifying nature of this environment was causing an increase in concentration because of evaporation. Alternatively, HS-, the predominant sulfur species in the aqueous solution, could have reacted with water and evolved as H2S gas, effectively decreas-
Figure 2. (a) UV-visible spectra of GNA solutions produced with aged (blue) and fresh (red) sodium sulfide. Inset is the UV-visible spectra of fresh (red) and aged (blue) sodium sulfide solutions. (b) UVvisible absorption spectra of GNA solutions produced by the reduction of HAuCl4 by varying concentrations of sodium thiosulfate. Inset is the UV-visible spectra of GNA solutions produce with sodium thiosulfate (orange) and sodium dithionate (green).
ing the concentration of the reductant in the sodium sulfide solution. Therefore, our initial experiments attempted to determine the affect of aging by varying the concentration of the sodium sulfide solution added. Changes in concentration of fresh Na2S solutions of as much as 1 order of magnitude were used; however, none of these reactions produced any useful results. While these two processes are likely occurring, they are clearly not the defining mechanism responsible for the improved reaction results with aged solutions. There is clearly a more complex change taking place. The first evidence to show a chemical change to the sodium sulfide solution was observed in the UV-visible spectra of aged and fresh solutions (Figure 2a, inset). The spectrum of the fresh Na2S solution (red trace) is clearly more intense and red-shifted
8896 J. Phys. Chem. C, Vol. 111, No. 25, 2007 relative to the aged solution (blue trace). While we may infer that the absolute concentration may be decreasing because of the decrease in OD, chemically, this is nonspecific data, meaning we cannot draw conclusions as to what types of changes are taking place. However, it is clear that there are chemical alterations occurring with time which are undoubtedly responsible for the observed variation in synthetic results. Chemically specific information is required to determine the exact nature of this change. 3.2. Sodium Thiosulfate as Reductant. Because sulfur, by its nature, forms relatively reactive compounds, we thought that perhaps, over time, the sulfur which is present in solution may react to form other species. For instance, HS- may either react with other HS- ions to form polymerized sulfur chains or react with atmospheric oxygen to create oxidized sulfur species. Of these two examples, the latter is a more likely explanation. Polymerized sulfur is generally lower in reactivity than many other sulfur species and has not been reported to form from S2- in aqueous solution. If this were the dominant product of aging, we would likely not see the spontaneous reduction of HAuCl4 to Au0. On the other hand, sulfur oxide species are fairly reactive, and, in fact, upon examining early work on sodium sulfide solutions, one product of oxygen exposure continually appeared, sodium thiosulfate, Na2S2O3.90 Sodium thiosulfate is a commonly found reagent, used in vast quantities in the photographic process as a silver ion scavenger, or “fixing solution”, and in leaching gold from mines.91 Sodium thiosulfate has been relatively unused in the synthesis of nanomaterials with a few exceptions.92,93 By exchanging thiosulfate 1:1 with sodium sulfide, we found that nearly identical results were obtained to GNAs synthesized with Na2S solutions that were aged 18 months (Figure 2b). TEM images show that thiosulfate as reductant produces GNAs as well; however, again because of aggregation processes during sample preparation, there is little visible difference from Na2S produced GNAs. Because thiosulfate is a more stable sulfur species than sodium sulfide, aging had no affect on the optical properties of the resulting GNA solutions. This reaction was performed hundreds of times from one solution of sodium thiosulfate over approximately 1 year with no degradation or change in synthetic results observed. Each time the reaction is performed at a given concentration, consistent results are obtained. Because of this new consistency, we are now able to accurately control the position of the EPB by altering the volume of thiosulfate added (Figure 2b). At increased thiosulfate concentrations, we observed a blue shift in the EPB and a red shift with lower concentrations. It is clear from Figure 2b that while changing the concentration of thiosulfate affects the position of the EPB, it also affects the relative intensity between the transverse band and the near-IR resonance (spectra are shifted in absorption for clarity). It is currently unclear why the intensity of the near-IR band changes as it blue shifts; however, it is most likely due to changes in shape, size, and interparticle interaction with varying concentrations of thiosulfate. While this shows that thiosulfate can produce similar results to aged sodium sulfide, it is not completely conclusive evidence that the same structures are being produced and that thiosulfate is the only product of aging. There are several oxide species which may result from this oxidation. Another species that can form from the oxidation of Na2S solutions is dithionate, S2O62-, so as a further control, it was deemed useful to determine what product(s) will form as a result of replacing sodium sulfide with dithionate. Similar to the thiosulfate synthesis, sodium sulfide was replaced 1:1 with sodium dithionate in the GNA reaction to
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Figure 3. NEXAFS spectra of aged (blue) and fresh (red) sodium sulfide solutions dried onto a Ta substrate. Crystalline sodium sulfide and sodium thiosulfate are shown in orange and green, respectively.
test its ability to produce the desired results. The resulting solutions, at several different dithionate concentrations, were a deep red color, similar to nonaggregated gold nanoparticles. However, from the absorption profile (Figure 2b, inset) the plasmon band is quite broad with a long tail in the red, indicating that either the polydispersity of the sample is very high, meaning many shapes or sizes of individual particles were produced, or it is very weakly aggregated.94 In either case, it is clear from this experiment that while it is possible that dithionate is present in some amount in the aged sodium sulfide solution, it is not the major reductant responsible for the controlled aggregation of gold nanoparticles observed. In order to determine conclusively if the final product of the aging of sodium sulfide solutions is indeed thiosulfate, a chemically selective technique is required to better probe this process. For this, we chose NEXAFS as a method to examine the aging process. Because NEXAFS is extremely sensitive to chemical state, we should be able to compare the aged and fresh sodium sulfide solutions to determine if a chemical change is indeed taking place. We also compared the aged Na2S solution to a sample of thiosulfate to determine if they are similar. For comparison, we performed experiments on both freshly made and aged (16 months) sodium sulfide solutions (Figure 3, red and blue traces, respectively). The similarities in NEXAFS are clear in Figure 3; however, there are some significant changes between these two spectra. The large peaks at 174 and 183 eV observed in all three spectra have been reported in the literature to be associated with the SO42- or SO32- species.95 This suggests that oxidation has occurred in both Na2S solutions. These solutions were dried onto a Ta substrate under ambient conditions; therefore, some “aging” was likely taking place during this drying time. The spectral features are in general less pronounced in the fresh Na2S solution suggesting that the sample is evolving over time. In order to better determine what species are responsible for these peaks, we examined both crystalline sodium sulfide and crystalline sodium thiosulfate. It is interesting to note that in the spectrum of crystalline sodium sulfide (Figure 3, orange trace) there are no apparent peaks at 174 or 183 eV unlike the dried, fresh Na2S solution spectrum in Figure 3. Clearly, even drying a fresh solution is enough to cause some chemical
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Figure 4. UV-visible spectra of GNA solutions as-synthesized (red) and after PSHB (blue). The decrease in intensity is shown as red highlighting while the areas of increased intensity are indicated by blue.
change. In the spectrum of crystalline sodium thiosulfate (Figure 3, green trace), the peaks at 174 and 183 eV noted above are also evident. It is important to note that the spectrum from the solution of Na2S2O3 (data not shown) was identical to that of crystalline Na2S2O3. By comparing the aged sodium sulfide solution NEXAFS to that of the crystalline sodium thiosulfate (green and blue traces, respectively), it becomes clear that they are nearly identical with the only major difference being slightly less pronounced features for the aged Na2S solution. These differences are likely due to the fact that the aging process is ongoing and that conversion is not complete to sodium thiosulfate. 3.3. Persistent Spectral Hole Burning (PSHB): Aggregates without Stabilizer. We recognized early on in our study of the transient absorption properties of the GNAs that the EPB is inhomogeneously broadened by the many different sized and shaped particles present because of the random nature of aggregation. Without inhomogeneous broadening, the oscillations would most likely not have been observed in the timeresolved experiment. We are clearly probing only a subset of the GNAs in the sample. This subset must be of a particular size and shape, all of which absorb at a similar wavelength with a fairly narrow line width, or again, the oscillations would not be observed. To probe this inhomogeneity of the EPB implied by the transient absorption data, we used a technique called persistent spectral hole burning (PSHB). In PSHB, a sample of the aggregated nanoparticles with a large polydispersity and hence an inhomogeneously broadened absorption band is illuminated with a femtosecond laser on resonance with the nearIR band. If the laser is of sufficient intensity, the subset of particles on resonance will melt, distort, or otherwise change. In the case of the GNAs, it was our hope that we would only see the subset which we were probing with transient absorption decrease in intensity, while the rest of the EPB remained constant. The first experimental attempt with this technique succeeded and greatly supported our hypothesis. Within 1 h of illuminating the sample with ∼200 mW (corresponding to a power density of 0.2 mJ/cm2) of amplified femtosecond pulses, a drastic decrease in absorbance was observed at 800 nm, the approximate peak of our femtosecond beam (Figure 4). This showed conclusively that a subset of GNAs of one size or shape can be
Figure 5. Schematic representation of the hole burning mechanism. The schematic shows three sets of states. The initial state (left) shows three model aggregate shapes with different optical absorptions represented in the model spectrum (left, bottom) as the three individual absorption bands as well as the ensemble spectrum. The transitory state (center) includes the introduction of an ∼100 fs, 800 nm hole burning laser which is on resonance with the center aggregate and results in the destruction of this aggregate. The model spectrum (center, bottom) shows a depletion of the center band on resonance with the laser. The final state (right) shows that the constituents of the destroyed aggregate reform into a structure off resonance with the laser resulting in a final optical absorption (right, bottom) that is substantially different than that of the initial state. It should be noted that this is not meant to be a comprehensive illustration of all possible mechanisms; this is merely one route a small set of aggregates may undertake during hole burning.
probed and, apparently, significantly altered. An interesting and unexpected observation, however, was the increase in absorbance to the red and blue of the optical hole burned in the spectrum. This increase in absorbance is shown in blue shading in Figure 4, while the decreased absorbance is shown in red shading. While one might expect the absorption of the GNAs, which are likely breaking apart into smaller aggregates, to blue shift, the appearance of increased intensity to the red was initially puzzling. It was not clear how this increase in absorption to the red could be attributed to the hole burning, especially since the transverse plasmon, the one representing the individual particles which make up the GNAs, was not changing. The near invariance of the transverse band implies that the nanoparticle concentration is static since there is no loss of water due to evaporation or destruction by melting of the nanoparticles due to heating, as will be discussed further. The only explanation which matches these facts is that the constituent particles that form the GNA break apart and then re-aggregate off resonance with the laser as shown schematically in Figure 5. This change in absorbance is permanent over the time scale we observed them, weeks to months, which implies that, if our hypothesis is true, these re-aggregated particles are completely stable. The other possibility which could explain these results is that on resonance GNAs are melting or welded together, which could also change their absorption properties. This hypothesis was tested with TEM. By comparing TEM images before and after the hole burning, it is clear that there is no change in the apparent
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Figure 6. (a) TEM images of GNAs as-synthesized by the Na2S and HAuCl4 reaction. (b) GNAs after undergoing PSHB. There are no significant differences in structure before and after laser irradiation.
aggregate or individual particle structure (Figure 6a,b). There were no welded GNAs or other structures that appeared to have been melted in any TEM image of GNAs irradiated at any power used in this study. Many triangular and rod shaped particles were still present. If melting were taking place, one would expect these to coalesce into spherical particles.96-98 While some claim that the EPB is due to tabular and triangular particles such as those in Figure 6, these data clearly refute this.86 If the EPB were only due to triangles, hole burning would decrease their concentration in the samples and undoubtedly, some triangles would be observed in a partially melted state. This, however, was not the case. No change in triangle or tabular particle concentration was apparent in TEM studies and no partially melted triangles were observed. Clearly, these triangular particles will have a near-IR absorption of their own; however, they are probably part of larger aggregated structures, and therefore this absorption dominates over their own. This is further proof that the GNAs are breaking apart and reforming into new GNAs. With this information in hand, it was important to understand the fundamental process responsible for the reformation of GNAs. First, we wanted to determine if altering the aggregate structures was a linear or nonlinear process. If peak power rather than average power is more important, then multiphoton processes are clearly responsible for the observed changes. In order to make this determination, we placed a similar sample of GNAs in a non-amplified femtosecond beam at the output of a Ti:sapphire oscillator with 600 mW average power and 0.2 µJ/cm2 peak power. After 2 h, there was no observable change in the spectrum of the solution even with a 3-fold increase in average power over the amplified beam. This shows conclusively that multiphoton processes are dominant in the destruction and reformation of GNAs. It is clear that low peak power illumination will not initiate this change, even at higher average powers. Next, we looked at how a linear change in peak power would affect the hole burning process. By altering the power while keeping illumination time fixed at 1 h for different aliquots of the same GNA sample, we were able to examine the effect of this change in photon flux. What resulted was a gradual decrease in the amount of hole burning that would take place within the 1 h time period (Figure 7). However, there was no observed threshold where hole burning ceased over the power range we used. In fact, there was a linear decrease in absorbance at 800
Figure 7. PSHB of GNAs for 1 h at varying powers. The linear decrease in absorption at 800 nm is illustrated in the inset with the black line being a best fit to guide the eye.
nm with increasing power (Figure 7, inset). It should be noted that, while the general trend of the PSHB at different powers is a blue shift, the sample irradiated with the lowest power seems to slightly red shift. The reason for this is not entirely clear; however, it may be due to a narrower subset of aggregates being affected because of the low power. Because of this, the increase in absorption begins closer to the resonance peak at 800 nm resulting in an apparent red shift. Also, note the transverse resonance is unaltered in intensity regardless of laser power. If there were any melting of non-spherical particles, core/shell, tabular, or otherwise, there would be a concurrent increase in intensity of the transverse band corresponding to an increase in spherical particle concentration.96,99,100 Even though our group has shown ample evidence that the reduction of HAuCl4 by Na2S forms aggregates of Au nanoparticles38,39,76 let us examine the facts presented by our hole burning data without the aid of the GNA model. It has been shown in the literature that it is possible to melt or photofragment spherical nanoparticles by the use of various wavelengths and pulse widths.101-105 However, we need to point out a few salient features in our PSHB experiments. First, the fluences used here are not high enough to melt spherical particles as has been reported previously.101-105 Also, the excitation wavelength is not on resonance with any spherical particle absorption which is necessary for melting to take place. Second, the highest power used here, 0.2 mJ/cm2, corresponds to the lowest power used to “melt” or alter Au nanorods.100 Third, for the melting of nanoshells (Au over a silica core), the lowest power used for melting is a factor ∼20 times larger than our highest power used in the current study.99 Further, our TEM shows no obvious indication of melting of any type of particle (Figure 6), so we can safely assume that we are not in a regime that “melts” or otherwise physically damages the nanomaterial in any way. Let us assume for the moment that nanoshells were formed from the reaction in question. The obvious prediction of a hole burning or melting experiment is that melting and hence a change in the position of the optical resonance should only be observed at power densities at or above 3.6 mJ/cm2 since this was the lowest power reported that induced change in the SiO2/
Reduction of HAuCl4 by Na2S Au core/shell system.99 However, a change is indeed observed in the near-IR band down to 0.1 mJ/cm2 in striking contrast to the above prediction. This power is ∼40 times smaller than what was used in the nanoshell study.101 As stated previously, there is no concurrent change observed for the particles in electron microscopy. These results are extremely interesting and lead us to conclude as follows. The major product from the reduction of HAuCl4 by Na2S reaction is not a core/shell structure. No major change should have been observed at the powers used in this investigation if a core/shell system was present. As has already been pointed out, the other nanostructure that displays a near-IR signature is an aggregate structure of spherical gold particles. We are, of course, neglecting the minority component of triangles and other shapes that we pointed out previously do not contribute substantially to the near-IR band and are not altered in any manner by laser irradiation. Since this aggregate structure is a “loose” affiliation of particles, the power needed to alter those types of structures that are on resonance with the laser is small. However, the bonding between particles is not so weak as to be completely disrupted because the aggregates can and do reform off resonance as our optical spectroscopy shows. The most consistent type of nanostructure that would display the type of behavior observed is that of aggregates of gold nanoparticles. 3.4. Surface Chemistry Effects on Hole Burning. An interesting point to understand is the role of the GNA surface chemistry in the hole burning mechanism. We have proposed that individual GNAs are breaking apart and then reforming into different structures. This is a claim that has not been made for any other aggregated system and for good reason. In general, aggregation is an irreversible change to the colloidal solution. Whether aggregation is induced by the disruption of surface capping or by intentional molecular linking, there is rarely a way to disaggregate the system, let alone have it reform into new structures. This particular system is wholly unique because of the sulfur oxide species on the surface which provide stabilization yet at the same time also cause self-assembly. There are clearly a variety of species on the surface which have different functions as we have shown previously. Some species must cause the individual particles to agglomerate, while others stabilize the individual GNAs. If only one type of surface species were present, either the particles would be completely stabilized, forming a normal colloidal solution, or all the particles would aggregate excessively and precipitate. There is some equilibrium that stabilizes this system and makes it unique. To further probe the surface chemistry, a stabilizing polymer was added to the GNA solution before performing the PSHB experiment with 200 mW average power and 0.2 mJ/cm2 peak power as before. We chose PVP to stabilize the particles because it is relatively inert, water soluble, and has been found to stabilize colloidal solutions well. By coating the individual nanoparticles themselves as well as entire GNAs with PVP, we were hoping that, upon carrying out the PSHB, we would no longer see reformation of GNAs because the unique surface species that allow re-aggregation would be passivated. This, in fact, is exactly what happened as is apparent from Figure 8a. While there is still a dramatic decrease in absorbance at 800 nm, there is no concomitant increase in absorbance to the blue as was observed in the absence of PVP. This same phenomenon was observed with PVP concentrations as low as 100 nM which, assuming an average particle size of 20 nm and a 6 nm2 surface area per 10 000 molecular weight PVP molecule, represents approximately one monolayer of polymer on the individual particle surfaces. At concentrations below this, an increase in
J. Phys. Chem. C, Vol. 111, No. 25, 2007 8899
Figure 8. PSHB of GNA solutions with PVP, a stabilizing polymer (a), and the as-synthesized GNAs (b). Spectra were recorded at constant power, 0.2 mJ/cm2, over time. The change in absorption at 800 nm versus time is plotted in (c) which clearly shows a dramatic difference in the rate of intensity depletion at 800 nm between GNA solutions with PVP (blue squares) and GNA solutions without PVP (red circles). The black lines are best fit single-exponential decays yielding time constants (τ1/e) of 24.5 min for GNAs with PVP and 77 min for those without PVP. This is an over 3-fold increase in the time constant between the GNAs with polymer and the GNAs without polymer.
intensity to the blue and red was observed again as in the GNAs without any stabilizer. With this data, we can say with some certainty that, rather than merely breaking apart or melting, these GNAs are actually reforming new structures off resonance with the incident laser. Not only does adding PVP change the
8900 J. Phys. Chem. C, Vol. 111, No. 25, 2007 resulting optical properties, but also the time required to perform the hole burning is altered. Comparing parts a and b of Figure 8, we find there is clearly a similar trend with increasing illumination time, namely, a decrease in absorbance at 800 nm as illumination time increases. Again, we must point out that in GNA solutions both with and without PVP there is no change in the intensity of the transverse band indicating that there is no large alteration in the concentration of Au spherical particles and, hence, no melting. However, there is an interesting trend observed when we plot the OD at 800 nm versus illumination time (Figure 8c). The solution which contained PVP (blue squares) decreased in OD much more rapidly than those without any stabilizer other than what sulfur oxide species already are present on the surface from the synthesis (red circles). In fact, fitting the data points to a singleexponential decay, we find that non-stabilized solutions have a time constant (τ1/e) of 77 min while solutions which contained PVP have a time constant of 24.5 min. The sample volumes were the same for each sample so that a direct comparison may be made. This is a decrease of more than one-third between the GNAs with PVP and those without. Our hypothesis as to why this drastic change is observed involves the random nature of GNA reformation. When the GNAs are disrupted in the laser beam, they undergo some random reorientation, most likely diffusion controlled. As this happens, there is a good chance that the particle will reform GNAs that are on resonance with the laser but again will be disrupted. Meanwhile, aggregates are diffusing through the solution and will most likely diffuse into and out of the illumination volume. Between diffusion and random reformation, it may take a significant number of passes through the laser beam before the aggregates will be disrupted sufficiently to be pushed completely off resonance. When the stabilizing polymer is in solution, however, every time an aggregate is disrupted, the PVP will coat the newly exposed surface of the individual particles composing the aggregate and prevent reformation of the GNAs. This is possibly a fast, one pass through the illumination volume process. As soon as the resonant particle passes through the beam, it will immediately break apart or at least break apart within fewer laser shots than solutions without PVP and hence become off resonance with the laser. Because of this, the stabilized particles will shift in absorption more quickly, while the as-synthesized particles will take longer, apparently three times as long. All of these are entirely new phenomena, ones unique to this particular system. This observation, as far as we are aware, is very unlikely with core/shell or other single-particle structures and can only take place with aggregated structures with truly unique surface properties. 4. Conclusion In every instance, we have found that the reaction of sodium sulfide and chloroauric acid produces GNAs. The strong interparticle interaction in these GNAs is responsible for the near-IR absorption band which in the past has been attributed to Au2S/Au core/shell nanoparticles. While we concede that this is indeed a possible result of this reaction given the complicated chemistry involved, we have found after several years of study no evidence that this is the common product formed; in fact, we have yet to observe a single instance of core/shell structures. Despite this controversy, articles are still published which do not acknowledge the aggregate model, claiming without further characterization that core/shell particles have been formed. We have presented here further evidence to show that, in general, only GNAs are formed. By determining the nature of
Schwartzberg et al. the sodium sulfide “aging”, which has been shown necessary to produce samples of high optical quality, we have been able to controllably tune the near-IR plasmon band with greatly improved consistency over previous methods. Despite this new control, we have not observed any structures by TEM but aggregated gold nanoparticles mixed with low concentrations of triangular and rod shaped particles. Furthermore, we have shown that with PSHB we are able to alter GNAs with femtosecond laser irradiation while creating aggregates which absorb to the blue and red of the resonant laser. Both TEM and PSHB data show that no melting of the nanostructures is occurring over the laser intensity range used in this study. By adding a polymer to this solution, individual particles are stabilized after the disruption of the GNAs by the femtosecond laser and are unable to reform, leading only to a decrease in absorption of the EPB on resonance with the laser, with no increases elsewhere. These phenomena are unlikely to occur with any structure other than aggregates. With these results, plus our work over the last several years, there should be little doubt that, in general, the product of sodium sulfide with chloroauric acid is GNAs. Acknowledgment. We acknowledge financial support for this project by the Petroleum Research Fund administered by the American Chemical Society, National Science Foundation, University of California Santa Cruz Faculty Research Fund (J.Z.Z.), and the SEGRF Fellowship of Lawrence Livermore National Labs (LLNL) (A.M.S.). Work at LLNL was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory, under Contract No. W-7405-Eng-48. T.v.B. was supported by the Office of Basic Energy Sciences, Division of Materials Science, under the auspices of the U.S. Department of Energy. The work conducted at the Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences Division, of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098 at Lawrence Berkeley National Laboratory. References and Notes (1) Kerker, M. Appl. Opt. 1991, 30, 4699. (2) McHugh, C. J.; Keir, R.; Graham, D.; Smith, W. E. Chem. Commun. 2002, 580. (3) Haes, A. J.; Hall, W. P.; Van Duyne, R. P. Laser Focus World 2005, 41, 105. (4) Yonzon, C. R.; Stuart, D. A.; Zhang, X. Y.; McFarland, A. D.; Haynes, C. L.; Van Duyne, R. P. Talanta 2005, 67, 438. (5) Seballos, L.; Zhang, J. Z.; Sutphen, R. Anal. Bioanal. Chem. 2005, 383, 763. (6) Quinten, M. Appl. Phys. B: Lasers Opt. 2001, 73, 317. (7) Kottmann, J. P.; Martin, O. J. F.; Smith, D. R.; Schultz, S. J. Microsc. (Oxford, U.K.) 2001, 202, 60. (8) Sun, Y. G.; Xia, Y. N. Anal. Chem. 2002, 74, 5297. (9) Yee, S. S.; Homola, J.; Gauglitz, G. Sens. Actuators, B 1999, 54, 1. (10) Murphy, C. J.; Sau, T. K.; Gole, A.; Orendorff, C. J. MRS Bull. 2005, 30, 349. (11) van der Zande, B. M. I.; Bohmer, M. R.; Fokkink, L. G. J.; Schonenberger, C. J. Phys. Chem. B 1997, 101, 852. (12) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957. (13) Orendorff, C. J.; Gearheart, L.; Jana, N. R.; Murphy, C. J. Phys. Chem. Chem. Phys. 2006, 8, 165. (14) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (15) Yu, Y. Y.; Chang, S. S.; Lee, C. L.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661. (16) Prodan, E.; Nordlander, P.; Halas, N. J. Nano Lett. 2003, 3, 1411. (17) Salgueirino-Maceira, V.; Correa-Duarte, M. A.; Farle, M.; LopezQuintela, A.; Sieradzki, K.; Diaz, R. Chem. Mater. 2006, 18, 2701. (18) Zhang, J. H.; Liu, J. B.; Wang, S. Z.; Zhan, P.; Wang, Z. L.; Ming, N. B. AdV. Funct. Mater. 2004, 14, 1089. (19) Sun, Y. G.; Mayers, B.; Xia, Y. N. AdV. Mater. 2003, 15, 641.
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