Surface Plasmon Fields and Coupling in the Hollow Gold

Feb 4, 2010 - Creighton , J. A.; Blatchford , C. G.; Albrecht , M. G. J. Chem. Soc., Faraday Trans. 2 1979, 75, 790. [Crossref], [CAS]. 8. Plasma reso...
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Surface Plasmon Fields and Coupling in the Hollow Gold Nanoparticles and Surface-Enhanced Raman Spectroscopy. Theory and Experiment† M. A. Mahmoud, B. Snyder, and M. A. El-Sayed* Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 ReceiVed: NoVember 16, 2009; ReVised Manuscript ReceiVed: January 19, 2010

Most gold nanoparticles have surface plasmon fields only surrounding their surfaces. Recently, hollow nanoparticles have been studied, such as gold nanocages (AuNC) and gold nanoframes (AuNF). Those particles have two types of surfaces, one facing the outside and the other within the cavity. Their coupling provides a surface field inside the hollow particle and on the outside surface. Using DDA computational method, we have shown that the coupling between these fields gives field intensities and distribution inside and outside the nanoparticles that are sensitive to the thickness (the distance between the two surfaces) as well as the nanoparticle size. For small sizes, the coupling between the fields on the opposite sides of the cage is detected. These effects are detected by following the changes in the experimentally observed surface plasmon resonance spectra of these nanoparticles and the surface-enhanced Raman spectra of adsorbed molecules. The effect of the interaction between the external and internal field as well as the available surface area inside and outside the nanoparticle effects on the Raman-enhancement is detected by comparing the Raman intensities dependence on the interparticle distance with those observed on solid nanocubes surfaces. Introduction One of the newly discovered properties of metals on the nanoscale is the surface plasmon resonance phenomena observed for gold nanoparticles and silver nanoparticles. On the nanoscale, these nanoparticles confine photons which give rise to coherent resultant oscillation of their conduction band electrons resulting in very strong electromagnetic fields around their surface.1 These fields enhance not only the radiative properties of these nanoparticles, but also those of surrounding electronic systems like in surface-enhanced raman spectroscopy (SERS). Fleischman et al.2 first observed Raman signal enhancement of pyridine adsorbed on the surface of a roughened silver electrode, which they have attributed to the increase in the surface area. Van Duyne et al.3-5 and Creighton et al.6-8 independently showed that the increase in the surface area was not sufficient to produce the large Raman intensity. The origin of SERS was examined by Moskovits et al.9,10 who proposed in many reports two mechanisms for Raman enhancement, the chemical and the electromagnetic mechanism.11,12 The rough surface electrode was proposed to have the nanoscale features giving rise to electromagnetic field enhancement mechanism. The developments and progress in the nanoscience facilitate the understanding of the SERS mechanism.13 Since the metallic nanoparticles have a surface plasmon field generated by photon excitation, this field is responsible for the electromagnetic mechanism of SERS.14,15 Aggregation of silver and gold nanoparticles is known to increase the field intensity, and thus the SERS enhancement.16 This is due to the plasmon field coupling of the individual particles couple, thereby generating a large plasmon field.17 Many reports have proposed the presence of hot spots on the surface of nanoparticles that are responsible for SERS.18-20 Some researchers reported that the active hot spots requires †

Part of the “Martin Moskovits Festschrift”. * To whom correspondence should be addressed. E-mail: melsayed@ gatech.edu.

aggregation of the nanoparticles,16,21-23 while others detected the hot spots even in case of single particles.14,24,25 Brus et al.26-31 showed that active hot spots occur at the interstitial spots between two nanoparticles where the fields are most intense. The plasmon field around gold nanorods was measured by electron-beam mapping.32 Recently, our group observed that in some cases the aggregation enhance the SERS signal, for example, silver nanocubes,33 while in other cases the aggregation of nanoframes reduces the SERS signal.17 SERS chips are required to make SERS widely used in commercial applications. The SERS chips should have nanoparticles assembled in a certain arrangement. Electron beam lithography (EBL) is used to fabricate some shapes of nanoparticles at different interparticle separation to be used in SERS reasearch.17 The Langmuir-Blodgett (LB) technique has also been used to assemble the colloidally prepared nanoparticles with different shapes and sizes and collected at different LB pressures giving rise to different interparticle separations.34-37 Because of the hollow design, gold nanocages have attracted great attention especially because of the tunability of their surface plasmon resonance spectra in the visible and the NIR regions.38 Xia and his group synthesized many sizes and shapes of hollow gold nanostructures and used them in many applications.39 Halas and co-workers40 were the first to show the tunability of the plasmon of gold shells on silica cores and polymer beads. The plasmon band is shown to red shift as the ratio of thickness of the gold to the diameter of the nanoparticles increases and the reason for the red shift was discussed in detail.41 In the present work, we studied the 40 nm gold nanoframe (AuNF) and 50 nm gold nanocages (AuNC) with the DDA simulation as a function of the wall thickness. Both the surface plasmon resonance (SPR) and the field enhancement factors were calculated. AuNF with wall thickness of 10 nm and AuNC with wall thickness of 5 nm were selected to study both their SPR and their field enhancement factors. These particles were

10.1021/jp9109018  2010 American Chemical Society Published on Web 02/04/2010

Hollow Gold Nanoparticles prepared by bottom up colloidal methods and their experimental SPR spectra were compared with DDA simulation results. The effect of coupling between surface field across the wall thickness and between the internal surfaces for small or thick nanocages was observed both theoretically and experimentally. The SERS of thiophenol adsorbed on the surfaces of these nanoparticles were experimentally examined after being assembled into monolayers at different interparticle separation by using Langmuir-Blodgett technique. The results were compared with those using 75 nm gold nanobox (AuNB) to understand the interactions between the internal surface plasmon field and Raman enhancements for hollow nanoparticles as they get closer to one another. Experimental Section Gold nanocages with different cage size and wall thickness were prepared from the silver nanocubes by galvanic replacement.42 The silver cubes acted as a template. Silver nanocages were prepared by heating 30 mL of ethylene glycol (EG) at 150 °C for 1 h, followed by the addition of a solution of 0.2 g polyvinyl pyrrolidone (PVP) (molecular weight of ∼55 000 g) dissolved in 10 mL EG. The resulting solution was heated until the temperature rose to 150 °C. Then 0.4 mL sodium sulfide (3 mM) dissolved in EG was added. To prepare different sizes of AgNCs to be used as a template for AuNCs with size 40, 50, and 75 nm; 2, 2.5, and 3 mL of 282 mM silver nitrate, dissolved in EG, was injected slowly into the reaction mixture, respectively.43,44 The silver ions were reduced completely after 15 min, producing AgNC. For purification of AgNCs, 10 mL from the AgNCs solution was diluted with acetone and centrifuged. The particles were then redispersed in water. The precipitate was dispersed in 50 mL of deionized water. To prepare the 40 nm AuNF with wall thickness of 10 nm, the solution of purified 40 nm AgNCs was brought to boiling and a 10 mg/L hydrogen tetrachloroaurate solution was injected slowly until the absorption spectrum of the solution was shifted to 820 nm.44 The cleaned 50 nm AgNC template was used to prepare the 50 nm AuNC with 5 nm wall thicknesses. The gold ion salt was added to the AgNC solution with stirring until the optical absorption peak red shifted to 814 nm.37 The 75 nm AuNB was prepared by the same way as AuNC, but the 75 nm AgNC was used as a template and the gold salt was added until the absorption peak shifted to 800 nm. The gold nanoparticle (AuNF, AuNC, and AuNB) solutions were continuously refluxed until their absorption spectrum became stable. Vigorous magnetic stirring was maintained throughout the synthesis. The Langmuir-Blodgett monolayer was prepared by a Nima 611D trough filled with ultrapure water. The surface pressure was measured with a Wilhelmy plate attached to a D1L-75 model pressure sensor. Nanoparticles in chloroform (1 mL) were sprayed over the water surface, and after 20 min the surface pressure-surface area curve (isotherm) was plotted by decreasing the area at which the particles were dispersed mechanically and measuring the surface pressure at each surface area. The LB film was transferred to the quartz and silicon substrates (cleaned with piranha solution 30% H2O2 + 70% H2SO4 volume ratio) using the vertical dipping method with the pressure kept constant during the deposition. Thiophenol (Sigma-Aldrich) was placed in a small receptacle affixed in the center of a Petri-dish, and the quartz substrates covered with the nanoparticles monolayer were placed near the periphery of the dish. This was then tightly covered. After 12 h, the thiophenol vapor was adsorbed on the surface of particles. A Holoprobe Raman microscope (Kaiser Optical Systems) with

J. Phys. Chem. C, Vol. 114, No. 16, 2010 7437 785 nm laser excitation was used for surface Raman measurements. For the UV-vis absorption measurements, a Carry UV-vis-NIR (Cary 500, Version 8.01) was used. A Zeiss Ultra60 was used for SEM image collection. Results and Discussion Theoretical Calculation. A. Surface Plasmon Resonance Spectra and Field Properties of Gold Nanocages and Nanoframes. The SPR of the plasmonic nanoparticles depends on their shape and size, for example, different metallic nanoparticles have been prepared to cover the visible and NIR regions for sensing purposes.31,42,45,46 In some applications, for example, SERS, the SPR induces the large surface fields whose intensity increase the SERS enhancements.27,31 The overlap of the field of neighboring particles enhances the Raman signal much more than that of the single particle.18 For this reason, many researchers studied theoretically the plasmon field enhancement factor generated after exciting the plasmon of one or a pair of nanoparticles. Two simulation models are commonly used to calculate the absorption and scattering of the electromagnetic waves of particles as well the plasmon field enhancement factor using the discrete dipole approximation (DDA)47 and the finite difference time domain (FTDA) methods.48 It is well known that the size and shape of the plasmonic nanoparticle greatly affect the SPR peak position17 and the field enhancement factor. We showed recently by the DDA calculation that 80 nm AuNF has two plasmon fields (inside and outside the frame shaped particle) and the simulated calculation agreed with the experimental results.17 It was previously discovered that as the gold nanoshell thickness decreases, the SPR wavelength shifts to the red.40 In the present work, we calculated the dependence of SPR of AuNC and AuNF on the wall thickness and the size of these structures. The problem that was addressed is whether or not the wall thickness and the separation of the inner wall in the cage type nanoparticles have an effect on the field enhancement factor as well the SPR peak position. The DDA calculation was carried out for different wall thickness of AuNF with a wall length of 40 nm, the SPR peak position was found to be at 710, 760, 831, and 910 nm for wall thickness of ∼10, 8, 9, and 7 nm, respectively. Figure 1 summarizes the results obtained from the calculation. In Figure 1A, the SPR spectrum included both absorption and scattering, the peak intensity ratio between the scattering and the extinction peaks increased as the wall thickness increased. But the observed unexpected large shift in the SPR peak position should be addressed in detail by utilizing the field enhancement factor calculation. This is because of the AuNF has plasmon field inside and outside the particles. Figure 1B-E shows the contour map for the plasmon field distribution of 40 nm AuNF with wall thickness of 7, 8, 9, and 10 nm, respectively. The color scale bar is the same; however, the dark red is for large field intensity. From the plasmon field calculation, it was concluded that field intensity on the outside corners of the frame is more intense than the outside walls in all the different particles with different wall thicknesses. This observation is similar to the reported calculations carried out on other nanoparticles of different shapes (cubes, prisms, etc.) having sharp corners. An important observation is found in comparing B, C, and D in Figure 1. The field in panel B, which has the smallest thickness, seems to have the largest intensity inside and out. This is due to the strong intrawall surface coupling between the outside and inside surface plasmon fields as the thickness

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Figure 1. The dependence of SPR spectrum (A) and the field enhancement contour map (B-D) on the wall thickness of 40 nm AuNF. (A) The extinction spectra (black), absorption (red), and scattering (green) of 40 nm AuNF with wall thickness of ∼7, 8, 9, and 10 nm. (B-E) The field enhancement map contour for AuNF with wall thickness of ∼7, 8, 9, and 10 nm, respectively. The scale bar is 0 (blue) to 2300 units (dark red), the polarization is along the y direction, and exciting light is along the y direction.

Figure 2. (A) The extinction spectra (black), absorption (red), and scattering (green) of 50 nm AuNC with wall thickness of 5, 8, and 10 nm. (B,D,F) The field enhancement contour map on the outside surface of AuNC with wall thickness of 5, 8, and 10 nm, respectively. (C,E,G) The field enhancement map contour inside the AuNC with wall thickness of 5, 8, and 10 nm, respectively. The scale bar is 0 (blue) to 1000 units (dark red), except for (B) in which the red is 2500 units.

(or the distance between the two surface fields) becomes smallest. As thickness increases, the distance increases, the intrawall surface coupling decreases, and the field inside the frame decrease (less intense red color). In panel E however, it is observed that the field inside increases compared to the field in panel D. The thickness in the panel E frame is larger than panel D, but the dimension for the overall outside sizes of the frames are the same (40 nm), the inside interwall distances within the frame decreases. This could be due to the coupling between the inside surfaces within the frame. This effect should be enhanced more in smaller empty cubic frames. In panel B, there seem to be intersurface coupling as well. This is due to the stronger fields resulting from the coupling of the two surfaces of each frame. The above different field couplings at different wall thickness and separations show its effect on the SPR spectrum. The

spectrum becomes less intense and blue shifted as the thickness becomes larger. However at the largest thickness (and smallest interwall separation) the spectrum becomes sharper with higher extinction maximum. The reason for the sharpness is not understood but one observation of the field distribution suggests that in Figure 2E, the field intensity is more homogeneous than for the other three. The gold nanocage (AuNC) is a unique shape of the nanoparticle; it is empty inside and has holes on its walls to allow molecules to adsorb on the inside surface of the walls as well on the outside. The DDA calculation for 50 nm external wall length AuNC with 10 nm × 10 nm hole on each of its walls was carried out for different wall thicknesses. Figure 2A shows the SPR spectra for 50 nm AuNC with wall thickness of 5, 8, and 10 nm. A red shift was observed as the wall thickness decreases (as in case of AuNF). Thus, the SPR peak shifted

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Figure 3. (A) The DDA calculation of the SPR spectra of a pair of AuNF placed at separation gap of 4, 8, 20, 40, 60, 80, 100, 120 nm. (B) The DDA calculation of the SPR spectra a pair of AuNF placed at separation gap of 5, 10, 25, 50, 75, 100, 125, 150 nm.

from 574 to 592 nm when the wall thickness decreased from 10 to 8 nm. A red shift in the SPR peak of AuNC was detected (the peak is at 656 nm) when the wall thickness became thinner (5 nm). Unlike the AuNF, the scattering ratio increased as the wall thickness decreased. The field enhancement factor was calculated on the outside surface of AuNC and 1 nm away from the inside surface of the cage. Figure 2B,D,F shows the field distribution map counters on the outside surface of 50 nm AuNF with wall thickness of 5, 8, and 10 nm, respectively. The 5 nm wall thickness AuNC has a field enhancement factor value five times larger than that for other nanocages (8 and 10 nm wall thickness), both of which seem to have the same enhancement factor value. However, the field inside the nanocages increases as the wall thickness decreases (see Figure 2C,E,G). We believe that the simulated hot spots formed inside the frames, but not in the cages, result from the stronger focusing effect of the field on the surface of the frame, which becomes more diffusive on the larger surface area of the cage faces. B. Plasmonic Coupling in Nanoparticle Pairs. The plasmon field of the plasmonic nanoparticles couple when the particles are separated with distances less than 2.5 times the particle diameter.17 When a pair of plasmonic nanoparticles comes together, their plasmon fields couple and a new field is generated, which is more intense than the individual plasmon field intensity of the single particle. Moreover, the plasmon field distribution varies and the highest field density is observed in the region in between the two particles.15 The SPR peak of the particle is red shifted due to this coupling. There have been some previous calculations on the field intensity and its distribution for pairs of AuNF.17 In this paper, the calculations were carried out on different pairs to describe the coupling between the hollow nanogold, that is, a two pair system consists of 40 nm AuNF with 10 nm thick walls and 50 nm AuNC with 5 nm wall thickness. Figure 3A shows the DDA calculation of the SPR spectra of the AuNF pair (SPR peak position of the single particle is at 710 nm) placed at different separations. The SPR peak position is red shifted to 800, 772, 742, 726, 719, 715, 713, and 712 nm when the gap distance between the particles is 4, 8, 20, 40, 60, 80, 100, and 120 nm, respectively. When placed at separation distances of 5, 10, 25, 50, 75, 100, 125, and 150 nm, the pair of AuNC (with SPR peak at 655 nm) shifts to 772, 743, 670, 671, 664, 661, 659, and 657 nm as

shown in Figure 3B. The reason for the red shift in the SPR maximum peaks is the coupling between the plasmon fields on the outside surface of each particle (similar to hybridization of orbitals). The interesting observation in Figure 3A is that while the one SPR spectrum band shifted at different particle separation distance in the case of AuNF, in the AuNC pair two peaks were observed. The field distribution calculates the appearance of two resolved SPR peaks, especially at 10 nm separation at which the two peaks appear clearly. The two peaks at shorter distances were assigned to the coupling of the outside fields (the long wavelength band) and the inside fields (the short wavelength band). Experimental Observation A. The Plasmon Absorption Spectra. AuNF with size of 40 nm and wall thickness of 10 nm has an SPR peak around 820 nm in aqueous solution; however, the SPR peak is blue shifted to 707 nm on a quartz substrate due to the change in the dielectric constant of the medium. Figure 4A shows the optical spectrum of the AuNF monolayer, assembled by using L-B trough at different surface percent coverage on the surface of quartz substrate. The AuNF monolayers have more intense plasmon peaks in the range between 550 to 850 nm and a broadband at longer wavelength. The more intense peak position was red shifted from 707 to 717 nm by increasing the percent of coverage from 21 to 40%. While the samples with percent of covered area of 25, 29, 32, and 34% have SPR peak maximum present in between, the red shift increases with increasing the percent of covered area of the substrate with the AuNF. The change in the percent of covered area changes the interparticle separation. The percent of covered area was calculated from the SEM image. Figure 5A-C shows the SEM image of AuNF monolayer assembled with a percent of coverage area of 21, 32, and 40%; the rest of images are in Supporting Information Figure S1. The DDA calculation for the single particle showed a narrower SPR peak compared with the experimental spectra. The broad experimental spectrum could be because of three possible reasons. The first is due to the broad inhomogeneous size distribution, since it is very hard to prepare 100% monodispersed particles. The second reason could be due to variation in the coupling between the single particles that are arranged in dimers or trimers, which have an SPR peak at

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Figure 4. The observed optical spectrum of gold nanoparticles monolayer assembled on the surface of quartz substrate at different percent of covered area (A) AuNF, (B) AuNC, and (C) AuNB.

Figure 5. (A-C) SEM images of AuNF monolayer covered 21, 32, and 40%, respectively, from a quartz substrate. (D-F) SEM images of AuNC monolayer covered 5, 10, and 34%, respectively, from a quartz substrate. SEM images of AuNB monolayer covered 4, 16.5, and 22%, respectively, from a quartz substrate.

longer wavelengths compared to single particle. The third is a result of the dielectric of the substrate which was not taken into account in this calculation, as its effect was concluded previously to be relatively small.49 The more intense SPR peak of AuNF includes single particles and an equilibrium concentration of aggregates with small numbers of particles (two or three). The broadband appearing at longer wavelengths can be assigned to large aggregates with many particles coupled together. The SPR peak of 50 nm AuNC with wall thickness of 5 nm is shifted from 814 nm in water to 695 nm when moved to the surface of quartz substrate. Monolayers of assembled AuNC on the surface of quartz substrate with percent of surface coverage area of 5, 7, 10, 22, 25, and 34%, were also found to have a more intense plasmon peak in the range from 550 to 850 nm. In addition, a broadband is observed at longer wavelength. Figure 5D-F shows the SEM imaging of 5, 10, and 34%, respectively (see also Supporting Information Figure S2). The DDA calculation for a pair of AuNC sets at different interparticle separation gap showed that, at 10 nm gap a

spectrum with resolved two peaks was observed. The experimental results shown in Figure 4B supported this calculation in which two SPR peaks were observed for all the AuNC samples assembles at different percent of covered area. Seventy-five nanometer gold nanoboxes (AuNB) were prepared and assembled at different surface pressures on the surface of quartz substrate. The SPR shifted from 800 nm in water medium to 710 nm on the surface of quartz substrate. The SEM imaging (Supporting Information Figure S3) shows that the percent of covered area by the particles are 4, 8, 12.5, 16.5, 20, and 22%. Figure 5I-H shows the SEM images of AuNB assembled on the surface of quartz substrate with percent of coverage area of 4, 16.5, and 22%, respectively. An intense SPR peak was observed in the range of 500 to 800 nm corresponding to the single particles and group of small aggregates. Also, a broadband appeared at low energy corresponding to the large aggregates (Figure 4C). B. Surface-Enhanced Raman of Thiophenol Adsorbed on the Surface of Gold Nanoparticles. Many sizes and shapes of nanoparticles are used as Raman substrate enhancers. However,

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Figure 6. The SERS of thiophenol adsorbed on the surface of (A) AuNF, (B) AuNC, (C) AuNB at different percent of coverage area on quartz substrates.

Figure 7. The SERS band intensities of thiophenol adsorbed on the surface of (A) AuNF, (B) AuNC, (C) AuNB at different percent of coverage area from quartz substrates. The intensity scale at low percent of coverage is different due to the change on the available surface area inside and outside the nanoparticles.

some conditions have to be met to observe a strong SERS spectrum. The first condition is nanoparticles or aggregates must have their wavelength maxima in the wavelength range of both laser used and the scattered Raman photons.17 The second condition is to assemble the nanoparticles on a substrate that fixes them during the measurement. The plasmon field intensity depends on the interparticle separation distance, so if the distance is varied during the SERS measurement or from one analyte to another, the SERS spectrum will not be an accurate analytical technique. In the previous sections, the optical properties of three different shapes of gold nanoparticles were studied. Those samples were used as SERS substrate for thiophenol. Thiophenol can be adsorbed on the surface of those nanoparticles. Figure 6A shows the SERS spectrum of thiophenol adsorbed on the surface of AuNF monolayer with surface coverage of 21, 24, 29, 32, 34, and 40% on quartz substrates. The SERS signal increased as the percent of coverage increased. Similarly Figure 6B shows the SERS spectrum of thiophenol adsorbed on the surface of AuNC assembled on quartz substrates at percent of coverage of 5, 7, 10, 22, 24, and 34%. The SERS signal of thiophenol adsorbed on the surface of AuNB with percent of coverage of 4, 8, 12.5, 16.5, 20, and 22% are presented in Figure 6C. The preliminary observation from Figure 6 is the Ramanenhanced signals on AuNF is weaker than that on AuNC or AuNB. This might be explained by the fact that the number of adsorbed molecules could be different as the available surface

area available for thiophenol is different for different particles. The ratios of the total surface areas of the AuNC and AuNB to that of the AuNF are found to be 1:1.8:2.3, respectively. This means that the AuNB is expected to adsorb more thiophenol molecules than AuNC, while the AuNF adsorb less than AuNC. Half of the available surface areas in case of AuNC and AuNF are inside the nanoparticles, while the other half are outside. The available surface area of the nanoparticles has an effect on the observed SERS intensity, as the SERS peak intensity increases with increasing concentration of adsorbed thiophenol. But if the surface area has the major effect, the relationship between the SERS peak intensities and the percent of the surface coverage should be linear in all gold nanoparticles shapes. Figure 7 shows a nonlinear relationship between the SERS band intensities and the percent of coverage. This nonlinear relationship is due to the dependence of the surface plasmon field coupling between the nanoparticles on their shape.33 The optical measurement and the SEM show that, the interparticle separation distance decreases and the number of aggregates increases as the percent of coverage area increases. The plasmon field enhancement factor intensity depends on the distance between the particles. However, the SERS signal enhancement factor is directly proportional to the square of the plasmon field enhancement factor intensity and the degree of aggregation as well as the overlap between the laser wavelength and that of the plasmon scattered wavelength.17 To study the correlation between the field enhancement factor and the SERS

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Figure 8. (Top) The DDA calculation result for a pair of 40 nm AuNF with wall thickness of 10 nm, placed at separation of 4 and 20 nm (A and B, respectively) this shows that field inside is getting stronger on the expense the interparticle fields as the pair separate from one another. (Bottom) The DDA calculation for a pair of 50 nm AuNC with wall thickness of 5 nm, placed at separation of 5 and 10 nm (C and D, respectively).

signal, the relationship between the SERS band intensities for thiophenol adsorbed on the surface of the nanoparticles and the percent of coverage area is plotted. Figure 7A shows the SERS band intensities of thiophenol adsorbed on the surface of AuNF assembled at different interparticle separation. However those SERS band intensities behave in a similar manner. For simplification, we use the SERS band around 1072 cm-1 in our description of the relationship between the SERS band intensities and the percent of covered area. The intensity of this SERS band increased sharply with a slope of 42 until 29% coverage, and then increased with smaller slope (16.5) until 34% coverage. As the percent of coverage increased beyond 34% the SERS band intensities increased rapidly with a slope of 93. The 1071 cm-1 SERS band intensity of thiophenol adsorbed on the surface of AuNC behaved in a similar manner as the AuNF. However the SERS band intensities increased sharply with a slope of 127 to 10% coverage, then with reduced slope (94) to 25%, and then suddenly increased after that; the slope increased to 336 (Figure 7B). On the other hand, the SERS band intensities of thiophenol adsorbed on the surface of AuNB monolayer shown in Figure 7C were found to increase gradually with a slope of 91 until 13% coverage and then increased sharply afterward (the slope of this part is 247). The sudden increase at higher coverage of all the different nanoparticles can be explained by the effect of aggregation that enhances the SERS spectrum of all particles both due to the increase in the nanoparticle density of the particles scattering the laser light as well as the increased plasmon fields. To understand these unusual changes of the SERS enhancement with the interparticle separation prior to aggregation, the field enhancement factor was calculated for each system.

According to the DDA results, the single AuNF has fields inside and outside the nanoparticle.17,50 When a pair of AuNFs with edge length of 40 nm and wall thickness of 10 nm are placed face-to-face at 4 nm separation, their plasmon fields couple with one another, generating a stronger field. Opposite to the results of the 80 nm AuNF published earlier,17 both the field inside and the field outside of the frames are enhanced compared with the particles having 200 nm separation. However the field enhancement factor outside is about twice as large as the field inside. This means that the aggregation of AuNF at distances less than 4 nm increases the plasmon field, and hence increases the SERS efficiency. But when the separation between the particles increases, both the field inside and outside the particle decrease. The field inside becomes stronger than the field outside. As the separation distance increases, the field inside increases, and the field outside decreases. This calculation agrees with the SERS results, when the SERS band intensities increase, large slope at percent of coverage of AuNF higher than 34%. At high coverage, many particles are present at small separation; this supports large SERS enhancement for the thiophenol molecule adsorbed both inside and outside the wall of the frame. At lower percent of coverage between 29 and 34%, the majority of the AuNFs are present at distances that both the field inside and outside decreased (as shown in DDA), therefore the Raman bands of thiophenol absorbed inside and outside the frame enhanced with lower efficiency. However the relationship between the SERS band intensities and percent of coverage area increases with low slope (16). Ultimately when the majority of the particles are present at larger distances, the SERS band intensities increase with higher slope (41), and this is because both the field inside and outside increase, but the field inside is

Hollow Gold Nanoparticles stronger. Figure 8A,B shows the field enhancement factor contour map for a pair of 40 nm AuNF present at separation distance of 4 and 20 nm, respectively. The DDA calculation for 50 nm AuNC pairs with 5 nm wall thickness present at different separation gap shows that when the separation distance is 5 nm both the field outside and inside the cages pair become high. When the distance between the cages increases the fields inside become very strong while the field outside the cage drops especially at 10 nm separation. This explains the observation of the two sharp SPR peaks in the DDA calculation at 10 nm separation distance. As the distance between the particles increases, the field outside increases while the field inside decreases. Figure 8C,D shows the field enhancement factor contour map for 50 nm AuNF with 5 nm wall thickness simulated by DDA at separation distance of 5 and 10 nm, respectively. In comparison to the field enhancement simulated by DDA for AuNC with the SERS results, the SERS signal is efficient at low percent of coverage area because the field inside and outside is valuable (the outside field is more efficient). So the thiophenol molecules adsorbed outside the cage at small separation are more enhanced compared with others located inside the cage. At small separation (higher percent of coverage area), the field inside becomes much stronger than the field outside, but both of those fields are less intense compared to that in of the highly separated pairs. This is the reason for increase of the SERS band intensity with lower slope. Finally, when the particles are aggregated at distances smaller than 5 nm, the SERS signal reaches its highest efficiency. Acknowledgment. This work was supported by the Division of Materials Research of the National Science Foundation (No. 0138391). Supporting Information Available: SEM images of AuNF, AuNC, and AuNB monolayers. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters. In Materials Science; Springer: New York, 1995, Vol. 25. (2) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26, 163. (3) Jeanmaire, D. L.; Van Duyne, R. P. J. Am. Chem. Soc. 1976, 98, 4034. (4) Jeanmaire, D. L.; Van Duyne, R. P. J. Am. Chem. Soc. 1976, 98, 4029. (5) Jeanmaire, D. L.; Van Duyne, R. P. J. Electroanal. Chem. Interfacial Electrochem. 1977, 84, 1. (6) Albrecht, M. G.; Creighton, J. A. J. Am. Chem. Soc. 1977, 99, 5215. (7) Albrecht, M. G.; Creighton, J. A. Electrochim. Acta 1978, 23, 1103. (8) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 2 1979, 75, 790. (9) Moskovits, M.; Suh, J. S. J. Phys. Chem. 1984, 88, 5526. (10) Moskovits, M.; Suh, J. S. J. Am. Chem. Soc. 1985, 107, 6826. (11) Moskovits, M. ReV. Mod. Phys. 1985, 57, 783. (12) Wolkow, R. A.; Moskovits, M. J. Chem. Phys. 1986, 84, 5196. (13) Moskovits, M. J. Raman Spectrosc. 2005, 36, 485. (14) Nie, S.; Emory, S. R. Science (Washington, D.C.) 1997, 275, 1102.

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