Surface-Enhanced Raman Spectroscopy on Two-Dimensional

J. Phys. Chem. C 2010, 114, 10463–10477

10463

Surface-Enhanced Raman Spectroscopy on Two-Dimensional Networks of Gold Nanoparticle-Nanocavity Dual Structures Supported on Dielectric Nanosieves Yingshun Li,† Huimin Su,‡ Kam Sing Wong,‡ and Xiao-Yuan Li*,†,§ Department of Chemistry, Department of Physics, and The William Mong Institute of Nano Science and Technology, The Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, The People’s Republic of China ReceiVed: January 19, 2010; ReVised Manuscript ReceiVed: May 3, 2010

We report a facile method for the fabrication of 2-dimensional networks of Au nanoparticle-nanocavity dual structures supported on dielectric nanosieves. The optical extinctions of the as-fabricated network films were found to be dominated by optical transmission, a signature characteristic to nanocavities. The surfaces of the as-fabricated Au films were found to be hydrophobic toward water with a measured contact angle of 125.4° but quite wettable with ethanol with a contact angle of 9.5° at room temperature and are readily convertible to totally hydrophobic or hydrophilic when chemically modified with self-assembled monolayer of molecules with desired functional groups. High quality surface-enhanced Raman spectra (SERS) from a monolayer of self-assembled 4-mercaptobenzoic acid (4-MBA), an electrostatic double layer of 4-mercaptobenzoic acid and rhodamine-6G (4-MBA-R6G), and nonspecifically adsorbed R6G were obtained, respectively, demonstrating that the dual structured network films are active, stable, and uniform as substrates in SERS. Employing the SERS spectra from a monolayer of self-assembled 4-MBA, the enhancement factors of 106 and 105 were achieved with excitations at 632.8 and 785 nm, respectively. Discrete dipole approximation (DDA) calculations were conducted to examine the electric field intensity distributions on linearly and circularly aggregated nanoparticles, two typical local aggregation patterns of nanoparticles in the as-fabricated network films. The DDA calculations reveal an important relationship between three factors: the generation of the hottest E-field spot on a linear chain aggregate, the position of the extinction peak of the aggregate, and the excitation wavelength used. It was found that the hottest spot on a linear aggregate is generated only when the excitation wavelength is in resonance with the extinction peak of the aggregate. An “effective aggregation number” scale is proposed to measure the effectiveness of the aggregation of nanoparticles in 1- and 2-dimensions at a given excitation. A “hot-spot” delocalization picture is proposed for the nanocavities formed by circular aggregation of nanoparticles to account for the observed isotropic SERS on the as-fabricated films. Introduction Surface-enhanced Raman spectroscopy (SERS) has been established as a powerful technique in ultrasensitive chemical and biological analyses1-4 with the demonstrated capability of detection down to single molecule level.5-7 One of the lasting issues in SERS has been the development and fabrication of SERS-active substrates, both for addressing various basic issues in SERS and for meeting the requirements of diverse applications.8 Desirable SERS substrates should be not only highly SERS-active, structurally well-defined and uniform, optically, and chemically stable but also simple to fabricate with lowcost and high-throughput.8 Of particular interest are the various two-dimensionally organized noble metal nanostructures for SERS study and applications, fabricated by methods including electron beam lithography,9-11 focused ion-beam lithography,12-19 photolithography,20 nanoimprint lithography,21-25 nano/microsphere lithography,26-29 Ag film on nanospheres,30-32 selforganized nanostructures,33-35 and the synthesis/fabrication on nanostructured templates.36-40 While diverse geometric patterns of two-dimensional organization of noble metal nanostructures * To whom correspondence should be addressed. E-mail: chxyli@ ust.hk. † Department of Chemistry. ‡ Department of Physics. § The William Mong Institute of Nano Science and Technology.

have been demonstrated for SERS, the basic nanostructured units normally possess one of the two, seldom both, topologies, protrusion and trench, respectively. The protruded structural units include nanodots of various sizes and shapes, nanorods and nanowires of various diameters and lengths, and gratings of various shapes, supported on either dielectric or conducting substrates. The trenched structural units include nanocavities (or nanoholes) and nanopores of various shapes and sizes created on smooth noble metal films. Relevant to this work are the twodimensional arrays of nanostructures fabricated by employing nanostructured templates.41-48 Nanoporous inorganic anodized aluminum oxide (AAO) and polymeric membranes with various pore diameters are two typical classes of templates employed, from which two-dimensional organization of the nanostructures was fabricated.49-52 In this report, we demonstrate for the first time that, employing the inorganic nanosieves membranes (the dielectrics) as the templates, a uniform film of two-dimensional networks of Au nanoparticles (the protruded structures) and nanocavities (the trenched structures) can be conveniently fabricated by dry atom sputtering technique. The fabricated nanoparticle-nanocavity dual structured network films show both the characteristic optical properties of nanoparticles and nanocavities and can be used as highly active, stable, uniform, and isotropic substrates in SERS applications. The SERS enhancement factors at two wavelengths 632.8 and 785 nm,

10.1021/jp100499z  2010 American Chemical Society Published on Web 05/18/2010

10464

J. Phys. Chem. C, Vol. 114, No. 23, 2010

respectively, were evaluated using the self-assembled monolayer of 4-mercatobenzoic acid (4-MBA) as the surface-probing molecules. We employed the discrete dipole approximation (DDA) calculation to study the local electric field intensity distributions on two typical local aggregation patterns, linear and circular, of Au nanoparticles in the as-fabricated dualstructured network films with the goal to gain a better understanding of their optical and SERS properties. To examine the underlying relation between the condition for hot spot generation and the optical extinction of linear aggregates, the extinction spectra of linear chain aggregates were also calculated. It was found that the hottest spot on a linear chain aggregate of Au nanospheres is generated only when the excitation wavelength is in resonance with the extinction peak of the aggregate. Experimental Methods Materials. 4-Mercaptoacetic acid (4-MAA, 97%) was obtained from Aldrich. Ethanol (99.9%) was purchased from Merck Chemicals. Rhodamine 6G (R6G, 99%) was purchased from Sigma. All chemicals were used as received. Nanoporous alumina membranes with average pore sizes (PAM, Anodisc) of 200, 100, and 20 nm were purchased from Whatman (Whatman International Ltd.). Water used in sample preparation was first deionized and then filtrated with a Barnstead NANOpure system to 18.2 MΩ · cm resistivity. Thiol compounds used were soluble in ethanol and/or in ultrapure water. The solutions were each time freshly prepared before sample preparation and characterization. Preparation of Two-Dimensional Networks of Au Nanoparticles Films on Porous Alumina Membranes (AuNP2dNW|PAM). The PAMs with pore diameters of ∼100 nm (PAM100) and ∼200 nm (PAM200) at the branched side were used as the substrate for the deposition of Au nanoparticles. The round-shaped membranes are all of a diameter of 13 mm with a thickness of 60 µm. Before metal deposition, the substrates were rinsed in ethanol and ultrapure water alternatively several times, dried in ambient air, and then fixed on a glass plate. The AuNP-2dNW|PAM were then fabricated by argon sputtering film deposition with Denton Discovery 18 thin film sputtering system in base vacuum of approximately 5 × 10-6 Torr. The deposition rate was set at 7.2 nm/min. After the desired thickness were obtained, the AuNP-2dNW|PAM samples were kept in a vacuum box for storage prior to other characterizations. SEM Characterization. The as-fabricated AuNP-2DNW|PAM samples were cut into small pieces, and the pieces were mounted on the SEM specimen holder by a silver conductive paste and then dried naturally. SEM characterization was performed on a model JSM-6700F FE-SEM instrument operated in ultrahigh vacuum conditions (∼10-5 Pa) with an accelerating voltage of 5-10 kV. The magnification of each SEM image is indicated in the figure legend. Thin-Film X-ray Diffraction (XRD) Measurement. The asfabricated samples were cut into 10 mm × 10 mm plates and mounted on a sample stage by elastic adhesive. The XRD were measured on a thin film X-ray diffractometer (X’pert Pro, PAnalytical) equipped with a Cu KR (λ ) 1.54 Å) X-ray source. 2θ was scanned from 36 to 80° covering the main diffractive peaks of Au lattice. Contact Angle (CA) Measurement. The wettability of the as-fabricated metal films on dielectric nanosieves with and without surface modification by hydrophilic/hydrophobic molecules was measured on a contact angle measuring system (G10,

Li et al. Kruss) equipped with a CCD camera for the image collection. The measurements were conducted at a controlled temperature (22 ( 2 °C) and relative humidity (60 ( 10%). Optical Transmission and Reflectance Measurements. The optical transmission and reflectance spectra from visible to infrared in mid- and near-IR region were measured on two series (PAM100 and PAM200) of AuNP-2dNW samples with the average film thickness of 10, 20, 30, 40, and 50 nm, respectively. The PAM substrate was removed from AuNP-2dNW|PAM by a sodium hydroxide solution. The AuNP-2dNW films were rinsed several times in ultrapure water to remove the residues of unreacted sodium hydroxide and were then mounted on a quartz plate. The reflectance and transmission spectra were measured on a FT-6000 FT-IR spectrometer (Bio-Rad) in the range of NIR and mid-IR, and on a homemade optical setup in the visible range. SERS Spectra Measurement. All Raman and SERS spectra were acquired on three sets of Renishaw micro-Raman spectrometers (Renishaw plc, Gloucestershire, U.K.) all equipped with thermoelectric cooling CCD detectors. The excitations used were 632.8 nm from a cw He/Ne laser (Spectra-Physics/Model 127; Imax ) 60 mW) and 785 nm from a cw semiconductor diode laser (Renishaw Plc/Model HPNIR785; Imax 500 mW), respectively. For all excitations, an Olympus ULWD 50× objective (NA 0.55; WD 8 mm) was used. All SERS and Raman spectra were measured at room temperature 22 ( 2 °C unless specified otherwise. The spectra acquisition parameters, including the laser power at the sample, the integration time of each spectrum and the number of integrations collected for each spectrum, are given in each figure legend. Preparation of the Samples for SERS Measurement. The self-assembled monolayer of 4-MBA was prepared by immersing the as-fabricated AuPN-2dNW|PAM substrates in an ethanol solution containing 1 mM of 4-MBA for ∼24 h. The 4-MBAAuNP-2dNW|PAM was then rinsed with absolute ethanol and ultrapure water alternatively for several times to remove the nonspecifically adsorbed molecules. For R6G electrostatically adsorbed on 4-MBA-AuNP2dNW|PAM, the latter was incubated in a 10-4 M R6G aqueous solution (∼pH 7) for 5 min and followed by rinsing with ultrapure water three times. At neutral pH, the self-assembled monolayer of 4-MBA exists in the salt form to enhance the electrostatic assembly of the positively charged R6G on the negatively charged 4-MBA molecules. For direct and nonspecific adsorption of R6G on the bare AuNP-2dNW|PAM substrate, the latter was incubated in a 10-4 M R6G aqueous solution for 5 min and followed by rinsing with ultrapure water for three times. Such a step was repeated three times to obtain the sample for SERS measurement. Determination of the Surface Enhancement Factor (EF) in SERS. Two square quartz cuvettes (5 mm, 700 µL inner volume) with PTFE caps were used for the measurement of SERS and normal Raman spectra of the 4-MBA compound. The cuvettes and glass plates were cleaned by immersion in concentrated sulfuric acid containing potassium dichromate for 30 min and then abundantly washed with deionized water and ultrapure water and dried in air. For SERS measurement, the SERS-active substrates 4-MBA-AuNP-2dNW|PAM were mounted on a glass plate and put in one cuvette filled with absolute ethanol, covered by a cap, and sealed with parafilm. For normal Raman spectra measurement, a piece of blank and precleaned PAM membrane was mounted on a glass plate and put in the second cuvette filled with a solution of 0.3 M 4-MBA in ethanol. For the evaluation of the “effective volume” in the normal

Au Nanoparticle-Nanocavity Dual Structures

J. Phys. Chem. C, Vol. 114, No. 23, 2010 10465

Figure 1. Field-emission SEM images of the PAM100 Anopore membrane substrate on the active (front) side (A) and Au nanoparticle network films with an average thickness of 10 nm (B), 20 nm (C), 30 nm (D), 40 nm (E), and 50 nm (F), respectively, on the PAM100 substrate. The scale bar is 100 nm, and the magnification is ×60 000. The average particle diameters are, from (B) to (F), 30.6, 36.4, 48.2, 55.4, and 64.8 nm, respectively. The filling factors are, from (B) to (F), 0.31, 0.40, 0.52, 0.60, and 0.65, respectively.

Raman spectra of 4-MBA solution, a small piece of silicon wafer (∼3 × 10 mm, thickness ) 400 µm) was put in the cuvette filled with absolute ethanol. The intensities of the 520 cm-1 band of the Si wafer at the focal point and various defocal distances were then collected. The details are given in the text and in the Supporting Information. Results and Discussion Structure and Morphology of Two-Dimensional Networks of Au Nanoparticles Supported on Dielectric Nanosieves. Two-dimensional networks of Au nanoparticle-nanocavity supported on PAM nanosieves were fabricated and characterized for three types of substrates with average pore diameters of 200 nm (PAM200), 100 nm (PAM100), and 20 nm (PAM20) at the active (branched) sides, respectively, and with different average metal film thicknesses, respectively. Figures 1 and 2 display the Au films fabricated on PAM100 (Figure 1) and PAM200 (Figure 2) substrates with average film thicknesses of 10 nm

(B), 20 nm (C), 30 nm (D), 40 nm (E), and 50 nm (F), respectively. For the purpose of comparison, the SEM images of the PAM100 (Figure 1A) and PAM200 (Figure 2A) substrates on the active side (also the branched side) are also measured (10 Å thick Au metal was presputtered on the substrates for the electric conductivity required for SEM measurement). Several structural features of the as-fabricated Au films emerge from the SEM micrographs. First, the Au films on PAM substrates are mainly on the rims of the nanosieves, which are of an average thickness of ca. ∼30, ∼20, ∼10 nm, respectively, for PAM200, PAM100, and PAM20 substrates (data in the Supporting Information). Second, unlike an atomically flat substrate on which smooth and uniform thin metal films are normally formed by metal vapor deposition and/or slow sputtering, the Au films on PAM mainly consist of particles of nanometer size in good homogeneity at a given deposition condition. Thin film X-ray diffraction of the as-fabricated Au films suggests that the deposited Au nanoparticles are in good

10466


Li et al.

Figure 2. Field-emission SEM images of the PAM200 Anopore membrane substrate on the active (front) side (A) and Au nanoparticle films with an average thickness of 10 nm (B), 20 nm (C), 30 nm (D), 40 nm (E), and 50 nm (F), respectively, on the PAM200 substrate. The scale bar and magnification are the same as in Figure 1. The average particles diameters are the same as the counterparts in Figure 1 (from (B) to (F), 34.2, 41.2, 47.0, 56.8, and 65.8 nm, respectively). The filling factors are, from (B) to (F), 0.29, 0.30, 0.32, 0.42, and 0.49, respectively.

crystalline structure instead of amorphous packing (data in the Supporting Information). Third, the average particle size increases linearly with the average thickness of the deposited film, as illustrated in Figure 3A. The average diameters of Au nanoparticles, estimated from the SEM images, are 30, 36, 48, 56, and 64 nm, respectively, for samples B-F in Figure 1. Fourth, contrary to the average particle size, the average cavity size decreases with the increase of the deposited metal film thickness (Figure 3A). Therefore, the “surface” filling factor (FF) of the metal, defined as the ratio between the projected areas occupied by deposited Au and the total area of the substrate, increases with the increase of the average thickness of the deposited Au film (Figure 3B). The filling factors, estimated from the SEM images, are found to be ∼0.31, 0.40, 0.52, 0.60, and 0.65, respectively for Au films shown in B-F in Figure 1. Quasi-linear relationships between the measured metal FF and the average film thickness of the deposited gold films on both PAM100 and PAM200 substrates are shown in Figure 3B. Fifth, the average Au particle size does not vary

much from PAM100 to PAM200 substrate at a given deposition thickness of Au. The average particle size in Figure 2 for PAM200 series are, within the experimental uncertainty, almost like their counterparts in Figure 1 for the PAM100 series at a given deposition condition. Last, but not least, with the increase of the average Au film thickness, the Au nanoparticles link with each other along the rims of the substrate to form a nicely compact two-dimensional network of nanoparticle-nanocavity dual structures, in which the average diameters of nanocavities are dictated by that of the nanopores in the substrate and the average thickness of the metal film deposited. Taking individual particles as the smallest structural units of the observed 2-dimensional networks, the main local topological features (or the local 2-dimensional aggregation patterns) of the network are (i) nanoparticles linked in a quasi-linear chain with various lengths and particle numbers and (ii) nanoparticles linked in a ring-like shape around individual nanopores. These two local structural features suggest that the optical properties of the asfabricated Au particle networks would be dictated not only by



Figure 4. Cross view photographs of the contact angles for a drop of pure water on the PAM100 substrate without (A) and with (B) a 40 nm thick Au particle network film, and for a drop of absolute ethanol on the PAM100 substrate without (C) and with (D) a 40 nm thick Au particle network film.

Figure 3. Scattered point graph illustrating (A) the average diameters of Au nanoparticles (solid circles) and the average hole diameters (open circles) versus the average thickness of the deposited Au film for PAM100 series and (B) the filling factors versus the average thickness of the deposited Au film for both PAM100 (solid squares) and PAM200 (open squares) series of the as-fabricated samples.

that of individual particles but also by their linear and ringshaped aggregates. It should be noted that the “nanocavities” observed in the as-fabricated Au film are actually the 2-dimensional enclosures by linked nanoparticles and are different from the conventional arrays of nanoholes (or nanocavities) formed on a “homogeneous” metal film.12-17,56-60 Another important point to note is that, unlike the arrays of nanoholes fabricated on uniform metal films, the average thickness of the rim between cavities in the as-fabricated network films is significantly smaller than the cavity diameter. In addition, we observed,52 from the measurement of the conductivities of the as-fabricated Au films, that even for Au films with low filling factors below the film percolation threshold, the film is perfectly conductive, suggesting that a very thin layer of interlinked Au film is formed beneath Au nanoparticles at all the films examined in this study. Surface Wettability. The surfaces of micro- and nanoporous materials often display wettability quite different from that of smooth bulky materials.53,54 It has been shown that the nanostructured Au surface is superhydrophobic.55 We have measured the wettability of the as-fabricated Au network surface with the aim to guide the selection of solvent for the transfer of surfaceprobing molecules from a solution to the Au network surface either via self-assembled monolayer technique or by nonspecific electrostatic physi-adsorption. The contact angles of water and ethanol on the surfaces of both a blank PAM100 membrane and a 40 nm thick Au network on PAM100 membrane were measured, respectively (Figure 4). The blank PAM surface is indeed hydrophilic with a contact angle of 16.6° by water (Figure 4A) and almost 0° by absolute ethanol (Figure 4C). This is not surprising since the alumina surface of the PAM membrane is expected to possess many terminal polar hydroxyl groups. However, the deposited Au network film significantly changed the surface to quite hydrophobic with a water contact angle of 125.4° (Figure 4B). On the other hand, the Au network

Figure 5. Cross view photographs of contact angles for a drop of water on the monolayers of L-cysteine (A) and dodecanethiol (B) selfassembled on Au particle network film (40 nm thick) on the PAM100 substrate.

surface remains quite wettable to ethanol with a contact angle of ∼9.5° (Figure 4D). The hydrophobicity of the Au network on PAM is readily modifiable by the self-assembly of thiol molecules carrying the desired terminal functional groups, as demonstrated in Figure 5. The contact angles of the Au network film covered with a monolayer of self-assembled L-cysteine (with hydrophilic a-amino acid terminal group) and dodecanethiol (with hydrophobic alkyl terminal group) are found to be 18.3 and 133.6°, respectively. This study provided guidance for the selection of proper solvents in studies involving the transfer of molecules from a solution to the surface of the asfabricated network films such as those in surface-enhanced Raman spectroscopy of chemisorbed and physisorbed molecules (vide infra). Optical Transmission and Reflectance Characteristics. One of the most important and unique properties of nanostructured precious metals is their localized surface plasmon resonance (LSPR) when the size of nanostructures becomes comparable with or smaller than the mean free path of the conducting electrons in metal. However, metal nanoparticles and nanocavities (and/or nanoholes) possess negative and positive curvatures, respectively, at the metal-dielectric interfaces and sometimes show quite different characteristics of optical extinction. For example, while nanoparticles typically show peak profile in their optical extinction in the visible and near-IR wavelengths attributable to their LSPR,56 nanoholes (and nanocavities)

10468


Figure 6. Extinction spectra of Au nanoparticle network film (40 nm thick) supported on a quartz plate, prepared on PAM100 (A) and PAM200 (B) substrates, respectively. Shown in the lower right corners are the transmission and reflectance spectra of the corresponding samples. The insets in the upper left corners show the SEM images (×80 000) of corresponding samples.

usually show enhanced transmissions, and therefore valley profiles, in their optical extinction that are mediated also by surface plasmon.12-17,57-60 We have measured both the transmission and reflectance of the as-fabricated Au nanoparticle network films. Figure 6 displays the extinction spectra of two typical samples, 40 nmAu|PAM100 (Figure 6A) and 40 nmAu|PAM200 (Figure 6B), respectively. The corresponding transmission and reflectance are shown as the insets at the lower right corners. For the convenience of comparison, the SEM images of the samples are also shown at the upper left corners. The optical extinction spectra of both samples show a valley profile, instead of a peak profile, as the main characteristics in the visible region. The minima of the extinction valley are located at 496 nm (for 40 nmAu|PAM100) and 509 nm (for 40 nmAu|PAM200), respectively. This is expected from the measured transmissions (red traces in the insets) that display a peak profile with maxima located at the same respective positions. The reflectances, on the other hand, also display a valley profile similar to that of the extinction spectra. Transmission and reflectance profiles of similar profiles were observed for all the AuNP-2dNW films fabricated and examined in this study, regardless the metal filling factors of the films. From the structures of the AuNP-2dNW film as illustrated in the insets of Figure 6 as well as in Figures 1 and 2, it is expected that the observed extinction of the as-fabricated films should contain contributions from (a) the individual Au nanoparticles largely dictated by the particle size and shape, (b) the interparticle couplings mainly controlled by the interparticle distances as well as the 2-dimentional aggregation topology,

Li et al. (c) the individual nanocavities formed by the enclosure of nanoparticles around the rims of nanopores of the template with the cavity diameter and cavity’s interior surface morphology as the main factors of concern, (d) intercavity couplings controlled by cavity-cavity distances and 2-dimensional layout patterns of cavities, and (e) the effect of nanosieve substrate. Despite the complexity introduced by so many structural factors to the optical properties of the as-fabricated AuNP-2dNW films, the optical responses from the above factors can be grouped into two main categories with completely opposite behaviors in transmission and extinction. The first category, including the individual nanoparticles and their interparticle couplings, is expected to show a peak profile in the extinction and reflectance but a valley profile in the transmission in the visible spectrum originated from the LSPR and the elevated (or enhanced) extinction to the red edge of the peak profile originated from the interparticle coupling (or aggregation).56 The other category, including the individual nanocavities and their intercavity couplings, is expected to show a valley profile in the optical extinction and a peak profile in transmission (or enhanced transmission).12-17,57-60 Figure 6, as well as all the measured transmissions and reflectances of the as-fabricated AuNP-2dNW films with a wide range of filling factors, are dominated by the characteristic signature of the nanocavity behavior. Indeed, the transmissions shown in Figure 6, as well as the measured transmissions of all fabricated samples with different filling factors can be well reproduced by an “effective cluster model” that takes into account the optical responses from two structural phases, one with individual Au nanoparticles and another with ring-shaped cavities formed by the enclosures of intercontacted particles.52 SERS of the Self-Assembled Monolayer of 4-MBA and of Electrostatically Assembled R6G-4-MBA Double Layer on AuNP-2dNW Films. We have chosen two types of molecules, 4-MBA and R6G, respectively, to examine the SERS properties of the as-fabricated AuNP-2dNW films. 4-MBA was chosen for SERS study for three reasons. First, it forms a selfassembled monolayer on the Au surface,61 allowing a reasonably accurate estimation of the total amount of surface-adsorbed molecules under Raman irradiation which, in turn, benefits the estimation of enhancement factor in SERS. Second, it does not absorb in the visible and near-infrared wavelengths where the Raman excitations were used for acquiring SERS, avoiding the interference of the intrinsic resonance enhancement to the estimation of surface enhancement factor. Third, the carboxylic acid groups on 4-MBA, trans to the surface-adsorbing thiol group, can be readily converted to negatively charged carboxylate group at neutral and basic pH, facilitating the organization of the second layer of positively charged molecules, such as R6G, via electrostatic-driven layer-by-layer assembly. R6G was also used in the SERS study for several considerations. First, it possesses an electronic transition centered at ca. ∼526 nm in aqueous solution and therefore may have contribution from either resonance (or preresonance) effect in Raman scattering or fluorescence emission when excited at visible wavelengths. Second, the R6G molecule carries a positive charge and therefore allows the electrostatically driven adsorption of a monolayer on top of the self-assembled monolayer of 4-MBA in neutral or basic pH. Last, but not least, R6G can adsorb directly and nonspecifically on the as-fabricated AuNP-2dNW films, therefore allowing a comparative study with the monolayer of R6G electrostatically adsorbed on 4-MBA. Figure 7 displays the SERS spectra (trace a), obtained with excitation at 632.8 nm, of 4-MBA self-assembled on 40


Figure 7. SERS spectrum (trace a) from a self-assembled monolayer of 4-MBA on 40 nmAu|PAM100 film (A) and 40 nmAu|PAM200 (B), respectively, immersed in absolute ethanol. The normal Raman spectra of 0.3 M 4-MBA in ethanol (trace b) and absolute ethanol (trace c) in the presence of PAM100 (A) and PAM200 (B) substrates. All spectra were acquired with 632.8 nm excitation under parallel and same experimental conditions. Each trace is the accumulation of 5 spectra, each of which is acquired with 20 s integration time.

nmAu|PAM100 (Figure 7A) and 40 nmAu|PAM200 (Figure 7B), respectively. For the purpose of comparison, and also to be used in the estimation of the surface enhancement factor, normal Raman spectra of 0.3 M 4-MBA ethanol solution (trace b) and absolute ethanol (trace c) in the presence of blank substrates (PAM100 for Figure 7A and PAM200 for Figure 7B) are also displayed. Figure 8 shows the SERS spectra (trace a), obtained with excitation at 785 nm, of 4-MBA self-assembled on 40 nmAu|PAM100 (Figure 8A) and 40 nmAu|PAM200 (Figure 8B), respectively. The normal Raman spectra of 0.3 M 4-MBA ethanol solution (trace b) and absolute ethanol (trace c) in the presence of respective blank substrates were also acquired in the exact same condition as SERS. As can be seen from Figures 7 and 8, the SERS spectra of the self-assembled monolayer of 4-MBA show two characteristic strong peaks at 1075 and 1586 cm-1, respectively, which are readily assignable to the breathing and axial deformation modes of the phenyl ring, respectively.64 While both bands can be employed in the estimation of the EF in SERS spectra, we used mainly the band at ∼1586 cm-1 for it is less interfered by the solvent bands (vide infra). Figure 9 displays SERS spectra, acquired with the excitation at 632.8 nm, of the electrostatically adsorbed (sub)monolayer of R6G (spectrum B) on top of a monolayer of 4-MBA (spectrum A) self-assembled on the 40 nmAu|PAM100 substrate. As a comparison, the SERS spectra of R6G nonspecifically and directly adsorbed on the as-fabricated 40 nmAu|PAM100 is also shown (spectrum C). The SERS spectra of R6G shows its characteristic bands at 608, 771, 1185, 1308, 1357, 1506, and 1646 cm-1, respectively, which are readily assignable the ν53, ν65, ν115, ν117, ν146, and ν154 vibrational modes, respectively.65 Both the peak positions and the relative intensities are very


Figure 8. SERS (trace a) of a monolayer of self-assembled 4-MBA on 40 nmAu|PAM100 (A) and 40 nmAu|PAM200 (B), respectively, immersed in absolute ethanol. The normal Raman spectra of 0.3 M 4-MBA in ethanol (trace b) and absolute ethanol (trace c) both in the presence of the PAM substrate. All spectra were acquired with 785 nm excitation under parallel conditions. Each trace is the accumulation of 10 spectra, each of which is acquired with 10 s integration.

similar between spectra B and C and are consistent with the SERS spectra reported in the literature, suggesting that the R6G molecules are retained well both when electrostatically adsorbed onto a “soft” layer of 4-MBA and when adsorbed nonspecifically onto the bare “hard” AuNP-2dNW films. As expected for the Au substrate, no pronounced SERS was observed for the adsorbed R6G with excitation at 514.5 nm, at which fluorescence from R6G dominates the spectra. Estimation of the Surface Enhancement Factors (EF) in SERS. As for any newly developed substrates for SERS study and application, it is desirable to estimate the (average and wavelength-dependent) surface enhancement factor of our fabricated AuNP-2dNW films.66-72 While several methods have been reported for the estimation of SERS enhancement factors,10,66-72 we adopted an approach similar to that reported by Van Duyne et al.,66,67 Felidj et al.10,68,69 and Tian et al.70 We used the SERS spectra from the monolayer of 4-MBA self-assembled on the as-fabricated AuNP-2dNW films, as shown in Figures 7 and 8, to estimate the average SERS enhancement factor at 632.8 and 785 nm, respectively. The observed 4-MBA mode at 1596 cm-1, the strongest SERS band and the least interfered by the solvent bands, was used as the working band to estimate the EF with excitations at 632.8 and 785 nm, respectively. For the estimation of the “EF” from the far-field experiment (irradiation dimension . λexci), the EF per molecule at a radiation frequency of (ω0 - ωQ) can be estimated from66-69

EF(ω0 - ωQ) )

[ISERS(ωQ)/Nsurf] ISERS(ωQ)Nsol ) [INR(ωQ)/Nsol] INR(ωQ)Nsurf

(1) where ISERS(ωQ) and INR(ωQ) are the experimentally measured intensities for a band at ωQ from SERS and normal Raman

10470


Li et al.

Figure 10. Key parameters in the evaluation of EF in SERS spectra. (A) The evaluation of the total number of molecules irradiated in the focal area of the self-assembled monolayer of 4-MBA. (B) The evaluation of the total number of molecules in the “effective scattering volume” of normal Raman scattering.

Figure 9. SERS spectra of (A) self-assembled monolayer of 4-MBA, (B) submonolayer of R6G electrostatically adsorbed on 4-MBA monolayer, and (C) R6G directly adsorbed on 40 nmAu|PAM100, respectively. Excitation: 632.8 nm. Laser power: 0.1 mW (A and B) and 0.15 mW (C) at the sample. Integration time: 10 s for (A) and (B), 20 s for (C).

spectra, respectively, at a given excitation ω0. Nsurf and Nsol are the total number of molecules contributing to the observed ISERS(ωQ) from SERS and INR(ωQ) from normal Raman spectra, respectively. This equation (and also this method of estimating EF) is valid only when the systemic errors and discrepancies between the measurements of ISERS and INR are canceled out. While ISERS and INR can be accurately measured from SERS and normal Raman spectra at a parallel experimental setting, as shown in Figures 7 and 8, the evaluation of Nsurf and Nsol poses a challenge to the reliable estimation of EF via this approach. Figure 10 illustrates the main factors dictating the estimation of EF from this approach, namely, the estimation of the total number of adsorbed molecules irradiated under the laser focal spot (Figure 10A) and the total number of molecules contributing to the observed INR (Figure 10B). As can be seen from Figure 10A, to estimate Nsurf, the key parameters needed are “the effective substrate area”, Als, irradiated under the focused laser beam and the “average area” occupied per adsorbed molecule Amol. The Nsurf can be estimated from

Nsurf )

Als R · πrls2 ) ) Als · Fpd Amol β · πrmol2

(2)

where R is a correction factor accounting for the surface corrugation and the metal filling factor, rls is the radius of the

laser focal spot, rmol is the radius of the average molecular van der Waals cross section perpendicular to the main molecular axis (S-Ccarboxylate axis) and assuming that each adsorbate occupies a circular area equivalent to the van der Waals cross section area of a 4-MBA, β is a correction factor accounting for the tilting of adsorbed molecules with respect to the surface normal, Fpd is the packing density of a monolayer of adsorbed molecules (the number of adsorbed molecules per unit area) on the substrate surface. Figure 11 illustrates the assumptions adopted in the evaluation of R and β factors. The R factor is dictated by two contributions, the surface corrugation factor and the metal filling factor. The surface corrugation is accounted by assuming a monolayer of closely packed hemispheres (Figure 11A) whose diameter can be estimated from the SEM data. The “effective irradiated area”, Aeff, under the laser focal spot on the as-fabricated substrate is therefore the focal spot area (πrls2) (r ∼ 1 µm in our setup at 632.8 nm excitation) multiplied by a corrugation factor that depends on the packing of the Au nanoparticles. The corrugation factor is in the range between 1.785 for a square close packing and 1.907 for a hexagonal close packing of hemispheres. We have adopted 1.785 as the surface corrugation factor, but the use of 1.907 will not affect the order of the magnitude of the overall EF. The R factor is therefore obtained by the product of surface corrugation factor and the metal filling factor obtained from the SEM data, which are 0.60 and 0.42 for 40 nmAu|PAM100 and 40 nmAu|PAM200 substrates, respectively. The values of R factor thus estimated are 1.066 and 0.753 for 40 nmAu|PAM100 and 40 nmAu|PAM200 substrates, respectively. The β factor is determined mainly by the adsorption orientation (or tilting angle with respect to the surface normal) of adsorbates. To estimate Nsurf, or the range of Nsurf, two limiting cases of adsorbed 4-MBA orientations are considered,61-63 as illustrated in Figure 11B (vertical orientation, θ ) 0°) and Figure 11C (tilted orientation, θ ) 30°), respectively, for both of which the x axis is along the phenyl ring normal; y and z axes are in the phenyl plane but perpendicular to and parallel with the main molecular axis (S-Ccarboxylate), respectively. The tilting angle of



Figure 11. (A) Surface topology assumed in the calculation of the effective surface area of Au nanoparticles network film for the evaluation of EF in SERS. (B) (0°) and (C) (30°) illustrate the two limiting orientations of 4-MBA with respect to the surface normal in the calculation of the surface density of assembled 4-MBA. The circles around the normal represent the area occupied by one adsorbed 4-MBA molecule.

30° for 4-MBA is adopted from the literature.61-63 The values of β for two orientations are 1 and 3.84°, respectively. With the above R and β values, and taking the maximum van der Waals diameter, 2rmol, of the 4-MBA molecule to be 0.64 nm along the phenyl ring,61 the packing density was estimated to be 3.1 × 1014 for θ ) 0° and 8.1 × 1013 molecules cm-2 for θ ) 30° orientations, respectively. As a comparison, the packing density of benzenethiol on the Au (111) surface was reported between 6.8 × 1014 and 2.9 × 1014 molecules cm-2, based on the van der Waals radii of phenyl ring.62 Because of the highly corrugated (and locally curved) surface, we expect that the packing densities on our Au|PAM substrate is somewhat lower than that on a flat single crystal surface.63 The evaluation of Nsol in eq 1, on the other hand, is critically dependent on the estimation of an “effective scattering volume” of the 4-MBA solution, as illustrated in Figure 10B. Experimentally, this requires that the INR from Nsol used in eq 1 is measured in as parallel experimental conditions as possible to that of ISERS and Nsurf, especially the comparable thicknesses of the solution layers. The Nsol can be obtained from

Nsol ) cVeffNA

(3)

where c is the molar concentration of 4-MBA ethanolic solution (mol L-1). Veff is the “effective scattering volume” of the solution (L) and can be represented by a truncated cone of the focused laser beam. NA is Avogadro’s constant. The key parameter for the accurate estimation of Veff is the determination of “the effective focal depth”, hlfd, of the focused laser beam in solution, which can be represented as the height of a truncated cone as illustrated in Figure 10B. To estimate hlfd in our experimental setting, we employed a piece of single crystal silicon wafer placed flat inside a quartz cuvette the same as the one used in all the SERS and normal Raman measurements. The laser beam was then focused on the silicon surface to achieve the maximized intensity of the characteristic Si lattice mode at 520 cm-1. This intensity was recorded as I100(Si). The cuvette, and therefore the Si wafer surface, were then moved up (along z-axis) gradually and step-

by-step, accompanying the laser focal spot which was gradually “overfocused” beneath the silicon surface, and as a result, the intensity of 520 cm-1 peak decreases gradually along with the increase of the overfocal depth. We define “the effective focal depth” hlfd in our experimental setting as the overfocal depth at which the intensity of Si peak was lowered to 50% of I100(Si). The details of the experimental data for the determination of hlfd were given in the Supporting Information. For our microRaman setup, the working objective is ×50 with a numeric aperture of 0.55 and working distance of 8 mm. With these parameters, the hlfd is determined to be 15.7 µm for 632.8 nm excitation in our micro-Raman setup. Veff was estimated to be 2317 µm3 and Nsol is estimated to be 4.19 × 1011. Using the ISERS and INR measured for the 1587 cm-1 band from Figure 7A,B, the EFs at 632.8 nm excitation are estimated to be in the range (2.1-7.9) × 105 for 40 nmAu|PAM100 and (1.0-3.8) × 106 for 40 nmAu|PAM200, respectively. Using exactly the same procedure as above but on a separate micro-Raman setup constructed with a rectangular focal spot of 2 × 24 µm2 and equipped with 785 nm excitation, the hlfd was estimated to be 17.9 µm for a truncated rectangular pyramid. For this case, Nsurf is estimated to be in the range (0.41-1.59) × 108 and (0.29-1.12) × 108, respectively, for 40Au|PAM100 and 40Au|PAM200 substrates. The effective scattering volume of the truncated rectangular pyramid Veff is estimated to be 8989 µm3, and Nsol is estimated to be ∼1.62 × 1012. (See Supporting Information for the detailed experimental data and parameters.) Using the ISERS and INR measured for the 1587 cm-1 band from Figure 8, the EFs at 785 nm excitation are estimated to be in the range (0.26-1.0) × 106 for 40 nmAu|PAM100 and (0.31-1.2) × 105 for 40 nmAu|PAM200 substrates, respectively. For the estimation of EFs at both excitations, both SERS and normal Raman spectra were acquired in the same experimental setting for a given excitation. Therefore, the sources of error were minimized. The above values of the estimated EF in this study represent the lower limits of the actual EF of the as-fabricated network films for two reasons. First, the “effective scattering volumes” used to account for the observed INR were underestimated by virtue of the method used. Therefore, the Nsol used in eq 1 was actually underestimated and represents the lower limit of its actual value. Second, the Nsurf is overestimated because we assumed a perfect close packing of the monolayer of adsorbed 4-MBA molecules. It was reported that the packing efficiency of the self-assembled monolayer of thiol molecules on Au is actually mediated by the surface curvature and/or corrugation, and that the curved surface leads to lowered packing efficency.63 Since the surfaces of our as-fabricated network films are highly corrugated at the nanometer scale (Figures 1 and 2), we expect that the packing efficiency of the self-assembled 4-MBA are lower than the perfect close packing. Therefore, the Nsurf used in the estimation of EFs represents the upper limit of its actual value. With the Nsol and Nsurf estimated at their lower and upper limits, it can be anticipated from eq 1 that the estimated EFs should represent the lower limits of the actual EFs. Two interesting observations emerge from above estimated EFs obtained at two different excitation wavelengths and for two different substrate films, 40 nmAu|PAM100 and 40 nmAu|PAM200. First, EFs of two films display opposite behaviors at two excitation wavelengths. At 632.8 nm excitation, the EF from 40 nmAu|PAM100 (∼5 × 105) is about 1 order of magnitude smaller than that from 40 nmAu|PAM200 (∼2.4 × 106), while at 785 nm excitation, it is exactly the opposite, namely, the EF from 40 nmAu|PAM100 (∼6.3 × 105) is roughly

10472


Li et al.

Figure 12. DDA calculations of the electric field intensity (E2) distribution on the linear chain aggregates of 1 (A), 2 (B), 3 (C), and 6 (D) Au nanospheres, respectively. Each particle is 40 nm in diameter and represented by 33 000 point dipoles. Only the modes polarized along the long axes are displayed. (see text for the discussion.)

1 order of magnitude larger than that from 40 nm|PAM200 (∼7.5 × 104). Second, the EFs for 40 nmAu|PAM100 substrates are approximately the same within the estimation error at both 632.8 nm (∼5 × 105) and 785 nm (∼6.3 × 105) excitations, but for the 40 nmAu|PAM200 substrate the EF at 632.8 nm (∼2.4 × 106) is about 1 order of magnitude larger than that at 785 nm (∼7.5 × 104). These seemingly paradoxical observations can actually be accounted for by the structural and optical characteristics of the as-fabricated films as discussed below. From the structures of the as-fabricated Au network films (Figures 1 and 2), it could be anticipated that there are mainly two factors contributing to the observed EFs. The first factor is the LSPR originated from the extinctions of individual nanoparticles and the interparticle couplings among them. This factor normally gives rise to an extinction peak at ∼520 nm (for Au nanoparticles with an average size comparable to those in our samples) with enhanced tails into longer wavelengths dictated by the nature of aggregation.56 This factor is expected to be more pronounced (or dominant) in the observed EF at shorter excitation wavelength (e.g., 632.8 nm, but still >520 nm). The second factor is the enhanced transmission, and therefore decreased scattering, originated from the nanocavities. This factor is expected to play a greater role at longer excitation wavelengths (e.g., 785 nm) and for films with larger average pore diameter and smaller filling factors. Therefore, which of the two factors plays a dominant role in an observed EF is very much dependent on both the excitation wavelength and the average cavity diameter of the sample. For 40 nmAu|PAM100 film, the EF(632.8 nm) is roughly the same as the EF(785 nm) because the average cavity diameter of the substrate is small (comparing with that on PAM200 substrate) and the nanocavity-enhanced transmission at both excitations can be neglected. As a result, the EF is very much dependent on the average size of the Au nanoparticles and their aggregation patterns, not so much on the excitation wavelength. On the other hand, for the 40 nmAu|PAM200 film where the nanocavity-enhanced transmission cannot be neglected because of the larger cavity diameter, the EF(632.8 nm) is larger

than EF(785 nm) because the cavity-enhanced transmission is larger at 785 nm than at 632.8 nm. Also consistent with this argument is that at 785 nm excitation, EF from 40 nmAu|PAM100 is larger than that from 40 nmAu|PAM200 because the latter has a larger average pore diameter than the former, therefore a larger nanocavity-enhanced transmission and smaller LSPR-enhanced scattering. However, the observation that the EF(632.8 nm) from 40 nmAu|PAM100 is 1 order of magnitude smaller than that from 40 nmAu|PAM200 is intriguing and will be accounted for by the modeling using the discrete dipole approximation calculations (vide infra). One important observation relevant to the EF is the spatial uniformity and isotropy of enhancement over the whole SERS substrate, typically 13 mm in diameter, developed in this study. When the laser focal spot was moved spatially from one site to another on an as-fabricated SERS substrate at a given excitation condition or when the sample substrate is rotated with respect to the incident laser beam, comparable SERS spectra and EFs were obtained. This can be a very important advantage of our substrates in SERS-based analytical applications. This desirable property is attained because (i) the Au network films are composed of nanoparticles of similar sizes and nanocavities of similar diameters and (ii) the aggregation pattern, therefore the interparticle and intercavity couplings, are relatively uniform throughout the sample and are robustly stable with chemical modification and light irradiation, as can be seen from Figures 1 and 2. Optical Electric Field Distribution on AuNP-2dNW by Discrete-Dipole-Approximation (DDA) Calculations. To gain a better understanding of the optical properties as well as the SERS behaviors of the as-fabricated nanoparticle-nanocavity dual structured network films, we have calculated the electric field intensity distribution and the optical extinction spectra using DDA method,73-75 which has been shown to be a highly efficient computational tool for the study of optical properties of nanoparticles of arbitrary shape and their arrays.76-79 For the DDA calculation, we assumed the smallest structural unit to be

Au Nanoparticle-Nanocavity Dual Structures an Au nanosphere with a diameter of 40 nm (∼2 million Au atoms) composed of 33 000 point dipoles under the excitation of 632.8 nm. The incident light was assumed to have an intensity of 1 and propagate along the paper normal with a vertically polarized electric field in paper plane. The dielectric functions of the Au spheres are from a recent work by the same group of authors.52 The distributions of electric field intensity in two types of aggregation topologies of Au nanoparticles, linear and circular, which were typical local aggregation structures observed in the SEM of the as-fabricated films, were calculated. For the linear aggregation topology, we have calculated the field intensity distributions on linear aggregates containing 2-18 contacting nanospheres respectively, and the results for 2, 3, and 6 spheres are shown in Figure 12. As a control and comparison, the field intensity for a single sphere is also calculated (Figure 12A). The maximum intensities are calculated to be 19.1 on one individual particle (Figure 12A), 5159 at the contacting point of two particles (Figure 12B), and 13 601 at the contacting points of three particles (Figure 12C), respectively, with E polarized parallel with the chain axis. As expected, the field intensities with E perpendicular to the long axis are the same (16-17) for three cases. When more spheres are aggregated linearly, hot spots of different intensities appear. For an n-sphere linear aggregate, the total number of hot spots (the sphere-sphere contacting points) is n - 1, while the number of symmetrically unequivalent hot spots are n/2 (for n ) even) and (n - 1)/2 (for n ) odd), respectively. Taking a linear 6-sphere chain as an example, three sets of symmetrically unequivalent hot spots are expected. The field intensity maxima appeared in the contacting areas between the third and fourth (the central pair), second and third, and first and second sphere and are calculated to be 41 208, 34 319, and 17 493, respectively, roughly in a ratio of 2.4:2:1 (Figure 12D). Comparing with the intensity at the “hot spot” of the 2-sphere case, the “hot spots” on a linear 6-sphere chain have gained an 8, 6, and 3 fold boosts in intensity, respectively. When the electric field intensity at the hottest spot, which is also the highest intensity maximum on the chain, is plotted against the number of spheres in the linear aggregate, as shown in Figure 13A, a very interesting observation emerges, which clearly shows that the intensity of the hottest spot in a linear chain aggregate of Au nanospheres does not always go up monotonically with the number of nanospheres. It does go up drastically and monotonically in a sigmoidal manner with the number of nanospheres up to n ) 6, reaches a maximum at n ) 6, and then starts to decrease when the number of contacting nanospheres continues to increase. In other words, this result suggests that there exists an “optimal aggregation number” of six spheres for generating the hottest spot of electric field intensity in a linear contacting aggregate of Au nanoparticles each with a diameter of 40 nm and at 632.8 nm excitation. However, the total intensity summing over all the hot spots of a linear aggregate, represented as ΣImax, always increases monotonically with the number of nanospheres in the aggregate, as shown in Figure 13B (dot), but again, the “average hot intensity per nanosphere”, defined as ΣImax/n (or the “average intensity per hot spot defined as ΣImax/ (n - 1)), has an optimal maximum value of n ) 6 (the black square in Figure 13B). Although a 6-sphere linear aggregate gives rise to the hottest spot in the electric field intensity at the specific conditions employed in the above calculation, it also generates the largest inhomogeneous intensity distribution (or the most unevenly distributed intensities) among all the hot spots on the linear aggregate, with the ratio between the intensities of its three types


Figure 13. DDA calculations of the electric field intensities at the inter sphere contacts (the hot spots) for the linear chain aggregates of 2-18 spheres, respectively. (A) The intensity at the “hottest spot” versus the number of contacting spheres (n) in the linear chain. (B) The “average hot intensity per sphere” (ΣImax/n) and the total hot intensity, ΣImax, summed over all the inter sphere contacts versus the number of contacting spheres in the linear chain. Other parameters are the same as Figure 12.

of hot spots being ∼2.4:2:1. This inhomogeneity of intensity distribution among the hot spots can be a disadvantage in many SERS applications where a homogeneous and evenly distributed SERS EF is often desirable. This problem can be resolved to a certain extent by linear aggregates of more (n . 6) nanospheres, as nicely demonstrated in Figure 14, which shows the dependence of the hot spot intensity on the location of the hot spot on a chain for n ) even number (Figure 14A) and n ) odd number (Figure 14B), respectively. Figure 14 clearly shows that the linear chain aggregates with n . 6 can homogenize the intensity distribution on the hot spots on the chain aggregate. For example, for a linear aggregate of 18 spheres (n ) 18) (Figure 14A), 13 out of its 17 hot spots (or ∼76% of all the hot spots on a chain) have similar intensities (within 15% of difference). While for a linear aggregate of 15 spheres (n ) 15) (Figure 14B), 10 out of its 14 hot spots (or 71% of all the hot spots on a chain) have similar intensities (within 12% of difference). Put together, the DDA calculations on the electric field intensity distribution on a linear chain aggregate of Au nanoparticles (Figures 13 and 14) illustrate nicely the potential merits and drawbacks of aggregation number in SERS from linear aggregates of nanoparticles. To achieve the hottest spot or highest local EF in SERS at 632.8 nm excitation for Au nanospheres of 40 nm in diameter, a linear 6-spheres chain topology is preferred. However, to achieve a homogeneous intensity distribution with more evenly distributed EF in SERS,

10474


Li et al.

Figure 15. DDA calculation of the peak positions of the longitudinal mode in the extinction spectra of the linear chain aggregates of Au nanospheres. Other parameters are the same as Figure 12. The 632.8 nm excitation is in resonance with the extinction peak of the longitudinal mode of 6-sphere aggregate, generating the hottest spot on the linear chain (see text for the discussion).

Figure 14. DDA calculations of the electric field intensities at different intersphere contacts from the center-most (i ) 1) to the outer-most (i ) n/2 for n ) even; i ) (n - 1)/2 for n ) odd) spots. (A) n ) 2, 4, 6, 8, 10, 12, and 18. (B) n ) 3, 5, 7, 9, 11, and 15. Other parameters are the same as Figure 12.

it is better to have aggregates of longer chains. Figure 13 also suggests that, at a given set of conditions (the metal material, diameter, dielectric environment, excitation wavelength), an “effective aggregation number” can be defined by using the hottest spot intensities from linear aggregates of n ) 1 (the lowest) to 6 (the highest) as the ruler scale. The “effective aggregation number” can be considered as a measure of the equivalence of the aggregated nanoparticles for the generation of the hottest spot. For example, with the set of conditions used in our calculation, the intensity at the hottest spot on a linear 12-sphere chain is calculated to be 31 044, comparable to that on a 4-sphere chain (30 132). The “effective aggregation number” of a 12-sphere chain is thus ∼4. By the same measure, the “effective aggregation number” of linear 9-sphere and 18sphere chains are ∼5 and 3.4, respectively. An intriguing question thus arises as to why, at the conditions used in our calculation, a magic number of 6-sphere linear aggregation is required for the generation of the hottest spot on the linear aggregate. To address this question, we have calculated the extinction spectra of the linear chain aggregates of Au nanospheres at the same level of theory and approximation as used in Figures 12-14. As expected,80 while the peak positions of the transverse mode remain almost unchanged with the increase of the number of Au nanospheres in the linear chain aggregates, the peak position of the longitudinal mode shifts to longer wavelengths with the increase of the chain length, as depicted in Figure 15. It is immediately evident from Figure 15 that the 632.8 nm excitation used in the calculation of E-field distribution happened to be in an exact resonance with the extinction peak of the longitudinal mode of the 6-sphere linear

aggregate, but not with the extinction peaks of linear aggregates of any other chain length. This observation reveals an intrinsic relationship between three factors: the generation of the hottest E-field spot on a given nanoparticle aggregate, the peak position of the optical extinction spectra of the aggregate, and the wavelength used in the optical excitation. In other words, the hottest E-field spot on a given aggregate of nanoparticles is generated only when the excitation wavelength used is in resonance with the peak of the extinction spectra of the aggregate. Therefore, the hottest spot can be generated, in principle, on linear aggregates of any length as long as a proper excitation wavelength is used. For example, from Figure 15, the hottest spot on a linear chain of 8-contacting Au spheres with a diameter of 40 nm can also be generated if the excitation wavelength is at 648 nm. This relationship provides a nice guidance for the design and optimization of optimal SERS substrate fabricated from the aggregated noble metal nanoparticles. For the circular aggregation topology, we have calculated the field intensity distributions in rings composed of 6 and 12 nanospheres, respectively, and the results are shown in Figure 16. We chose these two cases of circular aggregation of nanoparticles because the inspection of the SEM images of the as-fabricated films with various filling factors (Figures 1 and 2) reveals that these two circular aggregations represent the lower and upper limits of the unit circular patterns. For a 6-sphere ring, with the incident light pointing at the center and propagate along the normal of the plane of ring, the calculated local field intensity maxima were 6768 (Figure 15, upper left) and 4321 for the modes with even and odd symmetry with respect to the horizontal plane, respectively, as compared with the 41 208 at the hottest spot on a linear 6-sphere aggregate. The “effective aggregation number” of a 6-sphere ring aggregate is therefore ∼2. For a 12-sphere ring, the calculated local field intensity maxima are 18 246 (Figure 15, lower left) and 36 511, respectively, for the two modes shown, as compared with the 31 044 at the hottest spot on a linear 12-sphere aggregate. The “effective aggregation number” of a 12-sphere ring, estimated from the ruler scale is ∼5. The above DDA calculations suggest that for a given number of nanospheres of a given size and given metal material, the highest local electric field intensity (the hottest spot on an aggregate) can be achieved either on a linear aggregate (e.g., for n ) 6 at the conditions adopted) or on a circular aggregate (e.g., n ) 12 at the conditions adopted), depending on the



Figure 16. DDA calculations of the electric field intensity distribution on the circular aggregates of 6 (A, B) and 12 (C, D) Au nanospheres, respectively. Two modes of the highest intensity but different polarizations are shown. Other parameters are the same as Figure 12.

“effective aggregation number” of nanospheres in the aggregate. Taking 6-sphere aggregation used in our calculation as an example, its linear and circular aggregates have an “effective aggregation number” of 6 (the largest possible) and ∼2, respectively. Therefore, a linear aggregate generates a hotter spot than that of a circular aggregate. But for 12-sphere aggregation, the “effective aggregation numbers” for linear and circular aggregates are ∼4 and ∼5, respectively. As a result, the circular aggregate generates a hotter spot than a linear one. This result can actually be anticipated since, for a 3-contactingsphere model, the constructive interference of the LSPR between the two 2-sphere subsystems is the largest (the fully coupled) for a linear arrangement (3-sphere angle ) 180°) and the smallest (the decoupled) for the orthogonal arrangement (3sphere angle ) 90°). For a hexagonal circular arrangement of 6-spheres, the 3-sphere angle is 120°, and the DDA calculation indicates that the 6-membered ring can be treated approximately as three independent 2-spheres (dimers), and the interference among the three dimers can be neglected. As a consequence, the “effective aggregation number” of a hexagonal ring-shaped 6-sphere aggregate is ∼2. For a 12-membered ring aggregate, the trisphere angle is 150°, and therefore a much stronger constructive coupling between nearby “dimers” is expected. In other words, the concept of the “effective aggregation number” can also be used as a measure of the electric field intensity of hottest spots on two-dimensional nonlinear aggregates such as the ring-shaped aggregates. The DDA prediction that much “hotter” local intensity spots are generated on a 12-membered ring than on a 6-membred ring provides a nice explanation for the observed disparity of the EFs estimated for 40 nmAu|PAM100 and 40 nmAu|PAM200, respectively, at 632.8 nm excitation. At 632.8 nm excitation, the LSPR from individual nanoparticles and couplings among

them dominates the observed enhancement in SERS. For the 40 nmAu|PAM100 sample, the average diameter of the nanocavity template (∼100 nm in PAM100) is much smaller that in 40 nmAu|PAM200 (∼200 nm in PAM200). As a consequence, the number of nanoparticles enclosing a nanocavity in the 40 nmAu|PAM100 sample is smaller, on average, than that required to form an enclosure in 40 nmAu|PAM200, as is indeed the case observed in their respective SEMs (Figures 1 and 2). On an average term, the larger rings formed by more nanoparticles in 40 nmAu|PAM200 film would generate “hotter” spots with higher local field intensities than the smaller rings on 40 nmAu|PAM100, since the former has a larger “effective aggregation number” than the latter. This would lead to larger EF on the former sample (40 nmAu|PAM200) than on the latter. We point out the significance of symmetry of aggregates in addressing the difference between the linear and circular aggregations in their local field intensity maxima. For the linear chain aggregation, the 2-fold rotational symmetry (C2) in 2-dimensions (or the inversion symmetry in 1-dimension) is the only symmetry element present. As a result, the intensity distribution on the linear chain of contacting nanoparticles is very anisotropic with maxima polarized along the long-axis direction. The number of symmetrically unequivalent hot spots is dependent on the number of aggregated nanospheres in the chain and is equal to (n - 1)/2 when n is odd and n/2 when n is even. The field intensity maximum is the highest at (for n ) even) or nearest to (for n ) odd) the center of the symmetry and decreases gradually toward the hot spots at the ends of the chain, as nicely demonstrated in the 3-sphere and 6-sphere linear chains in Figure 12. On the other hand, the ring-shaped aggregation possesses a much higher symmetry. For 6-membered and 12-membered rings, the highest symmetries are 6-fold (C6) and 12-fold (C12) rotational symmetry in 2-dimension,

10476


respectively. We can therefore envision that the consequence of the high symmetry in ring-shaped aggregation is that the hot spots on rings would behave “delocalized” along the ring rim in any far-field optical experiments on these substrates, very much like the behavior of delocalized electrons on a benzene ring or a planar aromatic macrocycle. This behavior can also be accounted for by the plasmon hybridization theory devised by Halas and Nordlander and their co-workers.81,82 Manifestation of such a “delocalized hot spot” along the ring aggregates in SERS is that the far-field SERS would behave isotropically, regardless of the orientation of the as-fabricated network films with respect to the laser polarization. Conclusions In this work, we devised and demonstrated a facile method for the fabrication of 2-dimensional networks of Au nanoparticle-nanocavity dual structures supported on dielectric nanosieves. The optical extinctions of the as-fabricated Au films were measured and were found to be dominated by the characteristics of nanocavities. The surfaces of the as-fabricated Au films were found to be hydrophobic toward water with a measured contact angle of 125.4° but quite wettable with ethanol with a contact angle of 9.5° at room temperature. We demonstrated that the surface wettability of the as-fabricated Au films is readily convertible to totally hydrophobic or hydrophilic when chemically modified with the self-assembled monolayer of molecules with desired functional groups. The as-fabricated Au films are active, stable, and uniform as substrates in SERS and can be made in a relatively large area (13 mm and 47 mm in diameters). High quality SERS spectra from a monolayer of self-assembled 4-MBA, a 4MBA-R6G electrostatic double layer, and nonspecifically adsorbed R6G are demonstrated, respectively. Employing the SERS spectra of the monolayer of selfassembled 4-MBA, the enhancement factors of 106 and 105 were achieved with the excitations at 632.8 and 785 nm, respectively. Discrete dipole approximation calculations were conducted to examine the electric field intensity distribution on two typical local aggregation structures, linear and circular aggregated nanoparticles, respectively. For linear aggregation of Au nanospheres with a 40 nm diameter at 632.8 nm excitation, we found that a 6-sphere aggregate generates the hottest spot, but with most unevenly distributed intensities on all the hot spots on the chain. On the other hand, linear aggregation with n . 6 (n ) the number of contacting Au spheres) would homogenize the intensity distribution among all the hot spots on a chain. The calculation of the extinction spectra for Au linear aggregates reveals that the hottest spot is generated only when the excitation wavelength is in resonance with the extinction peak of the aggregate. A “hot-spot” delocalization is proposed for the nanocavities composed of circular aggregation of nanoparticles, which is believed to be responsible for the observed isotropic SERS enhancement on the as-fabricated nanoparticle-nanocavity network films. The potential benefits of the as-fabricated metallic network films can be envisioned from this work. First, the fabrication method is facile and low-cost with a relative high throughput comparing with most of the methods for the fabrication of twodimensional nanostructured noble metal films. Second, the method is quite versatile and can be straightforwardly extended to other metal materials such as Ag and transition metals, as well as other nanoporous or nanostructured substrates (e.g., porous silica and carbon and porous polymeric membranes). Indeed, Ag films fabricated with the same method display very similar structural and optical characteristics as Au in this report.83

Li et al. Third, depending on the average diameters of the pores on the PAM substrate, there is a fairly good range of tunability for the average particle size and average cavity diameter, as well as the metal filling factor, allowing the optical properties and SERS EF being tuned and optimized. Last, but not least, the asfabricated network films of nanoparticle-nanocavity dual structures supported on dielectric nanoporous sieves are intrinsically suitable for preparing surfaces possessing heterogeneous two phases (a liquid or solution phase inside pores and a solid metallic phase of rims) and intrinsically active in thermocatalysis and electrocatalysis, both of which are important and desirable attributions in SERS-based and electrochemically based chemoand biosensing.83 Acknowledgment. Part of this work was presented in brief in the 236th National Meeting of the American Chemical Society, Philadelphia, PA, August 17-21, 2008, as Paper No. 236 in Symposium on Frontiers in Nanoscale Materials Analyses (Analytical Chemistry Division). Y.-S.L. and X-Y.L., “Nobel metallic nanoweb interfaces for chemo- and bio-sensing by surface-enhanced micro-Raman spectroscopy.” Part of this work was also included in the Ph.D. thesis of Y.S.L., documented in public in HKUST, Hong Kong, in April 2009. This work was partially supported by Research Grant Council of Hong Kong through project #601706 (X.Y.L.), #602309 (X.Y.L.), #603908 (K.S.W.), and the HKUST Research Project Competition # RPC06/07.SC17 (to X.Y.L. and K.S.W.). Supporting Information Available: Figure S1 and Figure S2 provide the SEM images of the active (front and branched) and support (back) sides of the Anopore membranes used in this study. Figure S3 is the thin film X-ray diffraction of the as-fabricated 40 nmAu|PAM100 sample. Figure S4 is the Raman spectra of single crystal Si wafer at different overfocal depths, and the evaluation of the effective focal depth from I50 in our microRaman spectrometer with excitation at 632.8 nm. Figure S5 is the same as Figure S4 except it was done on a separate microRaman spectrometer constructed with a rectangular focal spot of 2 × 24 µm2 and equipped with a 785 nm excitation. Figure S6 illustrates the proposed “effective aggregation number” ruler scale based on DDA calculation for linear aggregates of Au nanospheres with a diameter of 40 nm and at 632.8 excitation. Table S1 and Table S2 list all the parameters used in the estimation of EFs for two types of as-fabricated Au network films (40 nmAu|PAM100 and 40 nmAu|PAM200) at 632.8 nm (Table S1) and 785 nm (Table S2), respectively. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Stiles, P. L.; Dieringer, J. A.; Shah, N. C.; Van Duyne, R. P. Annu. ReV. Anal. Chem. 2008, 1, 601–626. (2) Kneipp, K.; Kneipp, H.; Kneipp, J. Acc. Chem. Res. 2006, 39 (7), 443–450. (3) Jain, P. K.; Huang, X. H.; El-Sayed, I. H.; El-Sayed, M. A. Acc. Chem. Res. 2008, 41 (12), 1578–1586. (4) Stewart, M. E.; Anderton, C. R.; Thompson, L. B.; Maria, J.; Gray, S. K.; Rogers, J. A.; Nuzzo, R. G. Chem. ReV. 2008, 108 (2), 494–521. (5) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Phys. ReV. Lett. 1997, 78 (9), 1667–1670. (6) Nie, S. M.; Emory, S. R. Science 1997, 275, 1102–1106. (7) Kneipp, J.; Kneipp, K. Chem. Soc. ReV. 2008, 37 (5), 1052–1060. (8) For a nice set of reviews on the contemporary issues in SERS and its applications, see articles in: (i) Faraday Discuss. 2006, 132, 1-340; (ii) Topics in Applied Physics; Kneipp, K.; Moskovits, M.; Kneipp, H., Eds.; 2006; Vol. 103; (iii) Chem. Soc. ReV. 2008, 37 (5), 873-1076. (9) Kahl, M.; Voges, E.; Kostrewa, S.; Viets, C.; Hill, W. Sens. Actuators, B 1998, 51 (1-3), 285–291.

Au Nanoparticle-Nanocavity Dual Structures (10) Felidj, N.; Aubard, J.; Levi, G.; Krenn, J. R.; Salerno, M.; Schider, G.; Lamprecht, B.; Leitner, A.; Aussenegg, F. R. Phys. ReV. B 2002, 65 (7), 075419. (11) Reilly, T. H.; Chang, S. H.; Corbman, J. D.; Schatz, G. C.; Rowlen, K. L. J. Phys. Chem. C 2007, 111 (14), 1689–1694. (12) Ebbesen, T. W.; Lezec, H. J.; Ghaemi, H. F.; Thio, T.; Wolff, P. A. Nature 1998, 391, 667–669. (13) Grupp, D. E.; Lezec, H. J.; Thio, T.; Ebbesen, T. W. AdV. Mater. 1999, 11 (10), 860. (14) Srituravanich, W.; Fang, N.; Sun, C.; Luo, Q.; Zhang, X. Nano Lett. 2004, 4 (6), 1085–1088. (15) Brolo, A. G.; Arctander, E.; Gordon, R.; Leathem, B.; Kavanagh, K. L. Nano Lett. 2004, 4 (10), 2015–2018. (16) Koerkamp, K. J. K.; Enoch, S.; Segerink, F. B.; van Hulst, N. F.; Kuipers, L. Phys. ReV. Lett. 2004, 92 (18), 183901. (17) Gordon, R.; Brolo, A. G.; McKinnon, A.; Rajora, A.; Leathem, B.; Kavanagh, K. L. Phys. ReV. Lett. 2004, 92 (3), 037401. (18) Brolo, A. G.; Kwok, S. C.; Moffitt, M. G.; Gordon, R.; Riordon, J.; Kavanagh, K. L. J. Am. Chem. Soc. 2005, 127 (42), 14936–14941. (19) Lesuffleur, A.; Kumar, L. K. S.; Brolo, A. G.; Kavanagh, K. L.; Gordon, R. J. Phys. Chem. C 2007, 111 (6), 2347–2350. (20) Tan, R. Z.; Agarwal, A.; Balasubramanian, N.; Kwong, D. L.; Jiang, Y.; Widjaja, E.; Garland, A. Sens. Actuators, A 2007, 139 (1-2), 36–41. (21) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Appl. Phys. Lett. 1995, 67 (21), 3114–3116. (22) Jeon, S.; Park, J. U.; Cirelli, R.; Yang, S.; Heitzman, C. E.; Braun, P. V.; Kenis, P. J. A.; Rogers, J. A. Proc. Natl. Acad. Sci. 2004, 101 (34), 12428–12433. (23) Stewart, M. E.; Mack, N. H.; Malyarchuk, V.; Soares, J.; Lee, T. W.; Gray, S. K.; Nuzzo, R. G.; Rogers, J. A. Proc. Natl. Acad. Sci. 2006, 103 (46), 17143–17148. (24) Alvarez-Puebla, R.; Cui, B.; Bravo-Vasquez, J. P.; Veres, T.; Fenniri, H. J. Phys. Chem. C 2007, 111 (18), 6720–6723. (25) Abu Hatab, N. A.; Oran, J. M.; Sepaniak, M. J. ACS Nano 2008, 2 (2), 377–385. (26) Hulteen, J. C.; Van Duyne, R. P. J. Vac. Sci. Technol., A 1995, 13 (3), 1553–1558. (27) Ormonde, A. D.; Hicks, E. C. M.; Castillo, J.; Van Duyne, R. P. Langmuir 2004, 20 (16), 6927–6931. (28) Bartlett, P. N.; Birkin, P. R.; Ghanem, M. A. Chem. Commun. 2000, 1671–1672. (29) Bartlett, P. N.; Baumberg, J. J.; Birkin, P. R.; Ghanem, M. A.; Netti, M. C. Chem. Mater. 2002, 14 (5), 2199–2208. (30) Dick, L. A.; McFarland, A. D.; Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2002, 106 (4), 853–860. (31) Zhang, X.-Y.; Whitney, A. V.; Zhao, J.; Hicks, E. M.; Van Duyne, R. P. J. Nanosci. Nanotechnol. 2006, 6 (7), 1920–1934. (32) Zhang, X. Y.; Young, M. A.; Lyandres, O.; Van Duyne, R. P. J. Am. Chem. Soc. 2005, 127 (12), 4484–4489. (33) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629–1632. (34) Malynych, S.; Luzinov, I.; Chumanov, G. J. Phys. Chem. B 2002, 106 (6), 1280–1285. (35) Li, X. L.; Xu, W. Q.; Zhang, J. H.; Jia, H. Y.; Yang, B.; Zhao, B.; Li, B. F.; Ozaki, Y. Langmuir 2004, 20 (4), 1298–1304. (36) Schierhorn, M.; Lee, S. J.; Boettcher, S. W.; Stucky, G. D.; Moskovits, M. AdV. Mater. 2006, 18 (21), 2829. (37) Lombardi, I.; Cavallotti, P. L.; Carraro, C.; Maboudian, R. Sens. Actuators, B 2007, 125 (2), 353–356. (38) Broglin, B. L.; Andreu, A.; Dhussa, N.; Heath, J. A.; Gerst, J.; Dudley, B.; Holland, D.; El-Kouedi, M. Langmuir 2007, 23 (8), 4563– 4568. (39) Gu, G. H.; Kim, J.; Kim, L.; Suh, J. S. J. Phys. Chem. C 2007, 111 (22), 7906–7909. (40) Lee, S. J.; Guan, Z. Q.; Xu, H.-X.; Moskovits, M. J. Phys. Chem. C 2007, 111 (49), 17985–11988. (41) O’Sulliva, J. P.; Wood, G. C. Proc. R. Soc. London, Ser. A 1970, 317, 511. (42) Thompson, G. E.; Furneaux, R. C.; Wood, G. C.; Richardson, J. A.; Goode, J. S. Nature 1978, 272, 433. (43) Furneaux, R. C.; Rigby, W. R.; Davidson, A. P. Nature 1989, 337, 147–149. (44) Masuda, H.; Fukuda, K. Science 1995, 268, 1466–1468. (45) Li, A. P.; Muller, F.; Birner, A.; Nielsch, K.; Gosele, U. J. Appl. Phys. 1998, 84 (11), 6023–6026.

J. Phys. Chem. C, Vol. 114, No. 23, 2010 10477 (46) Masuda, H.; Abe, A.; Nakao, M.; Yokoo, A.; Tamamura, T.; Nishio, K. AdV. Mater. 2003, 15 (2), 161. (47) Lee, W.; Ji, R.; Gosele, U.; Nielsch, K. Nat. Mater. 2006, 5 (9), 741–747. (48) Lei, Y.; Cai, W. P.; Wilde, G. Prog. Mater. Sci. 2007, 52 (4), 465– 539. (49) Brumlik, C. J.; Martin, C. R. J. Am. Chem. Soc. 1991, 113 (8), 3174–3175. (50) Almawlawi, D.; Liu, C. Z.; Moskovits, M. J. Mater. Res. 1994, 9 (4), 1014–1018. (51) Piao, Y.; Lim, H.; Chang, J. Y.; Lee, W. Y.; Kim, H. Electrochim. Acta 2005, 50 (15), 2997–3013. (52) Su, H. M.; Li, Y.-S.; Li, X.-Y.; Wong, K. S. Opt. Express 2009, 17 (24), 22223–22234. (53) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546– 551. (54) Wenzel, R. N. J. Phys. Chem. 1949, 53 (9), 1466–1467. (55) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z. Q.; Jiang, L.; Li, X.-Y. J. Am. Chem. Soc. 2004, 126 (10), 3064–3065. (56) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103 (21), 4212– 4217. (57) Genet, C.; Ebbesen, T. W. Nature (London) 2007, 445, 39–46. (58) Kwak, E. S.; Henzie, J.; Chang, S.-H.; Gray, S. K.; Schatz, G. C.; Odom, T. W. Nano Lett. 2005, 5 (10), 1963–1967. (59) Coe, J. V.; Rodriguez, K. R.; Teeters-Kennedt, S.; Cilwa, K.; Heer, J.; Tian, H.; Williams, S. M. J. Phys. Chem. C 2007, 111 (47), 17459– 17472. (60) Garcia de Abajo, F. J. ReV. Mod. Phys. 2007, 79, 1267–1290. (61) Lee, J. R. I.; Willey, T. M.; Nilsson, J.; Terminello, L. J.; De Yoreo, J. J.; van Buuren, T. Langmuir 2006, 22 (26), 11134–11141. (62) Whelan, C. M.; Smyth, M. R.; Barnes, C. J. Langmuir 1999, 15 (1), 116–126. (63) Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Bushnell, D. A.; Kornberg, R. D. Science 2007, 318, 430–433. (64) Michota, A.; Bukowska, J. J. Raman Spectrosc. 2003, 34 (1), 21– 25. (65) Watanabe, H.; Hayazawa, N.; Inouye, Y.; Kawata, S. J. Phys. Chem. B 2005, 109 (11), 5012–5020. (66) Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2003, 107 (30), 7426–7433. (67) McFarland, A. D.; Young, M. A.; Dieringer, J. A.; Van Duyne, R. P. Phys. Chem. B 2005, 109 (22), 11279–11285. (68) Felidj, N.; Truong, S. L.; Aubard, J.; Levi, G.; Krenn, J. R.; Hohenau, A.; Leitner, A.; Aussenegg, F. R. J. Chem. Phys. 2004, 120 (15), 7141–7146. (69) Laurent, G.; Fe´lidj, N.; Aubard, J.; Le´vi, G.; Krenn, J. R.; Hohenau, A.; Schider, G.; Leitner, A.; Aussenegg, F. R. Phys. ReV. B 2005, 71 (4), 045430. (70) Cai, W. B.; Ren, B.; Li, X. Q.; She, C. X.; Liu, F. M.; Cai, X. W.; Tian, Z. Q. Surf. Sci. 1998, 406 (1-3), 9–22. (71) Fang, Y.; Seong, N.-H.; Dlott, D. D. Science 2008, 321, 388–392. (72) Le Ru, E. C.; Blackie, E.; Meyer, M.; Etchegoin, P. G. J. Phys. Chem. C 2007, 111 (37), 13794–13803. (73) Draine, B. T.; Flatau, P. J. J. Opt. Soc. Am. A 1994, 11 (4), 1491– 1499. (74) Draine, B. T.; Flatau, P. J. J. Opt. Soc. Am. A. 2008, 25 (11), 2693– 2703. (75) Draine, B. T. and Flatau, P. J. User Guide to the Discrete Dipole Approximation Code DDSCAT 7.0, 2008, http://arXiv.org/abs/0809.0337v5; http://www.astro. princeton.edu/∼drain/DDSCAT. (76) Yang, W.-H.; Schatz, G. C.; Van Duyne, R. P. J. Chem. Phys. 1995, 103 (3), 869–875. (77) Jensen, T.; Kelly, L.; Lazarides, A.; Schatz, G. C. J. Cluster Sci. 1999, 10 (2), 295–317. (78) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107 (3), 668–677. (79) Zou, S. L.; Schatz, G. C. Top. Appl. Phys. 2006, 103, 67–87, and references cited therein. (80) Quinten, M.; Kreibig, U. Surf. Sci. 1986, 172 (3), 557–577. (81) Prodan, E.; Radloff, C.; Halas, N. J.; Nordlander, P. Science 2003, 302, 419–422. (82) Wang, H.; Brandl, D. W.; Nordlander, P.; Halas, N. J. Acc. Chem. Res. 2007, 40 (1), 53–62. (83) Li, Y.-S.; Su, H.-M.; Wong, K. S.; Li, X.-Y. Work to be submitted.

JP100499Z

Surface-Enhanced Raman Spectroscopy on Two-Dimensional

Recommend Documents