Article pubs.acs.org/Langmuir
Dense Two-Dimensional Silver Single and Double Nanoparticle Arrays with Plasmonic Response in Wide Spectral Range Krystyna Drozdowicz-Tomsia, Henrique T.M.C.M. Baltar, and Ewa M. Goldys* MQ Photonics, Macquarie University, North Ryde NSW 2109, Australia ABSTRACT: We report the properties of plasmons in dense planar arrays of silver single and double nanostructures with various geometries fabricated by electron beam lithography (EBL) as a function of their size and spacing. We demonstrate a strong plasmon coupling mechanism due to near-field dipolar interactions between adjacent nanostructures, which produces a major red shift of the localized surface plasmon resonance (LSPR) in silver nanoparticles and leads to strong maximum electric field enhancements in a broad spectral range. The extinction spectra and maximum electric field enhancements are theoretically modeled by using the finite element method. Our modeling revealed that strong averaged electric field enhancements of up to 60 in visible range and up to 40 in mid-infrared result from hybridization of multipolar resonances in such dense nanostructures; these are important for applications in surface enhanced spectroscopies.
I
asymptotic dependence of the LSPR position as a function of the spacing parameter (distance/diameter) has also been obtained.21 Such interparticle interaction is demonstrated by the fact that the relative plasmon wavelength shift (Δλ/λ) for polarization along the interparticle axis decays nearly exponentially with the interparticle gap. Moreover, Jain and El-Sayed21 have found that this interaction effectively decays at approximately 20% of particle size regardless of nanoparticle size, shape, metal type, or the dielectric constant of the medium. The near-exponential distance decay and universal scaling behavior of interparticle plasmon coupling were qualitatively explained on the basis of a dipolar-coupling model.23,34 They were attributed to the interplay of two factors: direct dependence of single-particle polarizability on the cubic power of the particle dimension and the decay of plasmonic near-field as the cubic power of the inverse distance. All previous studies of plasmon behavior of nanoparticle pairs in arrays was carried out for relatively large nanoparticles in 100− 300 nm size range34−39 and for large spacings between nanoparticle pairs where the interaction between nanoparticle pairs was negligible. The optical response of very dense arrays combined of relatively small nanoparticles (∼50 nm), where spacings within pairs and between them disappear or even become negative, is much less known. For example, it has recently been shown that a two-dimensional hexagonal closepacked array of gold nanoshells with nanoscale gaps between nanoparticles provides significant enhancements in both surface enhanced Raman spectroscopy (SERS) and surface enhanced
ntense efforts to understand the properties of nanoscale noble metals in the past few decades are driven by technological demand for new optical devices such as tunable filters,1 waveguides,2 photonic circuits,3 improved solar cells,4 as well as biological and chemical sensors,5−8 and substrates for surface-enhanced spectroscopies (SES).9−13 Their optical properties in visible spectral range are due to the localized surface plasmon resonance (LSPR). The LSPR is a collective resonant oscillation of the conduction electrons in a nanostructure in response to an incident optical field, which can be localized on a single nanoparticle or may involve many coupled nanoparticles. The electron oscillation frequency is highly dependent on the type of metal (typically gold, silver, and platinum)14,15 as well as size,16 shape,17 and the dielectric surroundings of a metal nanostructure,18,19 and it also strongly depends on nanoparticle arrangement and separation,20−23 which can precisely fine-tune the position of plasmon resonances and create strong electric field enhancements.24−26 Several groups investigated the optical properties of isolated and coupled gold and silver nanoparticles22−30 and their interactions in arrays produced by self-assembly,31 nanosphere lithography,32 and most recently by electron beam lithography (EBL).33−37 The first two methods are relatively simple and inexpensive, but they suffer from limited reproducibility as well as variability of the shape and arrangement of nanoparticles. Well-defined and separated periodic nanoparticle arrays produced by e-beam lithography were intensely studied by various authors.1,19,20,27,29,30,37 In particular, several groups have shown that plasmon position in such arrays of cylindrical metal nanoparticles exhibit a universal scaling behavior.21,22,28 In arrays of noninteracting particles, the plasmon shifts linearly to longer wavelengths as a function of the cylinder diameter (at constant height) or aspect ratio (diameter/height). For an isolated pair of interacting cylindrical nanoparticles, an © 2012 American Chemical Society
Special Issue: Colloidal Nanoplasmonics Received: January 18, 2012 Revised: March 22, 2012 Published: March 22, 2012 9071
dx.doi.org/10.1021/la300277m | Langmuir 2012, 28, 9071−9081
Langmuir
Article
Figure 1. Scanning electron micrographs of representative nanoparticle arrays produced by EBL at two different e-beam exposure doses. Structure A: ∼50 nm triangles (a), ∼50 nm circles (b), ∼50 nm double triangles (c), and ∼50 nm double circles (d). Structure B: ∼70 nm triangles (a), ∼70 nm circles (b), ∼70 nm double triangles (c), and ∼70 nm double circles (d). Nominal diameter of circles, 50 nm; nominal equilateral triangle side length, −50 nm; and nominal distances, 150 nm × 150 nm in 2D array for both single and double features with internal nominal spacing between double features 20 and 30 nm. Each individual shape pattern (consisting of single circles or single triangles, or double circles, or double triangles) was repeated to form 100 × 100 μm2 array area. Each four pattern structure was repeated at six different exposure doses to vary the size and spacing. (Images shown here are for two different exposure doses.)
range of structure designs, their modeling can be predictable and accurate owing to high reproducibility of EBL. This provides the understanding of the role of individual design parameters of the arrays, which is an essential step in designing and producing highly sophisticated plasmonic nanostructures with predictable, desirable optical properties. The dense arrays of double nanoparticles have been produced to display different type of interactions in following regimes: first, relatively well separated dimers; second, touching dimers and third conductively coupled dimers. The physically most interesting and the least studied regime, where the pairs of nanoparticles form a narrow, conducting link for interparticle plasmon coupling, has potentially useful consequences in terms of concentration of very large optical fields in the spatial region of the interconnection “neck”.23,30 We show that in this region the anomalous plasmon field enhancement effects occur at contacting surfaces between metal spheres (or cylinders), which produce strong electromagnetic field enhancements, up to 60 (when averaged, and up to 120 at single point) and they are an order of magnitude higher than that in the narrow gap between isolated nanoparticle pairs of the same size. The “hot spots” areas thus formed can be tuned by structure design for SES in a broad spectral range from ultraviolet to the infrared.
infrared absorption (SEIRA) and the effect of field enhancement is seen in a broad spectral range extending up to 10 μm.40 Intense efforts were also applied to theoretically simulate the behavior of plasmonic nanostructures. Computational models capable of simulating nanostructure geometries include the discrete source method (DSM)41 and a boundary element method (BEM).42,43 A widely used discrete dipole approximation (DDA)18,24,29 has some limitations and can only be applied to nanoparticles of specific shape, surrounded by a uniform dielectric environment. General, ab initio methods of numerically solving Maxwell’s equations for the actual, complex geometry on a discrete spatial grid or element functions such as the finite difference time-domain (FDTD)24 and finite element method (FEM) are becoming more commonly used,44 especially with increasing availability of commercial simulating packages such as COMSOL. In this work we investigate the behavior of LSPRs in silver nanoparticle array structures with small nanoparticles (30−80 nm size). The nanoparticles have circular and triangular cross sections and two different thicknesses, and they were arranged in two-dimensional dense arrays with 150 nm period. They were fabricated by EBL on an isolating quartz substrate to avoid interparticle interactions due to propagating surface modes. The structures were protected from environmental degradation by a thin aluminum oxide layer applied by atomic layer deposition (ALD). Here, we report the evolution of plasmon resonance as a function of shape, size, and interparticle coupling based on measurements of optical transmission with unpolarized and polarized light. The behavior of the experimentally observed LSPR is compared to theoretical calculations carried out by using the FEM method and maximum electric field enhancement is derived from the simulation results. In contrast to other works, the simulations were carried out for our actual structure geometries, including the details of their nonuniform dielectric environment, by the same method as used in our previous work.45 We show that, once the details of actual structure geometry are established and consistently applied to a
■
EXPERIMENTAL METHODS
The samples were fabricated using a JEOL JBX-9600 FS (JEOL, Japan) 100 kV electron beam lithography (EBL) system in the Cornell Nanofabrication Facility. A regular two-dimensional array pattern with a fixed period of 150 nm in both directions and varying feature size was produced in silver on quartz by a lift-off technique by using different exposures. The following procedure was employed. Two layers of positive photoresist: 2% poly(methyl methacrylate) (PMMA) in Anisole followed by 1% of PMMA in methyl isobutyl ketone (MBIK) were spin-coated on a silica substrate to a total thickness of 90 nm, and each was baked at 170 °C for 15 min. Further, 10 nm of gold was thermally evaporated to provide a conductive, electron transparent layer to avoid charging during the EBL exposure. The EBL was carried out at six different doses varying the beam current in equal steps from 9072
dx.doi.org/10.1021/la300277m | Langmuir 2012, 28, 9071−9081
Langmuir
Article
Figure 2. Position of plasmon resonances from experimentally measured absorption spectra. (A) Single circles from sample set 1 (∼20 nm height) and sample set 2 (∼40 nm height) as a function of nanoparticle diameter. Linear characteristics for 20 and 40 nm heights plotted in the graph (A) are for isolated silver cylindrical nanoparticles in uniform dielectric medium extracted from ref 27, for 20 nm particles and calculated for 40 nm for comparison. (B) single circles and single triangles from sample set 1 as a function of aspect ratio (diameter for circles and side length for triangles divided by height) and linear fits to experimental data. 1200 to 2890 μC/cm2. As the electron beam has a Gaussian shape, such increasing beam intensity produces nanostructures with varying feature size at the same grid spacing. After the removal of the Au layer in Transene gold etchant (solution of KI and I2 in DI water), the samples were developed in a solution of 1:3 water/isopropyl alcohol cooled to 4 °C with ultrasonication followed by isopropanol rinse and drying with a nitrogen gun. After visual inspection under an optical microscope, the samples were transferred to an Oxford RIE-80 (Oxford Instruments, U.K.) plasma system for 5 s descum in argon to remove any excess residue of unwanted photoresist. Then a ∼20 nm (sample 1) or a ∼40 nm (sample 2) silver layer was evaporated using 99.999% pure Ag in a CVC SC4500 combination thermal/e-beam evaporator at a rate of 0.5 Ǻ /s. The final lift-off and removal of the PMMA positive resist was carried out by soaking the samples at room temperature in methyl chloride, followed by acetone and isopropanol rinse. The samples were dried with a nitrogen gun. To prevent degradation of the Ag layer, the surface of the structures was protected by depositing 15 nm of Al2O3 by atomic layer epitaxy in an ALD Oxford FlexAl (Oxford Instruments, U.K.) system directly after the lift-off process at a rate 1 Ǻ /min. The combination of parameters used such as the thickness and type of resist, exposure dose, development conditions as well as time and the descum process all affect the final pattern size and edge profile shape. It is expected that, due to the same processing sequence, all nanoparticles have the same edge rounding at both top and bottom interfaces. Two types of arrays were prepared for this study. The first type contained regions of isolated cylindrical particles and equilateral triangles of varying size in the 30−80 nm range. The second pattern was produced the same way as the first one; however, the twodimensional array was asymmetric. It contained double circles and double triangles aligned along one axis, the latter with their tips pointing toward each other. Their nominal feature size was 50 nm with the spacing in between double feature matrix of 20 and 30 nm between pairs (see Figure 1). The period of all four patterns, single circles, single triangles, double circles, and double triangles, was kept constant at 150 nm in both directions. Increase in size of the pattern feature was accompanied by the reduction of spacing between features. The ebeam exposure dose was set to produce dense arrays of double nanoparticles which are initially separated, and start to form nearly touching pairs, followed by conductively coupled nanoparticles as the nanoparticle size is increased. The size of each single type of pattern within array was 100 μm × 100 μm. The scanning electron microscopy (SEM) images of the structures were obtained using a Zeiss Ultra60 SEM instrument (Carls Zeiss, AMT AG, Germany) without any sample preparation, and the height of the nanostructures was determined by atomic force microscopy
(AFM) measurements using a Digital Instruments Nanoscope III microscope (Digital Instruments, Santa Barbara, CA). The optical spectra were obtained by using the SEE2100 microspectrometer in transmission mode, across the spectral range of 360−900 nm using illumination from a mercury lamp. The spectra were collected via a 20× NA 0.40 objective from several small regions ∼8.0 × 8.0 μm2 close to center of the structure containing only a single type of pattern regions to produce averaged spectra reported here. The unpolarized and polarized illumination was used to study LSPR shifts as a result of changes in size and nanoparticle spacings. A linear polarizing glass filter from Newport was placed in the illumination path and carefully aligned with the axis of the nanostructures or rotated 90° to that axis to provide differently polarized illumination. The polarization parallel and perpendicular to the axis joining the doublet features is referred to as longitudinal polarization (LP) and transverse polarization (TP), respectively. The experimental data were compared with simulations carried out by using the three-dimensional, commercial FEM modeling package COMSOL Multiphysics 3.5a with the RF module. The Maxwell equations were numerically solved by using tetragonal mesh elements and periodic boundary conditions. The 3D nanostructure unit of 150 nm × 150 nm cross section containing quartz substrate, silver nanocylindrical particles, and Al2O3 capping layer was illuminated from the top by the orthogonally polarized plane wave and the power out was integrated at the bottom boundary at the wavelength range of 350−1100 nm in 10 nm steps. To eliminate interference effects, the transmitted power was corrected by calculating the power out from the same structure but without silver nanoparticles. The same methodology was applied to experimentally acquired data.
■
RESULTS AND DISCUSSION SEM Imaging. Figure 1 presents an overview of SEM images of Ag nanostructures with different shapes and arrangements fabricated in this study. As the substrate used is insulating, triangular shapes observed at high magnification appear to show rounded corners due to intense charging (Figure 1 A, B patterns a and c). The single pattern nanostructures for all exposure doses were separated; however, the doublet features in some patterns especially for distances of less than 10 nm have either partially or completely coalesced (Figure 1 c, d) and could not be resolved as separate doublets. Some of the patterns showed a mixture of circular and ellipsoidal nanoparticles with 1:2 aspect ratio. At increasing electron dose, they eventually merged forming quasi-continuous grating lines. The circular shapes produced at slightly lower dose preserved their geometry (Figure 1 b bottom right), but at
9073
dx.doi.org/10.1021/la300277m | Langmuir 2012, 28, 9071−9081
Langmuir
Article
Figure 3. FEM simulations of plasmons in a cylinder on a substrate with a 10 nm Al2O3 capping layer, diameter D = 60 nm, height h = 40 nm as a function of (a−f) cylinder shape, (g) cylinder radius, (h) silver layer (height of cylinder), and Al2O3 thicknesses. (a) Absorbance spectra for structures with top rounded, bottom rounded, and both edges rounded, curvature radius R = 10 nm. (b−f) Side view of EM field distributions at both absorbance peaks for these structures; (b) both edges rounded at the main peak (at 488 nm); (c) bottom rounded structure, electric field at peak (at 508 nm), (d) top rounded structure, electric field at peak (at 532 nm); (e) bottom rounded structure, electric field at peak (at 479 nm); (f) top rounded structure, electric field at peak (at 444 nm); (g) absorbance spectra for both edges rounded, different radii of curvature R = 0, 2, 8 nm as shown. Inset: evolution of peak blue-shift with varying radius of curvature. (h) Evolution of main peak for nanoparticles with 60 nm diameter, 10 nm radius of cylinder curvature and varying thickness of silver: 40, 30, and 20 nm with the same 10 nm thickness of Al2O3 capping layer (red curves), and the same silver thickness of 40 nm and varying thickness of Al2O3 capping layer of 12 and 15 nm (green curves).
distance/diameter ∼1), the interparticle coupling effects and a coupling-induced red shift were not observed. Theoretical Modeling of Plasmon Position for Arrays of Isolated Nanoparticles. To confirm that the position of plasmon resonance for the isolated nanoparticles in the array is related to the structure geometry and that the coupling between these nanoparticles is weak, we conducted a series of calculations by FEM using COMSOL software. By carrying out the series of simulations of single cylindrical Ag nanoparticles in 150 nm × 150 nm array first in free space, then on quartz substrate, then with an additional Al2O3 capping layer, we have identified the role of individual elements of nanostructure geometry and the effect they have on the absorbance spectra.45 The total red shift due to presence of quartz substrate and 15 nm thick Al2O3 capping layer was ∼120 nm (for a cylinder diameter of 60 and 40 nm height, data not shown) due to the high refractive indices of these materials. Such a red-shift is in agreement with earlier reports, which highlighted a major influence of a dielectric substrate46 and the Al2O3 layer.37 The simulations also showed that the radius of curvature of the cylinder edges appears to be one of the key parameters as it eliminates the two peak character of the simulated spectra, which brings them in agreement with experiment (see Figure 3a). The sharp edges at the substrate/silver cylinder interface also produce dramatic changes of absorbance spectra (Figure 3a, red curve), and two plasmon resonances at 90 nm spacing are observed (Figure 3d, f), as compared with 30 nm separation, when sharp edges are introduced on top of the cylindrical silver nanoparticle
high doses they resemble quasi-continuous gratings. The silver particle heights were 20 nm ± 2 nm (sample set 1) and 40 nm ± 2 nm (sample set 2) as measured by AFM. Plasmons in Single Nanoparticle Arrays for Unpolarized Light. Figure 2 shows the peak position of the unpolarized extinction spectra for single cylindrical and triangular particles placed in 150 nm × 150 nm arrays of various sizes ranging from ∼30 nm up to ∼80 nm. The spectra taken at TP and LP illumination were very similar, as expected. The spectral shape of the extinction spectrum (not shown here) can be well fitted by the Lorentzian curves with a narrow (48−52 nm) full width half-maximum (fwhm) indicating uniform size and distribution of Ag nanoparticles. The dipolar resonance peak shifts linearly toward longer wavelengths with increase of nanoparticle size, as plotted in Figure 2A, or the aspect ratio (diameter/height), as plotted in Figure 2B, for the nanoparticles of ∼20 nm height. The linear trend lines for our samples of the same height in both cases of single circles and single triangles have a similar slope, indicating that triangular particles with rounded edges and small size behave similarly as circular particles but with a smaller volume. In these structures, the measured plasmon peak positions were in good agreement with the LSPR positions for isolated cylindrical silver nanoparticles of 20 nm height on the substrate calculated in ref 27, where the authors used a similar average refractive index of the medium as in our work (see lines in Figure 2A for 20 and 40 nm cylinder thicknesses). Therefore, in these small particles at relatively large distances from one another (minimum 9074
dx.doi.org/10.1021/la300277m | Langmuir 2012, 28, 9071−9081
Langmuir
Article
attributed to SEM measurements accuracy and fabrication errors. We also performed FEM calculations for the same sequence of nanostructures as for the single nanoparticles in array but for changed boundary conditions on the sides from periodic to continuous. In such case the simulated plasmon position corresponds to noninteracting, isolated nanoparticles. Only for the largest 80 nm individual nanoparticles and the same nanoparticles in 150 × 150 nm array, ∼10 nm small red shift resulted from calculations (data not presented here), confirming that the effect of coupling in such arrays of single nanoparticles is weak for all the nanoparticles considered here. Such results are in agreement with previous works, which showed that additional red shift due to nanoparticle coupling is not observed when distance/diameter parameter is larger than 1.05.21,22 It applies also here as for all our individual particles in array, this parameter is larger than 1.8. Plasmons in Double Nanoparticles in Array Observed Experimentally in Polarized Light and Modeled Theoretically. We now describe the experimental results obtained for nanoparticle doublets in dense arrays of 150 nm × 150 nm period, which were characterized by optical extinction at various polarizations of incident light. In Figure 5, we show the extinction spectra for double circles and at selected sizes only. The experimental data are accompanied by the modeled spectra for silver nanostructures on substrate and caped by dielectric layer, which correspond to the following selected scenarios: nanoparticles separated at two different spacings (arrays formed by nanoparticles with 50 or 60 nm diameters), a combination of touching and separated nanoparticles (at 70 nm diameter), and conductively coupled nanoparticles of “dumbbell” shape (at 76 nm diameter) at small separations (4 nm). We first discuss the experimental measurements. The ∼50 nm double cylinders were clearly separated and according to array design (confirmed by SEM measurements) had two nonequal spacings in longitudinal direction of ∼20 nm and ∼30 nm, and ∼100 nm in transverse direction. Their extinction spectra are presented in Figure 5A together with theoretical simulations (red lines). In these structures, a single broad peak ∼470 nm is observed for unpolarized light at the same position as in single cylinders (red dotted lines). In TP, a small blue-shift of 25 nm and in LP a small red-shift of ∼30 nm were observed due to stronger coupling in longitudinal direction. These shifts were comparable to those reported in refs 25−27. As the nanoparticle diameter increased to ∼60 nm, the spacings in longitudinal directions were reduced to nominally ∼10 and ∼20 nm. Most such pairs were still separated with joined pairs observed in SEM only occasionally. For such close particle arrays, the unpolarized spectra presented a double peak (not shown here) and polarized spectra are shown in Figure 5B. The blue-shift under TP illumination was small, about 15 nm as compared the same size single particles in an array. In contrast, the shift in the LP direction was significant, about 115 nm. For even larger diameters of 70 nm (hence nominal spacings of 0 nm within pairs and 10 nm between pairs), large shifts of plasmon position were observed for both polarizations (see Figure 5C) compared with single particle arrays, of ∼50 and 180 nm in the TP and LP direction, respectively. With further increase of particle diameter to 76 nm (Figure 5D) in real fabricated nanostructures, the nanoparticles almost merged to form corrugated gratings. In such structures, the plasmon peak for the TP remained as in single particle arrays, but for the LP a very significant red-shift of ∼150 nm was observed for the dipolar
(Figure 3a, blue curve; Figure 3c, e). The presence of the substrate generates asymmetry in the electric field distribution within the silver nanoparticles, which is further enhanced by presence of sharp edges at that interface. Nelayah et al.47 have shown, using low-loss energy-filtering transmission electron microscopy imaging to map plasmon resonances on triangular silver nanoprism, that the corner, edge, and volume modes can be excited when the electron beam is focused in these areas. In our case, for cylindrical nanoparticles with the sharp edges, apart from the quadrupolar and dipolar volume modes, also the edge modes are observed in simulated results at shorter wavelength then the dipolar volume mode, and the separation between these modes is increased when the sharp edges are present at the interface with high refractive index substrate. Rounding must be introduced at both interfaces, metal/ substrate and metal/capping layer, in order to simulate experimentally observed single peak spectra (Figure 3a, black curve; Figure 3b). This introduces an additional effect of a blueshift, which increases with increasing radius of curvature (Figure 3g), but for the same radius such a blue-shift is independent of cylinder size.45 Small variations in the sample preparation procedure such as an increasing the Al2O3 layer thickness from 12 to 15 nm can lead to noticeable differences of the observed plasmon resonances (Figure 3g). We have also attempted to vary other parameters such as cylinder height in search for the best fit to the experimental data for cylinder diameter of 60 nm, and the results of these fittings are shown in Figure 3h. This fitting to the experimental data were carried out within the limits of validity of the SEM dimension measurements of ±5 nm, which are the result of strong charging. Finally, we have been able to achieve good agreement between modeled and experimental data for our entire sequence of single cylinder arrays by applying 10 nm rounding of the edges on both sides and assuming silver layer thickness of 40 nm (Figure 4) which is in good agreement with results
Figure 4. Calculated and experimentally measured absorbance spectra for single nanoparticles in a 150 nm × 150 nm array. Experimentally measured (black lines) and calculated extinction spectra for the series of nanoparticles with thickness of 40 nm, 15 nm of Al2O3, and varying diameter as described in the figure.
from AFM measurements. Such edge rounding is consistent with previous reports of oblate spheroid shape for cylindrical particles produced by EBL.25,27 The best fit with experimental data was achieved for nanoparticles with diameters of ∼65 nm and thickness of ∼40 nm, which is in good agreement with parameters used for simulation and also supports our estimate of SEM measurement accuracy of ±5 nm. Any residual small departures of LSPR peak positions and spectral shapes are 9075
dx.doi.org/10.1021/la300277m | Langmuir 2012, 28, 9071−9081
Langmuir
Article
Figure 5. Experimentaly measured and theoretically calculated spectra for double cylindrical nanoparticles in dense arrays with 40 nm Ag thickness and capped by 15 nm of Al2O3 produced in 150 nm × 150 nm array by EBL with nanoparticle diameters (A) 50 nm, (B) 60 nm, (C) 70 nm, and (D) 76 nm in top and bottom panels. Solid black curves, experimentally measured spectra in TP; black dashed lines, experimentally measured spectra in LP; red dashed lines, theoretically calculated spectra at LP; red solid lines, theoretically calculated spectra at TP; dotted red spectra, theoretically calculated respective single particle spectra in 150 nm × 150 nm array. Top panel also shows electric field distributions for 68 nm double nanoparticles in dense arrays in: XZ plane for quadrupular (at 405 nm) and dipolar (at 500 nm) modes (e, f), and in the XY plane for LP and TP polarizations at dipolar mode (g, f).
resonance, but also a second strong peak due to hybridization of quadrupolar resonances was observed at ∼450 nm. Our earlier simulations for a range of single particles in the 150 nm × x 150 nm array were able to reproduce experimental results using 40 nm thickness of silver layer, 10 nm radius of cylinder curvature, and 15 nm of the alumina layer thickness with good accuracy. We now applied the same set of parameters and varied the particle diameters and spacings in order to model different regimes of nanoparticle coupling. The results of these simulations are presented in Figure 5 together with experimental results for corresponding nanostructures. Both calculated and experimental data show the main peak with a small shoulder for both TP and LP polarizations corresponding to dipolar and quadrupolar resonances. In Figure 5e−h (for 50 nm double particles), we also presented simulated electric field distributions (top row Figure 5e, f) which clearly show that the small shoulder around 410 nm and the main peak at around 570 nm in the extinction spectrum are related to the quadrupolar and dipolar resonances, respectively. Depending on polarization, the normalized electric field distribution is concentrated in the gap between the cylindrical nanoparticles (for LP, Figure 5g) or extends perpendicularly to the axis joining nanoparticles (for TP, Figure 5h). The calculated spectra for 50 nm double cylinders agree very well with the experimental data. For larger nanoparticles 60, 70, and 76 nm (Figure 5B−D), only the position of the resonances is in good agreement but otherwise the calculated spectra are slightly more intense and narrower, especially for the LP. The differences between the strong resonances obtained theoretically and weaker features observed in the experimental array spectra are attributed to inhomogeneities in the experimental
sample which arise from the dispersion in the nanoparticle size and the resultant variations in the interparticle spacings as well as the EBL resolution limits, which are insufficient to produce features with gaps in 0−10 nm range. Such defects in the experimentally fabricated nanoparticle arrays can localize the plasmon modes and introduce inhomogeneous broadening. Any deviation from perfect periodicity will also break the coherence of the collective dipolar plasmon modes and can lead to destruction of the superradiance. According to theoretical calculations, when the nanoparticles come in close contact (70 nm, in our case), the plasmon resonance is strongly shifted to the infrared, and a second resonance appears at shorter wavelengths. The latter is attributed to the development of a higher-order plasmon mode with a complex spatial distribution of local electric field in the confined interparticle region. In the regime where nanoparticles are conductively coupled and form a “dumbbell” shape, the optical spectrum is dominated by the long wavelength resonance associated with the dipolar nature of the electronic excitation across the pair as a whole with a small band at shorter wavelengths related to a quadrupolar plasmon mode. The quadrupolar resonance for small, silver nanoparticles produced in our study is much weaker than that for nanoparticles with diameters in 100−200 nm range, presented by other authors.28,30 When nanoparticles start to touch or become conductively coupled, such a higher quadrupolar− quadrupolar hybridized mode becomes relatively strong and can be clearly observed in experimental data even for these small nanoparticles, especially when both experimental and simulated spectra are normalized (data not shown here). Despite small spacings between nanoparticles frequently causing their coalescence into grating-type structures rather 9076
dx.doi.org/10.1021/la300277m | Langmuir 2012, 28, 9071−9081
Langmuir
Article
than 2D arrays of nanoparticles, they still behave similarly as pairs of nanoparticles in the strong coupling regime, and they demonstrate strong, shifting plasmon resonances for both polarization directions. We note here that, in genuine gratings, only the TP polarization leads to the excitation of LSPRs, and for the polarization along the grating wire no plasmon excitation is observed;48 this is in contrary to our results. Finally, we note that the strong radiation damping effect observed in gratings can explain slightly lower intensities of plasmon resonances at LP observed in experimental data, when the nanoparticle pairs approach coalescence. Calculated Electric Near-Field Enhancement in Arrays of Double Nanoparticles. The simulated and experimental extinction spectra for complex arrays of double nanoparticles presented in the previous section give insights into interaction of light with these nanostructures observed in far field. For many applications such as SES, near-field effects resulting in localized strong electric field enhancements play a dominant role, as they are responsible for fluorescence or Raman signal enhancements. Strong coupling observed in dimers results in increased field enhancements in the gap between nanoparticles,23,24,26,41 or on the sides of the cylinders forming a junction for conductively coupled cylindrical nanoparticles.23 Several authors23−26,41 presented results of calculated electric field enhancements for coupled spherical nanoparticles at a single point in the middle of the gap between close nanoparticles. Hao and Schatz26 estimated based on DDA calculations that the maximum electric field enhancement around isolated, spherical Ag nanoparticle with 20 nm diameter was about 11 (value (125)1/2, in their work), but for such pair of nanoparticles, the maximum of electric field in the middle of 2 nm gap was around 100. Sundaramurthy et al.49 studied field enhancement and gap-dependent resonance in a system of two opposing tip-to-tip Au nanotriangles and found a good correlation between measured and experimental data with the maximum enhancements up to ∼40 times in the middle of 16 nm gap. As our structures are more complex and consist of cylindrical nanoparticles, where the areas of strong interaction are larger and presence of the substrate introduces asymmetry in the electric field distribution, we applied a different approach, and calculated maximum enhancements using averaging of electric field from larger areas. Figure 6 shows the maximum electric field enhancement in the arrays of double nanoparticles calculated in the following way. The normalized electric field was integrated in the surrounding metal nanoparticle regions as a function of wavelength. First, the normalized electric field was integrated in the Al2O3 subdomain surrounding silver nanoparticles and divided by the volume of that subdomain Then, using edge integration in COMSOL software we calculated the value of the (normalized) electric field at the internal and external edges where the particles are in close contact (maximum enhancement regions) and divided by the length of these edges to obtain averaged values for these regions. For conductively coupled nanoparticles, such calculations were additionally carried out for the external edge where the particles are joined together. The maximum value of the normalized electric field (local electric field/incident electric field) thus obtained corresponds to the maximum electric field enhancement. The obtained spectra of maximum electric field enhancements are very different than the extinction spectra observed in far field, as they are dominated by the electric fields resulting from
Figure 6. Maximum normalized electric field enhancements calculated from the simulated data using FEM for various sizes (indicated in the figure) of the double silver nanocylindrical particles in arrays as described in Experimental Methods. The 50, 60, and 68 nm double particles in array are modeled for structures on quartz substrate and with Al2O3 capping layer and a conductively coupled 76 nm double structure was modeled on quartz substrate both with and without the Al2O3 caping layer for comparison. Arrows (A−D) indicate peaks at: (A) 480 nm, (B) 500 nm, (C) 550 nm, and (D) 700 nm for various electric field distributions shown in detail in Figure 7.
strong coupling due to hybridized higher order modes, which produce these maximum enhancements. The maximum electric field enhancements were calculated for arrays of: 50, 60, and 68 nm double particles for structures on quartz substrate and with Al2O3 capping layer. An array of 76 nm double particles was calculated with and without the Al2O3 capping layer for comparison. The results for 50 and 60 nm double particles, which are still well separated, show a fairly low value of maximum electric field enhancement (below 10) comparable to that observed by Hao and Schatz,26 as enhancement is generated by weakly interacting single nanoparticles. With increasing particle size (to 68 nm), we observe a very strong electric field enhancement (see Figure 6 brown line) related to strong coupling between the nanoparticles in small (2 nm) gaps between them. The peak enhancement occurs at 590 nm, and the enhancement value is approximately 60. Several smaller peaks (A, B, C, D) can be seen at ∼480, ∼500, ∼590, and ∼700 nm, and the corresponding electric field distributions have been shown in Figure 7. It indicates that calculated maximum electric field enhancements are more sensitive than the extinction spectra to reveal coupling of higher order modes of individual nanoparticles. When the nanoparticles are no longer separated and form electric contacts with one another (“dumbbell” shape, at 76 nm diameters), the spectrum of maximum electric field enhancement is changed drastically. The highest value of maximum enhancement is observed along the edge where the particles are joined together and it is higher than that in the gaps between dumbbell nanoparticles (nominally 4 nm). Their electric field enhancement spectrum (Figure 6, dark blue line) shows two distinctive peaks at ∼480 and ∼670 nm with maximum enhancement value also as high as 60 (as averaged for the length of the junction edge, but with maximum calculated value at a single point as high as 140, see Figure 8C) and the corresponding electric field distributions at these wavelengths are shown in Figure 8. The minimum at ∼570 nm is related to 9077
dx.doi.org/10.1021/la300277m | Langmuir 2012, 28, 9071−9081
Langmuir
Article
Figure 7. Normalized electric field distribution in Z direction for the XZ cross section plane calculated for 68 nm double nanoparticles with an Al2O3 capping layer illuminated by plane wave polarized in X direction at various wavelengths of incident illumination corresponding to points indicated in Figure 6: (A) at 480 nm, showing coupling of octupolar mode, (B) 500 nm coupling of haxapolar mode, (C) 590 nm coupling of quadrupolar modes, (D) 700 nm, other hybridized mode.
Figure 8. Electric field distribution in the plane perpendicular to the axis joining 76 nm double nanoparticle in the middle of their conductive junction cross-section at following wavelengths: (A) 480 nm, (B) 570 nm, and (C) 670 nm corresponding to maxima/minima of spectrum in Figure 6 (dark blue line).
a dark mode.50,51 The dark mode appears when the value of electric field along the edge joining two nanoparticles is at its minimum. It takes place when two dipolar modes on adjoining nanoparticles are in a bonding configuration and in such configuration the oscillation for individual dipoles is nonradiative.52 The other two wavelengths corresponds to different hybridized modes, as it is observed by different large electric field charges, positive at 480 nm and negative at 670 nm (see Figure 8A and C). We assign them to electric field distributions in hybridized quadrupolar−quadrupolar mode at 480 nm and quadrupolar−dipolar mode at 670 nm as the dipolar mode for these nanoparticles is at ∼980 nm as seen from the extinction spectra (Figure 5D). It is interesting to observe that the 76 nm double nanostructure with and without the Al2O3 layer shows similar spectral shapes with about 200 nm spectral shift to the red for the sample with Al2O3 coating. The uncapped sample shows the onset of a broad featureless enhancement starting from 700
nm and extending into the infrared. The enhancement values in the infrared region are slightly lower than these observed in the visible region (about 30% lower). In the capped sample, the onset of this broad band appears at ∼1000 nm and is also expected to reach comparable maximum values of about 40. This broad band is attributed to “lightening rod” effect.53,54 Lightning rod effects are general properties of subwavelength metal gaps or asperities not associated with plasmons.41 In the case of two adjacent metal nanostructures, the incident electromagnetic field does not appreciably penetrate inside the metal, and therefore it is compressed or “squeezed” into the gap between the two nanostructures. This general field focusing occurs for all metals in the IR and terahertz regions, where metals are essentially perfect conductors. Lightning rod effects are purely geometrical and are essentially frequency independent for wavelengths much larger than the size of the nanoparticles. 9078
dx.doi.org/10.1021/la300277m | Langmuir 2012, 28, 9071−9081
Langmuir
Article
The electromagnetic “hot spots” arising from adjacent metal nanoparticle pairs result directly from the interaction of the plasmons of the two individual nanostructures to form a new “dimer” hybridized plasmon, in addition to the lightning rod effects. These two effects, lightning rod and hybridized plasmons, are responsible for the presence of hot spots, which lead to strong electric field enhancements. In the case of our dumbbell nanoparticles (76 nm cylinders, conductively joined), the calculated strong electric field enhancements in the mid-infrared region are observed in the gaps between two adjacent dumbbell nanoparticles. They originate from the lightening rod effects. Our findings that large electric field enhancements in metallic nanostructures at long wavelengths in nanoparticle arrays apply to finite aggregates of arbitrary shaped nanoparticles as long as the wavelength of the incident light is in the near to midinfrared are in agreement with those presented by Le and coworkers.40 It requires dense arrays, but results in an extremely versatile system for SES, which provides high field enhancements and can be achieved for different designs of the array of the unit cell and shape of the nanoparticles providing that the spacings/diameter ratio is below 0.02.41 The very significant shifts of plasmon resonances observed in polarized light for small variations of size/distance parameter in our dense asymmetric arrays indicate that strong Coulombic interactions take place between plasmon oscillations in these close nanoparticles. Such nanostructures can be used for very sensitive diagnostic and calibration in EBL where absolute size could be detected from the position of plasmon peak and resolution from the position and intensity of the LP resonance peak. The EBL asymmetric nanostructures also offer an additional level of flexibility for applications in fluorescence enhancement or sensing. In particular, it is significant that through a combination of interparticle coupling and the presence of substrate and capping layer, the LSPR in silver can be shifted to the NIR region covering a wide spectral range with accompanying strong electric field enhancements.
modes. The electric field, extinction spectra, and maximum near-field electric field enhancements were theoretically modeled by using the finite element method (FEM). We have shown in detail the process which has to be applied to find exact details of complex, multilayer nanostructure geometry, based on correlation of experimental, optical, and structural measurements and tedious fitting of simulation results to achieve good match with the structures of such geometry. It requires first to achieve good agreement for simpler, multilayer, single nanoparticle array sequence of various sizes, which then can be consistently applied to more complex dense double particle arrays produced in the same technological sequence. It is a first step in showing future possibilities of producing plasmonic nanostructures with accurately designed properties. As a result, a good agreement was found with respect to the predicted extinction peak wavelength trends and measured values. The FEM calculation, with its assumption of a perfectly periodic array, overestimates the radiative damping actually observed in the experimental samples. The calculations of maximum near-field electric field enhancements demonstrated that, for dense nanostructures, the presence of “hot spots” and lightening rod effects for arrays of conductively coupled nanoparticles are responsible for the presence of strong optical response in both visible and in mid-infrared due to dipolar and hybridization of higher order modes. Our work indicates that small silver nanoparticles with sharp plasmon resonances in the 450−520 nm range (in isolated arrays of the nanoparticles), when placed in well-designed dense arrays, will lead to the formation of very strong electric field enhancements in broad VIS and MIR spectral regions, which is attractive for surface enhanced spectroscopies. The expected maximum enhancements for these nanostructures are in the order of 2 × 105, which correspond to observed in simulations 140 times, at a single point, electric field enhancements.
■
■
AUTHOR INFORMATION
Corresponding Author
CONCLUSIONS The present paper illustrates a complex behavior of the planar arrays of noble metal nanostructures as a function of size and spacing and their theoretical modeling. It provides experimental evidence for a strong plasmon coupling mechanism in a dense two-dimensional nanoparticle arrays fabricated by EBL in which near-field dipolar interactions between adjacent particles at the smallest spacing tend to dominate. In addition to confirming the existence of such interactions, we quantified their influence on both the spectral position of the collective dipolar extinction peak and other plasmon characteristics. In particular, in dense doublet structures, we still observed that the spectral position of the extinction peak for far-field excitation shows a blue-shift for TP polarization perpendicular to the doublet axis and a redshift for the LP polarization, which can be understood by strongest Coulombic force interactions between the electrons in adjacent, closest particles. We measured and analyzed the spectral response of nanoparticle pairs in three different separation regimes: separated, touching, and conductively coupled nanoparticles placed in dense arrays. Despite the formation of almost coalescent gratings, the nanoparticle pairs still showed similar behavior characteristic to specific separation regimes as for previously studied isolated nanoparticle pairs. It indicates that electric fields created in such nanostructures are very strong, and result in the hybridization of their optical
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We acknowledge support of the ARC Grant DP0770902. This work was performed in part at the Cornell NanoScale Facility (CNF), a member of the National Nanotechnology Infrastructure Network, which is supported by the National Science Foundation (Grant ECS-0335765). We thank John Treichler, CNF EBL engineer, for support in fabrication of the nanostructures presented in this work.
■
REFERENCES
(1) Lu, H.; Liu, X.; Mao, D.; Wang, L.; Gong, Y. Tunable band-pass plasmonic waveguide filters with nanodisk resonators. Opt. Express 2010, 18, 17922−17927. (2) Maier, S.; Brongersma, M. L; Kik, P. G.; Meltzer, S.; Requicha, A. G.; Atwater, H. A. A Route to Nanoscale Optical Devices. Adv. Mater. 2001, 13, 1501−1505. (3) Ozbay, E. Plasmonics: Merging Photonics and Electronics at Nanoscale Dimensions. Science 2006, 311, 189−193. (4) Pillai, P.; Catchpole, K. R.; Trupke, T.; Green, M. A. Surface plasmon enhanced silicon solar cells. J. Appl. Phys. 2007, 10, 0931051−8.
9079
dx.doi.org/10.1021/la300277m | Langmuir 2012, 28, 9071−9081
Langmuir
Article
(5) Storhoff, J. J.; Elghanian, R.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L. One-Pot Colorimetric Differentiation of Polynucleotides with Single Base Imperfections Using Gold Nanoparticle Probes. J. Am. Chem. Soc. 1998, 120, 1959−1964. (6) Haes, A. J.; Van Duyne, R. P. Nanoscale Optical Biosensor: Sensitivity and Selectivity of an Approach Based on the Localized Surface Plasmon Resonance Spectroscopy of Triangular Silver Nanoparticles. J. Am. Chem. Soc. 2002, 35, 10596−1064. (7) Stewart, M. E.; Anderton, C. R.; Thompson, L. B.; Maria, J.; Gray, S. K.; Rogers, J. K.; Nuzzo, R. G. Nanostructured Plasmonic Sensors. Chem. Rev. 2008, 108, 494−521. (8) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Biosensing with plasmonic nanosensors. Nat. Mater. 2008, 7, 442−453. (9) Yan, B.; Thubagere, A.; Premasiri, W. R.; Ziegler, L. D; Dal Negro, L.; Reinhard, B. M. Engineered SERS Substrates with Multiscale Signal Enhancement: Nanoparticle Cluster Arrays. ACS Nano 2009, 5, 1190−1202. (10) Theiss, J.; Pavaskar, P.; Echternach, P. M.; Muller, R. E.; Cronin, S. B. Plasmonic Nanoparticle Arrays with Nanometer Separation for High-Performance SERS Substrates. Nano Lett. 2010, 8, 2749−2754. (11) Beermann, J.; Novikov, S. M.; Leosson, K.; Bozhevolnyi, S. I. Surface enhanced Raman microscopy with metal nanoparticle arrays. J. Opt. A: Pure Appl. Opt. 2009, 11, 075004−1−5. (12) 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. Self-Assembled Metal Colloid Monolayers: An Approach to SERS Substrates. Science 1995, 267, 1629−1632. (13) Banholzer, M. J.; Millstone, J. E.; Qin, L.; Mirkin, C. A. Rationally designed nanostructures for surface-enhanced Raman spectroscopy. Chem. Soc. Rev. 2008, 37, 885−897. (14) Ordal, M. A.; Bell, R. J.; Alexander, R. W. Jr.; Long, L. L.; Querry, M. R. Optical properties of fourteen metals in the infrared and far infrared: Al, Co, Cu, Au, Fe, Pb, Mo, Ni, Pd, Pt, Ag, Ti, V, and W. Appl. Opt. 1985, 24, 4493−4499. (15) Mulvaney, P. Surface Plasmon Spectroscopy of Nanosized Metal Particles. Langmuir 1996, 12, 788−800. (16) Liebsch, A. Electronic excitations at metal surfaces; Plenum Press: New York, 1997; ISBN 0306455455. (17) Link, S.; El-Sayed, M. A. Spectral Properties and Relaxation Dynamics of Surface Plasmon Electronic Oscillations in Gold and Silver Nanodots and Nanorods. J. Phys. Chem. B 1999, 103, 8410− 8426. (18) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment. J. Phys. Chem. B 2003, 107, 668− 677. (19) Jain, P. K.; El-Sayed, M. A. Noble Metal Nanoparticle Pairs: Effect of Medium for Enhanced Nanosensing. Nano Lett. 2008, 12, 4347−4352. (20) Rechberger, W.; Hohenau, A.; Leitner, A.; Krenn, J. R.; Lamprecht, B.; Aussenegg, F. R. Optical properties of two interacting gold nanoparticles. Opt. Commun. 2003, 22, 137−141. (21) Jain, P. K.; El-Sayed, M. A. Universal Scaling of Plasmon Coupling in Metal Nanostructures: Extension from Particle Pairs to Nanoshells. Nano Lett. 2007, 7, 2854−2858. (22) Yang, L.; Wang, H.; Yan, B.; Reinhard, B. M. Calibration of Silver Plasmon Rulers in the 1−25 nm Separation Range: Experimental Indications of Distinct Plasmon Coupling Regimes. J. Phys. Chem. C 2010, 114, 4901−4908. (23) Garcia de Abajo, F. J. Nonlocal Effects in the Plasmons of Strongly Interacting Nanoparticles, Dimers, and Waveguides. J Chem. Phys. C 2008, 112, 17983−17987. (24) Futamata, M.; Maruyama, Y.; Ishikawa, M. Local Electric Field and Scattering Cross Section of Ag Nanoparticles under Surface Plasmon Resonance by Finite Difference Time Domain Method. J. Phys. Chem. B 2003, 107, 7607−7617.
(25) Yang, W. H.; Schatz, G. C.; Van Duyne, R. P. Discrete dipole approximation for calculating extinction and Raman intensities for small particles with arbitrary shapes. J. Chem. Phys. 1995, 103, 869− 875. (26) Hao, E.; Schatz, G. C. Electromagnetic fields around silver nanoparticles and dimers. J. Chem. Phys. 2004, 120, 357−366. (27) Gunnarsson, L.; Rindzevicius, T.; Prikulis, J.; Kasemo, B.; Kall, M.; Zou, S.; Schatz, G. C. Confined Plasmons in Nanofabricated Single Silver Particle Pairs: Experimental Observations of Strong Interparticle Interactions. J. Phys. Chem. B 2005, 109, 1079−1087. (28) Olk, P.; Renger, J.; Wenzel, M., T.; Eng, L. M. Distance Dependent Spectral Tuning of Two Coupled Metal Nanoparticles. Nano Lett. 2008, 8, 1174−1178. (29) Jain, P. K.; Huang, W.; El-Sayed, M. A. On the Universal Scaling Behavior of the Distance Decay of Plasmon Coupling in Metal Nanoparticle Pairs: A Plasmon Ruler Equation. Nano Lett. 2007, 7, 2080−2088. (30) Atay, T.; Song, J.-H.; Nurmikko, A. V. Strongly Interacting Plasmon Nanoparticle Pairs: From Dipole-Dipole Interaction to Conductively Coupled Regime. Nano Lett 2004, 4, 1627−1631. (31) Grabar, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Kenneth, R.; Brown, R.; Freeman, G.; Fox, A. P.; Keating, C. D.; Musick, M. D.; Natan, M. J. Two-Dimensional Arrays of Colloidal Gold Particles: A Flexible Approach to Macroscopic Metal Surfaces. Langmuir 1996, 12, 2353−2361. (32) Jensen, T. R.; Malinsky, M. D.; Haynes, C. L.; Van Duyne, R. P. Nanosphere Lithography: Tunable Localized Surface Plasmon Resonance Spectra of Silver Nanoparticles. J. Phys. Chem. B 2000, 104, 10549−10556. (33) Lamprecht, B.; Schider, G.; Lechner, R. T.; Krenn, J. R.; Aussenegg, F. R. Metal Nanoparticle Gratings: Influence of Dipolar Particle Interaction on the Plasmon Resonance. Phys. Rev. Lett. 2000, 84, 4721−4724. (34) Haynes, C. L.; McFarland, A. D.; Zhao, L. L.; Van Duyne, R. P.; Schatz, G. C.; Gunnarsson, L.; Prikulis, J.; Kasemo, B.; Käll, M. Nanoparticle Optics: The Importance of Radiative Dipole Coupling in Two-Dimensional Nanoparticle Arrays. J. Phys. Chem. B 2003, 107, 7337−7342. (35) Gotschy, W.; Vonmetz, K.; Leitner, A.; Aussenegg, F. R. Thin films by regular patterns of metal nanoparticles: tailoring the optical properties by nanodesign. Appl. Phys. B: Lasers Opt. 1996, 63, 381− 384. (36) Acimovic, S. S.; Kreuzer, M. P.; González, M. U.; Quidant, R. Plasmon near-field coupling in metal dimers as a step toward singlemolecule sensing. ACS Nano 2009, 3, 1231−1237.1. (37) Whitney, A. V.; Elam, J. W; Zou, S.; Zinovev, A. V.; Stair, P. C.; Schatz, G. C.; Van Duyne, R. P. Localized Surface Plasmon Resonance Nanosensor: A High-Resolution Distance-Dependence Study Using Atomic Layer Deposition. J. Phys. Chem. B 2005, 109, 20522−20528. (38) Auguié, B.; Barnes, W. L. Diffractive coupling in gold nanoparticle arrays and the effect of disorder. Opt. Lett. 2009, 34, 401−403. (39) Zhao, L. L.; Kelly, K. L.; Schatz, G. C. The Extinction Spectra of Silver Nanoparticle Arrays: Influence of Array Structure on Plasmon Resonance Wavelength and Width. J. Phys. Chem. B 2003, 107, 7343− 7350. (40) Le, F.; Brandl, D. W.; Urzhumov, Y. A.; Wang, H.; Kundu, J.; Halas, N. J.; Aizpurua, J.; Nordlander, P. Metallic Nanoparticle Arrays: A Common Substrate for Both Surface-Enhanced Raman Scattering and Surface-Enhanced Infrared Absorption. ACS Nano 2008, 2, 707− 718. (41) Eremina, E.; Eremin, Y.; Wriedt, T. Simulations of light scattering spectra of a nanoshell on plane interface based on the discrete sources method. Opt. Commun. 2006, 267, 524−529. (42) Myroschynchenko, V.; Rodriguez-Fernandez, J.; PastorizaSantos, I.; Funston, A. M.; Novo, C.; Mulvaney, P.; Liz-Marzan, L. M.; Garcia de Abajo, F. J. Modelling the optical response of gold nanoparticles. J. Chem. Soc. Rev. 2008, 37, 1792−1805. 9080
dx.doi.org/10.1021/la300277m | Langmuir 2012, 28, 9071−9081
Langmuir
Article
(43) Myroschynchenko, V. V.; Carbo-Argibay, E.; Pastoriza -Santos, I.; Perez-Juste, J.; Liz-Marzan, L. M.; Garcia de Abajo, F. J. Modeling the optical response of highly faceted metal nanoparticles with a fully 3D Boundary Element Method. Adv. Mater. 2008, 20, 4288−4293. (44) Zhao, J.; Pinchuk, A. O.; McMahon, J. M.; Li, S; Ausman, L. K.; Atkinson, A. L.; Schatz, G. C. Methods for Describing the Electromagnetic Properties of Silver and Gold Nanoparticles. Acc. Chem. Res. 2008, 4, 1710−1720. (45) Goldys, E. M.; Calander, N.; Drozdowicz-Tomsia, K. Extreme Sensitivity of the Optical Properties of Metal Nanostructures to Minor Variations in Geometry Is Due to Highly Localized Electromagnetic Field Modes. J. Phys. Chem. C 2011, 115, 676−682. (46) Knight, M. W.; Wu, Y.; Lassiter, J. B.; Nordlander, P.; Halas, N. J. Substrates Matter: Influence of an Adjacent Dielectric on an Individual Plasmonic Nanoparticle. Nano Lett. 2009, 9, 2188−2192. (47) Nelayah, J.; Gu, L.; Sigle, W.; Koch, C. T.; Pastoriza-Santos, L.; Liz-Marzán, L. M.; van Aken, P. A. Direct imaging of surface plasmon resonances on single triangular silver nanoprisms at optical wavelength using low-loss EFTEM imaging. Opt. Lett. 2009, 34, 1003−1005. (48) Schider, G.; Krenn, J. R.; Gotschy, W.; Lamprecht, B.; Ditlbacher, H.; Leitner, A.; Aussenegg, R. F. Optical properties of Ag and Au nanowire gratings. J. Appl. Phys. 2001, 90, 3825−3830. (49) Sundaramurthy, A.; Crozier, K. B.; Kino, G. S.; Fromm, D. P.; Schuck, P. J.; Moerner, W. E. Field enhancement and gap-dependent resonance in a system of two opposing tip-to-tip Au nanotriangles. Phys. Rev B 2005, 72, 165409-1−6. (50) Chen, F.; Alemu, N.; Johnston, R, L. Collective plasmon modes in a compositionally asymmetric nanoparticle dimer. AIP Adv. 2011, 1, 032134-1−16. (51) Chu, M. W.; Myroshnychenko, V.; Chen, C. H.; Deng, J. P.; Mou, C. J.; García de Abajo, F. J. Probing Bright and Dark SurfacePlasmon Modes in Individual and Coupled Noble Metal Nanoparticles Using an Electron Beam. Nano Lett. 2009, 9, 399−404. (52) Jain, P. K.; Eustis, S.; El-Sayed, M. A. Plasmon Coupling in Nanorod Assemblies: Optical Absorption, Discrete Dipole Approximation Simulation, and Exciton-Coupling Model. J. Phys. Chem. B 2006, 110, 18243−18253. (53) Gersten, J.; Nitzan, A. Electromagnetic Theory of Enhanced Raman Scattering by Molecules Adsorbed on Rough Surfaces. J. Chem. Phys. 1980, 73, 3023−3037. (54) Liao, P. F.; Wokaun, A. Lightning Rod Effect in Surface Enhanced Raman Scattering. J. Chem. Phys. 1982, 76, 751−752.
9081
dx.doi.org/10.1021/la300277m | Langmuir 2012, 28, 9071−9081
