Hybrid states of propagating and localized surface plasmons at silver core/silica shell nanocubes on a thin silver layer Hansik Yun,1 Seung-Yeol Lee,1 Kyoung-Youm Kim,2 Il-Min Lee,1 and Byoungho Lee1,* 1
National Creative Research Center for Active Plasmonics Application Systems, Inter-University Semiconductor Research Center and School of Electrical Engineering, Seoul National University, Seoul 151-744, South Korea 2 Department of Optical Engineering, Sejong University, Seoul 143-747, South Korea * [email protected]
Abstract: Hybrid characteristics of propagating surface plasmons (PSPs) and localized surface plasmons (LSPs) appear at a combined structure of a thin silver (Ag) layer and silver core/silica shell nanocubes (AgNC@SiO2s) in the Kretschmann configuration, because the resonant condition of PSPs on the thin Ag layer is significantly modified by LSPs of the AgNC@SiO2s. We investigate theoretically and experimentally that due to the hybrid property, the slope and position of the minimum reflectance band can be controlled on a graph of incident angle versus wavelength of reflected light, by changing structural parameters. The hybrid properties of PSPs and LSPs have a potential to simultaneously detect surface plasmon resonance signals and fluorescence images. ©2014 Optical Society of America OCIS codes: (250.5403) Plasmonics; (240.6680) Surface plasmons; (160.4236) Nanomaterials.
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1. Introduction Plasmonics has received a great amount of attention as one of the leading candidates for a core technology to realize next-generation devices, such as optical integrated circuits, optical memory, solar cells and bio sensors, because i) surface plasmons travel through nano-scale structures with high speed, ii) they absorb a distinctive wavelength band from incident light, and iii) their absorption spectra are extremely sensitive to both material and structural characteristics [1, 2]. The surface plasmons are collective oscillations of electrons at an interface between metal and dielectric, which can be categorized into two classes: propagating surface plasmons (PSPs) and localized surface plasmons (LSPs). PSPs propagate continuously on the flat surface of a metal layer under the momentum-matching condition of surface plasmons and light, which can be coupled by prism or grating coupling methods. This propagating feature has been studied for plasmonic waveguide, nano-lasing and nano-
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focusing [3–5]. Contrary to PSPs, LSPs go around on the surface of discontinuous metal nanostructures by direct illumination of light. They have been investigated for plasmonic antenna, light harvesting, and medical diagnostics, because their spectral properties are deeply dependent on the size and shape of the structure [6–9]. Many studies about plasmonics and their applications have tended to focus only on one of two classes due to their exclusive properties including propagation and localization. However, research on plasmonic sensors can freely employ whichever type of surface plasmons, because both respond sensitively to a subtle change in an environmental refractive index [1, 2]. Sensing techniques using PSPs have been already commercialized in the industry. The commercialized surface plasmon resonance (SPR) sensors make a distinctive dip signal in a reflectance spectrum as a detecting result because of the momentum matching condition for PSP excitation [1, 2]. On the other hand, various kinds of studies for adapting LSPs to sensors have been conducted at laboratory-scales though they have not been commercialized yet [10, 11]. As one of the studies, metal-enhanced fluorescence method can be used in biomolecular detections because fluorophores around metal nanoparticles can be excited by the strong electromagnetic fields confined to the nanoparticles. Therefore, their fluorescence can have higher intensity than that without metal nanoparticles [12, 13]. Although the PSP-based sensor can give clear quantitative data for a bulk sample on a flat metal surface, the SPR dip signal can be weakened for a small amount of sample, or complex signal data cannot be distinguished for a mixed sample with various elements. On the other hand, the LSP-based sensor can detect small molecules with metal nanoparticles, but the fluorescence images are less quantitative than those of the SPR sensor.
Fig. 1. (a) Schematic illustration of the combined structure of AgNC@SiO2s on a thin Ag layer for dual detection of SPR signal and fluorescence image. (b) Schematic diagram of the structure and parameters for numerical simulations.
Therefore, complementary information of SPR signal and fluorescence image data can be acquired by utilizing both PSPs and LSPs. In order to provide the potential and possibility of the simultaneous dual detection, we propose a metallic nanostructure composed of a thin metal layer for PSP excitation and metal nanoparticles for LSP excitation, as shown in Fig. 1(a). Since not only the homogeneously distributed particles with short inter-particle distances cause LSP coupling among them but a narrow dielectric gap between the metal nanoparticles and the thin metal layer induces an interaction between PSPs and LSPs, it is expected that PSPs and LSPs will be simultaneously excited on the combined structure. Similar structures, where couplings of PSPs and LSPs occur, have been also studied: for example, hybrid resonances of PSPs and LSPs have been proved with dual or anticrossing bands [14–16], and the enhancement of perfect absorption or surface-enhanced Raman scattering originating from the resonances has been observed [17–19]. Most of the previous works analyzed the coupling characteristics from LSPs to PSPs since light vertically illuminates a nanoparticle-distributed side. However, in order to apply the proposed structure effectively to a conventional SPR sensor, hybrid behavior of the SPR signal should be observed under the Kretschmann configuration [1, 2]. Also, it needs to be considered from a perspective of the coupling from
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Received 14 Feb 2014; revised 22 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008383 | OPTICS EXPRESS 8385
PSPs to LSPs, and the enhancement of fluorescence has to be correlated with the hybridized spectrum band. In order to verify the feasibility of this concept, we investigate the proposed structure in both simulation and experiment as follows. In our previous work, we reported that intermediate characteristics between PSPs and LSPs were observed on a quasi-continuous metallic layer by closely packing only metallic nanocubes [20]. The approach using metallic nanoparticles is extended and applied to this study, which focuses on the coupling phenomena from PSPs on a thin metal layer to LSPs on nanoparticles, by inserting a thin metal layer between metallic nanocubes and a substrate and increasing inter-particle distances from almost zero to more than a few hundred nanometers. Through these extensions, we can acquire a single hybrid band of PSPs and LSPs to utilize both PSPs and LSPs at one resonant wavelength in a sensor, while PSPs and LSPs are excited at different resonant wavelengths due to two hybrid bands having PSP and LSP dominant properties, respectively, in the previous work. Therefore, the proposed structure becomes suitable for the dual detections of SPR signals and fluorescence images based on PSPs and LSPs, respectively, and this research presents the more practical perspective than the previous work dose. In detail, reflectance spectra and electric field profiles of the proposed structure are calculated in two-dimensional simulations, and experimental samples are prepared as a form of silver nanocubes (AgNCs) with silica (SiO2) shells (AgNC@SiO2s) on a thin Ag film. Their fluorescence image as well as spectral reflectance is measured in various angles of incident light. To accurately compare the results of the simulation and experiment, a statistical treatment for a spatial distribution of nanoparticles on a flat surface is conducted with the Weibull distribution model [21, 22]. Through the procedures mentioned above, we intend to demonstrate the effectuality of both simulation and experimental methods to confirm the feasibility of the dual detection using the proposed structure. 2. Methods 2.1 Simulations For the calculation of electromagnetic fields and reflectance on the proposed structure, our own-built rigorous coupled wave analysis (RCWA) tool is used [23, 24]. This numerical method firstly divides overall geometry as an array of layers, and calculate Fourier spectrum of each layer. Then, possible eigenmodes which can exist in those layers are obtained. Finally, coupling coefficients from incident wave to eigenmodes in each layer are calculated by using the extended transfer matrix method (ETMM). By appropriately summing propagating coupling coefficients backwardly for all of eigenmodes existing in the incident layer, we could obtain reflectance for a specific wavelength and incident angle. θ-λR map is calculated from the repeated process of the RCWA calculation, varying the condition of the incident angle and wavelength. 2.2 Synthesis of Ag nanocubes 50 mg/mL solution of silver nitrate (AgNO3, Sigma-Aldrich), 25 mg/mL of poly(vinyl pyrrolidone) (PVP, Sigma-Aldrich), and 5 mM solution of sodium hydrosulfide (NaHS, Sigma-Aldrich) in ethylene glycol (EG, J.T. Baker) were prepared. 8 mL of EG was heated at 150°C and stirred with a silicon coated magnetic bar for 1 h in a 20 mL glass vial. 0.05 mL of the sulfide solution, 1.8 mL of the PVP solution and 0.4 mL of the AgNO3 solution were sequentially injected into the heated EG solution. After 3-5 min of reaction time, the solution was quenched with cooling water. AgNCs were dispersed in ethanol after three times of rinsing processes of centrifugation, dilution, and sonication with acetone and ethanol. 2.3 Synthesis of SiO2 shell Three kinds of AgNC solutions which include undiluted, 1.25-fold and 20-fold diluted solutions were prepared by diluting the synthesized AgNC solution with ethanol. 25% solution of ammonia and 10% solution of tetraethoxysilane (TEOS, Sigma-Aldrich) in ethanol
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were prepared. 7.6 mL ethanol was stirred in 20 mL glass vial and then 3 mL of the AgNC solution, 0.2 mL of the ammonia solution and 0.6 mL of the TEOS solution were sequentially injected into the ethanol in the vial. The mixed solution was stirred at room temperature for 12 h. Two kinds of fluorescence dyes which include fluorescein isothiocyanate (FITC) and rhodamine B isothiocyanate (RITC) were bound to SiO2 shell surface of AgNC@SiO2s using 3-aminopropyl triethoxysilane (APTES, Sigma-Aldrich). The AgNC@SiO2 solution was stirred with 0.1 mL of APTES (2mM), 0.1 mL of FITC (1mM), and 0.1mL of RITC (1mM) solutions at room temperature for 5 h. After reaction, excess reactants were washed away in the rinsing process. 2.4 Preparation of AgNC@SiO2 on Ag layer Ag layer with thickness of 50 nm was evaporated on the glass substrate with e-beam evaporator (KVE-3004, Korea Vacuum Tech.). The Ag layer on the glass was spin-coated with the three AgNC@SiO2 solutions at 200 rpm for 20 sec and then dried at 100°C for 2 min. 2.5 Characterization and optical measurement Surface morphologies of the samples were measured with a high resolution transmission electron microscope (HR-TEM; JEM-3010, JEOL) and a field emission scanning electron microscopy (FE-SEM; JSM 6700F, JEOL). Optical far-field images were measured by a modified optical microscopy setup which has a charge-coupled device (CCD; XCD-SX90CR, Sony) and the objective lens with the magnitude of 100 and the numerical aperture of 0.8 (LMPlanFLN, OLYMPUS Corp.). The samples were illuminated from the bottom by white light of a halogen lamp with changing the incident angle from 40° to 70°. Reflectance spectra were measured using a spectrometer (SM240, Spectral Products) at the same angle as that of incident light. 3. Results and discussion 3.1 Configuration of proposed structure PSPs excited on a thin metal layer in the Kretschmann configuration can have enough intensity to detect a subtle change of the refractive index of an analyte on the thin metal layer, and LSPs excited on metal nanoparticles can confine enough electromagnetic fields to enhance the fluorescence intensity of fluorophores around the metal nanoparticles. It is possible that PSPs and LSPs share the strong electromagnetic fields as a form of their interaction or coupling in a metallic nanostructure if the structure is designed to be partially or concurrently available to the excitations of both PSPs and LSPs. Therefore, the proposed structure, which is a combination of a metal layer and nanoparticles, can be suggested as shown in Fig. 1(a). By using this structure, we expect that the SPR dip signal will be detected in reflectance mode with a spectrometer and the fluorescence image will be observed in transmittance mode with an objective lens and CCD camera. Figure 1(b) shows the detailed geometries of the structure with the defined structural parameters. We selected a cube as a structure of nanoparticles because it has several advantages for the research for the following reasons. First, a cube is an appropriate polyhedron to stably contact face to face with a flat surface. The face-to-face contact provides a constant and parallel gap between planes, contrary to the edge-to-face contact of sphere-shaped particles and a flat layer. In respect of plasmonic properties, the electromagnetic fields can be more evenly confined and distributed in the plane-plane gap than the point-plane gap. Second, metal nanocubes can be generally synthesized in a regular shape and narrow size distribution in experiment [25]. Silver was chosen as metal materials of a thin layer and cubic cores because it has the longest propagation length of PSPs in noble metals. SiO2 shell around AgNC separates AgNCs from the Ag layer so that plasmonic couplings can be induced between them [14–16, 26], and prevents a direct contact between fluorophores and AgNCs so that the quenching effect is avoided [12, 27]. The reason for wrapping the dielectric shells around individual AgNCs instead of coating the dielectric layer on an entire Ag layer is that the sensitivity of SPR dip
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Received 14 Feb 2014; revised 22 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008383 | OPTICS EXPRESS 8387
signal may be raised by leaving the bare Ag surface, except AgNC@SiO2 particles, as wide as possible. The main parameters that affect the plasmonic properties of the proposed structure include the size of AgNC core (d), the thickness of SiO2 shell (t), the thickness of Ag thin layer (T), the inter-particle distance between AgNC@SiO2s (D), and incident angle (θ). In respect of required data, reflectance spectra at various incident angles, which can be converted into a graph of incident angle versus wavelength of reflectance (θ-λR map), should be calculated and measured. The θ-λR map can be an appropriate indicator to represent the existence and change of the surface plasmons, because the light absorption by the excitation of the surface plasmons causes a decrease in reflectance at the interface between an Ag layer and glass substrate in total internal refraction (TIR) of the Kretschmann configuration [1, 2]. 3.2 Simulations: characteristics of the minimum reflectance band on θ-λR map We calculated the reflectance spectra ranging from 400 nm to 800 nm at the incident angles from 40° to 80° using a simulation tool, RCWA [23, 24], to make a θ-λR map for one structure with fixed parameters. Based on the assumption that AgNC@SiO2s are homogeneously distributed on an Ag layer, one period (P) contains a cube and is equal to the inter-particle distance (D), as shown in Fig. 1(b). Two-dimensional (2D) simulations were performed, because they reduce computational time significantly compared to three-dimensional (3D) calculations. (The formers are appropriate for enormous calculations in scanning various structural parameters.) Although 2D simulations cannot provide the exact solution for our structure, we expect that they can be helpful in understanding the tendency of the optical property in our system under transverse-magnetic (TM) polarization with various incident angles because it is known that 2D simulations are available for TM polarization incidence, while 2D calculation results are different with those derived in the 3D model for transverseelectric (TE) polarization incidence [28–30]. Figures 2(a)–2(g) show that the θ-λR map changes with a consistent trend as the interparticle distance of AgNC@SiO2 array decreases from 3000 nm to 60 nm when d, t, and T are 30 nm, 10 nm, and 50 nm, respectively. The blue band on the map indicates the behavior of SPR signal, since the minimum reflectance is caused by surface plasmons excited at a specific angle and wavelength of incident light. The position and slope of the band for 2000 nm of the inter-particle distance are almost the same as those for the bare Ag layer without the nanoparticles (Figs. 2(a) and 2(b)). This clearly shows that a single nanoparticle on a wide Ag layer without their mutual interactions cannot affect the excitation characteristics of PSPs on the Ag layer. Therefore, incident light recognizes the surface of AgNC@SiO2s on the Ag layer with 2000 nm of the inter-particle distance as the surface of the bare Ag layer, so that the same type of surface plasmons, PSPs, can be excited. As the inter-particle distance decreases, however, the LSPs localized to respective AgNCs begin to interact with each other and thus the shape of the minimum reflectance band changes accordingly, and the slope of the band becomes steeper. In order words, the slope of the band increases as the population density of the nanoparticles on the Ag layer increases or the interaction between LSPs gets stronger. This is quite reasonable if we recognize that the steeper slope of the band indicates that the excitation condition of surface plasmons becomes lesser sensitive to the incident angle (more sensitive to the wavelength of the incident light). Since the excitation of LSPs is almost independent of the incident angle [2, 31], the overall reflectance spectra becomes lesser dependent on the incident angles as the LSP effects become more dominant. In addition, contrary to the continuous band in the case of the bare Ag layer, the bands for Ag layers with nanoparticles, as shown in Figs. 2(b)–2(e), are discontinuous. The number of the small bands decreases in a sequence while the particles go close to each other, and finally a single band with a rapid slope is formed. This discontinuity results from the Wood's anomaly effect [32, 33], i.e., the periodic distribution of the nanoparticles and thus, it disappears when we introduce a random distribution in the inter-particle distances (D), as will be shown in the later part of this paper.
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Received 14 Feb 2014; revised 22 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008383 | OPTICS EXPRESS 8388
Fig. 2. Changes of the minimum reflectance band on θ-λR map depending on inter-particle distance (D) among AgNC@SiO2s. θ-λR maps for AgNCs (30 nm) with SiO2 shell (10nm) on Ag layer (50 nm) in (a) an infinite, (b) 2000 nm, (c) 800 nm, (d) 350 nm, (e) 200 nm, (f) 100 nm, and (g) 60 nm of inter-particle distance. Hy-field profiles at (h) 44° and (i) 69.5° of different incident angles and 580 nm of same wavelength of incident light. Black dotted circles marked on the maps of (b) and (f) indicate positions where Hy-field profiles of (h) and (i) were calculated, respectively.
In order to investigate the reason for the increase in the slope of the minimum reflectance band in the case of a short inter-particle distance, we calculated Hy-field profiles at the two points marked with black open circles on the map, as shown in Figs. 2(b) and 2(f), where the reflectance values become the minimum at the wavelength of 580 nm. In the case of D = 2000 nm, Hy-fields are not confined to but pass over a single AgNC@SiO2 as if it did not exist, as shown in Fig. 2(h). This means that only PSPs contribute to the minimum reflectance band. On the contrary, in the case of D = 100 nm, Hy-fields are not only propagating along the flat Ag surface but also strongly confined to the SiO2 gaps between the AgNCs and Ag layer, as shown in Fig. 2(i). In other words, the resonant condition of PSPs on the Ag layer is significantly modified by the interaction with LSPs of the AgNCs. Therefore, the surface plasmon mode in this case can be interpreted as a hybrid state, generated by the coupling from the PSPs of the bare Ag layer to the LSPs of the AgNCs. In the hybridization, the dipole mode of LSPs may have stronger influence than the quadrupole mode because the dipole mode tend to be preferred in face-to-face configuration [34, 35], and the dipole mode of LSPs mainly contributes to the coupling with PSPs when an array of anisotropic particles, such as nanodisks and nanoellipsoids, is near a metal layer [36, 37].
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Received 14 Feb 2014; revised 22 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008383 | OPTICS EXPRESS 8389
Fig. 3. Effect of (a)-(c) Ag core size (d), (d)-(f) SiO2 shell thickness (t), and (g)-(i) Ag layer thickness (T) on the minimum reflectance band. The AgNC core sizes are (a) 20 nm, (b) 30 nm, and (c) 40 nm. The SiO2 shell thicknesses are (d) 10 nm, (e) 25 nm, (f) 35 nm. The Ag layer thicknesses are (g) 30 nm, (h) 50 nm, and (i) 70 nm.
The size of AgNC mainly influences the position of the minimum reflectance band on θ-λR map or the resonance wavelength, as shown in Figs. 3(a)–3(c), while the inter-particle distance determines the slope of the band. The resonance wavelength increases as the core size increases. This effect of the core size can be easily explained by the fundamental theory of LSPs excited at metal nanoparticles. Considering the LSPs of Ag nanoparticles dispersed in a solution or on a substrate, the resonance wavelength generally increases as the particle size increases, because free electrons should oscillate a long path length on surface of the large Ag particle [2, 31]. In the similar way, the minimum reflectance band of the structure with a large AgNC core is located in the longer wavelength region than that with a small core. In addition, because the increase of AgNC core size shortens the relative inter-particle distance, the enhanced plasmonic coupling may be induced between each particle and can cause an additional red-shift of the band. However, even if the size of the AgNC changes, the field profile or the so-called decay constant in the shell part of the nanoparticle is not altered significantly. Therefore, the strength of the LSP interaction, which determines the slope of the minimum reflection band, remains almost the same, and thus we cannot expect any noticeable changes in the slope of the minimum reflection band. Meanwhile, thickening SiO2 shell also causes a red-shift of the band because the enlarged dielectric portion increases the effective decay length to relatively shorten the inter-particle distance (Figs. 3(d)–3(f)), and thickening Ag layer hinders the excitation of PSPs to weaken the band (Figs. 3(g)–3(i)).
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Received 14 Feb 2014; revised 22 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008383 | OPTICS EXPRESS 8390
Fig. 4. (a) TEM image and (b) size distribution of AgNC@SiO2s. FE-SEM images of AgNC@SiO2s coated on Ag layer with (c) 20-, (d) 1.25-fold dilutions, and (e) undiluted solutions. They are named NC1000, NC200, and NC117, respectively. The scale bars are (a) 50 nm and (c)-(e) 500 nm. (f) Image of total internal reflection set-up with the hemi-cylindrical prism, white light source, spectrometer, and microscope lens. The polarizer makes incident light be TM polarization, and the incident angle is changed from 40° to 70° by a motorized rotation stage. CCD camera, which is not shown here, is adapted to microscopy.
3.3 Experiments: measurement of the minimum reflectance band on θ-λR map We were able to understand the characteristics of the minimum reflectance band on θ-λR map as results of scanning the various structural parameters in numerical simulations. In order to prove the simulation results, we tried to experimentally observe the difference of the band slopes for the samples having different inter-particle distances, since the change of the band slope is the representative phenomenon of the plasmonic characteristics in the suggested structure. We prepared samples by spin-coating AgNC@SiO2 nanoparticle solutions with different concentrations on a thin Ag layer. AgNCs were synthesized with the polyol method [38] and surrounded with thin SiO2 shell in the Stöber process [39]. AgNC is a cubic structure and SiO2 shells are wrapped around the AgNC cores with approx. 6 nm of thickness, as shown in TEM images of Fig. 4(a). The average size and standard deviation of AgNC@SiO2s is approx. 36.4 ± 2.9 nm and the size distribution is shown in Fig. 4(b). It can also be confirmed that most of the AgNC@SiO2s contact face to face with a flat surface. AgNC@SiO2s are distributed with three different population densities on the Ag layer with 50 nm thickness, as shown in FE-SEM images of Figs. 4(c)–4(e). The population density can be controlled with the concentration of the nanoparticle-dispersed solution. Three kinds of solutions were prepared by adding ethanol to the synthesized AgNC@SiO2 solution, which includes 20-, 1.25-fold dilutions, and the undiluted solution. The FE-SEM images indicate that the population density increases while the inter-particle distances decrease in proportion to the concentration of AgNC@SiO2 solution. The average values and standard deviations of the inter-particle distances are 200.3 ± 84.9 nm and 117 ± 40.1 nm in Figs. 4(d) and 4(e), respectively. The values of the sample coated with 20-fold diluted solution could not be measured in Fig. 4(c) due to a large deviation of the inter-particle distances, and we supposed the average distance is 1000 nm. The three samples of Figs. 4(c)–4(e) were named NC1000, NC200, and NC117, respectively, for convenience. The distribution graphs of the interparticle distances will be presented in Fig. 6(a) in order to efficiently use them in a later statistical treatment. In addition, two kinds of fluorophores, which are FITC and RITC, were added to all of the samples in order to clearly distinguish the angle-dependent resonances and
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Received 14 Feb 2014; revised 22 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008383 | OPTICS EXPRESS 8391
fluorescence images. The transmittance images and the reflectance spectra of the samples are measured with the CCD camera and the spectrometer, which are adapted to the home-made optical microscopy setup, as shown in Fig. 4(f). The setup satisfies the Kretschmann configuration since the polarizer before the white light source keeps a TM polarization and a motorized rotation stage controls the incident angle.
Fig. 5. Experimental θ-λR maps of (a) NC1000, (b) NC200, and (c) NC117.
The reflectance spectra of NC1000, NC200, and NC117 were measured at least for five different samples by changing the incident angle, and then three experimental θ-λR maps were completed as shown in Fig. 5. Three experimental θ-λR maps display the clear SPR dips of the reflectance spectra and three different slopes of the minimum reflectance bands which are expressed in the series of SPR dips. In the case of NC1000, the SPR dips are relatively deep and the band slope is gentle, as shown in Fig. 5(a). The position and shape of the band are similar to those for the bare Ag layer with 50 nm thickness, because the sparsely distributed AgNC@SiO2s weakly affect the PSPs on the Ag layer. As the population density of the particles increases, i.e., the inter-particle distances decrease, the band slope increases, as shown in Figs. 5(b) and 5(c). Therefore, the experimental spectrum data can demonstrate the calculated results of Fig. 2 in spite of the weak spectral dips. The widths of the reflectance dips of NC117 are broader than those of NC1000 due to radiation losses and a spatial deviation of AgNC@SiO2s. The nanoparticles partially play a role as radiative scatterers to cause radiation losses in the coupling between PSPs and LSPs, and the deviation of the distributed AgNC@SiO2s disturbs the accurate periodicity to broaden the width of the spectral dips. Because a large deviation of the inter-particle distances in NC200 increases the spatial randomness more than that in NC117, the spectral dips of NC200 become much weaker than those of NC117. 3.4 Comprehensive analysis of plasmonic hybrid property There are some structural differences between simulation and experimental conditions in the strict sense. In experiment, AgNC@SiO2 particles are settled on the Ag layer with a certain distribution of the inter-particle distances because they are randomly scattered in the process of spin-coating, while they are located at exact lattice points with a constant distance in simulation. Although the sizes of AgNCs also have a certain distribution in experiment, the size distribution affects the plasmonic properties of the samples less than the distribution of the inter-particle distances because the latter is thirteen times wider than the former; for the
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Received 14 Feb 2014; revised 22 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008383 | OPTICS EXPRESS 8392
example of NC117, the deviations of the inter-particle distance and particle size distributions are 40.1 nm and 2.9 nm, respectively, as shown in Figs. 6(a) and 4(b). Therefore, we will consider only the distance distribution in calculation so as to compare the outcomes between simulation and experiment as accurately as possible. We measured the inter-particle distances of AgNC@SiO2s on the Ag layer from the FE-SEM image of NC117 in Fig. 4(e), and then calculated the average (μ) and standard deviation (σ) of the distance distribution with a statistical method, as shown in Fig. 6(a). The distribution does not follow a normal distribution because it is not bilaterally symmetric and has a tail in long inter-particle distances. Therefore, we introduced the Weibull distribution model to fit it to the real distribution of inter-particle distances. Since the Weibull distribution is typically determined by a scale parameter (α) and shape parameter (β) [21, 22], we were able to make the Weibull distribution similarly fit to the inter-particle distance distribution of NC117 through a reverse calculation of α and β from μ and σ of the experimental distribution in Fig. 6(a), as shown in Fig. 6(b). The modified structure, which has fifty particles of AgNC@SiO2 with random interparticle distances of the fitted Weibull distribution in one period, was calculated under the same conditions as the previous simulations without considering the distribution, as shown in Fig. 6(c). In Fig. 6, three different types of data, which are the experimental θ-λR maps, simulation θ-λR maps, and fluorescence images, are correlated for NC1000 and NC117, which have the contrasting population density of AgNC@SiO2s. The SPR dips in the experimental θ-λR maps are marked with circular yellow dots on the same positions of the calculated θ-λR maps in Figs. 6(d) and 6(h) to compare with each other. The slopes of the experimental bands almost coincide with those of the calculated bands although the minimum reflectance band is blurred and the intensity is weakened when the Weibull distribution is introduced to simulation. The band especially becomes blurrier at a short wavelength than at a long wavelength. The reason is that light of short wavelengths may be more sensitive to the nanoparticle distribution with the same standard deviation and then enables to recognize the distribution more sensitively. We can also confirm that the slope of the minimum reflectance band is steeper for NC117 than NC1000. The invariable property of the band position and slope, therefore, enables to measure the series of SPR dips and to confirm the existence of the band on the θ-λR map even though AgNC@SiO2s are randomly scattered in the experiment. The validity of 2D simulations can also be demonstrated by comparing the simulation data with these representative experimental results. Using these summarized data, we present a potential to simultaneously detect SPR signals and fluorescence images. Figures 6(e)–6(g) and 6(i)–6(k) are corresponding fluorescence images of the samples at different points on the bands. Most of all, the fluorescence intensity is stronger in NC117 than NC1000 because the electromagnetic fields localized on the particles stimulate the fluorophores, such as FITC and RITC, around AgNC@SiO2s. On average, the fluorescence intensity of NC117 is enhanced approximately 2.3 times higher than that of NC1000. Additionally, the steep slope of the band enables fluorophores to clearly emit their instinct color in a wide range of incident angles. The wavelength and color of fluorescence change with high intensity in the wide incident angles from ~600 nm of red by RITC at 51° to ~520 nm of green by FITC at 57° in Figs. 6(i)–6(k), while they unclearly change in the narrow angles between 44° and 42° in Figs. 6(e)–6(g). Therefore, we can acquire qualitative fluorescence images as well as quantitative SPR signals using the plasmonic hybrid characteristic of the proposed structure.
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Received 14 Feb 2014; revised 22 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008383 | OPTICS EXPRESS 8393
Fig. 6. (a) Inter-particle distance distribution of NC117, (b) the Weibull distribution calculated with experimental μ and σ of NC117, and (c) schematic illustration of a structure and parameters for simulation including the Weibull distribution. Comparison of three kinds of data for (d)-(g) NC1000 and (h)-(k) NC117. (d) and (h) are simulation θ-λR maps including the Weibull distribution factor. The circular yellow dots indicate the minimum reflectance dips in the experimental data of Fig. 5. The corresponding fluorescence images are measured at (e) 44°, (f) 43°, (g) 42°, (i) 57°, (j) 55°, and (k) 51° of incident angle. The scale bars are 5 μm.
4. Conclusion In this research, we investigated the hybrid states of PSPs and LSPs for the simultaneous dual detection of SPR signals and fluorescence images in the Kretschmann configuration. Although PSPs and LSPs generally have conflicting properties, i.e., propagation and localization of surface plasmon, they can coexist as a hybrid form of PSP and LSP on the proposed structure because the resonant condition of PSPs on the thin Ag layer is significantly modified by the interaction between LSPs of the AgNC@SiO2s. As results of scanning the structural parameters, such as the inter-particle distance and size of AgNC@SiO2s, we found that they affect the slope and position of the minimum reflectance band on θ-λR map. The simulation results were confirmed by experimentally measuring series of SPR signals and fluorescence images of AgNC@SiO2s on the Ag layer at various incident angles in the Kretschmann configuration. In particular, the real distribution of the inter-particle distances measured in experimental samples was fitted to the Weibull distribution model and then the statistical data were fed back to the calculation process. The recalculated results significantly agreed with the experimental θ-λR maps and fluorescence images. This study suggested a potential promise in usefully applying the plasmonic hybrid property to dual detections of SPR signals and fluorescence images. The proposed structure, which is the thin metal layer combined with nanoparticles having metal cubic cores and dielectric shells, will be used in controlling the minimum reflectance band for a certain purpose because the slope and position of the band can be freely tuned with structural parameters. In addition, if the standard deviation is reduced in the spatial distribution of the #206502 - $15.00 USD (C) 2014 OSA
Received 14 Feb 2014; revised 22 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008383 | OPTICS EXPRESS 8394
particles through improving fabrication processes, the sensitivity will also be improved owing to the narrower and clearer band, although it became broad or blur because the nanoparticles were randomly scattered in this research. Therefore, the plasmonic hybrid properties of PSPs and LSPs on the suggested structure are expected to be useful in various practical applications, such as biological and chemical sensors. Acknowledgments This work was supported by the National Research Foundation of Korea funded by Korean government (MSIP) through the Creative Research Initiatives Program (Active Plasmonics Application Systems).
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Received 14 Feb 2014; revised 22 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008383 | OPTICS EXPRESS 8395
National Creative Research Center for Active Plasmonics Application Systems, Inter-University Semiconductor Research Center and School of Electrical Engineering, Seoul National University, Seoul 151-744, South Korea 2 Department of Optical Engineering, Sejong University, Seoul 143-747, South Korea * [email protected]
Abstract: Hybrid characteristics of propagating surface plasmons (PSPs) and localized surface plasmons (LSPs) appear at a combined structure of a thin silver (Ag) layer and silver core/silica shell nanocubes (AgNC@SiO2s) in the Kretschmann configuration, because the resonant condition of PSPs on the thin Ag layer is significantly modified by LSPs of the AgNC@SiO2s. We investigate theoretically and experimentally that due to the hybrid property, the slope and position of the minimum reflectance band can be controlled on a graph of incident angle versus wavelength of reflected light, by changing structural parameters. The hybrid properties of PSPs and LSPs have a potential to simultaneously detect surface plasmon resonance signals and fluorescence images. ©2014 Optical Society of America OCIS codes: (250.5403) Plasmonics; (240.6680) Surface plasmons; (160.4236) Nanomaterials.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
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1. Introduction Plasmonics has received a great amount of attention as one of the leading candidates for a core technology to realize next-generation devices, such as optical integrated circuits, optical memory, solar cells and bio sensors, because i) surface plasmons travel through nano-scale structures with high speed, ii) they absorb a distinctive wavelength band from incident light, and iii) their absorption spectra are extremely sensitive to both material and structural characteristics [1, 2]. The surface plasmons are collective oscillations of electrons at an interface between metal and dielectric, which can be categorized into two classes: propagating surface plasmons (PSPs) and localized surface plasmons (LSPs). PSPs propagate continuously on the flat surface of a metal layer under the momentum-matching condition of surface plasmons and light, which can be coupled by prism or grating coupling methods. This propagating feature has been studied for plasmonic waveguide, nano-lasing and nano-
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Received 14 Feb 2014; revised 22 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008383 | OPTICS EXPRESS 8384
focusing [3–5]. Contrary to PSPs, LSPs go around on the surface of discontinuous metal nanostructures by direct illumination of light. They have been investigated for plasmonic antenna, light harvesting, and medical diagnostics, because their spectral properties are deeply dependent on the size and shape of the structure [6–9]. Many studies about plasmonics and their applications have tended to focus only on one of two classes due to their exclusive properties including propagation and localization. However, research on plasmonic sensors can freely employ whichever type of surface plasmons, because both respond sensitively to a subtle change in an environmental refractive index [1, 2]. Sensing techniques using PSPs have been already commercialized in the industry. The commercialized surface plasmon resonance (SPR) sensors make a distinctive dip signal in a reflectance spectrum as a detecting result because of the momentum matching condition for PSP excitation [1, 2]. On the other hand, various kinds of studies for adapting LSPs to sensors have been conducted at laboratory-scales though they have not been commercialized yet [10, 11]. As one of the studies, metal-enhanced fluorescence method can be used in biomolecular detections because fluorophores around metal nanoparticles can be excited by the strong electromagnetic fields confined to the nanoparticles. Therefore, their fluorescence can have higher intensity than that without metal nanoparticles [12, 13]. Although the PSP-based sensor can give clear quantitative data for a bulk sample on a flat metal surface, the SPR dip signal can be weakened for a small amount of sample, or complex signal data cannot be distinguished for a mixed sample with various elements. On the other hand, the LSP-based sensor can detect small molecules with metal nanoparticles, but the fluorescence images are less quantitative than those of the SPR sensor.
Fig. 1. (a) Schematic illustration of the combined structure of AgNC@SiO2s on a thin Ag layer for dual detection of SPR signal and fluorescence image. (b) Schematic diagram of the structure and parameters for numerical simulations.
Therefore, complementary information of SPR signal and fluorescence image data can be acquired by utilizing both PSPs and LSPs. In order to provide the potential and possibility of the simultaneous dual detection, we propose a metallic nanostructure composed of a thin metal layer for PSP excitation and metal nanoparticles for LSP excitation, as shown in Fig. 1(a). Since not only the homogeneously distributed particles with short inter-particle distances cause LSP coupling among them but a narrow dielectric gap between the metal nanoparticles and the thin metal layer induces an interaction between PSPs and LSPs, it is expected that PSPs and LSPs will be simultaneously excited on the combined structure. Similar structures, where couplings of PSPs and LSPs occur, have been also studied: for example, hybrid resonances of PSPs and LSPs have been proved with dual or anticrossing bands [14–16], and the enhancement of perfect absorption or surface-enhanced Raman scattering originating from the resonances has been observed [17–19]. Most of the previous works analyzed the coupling characteristics from LSPs to PSPs since light vertically illuminates a nanoparticle-distributed side. However, in order to apply the proposed structure effectively to a conventional SPR sensor, hybrid behavior of the SPR signal should be observed under the Kretschmann configuration [1, 2]. Also, it needs to be considered from a perspective of the coupling from
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Received 14 Feb 2014; revised 22 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008383 | OPTICS EXPRESS 8385
PSPs to LSPs, and the enhancement of fluorescence has to be correlated with the hybridized spectrum band. In order to verify the feasibility of this concept, we investigate the proposed structure in both simulation and experiment as follows. In our previous work, we reported that intermediate characteristics between PSPs and LSPs were observed on a quasi-continuous metallic layer by closely packing only metallic nanocubes [20]. The approach using metallic nanoparticles is extended and applied to this study, which focuses on the coupling phenomena from PSPs on a thin metal layer to LSPs on nanoparticles, by inserting a thin metal layer between metallic nanocubes and a substrate and increasing inter-particle distances from almost zero to more than a few hundred nanometers. Through these extensions, we can acquire a single hybrid band of PSPs and LSPs to utilize both PSPs and LSPs at one resonant wavelength in a sensor, while PSPs and LSPs are excited at different resonant wavelengths due to two hybrid bands having PSP and LSP dominant properties, respectively, in the previous work. Therefore, the proposed structure becomes suitable for the dual detections of SPR signals and fluorescence images based on PSPs and LSPs, respectively, and this research presents the more practical perspective than the previous work dose. In detail, reflectance spectra and electric field profiles of the proposed structure are calculated in two-dimensional simulations, and experimental samples are prepared as a form of silver nanocubes (AgNCs) with silica (SiO2) shells (AgNC@SiO2s) on a thin Ag film. Their fluorescence image as well as spectral reflectance is measured in various angles of incident light. To accurately compare the results of the simulation and experiment, a statistical treatment for a spatial distribution of nanoparticles on a flat surface is conducted with the Weibull distribution model [21, 22]. Through the procedures mentioned above, we intend to demonstrate the effectuality of both simulation and experimental methods to confirm the feasibility of the dual detection using the proposed structure. 2. Methods 2.1 Simulations For the calculation of electromagnetic fields and reflectance on the proposed structure, our own-built rigorous coupled wave analysis (RCWA) tool is used [23, 24]. This numerical method firstly divides overall geometry as an array of layers, and calculate Fourier spectrum of each layer. Then, possible eigenmodes which can exist in those layers are obtained. Finally, coupling coefficients from incident wave to eigenmodes in each layer are calculated by using the extended transfer matrix method (ETMM). By appropriately summing propagating coupling coefficients backwardly for all of eigenmodes existing in the incident layer, we could obtain reflectance for a specific wavelength and incident angle. θ-λR map is calculated from the repeated process of the RCWA calculation, varying the condition of the incident angle and wavelength. 2.2 Synthesis of Ag nanocubes 50 mg/mL solution of silver nitrate (AgNO3, Sigma-Aldrich), 25 mg/mL of poly(vinyl pyrrolidone) (PVP, Sigma-Aldrich), and 5 mM solution of sodium hydrosulfide (NaHS, Sigma-Aldrich) in ethylene glycol (EG, J.T. Baker) were prepared. 8 mL of EG was heated at 150°C and stirred with a silicon coated magnetic bar for 1 h in a 20 mL glass vial. 0.05 mL of the sulfide solution, 1.8 mL of the PVP solution and 0.4 mL of the AgNO3 solution were sequentially injected into the heated EG solution. After 3-5 min of reaction time, the solution was quenched with cooling water. AgNCs were dispersed in ethanol after three times of rinsing processes of centrifugation, dilution, and sonication with acetone and ethanol. 2.3 Synthesis of SiO2 shell Three kinds of AgNC solutions which include undiluted, 1.25-fold and 20-fold diluted solutions were prepared by diluting the synthesized AgNC solution with ethanol. 25% solution of ammonia and 10% solution of tetraethoxysilane (TEOS, Sigma-Aldrich) in ethanol
#206502 - $15.00 USD (C) 2014 OSA
Received 14 Feb 2014; revised 22 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008383 | OPTICS EXPRESS 8386
were prepared. 7.6 mL ethanol was stirred in 20 mL glass vial and then 3 mL of the AgNC solution, 0.2 mL of the ammonia solution and 0.6 mL of the TEOS solution were sequentially injected into the ethanol in the vial. The mixed solution was stirred at room temperature for 12 h. Two kinds of fluorescence dyes which include fluorescein isothiocyanate (FITC) and rhodamine B isothiocyanate (RITC) were bound to SiO2 shell surface of AgNC@SiO2s using 3-aminopropyl triethoxysilane (APTES, Sigma-Aldrich). The AgNC@SiO2 solution was stirred with 0.1 mL of APTES (2mM), 0.1 mL of FITC (1mM), and 0.1mL of RITC (1mM) solutions at room temperature for 5 h. After reaction, excess reactants were washed away in the rinsing process. 2.4 Preparation of AgNC@SiO2 on Ag layer Ag layer with thickness of 50 nm was evaporated on the glass substrate with e-beam evaporator (KVE-3004, Korea Vacuum Tech.). The Ag layer on the glass was spin-coated with the three AgNC@SiO2 solutions at 200 rpm for 20 sec and then dried at 100°C for 2 min. 2.5 Characterization and optical measurement Surface morphologies of the samples were measured with a high resolution transmission electron microscope (HR-TEM; JEM-3010, JEOL) and a field emission scanning electron microscopy (FE-SEM; JSM 6700F, JEOL). Optical far-field images were measured by a modified optical microscopy setup which has a charge-coupled device (CCD; XCD-SX90CR, Sony) and the objective lens with the magnitude of 100 and the numerical aperture of 0.8 (LMPlanFLN, OLYMPUS Corp.). The samples were illuminated from the bottom by white light of a halogen lamp with changing the incident angle from 40° to 70°. Reflectance spectra were measured using a spectrometer (SM240, Spectral Products) at the same angle as that of incident light. 3. Results and discussion 3.1 Configuration of proposed structure PSPs excited on a thin metal layer in the Kretschmann configuration can have enough intensity to detect a subtle change of the refractive index of an analyte on the thin metal layer, and LSPs excited on metal nanoparticles can confine enough electromagnetic fields to enhance the fluorescence intensity of fluorophores around the metal nanoparticles. It is possible that PSPs and LSPs share the strong electromagnetic fields as a form of their interaction or coupling in a metallic nanostructure if the structure is designed to be partially or concurrently available to the excitations of both PSPs and LSPs. Therefore, the proposed structure, which is a combination of a metal layer and nanoparticles, can be suggested as shown in Fig. 1(a). By using this structure, we expect that the SPR dip signal will be detected in reflectance mode with a spectrometer and the fluorescence image will be observed in transmittance mode with an objective lens and CCD camera. Figure 1(b) shows the detailed geometries of the structure with the defined structural parameters. We selected a cube as a structure of nanoparticles because it has several advantages for the research for the following reasons. First, a cube is an appropriate polyhedron to stably contact face to face with a flat surface. The face-to-face contact provides a constant and parallel gap between planes, contrary to the edge-to-face contact of sphere-shaped particles and a flat layer. In respect of plasmonic properties, the electromagnetic fields can be more evenly confined and distributed in the plane-plane gap than the point-plane gap. Second, metal nanocubes can be generally synthesized in a regular shape and narrow size distribution in experiment [25]. Silver was chosen as metal materials of a thin layer and cubic cores because it has the longest propagation length of PSPs in noble metals. SiO2 shell around AgNC separates AgNCs from the Ag layer so that plasmonic couplings can be induced between them [14–16, 26], and prevents a direct contact between fluorophores and AgNCs so that the quenching effect is avoided [12, 27]. The reason for wrapping the dielectric shells around individual AgNCs instead of coating the dielectric layer on an entire Ag layer is that the sensitivity of SPR dip
#206502 - $15.00 USD (C) 2014 OSA
Received 14 Feb 2014; revised 22 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008383 | OPTICS EXPRESS 8387
signal may be raised by leaving the bare Ag surface, except AgNC@SiO2 particles, as wide as possible. The main parameters that affect the plasmonic properties of the proposed structure include the size of AgNC core (d), the thickness of SiO2 shell (t), the thickness of Ag thin layer (T), the inter-particle distance between AgNC@SiO2s (D), and incident angle (θ). In respect of required data, reflectance spectra at various incident angles, which can be converted into a graph of incident angle versus wavelength of reflectance (θ-λR map), should be calculated and measured. The θ-λR map can be an appropriate indicator to represent the existence and change of the surface plasmons, because the light absorption by the excitation of the surface plasmons causes a decrease in reflectance at the interface between an Ag layer and glass substrate in total internal refraction (TIR) of the Kretschmann configuration [1, 2]. 3.2 Simulations: characteristics of the minimum reflectance band on θ-λR map We calculated the reflectance spectra ranging from 400 nm to 800 nm at the incident angles from 40° to 80° using a simulation tool, RCWA [23, 24], to make a θ-λR map for one structure with fixed parameters. Based on the assumption that AgNC@SiO2s are homogeneously distributed on an Ag layer, one period (P) contains a cube and is equal to the inter-particle distance (D), as shown in Fig. 1(b). Two-dimensional (2D) simulations were performed, because they reduce computational time significantly compared to three-dimensional (3D) calculations. (The formers are appropriate for enormous calculations in scanning various structural parameters.) Although 2D simulations cannot provide the exact solution for our structure, we expect that they can be helpful in understanding the tendency of the optical property in our system under transverse-magnetic (TM) polarization with various incident angles because it is known that 2D simulations are available for TM polarization incidence, while 2D calculation results are different with those derived in the 3D model for transverseelectric (TE) polarization incidence [28–30]. Figures 2(a)–2(g) show that the θ-λR map changes with a consistent trend as the interparticle distance of AgNC@SiO2 array decreases from 3000 nm to 60 nm when d, t, and T are 30 nm, 10 nm, and 50 nm, respectively. The blue band on the map indicates the behavior of SPR signal, since the minimum reflectance is caused by surface plasmons excited at a specific angle and wavelength of incident light. The position and slope of the band for 2000 nm of the inter-particle distance are almost the same as those for the bare Ag layer without the nanoparticles (Figs. 2(a) and 2(b)). This clearly shows that a single nanoparticle on a wide Ag layer without their mutual interactions cannot affect the excitation characteristics of PSPs on the Ag layer. Therefore, incident light recognizes the surface of AgNC@SiO2s on the Ag layer with 2000 nm of the inter-particle distance as the surface of the bare Ag layer, so that the same type of surface plasmons, PSPs, can be excited. As the inter-particle distance decreases, however, the LSPs localized to respective AgNCs begin to interact with each other and thus the shape of the minimum reflectance band changes accordingly, and the slope of the band becomes steeper. In order words, the slope of the band increases as the population density of the nanoparticles on the Ag layer increases or the interaction between LSPs gets stronger. This is quite reasonable if we recognize that the steeper slope of the band indicates that the excitation condition of surface plasmons becomes lesser sensitive to the incident angle (more sensitive to the wavelength of the incident light). Since the excitation of LSPs is almost independent of the incident angle [2, 31], the overall reflectance spectra becomes lesser dependent on the incident angles as the LSP effects become more dominant. In addition, contrary to the continuous band in the case of the bare Ag layer, the bands for Ag layers with nanoparticles, as shown in Figs. 2(b)–2(e), are discontinuous. The number of the small bands decreases in a sequence while the particles go close to each other, and finally a single band with a rapid slope is formed. This discontinuity results from the Wood's anomaly effect [32, 33], i.e., the periodic distribution of the nanoparticles and thus, it disappears when we introduce a random distribution in the inter-particle distances (D), as will be shown in the later part of this paper.
#206502 - $15.00 USD (C) 2014 OSA
Received 14 Feb 2014; revised 22 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008383 | OPTICS EXPRESS 8388
Fig. 2. Changes of the minimum reflectance band on θ-λR map depending on inter-particle distance (D) among AgNC@SiO2s. θ-λR maps for AgNCs (30 nm) with SiO2 shell (10nm) on Ag layer (50 nm) in (a) an infinite, (b) 2000 nm, (c) 800 nm, (d) 350 nm, (e) 200 nm, (f) 100 nm, and (g) 60 nm of inter-particle distance. Hy-field profiles at (h) 44° and (i) 69.5° of different incident angles and 580 nm of same wavelength of incident light. Black dotted circles marked on the maps of (b) and (f) indicate positions where Hy-field profiles of (h) and (i) were calculated, respectively.
In order to investigate the reason for the increase in the slope of the minimum reflectance band in the case of a short inter-particle distance, we calculated Hy-field profiles at the two points marked with black open circles on the map, as shown in Figs. 2(b) and 2(f), where the reflectance values become the minimum at the wavelength of 580 nm. In the case of D = 2000 nm, Hy-fields are not confined to but pass over a single AgNC@SiO2 as if it did not exist, as shown in Fig. 2(h). This means that only PSPs contribute to the minimum reflectance band. On the contrary, in the case of D = 100 nm, Hy-fields are not only propagating along the flat Ag surface but also strongly confined to the SiO2 gaps between the AgNCs and Ag layer, as shown in Fig. 2(i). In other words, the resonant condition of PSPs on the Ag layer is significantly modified by the interaction with LSPs of the AgNCs. Therefore, the surface plasmon mode in this case can be interpreted as a hybrid state, generated by the coupling from the PSPs of the bare Ag layer to the LSPs of the AgNCs. In the hybridization, the dipole mode of LSPs may have stronger influence than the quadrupole mode because the dipole mode tend to be preferred in face-to-face configuration [34, 35], and the dipole mode of LSPs mainly contributes to the coupling with PSPs when an array of anisotropic particles, such as nanodisks and nanoellipsoids, is near a metal layer [36, 37].
#206502 - $15.00 USD (C) 2014 OSA
Received 14 Feb 2014; revised 22 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008383 | OPTICS EXPRESS 8389
Fig. 3. Effect of (a)-(c) Ag core size (d), (d)-(f) SiO2 shell thickness (t), and (g)-(i) Ag layer thickness (T) on the minimum reflectance band. The AgNC core sizes are (a) 20 nm, (b) 30 nm, and (c) 40 nm. The SiO2 shell thicknesses are (d) 10 nm, (e) 25 nm, (f) 35 nm. The Ag layer thicknesses are (g) 30 nm, (h) 50 nm, and (i) 70 nm.
The size of AgNC mainly influences the position of the minimum reflectance band on θ-λR map or the resonance wavelength, as shown in Figs. 3(a)–3(c), while the inter-particle distance determines the slope of the band. The resonance wavelength increases as the core size increases. This effect of the core size can be easily explained by the fundamental theory of LSPs excited at metal nanoparticles. Considering the LSPs of Ag nanoparticles dispersed in a solution or on a substrate, the resonance wavelength generally increases as the particle size increases, because free electrons should oscillate a long path length on surface of the large Ag particle [2, 31]. In the similar way, the minimum reflectance band of the structure with a large AgNC core is located in the longer wavelength region than that with a small core. In addition, because the increase of AgNC core size shortens the relative inter-particle distance, the enhanced plasmonic coupling may be induced between each particle and can cause an additional red-shift of the band. However, even if the size of the AgNC changes, the field profile or the so-called decay constant in the shell part of the nanoparticle is not altered significantly. Therefore, the strength of the LSP interaction, which determines the slope of the minimum reflection band, remains almost the same, and thus we cannot expect any noticeable changes in the slope of the minimum reflection band. Meanwhile, thickening SiO2 shell also causes a red-shift of the band because the enlarged dielectric portion increases the effective decay length to relatively shorten the inter-particle distance (Figs. 3(d)–3(f)), and thickening Ag layer hinders the excitation of PSPs to weaken the band (Figs. 3(g)–3(i)).
#206502 - $15.00 USD (C) 2014 OSA
Received 14 Feb 2014; revised 22 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008383 | OPTICS EXPRESS 8390
Fig. 4. (a) TEM image and (b) size distribution of AgNC@SiO2s. FE-SEM images of AgNC@SiO2s coated on Ag layer with (c) 20-, (d) 1.25-fold dilutions, and (e) undiluted solutions. They are named NC1000, NC200, and NC117, respectively. The scale bars are (a) 50 nm and (c)-(e) 500 nm. (f) Image of total internal reflection set-up with the hemi-cylindrical prism, white light source, spectrometer, and microscope lens. The polarizer makes incident light be TM polarization, and the incident angle is changed from 40° to 70° by a motorized rotation stage. CCD camera, which is not shown here, is adapted to microscopy.
3.3 Experiments: measurement of the minimum reflectance band on θ-λR map We were able to understand the characteristics of the minimum reflectance band on θ-λR map as results of scanning the various structural parameters in numerical simulations. In order to prove the simulation results, we tried to experimentally observe the difference of the band slopes for the samples having different inter-particle distances, since the change of the band slope is the representative phenomenon of the plasmonic characteristics in the suggested structure. We prepared samples by spin-coating AgNC@SiO2 nanoparticle solutions with different concentrations on a thin Ag layer. AgNCs were synthesized with the polyol method [38] and surrounded with thin SiO2 shell in the Stöber process [39]. AgNC is a cubic structure and SiO2 shells are wrapped around the AgNC cores with approx. 6 nm of thickness, as shown in TEM images of Fig. 4(a). The average size and standard deviation of AgNC@SiO2s is approx. 36.4 ± 2.9 nm and the size distribution is shown in Fig. 4(b). It can also be confirmed that most of the AgNC@SiO2s contact face to face with a flat surface. AgNC@SiO2s are distributed with three different population densities on the Ag layer with 50 nm thickness, as shown in FE-SEM images of Figs. 4(c)–4(e). The population density can be controlled with the concentration of the nanoparticle-dispersed solution. Three kinds of solutions were prepared by adding ethanol to the synthesized AgNC@SiO2 solution, which includes 20-, 1.25-fold dilutions, and the undiluted solution. The FE-SEM images indicate that the population density increases while the inter-particle distances decrease in proportion to the concentration of AgNC@SiO2 solution. The average values and standard deviations of the inter-particle distances are 200.3 ± 84.9 nm and 117 ± 40.1 nm in Figs. 4(d) and 4(e), respectively. The values of the sample coated with 20-fold diluted solution could not be measured in Fig. 4(c) due to a large deviation of the inter-particle distances, and we supposed the average distance is 1000 nm. The three samples of Figs. 4(c)–4(e) were named NC1000, NC200, and NC117, respectively, for convenience. The distribution graphs of the interparticle distances will be presented in Fig. 6(a) in order to efficiently use them in a later statistical treatment. In addition, two kinds of fluorophores, which are FITC and RITC, were added to all of the samples in order to clearly distinguish the angle-dependent resonances and
#206502 - $15.00 USD (C) 2014 OSA
Received 14 Feb 2014; revised 22 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008383 | OPTICS EXPRESS 8391
fluorescence images. The transmittance images and the reflectance spectra of the samples are measured with the CCD camera and the spectrometer, which are adapted to the home-made optical microscopy setup, as shown in Fig. 4(f). The setup satisfies the Kretschmann configuration since the polarizer before the white light source keeps a TM polarization and a motorized rotation stage controls the incident angle.
Fig. 5. Experimental θ-λR maps of (a) NC1000, (b) NC200, and (c) NC117.
The reflectance spectra of NC1000, NC200, and NC117 were measured at least for five different samples by changing the incident angle, and then three experimental θ-λR maps were completed as shown in Fig. 5. Three experimental θ-λR maps display the clear SPR dips of the reflectance spectra and three different slopes of the minimum reflectance bands which are expressed in the series of SPR dips. In the case of NC1000, the SPR dips are relatively deep and the band slope is gentle, as shown in Fig. 5(a). The position and shape of the band are similar to those for the bare Ag layer with 50 nm thickness, because the sparsely distributed AgNC@SiO2s weakly affect the PSPs on the Ag layer. As the population density of the particles increases, i.e., the inter-particle distances decrease, the band slope increases, as shown in Figs. 5(b) and 5(c). Therefore, the experimental spectrum data can demonstrate the calculated results of Fig. 2 in spite of the weak spectral dips. The widths of the reflectance dips of NC117 are broader than those of NC1000 due to radiation losses and a spatial deviation of AgNC@SiO2s. The nanoparticles partially play a role as radiative scatterers to cause radiation losses in the coupling between PSPs and LSPs, and the deviation of the distributed AgNC@SiO2s disturbs the accurate periodicity to broaden the width of the spectral dips. Because a large deviation of the inter-particle distances in NC200 increases the spatial randomness more than that in NC117, the spectral dips of NC200 become much weaker than those of NC117. 3.4 Comprehensive analysis of plasmonic hybrid property There are some structural differences between simulation and experimental conditions in the strict sense. In experiment, AgNC@SiO2 particles are settled on the Ag layer with a certain distribution of the inter-particle distances because they are randomly scattered in the process of spin-coating, while they are located at exact lattice points with a constant distance in simulation. Although the sizes of AgNCs also have a certain distribution in experiment, the size distribution affects the plasmonic properties of the samples less than the distribution of the inter-particle distances because the latter is thirteen times wider than the former; for the
#206502 - $15.00 USD (C) 2014 OSA
Received 14 Feb 2014; revised 22 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008383 | OPTICS EXPRESS 8392
example of NC117, the deviations of the inter-particle distance and particle size distributions are 40.1 nm and 2.9 nm, respectively, as shown in Figs. 6(a) and 4(b). Therefore, we will consider only the distance distribution in calculation so as to compare the outcomes between simulation and experiment as accurately as possible. We measured the inter-particle distances of AgNC@SiO2s on the Ag layer from the FE-SEM image of NC117 in Fig. 4(e), and then calculated the average (μ) and standard deviation (σ) of the distance distribution with a statistical method, as shown in Fig. 6(a). The distribution does not follow a normal distribution because it is not bilaterally symmetric and has a tail in long inter-particle distances. Therefore, we introduced the Weibull distribution model to fit it to the real distribution of inter-particle distances. Since the Weibull distribution is typically determined by a scale parameter (α) and shape parameter (β) [21, 22], we were able to make the Weibull distribution similarly fit to the inter-particle distance distribution of NC117 through a reverse calculation of α and β from μ and σ of the experimental distribution in Fig. 6(a), as shown in Fig. 6(b). The modified structure, which has fifty particles of AgNC@SiO2 with random interparticle distances of the fitted Weibull distribution in one period, was calculated under the same conditions as the previous simulations without considering the distribution, as shown in Fig. 6(c). In Fig. 6, three different types of data, which are the experimental θ-λR maps, simulation θ-λR maps, and fluorescence images, are correlated for NC1000 and NC117, which have the contrasting population density of AgNC@SiO2s. The SPR dips in the experimental θ-λR maps are marked with circular yellow dots on the same positions of the calculated θ-λR maps in Figs. 6(d) and 6(h) to compare with each other. The slopes of the experimental bands almost coincide with those of the calculated bands although the minimum reflectance band is blurred and the intensity is weakened when the Weibull distribution is introduced to simulation. The band especially becomes blurrier at a short wavelength than at a long wavelength. The reason is that light of short wavelengths may be more sensitive to the nanoparticle distribution with the same standard deviation and then enables to recognize the distribution more sensitively. We can also confirm that the slope of the minimum reflectance band is steeper for NC117 than NC1000. The invariable property of the band position and slope, therefore, enables to measure the series of SPR dips and to confirm the existence of the band on the θ-λR map even though AgNC@SiO2s are randomly scattered in the experiment. The validity of 2D simulations can also be demonstrated by comparing the simulation data with these representative experimental results. Using these summarized data, we present a potential to simultaneously detect SPR signals and fluorescence images. Figures 6(e)–6(g) and 6(i)–6(k) are corresponding fluorescence images of the samples at different points on the bands. Most of all, the fluorescence intensity is stronger in NC117 than NC1000 because the electromagnetic fields localized on the particles stimulate the fluorophores, such as FITC and RITC, around AgNC@SiO2s. On average, the fluorescence intensity of NC117 is enhanced approximately 2.3 times higher than that of NC1000. Additionally, the steep slope of the band enables fluorophores to clearly emit their instinct color in a wide range of incident angles. The wavelength and color of fluorescence change with high intensity in the wide incident angles from ~600 nm of red by RITC at 51° to ~520 nm of green by FITC at 57° in Figs. 6(i)–6(k), while they unclearly change in the narrow angles between 44° and 42° in Figs. 6(e)–6(g). Therefore, we can acquire qualitative fluorescence images as well as quantitative SPR signals using the plasmonic hybrid characteristic of the proposed structure.
#206502 - $15.00 USD (C) 2014 OSA
Received 14 Feb 2014; revised 22 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008383 | OPTICS EXPRESS 8393
Fig. 6. (a) Inter-particle distance distribution of NC117, (b) the Weibull distribution calculated with experimental μ and σ of NC117, and (c) schematic illustration of a structure and parameters for simulation including the Weibull distribution. Comparison of three kinds of data for (d)-(g) NC1000 and (h)-(k) NC117. (d) and (h) are simulation θ-λR maps including the Weibull distribution factor. The circular yellow dots indicate the minimum reflectance dips in the experimental data of Fig. 5. The corresponding fluorescence images are measured at (e) 44°, (f) 43°, (g) 42°, (i) 57°, (j) 55°, and (k) 51° of incident angle. The scale bars are 5 μm.
4. Conclusion In this research, we investigated the hybrid states of PSPs and LSPs for the simultaneous dual detection of SPR signals and fluorescence images in the Kretschmann configuration. Although PSPs and LSPs generally have conflicting properties, i.e., propagation and localization of surface plasmon, they can coexist as a hybrid form of PSP and LSP on the proposed structure because the resonant condition of PSPs on the thin Ag layer is significantly modified by the interaction between LSPs of the AgNC@SiO2s. As results of scanning the structural parameters, such as the inter-particle distance and size of AgNC@SiO2s, we found that they affect the slope and position of the minimum reflectance band on θ-λR map. The simulation results were confirmed by experimentally measuring series of SPR signals and fluorescence images of AgNC@SiO2s on the Ag layer at various incident angles in the Kretschmann configuration. In particular, the real distribution of the inter-particle distances measured in experimental samples was fitted to the Weibull distribution model and then the statistical data were fed back to the calculation process. The recalculated results significantly agreed with the experimental θ-λR maps and fluorescence images. This study suggested a potential promise in usefully applying the plasmonic hybrid property to dual detections of SPR signals and fluorescence images. The proposed structure, which is the thin metal layer combined with nanoparticles having metal cubic cores and dielectric shells, will be used in controlling the minimum reflectance band for a certain purpose because the slope and position of the band can be freely tuned with structural parameters. In addition, if the standard deviation is reduced in the spatial distribution of the #206502 - $15.00 USD (C) 2014 OSA
Received 14 Feb 2014; revised 22 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008383 | OPTICS EXPRESS 8394
particles through improving fabrication processes, the sensitivity will also be improved owing to the narrower and clearer band, although it became broad or blur because the nanoparticles were randomly scattered in this research. Therefore, the plasmonic hybrid properties of PSPs and LSPs on the suggested structure are expected to be useful in various practical applications, such as biological and chemical sensors. Acknowledgments This work was supported by the National Research Foundation of Korea funded by Korean government (MSIP) through the Creative Research Initiatives Program (Active Plasmonics Application Systems).
#206502 - $15.00 USD (C) 2014 OSA
Received 14 Feb 2014; revised 22 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008383 | OPTICS EXPRESS 8395
