Optimal Interparticle Gap for Ultrahigh Field Enhancement by LSP

Nov 30, 2016 - To confirm this more precisely, scanning electron microscopy (SEM) measurements were performed to translate the AuNP percentage concent...
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Optimal Interparticle Gap for Ultrahigh Field Enhancement by LSP Excitation via ESPs and Confirmation Using SERS Sachin K. Srivastava,†,§ Anran Li,†,§ Shuzhou Li,*,† and Ibrahim Abdulhalim*,†,‡ †

NTU-HUJ-BGU NEW CREATE Programme, School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798 ‡ Department of Electrooptic Engineering & Ilse Katz Institute for Nanoscale Science and Technology, Ben Gurion University of the Negev, Beer Sheva 84105, Israel S Supporting Information *

ABSTRACT: We have predicted extremely high electromagnetic hot spots using the extended−localized coupled surface plasmon resonance configuration. With this unique configuration, we found that an array of particles shows the critical importance of the interparticle gap on the enhancement factor, which was confirmed experimentally using surface-enhanced Raman scattering (SERS). The extended plasmon wave excited in the Kretschmann−Raether configuration propagates on the silver film surface and couples with the gold nanoparticles dispersed on top through excitation of the localized plasmons. A monomolecular layer of 4-aminothiophenol sandwiched between the metal film and the nanoparticles showed an SERS enhancement factor of the order of 1010 per molecule in the hot spots. The configuration was optimized, both by simulations and experiments, with respect to the size of the nanoparticles and the interparticle distances. It is demonstrated that the ultrahigh SERS enhancement does occur only when the extended surface plasmon is coupled to the localized surface plasmon at an optimized interparticle gap. Further, highly sensitive detection of glycerol in ethanol is demonstrated using the optimum structure with a detection limit on the order of 10−12 to the weight percentage of ethanol, which is equivalent to detection of a few molecules. This ultrahigh enhancement is useful in realizing various highly sensitive biosensors when strong enhancement is required as well as in highly efficient optoelectronic and energy devices.

1. INTRODUCTION The field of plasmonics has fascinated researchers for more than a century. However, the development of the field for the last three decades has played an important role in bringing out intriguing new phenomena (such as extraordinary transmission, surface-enhanced spectroscopy, enhanced local heating, etc.) and variety of applications.1−9 Some of these applications are widely known in the field of sensing, wave-guiding, modulation, metamaterials, theranostics, localized heating, enhanced solar cells, etc.10−14 Plasmons are basically the quanta of longitudinal oscillations of free electrons in free electron-rich materials, such as metals.15 On the basis of the dimension of the metal and the configuration of the structure, various kinds of plasmonic excitations are possible. For example, plasmon oscillations at the interface of a two-dimensional metal surface and a dielectric medium are called propagating or extended surface plasmons © 2016 American Chemical Society

(ESPs), while those in metallic nanoparticles of dimensions much smaller than the wavelength of the incident light are called localized surface plasmons (LSPs).15,16 Both kinds of plasmons have interesting physical phenomena and applications to their credit. For example, ESPs have been extensively used in biosensing applications, while LSPs play a crucial role in electromagnetic field enhancements in enhanced spectroscopies and theranostics.15,17−19 Attempts were made to study the coupling between ESPs and LSPs.20−24 A large number of articles were reported for such studies, and most of the efforts had been toward exciting ESPs via LSP excitation.25−27 Such configurations of metal nanoparticles over metal film separated Received: August 16, 2016 Revised: November 4, 2016 Published: November 30, 2016 28735

DOI: 10.1021/acs.jpcc.6b08276 J. Phys. Chem. C 2016, 120, 28735−28742

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The Journal of Physical Chemistry C by few nanometers were solved as a mirror image problem, and the hot spot was considered to be between the nanoparticle and its mirror image.5,26−32 These analyses did not consider the excitation of ESPs which requires matching of the ESP wave vector to that of the incident light under appropriate conditions. Some reports did study the excitation of LSPs via coupling them to ESPs, as it was understood recently that this mode of coupling has a potential for ultrahigh enhancements and new dimensions of research.33−35 Sarkar et al. studied the coupling of propagating plasmons to metallic cylinders of varying heights and 50 nm diameters separated by about 200 nm spacing and reported the coupling between the ESPs and Bragg modes of the metallic structures array.35 Mock et al. reported the spectroscopic response of free space and prism excitations as a function of particle−film separation in spherical gold nanoparticles coupled to Au film.34 In the other study by Balci et al., the coupling between localized and propagating surface plasmons was reported for strong coupling.33 None of these studies could reach the optimum conditions of LSP excitation using ESPs. Moreover, the interparticle spacing between the nanoparticles plays an important role, which needed more serious observations.36,37 Very recently, our group has realized that excitation of LSPs via ESPs can lead to ultrahigh enhancements of electromagnetic fields.38 To realize such a coupling, one needs to excite ESPs to couple them to LSPs. This can simply be achieved by exciting ESPs in a Kretschmann−Raether (K−R) configuration, where the base of a high index right angle prism is coated with a thin layer of metal and the light incident at the interface of the metal and surrounding medium through the prism excites ESPs under momentum matching condition. Now, when metallic nanoparticles (MNPs) or other metallic nanostructures (such as holes) are kept in the neighborhood of the metallic film, at certain favorable conditions, the ESPs may couple to LSPs, exciting them on the surface of MNPs. The space between the continuous metal film and the MNP is called a hot spot, as the electromagnetic field in this space is manifold to that of the incident light. In our preliminary work, we demonstrated a surface-enhanced fluorescence (SEF) signal from Rhodamine 6G on such a configuration, where LSPs on 40 nm gold nanoparticles (AuNPs) were excited by the ESP field from closely placed continuous silver (Ag) film in a K−R configuration.38 Even though the concept of ultrahigh field enhancement was proven, the optimum limit was not yet achieved. Further, precise experimental demonstration of the findings of the simulations was a challenge as the determination and control of the number of molecules in the hot spot was difficult. In addition, the SEF signal is influenced by the bleaching phenomenon and other interactions between the fluorophore and the metal. Hence, the configuration needed more investigation, both theoretically and experimentally, using SERS because the SERS enhancement factor is now well confirmed to represent the enhancement of the electromagnetic field.25,39 To make it more precise and controlled, we performed simulations to optimize the size and interparticle distance (g) of AuNPs for the maximum field enhancements.

Figure 1. Schematic of the experimental setup.

continuous Ag film deposited on an SF11 glass substrate (of refractive index ∼1.78). The horizontal gap between two adjacent Au NPs is represented by g. The Ag-coated substrate loaded with the 4-ATP molecules and the AuNPs was fixed on the base of a right angle prism with the help of an indexmatching liquid. A collimated beam of p-polarized light from a fiber optic laser of 785 nm wavelength was incident on the structure through the prism, while the SERS signal was collected from the top of the prism, as shown in the schematic diagram. A very narrow band-pass filter was incorporated in the excitation beam to filter out the harmonics of the laser, while a long pass filter was used to let the laser light block and only the Stokes frequencies pass through, which were further fed to a Raman spectrometer. The light reflected from the interface was collected from the other end face of the prism by an optical power meter to determine the ESPR angle, which corresponds to the lowest value of the optical power. The electromagnetic field enhancement on such a configuration was estimated by the FDTD method40 by Lumerical Solutions, Inc. (Vancouver, Canada). In order to simulate the infinite Ag film decorated with the Au NPs, periodic boundary conditions were applied in all of the calculations. The dielectric functions of Au and Ag were obtained from Johnson and Christy41 and Palik,42 respectively. The tests for convergence of simulations were carefully performed to verify the accuracy and stability of the calculations. In this study, the separation between the Au NPs and the Ag film was considered to be 2 nm. However, the effect of AuNP separation from the Ag film on the electromagnetic enhancement was thoroughly studied and is presented in Figure S1 of the Supporting Information. In Figure 2, the variation of the magnitude of electromagnetic enhancement with the increase in the diameter of the Au nanoparticles is plotted. Figure 2a presents the electromagnetic field profiles for the ESPR−LSPR configuration with the AuNPs of different sizes. The colored bars represent the values of the enhanced power relative to incident electromagnetic power on a logarithmic scale. It is observed that with an increase in the size of the nanoparticle the electromagnetic power enhancement increases, reaches a maximum, and then decreases with further increase in size. It can be visualized more clearly in Figure 2b, where we have plotted the variation of the hot spot maximum power (max E2) as a function of particle diameter. It is obvious that around a particle diameter of about 80−100 nm the electromagnetic enhancement was the maximum. The reason behind such an enhancement is that when the particle size increases it leads to an increase in the LSPR wavelength, and at the optimum particle size, the coupling between the LSPs and the ESPs becomes the

2. RESULTS AND DISCUSSION Figure 1 shows the schematic diagram of the ESPR−LSPR configuration and the experimental setup. Following the theoretical design, in the optimum configuration, a monolayer of 4-aminothiophenol (4-ATP) molecule was sandwiched between AuNPs of 80 nm diameter and 47 nm thick 28736

DOI: 10.1021/acs.jpcc.6b08276 J. Phys. Chem. C 2016, 120, 28735−28742

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Figure 2. Effect of the Au nanoparticle diameter on the electromagnetic enhancement: (a) field profile; (b) variation of max (E2) with AuNP diameter.

oscillations occur, which might further couple to transverse hybridized modes to result in various superhybrid modes as a result of partial Bragg-like diffractions from the metallic nanoparticles decorated at the optimum structure.44 These factors, though, play a small role in deciding the optimum g. Even though the Max E2 is obtained for almost a single AuNP on the Ag surface, the optimal SERS enhancement is observed when there is a sufficient number of hot spots uncoupled with each other. The maximum value obtained at 1300 nm in the average field intensity is also a geometrical effect because after the particles are well isolated from each other any increase in g results in an increase in the total area, which then causes the average enhancement to begin decreasing. To confirm the theoretical findings, we used the 47 nm Ag/4-ATP/AuNP structures with varying AuNP sizes and then that with optimum AuNP size and different g values. To obtain such a configuration, first, the Ag-coated substrates were incubated in 1% 4-ATP solution in ethanol for 1 h and then incubated in the colloidal gold sols. The 4-ATP molecules bind to the Ag surface because of the thiol bonds, while the AuNPs being negatively charged bind electrostatically to the −NH2 moiety of 4-ATP, which in aqueous solution serves as NH3+. The anchoring of AuNPs depends on the degree of 4-ATP concentration on the Ag surface. To ensure a repeatable structure, the concentrations of the solutions, the incubation times, the shaker speed, etc., were kept the same for all the set of experiments. Further, complete coverage of the Ag surface by 4-ATP was ensured by choosing a concentration larger than the number of 4-ATP molecules required for the purpose. Briefly, to cover an Ag-coated slide of 14 × 21 mm2 dimension with 4ATP molecules of an area of 0.2 nm2, 1.47 × 1015 molecules are required. Considering the density of solid 4-ATP 1.18 g/cm3 and molecular weight 125.19 g/mol, a 1 mL solution of 1% 4ATP in ethanol has 5.68 × 1021 4-ATP molecules, a number larger than that required for full surface coverage. The incubation time for anchoring the Au NPs on Ag surface was 1 h. In Figure 4a, we demonstrate the SERS spectra recorded from 4-ATP on Ag/4-ATP/AuNP structures with AuNPs of 40, 80, and 150 nm diameters. The SERS bands of 4-ATP attached on the chip can be observed for all of the AuNP sizes. However, the maximum SERS enhancement is observed at 80 nm diameter, which is the same as predicted by the simulations. Hence, we chose the AuNPs of 80 nm diameter and fabricated the Ag/4-ATP/AuNP 80 nm structures with different concentrations of AuNPs to further study the effect of g on enhancement. The Ag/4-ATP substrates were incubated in the aqueous colloidal solutions of different percentages (v/v) of

maximum, which further leads to enhanced electromagnetic field in the hot spots. Following the simulations, the AuNPs of 80 nm diameter were chosen for further analysis. It is not only the size of the nanoparticles which is responsible for the enhanced coupling between the nanoparticles but also the interparticle distance (g), which plays a crucial role in determining the maximum enhancement at a particular wavelength. To investigate this point, we have further performed simulations to understand the effect of g on the performance of the coupled structure. In Figure 3, we have

Figure 3. Effects of the horizontal gap g between two Au NPs on the maximum (left) and average (right) electric field enhancement in Au NP array over Ag film at 785 nm with 34.645° incidence angle inside the prism.

plotted the variations of the maximum (left Y axis) and average (right Y axis) power enhancements as a function of g. It can be observed that with an increase in g the maximum E2 increases and becomes almost constant around g ≈ 1300 nm, while the average enhancement increases (see discussion below) to a maximum around g = 1300 nm and then starts to decrease. Based on the above-mentioned simulations, it can be concluded that the optimum structure for maximum enhancement at the wavelength 785 nm is with a nanoparticle of diameter 80−100 nm and g about 1300 nm. The first parameter, i.e., the diameter of the nanoparticle, is determined mainly by the wavelength and the surrounding dielectric medium as well as the interaction between LSPs themselves, which becomes negligible above a certain g, while the optimum g is affected by several parameters such as the wavelength, the particle size, the interaction between the LSPs on the different nanoparticles, and the total geometrical area. In such a configuration, a monomolecular cavity is formed between the metallic nanoparticle and a continuous metal film. Under suitable conditions, the plasmon and the cavity modes are hybridized.43 Further, due to ESPs, longitudinal 28737

DOI: 10.1021/acs.jpcc.6b08276 J. Phys. Chem. C 2016, 120, 28735−28742

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urations of varying g, confirmed by FESEM, was obtained and possessed closely similar SERS enhancements. Respective SERS spectra have been plotted in the Figure 4b for the Ag/4ATP/AuNP 80 nm substrates with varying concentrations (v/v) of AuNPs in water. It can clearly be observed that the SERS signal is not the same for all the AuNPs percentages, which clearly means that the SERS enhancement is a function of the AuNPs distribution over the Ag/4ATP substrate. It can more accurately be observed in the inset, where we have plotted the variation of the SERS signal at 1077 cm−1 Raman peak of 4-ATP with varying AuNPs % (v/v). It is observed that with an increase in the AuNPs percentage the SERS signal increases, reaches a maximum around 30% concentration, and then decreases with further increase in the AuNP concentration. The error bars represent the repeatability and reliability of the experiments, as they were estimated by a set of measurements on different days and times and considering the accuracies of the pipettes, laser, and the spectrometer. Hence, it is concluded that 30% (v/v) AuNPs solution provides the optimum g for maximum enhancement. To confirm this more precisely, scanning electron microscopy (SEM) measurements were performed to translate the AuNP percentage concentration to g. This has been discussed in Figure 5, but before we approach there, let us pause to mention one very important point here that such a high enhancement is observed only at the ESP resonance angle. To prove the point that there is a coupling of ESPs to LSPs to form a hot spot, the SERS spectra were recorded for on- and off-SPR conditions at an Ag/4-ATP/AuNPs configuration. Additionally, these experiments were supported by that performed on the Ag/4-ATP configuration for the SPR resonance case. The corresponding SERS spectra have been plotted in Figure 4c. It has been observed that no enhanced SERS signals are observed on SPR resonance conditions from Ag/4-ATP substrates when they are used without AuNPs. However, the ultrahigh SERS enhancement is observed on the SPR resonance when ESPs are excited at an angle of 48.37° on the SF11 substrate and Ag film interface and coupled to the LSPs. When the incidence angle is moved away by a few degrees (5° in the present case) to an offSPR resonance angle, no enhancement in the SERS signal is observed. This again backs the concept of ultrahigh enhancement due to ESP-LSP coupling. Figure 5 presents the SEM images for the various percentages of 80 nm AuNPs over the Ag/4-ATP substrate. The SEM images were processed with ImageJ45 freeware with a porosity analysis plugin46 to estimate the g distribution and the average g. In the same figure, we have attached the estimated histograms of particle distribution with different g values and average g values (gAv) with the respective AuNP concentrations. From the number, it can be concluded that gAv for optimum SERS enhancement which corresponds to the 30% concentration is around 1 μm. This is fairly close to the simulated results. A slight discrepancy from simulations occurs because they give the maximum enhancement in hot spots while the experiments consider the whole area of the structure illuminated by the beam, thereby leading to averaged field enhancement. Another important reason is the fact that the nanoparticles are not evenly dispersed on the surface, while the simulations were done for an ideal case of periodically assembled nanoparticles. The interparticle distance “g” is a crucial parameter for efficient coupling of ESPs to LSPs and hence ultrahigh SERS enhancement. Among various efforts for achieving high electromagnetic enhancements using coupling of ESPs to LSPs, this parameter was never considered to the best

Figure 4. SERS spectra (a) with variation in Au nanoparticle diameter; (b) from Ag/4-ATP/80 nm AuNP structure with varying AuNP solution concentration (inset: variation of the SERS peak signal@1077 cm−1 with AuNP concentration); (c) for on-SPR resonance at Ag/ 4ATP configuration and on- and off-SPR resonances at the optimum Ag/4ATP/AuNP structure.

AuNPs of 80 nm for 1 h to form a monolayer of AuNPs with different g values at the Ag/4ATP substrate. A colloidal solution of 80 nm AuNPs with 1.1 × 1010 particles/mL was diluted in water to achieve sols of varying concentrations ranging from 5 to 60% (v/v). The preparation of Ag/4-ATP/AuNPs substrates was repeated more than 5 times by Ag depositions and 4-ATP and AuNPs incubations on different days and times for a period of 5 months. With the calibrated concentrations and incubation time, good repeatability of the Ag/4-ATP/AuNPs config28738

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Figure 5. SEM images of Ag/4-ATP/80 nm AuNP structure with varying AuNP concentrations. The insets show the histogram of the particle distribution andthe averge g value (gAv).

of our knowledge, and that is why the electromagnetic enhancements reported were of the orders of ∼100.47 Study of a denser packing of AuNPs over the Ag film is less relevant as when the AuNP concentration over the Ag surface increases the overall thickness of the effective plasmonic film increases, leading to the deviation from the condition of optimum

thickness for ESPR. Hence, no ESPR, confirmed from the measurement of the optical power of light reflected from the interface, was observed for densely packed configurations. The optimized structures were used to demonstrate highly sensitive detection of glycerin in ethanol as a model experiment. The SERS spectra for different volume percentages of glycerin in 28739

DOI: 10.1021/acs.jpcc.6b08276 J. Phys. Chem. C 2016, 120, 28735−28742

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3. CONCLUSIONS The ultrahigh enhancement of the electromagnetic field near the gold nanoparticle predicted recently when LSPs are excited via ESPs is confirmed experimentally using SERS signals from the 4-ATP monomolecular layer. The enhancement correlates well with the extremely large electromagnetic field enhancements in the hot spots created between the metal nanoparticle and the continuous metal film. The ESP−LSP configuration was optimized with respect to the AuNP particle sizes, vertical distance between AuNPs and the metal film, and the interparticle distance (g) of AuNPs on Ag film. Further, the effect of the excitation wavelength on such configurations was compared with previous studies to make a generalized model. It was confirmed through control experiments that the ultrahigh SERS enhancement is achieved only for the optimum conditions of ESP-LSPR coupling. Highly sensitive detection of glycerin in ethanol was demonstrated as a model experiment. The present study can be used for designing specific configurations requiring ultrahigh field enhancements in the fields of biosensing, theranostics, energy applications and optoelectronics. This configuration can be applied to many other cases of LSP excitation through ESP waves, and we believe it opens a new niche in the plasmonics field and its applications.

ethanol were recorded using an experimental setup as shown in Figure 1 while adding the sample solution over the Ag/4-ATP/ AuNP chip. Sample solutions of glycerin in ethanol were prepared from volume percentages of 10−10−10−2. Further, the volume percentages of glycerin in ethanol were translated to weight percentages (wt %) varying from 1.6 × 10−10 to 1.6 × 10−2 for convenience of representation. In Figure 6, the SERS

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Figure 6. SERS spectra for varying weight percentages of glycerin in ethanol (inset: variation of the SERS signal at 1077 cm−1 Raman peak with Log10 (wt %)).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b08276. Variation of electromagnetic enhancement with increase in the spacer thickness between continuous metal film and the AuNP, effect of excitation wavelength on optimum g, effect of NP material on enhancement, and effect of metal film material on the enhancement and the estimation of enhancement factor (PDF)

spectra for varying wt % of glycerin in ethanol are presented. SERS spectra similar to that of 4-ATP with varying SERS enhancement were observed for changing glycerin wt %. The reason for similar SERS spectra is that the 4-ATP molecules were also present in the hot spots and contribute largely to the overall response. It is observed that with an increase in the glycerin wt %, the SERS enhancement increases. However, a quantitative estimation of increase in SERS signal with respect to wt % cannot be made from the raw data. For more precise estimation, the SERS spectra were referenced to zero to predict the actual change in SERS signal with increase in glycerin wt %. In the inset of Figure 6, we have plotted the variation of SERS signal at 1077 cm−1 Raman band with change in glycerin wt %. Since a large range of glycerin wt % was considered, the X-axis is taken to be logarithmic of the wt %. It is quite obvious from the inset that with an increase in concentration, the SERS signal increases, which is obviously due to an increase in the Raman signal with an increase in the amount of glycerin with increasing wt % on the chip. The error bars were added by incorporating the variation on SERS signals and accuracy of measurement of the pipettes. It can also be observed that we could monitor concentrations as small as 1.6 × 10−10 wt % of glycerin in ethanol easily, which is equivalent to about 700 glycerin molecules. Considering the signal-to-noise ratio of the spectrometer (450:1), we can estimate our detection limit to be of the order of 10−12 wt % and thus reaches the limit of detection of a few molecules. With further tight focusing, the system may be improved to even a single molecule detection limit. Because of its low viscosity, ethanol was chosen as solvent because we wanted to ensure that the glycerin molecules can undoubtedly reach into the hot spots.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +6567904380. *E-mail: [email protected]. Tel: +97286479803. ORCID

Shuzhou Li: 0000-0002-2159-2602 Author Contributions §

S.K.S. and A.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was conducted under the NTU-HUJ-BGU Nanomaterials for Energy and Water Management Programme in the Campus for Research Excellence and Technological Enterprise (CREATE), supported by the National Research Foundation, Prime Minister’s Office, Singapore.

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DOI: 10.1021/acs.jpcc.6b08276 J. Phys. Chem. C 2016, 120, 28735−28742