Electromagnetic Enhancement in Shell-Isolated Nanoparticle

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Electromagnetic Enhancement in Shell-Isolated NanoparticleEnhanced Raman Scattering from Gold Flat Surfaces Shu Chen, Zhilin Yang, Lingyan Meng, Jian-Feng Li, Christopher T Williams, and Zhong-Qun Tian J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b01254 • Publication Date (Web): 10 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Electromagnetic Enhancement in Shell-Isolated Nanoparticle-Enhanced Raman Scattering from Gold Flat Surfaces Shu Chen, 1 Zhilin Yang, 1, * Lingyan Meng, 1 Jianfeng Li, 2 Christopher T. Williams, 3 and Zhongqun Tian2 1

Department of Physics, Xiamen University, Xiamen 361005, China.

2

State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry,

College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China. 3

Department of Chemical Engineering, University of South Carolina, Columbia 29208, USA.

ACS Paragon Plus Environment

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ABSTRACT The electromagnetic (EM) enhancement in shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) was systematically analyzed for the first time by using the three dimensional finite difference time domain (3D-FDTD) method. Compared with a bare metallic nanoparticle directly contact to a gold flat surface, a hot spot with stronger EM enhancement occurs in the junction between silica shell-isolated nanoparticle and flat surface in SHINERS. The largest enhancement factors can be as high as nine orders of magnitude. The exact locations of hot spots and their EM enhancement strongly depend on the thickness of the silica shell, with an optimal thickness of 1 or 2 nm under 633 nm laser excitation. The dependence of the EM enhancement on both particle size and silica shell thickness was also investigated. The results further demonstrate that the SHINERS should be useful as a general diagnostic tool for surface science.


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INTRODUCTION Surface-enhanced Raman scattering (SERS)

1-4

affords a powerful tool to unveil molecular

structure and reaction pathways occurring on solid surfaces. Usually, massive spectral enhancement can be achieved only on metal nanostructures with roughness or tiny gaps between the adjacent metallic objects, 5-12 which support propagating or localized surface plasmon modes, 4, 13-16

respectively. However, these nanostructures are not good candidates to study the

interfacial structure and reactivity occurring on atomically flat surfaces. The desire to elucidate the intrinsic mechanisms of chemical reactions on either flat surfaces or SERS-inert catalysts has stimulated development of new SERS substrates over the last decade. Tip-enhanced Raman spectroscopy

(TERS)17 and

shell-isolated

nanoparticle-enhanced

Raman

spectroscopy

(SHINERS) 18 are two typical methods to carry out such investigations. The SHINERS method proposed by our group utilizes a SERS-active metal nanoparticle (NP) surrounded by a chemically inert dielectric shell (i.e., SiO2).18-20 Shell-isolated NPs above the flat surface acts as the Raman signal amplifiers, similar with a gold tip in TERS. Importantly, SHINERS not only keeps the main advantages (i.e. high detection sensitivity) of TERS but also creates more hot spots in the single crystal surface for SERS study.18 SHINERS combines the versatility of conventional SERS with the benefit of TERS, allowing the enhanced Raman signals from virtually any surface. The technique holds potential as part of the next generation of advanced spectroscopy in surface science, life sciences, food and drugs inspection, explosive screening and environment monitoring.18-22 In recent years, the novel optical properties of particle-film system have aroused the interest of researchers,23-27and the related results have contributed to the development of SHINERS technique to some extent. However, lots of scientific problems are still unclear in our previous experiments.18-20 For instance, why the 1nm


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silica shell can sharply increase the SERS enhancement compared to the bare nanoparticle? How the electromagnetic (EM) coupling between particle and metallic film happens with normal incidence of light? The underlying EM enhancement mechanism in SHINERS thus has become a significant obstacle to apply SHINERS as a reliable and powerful tool to quantitatively evaluate the concentration of the detected chemical substances. Herein, we provide a systematically theoretical analysis of the localized surface plasmon resonance (LSPR)-based optical properties and SERS properties of SHINERS by using the three dimensional finite difference time domain (3D-FDTD) method.28 SIMULATION METHOD AND MODEL The fundamental principle of FDTD is well documented in the literature.28 The FDTD method involves the discretization of Maxwell’s equations in both the time and the space domain in order to find the E and H fields at different positions and at different time-steps. This method can conveniently be applied to simulate the EM scattering and radiation from a target of complex shape as well as non-uniform dielectric objects by simply adjusting the number, size and material properties of the Yee cell.4,19.20 Numerical simulations in this work were performed with commercial Lumerical FDTD solutions (version 7.5) software. Perfectly matched layer (PML) boundary conditions are used in the FDTD simulation. The simulation time in all calculations was 1000 fs, which is long enough to ensure calculation convergence. To obtain accurate results in the simulations, the Yee cell size was set at 0.5 nm. In order to introduce the strong EM coupling in SHINERS, an individual Au@SiO2 NP placed on a gold film was chosen as the investigated object, as presented in Figure 1. Such


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nanostructures were fabricated and characterized for surface enhanced spectroscopy and electrochemical analysis. 29-31 The diameter of the gold core and the thickness of the silica shell

Figure 1. Schematic illustration of a typical SHINERS model for simulation: single Au@SiO2 NP on gold film. are denoted as D and h, respectively. The xy-plane dimension of the gold film with 100 nm thickness is infinite. A normally incident plane wave propagates along the negative z-axis direction with the polarization along the x-direction and its electric magnitude (Ein) is 1V/m. The optical constant of Au was taken from the CRC Handbook of Chemistry and Physics.32 To quantitatively study the EM enhancement on the single-crystal surface, two special points on the gold film were selected. Point A is the exact touching point between the particle and gold film, and point B is the maximum enhancement site on the gold film. RESULTS AND DISCUSSION A scattering spectrum is an effective way to characterize the SPR properties of nanostructures from the far field.25, 27 To clarify the SPR properties of SHINERS, the scattering


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Figure 2. (a) The scattering spectra of a single NP with D=80 nm on the gold film. The black and green curves correspond to the scattering spectra of bare gold particle and Au@SiO2 particle with 1 nm shell, respectively. The scattering spectra are normalized in the range 0~1. Electric field and vector distributions of steady state correspond to cross sectional view under (b) 633 nm and (c) 570 nm excitation. spectrum for an 80 nm gold NP with 1 nm silica shell on gold film is presented as a green curve in Figure 2 (a). There are three distinguished LSPR peaks at 535 nm, 570 nm and 633 nm. The first peak at 535 nm is ascribed to the ordinary gold plasmon band that always appears in the UV-Vis spectra for medium-sized Au nanosphere without a coupling effect.4 The remaining two peaks at 633 nm and 570 nm can be assigned to the dipole coupling mode and quadrupole


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coupling mode between the shell-isolated particle and the gold film, respectively.24 The origin of the two SPR modes can be further demonstrated by their corresponding electric field and electric vector distributions, as shown in Figure 2(b) and (c). It can be seen that the surface charges distribution of the core-shell NP behaves like oriented dipole and quadrupole mode respectively under SPR conditions. For comparison, the scattering spectrum of a bare NP (without silica shell) on the same gold film was also simulated and is shown as the black curve in Figure 2 (a). Besides the 535 nm peak, one band at 584 nm can be detected from the bare Au NP, which results from the dipole coupling. This band is blue-shifted relative to the mode of Au@SiO2 with 1 nm shell. Interestingly, the quadrupole coupling mode is not visible. This can be explained by the charge exchange effect between the NP and the gold film, which always results in a weak coupling effect in the region between NP and gold film.27 As a result of the coupling effect, the EM field confined at the junctions between NP and gold film can be enhanced, resulting in the formation of hot spots. The cross sectional view and plane-view of the EM field enhancement distributions of shell-isolated NP on gold film are shown in Figure 3. The EM field enhancement is defined as the M=Eloc/Ein, where Eloc and Ein are the magnitudes of the localized and incident fields, respectively. The SERS enhancement was defined as M4.3 Upon laser excitation at 633 nm, the strong coupling between dipole mode of the NP and the film results in a hot spot within the gap as shown in Figure 3(a). The strong EM field enhancement on the gold film can also be observed from top view as shown in Figure 3(b).


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Figure 3. Cross sectional view (a) and plane-view (b) of the EM enhancement distributions in single particle on gold film with D = 80 nm, h = 1 nm under 633 nm laser excitation. The above discussion shows that the existence of silica shell is very important. The reported

works imply the thickness of silica shell acting as the gap distance between NP and gold film is also a key factor in attaining intense SERS enhancement.25, 33We then quantitatively study the EM enhancement depending on silica shell by choosing point A and B on the flat gold surface. The result indicates that the junction region just below the NP does not correspond to the maximum field. The hot spot with maximum EM enhancement marked with point B deviates slightly from point A, distinguished from the schematic model in Figure 1 and the EM distributions in Figure 3. Moreover, the location of point B will change slightly as the increasing of silica shell thickness. The electric field enhancement spectra of point A and B depend on silica shell thickness from 0 nm to 20 nm as shown in Figure 4. The electric field enhancement is 2

replaced by M . For point A in Figure 4(a), the SERS enhancement for Au@SiO2 NP with 1 nm shell is the maximum and its enhancement factors (EFs) are about 1×107. However, there is no field enhancement when thickness of silica shell is 0 nm because of charge exchange between the gold NP and the gold film. For point B, the SERS enhancement as illustrated in Figure 4(b) is about 1x109 if the shell thickness is 1 nm, while the SERS EF is 1.44×106 without the silica


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shell. Obviously, the silica shell in SHINERS has effectively suppressed the charge exchange between the NP and gold film, resulting in a stronger EM field enhancement. For both points A and B, the electric enhancement gradually decreases due to the reduction of plasmon coupling by increasing

Figure 4. The electric field enhancements of points A and B are shown in (a) and (b) with h=0, 1, 2, 4, 8, 20 nm respectively; the diameter of particle is 80 nm. the SiO2 shell from 1 nm to 20 nm. Moreover, the dipole and quadrupole coupling modes happen in the electric field curves if the shell thickness is 1 or 2 nm [see Figure 4 (a) and (b)]. The quadrupole coupling mode gradually blue shifts and disappears eventually by increasing thickness of the SiO2 shell. Thus, the perfect thickness of SiO2 is 1 - 2 nm in SHINERS. It should be noted that the maximum SERS enhancement ratio between point B and A rapidly reduces


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from ~100 to 3.51, with the silica shell increases from 1 nm to 20 nm. Significantly, the ratio is about 108 at h=0 nm because of charge exchange.

Figure 5. The scattering spectrum is the black curve and the electric field intensity enhancement spectra of point A, B are the green and red ones corresponding to single particle with D = 80 nm, h = 1 nm placed on gold film. The scattering spectrum and the electric field enhancement for Au@SiO2 NP with 1 nm silica shell in Figure 2 and Figure 4 imply a strong correlation between the far field and near field. To quantitatively explore the relationship between the far and near fields in such a system, the dependence of the scattering efficiency and the electric field intensity enhancement at two points (A and B) on the excitation wavelength were calculated. As shown in Figure 5, the dipole coupling mode and quadrupole coupling mode all appear in far and near field. There are two peaks in the electric field enhancement spectra, while three peaks appear in the scattering spectrum. It should be noted that the quadrupole coupling mode is the main factor making contribution to the scattering intensity in the far field. Conversely, the dipole coupling mode overtakes the quadrupole coupling mode and becomes the key factor in SERS enhancement in


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near field, as shown in Figure 5 and Figure 4. The peak position of the quadrupole coupling mode is approximately matched in both fields, while the peak position of dipole coupling mode in the near field is weakly shifted comparing with that in scattering spectrum. This deviation phenomenon is also experimentally observed by the Van Duyne group.34, 35

Figure 6. The SERS enhancement distributions under 633 nm laser excitation with h = 0, 1, 2, 4, 8, 20 nm and D = 80 nm. Please note these distributions have been normalized by the maximum SERS values of h=1 nm. In order to determine the optimum configuration for SERS experiments, we also investigated the shell thickness effect of SERS enhancement distributions under 633 nm laser wavelength. The aforementioned EM distributions was replaced by the common logarithm of the EM enhancement of SERS (≈M4) from which one can determine the order of the maximum SERS EFs. As shown in Figure 6, by increasing the thickness of the silica shell from 1 nm to 20 nm, the plasmon coupling between the NP and gold film rapidly reduces, and the maximum enhancement does not locate at the junction of NP and film. The best SERS enhancement can be


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attained in gold-silica NP with a shell thickness of 1 or 2 nm, corresponding to maximum EFs of about 9 orders.

Figure 7. The electric field enhancement of point B in single particle with D = 40, 60, 80, 100, 120 nm on gold film and h = 1 nm. To further optimize the electric field intensity enhancement of point B, we studied the particle size effect of EM enhancement. By increasing the diameter of the NP from 40 to 120 nm, the electric field intensity gradually enhances and the SERS EFs are as high as 10 orders. The quadrupole coupling mode becomes dominant gradually with the rising NP size, accompanied by a red-shifted plasmon band (see Figure 7). It should be noted that the monotonous increasing tendency of the EM enhancement at point B will change when the particle size reaches a special size (~ above 200nm) according to our further calculations.


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Radiation damping, higher electric coupling modes of particle-film, even magnetic dipole mode between particle and film will play more and more important roles in the near and far field optical properties for SHINERS with larger sizes. CONCLUSIONS In conclusion, a systematic theoretical analysis of the EM enhancement of a single Au@SiO2 on a gold film based on SHINERS has been developed. According to the calculated results, dipole coupling and quadrupole coupling modes appear in the scattering spectrum due to strong plasmon coupling in the junction of NP and gold film. The silica shell with an optimum thickness of 1-2 nm effectively suppresses the charge transfer from NP and gold film, and is a key factor in obtaining SERS enhancement from the flat gold film. The different influences of SPR on the far and near fields were also examined. This study provides a strong theoretical framework that will underpin potential applications of SHINERS in surface science, as well as food safety, drug quality control, explosives detection, and environmental pollutant monitoring.

AUTHOUR INFORMATION

Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT

This work has been supported by the NSF of China (21173171 and11474239), the Fundamental Research Funds for the Central Universities (2012121013).


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Electromagnetic Enhancement in Shell-Isolated Nanoparticle

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