Surface Plasmon Resonance in Bimetallic CoreâShell Nanoparticles

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Surface Plasmon Resonance in Bimetallic Core-shell Nanoparticles Chao Zhang, Baoqin Chen, Zhi-Yuan Li, Younan Xia, and Yue-Gang Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b04232 • Publication Date (Web): 26 Jun 2015 Downloaded from http://pubs.acs.org on June 29, 2015

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The Journal of Physical Chemistry

Surface Plasmon Resonance in Bimetallic Core-Shell Nanoparticles Chao Zhang, Bao-Qin Chen, and Zhi-Yuan Li* Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Sciences, P. O. Box 603, Beijing 100190, China Younan Xia The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, and School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332, USA

Yue-Gang Chen Department of Physics, Guizhou University, Guiyang 550025, China *Corresponding author. Email address: [email protected]

Abstract Colloidal metal nanoparticles have unique surface plasmon resonance (SPR) properties for applications in optics, medicine, photo-catalysis and photovoltaics. In this work, we use the Mie theory to investigate the SPR properties of bimetallic core-shell nanoparticles with a spherical shape and consisting of Drude metals. We find that there exists a special SPR mode whose energy is concentrated at the interface between the core and the shell and call it the extraordinary SPR mode. This mode can interact with the conventional SPR mode whose energy is concentrated at the outer surface of the bimetallic nanoparticle, which is called the ordinary SPR mode. The ordinary and extraordinary SPR modes together determine the line shape of the extinction spectrum, as well as the shift of SPR peak (to shorter or longer wavelengths) as a function of various geometric parameters. When extended to practical noble metals such as Au, Ag, Pd and Pt, we find that the SPR of both Au@Pt and Au@Pd nanoparticles can occur in the visible region with high tunability, which is beneficial to the enhancement of photo-catalytic properties of both Pd and Pt. The theoretical studies would open up new avenues for engineering the plasmonic properties of bimetallic nanoparticles to enhance their applications related to fluorescence, Raman spectroscopy, optical sensing, photo-catalysis, photovaltaics, and biomedical research. 1 ACS Paragon Plus Environment


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I. Introduction Colloidal metal nanoparticles have found a broad range of applications related to surface plasmon resonance (SPR), which represents the collective oscillation of conduction electrons at the surface (or more accurately, at the interface between a metal nanoparticle and its dielectric surrounding) upon excitation by the incident light. The SPR effect can lead to strong confinement for the electromagnetic field and thus great enhancement for the local electric field near the metal surface within a sub-wavelength distance. Due to their capability to confine and enhance electromagnetic field, metal nanoparticles have been widely explored to enable and/or significantly augment many applications. In optics, they have been employed to enhance the intensity of fluorescence,1-3 enable new nonlinear optical processes,4,5 and even help achieve new sub-wavelength optical devices such as nano-lasers.6-8 In medicine, they have been applied to develop new diagnostic methods by taking advantage of SPR peak shift9 and surface-enhanced Raman scattering (SERS).10-13 In chemistry, they have been employed to enhance various photo-catalytic processes,14 and in photovoltaics they have been used to improve the efficiency of thin film solar cells.15,16 A metal nanoparticle always shows a maximum optical extinction at the SPR frequency, with the optical response being strongly dependent on the shape, size, composition, and structure of the nanoparticle. In an effort to tailor the SPR properties and thus improve their performance in various applications, people have developed a myriad of chemical methods for generating metal nanoparticles with different shapes, including spheres, rods, cubes, and decahedra. In addition, new methods have been demonstrated for controlling the composition of nanoparticles by synthesizing more complex structures. To this end, the Xia group has developed a method to prepare metal nanoparticles with hollow interiors and porous walls by using the galvanic replacement reaction between two different metals.17-23 They demonstrated the synthesis of Au-Ag alloyed nanoboxes and nanocages by employing Ag nanoparticles as sacrificial templates to reduce HAuCl4 in an aqueous solution. The nanoboxes have a hollow interior and smooth surface while the nanocages are characterized by porous walls. Many other types of bimetallic nanostructures have also been developed by various groups, including Au-Ag,24-31 Au-Pd,32-35 and Au-Pt nanoparticles.36,37 These bimetallic nanoparticles 2 ACS Paragon Plus Environment

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are of great interest and importance because their SPR properties are distinct from those of their constituent components. Among various types of bimetallic nanoparticles, those with a core-shell configuration have been widely studied and it has been shown that the coupling between the core and the shell could lead to SPR properties different from the nanoparticles made of each individual component.17,18,22-25 In this work, we used the Mie theory to investigate the SPR properties of bimetallic core-shell nanoparticles. We demonstrated the existence of a SPR mode at the interface between the core and the shell, which can be used to explain the unique optical response of core-shell nanoparticles. At the same time, we analyzed the SPR peak shift and local field distribution near the surface of core-shell nanoparticles with different geometric parameters under the resonance condition, from which the physical origin of their unique SPR feature could be identified. This paper is organized as follows. In Sec. II, we present a brief introduction to the theoretical model based on the Mie theory for calculating the optical properties of bimetallic core-shell nanoparticles. In Sec. III, we calculate the extinction spectrum and modal profile of a bimetallic core-shell nanoparticle with both the metals being described by the Drude model and we demonstrate that the SPR mode at the interface between the core and the shell can be used to explain the unique optical feature of a core-shell system. In Sec. IV, we use the model to study core-shell nanoparticles made of Au and Ag. We discuss the physical origin of their special optical properties and their difference from those of nanoparticles made of pure Au or Ag. In Sec. V, we investigate a bimetallic core-shell nanoparticle system where the Au core is coated by a Pd or Pt shell and examine their photo-catalytic efficiency in comparison with nanoparticles made of pure Pd or Pt. Finally, we summarize this work and draw some conclusions in Sec. VI.

II. Theoretical model and calculation method For simplicity and yet without losing generality, all the nanoparticles we consider in this paper have a spherical, concentric, core-shell structure, as illustrated in Fig. 1. In the classical electromagnetics regime, we assume a sharp interface between the two different metals M1 3 ACS Paragon Plus Environment


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and M2. The inner (outer) sphere has a radius and permittivity of r1 and ε1 (r2 and ε2). We assume that the nanoparticle is embedded in a homogeneous background of water with permittivity ε3=1.33.2 Using the Mie theory,38 we can calculate the extinction spectrum and the local field distribution of the nanoparticle under the illumination of an incident plane wave that is polarized along the x-axis and propagates along the z-axis. In a linear, isotropic, and homogeneous medium, the time-dependent harmonic electromagnetic field inside and outside a spherical particle can be represented by a combination of solutions to the vector wave equation under the spherical coordinates. The electromagnetic field in the space can be written as the superposition of a series of vector spherical harmonic waves: E m = E om + E em

(1)

H m = H om + H em

(2)

Eom (r , θ , φ ) = cos φπ n ( cos θ )  f mn jn ( ρm ) + vmn hn ( ρ m )  eˆθ

(3)

− sin φτ n ( cos θ )  f mn jn ( ρm ) + vmn hn ( ρm )  eˆφ

{

} ρ eˆ  ρ h ( ρ )  } ρ eˆ

Eem (r , θ , φ ) = −i cos φτ n ( cos θ ) g mn  ρ m jn ( ρ m )  + wmn  rm hn ( ρ m )  '

{

+i sin φπ n ( cos θ ) g mn  ρ m jn ( ρ m )  + wmn '

'

m

θ

'

m n

m

m

(4)

φ

H em (r , θ , φ ) = ( km ω ) sin φπ n ( cos θ )  g mn jn ( ρm ) + wmn hn ( ρm )  eˆθ + ( km ω ) cos φτ n ( cos θ )  g mn jn ( ρm ) + wmn hn ( ρm )  eˆφ ' '   ρ m jn ( ρ m )   ρ m hn ( ρ m )    eˆθ H om ( r ,θ , φ ) = −i ( km ω ) sin φτ n ( cos θ )  f mn + vmn ρm ρm     ' '   ρm jn ( ρ m )   ρ m hn ( ρ m )    eˆφ −i ( km ω ) cos φπ n ( cos θ )  f mn + vmn ρm ρm    

(5)

(6)

Here Em, Hm are the electric and magnetic fields in the mth layer, where m=1, 2, 3 denotes the core, shell, and the surrounding medium (water), respectively. km is the wave vector of light in the mth layer, which is defined as km=(εm)1/2(ω/c), with ω being the angular frequency of 1 the incident light and c being the speed of light in vacuum, and ρm=kmr. π n (cosθ ) = Pn sin θ ,

τ n (cos θ ) = dPn1 dθ , where Pn1 is the first kind of associated Legendre functions of degree n 4 ACS Paragon Plus Environment

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and order 1. jn is the first kind of spherical Bessel functions and hn is the first kind of spherical Hankel functions. f mn , g mn , v mn , and wmn are superposition coefficients of these vector spherical harmonics. eˆθ and eˆφ are the unit vector along the polar and azimuthal angle direction. According to the boundary conditions at each interface between adjacent layers, we have

Eom,θ |r =rm−1 = Eo( m−1),θ |r =rm−1

Eom,ϕ |r =rm−1 = Eo( m−1),ϕ |r =rm−1

,

H om,ϕ |r =rm−1 = Ho( m−1),ϕ |r =rm−1

,

Eem,ϕ |r =rm−1 = Ee( m−1),ϕ |r =rm−1

, ,

H om,θ |r =rm−1 = Ho( m−1),θ |r =rm−1

,

Eem,θ |r =rm−1 = Ee( m−1),θ |r =rm−1

,

H em,θ |r =rm−1 = H e( m−1),θ |r =rm−1 , H em,ϕ |r =rm−1 = H e( m−1),ϕ |r =rm−1 , m=2, 3. Here Eom,θ is the coefficient of the eˆθ field component, and other functions are similarly defined. We can derive the following linear relationship between the superposition coefficients:  jn ( ρ m )  '   km  ρ m jn ( ρ m )  ρ m µ m

hn ( ρ m )

  f mn   ' = km  ρ m hn ( ρm )  ρ m µm   vmn 

 jn ( ρ m −1 )   km −1  ρ m −1 jn ( ρ m −1 )  ' ρ m −1µ m −1   

  ρ j ( ρ )' ρ  m n m  m  j (ρ )k µ  n m m m

hn ( ρ m −1 )

  f( m −1) n    ' km −1  ρ m −1hn ( ρm −1 )  ρ m −1µ m −1   v( m −1)n 

'  ρ m hn ( ρ m )  ρ m   g mn   = hn ( ρ m ) km µm   wmn 

  ρ j ( ρ )' ρ   m−1 n m −1  m−1  j (ρ )k  n m−1 m −1 µm −1

'  ρ m−1hn ( ρ m−1 )  ρ m −1   g( m−1)n    hn ( ρ m −1 ) km−1 µm −1   w( m−1)n 

(7)

(8)

Considering the boundary condition in the infinite region away from the nanoparticle, the field is the incident plane wave (x-polarization, z-axis incidence), we have f3n = 1 and

g3n = 1 . Besides, since the field in the core is inwardly propagating spherical harmonic waves, we have v1n = 0 and w1n = 0 . After solving the two linear equations in Eqs. (7) and (8), we obtain the superposition coefficients, from which the electromagnetic field can be calculated everywhere inside and outside the sphere using Eqs. (1-6). The extinction cross section of the nanoparticle is given by these coefficients according to 5 ACS Paragon Plus Environment


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Cext =

2π k3 2

∞

∑ (2n + 1) Re{v

3n

+ w3n }

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(9)

n =1

III. Analysis of core-shell nanoparticles made of Drude metals Before considering any practical system comprised of real metals, we analyze core-shell nanoparticles made of model metals whose permittivity can be described using the ideal Drude model, and we call this model system Drude bimetallic nanoparticles. This simplification will allow us to easily identify the physical origin of any peculiar optical feature. The real and imaginary parts of permittivity for the Drude model we adopt here are

ε r = 1 − ω p 2τ 2 / (1 + ω 2τ 2 ) and ε i = ω p 2τ 2 / ω (1 + ω 2τ 2 )  , respectively, with ω p being the plasma frequency. For the two metals denoted as P1 and P2, their ω p correspond to wavelengths of λ p1 = 200 nm and λ p2 = 300 nm , whereas the relaxation time is taken as

τ = 10 −14 s . The size of the bimetallic nanoparticle is fixed with an outer radius of r2=10 nm while the inner radius r1 is varied as shown in Fig. 1. The extinction spectra calculated for the core-shell nanoparticles are shown in Fig. 2. Both the P1@P2 and P2@P1 nanoparticles show two resonance modes in the spectrum. For the P1@P2 nanoparticle with r1=7 nm, as can be found in Figs. 2a and 2b, the resonance peaks are located at 263 and 564 nm, respectively. For r1=9 nm, the resonance peaks are shifted to 280 and 487 nm, respectively. The results clearly show that the resonance peak at a shorter wavelength shows a considerable blue shift while the resonance peak at a long wavelength exhibits a significant red shift as the ratio between the outer and inner radii (x=r2/r1) is increased. As shown in Figs. 2c and 2d, however, the extinction spectra display an opposite trend of peak shift for the resonance peaks of P2@P1 nanoparticles as a function of x. According to the plasmon hybridization theory, which was first proposed by Nordlander and Halas,39 the extinction spectrum of a dielectric-metal core-shell nanoparticle would split into two resonance modes due to the interaction and coupling between the SPR modes of the inner and outer surfaces of the metal shell (each one of them would only exhibit a single SPR peak). In the dielectric-metal core-shell nanoparticle, the separation between the two resonance 6 ACS Paragon Plus Environment

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peaks would become smaller with increasing x. As shown in Figs. 2a and 2b, the peak shift for the two SPR bands of metal-metal core-shell bimetallic nanoparticles is opposite to what is reported for dielectric-metal core-shell nanoparticles. When we switch the metals used for the core and shell, an opposite trend is observed, as illustrated in Figs. 2c and 2d. Obviously, there is something new in the bimetallic core-shell nanoparticle system that is completely different from what was predicted for the dielectric-metal core-shell nanoparticles using the plasmon hybridization theory. In order to identify the physical origin of this unique SPR feature, we analyze the modal profiles under resonant conditions in detail. Figures 3 and 4 show the electric field modal profiles calculated for the P1@P2 and P2@P1 nanoparticles, respectively, at both short [panels (a) and (c)] and long [panels (b) and

(d)] resonance wavelengths. Two different values of r1 are considered. For the resonance at short wavelength, we can clearly see that the field is mainly concentrated at the interface between the two metals for the P1@P2 and P2@P1 nanoparticles. When x is increased, the field at the short resonance wavelength is still localized at the interface while the field outside the interface drops in intensity as shown in Figs. 3 and 4. This mode can be considered as the SPR at the interface between two different materials, albeit the interface is not between the usual dielectric and metal, but between two different metals. As this mode is somewhat different from the well-known SPR mode that exists on the metal-dielectric interface, we call it an extraordinary SPR mode. At λ p1 < λ p2 , we have ε p2 , r > 0 and ε p1 ,r < 0 at short wavelength. In the case of P1@P2, supposing that the thickness of the shell is very thin, we can estimate the resonance at short wavelength from the SPR peak of a small metal sphere, and find that λ1 = (1 / λ p1 2 + 2 / λ p2 2 ) / 3 − 1 / c 2τ 2  of

P2@P1,

the

resonance

λ2 = (2 / λ p 2 + 1 / λ p 2 ) / 3 − 1 / c 2τ 2  1

2

−1/2

−1/2

= 0.2529 µm . Similarly, in the situation

wavelength

is

estimated

to

be

= 0.2222 µm . Both values of the resonance peaks

approach the short resonance wavelengths shown in Figs. 2a and 2c when r1 → 0 . From this observation, we can claim that the SPR mode at the interface determines the resonance observed at short wavelength. For the resonance mode at long wavelength, as displayed in Figs. 2b and 2d, the 7 ACS Paragon Plus Environment


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calculated field mode profiles are shown in Figs. 3b and 3d for the P1@P2 configuration and in Figs. 4b and 4d for the P2@P1 configuration. All of them indicate that the field is concentrated at the outer surface of the particle, in contact with water. As this mode behaves very close to the well-known SPR mode whose energy is confined at the metal-dielectric interface, we call this mode the ordinary SPR mode. When x is increased, the field is still localized at the outer surface. In addition, the field in the inner core gradually becomes weaker. In this case, the resonance at long wavelength tends to match with the SPR peak of a small sphere made of a single metal and surrounded by water. We thus believe that the SPR at the outer surface of the bimetallic core-shell nanoparticle is responsible for the resonance observed at long wavelength. Now that we have already gained some ideas about the physical origin of the two plasmon resonances in bimetallic nanoparticles, namely, the ordinary and extraordinary SPR modes, we proceed to clarify what causes the varying trend of the extinction spectra with x increased. For the ordinary SPR mode the field is localized at the outer interface. When the thickness of shell becomes larger, namely with x increasing, the effective permittivity in the outer interface is increased, and it approaches to ε p2 when r2 → ∞ . This feature results in the red shift of the resonance wavelength of this SPR mode in the P1@P2 bimetallic nanoparticles. Similarly in the P2@P1 bimetallic nanoparticles, when x increases, the field would sense less P1 and thus induces the red shift of resonance wavelength as shown in Fig. 2(c). Because P1 is more metallic than P2, we can treat the outer two layers (P2 and water) as an effective single dielectric insulator medium in the P1@P2 situation. As the core-shell particle has a geometric configuration like the insulator-insulator-metal (IIM) structure, the field would extend to the insulator-like region much more than the metal-like region. With x increasing the effective permittivity that the outer field would sense increases, thus the resonance wavelength of the extraordinary SPR mode would blue shift. However, in the P2@P1 bimetallic nanoparticles the shell layer is metal-like and other two layers (the inner P2

core and outside water) are insulator-like, thus the bimetallic particle could be seen as an IMI-like structure. As a result, the field would extend less in the metal-like region and more 8 ACS Paragon Plus Environment

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in the insulator region. So when x increases, the resonance wavelength of the extraordinary SPR mode would red shift due to less interaction of field with the outer interface.

IV. Analysis of Ag@Au and Au@Ag core-shell nanoparticles In the last section, we have discussed the optical properties of core-shell nanoparticles constructed from two ideal Drude metals. Now we turn our attention to more practical systems based upon Ag and Au, which are widely used for SPR applications in the visible region because of their high densities of conduction electrons. We consider both Ag@Au and Au@Ag configurations, with a focus on the difference between the bimetallic systems and nanoparticles made of pure Ag or Au by closely examining their extinction spectra and field distributions. The core-shell nanoparticles are set with a fixed value of 10 nm for r2 while the values of r1 are increased from 0 to 10 nm. The calculated extinction spectra are shown in Figs. 5a and 5b, respectively, for Ag@Au and Au@Ag configurations. When the shell is thin, we can clearly observe two resonance modes for the core-shell nanoparticles. As the shell thickness is increased, one of the resonance modes will disappear. This result is similar to what was observed experimentally in Ref. [23]. For the Ag@Au nanoparticles (Fig. 5a), the SPR peak at long wavelength shows a red shift while the peak at short wavelength shows a blue shift when the value of x is increased. As depicted in Fig. 5b, the Au@Ag nanoparticles display opposite trends. Since Ag is more metal-like than Au in the spectral region of 350−550 nm, the Ag@Au configuration ( ε1 = ε Au and ε 2 = ε Ag ) is similar to the situation of P1@P2 discussed in the last section. As shown in both Fig. 2a and Fig. 5a, there are two resonance modes in the spectra for such core-shell nanoparticles. When x is increased, the resonance peak at long wavelength, which originates from the ordinary SPR mode, shows a red shift while the peak at short wavelength, which originates from the extraordinary SPR mode, shows a blue shift. At the same time, we can see from Fig. 6 that the field is concentrated at the interface between the two metals for the extraordinary SPR mode whereas the field for the ordinary SPR mode is concentrated at the outer surface. All the features in the electric field modal 9 ACS Paragon Plus Environment


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profiles are similar to those derived for the P1@P2 system. Accordingly, we can conclude that in the Ag@Au nanoparticles, the mode at long wavelength, namely, the ordinary SPR mode, is induced by the resonance at the interface between the Au shell and water while the mode at short wavelength, namely, the extraordinary SPR mode, arise from the resonance at the interface between the two metals. Now we turn our attention to the Au@Ag nanoparticles. As expected, the Au@Ag and P2@P1 systems show similar features in both the extinction spectra and field modal profiles.

As shown in Fig. 5b, the ordinary SPR mode exhibited a blue shift while the extraordinary SPR mode displayed an opposite trend, which is consistent with Fig. 2b. Besides, the local field profiles in Fig. 7 for both modes are similar to the feature shown in Fig. 4. The similarity indicates that we can use the models developed in Sec. III for the core-shell nanoparticles made of Drude metals to account for the extinction spectra and field modal profiles of both the Ag@Au and Au@Ag nanoparticles. Despite the similarity, there are some major differences between the Ag-Au bimetallic systems and the core-shell nanoparticle made of Drude metals. When x is increased to 2, the extraordinary SPR mode approaches the ordinary SPR mode for the Au@Ag nanoparticle, so we would only observe one resonance peak in the extinction spectrum shown in Fig. 5b and the wavelength of this resonance peak is close to the SPR wavelength of a Ag nanoparticle surrounded by water. It would be misleading to conclude from this phenomenon that the SPR of Ag is responsible for the resonance mode at short wavelength. This is just a coincidence because the the extraordinary SPR mode peak overlaps with the ordinary SPR mode peak of Au@Ag nanoparticle. When x increases, the resonance wavelength of the ordinary and extraordinary SPR modes are both moving to the Ag resonance wavelength, as illustrated in Fig. 5b. So when x increases to 2, the extinction spectrum seems to exhibit only one resonance mode. In real cases, core-shell nanostructures with fixed core size but changed shell thickness are more easily synthesized. Therefore, we would like to consider the extinction spectra of the Ag@Ag and Au@Ag bimetallic nanoparticle with fixed core size (r1=10 nm) and valued shell thickness (r2=11~17 nm). As illustrated in figure 8, for the Ag@Au nanoparticles with x 10 ACS Paragon Plus Environment

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increasing, the resonance peak of the extraordinary SPR mode shows a blue shift while the resonance peak of the ordinary SPR mode shows a red shift. For the Au@Ag nanoparticles with x increasing, the resonance peak of the extraordinary SPR mode shows a red shift while the resonance peak of the ordinary SPR mode shows a blue shift. And these peaks movement coincide with Figure 5. There are some similar experiments result, i.e. in Ref.24 the UV–Vis spectra of Au@Ag nanoparticle with various mass ratio.

V. Analysis of Au@Pd and Au@Pt core-shell nanoparticle Both Pd and Pt are commonly used metal catalyst. Recently there have been many researches about Au@Pd32-35 and Au@Pt37 core-shell nanoparticle because these particles might become a new category of catalysts considering the enhanced catalytic effects involved in these bimetallic nanoparticles. To have a clarified idea of the physical origin of enhanced catalysis functionality, we calculate and analyze the optical properties of the Au@Pd and Au@Pt core-shell nanoparticle at visible wavelength, which is essential for the photo-catalysis functionality. The core of both bimetallic nanoparticles is made from gold because the outer Pd and Pt shell that has good catalysis functionality could connect with other reactants. We will use the major physics developed in Sec. III to analyze the calculation result for these bimetallic nanoparticles by means of Mie theory and assess their photo-catalysis efficiency. Figures 9 and 10 show the calculated extinction spectra and the resonance modal profile of the Au@Pd and Au@Pt bimetallic nanoparticle. In Fig. 9a we find that at the visible wavelength there is only one resonance, which is located at 537 nm and 556 nm for Au@Pd with r1=9 nm and r1=7 nm, respectively. For a pure Pd nanoparticle there is no SPR effect in this visible wavelength regime, as shown in Fig. 9a. Figures 10a and 10c show clearly that the field is strongly concentrated at the Au/Pd interface and this is induced by the extraordinary SPR mode. As the real part of palladium permittivity at the visible wavelength is positive and bigger than water, only the extraordinary SPR mode exists and the resonance red shifts when x increases. When the shell thickness is small, as can be seen in Fig. 10a, the field would be enhanced at the outer surface of palladium layer, and this could promote the 11 ACS Paragon Plus Environment


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photo-catalysis process near the surface at visible wavelength. But when the thickness of the shell is increased, the field at surface would decay, and this is not good for high efficiency photo-catalysis process. As shown in Fig. 9b, the Au@Pt bimetallic nanoparticle of r1=9 nm and r1=7 nm has the resonance wavelength located at 516 nm and 490 nm, respectively. We can find that there is also no SPR effect at visible wavelength for pure Pt nanoparticles. Figures 10b and 10d illustrate that the field are concentrated at the water/Pt interface and this is induced by the metallic feature of platinum material at the visible wavelength. The resonance is an ordinary SPR mode and would have a blue shift when x increases. With the thickness being smaller the local field gets stronger at the surface, and this is beneficial for photo-catalysis.

VI. Conclusion In summary, we have adopted the Mie theory to calculate and analyze the extinction spectrum and modal profile of a series of spherical core-shell bimetallic nanoparticles. For simplicity and clarity, we first consider bimetallic particles whose permittivity is described by the ideal Drude model. It was found that two resonance peaks exist in the extinction spectrum and they can be attributed to two different SPR modes in the bimetallic nanoparticle. The mode at short wavelength, which is called the extraordinary SPR mode, has the electric field energy concentrated at the interface between the core and shell metals, and while the mode at long wavelength, which is called the ordinary SPR mode, has the electric field energy concentrated at the outer surface of the nanoparticle. Because of this field localization feature, these two modes exhibit opposite trends in moving the peak position, either red shift or blue shift, when the relative thickness of the two metals is varied. Furthermore, the trend of peak shifting also depends on which metal is in the core and which metal is in the shell. The different optical response characteristics of two metals in the ultraviolet and visible regions could lead to the appearance of SPR peaks over a broad spectrum, where the plasma frequency, ω p , plays a particularly important role, as well as the complicated electromagnetic interactions between the core and the shell. We have further considered bimetallic nanoparticles made from practical noble metals 12 ACS Paragon Plus Environment

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such as Ag, Au, Pt, and Pd. Systematic and careful analyses over the extinction spectra and electric field modal profiles for Ag@Au and Au@Ag bimetallic nanoparticles suggest that their behaviors are very similar to those of the Drude bimetallic nanoparticles. The SPR peaks are tunable over a broad range in the visible region when the relative thickness of Ag and Au layer are varied. This power of SPR manipulation in Ag@Au and Au@Ag bimetallic nanoparticles can be harnessed to create some attractive optical properties and functionalities that are difficult to achieve with a single-component Ag or Au nanoparticle. The feature of double SPR modes, together with its wide tunability and the associated enhancement in local field, can be employed to enhance some light-matter interaction processes that need two or more simultaneous enhancement channels (usually at different wavelengths) in order to maximize the efficiency. The molecular fluorescence is a good example, where it has been shown that double SPR modes can enhance the efficiency more significantly than with a single SPR mode.3 To explore other aspects of plasmonic functionality and application by using bimetallic nanoparticles, we have also considered Au@Pt, Au@Pd nanoparticles with an emphasis on manipulating SPR in the visible band and associated local field enhancement, because these two features are very important for improving the photo-catalysis functionality of Pt and Pd. Detailed comparative studies show that it is easy for Au@Pd and Au@Pt bimetallic nanoparticles to have their SPR appearing in the visible band and at the same time have a broad tunability, which is very difficult for pure Pd and Pt nanoparticles to achieve. In some sense, these bimetallic nanoparticles allow merging individual merits of each composite metal, namely the appearance and tunability of SPR in the visible band of Au and photo-catalysis capability of Pt and Pd, into a single nanoscale system and at the same time enable their mutual enhancement due to electromagnetic interaction. With the development of nanofabrication and colloid nanoparticle synthesis technologies, more and more applications of SPR are enabled in many areas. Our studies clearly show that the scheme of bimetallic nanoparticle offers a new routine to fully utilize and merge the merit of functionality of each independent composite metal material in regard to optical, electrical and chemical properties to support multiple functionalities. Due to various optical, physical, 13 ACS Paragon Plus Environment


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and chemical coupling and interaction, synergistic effects between core and shell materials would result in some enhanced functionalities that are better than those of the single component. As to the plasmonic properties that concern us in this work, the bimetallic nanoparticles suggest that we could combine the resonance structure (silver or gold nanostructure at optical regime) and non-resonance material (platinum, palladium, or graphene at optical regime) into a single system. Special characteristics of plasmonic tunability and field enhancement can be created in these bimetallic nanostructures and they could help us to create more effective and controllable devices at optics, biology, photo-catalysis and photovoltaics. Theoretical calculation and analysis can offer a promising design routine to create more efficient devices by modulating the shape, size, material in these bimetallic nanostructures. In a broader point of view, composite nanomaterials and nanostructures would offer new ways to engineer attracting optical, chemical or electronic features that combine the merit of each component, which in turn could be harnessed to build a series of new devices with better optical, chemical, or electronic functionalities.

Acknowledgment This work is supported by the 973 Program of China at No. 2013CB632704 and the National Natural Science Foundation of China at No. 11434017.

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Figure 1. The schematic diagram of a spherical bimetallic core-shell nanoparticle placed in homogeneous background water. The core and shell metals are M1 and M2, with permittivity is ε1 and ε2, respectively. The inner and outer radii are r1 and r2. A plane wave with the electric field polarized along the x-axis propagates along the z-axis direction to excite surface plasmon resonance in the bimetallic nanoparticle.



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Figure 2. Extinction spectra calculated for bimetallic core-shell nanoparticles made from Drude metals P1 and P2 with various inner radius (r1) and fixed outer radius r2=10 nm. (a) The extinction spectra for the P1@P2 core-shell nanoparticle in the wavelength range from 200 to 300 nm. (b) The extinction spectra for the P1@P2 core-shell nanoparticle in the wavelength range from 300 to 1000 nm. When the parameter x=r2/r1 increases, the resonance peak at short wavelength which originates from the extraordinary SPR mode, shows a blue shift while the resonance peak at long wavelength, which originates from the ordinary SPR mode, shows a red shift. (c) The extinction spectra for the P2@P1 core-shell nanoparticle in the wavelength range from 200 to 300 nm. (d) The extinction spectra for the P2@P1 core-shell nanoparticle in the wavelength range from 300 to 1000 nm. As x is increased, the resonance peak of the extraordinary SPR mode shows a red shift while the resonance peak of the ordinary SPR mode shows a blue shift.


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Figure 5. Extinction spectra calculated for Ag@Au and Au@Ag nanoparticles with various inner radius (r1) and fixed outer radius r2=10 nm. (a) The extinction spectra of the Ag@Au nanoparticles. As x is increased, the resonance peak of the extraordinary SPR mode shows a blue shift while the resonance peak of the ordinary SPR mode shows a red shift. (b) The extinction spectra of the Au@Ag nanoparticles. As x is increased, the resonance peak of the extraordinary SPR mode a red shift while the resonance peak of the ordinary SPR mode shows a blue shift.



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Figure 6. Local electric field modal profile calculated at the resonance wavelengths of Ag@Au nanoparticles. (a) The local field intensity pattern at 388 nm for Ag@Au with r1=9 nm, (b) the local field intensity pattern at 496 nm for Ag@Au with r1=9 nm, (c) the local field intensity pattern at 382 nm for Ag@Au with r1=7 nm, (d) the local field intensity pattern at 507 nm for Ag@Au with r1=7 nm.


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Figure 8. Extinction spectra calculated for Ag@Au and Au@Ag nanoparticles with fixed inner radius r1=10 nm and various outer radius (r2). (a) The extinction spectra of the Ag@Au nanoparticles. As x is increased, the resonance peak of the extraordinary SPR mode shows a blue shift while the resonance peak of the ordinary SPR mode shows a red shift. (b) The extinction spectra of the Au@Ag nanoparticles. As x is increased, the resonance peak of the extraordinary SPR mode a red shift while the resonance peak of the ordinary SPR mode shows a blue shift.


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Page 27 of 29

Cext(10-3µm2)

0.8

r1=10 nm

(a)

r1=9 nm r1=8 nm

0.6

r1=7 nm r1=6 nm r1=5 nm

0.4

r1=4 nm r1=3 nm r1=2 nm

0.2

r1=1 nm r1=0 nm

0.0 300

400

500

600

700

800

wavelength(nm) 0.8

-3

Cext(10 µm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


r1=10 nm

(b)

r1=9 nm r1=8 nm

0.6

r1=7 nm r1=6 nm r1=5 nm

0.4

r1=4 nm r1=3 nm r1=2 nm

0.2

r1=1 nm r1=0 nm

0.0 300

400

500

600

700

800

wavelength(nm)

Figure 9. Extinction spectra calculated for Au@Pd and Au@Pt nanoparticles with different values r1 and fixed outer radius r2=10 nm. (a) The extinction spectra of Au@Pd nanoparticles. There is only one resonance mode in the visible spectrum and the resonance peak shows a red shift as x is increased. (b) The extinction spectra of the Au@Pt core-shell nanoparticles. There is only one resonance mode in the visible spectrum and the resonance peak shows a blue shift with increasing x.



20

10

3.9

10

3.9

0

2.8

0

2.8

-10

1.6

-10

1.6

0.5

-20 -20

y(nm)

(a)

-20 -20

20

-10

0

x(nm)

10

20

y(nm)

5.0

20

5.0

(b)

-10

0

x(nm)

10

20

0.5

20

10

3.9

10

3.9

0

2.8

0

2.8

-10

1.6

-10

1.6

0.5

-20 -20

(c)

-20 -20

-10

0

x(nm)

10

20

y(nm)

5.0

y(nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5.0

(d)

-10

0

x(nm)

10

20

0.5

Figure 10. Local electric field modal profile calculated at the resonance wavelengths of Au@Pd and Au@Pt nanoparticles. (a) The local field intensity pattern at 537 nm for Au@Pd with r1=9 nm, (b) the local field intensity pattern at 516 nm for Au@Pt with r1=9 nm, (c) the local field intensity pattern at 556 nm for Au@Pd with r1=7 nm, (d) the local field intensity pattern at 490 nm for Au@Pt with r1=7 nm.


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TOC


Surface Plasmon Resonance in Bimetallic CoreâShell Nanoparticles

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