Transverse surface plasmon resonances (SPR) in Au–Ag and Ag–Au core–shell structure nanowires have been investigated by means of quasi-static theory. There are two kinds of SPR bands resulting from the outer surface of wall metal and the interface between core and wall metals, respectively. The SPR corresponding to the interface, which is similar to that of alloy particle, decreases and shifts obviously with increasing the wall thickness. However, the SPR corresponding to the outer surface, which is similar to that of pure metal particle, increases and shifts slightly with increasing the wall thickness. A mechanism based on oscillatory surface electrons under coulombic attraction is developed to illuminate the shift fashion of SPR from bimetallic core–shell interface. The net charges and extra coulombic force in metallic wall affect the SPR energy and the shift fashion.
KeywordsSurface plasmon resonance (SPR) Interface Bimetallic Core–shell structure Nanowires
The optical properties of bimetallic Au–Ag nanoparticles are currently of considerable interest and the subject of a significant literature due to the potential uses of the tunable light absorption, scattering and local field enhancement in the UV-visible region [1, 2]. There are two types of bimetallic nanoparticles: alloy particles (particles with a homogeneous distribution of two kinds of metals) and core–shell structure particles (particles with heterogeneous arrangement of two kinds of metals leading to core–shell structure) . The composition or shell thickness dependent surface plasmon resonance (SPR) absorption spectra of both core–shell type and alloy model Au–Ag nanoparticles have already been studied experimentally and theoretically [3–6].
As we know, only one SPR peak occurs at around 520 nm for pure Au spherical nanoparticles . Similarly, only one SPR peak occurs at around 400 nm for pure Ag spherical nanoparticles . In bimetallic Au–Ag nanoparticles, the SPR band depends on the composition and the distribution of the two metals . For alloy type particles, the two kinds of metals are homogeneously distributed over the whole volume on an atomic scale, so there is still only one SPR peak located between those of pure Au and Ag nanoparticles. This SPR in alloy type Au–Ag bimetallic nanoparticles may shift from 400 nm to 520 nm linearly by changing the molar fraction of Au from 0 to 100% .
For core–shell structure particles, one of the two metals constitutes the core of the structure, and the other one the external shell, the SPR become complex because of the interface between core and shell. A direct approach to determining the interface is to see the core–shell structure by transmission electron microscopy (TEM), because a boundary between Ag and Au elements can be distinguished by bright and dark contrast in the TEM imaging . In the core–shell bimetallic Au–Ag nanoparticles, two SPR bands can be observed . Due to the two interacting metals at the interface, SPR peak positions for both silver and gold shift . For example, when Au nanoparticle was coated by an Ag shell, a blue shift for the original pure gold peak was observed. However, with increasing the amount of silver, the Ag shell thickness grows resulting in a red-shift of the SPR maximum for the silver fraction . At last, the Au–Ag core–shell structure bimetallic colloids show only one SPR at around 402 nm, which can be attributed to the SPR of silver particles alone . The question arises for the origin and the shift fashion of the two SPR bands. In the report of Hodak et al. , it is concluded that two types of collective electron oscillations occur. However, the mechanism and physical picture of these two kinds of SPR, especially the mechanism of collective electron oscillation at the interface, have not been studied in great detail so far. In this paper, we propose to investigate the origin of the SPR at the interface of Au–Ag core–shell structure bimetallic nanowire by calculating the wall thickness dependent absorption spectra. The mechanism based on oscillatory surface electrons under coulombic attraction has been used to illuminate the shift fashion of SPR bands that were observed experimentally and calculated theoretically.
Although the length of the nanowire in this calculation is infinite, the absorption cross sections per unit length of the nanowire can be finite [19, 20]. So we refer to such an absorption cross section per unit length as simply an absorption cross section in this study.
Results and Discussion
In order to investigate the origin of the shorter wavelength SPR peak of gold coated silver nanowire, we also calculated the absorption spectra of silver coated gold nanowire (i.e. Au–Ag core–shell structure nanowire), as shown in Fig. 3b. For Au–Ag core–shell structure nanowire, we also observed two absorption peaks in the visible region. However, increasing the Ag wall thickness leads to the longer wavelength peak blue shift distinctly and nonlinearly (the shift fashion is similar to that of shorter wavelength peak in Ag–Au core–shell structure nanowire), and leads to the shorter wavelength peak red shift slightly (the shift fashion is similar to that of longer wavelength peak in Ag–Au core–shell structure nanowire). These spectral characters have also been observed experimentally in Au core/Ag shell bimetallic nanoparticles by Pande et al. [12, 13].
By comparing Fig. 3a and b, we find the shorter wavelength peak at around 400 nm in Au–Ag core–shell structure nanowire and the longer wavelength peak at around 530 nm in Ag–Au core–shell structure nanowire have the same origin. Both of them come from the SPR of outer metallic wall. On the other hand, the longer wavelength peak shifting from 550 to 480 nm in Au–Ag core–shell nanowire and the shorter wavelength peak shifting from 410 to 450 nm in Ag–Au core–shell nanowire also have the same origin. Both of them come from the SPR of interface between metallic core and wall. When the metallic wire has been coated with another kind of metal, a core–shell structure comes into being and the surface of the wire has transformed into an interface between wire and wall. Due to the interacting of the two metals at the interface, the corresponding SPR peak shifts obviously with changing the ratio of wire and wall radius.
In conclusion, there are two kinds of transverse SPR in Au–Ag bimetallic nanowire. One is resulted from the outer surface of wall metal and is similar to that of pure metallic particle, the other is resulted from the interface between the core and wall metals and is similar to that of alloy particle. Quasi-static calculations show that, for Ag-Au core-shell structure nanowire, increasing the Au wall leads to the interface SPR red shifts obviously and decreases, whereas the outer surface SPR red shifts slightly and increases. For Au–Ag core–shell structure nanowire, increasing the Ag wall leads to the interface SPR blue shifts obviously and decreases, whereas the outer surface SPR red shifts slightly and increases. The shift fashion of SPR from bimetallic core–wall interface is different from alloy particles. The net charges and extra coulombic force in metallic wall affect the energy of SPR and leads the nonlinear shifting.
This work was supported by the National Natural Science Foundation of China under grant no. 10804091.