Localized Surface Plasmon Resonance Dependence on Misaligned Truncated Ag Nanoprism Dimer
Misaligned edge-to-edge dimers are the common products during the preparation of Ag nanoprism dimers using self-assembly method. However, in the self-assembly method, Ag nanoprisms are easily truncated because they are easy to oxidize in an acidic environment. In this work, modeling a truncated Ag nanoprism on a misaligned edge-to-edge dimer provides a better understanding of the effects of the truncation and misalignment on localized surface plasmon resonance (LSPR) of the dimer. The resonant wavelength and intensity of the dimer are flexibly modulated by changing the misalignment length of the dimer. As the misalignment length increases, a stronger peak at the shorter wavelength and a weaker one at the longer wavelength are observed. The resonant wavelengths and intensities of the two peaks are also flexibly tuned by adjusting the truncated length of the Ag nanoprism in the dimer. The results are numerically demonstrated based on the finite element method (FEM) and show promising potential for nanoswitch, multi-channel tunable biosensor and other nanodevice applications.
KeywordsLSPR Misalignment Truncation Ag Nanoprism dimer
Finite element method
Localized surface plasmon resonance
Silver and gold nanoparticles have attracted extensive attentions due to their unique optical properties which originate from their localized surface plasmon resonance (LSPR) effects. The conduction electrons in these nanoparticles collectively oscillate with the incident light causing an enhanced electric field that is localized around the surface of the nanoparticle [1, 2]. The LSPR effect of nanoparticles can be modulated by changing the nanoparticle’s size, shape, material, and the surrounding environment [3, 4, 5]. Among the numerous nanoparticles, nanoprisms (NPs) have attracted the most attention due to their anisotropic geometrical properties and tunable LSPR properties [6, 7, 8]. Because of the anisotropic geometrical properties, oscillating charges on the surface tend to accumulate around the NP’s tips making the localized electric field around NP more enhanced than in isotropic nanoparticles . The LSPR properties of NP are usually divided into in-plane dipolar resonance, in-plane quadrupolar resonance, and out-of-plane dipolar resonance. The in-plane dipolar resonance shows a redshift effect with increasing the edge length of NP [10, 11].
As two nanoparticles are brought close to each other, the enhanced electric field concentrates in the gap of the dimer. This phenomenon is known as hot spot effect which originates from the LSPR coupling effect between the nanoparticles. The dimers with remarkable hot spot effects can be easily prepared by e-beam lithography , nanosphere lithography , and self-assembly method [14, 15]. For the self-assembly method, metal nanoparticles with specific geometries are linked through the molecules with particular groups. Therefore, the gap in the dimer is as narrow as the molecular chain length. This further reduces the gap between two nanoparticles and causes a more remarkable hot spot effect [16, 17, 18].
The hot spot effect of the NP dimer as a typical dimer has been extensively studied, but most studies about the NP dimers concentrate on the tip-to-tip geometry [19, 20, 21, 22, 23]. However, during preparation process of the tip-to-tip dimer through the self-assembly method, there is a wide existence of NP dimers with edge-to-edge geometries . The self-assembly method often causes a randomly misaligned effect for the NP dimers. The randomly misaligned edge-to-edge Au NP dimers with the broken symmetry can induce a novel optical property . This result indicates that the double resonances can be switched by modulating the misalignment length of the dimer. The resonant positions can be tuned by changing the gap length and the thickness of Au NP. Due to the sensitivity of optical properties on the structural parameters, this finding paves a promising way for developing nanoswitches, nanomotors, nanorulers, and dual-channel biosensors. However, because of the inertia of the gold atom, the structural parameters of Au NP dimers are usually fixed once they are prepared.
Because Ag is easily oxidized, the tips of the Ag NP can be easily etched in the process of its preparation. This results to Ag NP transforming into an Ag truncated nanoprism (TNP). The change of the structural symmetry originating from the truncation effect induces some novel optical properties [8, 25]. The truncation effect can be introduced in the misaligned edge-to-edge Ag NP dimer to obtain a tunable device with novel optical properties. Here, effects of the truncation and misalignment on the LSPR edge-to-edge Ag TNP dimers are studied using FEM.
Structural Model of Misaligned Truncated Ag Nanoprism Dimer
The misaligned edge-to-edge dimer consists of two identical Ag TNPs with the following dimensions, edge length (L), misalignment length (l 1), and truncated length (l 2). The tips of Ag NP are cut off along a straight line with the initial edge length L = 130 nm and truncated length l 2 = 10 nm. To study the effect of the misalignment, a misalignment ratio R = l 1/L of Ag TNP dimer is varied from 0 to 1.5. As R approached 0 or 1, the Ag TNP dimers with the increased truncated length were modeled to study the influence of the truncation effect. Ag TNP transforms to a hexagonal nanoplate (HNP), as the edge length of Ag HNP (L 1 ) is equal to the truncated length l 2 = L/3. To further investigate the effect of the truncation and misalignment, Ag HNP dimers with the misalignment ratio R 1 = l 1/L 1 , ranging from 0 to 3, were simulated.
Finite Element Method for Misaligned Truncated Ag Nanoprism Dimer
FEM method using COMSOL Multiphysics is used to investigate the effect of misalignment length on the LSPR edge-to-edge Ag TNP dimer. The relative permittivity for Ag was obtained from the Drude model, ε(ω) = ε ∞ − ω p 2/[ω(ω + iγ)], where ε ∞ = 3.7 is the infinite frequency, ω p = 1.38 × 1016 is the bulk plasma frequency, and γ = 3.72 × 1013 is the oscillation damping of electrons .
The Ag TNP dimer was modeled on the x-y plane at z = 0 with air (n = 1) surrounding it. Using air around the dimer was done to simplify the computation process. The incident light, polarized along the y-axis, was directed normally along z-axis on the Ag TNP surface. Tuning of the wavelength was done between λ = 600 nm to λ = 1100 nm with a step of 4 nm.
Results and Discussion
Peak 1 in Fig. 2a strengthens further as R increases from 1 to 1.5 and its resonant wavelength is kept unchanged. Moreover, peak 2 continuously decreases until it disappears while its resonant wavelength decreases throughout. As R reaches 1.5, the double peaks degenerate into a single peak with a resonant wavelength equal to that of the Ag TNP monomer. When R is large than 1, the LSPR coupling interaction between Ag TNPs is extremely weak. In Fig. 2b, peak 2 plays the dominant role in ECS spectrum when R < 0.5 (the blue region) whereas peak 1 dominates when R > 0.5 (the yellow region). Therefore, the two peaks can be switched on or off by modulating the misalignment length of the dimer.
In summary, the LSPR effects of Ag TNP misaligned edge-to-edge dimers have been studied using FEM. The ECS spectrum of Ag TNP dimer possesses two peaks, which can be switched on or off by modulating the misalignment length. When R is less than 0.5, the longer wavelength peak plays the prominent role in the ECS spectrum. As R grows larger than 0.5, the shorter wavelength peak occupies the dominant role. Due to the truncation effect, the resonant wavelengths of the two peaks can be flexibly modulated by changing the truncated length. When the truncated length increases to a specific value, Ag TNP transforms into Ag HNP. The double peaks degenerate to a single peak, and the peaks of the Ag HNP dimer also can be switched by changing the misalignment length. The calculated results indicate that Ag TNP misaligned edge-to-edge dimers pave the way for a promising surface-enhanced Raman spectrum, nanoswitch, multi-channel tunable biosensor, other nanodevices, etc.
This study is financially supported by the National Natural Science Foundation of China (No. 11374074, 61308069) and the National Basic Research Program of China (No. 2013CB328702).
HYA, XJ, and XS conceived the idea. HYA, EO, PL, and XS calculated the results and made the conclusions. HYA, EO, and SL contributed to the preparation and revision of the manuscript. All the authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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