| Recently, what researchers focus on is a biosensor based on localized surface plasmon resonance of metal nanoparticles. Compare with other sensors, this sensor costs less and is portable; however, the sensitivity of the sensor is lower. How to improve the sensitivity of the sensor is the most important problem. As we all know, one of the most important influence of the sensor's sensitivity is the refractive index sensitivity of nanoparticles. The property of different materials is different. Using different materials in one nanostructure could change the localized surface plasmons resonance of nanoparticles and is one of the methods to improve the refractive index sensitivity of nanoparticles. The investigation of the localized surface plasmon resonance of two types of composition nanostructure is presented.The gold nanopaticles is difficult to be oxidated and good at combining with biological molecule but their sensitivity is bad, while the properties of the silver nanoparticles are opposite to that of the gold nanoparticles. If these two materials are used at the same time, all of the advantages could be kept. A type of Quadrangular frustum pyramid composition nanostructure with its bottom layer of silver and its top layer of gold is proposed. Based on discrete dipole approximation, localized surface plasmon resonances of metal composition nanoparticles induced by optical excitation is investigated, a detail discussion is given on the extinction properties and the refractive index sensitivity of quadrangular frustum pyramid nanoparticles in different size,metal composition ratio and so on. The result of calculation shows that the peak wavelength blue shifts as increasing the thickness of quadrangular frustum pyramid nanoparticles and red shifts as increasing the Ag concentration in the composition nanoparticles with the same shape. It is observed that if the thickness of quadrangular frustum pyramid nanoparticles with structure of gold adding to silver is more than the mean free path of conduction electrons, the excitation intensity of localized surface plasmon resonances won't change any more with increasing the thickness of added gold.Ag/SiO2/Ag composition nanoparticles composed of two silver layers and one SiO2 layer between them is another good candidate for LSPR sensors owing to the increased geometrical singularities or sharp edges and the coupling among layers. However, if the strong local field is contained within the SiO2 layer, the response to the change of local environment of this part field will disappear leading to the reduction of the refractive index sensitivity. In order to solve the problem, the part around sharp edges of SiO2 layer should be removed. A type ofâ… -shaped Ag/SiO2/Ag is present. Based on finite difference time domain method, localized surface plasmon resonances of Ag/SiO2/Ag nanoparticles induced by optical excitation is investigated, a detail discussion is given on the extinction properties, the enhancement of electromagnetic field and the refractive index sensitivity of Ag/SiO2/Ag nanocubes in different thickness of SiO2. The result of calculation shows that the plasmon tuning through thickness control is superior to that obtained through cube size control. Compared to single layered Ag nanocube, the extinction intensity of Ag/SiO2/Ag nanocubes is much higher (the most increased factor is about 1.5 times) and the factor of electromagnetic enhancement is much larger. Also the distributing of electromagnetic field of the two layers is different. In addition, the refractive index sensitivity of I-shaped Ag/SiO2/Ag nanocubes is increased about 100 nm/RIU compared to Ag/SiO2/Ag nanocubes. |
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