五边形截面的Ag纳米线局域表面等离子体共振模式

五边形截面的单晶Ag纳米线对ZnO量子点荧光具有增强的现象. 为解释这一现象, 利用时域有限差分法对五边形截面的Ag纳米线的局域表面等离子体共振模式进行了理论模拟. 结果表明, 五边形截面的Ag纳米线在紫外区域存在两个消光峰, 分别由Ag纳米线的横向偶极共振(340 nm)和四极共振(375 nm)引起; 这两个消光峰与ZnO量子点荧光增强峰相一致, 而且随着Ag纳米线的半径增大而红移; 消光峰对应的共振模式取决于Ag纳米线的截面形状; 根据Ag纳米线电场增强倍数与激发光波长变化关系曲线可知, 最大增强电场位于五边形截面的顶点处, 而边线处电场增强较小. 理论模拟的结果较好地解释了Ag纳米线/ZnO量子点体系的荧光增强现象, 也为Ag纳米线在提高半导体材料发光效率、生物探测等方面的应用提供有益的参考.

Localized surface plasmon resonance modes in Ag nanowires with pentagonal cross sections

Ag nanowires have attracted much attention due to their potential applications in spontaneous emission amplifiers, logic gates, single photon sources, and biomolecule detection. Single crystal Ag nanowires are prepared by chemical method. The Ag nanowires exhibit pentagonal cross sections with an average radius of 80 nm. Two enhanced emission peaks (345 and 383 nm) are observed in ZnO quantum dots when mixing with Ag nanowires. To explore the origination of the enhancement, the localized surface plasmon resonance modes of Ag nanowires are investigated theoretically by the finite difference time domain method. The extinction spectrum, electric field distribution and electric field enhancement factor versus excitation wavelength of Ag nanowires are simulated. The results show that the Ag nanowires have two extinction peaks in the ultraviolet region: the 340 nm peak originating from the transverse dipole resonance (DR) and the 375 nm peak belonging to the transverse quadrupole resonance (QR). The same extinction peaks are also observed in the experimental measurement, which are consistent with the emission enhancement peaks of ZnO quantum dots. Compared with that of the DR peak, the red shift of the QR peak becomes more obvious with the increase of Ag nanowire radius. The resonance mode of the extinction peak depends on the cross sectional shape of the Ag nanowire. In the case of the traditional Ag nanowire with circular cross section, DR is excited by long wavelength light while QR is excited by short wavelength light. According to the curves of electric field enhancement factor vs excitation wavelength, the maximum enhanced electric field is observed at the apex of the pentagonal section of Ag nanowire, and the enhancement factor reaches 180 times for excitation wavelength of 377 nm. However, the electric field at the pentagon edge is enhanced only by several times. The simulation results give a reasonable explanation to the emission enhancement in Ag nanowire/ZnO quantum dot system, and indicate that Ag nanowires can be applied to improving the luminescent efficiency of semiconductor materials, biological detection, etc.