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

Abstract

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.