Tailoring Optical Cross Sections Of Gold Nanords At A Targetplasmonic Resonance

The intensity of light and matter interactions in the nanoscale can be represented by optical cross section.These strong interactions rely on surface plasmon resonance(SPR),the collective oscillation of free electrons across the particle interface under the excitation of the incident light.The SPR is featured as a Lorentzian-shaped peak centered at a resonance wavelength.The resonance wavelength is dependent on the particle shape.Varying the shape in terms of aspect ratio R allows for tuning the longitudinal surface plasmon resonance(LSPR)of gold nanorods(AuNRs)from visible to the near infrared.The height of the Lorentzian peak,known as the optical cross section,is mainly determined by the particle size.Absorption and scattering both contribute to the overall extinction cross section,whereas the fraction of scattering and absorption in extinction is also size-dependent.In many applications the LSPR is required to be at a target wavelength depending on the excitation source and the photodetector.Size tuning as well as optical cross section tailoring the at a fixed target LSPR wavelength is therefore highly desirable.However,the size tuning is usually accompanied by the shift of the LSPR wavelength,requiring quantitative instructions for fine control.This paper is a research subject based on this problem.The following is the specific research content of this paper:(1)Herein,we report a method towards finely tailoring optical cross sections of AuNRsat a fixed target resonance wavelength.Using a binary surfactant mixture consisting 5-bromosalylic acid(BSA)and cetyltrimethylammonium bromide(CTAB).A systematic study was performed on the sum effects of the BSA concentration and the volume of growth solution,which gives a formula for quantitative instructions.The overgrowth of Au was found to occur more preferentially on the sides of AuNRs in presence of BSA in absence of Ag.This BSA-directed Au growth allows for fine control of the shape of AuNRsduring overgrowth,meaning that a fixed target LSPR wavelength can be achieved by controlling the combinations of BSA concentration and the volume of the growth solution.Based on the formula,we gave examples for successful synthesis of AuNRs with different optical cross sections at target LSPR wavelengths.(2)In order to determine the size of the gold nanorods,we further derived the nonlinear relationship between the aspect ratio and the width of the gold nanorods under fixed LSPR wavelength through FEM numerical simulation.The ratio of optical cross section to physical cross section of the gold nanorods is calculated and plotted as a function of the width of the gold nanorods.The result clearly indicates that AuNRs with the width of 30 nm possess a highest efficiency in terms of optical per physical cross section.