Controlled Synthesis And Optical Properties Of Elliptical Gold Nanowires

Different from bulk materials,nano materials have small size effect,surface effect,dielectric local effect and quantum size effect,so they show unique physical and chemical properties.For one-dimensional nano materials,their shape and size are important factors to determine their optical properties.The local and propagating surface plasmons are significantly affected by the geometry and size.Shape constraints will hinder the effective utilization of light waves by nano-optoelectronic devices.Theoretical studies show that compared with circular nanowires,the surface plasmon polaritons on elliptical nanowires have much less loss at optical frequencies.Moreover,the symmetry breaking of elliptical one-dimensional nanomaterials will bring anisotropic surface plasmon resonance.This kind of breaking is very similar to the charge distribution anisotropy of ellipsoids,elliptical disks and elliptical rings.The charge distribution anisotropy of the latter brings many highly adjustable optical properties,which are used in light absorption,photocatalysis,light collection and plasma resonance.As far as we know,there is still a lack of controllable means for the preparation of elliptical plasmonic one-dimensional nanomaterials,and its optical properties are not very clear.To solve this problem,in this thesis,the ion track template stretching method is proposed to prepare elliptical nanowires,and the important parameters such as ellipticity,size and array density are successfully adjusted.On this basis,we studied the optical properties of the surface plasmons,and studied the influence of the main parameters on the optical properties and the mechanism.There are three main research results in this thesis:(1)Fabrication and parameter control of elliptical gold nanowires.It is found that the ellipticity of elliptical gold nanowires can be controlled by controlling the elongation of the template.For the PC template with cylindrical channel diameter of105 nm,elliptic gold nanowires with ellipticity of 2.0,2.1,2.3 and 2.6 were prepared by controlling the stretch ratio.By controlling the etching time,cylindrical channels with different initial diameters were obtained,and then the elliptic size was controlled.Elliptic gold nanowires with ellipticity of 2.1 and short axis diameters of 33 nm,117 nm,173 nm and 225 nm were prepared.Experimental results show that the stretching template can not only control the length of the major axis,but also affect the length of the minor axis to a certain extent.However,this change is one order of magnitude smaller than the major axis and controllable,so the ellipticity and size can be controlled precisely.In addition,the density and length of the elliptical gold nanowire array were regulated by the irradiation ion fluence and deposition time respectively.Transmission electron microscopy analysis showed that the lattice fringe spacing of gold nanowire was about 0.235 nm(corresponding to the [111] crystal plane of gold).These results indicate that the ion track template stretching method is a reliable method for controllable fabrication of elliptical nanowires.(2)Optical properties of elliptical gold nanowire arrays.In this part of the work,we firstly carried out the study of spectral measurement method,and finally adopted the “polarizer + polarizer” double polarizer measurement scheme to reduce the influence of polarizer and stretch template on the final spectral results.It is found that the spectrum of elliptical gold nanowire array has polarization anisotropy and the cross section size and ellipticity have significant influence on the anisotropy.When the minor-axis size is shortened from 140 nm to 50 nm,the gap between the major-axis peak and the minor-axis peak increases from 100 nm to 150 nm,indicating that the polarization anisotropy of the small-size elliptical gold nanowires is more significant,which is due to the enhancement of surface plasmons caused by the reduction in size.When the minor-axis diameter is 140 nm,the ellipticity increases from 1.8 to 2.9,and the corresponding peak gap expands from 100 nm to 200 nm,indicating that the elliptic gold nanowires with large ellipticity have more significant spectral anisotropy.This is because the elliptical gold nanowires with high ellipticity have a large size difference between the minor axis and the major axis,thus increasing the difference of surface plasmons resonating along the minor axis and the major axis.(3)Optical properties of single elliptical gold nanowires.In this part of the work,the single particle dark field scattering spectroscopy experiment and finite difference time domain(FDTD)simulation method are used to reveal the Fabry-Perot resonance principle behind the spectral results,and the influence of the size parameter variation on the optical properties is analyzed and studied.It is found that there are similarities and differences between the spectral results of elliptical gold nanowires lying flat and upright,the former has one peak and the latter has three peaks.Corresponding spectral and field intensity simulation results show that this phenomenon is mainly caused by the existence of multiple Fabry-Perot resonance modes at finite length.The optical simulation results show that the increase of the elliptic gold nanowire length not only causes the red shift of the peak position,but also generates new Fabry-Perot resonance modes.The change of the major and minor axes of the ellipse will not cause a new Fabry-Perot resonance but will make the corresponding major-axis peak or minor axis peak redshift.In this thesis,elliptical gold nanowires with different ellipticity,size,array density and length have been prepared by the ion track template stretching method.Due to the symmetry break of the elliptic cross section,the spectrum of elliptical gold nanowires is characterized by polarization anisotropy and multi-peak response.The principle beneath this is derived from multiple Fabry-Perot resonance modes excited at the minor and major axes of the elliptical nanowires,respectively.The high quality elliptical nanowires prepared by this method are expected to be applied to polarized optical devices and solar energy absorption and storage.