Al-Au Heterogeneous Dimer-trimer Nanostructure for SERS

Abstract

Background: Tunability in resonance wavelength and the enhancement of the electromagnetic field intensities around the surface are two unique properties which make metal as a plasmonic material. A theoretical investigation on the LSPR and field enhancement for heterogeneous dimer–trimer metallic nanostructure by constituting Al and Au as two different plamsonic materials has been studied. Since electrons in Al exhibit free behavior for LSPR of Au, therefore, they influence the electric field magnitude generated by Au LSPR. Methods: The electromagnetic simulations reported in this paper were performed using the FDTD Solutions (version 7.5.1), a product of Lumerical Solutions Inc., Vancouver, Canada. We adopted a cubic Yee cell of 1 nm side and a time step Δt= 1.31•10-18 s, bounded by Courant condition. Results: The extinction spectrum shows LSPR peak over UV-visible region for isotropic nanostructure which shifts to NIR region for anisotropic shape nanostructure. The spherical shape hetero dimer nanostructure shows enhancement factor ~ 3.9 X 105 whereas it increases to ~ 6.2 X 106 for anisotropic shape at 610 nm. The field distribution corresponding to the trimer nanostructure reveals a large dipolar field distribution on each of the three nanoparticles, oscillating approximately in-phase. The spherical shape Al-Au-Al shows enhancement factor ~ 8.5 X 106 at 571 nm. The anisotropic shape increase the enhancement factor to ~ 2.4 X 107 at peak wavelength 700 nm i.e. tuning the plasmon wavelength towards NIR region. Conclusion: The tunability in plasmon wavelength and field enhancement factor has been evaluated for heterogeneous nanostructure over wider spectrum range i.e. DUV-Visible-NIR using Au-Al dimer and trimer nanostructure. The isotropic shape Au-Al hetero nanostructure shows larger enhancement in the UV-visible region, whereas the anisotropic shape nanostructure contributes towards the NIR region.