Study of Localized Surface Plasmons in Gold Nanorods

Das, Anwesh (2016) Study of Localized Surface Plasmons in Gold Nanorods. Masters thesis, Indian Institute of Science Education and Research Kolkata.

PDF (MR dissertation of Anwesh Das (14MR003)) MR__Thesis_Anwesh_Das_14MR003.pdf - Submitted Version Restricted to Repository staff only Download (4MB)

Abstract

Metals typically reflect in the low frequency region of the electromagnetic (light) spectrum, whereas they are transparent to light in the deep Ultra Violet region. However as the size of particles is reduced to the nanometer regime, they can show both absorptive and scattering behavior in the near-UV and Visible parts of the Electro-magnetic spectrum. In nanostructures absorption and scattering leads to an extinction peak in the absorption spectrum at the same frequency. This extinction is not seen in their bulk counterparts. Theoretical simulations and experiments have been conducted for all known metals. Noble metal nanoparticles form an interesting class of particles because their extinction peak lies in the visible region. Therefore they show bright colours in visible light. Silver and Gold nanoparticles in vacuum show an extinction peak at approximately 3.6 eV and 2.38 eV, respectively. This extinction peak arises due to activation of normal modes of sub-wavelength particles or surface plasmon modes. The extinction in silver is due to surface plasmon modes only. For gold, the extinction has contributions from surface plasmons and electronic inter-band transitions from d to sp orbitals. Surface plasmons in nanoparticles are localized within a single nanoparticle and are hence known as localized surface plasmons.Gustav Mie in 1908 had formulated the extinction and scattering of an arbitrary sized sphere by solving the maxwell’s equations. The behavior of noble metal nanostructures can be described within the Mie scattering theory. The formulation in small particle is further simplified when the diameter of the particle is approximately 1/100th times the wavelength of the interacting field. The extinction can simply be formulated in terms of size parameter of the particle, dielectric function of the particle and dielectric function of the surrounding medium by ignoring the retardation effects. In order to observe and study these effects, my thesis presents an investigation of gold nanostructures (spheres and spheroids) in aqueous solution. The particles were synthesized using the seeded method for colloidal gold nanorods. Samples have been characterized using XRD, UV-Vis absorption spectroscopy and FESEM. The spheres and the nanorods show one and two extinction peaks in the UV-Visible spectrum. These peaks arise primarily due to oscillation of electrons along the transverse and longitudinal axes of the nanostructure. The growth dynamics of gold nanorods has been studied using UV-Visible absorption spectroscopy. Investigation has also been carried out to study and tune the Longitudinal Surface Plasmon Resonance (LSPR) of gold nanorods within the Vis-NIR region (520 nm - 790 nm) of the electromagnetic spectrum.

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