A method for sizing core-shell Ag-Ag2O single nanoparticle based on optical extinction spectroscopy

J. M. J. SANTILLÁN; L. B. SCAFFARDI; D. C. SCHINCA

Buenos Aires

Congreso; XIII Giambiagi School, Nanophotonics, july 18-22, 2011, Buenos Aires, Argentina; 2011

Universidad de Buenos Aires, UBA

Metallic nanoparticles (Nps) are of great interest owing to the ability of controlling optical fields on the nanometer scale. Plasmon resonance optical properties, such as peak wavelength, full width at half maximum (FWHM) and contrast (defined below as the relation between the maximum and the minimum of the extinction spectra), depend on the material, size, shape and structure of the Nps, as well as on the surrounding media. This paper develops a parametric method for determining the core radius and shell thickness in small silver–silver-oxide core–shell Nps (r < 10 nm) based on single particle optical extinction spectroscopy. Using the electrostatic approximation of Mie theory, core–shell single particle extinction spectra were calculated for a silver particle’s core size smaller than about 20 nm and different thicknesses of silver oxide around it.  The study of the relationship between plasmon peak wavelength, FWHM and contrast of the extinction spectra as a function of core radius and shell thickness reveals that plasmon peak wavelength is strongly dependent on shell thickness, whereas FWHM and contrast depend on both variables. Even a very thin oxide layer shifts the plasmon peak noticeably, enabling plasmon tuning with appropriate shell thickness. This characteristic, together with the behaviour of FWHM and contrast of the extinction spectra can be combined into a parametric method for sizing both core and shell of single silver Nps in a medium using only optical information. In turn, shell thickness can be related to oxygen content in the Np’s surrounding media. The importance of the method lays in the possibility of monitoring the growth of the silver-oxide layer around small spherical silver Nps in real time. The method proposed is applied to size silver Nps from single particle extinction spectrum. The results are compared with full optical spectrum fitting using the electrostatic approximation in Mie theory. In this context we have successfully reproduced experimental Ag single Np extinction spectrum, based on a core–shell Ag–Ag2O structured Np using Palik’s dielectric function. For the studied case, core and shell radii are r1 = 22 nm and r2 = 22.9 nm, respectively. Taking into account the Johnson and Christy (J-C) dielectric function for comparative study, we conclude that it does not seem to be able to reproduce the same spectrum, neither for bare core nor core–shell Nps, at least if accepted values for the damping constant correction factor are used. The method may be the basis for developing a plasmonic sensor for O2 concentration based on Ag single NP spectroscopy.