Analysis of optical properties of spherical coated silver nanoparticles

DANIEL C. SCHINCA; LUCÍA B. SCAFFARDI; JORGE O. TOCHO

Buenos Aires, Argentina

Conferencia; 10th International Conference on Near-Field Optics, Nanophotonics and related Techniques; 2008

Facultad de Ciencias Exactas y naturales, Universidad Bs As (UBA)

10th International Conference on Near-Field Optics, Nanophotonics and related Techniques Buenos Aires, Argentina, September 1-5, 2008                                                                                   ANALYSIS OF OPTICAL PROPERTIES OF SPHERICAL COATED SILVER NANOPARTICLES.   Daniel C. Schinca1,2, Lucía B. Scaffardi1,2 and Jorge O. Tocho1,3 1CIOp (CONICET, CIC), c.c. 124, 1900 La Plata, Argentina 2 Área Departamental de Ciencias Básicas, Facultad de Ingeniería, Universidad Nacional de La Plata, Argentina 3 Departamento de Física, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Argentina e-mail: daniels@ciop.unlp.edu.ar ABSTRACT A great amount of research has been directed toward a multitude of potential uses for nanomaterials through the engineering of the properties of such materials by controlling their size. Suspended metallic nanoparticles (Nps) exhibit well known optical properties that depend on size, shape and refractive index of particles and surrounding media. These characteristics allow determining size of the particles through the analysis of extinction spectra of the plasmon, when the optical properties are known, or inversely, when particle size is known, it is possible to derive the optical properties through extinction spectra. Within this area, core-shell Nps are a growing matter of research, because of their optical, electrical and thermal properties, or highly sensitive pressure and temperature dependent conductivity, among others. This work presents a detailed comparative study between the behaviour of the calculated extinction spectra of coated and uncoated silver nanospheres as a function of the metallic core radius, coating dielectric thickness and refractive index of the surrounding media and coating.  The effects of these parameters on plasmon peak position (central wavelength) and on the resonance width (FWHM) are analyzed in particular for Nps with size smaller than 10 nm radius. The silver nanospheres extinction spectra were calculated using dipolar approximation taking into account a modification of the dielectric function due to size effect. Finally, the results of this analysis allow proposing a method to determine the mean radius of the silver Nps in the range 1 to 10 nm.  These studies were applied to two kind of silver Nps samples: one obtained by chemical processes (with coating) in ethanol, and the other without coating obtained by ablation process with a Ti:Za ultrashort pulses. Chemical methods tend to yield small Nps with a controllable and stabilized size by covering it with a shell of different organic compounds. On the other hand, laser ablation methods produce in some cases Nps with a broader distribution but a clean surface, which is more appropriate for functionalization in different industrial and biological applications.