SIZE EFFECTS ON THE OPTICAL PROPERTIES OF METAL NANOPARTICLES. APPLICATIONS TO SIZING BY ANALYSIS OF EXTINCTION SPECTRA

LUCÍA B. SCAFFARDI; JORGE O. TOCHO

Progress in Nanotechnology Research

Nova Science Publishers

Lugar: New York; Año: 2006; p. 1 - 40

Abstract Small metallic particles present striking optical properties well known from antique times. Especially important were noble metals from which strong absorption in the visible were used to produce beautiful colored glasses. Nowadays, advances in the production, manipulation and measurements on the nanometer scale have revitalized the study of small metallic particles. Applications of nanoparticles to biological sensors and optoelectronic devices, among other, are now receiving increasing attention. Nanostructured metals are believed to be one of the key ingredients for future optoelectronic devices. From the beginning of the last century it is known that optical properties of small particles must be assigned to both the specific constitutive element and to the size and shape of the particles present in the material. Strong absorption of metallic nanoparticles are due to resonances between electromagnetic field and collective oscillations of the free electrons, phenomena known classically as surface modes or surface plasmon in quantum mechanical language. It is surprising that classical electromagnetic treatment is enough to explain the details of the extinction spectra of nanoparticles. That is, the optical properties of the bulk material, given by the dielectric function or the refractive index, can be used to describe the behavior of the spectra of very small particles, down typically to 10 nm or 20 nm radius, depending on the specific material. For small enough particles, the mentioned procedure fails to reproduce the experimental extinction spectra and it is accepted that the optical properties must be modified to introduce size effects. For metal particles smaller than the mean free path of conduction electrons in the bulk metal, the mean free path can be dominated by collisions with particle boundary. The damping constant is increased due to additional collisions with the boundary of the particle. However, the plasma frequency is assumed to be independent of the size. This procedure introduces a modification for size in only the contribution of free electrons on the dielectric function. Good agreement with experimental spectra was obtained for gold particles bigger than 3 nm but the range can be extended for smaller particles if only the imaginary part of the dielectric function is modified. For noble metals, transitions of bound electrons to conduction levels contribute appreciably to the dielectric function. In this chapter, the contributions of both free and bound electrons to the dielectric function and its change with size will be explored. The influence of the size dependant dielectric function on the extinction spectra of gold nanoparticles will be considered, and the values of the different parameters involved in the model will be determined. As these parameters modify mainly the contrast between the maximum and the minimum in the extinction spectra near resonance, the measure of this contrast can be used to determine the size of metallic nanoparticles. Results shown in this chapter can be used as a useful tool to determine the mean radius of gold nanoparticles between 0.5 nm and 5 nm from the analysis of the spectral extinction measurements. These results extend to smaller values the range of radii of metallic particles covered by scattering methods in one order of magnitude.