INVESTIGADORES
SCAFFARDI Lucia Beatriz
capítulos de libros
Título:
SIZE EFFECTS ON THE OPTICAL PROPERTIES OF METAL NANOPARTICLES. APPLICATIONS TO SIZING BY ANALYSIS OF EXTINCTION SPECTRA (CAP. 9)
Autor/es:
LUCÍA B. SCAFFARDI; JORGE O. TOCHO
Libro:
Progress in Nanotechnology Research
Editorial:
Nova Science Publishers
Referencias:
Lugar: New York; Año: 2007; p. 249 - 276
Resumen:
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.