Theoretical Calculation of Optical Absorption of Noble Metal Nanoparticles Using a Simple Model: Effects of Particle Size and Dielectric Function

摘要

Noble metal nanoparticles (NPs) exhibit size-dependent properties that are of interest for applications ranging from nanophotonics, biology, sensing, spectroscopy, and solar energy harvesting. To understand the effects of the size of the NPs and dielectric function of the surrounding medium on the optical absorption (OA) of Ag, Au, and Cu NPs embedded in silica matrix we have carried out a detail study, theoretically, using a simple model. For the simulations of the OA spectra, a modified Maxwell-Garnett type effective medium theory has been used for a range of 1 to 50 nm in radii of the NPs. The OA spectra corresponding to a radius of 2 nm and higher indicate the presence of surface plasmon resonance (SPR) bands. This SPR band is seen to shift towards lower wavelength (blue-shift) in case of Ag and Au NPs and towards higher wavelength (redshift) in case of Cu NPs with decrease in NP radius from a value of 10 nm. These shifts of the SPR peaks are explained on the basis of electron damping by the radiation effects and spill-out effects, respectively. In addition, for all the three cases, the bandwidth of the SPR peak has been found to increase linearly with increase in inverse of the NP radius up to a size of about 20 nm. This linear behaviour is mainly due to the damping of the conduction electron oscillations. An initial increase in SPR intensity and then decrease of it with increase in the dielectric function of the surrounding medium of the NPs have been seen for all three cases. Such changes in the SPR peak position and the intensity of the noble metal NPs can be successfully utilized for the practical applications in the optoelectronic devices and others.