Optical Properties of Nanoparticles(Studied using Mie Theory)

by Achal Bhardwaj

AbstractIn this work, the optical properties of nanoparticles was studied using Mie theory. This famous theory is suited to compute accurately the optical cross sections of a spherical particle. According to this theory, we can calculate the place of surface plasmon resonance in optical spectra of spherical nanoparticle. Matlab was used for simulation of some optical properties like scattering, absorbtion and extinction cross section area and of nanoparticles. Results show that front surface located nanoparticles Ag and Au are the most widely used materials due to their surface plasmon resonances located in the visible range and therefore interact more strongly with the peak solar intensity.

About Authors

Hossein Ghaforyan

Department of Physics, Payame Noor University,

Sara Mohammadi Bilankohi

Department of Physics, Payame Noor University,

Majid Ebrahimzadeh

Department of Physics, Payame Noor University,

Introduction

The optical properties of nanoparticles are highly dependent on the particle size, shape, chemical composition, and the local dielectric environment.

it is possible to selectively tune these properties to suit a given application

Many studies have explored the optical properties of metal nanoparticles using simulations, that are useful in applications including solar cell, imaging, sensing and constructing nanostructures.

Metal nanoparticles are widely used to construct structures that possess unique electronic, photonic and catalytic properties such as local surface plasmon resonance (SPR).

Surface plasmon resonance of metallic nanoparticles is one of the reasons of their unique optical properties.

Surface Plasmon Resonance (SPR)What is SPR?It is the resonant oscillation of conduction electrons at the interface between a negative and positive permittivity material simulated by incident light.

The resonance condition is established when the frequency of incident photons matches the natural frequency of surface electrons oscillating against the restoring force of positive nuclei.

Why it is important??Investigating the relationship between the sizes of nanoparticle with surface plasmon frequency in simple shapes such as spherical shape of nanoparticles is possible using the Mie theory.

According to this theory, we can calculate the place of surface plasmon resonance in optical spectra of metallic spherical nanoparticle.

Mie Theory

According to this theory, we can calculate the place of surface plasmon resonance in optical spectra of metallic spherical nanoparticle. Matlab program was used for simulation of some optical properties such as scattering and extinction cross section area and influence of embedded metal nanoparticles on FeS2 (Pyrite) solar cells.

Methodology of StudyThe Mie scattering theory allows describing the scattering of a plane monochromatic wave by a homogeneous sphere surrounded by a homogeneous medium for any particle radius and of any material.

Mie scattering theory does not deal with the problem of surface electron density oscillations (surface plasmons) coupled to surface localized electromagnetic fields, although usually positions of successive peaks appearing in light scattering spectra of conducting particles obtained with Mie theory, are interpreted as directly related to positions of surface plasmon resonances.

Once the Mie coefficients are determined, we can calculate the extinction, absorption and scattering cross sections or the electromagnetic fields inside and outside of the spherical particle.

Mie theory codes were used to simulate the optical properties of spheres nanoparticles.When the dimensions of the particles are smaller than the light wavelength it is possible to employ the so-called quasistatic approximation.

Mie theory codes were used to simulate the optical properties of spheres nanoparticles.When the dimensions of the particles are smaller than the light wavelength it is possible to employ the so-called quasistatic approximation.

Otherwise if the investigated structures have spatial dimensions say 50 nm, we can connect the macroscopic dielectric function with the microscopic polarizability (alpha).

The elastic scattering of light can then be described in terms of Rayleigh scattering. Expressing the dipole moment P through the local microscopic electric field on one hand, and connecting it with the dielectric function (through D = εE = E+4πP) on the other hand, leads to the Clausius-Mossotti relation for spherical particles.

ObservationsBelow are the Simulated optical extinction (black lines), scattering (red lines) and absorption (green lines) efficiencies of 10 nm and 100 nm diameter Ag spheres.

Below are the Simulated optical extinction (black lines), scattering (red lines) and absorption (green lines) efficiencies of 10 nm and 100 nm diameter Au spheres.

Influence of metal nanoparticle embedded on FeS 2 (Pyrite) Solar Cells

Pyrite, formally known as Iron disulfide, is the most abundant naturally occurring of the sulfide minerals.

It has a crystal structure that resembles the fluorite structure. Iron disulfide has a yellow-brass, metallic luster that is sometimes incorrectly recognized as gold.

Due to this mistaken identity it is often referred to as “fool’s gold”.As the result of sparks generated when struck against metal, pyrite was used as a source of ignition for early firearms.

We considered the metal nanoparticles are on FeS 2 surface.

Below figure shows the calculated extinction spectrum of a 60 nm diameter Ag and Au nanosphere embedded in FeS 2 .

ConclusionFor solar cell applications, we need high scattering and low absorption across the solar spectrum. Scattering in the near-infrared (NIR) is particularly desirable in solar cell, because this property lead to produce current at night too. As a result Ag and Au are more preferred for solar applications.

Thank You