Studying The Extinction Properties Of Silver Nanoparticles Using A Method Known As The Discrete Dipole Approximation (DDA)

The optical properties of metal nanoparticles have long been of interest in physical chemistry, starting with Faraday's investigations of colloidal gold in the middle 1800s. in 1908, Mie presented a solution to Maxwell's equations that describes the extinction spectra (extinction = scattering + absorption) of spherical particles of arbitrary size. Mie's solution remains of great interest to this day, it is the only simple, exact solution to Maxwell's equations that is relevant to particles. In addition,most of the standard colloidal preparations yield particles that are approximately spherical, and most of the optical methods for characterizing nanoparticle spectra probe a large ensemble of these particles. This leads to results that can be modeled reasonably well using Mie theory. More recently, new lithographic techniques as well as improvements to classical wet chemistry methods have made it possible to synthesize noble metal nanoparticles with a wide range of sizes, shapes, and dielectric environments. there has been growing interest in characterizing the optical properties of metal nanoparticles that are made using lithographic methods such as nanosphere lithography,e-beam lithography, and other methods, which produce well-defined sizes and nonspherical shapes without aggregation. In addition, variations on classical wet chemistry techniques have been developed that give high yields of nonspherical particles, especially rods and triangles.the modern generation of metal nanoparticle science,including applications to medical diagnostics6 and nanooptics, has provided new challenges for theory. All of these factors motivate the need for theory that can describe the electrodynamics of nanoparticles of arbitrary shape and size subject to a complex external dielectric environment. In this feature article, we describe recent progress in the theory of nanoparticle optical properties, emphasizing especially the linear optical properties (extinction, absorption, scattering,)of isolated silver particles of arbitrary shape with sizes up to a few hundred nanometers. This paper begins with a qualitative discussion of the use of simple electrostatic theories and models-Mie theory and M-G theory. We then give a brief description of modern numerical techniques that can treat arbitrary particles and environments, and then, we describe applications of these techniques to problems of recent interest, using a method known as the discrete dipole approximation (DDA). Our applications include studies of the particle size and shape dependence of absorption, extinction, and scattering efficiencies , which can be directly compared with experiments. In this work, the main features in the optical spectra have been investigated depending of the geometry and size of the nanoparticles. The origin of the optical spectra are discussed in terms of the size, shape, and material properties of each nanoparticle, showing that a nanoparticle can be distinguish by its optical signatur. In most cases, we have clearly identified the main optical signature associated to each geometry. We find,as it might have been expected, that the spectra are more complex as the particle has less symmetry and/or has more vertexes. Our main goal is 2-fold: first we show how the main peaks of the optical spectra can be associated to the shape and size of the nanoparticle, as well as its material properties, and second, we show in all these spectra the relative importance of absorption and scattering processes. we attach a multipolar character to the different excitations responsible for the charateristic features of the spectra by comparing results for particles with the same shape but with different size. We have also paid a particular attention to the accuracy of our calculations showing...