Field enhancement in metal nanoparticles for hot electrons generation rate

In recent years, the prediction about the shortage of fossil oil, the increase of its extraction price and the environmental damage caused by fossil oil based product has been the reason of a novel wave of research in the renewable energy field. Among the many renewable sources hydrogen has became quite popular. Indeed, hydrogen combustion give as results heat and water, that can be again dissociated into oxygen and hydrogen. Hydrogen can become an efficient fuel in everyday future life but there are some issues related to its use as fuel. It has to be accumulated in his liquid form which is extremely flammable and the currently used hydrogen production methods, such as steam reforming, are energetically expensive and not eco-friendly. An alternative method to produce hydrogen is the photon-induced hydrolysis. Hydrolysis has gained a lot of attention in recent years as it would be a method that directly convert solar energy, which is an almost infinite energy source, into the chemical energy needed to break down water molecules. The basic idea is to use the photogenerated hot electrons inside metal nanostructures to drive the hydrolysis in surface molecules on top of the metallic surface. Indeed, when an electromagnetic wave impinges a metallic surface the electrons inside the metal jump to excited states. In case of bulk metal this phenomenon can be explained invoking surface plasmons, which are collective oscillations of the electron cloud. These electrons, nevertheless, don’t have enough energy to drive an hydrolysis reaction. However, if metallic nanocrystals are used then hot carriers make their appearance since the crystal momentum is not conserved anymore. These are the so called hot carriers which can drive the hydrolysis of water surface molecules. It is known that the density of photogenerated hot carriers is strongly dependent on material and shape of the nanostructure. The aim of this work is to further investigate the shape dependence of photon-induced hydrolysis and understand how this process can be optimized by mean of numerical simulations. It has been shown experimentally that the presence of the so called hot spots further enhances the hot electron generation rates. Therefore, firstly the properties of the elementary geometries, in terms of broadness of surface fields are examined by mean of a first set of finite difference time domain simulations. The next natural step is the study of 2d arrays of nanoparticles, which are of practical interest when it comes to hydrogen generation. It is indeed well known that the introduction of disorder in optical media gives rise to a strong localization of light just like the disorder in ion lattice potentials gives rise to the so called Anderson localization. Inspired by that, among all possible kind of 2d arrays, 2D disordered arrays, in which either the size of the nanoparticles or their position is chosen according to some probability distribution, have been chosen. The role of disorder has been quantified by mean of the the photonic localization length that has allowed to find the parameters that best enhance the broadness of the hot electron generation rate as well as the electromagentic field enhancement at the nanoscale.