Toxic-Metal-Free Nanocrystals for Technological Applications

Throughout the years, semiconductor NCs emerge to be promising building blocks for a wide range of applications due to their unique electronic and optical properties at nanoscale. Particularly, colloidal quantum dots (CQDs) have drawn the interest of researchers since, apart from their unprecedented size, shape-tunable and composition-dependent properties, require cheap and easy synthesis, making them preferable for device fabrication. However, the majority of the materials that are currently used are either toxic or heavy-metal-based, which limits the application of devices and their commercialization.
Taking this into consideration, we decided to investigate new nanomaterials that fulfill a vital criterion: to be environmentally friendly, free of toxic or heavy-metal elements. In this way, their currently used counterparts can be replaced with these new materials and hence expand the commercialization of devices. The atomistic semi-empirical pseudopotential method (SEPM) is used for this investigation of mainly In-based and Ga-based materials. Our results are compared with available theoretical and experimental data. In this way, we enrich the knowledge on unexplained or insufficiently explained aspects in the properties of such NCs, proposing potential applications in realistic systems according to the calculated properties. In this theoretical research we characterize a new nanomaterial that has not been synthesized before in colloidal form. GaSb colloidal QDs
is an example of an environmentally friendly material that has high potential once synthesized. We predict a direct-to-indirect confinement-induced bandgap transition in the reciprocal space, at relatively large NC sizes (R < 36 A) due to strong quantum confinement effects. We show that emission can be tuned throughout the visible spectrum and that is accompanied by large Stokes’s shifts and long radiative lifetimes, attributed to the indirect nature of the bandgap. These results suggest that GaSb NCs can be ideal candidates in solar cells applications and memory storage, but most importantly, in a photocatalytic CO2 reduction with water. Additionally, our investigation included the modelling of InP NCs that have been widely used for decades in many applications. Considering the poor surface passivation of such NCs at the experimental level and that particular optical properties lack a satisfactory explanation, we applied the SEPM predicting an unprecedented relationship between the surface of the NC and the relative optical properties. By applying a newly derived passivation set achieving an ideally passivated surface,
we show that colloidal InP NCs can still exhibit low QY, depending on their surface composition (i.e., to whether the surface is In- or Prich), a prediction that gives new insights into the optical properties of this material and highlights the importance of NC surface. We expanded our investigation to different structures: we modelled 2D QD films made of In-based and Ga-based materials, following an atomistic tight-binding model (TBM). We show how toxic-free NCs can find application in band-like carrier transport, proposing potential alternatives for specific toxic materials that are currently used. In this investigation we show how the NC composition and stoichiometry can enhance the carrier mobility and lead to QD films more resilient to temperature changes. An ongoing investigation in the optical properties of GaP NCs showed that these NCs exhibit long radiative lifetimes and fast Auger recombination lifetimes. The aim of this work is to provide the first insights of such processes in GaP NCs, that are considered, among others, a potentially promising candidate for replacing toxic elements. The purpose of this thesis is to provide useful information about the unique properties of environmentally friendly nanomaterials, with the main objective to help experimental groups to replace their toxic counterparts.