Magnetismo em nanocristais dopados com impurezas não magnéticas e propriedades estruturais, eletrônicas e ópticas de pontos quânticos de fósforo negro

Abstract

The rst part of this thesis aims to study of Silicon, Carbon and Germanium nanocrystals doped with a single impurity (Boron or Phosphorus) com sizes ranging between 7.3Å e 35Å. The calculations were performed using Density Functional Theory as implemented in SIESTA code. Our results show that the doped nanocrystals exhibit size-dependent magnetic dipole moments, vanishing for large nanocrystal diameters. The rate of dipole momentreductiondependsonthenanocrystalspecimen. Inallcases, thebulkcalculations providedzeromagneticdipolemoment. Wealsoperformedcalculationswheretheposition of the impurity is changed within the nanocrystal of silicon and germanium, and we obtained strong oscillation of the magnetic dipole moment. In the second part we present a theoretical study of the charging e ects in single and double layer black phosphorus quantum dots (BPQDs) with lateral sizes of 2 nm and 3 nm. We demonstrate that the charging of BPQDs are able to store up to an N max electron (that depends on the lateral size and number of layers in the QD), after which structural instabilities arises. For example, 3 nm wide hydrogen-passivated single layer BPQDs can hold a maximum of 16 electrons, and an additional electron causes the expelling of hydrogen atoms from the QD borders. We also calculated the addition energy (EA) spectrum. For single layer QDs with 2 and 3 nm of lateral sizes, the average EA is around 0.4 eV and 0.3 eV, respectively. For double layer QDs with the same sizes, the average EA is around 0.25 eV and 0.2 eV, respectively. These energies are much larger than the thermal energy at room temperature (≈ 30 meV), satisfying the condition of the e ect Coulomb blockade. These results are important for future experimental studies, in order to optimize the operation of new devices. We also investigated the optical properties of PQFNs deposited on substrates of SiO2 and Si, where the results make clear the dramatic e ect of substrates and the number of layers of BPQDs in the excitonic gaps and excitonic spectrum.