J. Dimuna, T. Boyett, I. Miotkowski, A.K. Ramdas, T. Pekarek, and J. T. Haraldsen
Dr. Jason Haraldsen | College of Arts and Sciences | Department of Physics
Using computational and experimental techniques, we examine the nature of the 2+ oxidation of Ti-doped CdSe. Through stoichiometry and confirmed through magnetization measurements, the weakly-doped material of Cd1-xTixSe (x = 0.0043) shows the presence of a robust spin-1 magnetic state of Ti, which is indicative of a 2+ oxidation state. Given the obscure nature of the Ti2+ state, we investigate the electronic and magnetic states using density functional theory. Using a generalized gradient approximation with an onsite potential, we determine the electronic structure, magnetic moment density, and optical properties for a supercell of CdSe with an ultra-low concentration of Ti. We find that, in order to reproduce the magnetic moment of spin-1, an onsite potential of 4-6 eV must be in included in the calculation. Furthermore, the electronic structure and density of states shows the presence of a Ti-d impurity band above the Fermi level and a weakly metallic state for a U = 0 eV. However, the evolution of the electronic properties as a function of the Hubbard U shows that the Ti-d drop below the Fermi around 4 eV with the onset of a semiconducting state. The impurity then mixes with the lower valence bands and produces the 2+ state for the Ti atom.
Hello, and thank you for clicking on my presentation. My name is John Dimuna and I am working on a computational condensed matter project with the physics department at UNF. In this project, we used a powerful computer to simulate the motion of elections through a method called density functional theory. The material we examined was titanium doped cadmium selenide, which is a semiconductor. A semiconductor is a material that will allow electrons to move through the materials under certain conditions. Starting with a large 100 atom cadmium selenide structure, we replaced one cadmium atom with titanium. This process is called doping. Titanium has less electrons than the cadmium atoms being replaced in the host lattice, therefore this is n-type doping specifically. When titanium shares its electrons with the surrounding lattice, the titanium atom becomes charged. This is known as an oxidation state. Titanium is known to exhibit many oxidation states with some naturally occurring and others more difficult to induce. However, when experimentally substituted into our material, titanium is observed experimentally to take a +2 state while keeping the material in a semiconducting state. When running simulations of the material, we found that the titanium occupied a state of +1 and observed through the electronic structure and other measured characteristics that the bulk material had metallic properties. Since this did not match the experimental data, it was clear that the crystal structure required an onsite potential, known as a Hubbard U potential. This potential tells the titanium to hold on to its electrons more tightly. After applying this potential, the titanium atom shifted towards the +2 state and the electronic structure produced a semiconducting state, which is consistent with the observations from the experiment. Therefore, we are able to show that this crystal produces a very obscure titanium by creating a strong potential at the titanium site. This effect was observed in experiment but these calculations allow us to understand the nanoscopic nature of this effect.