Atomistic modeling of the structural and electronic properties of Cr-based oxides and their potential application as TCO-materials.
12 October 2017
Campus Groenenborger, U0.25 - Groenenborgerlaan 171 - 2020 Antwerpen (route: UAntwerpen, Campus Groenenborger
Organization / co-organization:
Department of Chemistry
Erik Neyts & Bart Partoens
Public defence of the PhD thesis of Mrs. Samira Dabaghmanesh - Faculty of Science - Department of Chemistry
This thesis focuses on the study of the electronic and structural properties of Cr-based oxides within density functional theory (DFT) and molecular dynamics simulations. We examine different DFT-based dispersion correction approaches to compute the bulk and molecular adsorption properties of three famous corundum-type oxides Cr2O3, α-Fe2O3 and α-Al2O3. Our study highlights the importance of van der Waals (vdW) interactions in the adsorption of molecules. Although the vdW contribution in the adsorption of CH3 (as a chemisorption interaction) is less important compared to the adsorption of benzene (as a physisorption interaction), we find this contribution is not negligible.
Next, we investigate Cr2O3 and LaCrO3 which have been recently received great experimental interests as potential candidates for p-type transparent conducting oxides (TCOs). Doped Cr2O3 has been shown to be a p-type TCO. Its conductivity, however, is low mainly due to a high effective hole mass and a flat valence band. Using first principles calculations we show that alloying Cr2O3 with sulfur increases the valence band dispersion and results in huge reduction in its hole effective mass. We introduce Cr4S2O4 with an optical band gap of 3.08 eV and an effective hole mass of 1.8 me, as a new p-type TCO host candidate.
In a separate study we perform first principles calculations and obtain the electronic structure and formation energy of various point defects in LaCrO3. Sr-doped LaCrO3 has been experimentally introduced as a new p-type TCO. Our results for the formation energies show that in addition to Sr, two more divalent defects, Ca and Ba, substituting for La in LaCrO3, behave as shallow acceptors. We further demonstrate that under oxygen-poor growth conditions, these shallow acceptors will be compensated by intrinsic donor-like defects (an oxygen vacancy and Cr on an oxygen site), but in the oxygen-rich growth regime the shallow acceptors have the lowest formation energies between all considered defects and will lead to p-type conductivity.
In the last chapter of the thesis we employ classical molecular dynamics simulations based on a recently developed force field to study the structural properties of Cr2O3 nanoclusters over graphene and carbon nanotubes. Carbon supported metal oxide nanoparticles hold promise for various future applications in diverse areas including spintronics, catalysis and biomedicine. These applications, however, typically depend on the structure and morphology of the nanoparticles. We for the first time demonstrate that the Cr2O3 nanoclusters tend to aggregate over both freestanding graphene and carbon nanotubes and form larger nanoclusters. These large nanoclusters are characterized by their worm-like shape with a lattice constant similar to that of bulk Cr2O3.