Koushik Majhi

Putting DFT to the test: a first-principles study of electronic, magnetic, and optical properties of Co3O4.

First-principles density functional theory (DFT) and a many-body Greens function method have been employed to elucidate the electronic, magnetic, and photonic properties of a spinel compound, Co3O4. Co3O4 is an antiferromagnetic semiconductor composed of cobalt ions in the Co(2+) and Co(3+) oxidation states. Co3O4 is believed to be a strongly correlated material, where the on-site Coulomb interaction (U) on Co d orbitals is presumably important, although this view has recently been contested. The suggested optical band gap for this material ranges from 0.8 to 2.0 eV, depending on the type of experiments and theoretical treatment. Thus, the correlated nature of the Co d orbitals in Co3O4 and the extent of the band gap are still under debate, raising questions regarding the ability of DFT to correctly treat the electronic structure in this material. To resolve the above controversies, we have employed a range of theoretical methods, including pure DFT, DFT+U, and a range-separated exchange-correlation functional (HSE06) as well as many-body Greens function theory (i.e., the GW method). We compare the electronic structure and band gap of Co3O4 with available photoemission spectroscopy and optical band gap data and confirm a direct band gap of ca. 0.8 eV. Furthermore, we have also studied the optical properties of Co3O4 by calculating the imaginary part of the dielectric function (Im(ε)), facilitating direct comparison with the measured optical absorption spectra. Finally, we have calculated the nearest-neighbor interaction (J1) between Co(2+) ions to understand the complex magnetic structure of Co3O4.

Visualization Based Data Mining for Comparison Between Two Solar Cell Libraries

Material informatics may provide meaningful insights and powerful predictions for the development of new and efficient Metal Oxide (MO) based solar cells. The main objective of this paper is to establish the usefulness of data reduction and visualization methods for analyzing data sets emerging from multiple all‐MOs solar cell libraries. For this purpose, two libraries, TiO2|Co3O4 and TiO2|Co3O4|MoO3, differing only by the presence of a MoO3 layer in the latter were analyzed with Principal Component Analysis and Self‐Organizing Maps. Both analyses suggest that the addition of the MoO3 layer to the TiO2|Co3O4 library has affected the overall photovoltaic (PV) activity profile of the solar cells making the two libraries clearly distinguishable from one another. Furthermore, while MoO3 had an overall favorable effect on PV parameters, a sub‐population of cells was identified which were either indifferent to its presence or even demonstrated a reduction in several parameters.