Self-energy and excitonic effects in the electronic and optical properties ofTiO2crystalline phases

Abstract

We present a unified ab initio study of electronic and optical properties of TiO 2 rutile and anatase phases with a combination of density-functional theory and many-body perturbation-theory techniques. The consistent treatment of exchange and correlation, with the inclusion of many-body one-particle and two-particles effects in self-energy and electron-hole interaction, produces a high-quality description of electronic and optical properties, giving, for some quantities, the first available estimation for this compound. In particular, we give a quantitative estimate of the electronic and direct optical gaps, clarifying their role with respect to previous measurements obtained by various experimental techniques. We obtain a description for both electronic gap and optical spectra that is consistent with experiments by analyzing the role of different contributions to the experimental optical gap and relating them to the level of theory used in our calculations. We also show the spatial properties of excitons in the two crystalline phases, highlighting the localization character of different optical transitions. This paper aims at understanding and firmly establishing electro-optical bulk properties, yet to be clarified, of this material of fundamental and technological interest for green energy applications.

Hubbard-Ucorrected Hamiltonians for non-self-consistent random-phase approximation total-energy calculations: A study of ZnS,TiO2, and NiO

“…The O-2s states lie far (17 eV) below the VBM. These electronic structure features have been observed and discussed in numerous other works [67][68][69][70][71][72].…”

Clocking the Ultrafast Electron Cooling in Anatase Titanium Dioxide Nanoparticles

“…On the other hand, many-body correlations have been thought to be negligible in this material and, as such, they remained widely unexplored. Recently, by employing state-of-the-art experimental [21] and computational techniques [21][22][23][24][25], the substantial role of electron-hole Coulomb correlations was unravelled in the anatase polymorph of TiO 2 . Strongly bound direct excitons were found to emerge with a binding energy exceeding 150 meV and a pronounced coupling to the phonon degrees of freedom [21,26].…”

“…and robust against temperature and defects, they are expected to move freely on the (001) plane [21,23]. As such, they may offer an efficient source to mediate the flow of energy at the nanoscale in engineered devices based on anatase TiO 2 .…”

“…Based on the band structure of anatase TiO 2 [23], the single-particle states contributing to the a-axis exciton lie along the Γ-Z direction of the Brillouin zone (Fig. 3).Due to the symmetry of the p-d wavefunctions, however, the states exactly at the Γ point are symmetry forbidden and do not contribute to the exciton state [23]. Once a nonequilibrium distribution of uncorrelated electron-hole pairs is created, the electrons are expected to thermalize to the bottom of the CB at Γ and the holes to the top of the valence band (VB) close to X.…”

“…Central for this is to clarify the electron-hole populations giving rise to the exciton collective state. Based on the band structure of anatase TiO 2 [23], the single-particle states contributing to the a-axis exciton lie along the Γ-Z direction of the Brillouin zone (Fig. 3).…”