Oxygen Deficiencies in Titanium Oxide Clusters as Models for Bulk Defects

Abstract

TD-DFT calculations were performed on neutral TinO2n, TinO2n-1, and TinO2n-2 clusters, where n ≤ 7. Our calculations show that the TinO2n clusters are closed shell systems containing empty d orbitals and that the partially filled d orbitals of the suboxide clusters have a profound effect on their structural, electronic, and topological properties. The low energy photoexcitations of TinO2n clusters are all O-2p to Ti-3d transitions, while the open-shell suboxide clusters are all characterized by d-d transitions that occur at a much smaller optical gap. Upon photoabsorption, the localization of the hole is accompanied by a local bond elongation, i.e., polaron formation, whereas d-electrons are generally delocalized among the cluster. Several of the compact of the TinO2n-2 structures contain higher symmetry which is reflected in their relative stability in the experimental cluster distribution. In particular, the tetrahedral symmetry of the optimized ground state structure for Ti4O6 inhibits charge carrier localization and therefore contains higher stability.

Effect of oxidation on excited state dynamics of neutral TinO2n−x (n < 10, x < 4) clusters

“…The cluster distribution is consistent with previous studies, 30 with the highest intensity peaks generally composed of TinO2n-1 or TinO2n-2, and is in agreement with their stabilities. 15,31 Neutral stoichiometric (TinO2n) clusters have previously only been recorded experimentally using single photon (VUV) ionization thought to be void of fragmentation, 32 supporting that fragmentation is not significant in our experiment. Thus, the formation of O deficient clusters is due to kinetic limitations during the growth of clusters in the ablation plasma.…”

“…The cluster geometries and electronic structures were calculated using time-dependent density functional theory and CAM-B3LYP functional as described in detail in other publications. 26,31 The two terminal O atoms in TinO2n clusters have the lowest binding energy, making the geometry of TinO2n-1 clusters similar to TinO2n but lacking one dangling O atom. The lack of one dangling O atom causes the HOMO to shift from an O 2p-orbital onto a Ti 3d-orbital making both the HOMO and LUMO primarily localized on Ti atoms.…”

“…The lack of one dangling O atom causes the HOMO to shift from an O 2p-orbital onto a Ti 3d-orbital making both the HOMO and LUMO primarily localized on Ti atoms. 31,35 Therefore, photoexcitation in suboxides is a d-d transition and occurs with a much smaller optical gap. 35 The TinO2n-…”

Communication: Effect of Oxidation on Excited State Dynamics of Neutral TinO2n-x (n<10, x<4) Clusters

“…Surprisingly, despite the large changes to the electronic and structural characteristics of the clusters as they gain and lose O atoms, 26,31 their excited state dynamics remain roughly consistent. The transient signals for all clusters contain a fast (35 fs) response and a sub-ps relaxation lifetime (τ).…”

“…The cluster geometries and electronic structures were calculated using time-dependent density functional theory and CAM-B3LYP functional as described in detail in other publications. 26,31 The two terminal O atoms in TinO2n clusters have the lowest binding energy, making the geometry of TinO2n-1 clusters similar to TinO2n but lacking one dangling O atom. The lack of one dangling O atom causes the HOMO to shift from an O 2p-orbital onto a Ti 3d-orbital making both the HOMO and LUMO primarily localized on Ti atoms.…”