Oscillation in Excited State Lifetimes with Size of Sub-Nanometer Neutral (TiO2)n Clusters Observed with Ultrafast Pump-Probe Spectroscopy

Abstract

<div>Neutral titanium oxide clusters of up to 1 nm in diameter (TiO<sub>2</sub>)<sub>n</sub>, with n < 10, are produced in a laser vaporization source and subsequently ionized by a sequence of femtosecond laser pulses. Using 400 nm pump, 800 nm probe lasers, the excited state lifetimes of neutral (TiO<sub>2</sub>)<sub>n</sub> clusters are measured. All clusters exhibit a rapid relaxation lifetime of ~30 fs, followed by a sub-picosecond lifetime that we attribute to carrier recombination. The excited state lifetimes oscillate with size, with even numbered clusters possessing longer lifetimes. Density functional theory calculations show the excited state lifetimes are correlated with electron-hole pair localization or polaron-like formation in the excited states of neutral clusters. Thus, structural rigidity is suggested as a feature for extending excited state lifetimes in titania materials.</div>

Oxygen Deficiencies in Titanium Oxide Clusters as Models for Bulk Defects

“…In contrast, the excited d-electron density of suboxides is typically delocalized across many, if not all, of the Ti atoms in the system. Interestingly, this behavior is starkly different than both the stoichiometric clusters 29 and extended surfaces, 57 where excess electrons are typically pinned to one Ti 3+ site. Thus, tightly bound excitons only appear in the TinO2n clusters.…”

“…Detailed information on the experimental setup was previously described. 49 The static mass spectrum was published previously 29 and reveals the presence of neutral clusters containing several stoichiometries, with the TinO2n-1 series generally being the most prominent. The TinO2n series are the least intense cluster series in the distribution obtained through femtosecond ionization of neutral clusters produced via laser ablation.…”

“…7,23 Our recent work on closed shell neutral TinO2n clusters revealed the electronic properties of the S1 excited state are correlated with the UV-excited state subpicosecond lifetimes. 29 However, suboxides retain partially filled d-orbitals, which lead to profound changes in their electronic and photoabsorption properties. Here, we extend our previous theoretical approach 29 to the study of suboxide clusters, as models for O vacancies, to determine the effect that partially filled d-orbitals have on the electronic and structural properties of titanium oxide.…”

“…29 However, suboxides retain partially filled d-orbitals, which lead to profound changes in their electronic and photoabsorption properties. Here, we extend our previous theoretical approach 29 to the study of suboxide clusters, as models for O vacancies, to determine the effect that partially filled d-orbitals have on the electronic and structural properties of titanium oxide.…”