FERMI | TeraFERMI Elettra | SISSI-Mat The introduction of oxygen vacancies has a significant impact on the activity of titanium dioxide, a prototype photocatalytic material, by generating in-gap states that extend the photoresponse well into the visible and infrared regions. Here, we employ terahertz (THz) short pulses to explore the electron dynamics around the oxygen vacancies. We investigated anatase oxygen-deficient TiO2-x thin films, by first measuring their steadystate optical reflectivity over a broad frequency range, from THz to ultraviolet region (12 meV – 6.5 eV) at the SISSIMaterial science beamline at Elettra and at the Sapienza University of Rome. The optical reflectivity shows three main features, namely a free electrons Drude term, a mid-infrared band (MIR), that is usually interpreted in term of polaron formation, due to the interaction of excess electrons delocalized over several Ti atoms with O2- ions, and a near-infrared band, associated to the presence of a deep localized electron, usually referred as defect state (DS), trapped in a structural lattice distortion induced by the vacancy. We refer to a polaron as an electron “dressed” with its self-induced polarization, due to the displacement of the ions caused by the electronic charge in an ionic crystal or a polar semiconductor. Secondly, we addressed THz pump-infrared (IR) probe (780 nm) measurements at the TeraFERMI beamline at FERMI. We found that the area of the transient reflectivity response (integrated over the time interval 0-30 ps) directly scales with the intensity of the DS component. This suggests that those time-resolved measurements probe the dynamics of the defect state after THz photoexcitation. Moreover, we found that the 780 nm probe signal exhibits at least three different time scales (see Fig. 1 b, lower panel). A first one is a sharp rise time after THz excitation, providing a time scale ≤ 100 fs, which corresponds to the rise time of the pump THz electric field. After photoexcitation, relaxation processes occur, corresponding to a short time scale in the range 0.2-0.4 ps for all the samples. This relaxation time is longer than the decay of the (almost) symmetric single-cycle THz-pump pulse. After this decay, the transient reflectivity becomes almost flat, compatible with a long relaxation with time scale values in the order of ns. The most intuitive interpretation of the experimental data is in terms of polaron excitation by a THz field, as described in the sketch of Fig. 1 (a). The defect state can be seen as a small polaron localized at the oxygen vacancies. The THz pump field interacts with both the electrons and the ions, providing a displacement strong enough as to lead electrons to climb the defect state potential well and eventually to escape from the oxygen vacancy site. The photoionized free charge is now allowed to delocalize within the Ti- 3d conduction band. The underlying strongly polar lattice leads to large polaron formation, accompanied by a structural distortion extended over several Ti sites. While the polaron formation occurs between 200 and 400 fs, as found also in previous literature, the time required to the large polaron to relax back to the original defect state is few ns. This time scale represents indeed the time needed for a scattering event to take place between the large polaron and an oxygen vacancy, thus allowing the relaxation to the deep defect state to occur. This picture is corroborated by the use of a 1560 nm probe, a wavelength falling in the spectral range between the MIR and the DS bands. The response has a positive sign, contrary to the signal probed at 780 nm - see Fig. 1 (b) -, while the long steplike dynamics after a very short and sharp rise resemble those occurring in the 780 nm probed response. The analysis of the sign of the pump-probe signal for the different probe energies confirms indeed the polaronic interpretation, as schematically represented in Fig. 1 (c). Here we report the variation of the optical conductivity between the one before pump, corresponding to the steady-state measured one, and the one after pump, i.e. a fictitious conductivity constructed by arbitrarily reducing the spectral weight of the defect state component, while increasing of the same amount the mid-infrared band in order to conserve the total spectral weight. A reconstruction of the corresponding transient reflectivity is shown in the lower panel, hence highlighting that such a redistribution of the spectral weight induces a reduction of the transient reflectivity at 780 nm and an increase of the transient reflectivity at 1560 nm, consistent with the experimental data (Fig. 1 b). This confirms the drop of the spectral weight after the THz pump pulse, associated to the defect state and accompanied by an increase of the mid-infrared spectral weight, associated to the large polaron band. This happens when the electron is photoionized from the defect state potential well by thereby forming the long-lived metastable polaronic state. We demonstrated that in anatase oxygen-deficient titanium dioxide, deeply confined charges are converted into delocalized, long-lived metastable states with nanosecond dynamics. The approach of our experiment is completely new, being based on a pumpprobe experiment where a sub-gap terahertz pump beam is used for the first time. Our new finding impacts the exploitation of oxygen-deficient titanium dioxide, a functional material largely usable for solar energy conversion, as well as in novel opto-electronic applications, ranging from electrooptic resistive switching to sensing devices with processing functions operating in a very broad spectrum, extending well above and below the visible range. Terahertz driven nanosecond dynamics of oxygen defect state in anatase TiO2−x Elettra Highlights 2024–2025 36
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