Nonequilibrium Dynamics of Collective Excitations in Quantum Materials by Edoardo Baldini

Author:Edoardo Baldini
Language: eng
Format: epub, pdf
Publisher: Springer International Publishing, Cham

Having established the dominant role played by CB electrons in blocking the excitonic transitions allows one to retrieve valuable information on the electron recombination dynamics. Indeed, the A signal at the excitonic resonance can be used as a measure of the photoexcited electron concentration changing with time. Representative temporal traces at a probe photon energy of 3.88 eV and for different excitation densities are shown in Fig. 4.23c and normalized with respect to their maximum. We observe that below 40 ps the bleach recovery accelerates with fluence, which is indicative of higher-order recombination processes for the charge carriers, such as bimolecular and Auger recombination. Indeed, the recombination dynamics in semiconductors and insulators proceeds via single-carrier nonradiative processes (trapping at impurity states), two-body radiative (bimolecular) mechanisms and nonradiative trap-Auger recombination processes and three-body band-to-band Auger processes [87]. Since anatase TiO is an indirect bandgap insulator, band-to-band radiative recombination is known to be extremely inefficient. Thus, the only radiative recombination pathways that can take place at RT are those involving a delocalized carrier in a band and a localized carrier trapped at a defect state. To verify the efficiency of the radiative recombination processes, we measured the photoluminescence obtained in colloidal anatase TiO NPs by femtosecond fluorescence up-conversion. As it is well established that the spectral content of the photoluminescence does not depend on the pump photon energy for above-gap excitation, we illuminate the NPs with a pump pulse centered around 4.66 eV. The broad photoluminescence spectrum at a time delay of 1 ps is shown in Fig. 4.25a. The photoluminescence appears only in the visible regime and is completely absent in the spectral region eV. It retains the form of a broad band centered around 2.24 eV, which is characterized by an extremely weak intensity. Figure 4.25b displays the temporal traces at 1.91, 2.18 and 2.58 eV. The photoluminescence signal rises within our experimental temporal resolution (200 fs) and decays bi-exponentially with time constants of 2 ps and 30–40 ps. Since the PL band is centered around 2.24 eV, it can be readily assigned to an extrinsic radiative recombination channel that involves carriers trapped at defect states, ruling out the involvement of any self-trapped exciton recombination process. By evaluating the photoluminescence quantum yield for time delays below 500 ps, we find it to be of the order of , i.e. extremely weak. As a consequence, it is straightforward to assume that the recombination dynamics at early time delays in anatase TiO is entirely governed by Auger mechanisms, as already inferred from transient absorption spectroscopy studies in the infrared and visibile for highly-excited anatase TiO NPs and thin films [64, 70]. It is also consistent with the spectral dependence of the and relaxation components (Fig. 4.21d), whose broad shape hints to energetically redistributed carriers over a wider phase space during the Auger processes. In contrast, the longer and relaxation components can be assigned to electron trapping processes at defect states, which lead to bleach recovery by emptying the phase space involved in the exciton state.

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