Clocking the Ultrafast Electron Cooling in Anatase Titanium Dioxide Nanoparticles

Abstract

The recent identification of strongly bound excitons in room temperature anatase TiO 2 single crystals and nanoparticles underscores the importance of bulk many-body effects in samples used for applications. Here, for the first time, we unravel the interplay between many-body interactions and correlations in highly-excited anatase TiO 2 nanoparticles using ultrafast two-dimensional deepultraviolet spectroscopy. With this approach, under non-resonant excitation, we disentangle the optical nonlinearities contributing to the bleach of the lowest direct exciton peak. This allows us to clock the ultrafast timescale of the hot electron thermalization in the conduction band with unprecedented temporal resolution, which we determine to be < 50 fs, due to the strong electronphonon coupling in the material. Our findings call for the design of alternative resonant excitation schemes in photonics and nanotechnology.1 arXiv:1703.07818v2 [cond-mat.mes-hall] 13 Jan 2018In the last decades, anatase TiO 2 has attracted huge interest as one of the most promising materials for a variety of challenging applications, ranging from photocatalysis [1,2] and photovoltaics [3] to sensors [4,5]. Since these technologies involve charge transport, thermalization and localization, they call for studies of the fast electron and hole dynamics, which provide a deep knowledge of the nature of the photogenerated/injected charge carriers and of the energy balance therein. These processes intimately depend on the details of the electronic structure and the presence of many body-effects in the material. Since anatase TiO 2 is a d 0 transition metal oxide, strong electron-electron correlations do not play a substantial role in the electronic structure [6]. Hence, this solid can be classified within the simple band insulator scheme, in which the forbidden energy gap arises as a result of band theory and is not a consequence of the strong on-site Coulomb interaction. However, different to conventional band insulators, anatase TiO 2 represents a peculiar example in which electron-phonon interaction and electron-hole correlation become relatively strong and influence the optical spectra. On the one hand, the presence of a moderately large electron-phonon coupling in anatase TiO 2 has often been invoked to interpret experimental results naturally pointing to the polaronic (self-trapped) picture [7][8][9][10][11][12][13]. Notable examples include the low temperature green photoluminescence (PL) due to self-trapped excitons [14][15][16][17][18][19][20] and the room temperature electron mobilities whose values are limited by strong scattering with phonons [10]. 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...

Intrinsic intermediate gap states of TiO2 materials and their roles in charge carrier kinetics

“…Many works studied the chemical entities of gap states, their role in charge carrier kinetics, as well as the connection with photocatalytic properties. The electronic structure and associated charge carrier kinetics caused by gap states can be studied by: transient absorption (TA) spectroscopy [71]; electron paramagnetic resonance (EPR) [72]; photoluminescence (PL) [73]; electrochemical techniques; photoelectron spectroscopy; surface photovoltage spectroscopy (SPS) [74]; microwave photoconductivity (MPC) [75] and the other methods [76][77][78]. The exact chemical entities and the spatial configuration of gap states cannot be directly revealed with these statistical techniques.…”

Giant Exciton Mott Density in Anatase TiO2

“…As in TiO 2 the intraband carrier relaxation is complete within 50 fs (due to the strong electron-phonon coupling) [70,71], one may argue that the results obtained at 700 fs are not representative of the stated excitation density. Indeed, recombination mechanisms such as carrier trapping or three-body Auger processes may have already decreased the actual carrier density contributing to the exciton screening in this indirect-gap material.…”

Electron Dynamics in Anatase TiO2 Nanoparticles by Ultrafast Broadband Deep-Ultraviolet Spectroscopy

“…Not obvious in these figures is the occurrence of coherent acoustic phonons, which have been recently analysed [3]. Finally, by unravelling the origin of the bound exciton bleaching in nanoparticles [4], we can now use their spectral signatures as substrate-sensitive probes of charge injection in photovoltaics based on sensitized transition metal oxides. In this respect, we recently demonstrated the effectiveness of our approach by probing the interfacial electron transfer in dye-sensitized anatase TiO2 nanoparticles via the bleaching of the excitonic features upon visible pumping of an external dye (N719) adsorbed to the nanoparticles ( Fig.…”