The nature of radiative transitions in TiO<sub>2</sub>-based nanosheets
Journal Of The American Chemical Society (submitted), (2012)
The nature of radiative transitions in TiO2-based nanosheets
Due to the rapid progress in the synthesis methods, TiO2-based nanomaterials are nowa- days the object of a strong research interest for their promising performances in photocatal- ysis and photovoltaics applications. In particular large quantities of two-dimensional (2D) nanosheets (NSs) can be produced and used for further assembling of new nanostructured materials (Nanotubes, Nanorods,..) with different morphologies and functionalities. Beside anatase-based nanosheets with up to 90% of the (001) surface exposed, titania and titanate 2D nanosheets have been recently synthesized and gained great attention. The analysis of the rela- tion between atomic structures and opto-electronic properties is a fundamental prerequisite for rationalizing experimental data and improving devices performances. At variance with most of the calculations published on these nanomaterials, we perform here ab-initio excited-state sim- ulations based on Many Body Perturbation Theory (MBPT) approaches: for selected anatase and lepidocrocite nanosheet atomic models we discuss the electronic and optical properties.We show how, due to the reduced dimensionality, many-body effects play a fundamental role for a correct interpretation of the experimental observations. Moreover by means of MBPT cal- culations on top of finite-temperature ab-initio molecular dynamics simulations, we show how the inclusion of lattice vibrations in the optical spectra calculation, provides a clear evidence of the large Stokes-shifts experimentally observed. A detailed analysis of the the snapshots reveals that the optical peaks below the absorption onset are mainly related to anatase-like atomic structures with the presence of symmetric Ti-O-Ti bonds at the surface. Still finite temperature simulations confirm the excitonic nature of the optical spectra for these anatase derived low-dimensional materials.
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