Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - exciTitania (Excitonic quasiparticles in Titania)

Teaser

Titanium dioxide, also known as titania, is a one-of-a-kind metal oxide and semiconductor material employed in fields like photovoltaics, catalysis and air purification. To understand the unique properties of this material when exposed to light, we need to examine carefully...

Summary

Titanium dioxide, also known as titania, is a one-of-a-kind metal oxide and semiconductor material employed in fields like photovoltaics, catalysis and air purification. To understand the unique properties of this material when exposed to light, we need to examine carefully the nature of its elementary charge excitations. Although titania has been studied in depth during the last decades, there are many unclear aspects about its optical response, as well as serious misconceptions on the chemical and physical processes triggered during its interaction with light. A deep understanding of the excitonic properties of TiO2 is of high relevance to achieve major advances in the aforementioned fields and may lead to the fabrication of new devices with enhanced performance in energy conversion applications.

In exciTitania, we have worked towards the clarification of some intriguing characteristic of TiO2, including the anomalous dependence of its electronic and optical properties with the temperature, and its unexpected behaviour respecting the coherent motion of its crystal lattice. We have employed state-of-the-art theoretical calculations in close collaboration with researchers conducting advanced spectroscopy measurements.

We have got remarkable new insights into the excitonic nature of TiO2. We discovered novel properties of titania single crystal and nanoparticles which could be used, for example, to build low cost sensors with specific functions. We also elucidated the mechanisms behind the generation of the coherent motions of the atoms in the material. Our investigation has also shed light to the migration of the charge carriers in TiO2, which, with further research, may explain why this semiconductor has so exceptional features for the degradation of air pollutants.

Work performed

We performed first-principle calculations to study the electronic and optical properties of TiO2, with special emphasis in the interplay between the charge excitations and the coherent vibration of the lattice (phonons). Especially, we have sought to improve the theoretical description of the interaction between electrons and phonons. This is a very challenging task when done at a high level of theoretical accuracy, but necessary to correctly characterise the optical properties of titania and their dependence with the temperature. Furthermore, we have studied the mobility and transport of the charge carriers in TiO2, and compared the properties for the migration in different directions inside the crystal.

We found the deformation potential coupling as the dominant mechanism for the generation and detection of coherent acoustic phonons in TiO2 nanoparticles and single crystals. These coherent acoustic phonons are responsible for a huge modulation of the exciton peak amplitude as well as a giant exciton energy shift in bulk TiO2. The detection and generation of these phonons are observed close to an exciton resonance at room temperature. Our calculations reveal that this exciton resonance exhibits extraordinarily large photo elastic coefficients, comparable to those found for quantum confined nanoparticles in the visible spectrum. These findings pave the way for the design of exciton control schemes using strain pulses.

For the proper description of the electron-phonon interaction in TiO2, we have joined efforts with theoreticians working on a framework that can describe the effects of multiple phonons using a single arrangement of the atomic positions in a supercell. The combination of this scheme with many body perturbation theory has resulted so far in the accurate account of the zero point renormalisation of the electronic gap of TiO2. This had never been accomplished before.

The results obtained within this project have been presented in the following conferences and seminars:
-2017 Seminar at Duke University, USA, “Excitonic quasiparticles in TiO2”
-2018 Seminar at UPV/EHU, Basque Country, Spain “Excitonic quasiparticles in TiO2”
-2018 Seminar at University of Oxford, UK “Excitonic quasiparticles in TiO2”
-2018 9th International Conference on Spontaneous Coherence in Excitonic Systems, Montreal, Canada “Excitonic quasiparticles in TiO2”
-2019 DESY NanoLab annual kickoff meeting, Zeuthen, Germany “Bulk and 2028surface properties of TiO2 photocatalyst”
-2019 World Congress on Laser, Optics and Photonics, Barcelona, Spain “Excitonic quasiparticles in TiO2”

All results and internal reports have been brought multiple time to discussion with our collaborators for their further exploitations. Some of our findings have been published in high impact international journals with open access. The rest will be published in the near future, always making them accessible free of charge.

Final results

Titania is widely employed in many fields, specially in light-harvesting applications. A better understanding of this material, which is easy and cheap to obtain, may lead to important advances in the atmosphere decontamination and renewable energy segment. Hence, there is a great effort put into investigating its properties in details. However, many aspects of TiO2 remain elusive and the use of sophisticated methodologists applied to its study continues to be a challenge. With this project, we have come closer to unraveling the complex and anomalous behaviour of titania. We have paved the road to new investigations that could potentially lead to the fabrication of more efficient TiO2-based devices and even novel applicabilities in unexplored fields. We have thrown light not only to the electronic, optical and photocatalytic properties of titania, but also to the field of excitonics in general. We expect our work to have an important impact not only in the succeeding study of TiO2, but also in the fields of photovoltaics and photocatalysis. The framework we are using in the wake of this project for the investigation of the electron-phonon coupling is not limited to TiO2. It can be transferred also to the research of other direct and indirect gap semiconductors. In this sense, our work could have an impact beyond our primarily target of study and contribute to the better knowledge of the properties of solid state matter.

Website & more info

More info: http://nano-bio.ehu.es/.