Strongly bound excitons in anatase TiO2 single crystals and nanoparticles

April 13, 2017

Shedding light on the absorption of light by titanium dioxide

Anatase TiO₂ is involved in a wide range of applications, ranging from photovoltaics and photocatalysis to self-cleaning glasses, and water and air purification. All of these are based on the absorption of light and its subsequent conversion into electrical charges. Given its widespread use in various applications, TiO₂ has been one of the most studied materials in the twentieth century, both experimentally and theoretically. 

When light shines on a semiconducting material such as TiO₂, it generates either free negative (electrons) and positive (holes) charges or a bound neutral electron-hole pair, called an exciton. Excitons are of great interest because they can transport both energy and charges on a nanoscale level, and form the basis of an entire field of next-generation electronics, called “excitonics”. The problem with TiO₂ so far is that we have not been able to clearly identify the nature and properties of the physical object that absorbs light and characterize its properties.

The group of Majed Chergui at EPFL, along with Swiss and international colleagues, have shed light on this long-standing question by using a combination of cutting-edge experimental methods: steady-state angle-resolved photoemission spectroscopy (ARPES), which maps the energetics of the electrons along the different axis in the solid; spectroscopic ellipsometry, which determines the optical properties of the solid with high accuracy; and ultrafast two-dimensional deep-ultraviolet spectroscopy, used for the first time in the study of materials, along with state-of-the-art first-principles theoretical tools.

Figure caption: Lattice structure of anatase TiO₂ with a graphical representation of the 2D exciton that is generated by the absorption of light (purple wavy arrow). This 2D exciton is the lowest energy excitation of the material.

They discovered that the threshold of the optical absorption spectrum is due to a strongly bound exciton, which exhibits two remarkable novel properties: First, it is confined on a two-dimensional (2D) plane of the three-dimensional lattice of the material. This is the first such case ever reported in condensed matter. And secondly, this 2D exciton is immune against temperature and defects as it is present in any type of TiO₂ — single crystals, thin films, and even nanoparticles used in devices.

This “immunity” of the exciton to long-range structural disorder and defects implies that it can store the incoming energy in the form of light and guide it at the nanoscale in a selective way. This promises a huge improvement compared to current technology, in which the absorbed light energy is dissipated as heat to the crystal lattice, making the conventional excitation schemes extremely inefficient.

Furthermore, the properties of the newly discovered exciton are very sensitive to a variety of external and internal stimuli in the material (temperature, pressure, excess electron density), paving the way to a powerful, accurate and cheap detection scheme for sensors with an optical read-out.

“Given that it is cheap and easy to fabricate anatase TiO₂ materials, these findings are crucial for many applications and beyond”, says Majed Chergui. “To know how electrical charges are generated after light is absorbed is a key ingredient for efficient photocatalysts.”

News sites reporting on the publication:
PhysOrg
AZO Nano
Nanotechnology World Organisation
GIT Laboratory Journal

Reference: Baldini, E, L Chiodo, A Dominguez, M Palummo, S Moser, M Yazdi-Rizi, G Auböck, B P P Mallett, H Berger, A Magrez, C Bernhard, M Grioni, A Rubio, and M Chergui, Strongly bound excitons in anatase TiO2 single crystals and nanoparticles. Nature Communications, (2017) 8: 13 (10.1038/s41467-017-00016-6) Baldini-2017.

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