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 national 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.