Anatase, a mineral form of TiO2, is one of the most studied materials for light-energy conversion applications. However, the nature of its fundamental charge excitations is still unknown and is crucial to the understand of whether the light absorption mechanisms in anatase are due to uncorrelated electron–hole pairs or bound excitons, and to determine their character.
A multinational team of European researchers have combined multiple spectroscopic techniques alongside ab-initio calculations to determine the nature of the excitons in anatase.
The field of excitonics has gained traction in recent years due to the unique properties that excitons produce during conversion and energy transport processes. Anatase is one of many materials that possess a superior ability to convert light into other forms of energy. Researchers have been trying to improve the opto-electronic performance of anatase, but the fundamental electronic and optical excitations at the microscopic level are not fully understood.
Anatase is one of the most common mineral morphologies of titanium dioxide (TiO2). Anatase possesses a tetragonal unit cell and can become a network of edge- and corner-sharing TiO6 octahedra.
Anatase has an electronic structure with almost flat bands along the 3D Brillouin zone (BZ), Ti-3d and O-2p orbital interactions that form bilayers perpendicular to the (001) crystallographic plane, and a strong optical anisotropy for light polarised in, and perpendicular to, the (001) plane.
In anatase, the coordination of the TiO6 octahedra is less compact than other TiO2 crystal arrangements, e.g. rutile, where each octahedron is coordinated with eight neighbours. Although the compression of the lattice is minimal compared to other forms, the effects on the spatial properties of the elementary charge excitations is great.
The researchers used a combination of Angle-resolved photoemission spectroscopy (ARPES) (Electronic Structure Factory), Spectroscopic ellipsometry (SE) (Woollam VASE ellipsometer), Ultrafast 2D UV spectroscopy (KMLabs, Halcyon + Wyvern500) and density functional theory (DFT) ab-initio calculations, to demonstrate that the direct optical gap of the single crystals is dominated by a strongly bound exciton rising over the continuum of the indirect (photon-mediated) interband transitions.
The combination of the experimental measurement and many body theory measurements allowed the researchers to discover that the bound excitons were found to possess an intermediate character between the Wannier–Mott and Frenkel regimes.
The chain-like structure of anatase leads to unique characteristics of the band structure and hinders the delocalisation of the bound exciton in three-dimensions, which spans many unit cells. This essentially forms a unique two-dimensional (2D) wavefunction in the three-dimensional (3D) lattice, with a high localisation of excitons in the c-axis of the unit cell. The result is an enhancement of the bound exciton in a way that is similar to the low-dimensional effect in semiconductor quantum structures.
The formation of the bound excitons is due to a combination of the electronic structure of anatase and the screening process utilised. The degree of excitonic spatial delocalisation is influenced by the crystal structure, as the packing of the polyhedra containing the atoms in the excitonic transitions is related to the band structure. These observations also fit well with the exciton physics of titanates in general.
The presence of bound excitons in anatase gives rise to many-state transitions and the formation of a bound collective of exciton states. This normally occurs when the velocities of the lowest conduction band exciton and the highest valence band exciton are identical in a given part of the Brillouin zone. The band states within anatase are actually parallel in extended portions of the Brillouin zone and contribute density of states to the collective transition derived from the bound excitonic species.
A future consideration for the researchers is the effect that phonons have on the exciton width and lineshape and whether the optical properties are affected by tuning the exciton–phonon coupling through mechanical strain.
Baldini E., Chiodo L., Dominguez A., Palummo M., Moser S., Yazdi-Rizi M., Auböck G., Mallett B. P. P., Berger H., Magrez A., Bernhard C., Grioni M., Rubio A., Chergui M., Strongly bound excitons in anatase TiO2 single crystals and nanoparticles, Nature Communication, 2017, 8, 13
Image Credit: Shutterstock.com/cybrain