Identification of large polarons and exciton polarons in rutile and anatase polymorphs of titanium dioxide

TL;DR

This study uses advanced ab initio methods to identify new types of polarons in TiO2 polymorphs, expanding understanding of charge transport mechanisms crucial for various applications.

Contribution

It reveals three novel polaron species in TiO2, including large hole, quasi-2D electron, and exciton polarons, beyond the previously known small polarons.

Findings

Identification of a large hole polaron in rutile

Discovery of a large quasi-2D electron polaron in anatase

Detection of a large exciton polaron in anatase

Abstract

Titanium dioxide (TiO2) is a wide-gap semiconductor with numerous applications in photocatalysis, photovoltaics, and neuromorphic computing. The unique functional properties of this material critically depend on its ability to transport charge in the form of polarons, namely narrow electron wavepackets accompanied by local distortions of the crystal lattice. It is currently well established that the most important polymorphs of TiO2, the rutile and anatase phases, harbor small electron polarons and small hole polarons, respectively. However, whether additional polaronic species exist in TiO2, and under which conditions, remain open questions. Here, we provide definitive answers to these questions by exploring the rich landscape of polaron quasiparticles in TiO2 via recently developed ab initio techniques. In addition to the already known small polarons, we identify three novel species, namely a large hole polaron in rutile, a large quasi-two-dimensional electron polaron in anatase, and a large exciton polaron in anatase. These findings complete the puzzle on the polaorn physics of TiO2 and pave the way for systematically probing and manipulating polarons in a broad class of complex oxides and quantum materials.