title: Direct view at excess electrons in TiO2 rutile and anatase.
abstract: A combination of scanning tunneling microscopy and spectroscopy and density functional theory is used to characterize excess electrons in TiO2 rutile and anatase, two prototypical materials with identical chemical composition but different crystal lattices. In rutile, excess electrons can localize at any lattice Ti atom, forming a small polaron, which can easily hop to neighboring sites. In contrast, electrons in anatase prefer a free-carrier state, and can only be trapped near oxygen vacancies or form shallow donor states bound to Nb dopants. The present study conclusively explains the differences between the two polymorphs and indicates that even small structural variations in the crystal lattice can lead to a very different behavior.
authors.affiliation: Institute of Applied Physics, Vienna University of Technology, Wiedner Hauptstrasse 8-10/134, 1040 Vienna, Austria.
authors.forename: Martin
authors.initials: M
authors.lastname: Setvin
authors.affiliation: Faculty of Physics and Center for Computational Materials Science, Universität Wien, Sensengasse 8/8-12, A-1090 Wien, Austria.
authors.forename: Cesare
authors.initials: C
authors.lastname: Franchini
authors.affiliation: Institute of Applied Physics, Vienna University of Technology, Wiedner Hauptstrasse 8-10/134, 1040 Vienna, Austria.
authors.forename: Xianfeng
authors.initials: X
authors.lastname: Hao
authors.affiliation: Institute of Applied Physics, Vienna University of Technology, Wiedner Hauptstrasse 8-10/134, 1040 Vienna, Austria.
authors.forename: Michael
authors.initials: M
authors.lastname: Schmid
authors.affiliation: Materials Department, University of California, Santa Barbara, California 93106-5050, USA.
authors.forename: Anderson
authors.initials: A
authors.lastname: Janotti
authors.affiliation: Faculty of Physics and Center for Computational Materials Science, Universität Wien, Sensengasse 8/8-12, A-1090 Wien, Austria.
authors.forename: Merzuk
authors.initials: M
authors.lastname: Kaltak
authors.affiliation: Materials Department, University of California, Santa Barbara, California 93106-5050, USA.
authors.forename: Chris G
authors.initials: CG
authors.lastname: Van de Walle
authors.affiliation: Faculty of Physics and Center for Computational Materials Science, Universität Wien, Sensengasse 8/8-12, A-1090 Wien, Austria.
authors.forename: Georg
authors.initials: G
authors.lastname: Kresse
authors.affiliation: Institute of Applied Physics, Vienna University of Technology, Wiedner Hauptstrasse 8-10/134, 1040 Vienna, Austria.
authors.forename: Ulrike
authors.initials: U
authors.lastname: Diebold