Tunable Polaronic Conduction in AnataseTiO2

Abstract

Oxygen vacancies created in anatase TiO2 by UV photons (80 -130 eV) provide an effective electrondoping mechanism and induce a hitherto unobserved dispersive metallic state. Angle resolved photoemission (ARPES) reveals that the quasiparticles are large polarons. These results indicate that anatase can be tuned from an insulator to a polaron gas to a weakly correlated metal as a function of doping and clarify the nature of conductivity in this material.The anatase structural phase of titanium dioxide (TiO 2 ) can be the key element in novel applications. Whereas extensive work has been focused on its famous photocatalytic behavior [1-3], more and more proposed devices, such as memristors [4], spintronic devices [5], and photovoltaic cells [6][7][8], rely on its less well-known electronic properties. In particular, anatase has been recently suggested as a candidate for replacing the In-based technology for transparent conducting oxides [9] in a wide range of applications from solar cell elements, to light-emitting devices, to flat panels, to touch-screen controls [10]. The crucial quantity for the figure of merit in these devices is conductivity, and it is therefore of major interest to understand and control the electronic properties of pristine and doped anatase.Stoichiometric anatase is an insulator with a 3.2 eV band gap [11] but oxygen vacancies, typically present with concentrations in the 10 17 cm −3 range [12,13], create a shallow donor level ∼10 meV below the conduction band (CB) [14]. Since large single crystals became available for transport studies, a better insight has been gained on the influence of these donors on the electronic response of anatase. Above ∼60 K, the electrons thermally excited into the CB give rise to metallic-like transport. At lower temperatures, the anomalous increase of resistivity indicates that the charge carriers are not bare electrons but polarons [14], i.e., electrons coherently coupled to a lattice distorsion induced by the Coulomb interaction. Understanding the properties of such composite particles in anatase is important to better engineer the material for targeted applications, where the low electron mobility often represents the overall performance bottleneck. We will also demonstrate that, from the point of view of fundamental physics, anatase represents an excellent model compound to study the behavior of the "rare" large polaron quasiparticles (QPs), intermediate between localized small polarons and free electrons.We performed ARPES measurements on TiO 2 single crystals ( Fig. 1(a)) and thin films grown in situ on insulating LaAlO 3 and conducting Nb-doped SrTiO 3 substrates. Clean (001) surfaces were prepared as described in Suppl. Inf. The results presented have been obtained consistently both for single crystals and thin films, and therefore reflect intrinsic properties of the anatase phase, independent of the sample preparation method. While oxygen defects are always present to some extent after the surface preparation, we have found that exposure to UV photons...

Clocking the Ultrafast Electron Cooling in Anatase Titanium Dioxide Nanoparticles

“…At high density, the polaronic quasiparticle are observed to collapse into an electron liquid coupled to the phonon modes. Within the single-particle limit probed by ARPES, the crossover takes place around n = 10 19 cm −3 [11]. In the two-particle limit of optical absorption, this threshold may be even lower, leading to the emergence of a well-defined Drude response for the photocarriers [34] instead of the characteristic absorption features of large polarons [60].…”

“…The time resolution is 150 fs and the photoexcited carrier density is n = 5.7 × 10 19 cm −3 . This high-excitation density regime is chosen to avoid polaronic effects within the bands and promote the formation of a mobile electron-hole liquid [11]. Figure 1(a) shows ultrafast 2D deep-UV pump-probe colour-coded maps at different time delays (1 ps, 100 ps and 500 ps).…”

“…However, different to conventional band insulators, anatase TiO 2 represents a peculiar example in which electron-phonon interaction and electron-hole correlation become relatively strong and influence the optical spectra. On the one hand, the presence of a moderately large electron-phonon coupling in anatase TiO 2 has often been invoked to interpret experimental results naturally pointing to the polaronic (self-trapped) picture [7][8][9][10][11][12][13]. Notable examples include the low temperature green photoluminescence (PL) due to self-trapped excitons [14][15][16][17][18][19][20] and the room temperature electron mobilities whose values are limited by strong scattering with phonons [10].…”

“…In TiO 2 , the optical phonons that are most strongly coupled to the electronic degrees of freedom are those belonging to the branches of the E u and A 2u modes [8,11,59]. Due to this coupling, in the low carrier density limit, the CB electrons form well-defined large polaron quasiparticles, characterized by a dimensionless polaron coupling constant α ∼ 2 [11]. This constant determines the ratio between the polaron self-energy and the LO phonon energy, thus defining an intermediate-to-strong electron-phonon coupling regime for the electrons in anatase TiO 2 .…”

“…In nonpolar semiconductors (such as Ge and Si) phonon scattering occurs through the optical phonon deformation potential interaction, while in partially-ionic polar systems (such as anatase TiO 2 ) the most effective coupling mechanism is represented by the electron-longitudinal optical (LO) phonon scattering described by the Fröhlich interaction [58]. In TiO 2 , the optical phonons that are most strongly coupled to the electronic degrees of freedom are those belonging to the branches of the E u and A 2u modes [8,11,59]. Due to this coupling, in the low carrier density limit, the CB electrons form well-defined large polaron quasiparticles, characterized by a dimensionless polaron coupling constant α ∼ 2 [11].…”