Titanium dioxides is an attractive material due to its potential applications in optoelectronic devices (such as flat panel displays, organic light-emitting diodes), and especially in dye sensitized solar cells. Although numerous new phenomena have been observed in this system, its electronic properties still represent a few puzzles. Comprehending and engineering the conductivity of this material would be an important step towards a broader range of applications.
On the fundamental side, there is continuous interest in the charge propagation in polar materials like TiO2 where mobile charge carriers are introduced by doping and show a polaronic character. At one end, the strong interaction between conduction electrons and phonons leads to the formation of small polarons and carrier localization through self-trapping. For weaker coupling, charge carriers remain mobile but have an enhanced effective mass meff, as they travel through the crystal as large polarons. These quasiparticles are particularly attractive, because they can be studied by conventional transport measurements which have relevance in electronic applications.
In our laboratory we managed to grow single crystals of the anatase of TiO2. The material is a 3.2 eV bandgap insulator and it is transparent in the visible (see fig. 1). In our earlier reports it has been shown that despite the transparent character it shows metal-like conductivity above 60 K. This was considered to originate from the oxygen vacancies which give rise to a shallow donor level just below the conduction band minimum (CBM). It was suggested that the charge carriers in the conduction band are of polaronic nature since anatase is an ionic, polar material. The large polaron character of the charge carriers is reflected by the room temperature resistivity of 0.2-0.9 Ohmcm which is high for a metallic system and by the very high Seebeck-coefficient (S) which has an unusual temperature dependence (fig 2).
Figure 1. Large transparent single crystal of the anatase phase of TiO2. It was thinned down to 40 µm.
Figure 2. Temperature dependence of the resistivity and Seebeck coefficient of single crystal of the anatase phase of TiO2.
The large polaronic nature of the charge carriers beyond the conventional transport measurements is investigated by high pressure studies, optical measurements, pumped-probe studies and Scanning Tunneling Spectroscopy measurements. In these investigations our partners are Prof. J. Demsar (Uni Konstanz), P. F. Carbone (EPFL).
Doped anatase crystals
The electronic transport and light absorption of anatase strongly depend on the dopant elements. We managed to dope anatase in single craystalline form with various atoms (see figure below. The scalebar is 1 mm). Their investigation is in progress.
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