Self-reduction of the native TiO2 (110) surface during cooling after thermal annealing - in-operando investigations.

We investigate the thermal reduction of TiO2 in ultra-high vacuum. Contrary to what is usually assumed, we observe that the maximal surface reduction occurs not during the heating, but during the cooling of the sample back to room temperature. We describe the self-reduction, which occurs as a result of differences in the energies of defect formation in the bulk and surface regions. The findings presented are based on X-ray photoelectron spectroscopy carried out in-operando during the heating and cooling steps. The presented conclusions, concerning the course of redox processes, are especially important when considering oxides for resistive switching and neuromorphic applications and also when describing the mechanisms related to the basics of operation of solid oxide fuel cells.

Local surface conductivity of transition metal oxides mapped with true atomic resolution.

The introduction of transition metal oxides for building nanodevices in information technology promises to overcome the scaling limits of conventional semiconductors and to reduce global power consumption significantly. However, oxide surfaces can exhibit heterogeneity on the nanoscale e.g. due to relaxation, rumpling, reconstruction, or chemical variations which demands for direct characterization of electronic transport phenomena down to the atomic level. Here we demonstrate that conductivity mapping is possible with true atomic resolution using the tip of a local conductivity atomic force microscope (LC-AFM) as the mobile nanoelectrode. The application to the prototypical transition metal oxide TiO2 self-doped by oxygen vacancies reveals the existence of highly confined current paths in the first stage of thermal reduction. Assisted by density functional theory (DFT) we propose that the presence of oxygen vacancies in the surface layer of such materials can introduce short range disturbances of the electronic structure with confinement of metallic states on the sub-nanometre scale. After prolonged reduction, the surfaces undergo reconstruction and the conductivity changes from spot-like to homogeneous as a result of surface transformation. The periodic arrangement of the reconstruction is clearly reflected in the conductivity maps as concluded from the simultaneous friction force and LC-AFM measurements. The second prototype metal oxide SrTiO3 also reveals a comparable transformation in surface conductivity from spot-like to homogeneous upon reduction showing the relevance of nanoscale inhomogeneities for the electronic transport properties and the utility of a high-resolution LC-AFM as a convenient tool to detect them.

Electrically controlled transformation of memristive titanates into mesoporous titanium oxides via incongruent sublimation.

Perovskites such as SrTiO3, BaTiO3, and CaTiO3 have become key materials for future energy-efficient memristive data storage and logic applications due to their ability to switch their resistance reversibly upon application of an external voltage. This resistance switching effect is based on the evolution of nanoscale conducting filaments with different stoichiometry and structure than the original oxide. In order to design and optimize memristive devices, a fundamental understanding of the interaction between electrochemical stress, stoichiometry changes and phase transformations is needed. Here, we follow the approach of investigating these effects in a macroscopic model system. We show that by applying a DC voltage under reducing conditions on a perovskite slab it is possible to induce stoichiometry polarization allowing for a controlled decomposition related to incongruent sublimation of the alkaline earth metal starting in the surface region. This way, self-formed mesoporous layers can be generated which are fully depleted by Sr (or Ba, Ca) but consist of titanium oxides including TiO and Ti3O with tens of micrometre thickness. This illustrates that phase transformations can be induced easily by electrochemical driving forces.

Tuning the surface structure and conductivity of niobium-doped rutile TiO2 single crystals via thermal reduction.

We report on the systematic exploration of electronic and structural changes of Nb-doped rutile TiO2(110) single crystal surfaces due to the thermoreduction under ultra-high vacuum conditions (without sputtering), with comparison to undoped TiO2(110) crystals. It has been found that the surface of the doped sample undergoes a previously unknown transition during reduction above 850 °C, as provided by LEED, STM and LC-AFM. This transition involves a change from heterogeneous conductivity (due to the presence of conducting filaments) to homogeneous conductivity, connected with a new (4 × 2) reconstruction of rows parallel to the [001] direction. DFT calculations suggest substitution of Ti by Nb atoms in the first atomic layer. Due to the strong reducing conditions during annealing, oxygen is released from the crystal and Nb diffuses from the subsurface into the bulk, agglomerating however on the surface, as shown by SIMS depth profiling. We present that 0.5% Nb doping significantly influences the reduction process and in turn the structural properties of the surface by supporting the evolution of the new reconstruction. It is shown that the thermal treatment of TiO2:Nb under low oxygen partial pressure gives an opportunity to tune the electrical conductivity and work function of the surface.

Homogeneity and variation of donor doping in Verneuil-grown SrTiO3:Nb single crystals.

The homogeneity of Verneuil-grown SrTiO3:Nb crystals was investigated. Due to the fast crystal growth process, inhomogeneities in the donor dopant distribution and variation in the dislocation density are expected to occur. In fact, for some crystals optical studies show variations in the density of Ti(3+) states on the microscale and a cluster-like surface conductivity was reported in tip-induced resistive switching studies. However, our investigations by TEM, EDX mapping, and 3D atom probe reveal that the Nb donors are distributed in a statistically random manner, indicating that there is clearly no inhomogeneity on the macro-, micro-, and nanoscale in high quality Verneuil-grown crystals. In consequence, the electronic transport in the bulk of donor-doped crystals is homogeneous and it is not significantly channelled by extended defects such as dislocations which justifies using this material, for example, as electronically conducting substrate for epitaxial oxide film growth.