Mapping the conducting channels formed along extended defects in SrTiO3 by means of scanning near-field optical microscopy.

Mixed ionic-electronic-conducting perovskites such as SrTiO3 are promising materials to be employed in efficient energy conversion or information processing. These materials exhibit a self-doping effect related to the formation of oxygen vacancies and electronic charge carriers upon reduction. It has been found that dislocations play a prominent role in this self-doping process, serving as easy reduction sites, which result in the formation of conducting filaments along the dislocations. While this effect has been investigated in detail with theoretical calculations and direct observations using local-conductivity atomic force microscopy, the present work highlights the optical properties of dislocations in SrTiO3 single crystals. Using the change in optical absorption upon reduction as an indicator, two well-defined arrangements of dislocations, namely a bicrystal boundary and a slip band induced by mechanical deformation, are investigated by means of scanning near-field optical microscopy. In both cases, the regions with enhanced dislocation density can be clearly identified as regions with higher optical absorption. Assisted by ab initio calculations, confirming that the agglomeration of oxygen vacancies significantly change the local dielectric constants of the material, the results provide direct evidence that reduced dislocations can be classified as alien matter embedded in the SrTiO3 matrix.

Detection of Fe2+ valence states in Fe doped SrTiO3 epitaxial thin films grown by pulsed laser deposition.

We present an X-ray absorption spectroscopy study on Fe-doped SrTiO3 thin films grown by pulsed laser deposition. The Fe L2,3 edge spectra are recorded for doping concentrations from 0-5% after several annealing steps at moderate temperatures. The Fe valence state is determined by comparison with an ilmenite reference sample and calculations according to the charge transfer multiplet model. We found clear evidence of Fe(2+) and Fe(3+) oxidation states independently of the doping concentration. The Fe(2+) signal is enhanced at the surface and increases after annealing. The Fe(2+) configuration is in contrast to the mixed Fe(3+)/Fe(4+) valence state in bulk material and must be explained by the specific defect structure of the thin films due to the kinetically limited growth which induces a high concentration of oxygen vacancies.

Spectroscopic study of the electric field induced valence change of Fe-defect centers in SrTiO3.

The electrochemical changes induced by an electric field in Fe-doped SrTiO(3) have been investigated by X-ray absorption spectroscopy (XANES and EXAFS), electron paramagnetic resonance (EPR) and Raman spectroscopy. A detailed study of the Fe dopant in the regions around the anode and cathode reveals new insights into the local structure and valence state of Fe in SrTiO(3) single crystals. The ab initio full multiple-scattering XANES calculations give an evidence of the oxygen vacancy presence in the first coordination shell of iron. Differences in the length and disorder of the Fe-O bonds as extracted from EXAFS are correlated to the unequivocal identification of the defect type by complementary spectroscopical techniques to identify the valence state of the Fe-dopant and the presence of the Fe - V(Ö) complexes. Through this combinatorial approach, novel structural information on Fe - V(Ö) complexes is provided by X-ray absorption spectroscopy, and the relation of Fe-O bond length, doping level and oxidation state in SrTi(1-x)Fe(x)O(3) is briefly discussed.

Redox-Based Resistive Switching Memories - Nanoionic Mechanisms, Prospects, and Challenges.

This review article introduces resistive switching processes that are being considered for nanoelectronic nonvolatile memories. The three main classes are based on an electrochemical metallization mechanism, a valence change mechanism, and a thermochemical mechanism, respectively. The current understanding of the microscopic mechanisms is discussed and the scaling potential is outlined..