Scientific Highlights 2017

3 April 2017

LaTiOxNy thin film model systems for photocatalytic water splitting: physicochemical evolution of the solid-liquid interface and the role of the crystallographic orientation

The size of the band gap and the energy position of the band edges make several oxynitride semiconductors promising candidates for efficient hydrogen and oxygen production under solar light illumination. The intense research efforts dedicated to oxynitride materials have unveiled the majority of their most important properties. However, two crucial aspects have received much less attention. One is the critical issue of the compositional/structural surface modifications occurring during operation and how these affect the photoelectrochemical performance. The second concerns the relation between the electrochemical response and the crystallographic surface orientation of the oxynitride semiconductor. These are indeed topics of fundamental importance since it is exactly at the surface where the visible light-driven electrochemical reaction takes place.
In contrast to conventional powder samples, thin films represent the best model system for these investigations. This study reviews current state-of-the-art of oxynitride thin film fabrication and characterization before focusing on LaTiO2N selected as representative photocatalyst. We report the investigation of the initial physicochemical evolution of the surface. Then we show that, after stabilization, the absorbed photon-to-current conversion efficiency of epitaxial thin films can differ by about 50% for different crystallographic surface orientations and be up to 5 times larger than for polycrystalline samples.
Keywords: solar water splitting; oxynitride thin film; pulsed laser deposition; photoelectrochemical water splitting;;

Facility: Thin Films and Interfaces, LMX, LMU, ENE, Univ. Bern, Kyushu University

Reference: M. Pichler et al., Adv. Funct. Mater., 1605690 (2017)

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20 March 2017

Tuning the multiferroic mechanisms of TbMnO3 by epitaxial strain

A current challenge in the field of magnetoelectric multiferroics is to identify systems that allow a controlled tuning of states displaying distinct magnetoelectric responses. Here we show that the multiferroic ground state of the archetypal multiferroic TbMnO3 is dramatically modified by epitaxial strain. Neutron diffraction reveals that in highly strained films the magnetic order changes from the bulk-like incommensurate bc-cycloidal structure to commensurate magnetic order. Concomitant with the modification of the magnetic ground state, optical second-harmonic generation (SHG) and electric measurements show an enormous increase of the ferroelectric polarization, and a change in its direction from along the c- to the a-axis. Our results suggest that the drastic change of multiferroic properties results from a switch of the spin-current magnetoelectric coupling in bulk TbMnO3 to symmetric magnetostriction in epitaxially-strained TbMnO3. These findings experimentally demonstrate that epitaxial strain can be used to control single-phase spin-driven multiferroic states.
Keywords: multiferroics; thin films; strain; PLD; magnetic ordering; second harmonic generation;

Facility: SINQ

Reference: K. Shimamoto et al, Scientific Reports 7, 44753 (2017)

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