The Thin Films and Interfaces Group

The conversion of solar light into hydrogen by photoelectrochemical water splitting is one of the potential strategies that can allow the development of a carbon-neutral energy cycle. Oxynitride semiconductors are promising materials for this application, although important limitations must still to be addressed. One of the most important issues is physicochemical degradation of the semiconductor, at the interface with water, where the electrochemical reactions occur. In this regard, thin films, with well-defined and atomically flat surfaces, are invaluable tools for characterizing material properties and degradation mechanisms, while identifying strategies to mitigate detrimental effects. Thin oxynitride films may allow the use of complementary characterizations, not applicable to conventional powder samples. In particular, the study of the solid–liquid interface can benefit enormously from the use of thin films for synchrotron-based surface-sensitive X-Ray scattering methods and neutron reflectometry. These investigation approaches promise to speed up the design and discovery of new materials for the production of solar fuels, while paving the way for similar applications in other research fields. This work aims at reviewing the literature contributions on oxynitride thin films for solar water splitting summarizing what is learnt so far and suggesting experimental strategies to unveil what is still not clear.

Orthoferrites are a class of magnetic materials with a magnetic ordering temperature above 600 K, predominant G-type antiferromagnetic ordering of the Fe-spin system and, depending on the rare-earth ion, a spin reorientation of the Fe spin taking place at lower temperatures. DyFeO3 is of particular interest since the spin reorientation is classified as a Morin transition with the transition temperature depending strongly on the Dy-Fe interaction. Here, we report a detailed study of the magnetic and structural properties of microcrystalline DyFeO3 powder and bulk single crystal using neutron diffraction and magnetometry between 1.5 and 450 K. We find that, while the magnetic properties of the single crystal are largely as expected, the powder shows strongly modified magnetic properties, including a modified spin reorientation and a smaller Dy-Fe interaction energy of the order of 10 μeV. Subtle structural differences between powder and single crystal show that they belong to distinct magnetic space groups. In addition, the Dy ordering at 2 K in the powder is incommensurate, with a modulation vector of 0.0173(5) c∗, corresponding to a periodicity of ∼58 unit cells.

The gas-phase reaction dynamics and kinetics in a laser induced plasma are very much dependent on the interactions of the evaporated target material and the background gas. For metal (M) and metal–oxygen (MO) species ablated in an Ar and O₂ background, the expansion dynamics in O₂ are similar to the expansion dynamics in Ar for M⁺ ions with an MO⁺ dissociation energy smaller than O₂. This is different for metal ions with an MO⁺ dissociation energy larger than for O₂. This study shows that the plume expansion in O₂ differentiates itself from the expansion in Ar due to the formation of MO⁺ species. It also shows that at a high oxygen background pressure, the preferred kinetic energy range to form MO species as a result of chemical reactions in an expanding plasma, is up to 5 eV.

Many in-memory computing frameworks demand electronic devices with specific switching characteristics to achieve the desired level of computational complexity. Existing memristive devices cannot be reconfigured to meet the diverse volatile and non-volatile switching requirements, and hence rely on tailored material designs specific to the targeted application, limiting their universality. “Reconfigurable memristors” that combine both ionic diffusive and drift mechanisms could address these limitations, but they remain elusive. Here we present a reconfigurable halide perovskite nanocrystal memristor that achieves on-demand switching between diffusive/volatile and drift/non-volatile modes by controllable electrochemical reactions.

Perovskite oxynitride semiconductors have attracted huge interest recently as promising photoelectrode materials for photoelectrochemical (PEC) water splitting. Oxynitride thin films grown by physical vapor deposition are ideal model systems to study the fundamental physical and chemical properties of the surface of these materials, including their evolution. Using a combination of high-sensitivity low-energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS), the surface evolution of LaTiO_xN_y (LTON) and CaNbO_xN_y (CNON) thin films before and after the PEC characterizations is monitored. This work provides therefore insight into the surface characteristics and evolution of LTON and CNON oxynitride thin films as photoelectrodes for PEC applications.