Lluís López-Conesa

Band engineered epitaxial 3D GaN-InGaN core-shell rod arrays as an advanced photoanode for visible-light-driven water splitting.

3D single-crystalline, well-aligned GaN-InGaN rod arrays are fabricated by selective area growth (SAG) metal-organic vapor phase epitaxy (MOVPE) for visible-light water splitting. Epitaxial InGaN layer grows successfully on 3D GaN rods to minimize defects within the GaN-InGaN heterojunctions. The indium concentration (In ∼ 0.30 ± 0.04) is rather homogeneous in InGaN shells along the radial and longitudinal directions. The growing strategy allows us to tune the band gap of the InGaN layer in order to match the visible absorption with the solar spectrum as well as to align the semiconductor bands close to the water redox potentials to achieve high efficiency. The relation between structure, surface, and photoelectrochemical property of GaN-InGaN is explored by transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES), current-voltage, and open circuit potential (OCP) measurements. The epitaxial GaN-InGaN interface, pseudomorphic InGaN thin films, homogeneous and suitable indium concentration and defined surface orientation are properties demanded for systematic study and efficient photoanodes based on III-nitride heterojunctions.

3D Visualization of the Iron Oxidation State in FeO/Fe3O4 Core–Shell Nanocubes from Electron Energy Loss Tomography

The physicochemical properties used in numerous advanced nanostructured devices are directly controlled by the oxidation states of their constituents. In this work we combine electron energy-loss spectroscopy, blind source separation, and computed tomography to reconstruct in three dimensions the distribution of Fe(2+) and Fe(3+) ions in a FeO/Fe3O4 core/shell cube-shaped nanoparticle with nanometric resolution. The results highlight the sharpness of the interface between both oxides and provide an average shell thickness, core volume, and average cube edge length measurements in agreement with the magnetic characterization of the sample.

Multiple strain-induced phase transitions inLaNiO3thin films

Strain effects on epitaxial thin films of LaNiO3 grown on different single crystalline substrates are studied by Raman scattering and first-principles simulation. New Raman modes, not present in bulk or fully-relaxed films, appear under both compressive and tensile strains, indicating symmetry reductions. Interestingly, the Raman spectra and the underlying crystal symmetry for tensile and compressively strained films are different. Extensive mapping of LaNiO3 phase stability is addressed by simulations, showing that a variety of crystalline phases are indeed stabilized under strain which may impact the electronic orbital hierarchy. The calculated Raman frequencies reproduce the principal features of the experimental spectra, supporting the validity of the multiple strain-driven structural transitions predicted by the simulations.

Growth, structure, luminescence and mechanical resonance of Bi2O3 nano- and microwires

α-Bi2O3 hierarchical structures formed by multibranched micro- and nanowires have been grown by an evaporation–deposition method. The morphology, composition, structure, and several physical properties of the obtained nanowires have been investigated. The relative intensities of photoluminescence bands related to Bi2+ ions and oxygen vacancies have been found to depend on the composition of the precursor used in the growth process. In situ scanning electron microscope measurements of the mechanical resonance frequency of the microwires have been used to determine their Youngs modulus, which was found to depend on the wire dimensions, with values ranging from 11 GPa to 284 GPa. The quality factors measured suggest that Bi2O3 wires may have potential applications as micromechanical resonators.

Structural and compositional properties of Er-doped silicon nanoclusters/oxides for multilayered photonic devices studied by STEM-EELS

High resolution scanning transmission electron microscopy with an aberration corrected and monochromated instrument has been used for the assessment of the silicon-based active layer stack for novel optoelectronic devices. This layer contains a multilayer structure consisting of alternate thin layers of pure silica (SiO2) and silicon-rich silicon oxide (SRO, SiOx). Upon high temperature annealing the SRO sublayer segregates into a Si nanocluster (Si-nc) precipitated phase and a SiO2 matrix. Additionally, erbium (Er) ions have been implanted and used as luminescent centres in order to obtain narrow emission at 1.54 μm. Our study exploits the combination of high angle annular dark field (HAADF) imaging with a sub-nanometer electron probe and electron energy loss spectroscopy (EELS) with an energy resolution below 0.2 eV. The structural and chemical information is obtained from the studied multilayer structure. In addition, the instrumental techniques for calibration, deconvolution, fitting and analysis of the EELS spectra are explained in detail. The spatial distribution of the Si-nanoclusters (Si-ncs) and the SiO2 barriers is accurately delimited in the multilayer. Additionally, the quality of the studied multilayer in terms of composition, roughness and defects is analysed and discussed. Er clusterization has not been observed; even so, blue-shifted plasmon and interband transition energies for silica are measured, in the presence of Er ions and sizable nanometer-size effects.

Atomic scale determination of cation inversion in spinel-based oxide nanoparticles.

The atomic structure of nanoparticles can be easily determined by transmission electron microscopy. However, obtaining atomic-resolution chemical information about the individual atomic columns is a rather challenging endeavor. Here, crystalline monodispersed spinel Fe3O4/Mn3O4 core-shell nanoparticles have been thoroughly characterized in a high-resolution scanning transmission electron microscope. Electron energy-loss spectroscopy (EELS) measurements performed with atomic resolution allow the direct mapping of the Mn2+/Mn3+ ions in the shell and the Fe2+/Fe3+ in the core structure. This enables a precise understanding of the core-shell interface and of the cation distribution in the crystalline lattice of the nanoparticles. Considering how the different oxidation states of transition metals are reflected in EELS, two methods of performing a local evaluation of the cation inversion in spinel lattices are introduced. Both methods allow the determination of the inversion parameter in the iron oxide core and manganese oxide shell, as well as detecting spatial variations in this parameter, with atomic resolution. X-ray absorption measurements on the whole sample confirm the presence of cation inversion. These results present a significant advance toward a better correlation of the structural and functional properties of nanostructured spinel oxides.