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The nanoscale magnetic configuration of self-assembled groups of magnetite 40 nm cubic nanoparticles has been investigated by means of electron holography in the transmission electron microscope (TEM). The arrangement of the cubes in the form of chains driven by the alignment of their dipoles of single nanocubes is assessed by the measured in-plane magnetic induction maps, in good agreement with theoretical calculations This research received funding from the Xunta de Galicia, Program for Development of a Strategic Grouping in Materials (AeMAT, Grant No. ED431E2018/08), the Agencia Estatal de Investigación (project PID2019-109514RJ-100), the Spanish Ministry of Economy and Competitiveness, grant number MAT2010-16407, and from the European Union Seventh Framework Programme under Grant Agreement 312483-ESTEEM2 (Integrated Infrastructure Initiative–I3) SI

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 and Fe ions in a FeO/FeO 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., This work has been carried out in the frame of the Spanish research projects MAT2013-41506, MAT2013-48628-R, FIS2013-46159-C3-3-P, and CSD2009-00013, and Catalan Government support from the SGR2014-672 and 2014-SGR1015 projects is acknowledged. We also acknowledge the support received from the European Union Seventh Framework Program under Grant Agreement 312483 - ESTEEM2 (Integrated Infrastructure Initiative-I3). G.S.A. was partially supported by the Knut and Alice Wallenberg Foundation (Project: 3DEMNATUR). M.E. acknowledges the Spanish Ministry of Science and Innovation through the Juan de la Cierva Program. ICN2 acknowledges support from the Severo Ochoa Program (MINECO, Grant SEV-2013-0295). F.d.l.P. and C.D. acknowledge funding from the ERC under grant no. 259619 PHOTO EM. C.D. acknowledges the Royal Society for funding

A mechanism for the formation and crystallization processes of bone-like apatite grown on non-stoichiometric silicocarnotite (SC) is here proposed. Single-phase SC powders and ceramics were obtained from fixed mixtures of hydroxyapatite and bioactive glass 45S5. The bioactive behavior of SC was assessed by immersion in Hank´s solution at different times. Afterward, a systematic theoretical-experimental study of the structural properties at the micro and nanoscale using TEM was performed and correlated with SEM, EDX, XRD, and Raman techniques to determine the apatite mineralization process from the SC phase. The initial stage of apatite formation from SC was identified as the hydration and further polymerization of silanol groups, resulting in a silica-based hydrogel, which plays a critical role in the ionic exchange. As a result of the adsorption of ionic species from the medium into the silica-based hydrogel, the precipitation of crystalline apatitic structures starts through the emergence of newly formed SC nanocrystals, which act as a template for the crystallization process of a substituted apatite with SC-like structure. Then, due to the polymorphism between SC and HAp structures, the apatite layer retains the SC periodic arrangement following an epitaxial-like growth mechanism. Identification of the apatite layer formation mechanism is critical to understand its physical and chemical properties, which controls the long-term dissolution/precipitation rate of bioactive materials and their performance in the biological environment.

Transparent metallic oxides are pivotal materials in information technology, photovoltaics, or even in architecture. They display the rare combination of metallicity and transparency in the visible range because of weak interband photon absorption and weak screening of free carriers to impinging light. However, the workhorse of current technology, indium tin oxide (ITO), is facing severe limitations and alternative approaches are needed. AMO perovskites, M being a nd transition metal, and A an alkaline earth, have a genuine metallic character and, in contrast to conventional metals, the electron-electron correlations within the nd band enhance the carriers effective mass (m*) and bring the transparency window limit (marked by the plasma frequency, ω*) down to the infrared. Here, it is shown that epitaxial strain and carrier concentration allow fine tuning of optical properties (ω*) of SrVO films by modulating m* due to strain-induced selective symmetry breaking of 3d-t(xy, yz, xz) orbitals. Interestingly, the DC electrical properties can be varied by a large extent depending on growth conditions whereas the optical transparency window in the visible is basically preserved. These observations suggest that the harsh conditions required to grow optimal SrVO films may not be a bottleneck for their future application.

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., The authors acknowledge funding from the Spanish Ministerio de Economía y Competitividad (MINECO) through the projects MAT2016-77391-R and MAT2016-79455-P/FEDER (UE). This work has also been carried out in the framework of the projects 2017-SGR-292 and 2017-SGR-776 from the Generalitat de Catalunya. A.L.O. acknowledges the MINECO through the Juan de la Cierva Program (IJCI-2014-21530). M.W. acknowledges the European Union Seventh Framework Programme under grant agreement no. 312483-ESTEEM2. ICN2 is funded by the CERCA Programme/Generalitat de Catalunya. ICN2 also acknowledges support from the Severo Ochoa Program (MINECO, grant SEV-2013-0295).

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 Young's 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.

In this work, the fabrication and the structural, optical and electrical properties of Al-Tb/SiO2 nanomultilayers have been studied. The nanomultilayers were deposited by means of electron beam evaporation on top of p-type Si substrates. Optical characterization shows a narrow and strong emission in the green spectral range, indicating the optical activation of Tb3+ ions. The electrical characterization revealed conduction limited by the electrode, although trapped-assisted mechanisms can also contribute to transport. The electroluminescence analysis revealed also emission from Tb3+ ions, a promising result to include this material in future optoelectronics applications as integrated light-emitting devices. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim., This work was financially supported by the Spanish Ministry of Economy and Competitiveness (TEC2016-76849-C2-1-R and MAT2015-71035-R). O.B. also acknowledges the subprogram “Ayudas para Contratos Predoctorales para la Formaci on de Doctores” from the Spanish Ministry of Economy and Competitiveness for economical support.

Recently, colored H-doped TiO2 (H-TiO2) has demonstrated enhanced photoelectrochemical (PEC) performance due to its unique crystalline core—disordered shell nanostructures and consequent enhanced conduction behaviors between the core-shell homo-interfaces. Although various hydrogenation approaches to obtain H-TiO2 have been developed, such as high temperature hydrogen furnace tube annealing, high pressure hydrogen annealing, hydrogen-plasma assisted reaction, aluminum reduction and electrochemical reduction etc., there is still a lack of a hydrogenation approach in a controlled manner where all processing parameters (temperature, time and hydrogen flux) were precisely controlled in order to improve the PEC performance of H-TiO2 and understand the physical insight of enhanced PEC performance. Here, we report for the first time a controlled and local rapid thermal annealing (RTA) approach to prepare hydrogenated core-shell H-TiO2 nanorods grown on F:SnO2 (FTO) substrate in order to address the degradation issue of FTO in the typical TiO2 nanorods/FTO system observed in the conventional non-RTA treated approaches. Without the FTO degradation in the RTA approach, we systematically studied the intrinsic relationship between the annealing temperature, structural, optical, and photoelectrochemical properties in order to understand the role of the disordered shell on the improved photoelectrochemical behavior of H-TiO2 nanorods. Our investigation shows that the improvement of PEC performance could be attributed to (i) band gap narrowing from 3.0 to 2.9 eV; (ii) improved optical absorption in the visible range induced by the three-dimensional (3D) morphology and rough surface of the disordered shell; (iii) increased proper donor density; (iv) enhanced electron–hole separation and injection efficiency due to the formation of disordered shell after hydrogenation. The RTA approach developed here can be used as a suitable hydrogenation process for TiO2 nanorods/FTO system for important applications such as photocatalysis, hydrogen generation from water splitting and solar energy conversion. Electronic supplementary material The online version of this article (doi:10.1186/s11671-017-2105-x) contains supplementary material, which is available to authorized users.

Multilayers consisting of silicon nanocrystals (Si NCs) and SiO2 are successfully fabricated by electron beam evaporation, using pure Si and SiO2 targets in an oxygen-rich atmosphere for alternately depositing silicon-rich oxide (SRO) layers and SiO2 barriers, respectively. A post-deposition annealing process is carried out at different temperatures in order to achieve the Si precipitation in the form of nanocrystals. The stoichiometry of the layers is determined by X-ray photoelectron spectroscopy, which confirms the controlled silicon oxidation in order to attain SRO layers. Transmission electron microscopy and Raman-scattering measurements confirm the presence of crystalline Si-nanoprecipitates. Photoluminescence spectra from the Si NC samples can be deconvolved into two contributions, whose dynamics suggest that two different luminescent centers are responsible for the optical emission of the samples. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, This work was financially supported by the Spanish Ministry of Economy and Competitiveness (TEC2016‐76849‐C2‐1‐R and MAT2015‐71035‐R). O.B. also acknowledges the subprogram “Ayudas para Contratos Predoctorales para la Formación de Doctores” from the Spanish Ministry of Economy and Competitiveness for economical support. J.L.F. also acknowledges the subprogram “Ayudas para la Formación de Profesorado Universitario” from the Spanish Ministry of Education, Culture and Sports for economical support.

Charge transport in nanocrystalline SiC with and without embedded Si nanocrystals (Si NCs) prepared by annealing of $a\ensuremath{-}\mathrm{S}{\mathrm{i}}_{1\ensuremath{-}x}{\mathrm{C}}_{x}:\mathrm{H}$ precursors is studied using temperature-dependent current-voltage measurements supported by electron spin resonance and mass spectrometry data. Transport is Ohmic in all films at all temperatures and the temperature dependence of conductivity shows that the materials behave as disordered semiconductors, exhibiting extended-state transport at high temperature and variable-range hopping transport at low temperature. Grain-boundary-, surface-, and interface-dominated transport is systematically ruled out. Films are $n$ type, and films with Si NCs exhibit up to ${10}^{3}$ times higher conductivity (up to $0.1\phantom{\rule{0.16em}{0ex}}\mathrm{S}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$) after exposure to a hydrogen plasma which passivates dangling bonds, particularly on Si NCs. A forming gas anneal does not have such an effect, indicating that atomic rather than molecular hydrogen is required. The conductivity of SiC films without Si NCs is largely unchanged by passivation and the Fermi level is not raised nearly as closely to the conduction band. This is attributed to a type I band offset between Si NCs and SiC that leads to extended-state conduction in films with Si NCs taking place in a Si network. This is confirmed by the dependence of the extended-state mobility on the volume fraction of excess Si. Variable-range hopping is relatively insensitive to the presence of excess Si and is hence considered to take place via shallow defect states throughout the volume of the films. The high conductivities are found to be a consequence of background doping by oxygen and nitrogen.

In this work we present a structural, optical and electrical characterization of Si x C 1− x /SiC multilayer systems with different silicon content. After the deposition process, an annealing treatment was carried out in order to induce the silicon nanocrystals formation. By means of energy-filtered transmission electron microscopy (EFTEM) we observed the structural morphology of the multilayers and the presence of crystallized silicon nanoprecipitates for samples annealed up to 1100 °C. We discuss the suitability of optical techniques such as Raman scattering and reflectance and transmittance ( R&T ) for the evaluation of the crystalline fraction of our samples at different silicon excess ranges. In addition, the combination of R&T measurements with simulation has proved to be a useful instrument to confirm the structural properties observed by EFTEM. Finally, we explore the origin of the extremely high current density revealed by electrical measurements, probably due to the presence of an undesired defective SiC y O z ternary compound layer, already supported by the structural and optical results. Nevertheless, the variation of the electrical measurements with the silicon amount indicates a small but significant contribution from the multilayers.

We report on a growth study of MgO films deposited on Al${}_{2}$O${}_{3}$(0001) substrates by magnetron sputtering. The films exhibited a preferred rocksalt MgO(111) orientation. Surprisingly, depending on the O${}_{2}$ gas flow ratio, a structure of graphiticlike wurtzite MgO(0001) has been revealed. The observed Mg-O perpendicular bond length reduction is accompanied by an atomically flat surface morphology for the development of MgO(111) films; the transition to the bulk rocksalt structure occurs in the 3--6 nm coverage range. Previously, relaxation of the electrostatic instability of MgO(111) films accompanied by an in-plane lattice increase has been suggested theoretically [Phys. Rev. Lett. 98, 205701 (2007)]. Here, relying on ab initio calculations, we infer that Mg vacancies facilitate the lattice match with the substrate. This mechanism suggests methods to engineer oxide heterostructures.

Superlattices of Si-rich silicon nitride and Si3N4 are prepared by plasma-enhanced chemical vapor deposition and, subsequently, annealed at 1150 °C to form size-controlled Si nanocrystals (Si NCs) embedded in amorphous Si3N4. Despite well defined structural properties, photoluminescence spectroscopy (PL) reveals inconsistencies with the typically applied model of quantum confined excitons in nitride-embedded Si NCs. Time-resolved PL measurements demonstrate 105 times faster time-constants than typical for the indirect band structure of Si NCs. Furthermore, a pure Si3N4 reference sample exhibits a similar PL peak as the Si NC samples. The origin of this luminescence is discussed in detail on the basis of radiative defects and Si3N4 band tail states in combination with optical absorption measurements. The apparent absence of PL from the Si NCs is explained conclusively using electron spin resonance data from the Si/Si3N4 interface defect literature. In addition, the role of Si3N4 valence band tail states as...