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Expanding the activity of wide bandgap semiconductors from the UV into the visible range has become a central goal for their application in green solar photocatalysis. The hybrid plasmonic/semiconductor system, based on silver nanoparticles (Ag NPs) embedded in a film of CeO2, is an example of a functional material developed with this aim. In this work, we take advantage of the chemical sensitivity of free electron laser (FEL) time-resolved soft X-ray absorption spectroscopy (TRXAS) to investigate the electron transfer process from the Ag NPs to the CeO2 film generated by the NPs plasmonic resonance photoexcitation. Ultrafast changes (

The femtosecond evolution of the electronic temperature of laser-excited gold nanoparticles is measured, by means of ultrafast time-resolved photoemission spectroscopy induced by extreme-ultraviolet radiation pulses. The temperature of the electron gas is deduced by recording and fitting high-resolution photo emission spectra around the Fermi edge of gold nanoparticles providing a direct, unambiguous picture of the ultrafast electron-gas dynamics. These results will be instrumental to the refinement of existing models of femtosecond processes in laterally-confined and bulk condensed-matter systems, and for understanding more deeply the role of hot electrons in technological applications.

A ferromagnetic (FM) thin film deposited on a substrate of $\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3}\text{\ensuremath{-}}{\mathrm{PbTiO}}_{3}$ (PMN-PT) is an appealing heterostructure for the electrical control of magnetism, which would enable nonvolatile memories with ultralow-power consumption. Reversible and electrically controlled morphological changes at the surface of PMN-PT suggest that the magnetoelectric effects are more complex than the commonly used ``strain-mediated'' description. Here we show that changes in substrate morphology intervene in magnetoelectric coupling as a key parameter interplaying with strain. Magnetic-sensitive microscopy techniques are used to study magnetoelectric coupling in Fe/PMN-PT at different length scales, and compare different substrate cuts. The observed rotation of the magnetic anisotropy is connected to the changes in morphology, and mapped in the crack pattern at the mesoscopic scale. Ferroelectric polarization switching induces a magnetic field-free rotation of the magnetic domains at micrometer scale, with a wide distribution of rotation angles. Our results show that the relationship between the rotation of the magnetic easy axis and the rotation of the in-plane component of the electric polarization is not straightforward, as well as the relationship between ferroelectric domains and crack pattern. The understanding and control of this phenomenon is crucial to develop functional devices based on FM/PMN-PT heterostructures.

In this work, we present an investigation on the effects of thermal annealing on the magnetic response of Lithium Niobate/Fe samples. Fe thin films have been deposited on Lithium Niobate Z-cut ferroelectric substrates by vapor phase epitaxy. A series of annealing treatments were performed on the samples, monitoring the evolution of their magnetic properties, both at the surface and on the volume. The combination of structural, magnetic, chemical and morphological characterizations shows that the modification of the chemical properties, i.e. the phase decomposition, of the substrate upon annealing affects drastically the magnetic behavior of the interfacial Fe layer. By tuning the annealing temperature, the magnetic coercive field value can be increased by an order of magnitude compared to the as-grown value, keeping the same in-plane isotropic behavior. Since no evident differences were recorded in the Fe layer from the chemical point of view, we attribute the origin of this effect to an intermixing process between a fragment of the substrate and the Fe thin film upon critical temperature annealing, process that is also is responsible for the observed changes in roughness and morphology of the magnetic thin film.

In the framework of piezoelectric/ferromagnetic patterned heterostructures, the purpose of this work is to electrically control the magnetic properties by tuning the morphology, especially by modifying the magnetic shape anisotropy through patterned strain. We have thus designed and studied a heterostructure with bottom nano-striped and top full film electrodes. ZnO piezoelectric and CoFeB magnetic materials were chosen to respond at critical criteria of its geometry. In addition, numerical simulations and magnetostatic calculations were performed to understand the reproduction of the pattern across the multiferroic heterostructure. Calculations have shown that the geometry of the heterostructure presents strict constraints, as for instance the distance between stripes versus the piezoelectric thickness. This study is a preliminary step towards reversible patterning of magnetic properties.

Converse magnetoelectric coupling in artificial multiferroics is generally modeled through three possible mechanisms: charge transfer, strain mediated effects or ion migration. Here the role played by electrically controlled morphological modifications on the ferromagnetic response of a multiferroic heterostructure, specifically FexMn1-x ferromagnetic films on piezoferroelectric PMN-PT [001] substrates, is discussed. The substrates present, in correspondence to electrical switching, fully reversible morphological changes at the surface, to which correspond reproducible modifications of the ferromagnetic response of the FexMn1-x films. Topographic analysis by atomic force microscopy shows the formation of surface cracks (up to 100 nm in height) upon application of a sufficiently high positive electric field (up to 6 kV cm-1). The cracks disappear after application of negative electric field of the same magnitude. Correspondingly, in operando X-ray magnetic circular dichroic spectroscopy at Fe edge in FexMn1-x layers and micro-MOKE measurements show local variations in the intensity of the dichroic signal and in the magnetic anisotropy as a function of the electrically driven morphological state. This morphologic parameter, rarely explored in literature, directly affects the ferromagnetic response of the system. Its proof of electrically reversible modification of the magnetic response adds a new possibility in the design of electrically controlled magnetic devices.

In this work we report the interfacial charge transfer between the Fe core and Ag shell in self-organized nanoparticles on MgO films on Mo(001). Predeposited Fe nanoparticles organize in a square network with long-range order on the oxide surface guided by the MgO coincidence lattice. When Ag is added, it covers the Fe nanoparticles, forming a shell. By means of XPS and UPS we show that a charge transfer occurs between the Fe core and the Ag shell, determining the oxidation of part of the Fe atoms and a negative charging of the Ag shell. This is confirmed by band bending and core level shifts. As a consequence of the Fe@Ag morphology and composition the plasmonic response of the nanoparticles is modified with respect to pure Ag nanoparticles.

The coupling with plasmonic metal nanoparticles (NPs) represents a promising opportunity to sensitize wide band gap oxides to visible light. The processes which come into play after the excitation of localized surface plasmon resonances (LSPRs) in the NPs largely determine the efficiency of the charge/ energy transfer from the metal NP to the oxide. We report a study of plasmon-mediated energy transfer from mass-selected silver NPs into the cerium oxide matrix in which they are embedded. Femtosecond transient absorption spectroscopy is used to probe the dynamics of charge carrier relaxation after the excitation of the LSPR of the silver nanoparticles and to evaluate the plasmon-mediated electron transfer efficiency from the silver nanoparticles to the cerium oxide. High injection efficiencies in the 6-16% range have been identified for excitation between 400 and 600 nm. These high values have been explained in terms of plasmon-mediated direct electron injection as well as indirect hot electron injection from the NPs to the oxide. The information obtained provides an important contribution towards a knowledge- driven design of efficient cerium oxide based nanostructured materials for solar to chemical energy conversion.

Transparent conductive oxides are a class of materials that combine high optical transparency with high electrical conductivity. This property makes them uniquely appealing as transparent conductive electrodes in solar cells and interesting for optoelectronic and infrared-plasmonic applications. One of the new challenges that researchers and engineers are facing is merging optical and electrical control in a single device for developing next-generation photovoltaic, optoelectronic devices and energy-efficient solid-state lighting. In this work, the authors investigated the possible variations in the dielectric properties of aluminum-doped ZnO (AZO) upon gating by means of spectroscopic ellipsometry (SE). The authors investigated the electrical-bias-dependent optical response of thin AZO films fabricated by magnetron sputtering within a parallel-plane capacitor configuration. The authors address the possibility to control their optical and electric performances by applying bias, monitoring the effect of charge injection/depletion in the AZO layer by means of in operando SE versus applied gate voltage.

The conduction and optoelectronic properties of transparent conductive oxides can be largely modified by intentional inclusion of dopants over a very large range of concentrations. However, the simultaneous presence of structural defects results in an unpredictable complexity that prevents a clear identification of chemical and structural properties of the final samples. By exploiting the unique chemical sensitivity of Hard X-ray Photoelectron Spectra and Near Edge X-ray Absorption Fine Structure in combination with Density Functional Theory, we determine the contribution to the spectroscopic response of defects in Al-doped ZnO films. Satellite peaks in O1s and modifications at the O K-edge allow the determination of the presence of H embedded in ZnO and the very low concentration of Zn vacancies and O interstitials in undoped ZnO. Contributions coming from substitutional and (above the solubility limit) interstitial Al atoms have been clearly identified and have been related to changes in the oxide stoichiometry and increased oxygen coordination, together with small lattice distortions. In this way defects and doping in oxide films can be controlled, in order to tune their properties and improve their performances.

X-ray photoelectron spectroscopy reveals spontaneous atom diffusion from a Mo(001) support into a MgO thin film doped with transition-metal ions. The amount of interfacial mixing depends on the nature of the dopants and is considerably larger for Fe than for Cr impurities. DFT calculations find the reason for Mo diffusion in the ability of the dopants to change oxidation state. Cr exclusively occurs as 3+ ion in the rocksalt lattice, whereby the charge mismatch to native Mg2+ ions is compensated for by Mg vacancies. Iron, on the other hand, switches its thermodynamically preferred configuration from 3+ to 2+ with increasing temperature. As a result, Mo atoms from the support move into the Mg vacancies upon sample annealing and become oxidized via charge transfer into the Fe3+ species. Our study unravels a new charge-compensation scheme in doped oxides that proceeds via chemical intermixing at a metal-oxide interface. The mechanism may rationalize the often observed inactivity of doped oxides in charge-transfer reactions.

International audience; The coincidence lattice formed between a crystalline MgO film and a Mo(001) support is found to be an ideal template to produce long-range ordered ensembles of Fe and Cr particles for electronic, magnetic and chemical applications. The structural and electronic properties of this super-lattice are analyzed by means of scanning tunnelling microscopy and density functional theory. The different registers of atoms at the interface induce periodic changes in the MgO lattice parameter, in the metal-oxide binding length, and the workfunction. These variations modulate the adsorption landscape for Cr and Fe atoms and give rise to the observed ordering phenomena. Preferred nucleation and growth is revealed in those regions of the coincidence lattice that have a small lattice mismatch with the ad-particles and enable electron-transfer processes with the support.

Using scanning tunneling microscopy and density functional theory (DFT), we have analyzed the local electronic properties of (111)-oriented MgO nanoislands on Au(111). Conductance and barrier-height measurements revealed substantial modulations in the electronic structure and electrostatic potential across the islands, with particularly high and low values for band onsets and surface potential occurring at the perimeter and in the island center, respectively. DFT calculations showed that MgO(111) monolayer structures exhibit a strongly reduced distance between the Mg${}^{\ensuremath{\delta}+}$ and O${}^{\ensuremath{\delta}\ensuremath{-}}$ plane as compared to bulk MgO, which in turn suppresses the polar character of the film. The spatial modulations in the electronic properties originate from gradual changes of the interface registry when approaching the island edges, driven by a small mismatch between the Au(111) and MgO(111) lattices. At the periphery of the islands, additional effects such as band shifts and low-lying electronic states are observed, which arise from the interplay of residual edge polarity and unsaturated chemical bonds. We expect that the peculiar edge properties of MgO(111) islands are decisive for the chemical behavior of the nanostructures.