Eng. Cristian TACHE

Carbon dioxide (CO2) is reversibly adsorbed and desorbed from ferroelectric (001) oriented, BaO-terminated barium titanate, as revealed in real time by high resolution and ultrafast photoelectron spectroscopy and certified by low energy electron diffraction. Desorption proceeds when the substrate is heated above its Curie temperature. The amount of CO2 adsorbed is derived to be between one molecule for a surface BaO unit cell (adsorption below room temperature) and one molecule for two unit cells (adsorption above room temperature). The molecule is bound with its carbon to surface oxygen, forming a CO3 structure. The BaTiO3(001) surface is unaffected by repeated cycles of adsorption-desorption. The relatively high amount of CO2 adsorbed and the stability of the substrate after repeated adsorption and desorption processes promotes barium titanate as a promising candidate for decarbonization technologies.

Atomically clean SrTiO3(001) is characterized by low energy electron diffraction, core level and valence band photoelectron spectroscopy, the latter also with spin resolution. Samples prepared by a sputtering-annealing procedure exhibited in-gap states in the valence band spectra, Ti3+ components in Ti 2p core level spectra and a noticeable spin asymmetry in the 3-9 eV binding energy range, which corresponds to valence states of mainly O 2p character. Upon annealing in oxygen, the spin asymmetry vanishes, accompanied by the intensity decrease of the contribution of titanium low ionization states and of in-gap states, indicating that these three phenomena are mutually connected. The observed spin asymmetry may be generated by indirect exchange mediated by the in-gap states between O 2p orbitals, or by the partial Ti 3d character of these states, which acquire non-zero spin in case of incomplete oxygen coordination.

Carbon monoxide is adsorbed at room temperature on graphene formed on atomically clean Pt(001)-hex by chemical vapor deposition, starting with ethylene, in ultrahigh vacuum. The graphene formation is characterized in situ by high resolution photoelectron spectroscopy (HRPES), by low energy electron diffraction (LEED) and by near-edge X-ray absorption fine structure (NEXAFS). The formation of graphene destroys the hex reconstruction of Pt(001) and graphene exhibits totally in-plane sp(2) bonding. CO adsorption is characterized by HRPES and a rigid shift towards higher binding energies by about 96 meV is experienced by Pt 4f core levels, together with a shift towards lower binding energy by 36 meV of the C 1s level corresponding to graphene, while the amplitude analysis of carbon and platinum peaks suggests the intercalation of carbon oxide between graphene and the metal substrate. The presence of oxidized carbon is evidenced by a separate component in the C 1s spectrum (attributed to carbon bond to oxygen) and by the occurrence of the O 1s signal. The coverage expressed in terms of the ratio of the integral amplitudes of the carbon bond to oxygen to the amplitude of the carbon from graphene approaches 3 %, yielding a CO coverage of Pt(001) of about 0.12 monolayer. The derived atomic ratio (O 1s):(C 1s bond to O) is initially close to 1, then evolves in time towards values close to 2, which means that CO is progressively oxidized upon adsorption and irradiation with soft X-rays. The relative amount of oxygen and oxidized carbon decreases in time under irradiation with soft X-rays. Weakly bound graphene on incommensurate metal surfaces may be used as atomic scale nanoreactors for trapping and immediate oxidation of carbon monoxide.

Manganese is deposited at high temperature on (001) oriented ferroelectric lead zirco-titanate prepared in two different ways: sputter-annealed or just simply annealed in ultrahigh vacuum. Room temperature ferromagnetism (FM) is obtained for Mn deposited on sputter-annealed substrates, while for the other sample preparation a paramagnetic behaviour is obtained. Also, for the first case a clear inwards component of the polarization P( - ) is observed by X-ray photoelectron spectroscopy and piezoresponse force microscopy. Composition analysis evidenced formation of Pb vacancies in the case of FM - P(( - ) )sample, consistent with hole formation near the surface, needed both to stabilize the inwards polarization state and to intermediate ferromagnetism between Mn2+ ions. The indirect exchange ferromagnetism mediated by holes is stronger, most probably because the interaction energy is proportional with the carrier effective mass. Also, whereas in the case of unsputtered substrate a stable surface Mn oxide is formed, defect formation by sputtering seems to favor Mn migration inside the sample. This also induces the formation of a thin film where ferromagnetism and the orientation of ferroelectric polarization might have the same origin, i. e. holes accumulated near the outer surface.

The growth of graphene by molecular beam epitaxy from an elemental carbon precursor is a very promising technique to overcome some of the main limitations of the chemical vapour deposition approach, such as the possibility to synthesize graphene directly on a wide variety of surfaces including semiconductors and insulators. However, while the individual steps of the chemical vapour deposition growth process have been extensively studied for several surfaces, such knowledge is still missing for the case of molecular beam epitaxy, even though it is a key ingredient to optimise its performance and effectiveness. In this work, we have performed a combined experimental and theoretical study comparing the growth rate of the molecular beam epitaxy and chemical vapour deposition processes on the prototypical Ir (111) surface. In particular, by employing high-resolution fast X-ray photoelectron spectroscopy, we were able to follow the growth of both single- and multi-layer graphene in real time, and to identify the spectroscopic fingerprints of the different C layers. Our experiments, supported by density functional theory calculations, highlight the role of the interaction between different C precursor species and the growing graphene flakes on the growth rate of graphene. These results provide an overview of the main differences between chemical vapour deposition and molecular beam epitaxy growth and thus on the main parameters which can be tuned to optimise growth conditions.

We report the structural, electronic and magnetic investigation using extended X-ray absorption fine structure spectroscopy (EXAFS), photoelectron microscopy, spin-resolved photoemission and magneto-optical Kerr effect on the properties of MnGe systems obtained by molecular beam epitaxy deposition of manganese on Ge(001) wafers annealed on temperatures between 50 and 450 C. Magnetic ordering can be achieved when the substrate temperature is higher than 250 C, when the manganese tends to diffuse into the Ge matrix and segregate in MnGe-like compounds, as proved by EXAFS. High spatial resolution photoelectron spectroscopy reveals Mn inhomogeneities in the 5-10 m range, even though Mn is found mostly in the same chemical state all over the surface.

Atomically clean lead zirco-titanate PbZr0.2Ti0.8O3 (001) layers exhibit a polarization oriented inwards P(-), visible by a band bending of all core levels towards lower binding energies, whereas as introduced layers exhibit P(+) polarization under air or in ultrahigh vacuum. The magnitude of the inwards polarization decreases when the temperature is increased at 700 K. CO adsorption on P(-) polarized surfaces saturates at about one quarter of a monolayer of carbon, and occurs in both molecular (oxidized) and dissociated (reduced) states of carbon, with a large majority of reduced state. The sticking of CO on the surface in ultrahigh vacuum is found to be directly related to the P(-) polarization state of the surface. A simple electrostatic mechanism is proposed to explain these dissociation processes and the sticking of carbon on P(-) polarized areas. Carbon desorbs also when the surface is irradiated with soft X-rays. Carbon desorption when the polarization is lost proceeds most probably in form of CO2. Upon carbon desorption cycles, the ferroelectric surface is depleted in oxygen and at some point reverses its polarization, owing to electrons provided by oxygen vacancies which are able to screen the depolarization field produced by positive fixed charges at the surface.

Carbon layers grown on lead zirco-titanate (PZT) weakly interact with the substrate and exhibit a nearly two dimensional character, up to a carbon surface density approaching that of graphene. The first feature is evidenced by X-ray photoelectron spectroscopy, and the second by angle resolved near-edge-absorption spectroscopy (NEXAFS). The binding energies and lineshape parameters of C 1s are similar to that of graphene. The dichroism of C K-edge NEXAFS shows the prevalence of in-plane sp(2) bonds for layers whose effective coverage is below the graphene surface density. The polarization state of the substrate, oriented outwards, is preserved upon carbon deposition. The surface Pb content is strongly affected by the carbon ad-layers.

The ability of photoelectron spectro-microscopy with sub-micrometer lateral resolution to identify ferroelectric domains by analysis of surface band bendings is demonstrated on lead zirco-titanate PZT(1 1 1) thin films grown by pulsed laser deposition. Conventional synchrotron radiation X-ray photoelectron spectroscopy allowed one to derive the surface composition of the sample and evidenced shifts toward higher binding energy when the sample is subject to intense soft X-ray beam. A basic model is developed which supposes that photogenerated carriers reduce the depolarization field, yielding a lower torque applied to the ferroelectric polarization. As a consequence, the out-of-plane component of the polarization increases. Domain migration during irradiation with soft X-ray is inferred from the relative amplitude of the components with different binding energy. When the flux density of soft X-ray is on the order of 1011 photons/(s mu m(2)), metal Pb clusters are formed at the surface on areas with the out-of-plane component of the polarization pointing outwards only. (C) 2015 Elsevier B.V. All rights reserved.

This paper analyses in detail the core levels evolution of Pb(Zr,Ti)O-3, i.e. Pb 4f, Zr 3d, Ti 2p, O 1s in various conditions: absolutely freshly prepared sample, sample stored under air, and the effects of in vacuum annealing. The aim of the study is to quantify separately the chemical reactivity at the surface and the band bending effects due to the ferroelectric polarization. It is found that freshly prepared samples present mostly inwards (down arrow) polarization. This phenomenon is mostly revealed by the Ti 2p and O 1s spectra, manifested as a distinct component with 1.8 eV lower binding energy in the O 1s binding energy and by 1.1 eV in the Ti 2p binding energy. Sample aging under air suppresses the inwards polarization, and most signal comes from surfaces not presenting ferroelectric permanent polarization perpendicular to the sample surface. This process conducts also to the formation of Pb(CO3)(2) on the surface. Annealing to temperatures up to 400 degrees C stabilizes a surface composed by a main part of surface without polarization perpendicular to the surface, and with some areas presenting outwards (up arrow) polarization. These areas have, most probably, different terminations, the polarized area being (Ti,Zr)O-2 terminated. (C) 2013 Elsevier B.V. All rights reserved.

TiO2:Fe thin films prepared by pulsed laser deposition exhibit in some case light dependent saturation magnetization, as determined from Kerr magnetometry measurements performed in dark or by illuminating the sample. This phenomenon is studied in correlation with local atomic structure investigated by extended X-ray absorption fine structure, composition and chemical state analyzed by X-ray photoelectron spectroscopy and by X-ray absorption near-edge structure. It is found that light-controllable magnetism is a property of a mixture of Fe and oxidized Fe clusters embedded in the anatase TiO2 matrix.

Generally, the uranifer tailings dump are not protected. During the time appear chemical and physical modifications which favour spreading of the radioactive contaminant into the environment, because of the waters provided from precipitations with pH variation (acid rainfall, snow), solar radiance, variation of temperature and humidity, interaction of different minerals from the waste rock with infiltration water during the flow, leading to precipitations phenomenon, ionic exchange, adsorption and re-dissolving [2]. As a rule, the speed of radionuclides diffusion from radioactive tailings dump depends on each of these factors and is very specific to the hydro and geochemistry of the nearest rocks [7]. Because of that, the scientific research is very difficult and will obliges us to study through the simplified physical systems but maintaining the important parameters which are influencing the diffusion process of the contaminant in the environment. The main objective of this paper is to characterize hydro and geochemistry properties controlling the migration of radionuclides extracted from tailings dump into the environment, including the processes of sorption and adsorption and neglecting other physical processes as precipitation, ion exchange or some other geochemical interactions within the system. For this purpose we made studies on Ra-226 and U-nal extraction from uranifer tailings dump analyzed in pure water in dynamic system, studies about the influence of clay presence on speed of diffusion process in sorption and adsorption phenomenon. Finally we made a prediction for long term about variation of concentrations of Ra-226 and U-nal based on experimental data using some software programs from literature [8].