We employed operando soft X-ray absorption spectroscopy (XAS) to monitor the changes in the valence states and spin properties of LaMn1–xCoxO3 catalysts subjected to a mixture of CO and O2 at ambient pressure. Guided by simulations based on charge transfer multiplet theory, we quantitatively analyze the Mn and Co 2p XAS as well as the oxygen K-edge XAS spectra during the reaction process. The Mn sites are particularly sensitive to the catalytic reaction, displaying dynamics in their oxidation state. When Co doping is introduced (x ≤ 0.5), Mn oxidizes from Mn2+ to Mn3+ and Mn4+, while Co largely maintains a valence state of Co2+. In the case of LaCoO3, we identify high-spin and low-spin Co3+ species combined with Co2+. Our investigation underscores the importance to consider the spin and valence states of catalyst materials under operando conditions.
Polarization dependent x-ray absorption spectroscopy was used to study the magnetic ground state and the orbital occupation in bulk-phase VI3 van der Waals crystals below and above the ferromagnetic and structural transitions. X-ray natural linear dichroism and x-ray magnetic circular dichroism spectra acquired at the V $L_{2,3}$ edges are compared against multiplet cluster calculations within the frame of the ligand field theory to quantify the intra-atomic electronic interactions at play and evaluate the effects of symmetry reduction occurring in a trigonally distorted VI6 unit. We observed a non zero linear dichroism proving the presence of an anisotropic charge density distribution around the V3+ ion due to the unbalanced hybridization between the vanadium and the ligand states. Such hybridization acts as an effective trigonal crystal field, slightly lifting the degeneracy of the $t_{2g}^2$ ground state. However, the energy splitting associated to the distortion underestimates the experimental band gap, suggesting that the insulating ground state is stabilized by Mott correlation effects rather than via a Jahn–Teller mechanism. Our results clarify the role of the distortion in VI3 and establish a benchmark for the study of the spectroscopic properties of other van der Waals halides, including emerging 2D materials with mono and few-layers thickness, whose fundamental properties might be altered by reduced dimensions and interface proximity.
In this work, which follows Part I that is dedicated to the precatalyst, we investigate the electronic properties and the accessibility of the Ti active sites in a highly active silica-supported Ziegler–Natta catalyst for industrial polyethylene production, applying a multi-scale, multi-technique approach. Complementary electronic spectroscopies (i.e. Ti K-edge XANES, Ti L2,3-edge NEXAFS and DR UV–Vis-NIR) reveal the coexistence of several titanium phases, whose relative amount depends on the concentration of the alkyl aluminum activator. In addition to β-TiCl3-like clusters and monomeric Ti(IV) sites, which are already present in the precatalyst, isolated Ti(III) sites and α-TiCl3-like clusters are formed in the presence of the activator. Two families of alkylated Ti(III) sites characterized by a different electron density are detected by IR spectroscopy of adsorbed CO, and two types of Ti-acyl species are formed upon CO insertion into the Ti-alkyl bond, characterized by a different extent of η2-coordination. The whole set of data suggests that TiCl3 clusters are preferentially formed at the exterior of the catalyst particles, likely as a consequence of Ti(III) mobility in the presence of strong Lewis acids, in most cases hampering the spectroscopic detection of isolated Ti(III) sites. In contrast, only monomeric Ti(III) sites are formed at the interior of the catalyst particles, characterized by a high electron density evocative of the presence of electron donors in the close proximity (e.g. aluminum alkoxide by-products). These sites are less accessible because of diffusion limitations, and only become visible by surface-sensitive spectroscopic methods (such as Ti L2,3-edge TEY-NEXAFS) upon the fragmentation of the catalyst particles.
Magnesium chloride is a prototypical deliquescent material whose surface properties, although central for Ziegler–Natta cataysis, have so far remained elusive to experimental characterization. In this work, we use surface-selective X-ray absorption spectroscopy (XAS) at ambient pressure in combination with multivariate curve resolution, molecular dynamics, and XAS theoretical methods to track in real time and accurately describe the interaction between water vapor and the MgCl2 surface. By exposing MgCl2 to water vapor at temperatures between 595 and 391 K, we show that water is preferentially adsorbed on five-coordinated Mg2+ sites in an octahedral configuration, confirming previous theoretical predictions, and find that MgCl2 is capable of retaining a significant amount of adsorbed water even under prolonged heating to 595 K. As a consequence, our work provides first experimental insights into the unique surface affinity of MgCl2 for atmospheric water. The developed technique is proven highly sensitive to the modifications induced by adsorbates on a given low-Z metal based surface and may be useful in the toolbox required to disentangle the mechanisms of interfacial chemical processes.
This thesis completes my work as doctoral student of the Scuola di Dottorato in Fisica, Astrofisica e Fisica Applicata at the Universit`a degli Studi di Milano, that has been carried out since November 2019 at the Istituto Officina dei Materiali of the Consiglio Nazionale delle Ricerche (IOM-CNR) in the premises of the Elettra - Sincrotrone Trieste and FERMI@Elettra infrastructures and in the framework of the NFFA facility.
My experimental activity employed complementary spectroscopy and polarimetry techniques oriented to address the characterisation of electronic and spin properties of systems with decreasing dimensionality. This programme has been conducted by exploiting state-of-the-art infrastructures to generate visible, UV and EUV ultrashort pulses (tabletop lasers and HHG at NFFA-SPRINT laboratory) and soft X-ray synchrotron light (at Elettra, Diamond and ESRF synchrotron light sources).
I used photoemission as the main tool in my investigation, supplementing my results with absorption spectroscopy. I focused on three materials, Fe(001)-p(1x1)O/MgO, EuSn2P2 and VI3, of high interest in modern and next-generation magnetic devices.
In the three systems I studied the electronic band structure to identify key features hinting at the bound electrons behaviour. I investigated the properties of the magnetically ordered phases and found evidence of the reduced dimensionality in the emergence of atypical spin ordering and the increasingly manifest electron correlation phenomena.
The information retained by band electrons is critical to access the spin polarisation of the bands and to give insight into the effects of spatial confinement on the spin degree of freedom.
Interfaces between water and materials are ubiquitous and are crucial in materials sciences and in biology, where investigating the interaction of water with the surface under ambient conditions is key to shedding light on the main processes occurring at the interface. Magnesium oxide is a popular model system to study the metal oxide–water interface, where, for sufficient water loadings, theoretical models have suggested that reconstructed surfaces involving hydrated Mg2+ metal ions may be energetically favored. In this work, by combining experimental and theoretical surface-selective ambient pressure X-ray absorption spectroscopy with multivariate curve resolution and molecular dynamics, we evidence in real time the occurrence of Mg2+ solvation at the interphase between MgO and solvating media such as water and methanol (MeOH). Further, we show that the Mg2+ surface ions undergo a reversible solvation process, we prove the dissolution/redeposition of the Mg2+ ions belonging to the MgO surface, and we demonstrate the formation of octahedral [Mg(H2O)6]2+ and [Mg(MeOH)6]2+ intermediate solvated species. The unique surface, electronic, and structural sensitivity of the developed technique may be beneficial to access often elusive properties of low-Z metal ion intermediates involved in interfacial processes of chemical and biological interest.
Space and mirror charge effects in time-resolved photoemission spectroscopy can be modeled to obtain relevant information on the recombination dynamics of charge carriers. We successfully extracted from these phenomena the reneutralization characteristic time of positive charges generated by photoexcitation in CeO2-based films. For the above-band-gap excitation, a large fraction of positive carriers with a lifetime that exceeds 100 ps are generated. Otherwise, the sub-band-gap excitation induces the formation of a significantly smaller fraction of charges with lifetimes of tens of picoseconds, ascribed to the excitation of defect sites or to multiphoton absorption. When the oxide is combined with Ag nanoparticles, the sub-band-gap excitation of localized surface plasmon resonances leads to reneutralization times longer than 300 ps. This was interpreted by considering the electronic unbalance at the surface of the nanoparticles generated by the injection of electrons, via localized surface plasmon resonance (LSPR) decay, into CeO2. This study represents an example of how to exploit the space charge effect in gaining access to the surface carrier dynamics in CeO2 within the picosecond range of time, which is fundamental to describe the photocatalytic processes.
This work presents an original approach to preparing pure and Ni-doped CeO2 nanoparticles (NPs) that can be directly drop-casted on a substrate or calcined to form powders. The reduction of the NPs in H2 is very different than the one usually anticipated for supported Ni–CeO2 catalysts. In situ soft X-ray absorption and infrared spectroscopies revealed that the reduction of Ce4+ into Ce3+ in H2 proceeds via simultaneous oxidation of Ni2+ ions into Niδ+ (2<δ<3). Comparison with reference samples indicates that Ce4+ ions reduction is promoted over Ni-doped CeO2 NPs, whereas that of Ni2+ is hindered. Theoretical simulation of Ni L-edge spectra suggested that Ni dopant into ceria is in a square planar four-coordinate environment, in contrast to the familiar octahedral symmetry of bulk nickel oxides. Our results reveal that the surface chemistry of Ni-doped CeO2 is quite distinct as compared to that of the individual bulk oxides, which potentially can lead to a different performance of this material, notably in catalytic applications.
The ever-growing demand for Li-ion batteries requires high-capacity electrode materials that should also be environmentally benign, Co-free, secure and durable, to achieve an optimal compromise between sustainability and functional performances. Spinel LiMn2O4 (LMO) is a state-of-the-art material, which, in principle, could satisfy such requirements. However, an undesired cubic-tetragonal phase transition favors Jahn-Teller (J-T) spinel distortion, leading to severe capacity reduction upon cycling below 3 V. Here, we propose a novel dual-doping strategy for LMO, based on the partial substitution of Mn(III) with Fe(III) and Ti(IV) to design new active materials for high-capacity cathodes, namely LiFexMn2-x-yTiyO4 (LFMT), with Li/Mn ratio ranging between 1 and 1.7. The substitution of Mn with Fe and Ti suppresses the J-T distortion, which is often still evident in the case of Ti-doped LMO. This allows cycling in a wider voltage range (4.8-1.5 V), thus resulting in higher capacity and significantly improved stability. The lithiation mechanisms were investigated by combining ex-situ X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS analyses). It demonstrated that the only redox-active metal is Mn, while Fe and Ti are electrochemically inactive. The extensive electrochemical lithiation/delithiation of the LFMT compositions brought to unprecedented results, which give evidence of stabilizing cation disorder through the formation of Mn-rich and Mn-poor domains, which leades to two spinel phases with different Mn:Ti ratios. These insights into the lithiation mechanism pave the way for a better understanding of the doping chemistry and electrochemistry of Mn-based spinels as cathode materials for Li-ion batteries.
High entropy oxides (HEOs) are an emerging class of materials constituted by multicomponent systems that are receiving special interest as candidates for obtaining novel and desirable properties. In this study we present a detailed investigation of the relevant intermediates arising at the surface of the prototypical HEO Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O during low-temperature CO oxidation. By combining Cu L2,3-edge operando soft X-ray absorption spectroscopy (soft-XAS) with density functional theory simulations, we propose that upon HEO exposure to CO at 235 °C reduced Cu(I) sites arise mostly coordinated to activated CO molecules and partly to bidentate carbonate species. When the HEO surface is then exposed to a stoichiometric mixture of CO+1/2 O2 at 250 °C, CO2 is produced while bidentante carbonate moieties remain interacting with the Cu(I) sites. We structurally characterize the carbonate and CO preferential adsorbtion geometries on the Cu(I) surface metal centers, and find that CO adopts a bent conformation that may energetically favor its subsequent oxidation. The unique surface, structural and electronic sensitivity of soft XAS together with the developed data analysis work-flow may be beneficial to characterize often elusive surface properties of systems of catalytic interest.
Hybridization of electronic states and orbital symmetry in transition metal oxides are generally considered key ingredients in the description of both their electronic and magnetic properties. In the prototypical case of La0.65Sr0.35MnO3 (LSMO), a landmark system for spintronics applications, a description based solely on Mn 3d and O 2p electronic states is reductive. We thus analyzed elemental and orbital distributions in the LSMO valence band through a comparison between density functional theory calculations and experimental photoelectron spectra in a photon energy range from soft to hard x rays. We reveal a number of hidden contributions, arising specifically from La 5p, Mn 4s, and O 2s orbitals, considered negligible in previous analyses; our results demonstrate that all these contributions are significant for a correct description of the valence band of LSMO and of transition metal oxides in general.
Although Ziegler–Natta (ZN) catalysts play a major role in the polyolefin market, a true understanding of their properties at the molecular level is still missing. In particular, there is a lack of knowledge on the electronic properties of Ti sites. Theoretical calculations predict that the electron density of the Ti sites in the precatalysts correlates with the activation energy for olefin insertion in the Ti-alkyl bond generated at these sites after activation by Al-alkyls. It is also well known that the effective charge on the Ti sites in the activated catalysts affects the olefin π-complexation. In this contribution, we exploit two electronic spectroscopies, UV–vis and Ti L2,3-edge near-edge X-ray absorption fine structure (NEXAFS), complemented with theoretical simulation to investigate three ZN precatalysts of increasing complexity (up to an industrial system) and the corresponding catalysts activated by triethylaluminum (TEAl). We provide compelling evidence for the presence of monomeric 6-fold-coordinated Ti4+ species in all of the precatalysts, which however differ in the effective charge on the Ti sites. We also unambiguously demonstrate that these sites are reduced by TEAl to two types of monomeric 5-coordinated Ti3+, either alkylated or not, and that the former are involved in ethylene polymerization. In addition, small TiCl3 clusters are formed in the industrial catalyst, likely due to the occurrence of severe reducing conditions within the catalyst pores. These data prove the potential of these two techniques, coupled with simulation, in providing an accurate description of the electronic properties of heterogeneous ZN catalysts.
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.
Preferential oxidation of CO (COPrOx) is a catalytic reaction targeting the removal of trace amounts of CO from hydrogen-rich gas mixtures. Non-noble metal catalysts, such as Cu and Co, can be equally active to Pt for the reaction; however, their commercialization is limited by their poor stability. We have recently shown that CoO is the most active state of cobalt for COPrOx, but under certain reaction conditions, it is readily oxidized to Co3O4 and deactivates. Here, we report a simple method to stabilize the Co2+ state by vanadium addition. The V-promoted cobalt catalyst exhibits considerably higher activity and stability than pure cobalt. The nature of the catalytic active sites during COPrOx was established by operando NAP-XPS and NEXAFS, while the stability of the Co2+ state on the surface was verified by in situ NEXAFS at 1 bar pressure. The active phase consists of an ultra-thin cobalt-vanadate surface layer, containing tetrahedral V5+ and octahedral Co2+ cations, with an electronic and geometric structure that is deviating from the standard mixed bulk oxides. In addition, V addition helps to maintain the population of Co2+ species involved in the reaction, inhibiting carbonate species formation that are responsible for the deactivation. The promoting effect of V is discussed in terms of enhancement of CoO redox stability on the surface induced by electronic and structural modifications. These results demonstrate that V-promoted cobalt is a promising COPrOx catalyst and validate the application of in situ spectroscopy to provide the concept for designing better performing catalysts.
Solid oxide photoelectrochemical cells (SOPECs) with inorganic ion-conducting electrolytes provide an alternative solution for light harvesting and conversion. Exploring potential photoelectrodes for SOPECs and understanding their operation mechanisms are crucial for continuously developing this technology. Here, ceria-based thin films were newly explored as photoelectrodes for SOPEC applications. It was found that the photoresponse of ceria-based thin films can be tuned both by Sm-doping-induced defects and by the heating temperature of SOPECs. The whole process was found to depend on the surface electrochemical redox reactions synergistically with the bulk photoelectric effect. Samarium doping level can selectively switch the open-circuit voltages polarity of SOPECs under illumination, thus shifting the potential of photoelectrodes and changing their photoresponse. The role of defect chemistry engineering in determining such a photoelectrochemical process was discussed. Transient absorption and X-ray photoemission spectroscopies, together with the state-of-the-art in operando X-ray absorption spectroscopy, allowed us to provide a compelling explanation of the experimentally observed switching behavior on the basis of the surface reactions and successive charge balance in the bulk.
THORONDOR is a data treatment software with a graphical user interface (GUI) accessible via the browser‐based Jupyter notebook framework. It aims to provide an interactive and user‐friendly tool for the analysis of NEXAFS spectra collected during in situ experiments. The program allows on‐the‐fly representation and quick correction of large datasets from single or multiple experiments. In particular, it provides the possibility to align in energy several spectral profiles on the basis of user‐defined references. Various techniques to calculate background subtraction and signal normalization have been made available. In this context, an innovation of this GUI involves the usage of a slider‐based approach that provides the ability to instantly manipulate and visualize processed data for the user. Finally, the program is characterized by an advanced fitting toolbox based on the lmfit package. It offers a large selection of fitting routines as well as different peak distributions and empirical ionization potential step edges, which can be used for the fit of the NEXAFS rising‐edge peaks. Statistical parameters describing the goodness of a fit such as χ2 or the R‐factor together with the parameter uncertainty distributions and the related correlations can be extracted for each chosen model.
Ti silicates, and in particular Titanium Silicalite‐1 (TS‐1), are nowadays important catalysts for several partial oxidation reactions in the presence of aqueous H 2 O 2 as oxidant. Despite the numerous studies dealing with this material, some fundamental aspects are still unfathomed. In particular, the structure and the catalytic role of defective Ti sites, other than perfect tetrahedral sites recognized as main active species, has not been quantitatively discussed in the literature. In this work, we assess the structural features of defective Ti sites on the basis of electronic spectroscopies outcomes, as interpreted through quantum‐mechanical simulation. We disclose here strong evidences that the most common defective Ti sites, often reported in the TS‐1 literature, are monomeric Ti centers, embedded in the zeolite framework, having a distorted octahedral local symmetry.
Ambient pressure operando soft X-ray absorption spectroscopy (soft-XAS) was applied to study the reactivity of hydroxylated SnO2 nanoparticles towards reducing gases. H2 was first used as a test case, showing that gas phase and surface states can be simultaneously probed: soft-XAS at the O K-edge gains sensitivity towards the gas phase, while at the Sn M4,5-edges tin surface states are explicitly probed. Results obtained by flowing hydrocarbons (CH4 and CH3CHCH2) unequivocally show that these gases react with surface hydroxyl groups to produce water without producing carbon oxides, and release electrons that localize on Sn to eventually form SnO. The partially reduced SnO2-x layer at the surface of SnO2 is readily reoxidised to SnO2 by treating the sample with O2 at mild temperatures (> 200 °C), revealing the nature of “electron sponge” of tin oxide. The experiments, combined with DFT calculations, allowed devising a mechanism for dissociative hydrocarbon adsorption on SnO2, involving direct reduction of Sn sites at the surface via cleavage of C-H bonds, and the formation of methoxy- and/or methyl-tin species at the surface.
Bulk PtSn4 has recently attracted the interest of the scientific community for the presence of electronic states exhibiting Dirac node arcs, enabling possible applications in nanoelectronics. Here, by means of surface-science experiments and density functional theory, we assess its suitability for catalysis by studying the chemical reactivity of the (0 1 0)-oriented PtSn4 surface toward CO, H2O, O2 molecules at room temperature and, moreover, its stability in air. We demonstrate that the catalytic activity of PtSn4 is determined by the composition of the outermost atomic layer. Specifically, we find that the surface termination for PtSn4 crystals cleaved in vacuum is an atomic Sn layer, which is totally free from any CO poisoning. In oxygen-rich environment, as well as in ambient atmosphere, the surface termination is a SnOx skin including SnO and SnO2 in comparable amount. However, valence-band states, including those forming Dirac node arcs, are only slightly affected by surface modifications. The astonishingly beneficial influence of surface oxidation on catalytic activity has been demonstrated by electrocatalytic tests evidencing a reduction of the Tafel slope, from 442 down to 86 mV dec−1, whose origin has been explained by our theoretical model. The use of surface-science tools to tune the chemical reactivity of PtSn4 opens the way toward its effective use in catalysis, especially for hydrogen evolution reaction and oxygen evolution reaction.
The mechanisms of CO oxidation on the Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O high entropy oxide were studied by means of operando soft X-ray absorption spectroscopy. We found that Cu is the active metal, and that Cu(II) can be rapidly reduced to Cu(I) by CO when the temperature is larger than 130 °C. Co and Ni do not have any role in this respect. The Cu(II) oxidation state can be easily but slowly recovered by treating the sample in O2 at ca. 250 °C. However, it should be noted that CuO is readily and irreversibly reduced to Cu(I) if treated in CO at T>100 °C. Thus, the main conclusion of this work is that the high configurational entropy of Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O stabilizes the rock-salt structure and permits the oxidation/reduction of Cu to be reversible, thus permitting the catalytic cycle to take place.
Implementation of in-situ and operando experimental set-ups for bridging the pressure gap in characterization techniques based on monitoring of photoelectron emission has made significant achievements at several beamlines at Elettra synchrotron facility. These set-ups are now operational and have been successfully used to address unsolved issues exploring events occurring at solid–gas, solid–liquid and solid-solid interfaces of functional materials. The sections in the article communicate the research opportunities offered by the current set-ups at APE, BACH, ESCAmicroscopy and Nanospectroscopy beamlines and outline the next steps to overcome the present limits.
Palladium ditelluride (PdTe2) is a novel transition‐metal dichalcogenide exhibiting type‐II Dirac fermions and topological superconductivity. To assess its potential in technology, its chemical and thermal stability is investigated by means of surface‐science techniques, complemented by density functional theory, with successive implementation in electronics, specifically in a millimeter‐wave receiver. While water adsorption is energetically unfavorable at room temperature, due to a differential Gibbs free energy of ≈+12 kJ mol−1, the presence of Te vacancies makes PdTe2 surfaces unstable toward surface oxidation with the emergence of a TeO2 skin, whose thickness remains sub‐nanometric even after one year in air. Correspondingly, the measured photocurrent of PdTe2‐based optoelectronic devices shows negligible changes (below 4%) in a timescale of one month, thus excluding the need of encapsulation in the nanofabrication process. Remarkably, the responsivity of a PdTe2‐based millimeter‐wave receiver is 13 and 21 times higher than similar devices based on black phosphorus and graphene in the same operational conditions, respectively. It is also discovered that pristine PdTe2 is thermally stable in a temperature range extending even above 500 K, thus paving the way toward PdTe2‐based high‐temperature electronics. Finally, it is shown that the TeO2 skin, formed upon air exposure, can be removed by thermal reduction via heating in vacuum.
Cu2ZnSnS4 (CZTS) nanocrystals (NCs) were produced via hot-injection from metal chloride precursors. A systematic investigation of the influence of synthesis conditions on composition, size and microstructure of CZTS NCs is presented. The results show that the solvent amount (oleylamine) is a key parameter in the synthesis of this quaternary chalcogenide: a low solvent content leads to CZTS NCs with a prominent kesterite phase with the desired composition for use as absorber material in thin film photovoltaic cells. It is also observed that lowering the injection temperature (250 °C) favours formation of CZTS NCs in the wurtzite phase. The effect of different high temperature thermal treatments on the grain growth is also shown: large crystals are obtained with annealing in inert atmosphere, whereas nanocrystalline films are obtained introducing sulphur vapour during the heat treatment. A correlation between the grain dimension and the carbonaceous residues in the final films is investigated. It is shown that the grain growth is hindered by organic residues, amount and nature of which depend on the heat treatment atmosphere. In fact, oleylamine is removed by a complex pyrolytic process, which is affected by the presence of sulphur vapour. The latter favours the stability of oleylamine residuals against its non-oxidative release.
Materials exhibiting nodal‐line fermions promise superb impact on technology for the prospect of dissipationless spintronic devices. Among nodal‐line semimetals, the ZrSiX (X = S, Se, Te) class is the most suitable candidate for such applications. However, the surface chemical reactivity of ZrSiS and ZrSiSe has not been explored yet. Here, by combining different surface‐science tools and density functional theory, it is demonstrated that the formation of ZrSiS and ZrSiSe surfaces by cleavage is accompanied by the washing up of the exotic topological bands, giving rise to the nodal line. Moreover, while the ZrSiS has a termination layer with both Zr and S atoms, in the ZrSiSe surface, reconstruction occurs with the appearance of Si surface atoms, which is particularly prone to oxidation. It is demonstrated that the chemical activity of ZrSiX compounds is mostly determined by the interaction of the Si layer with the ZrX sublayer. A suitable encapsulation for ZrSiX should not only preserve their surfaces from interaction with oxidative species, but also provide a saturation of dangling bonds with minimal distortion of the surface.
I explored the properties of systems that were fabricated aiming to exploit enhanced multiferroic behavior and potentially useful functionalities at room temperature. The systems of choice for this thesis were two prototypical multiferroic heterostructures composed by a ferromagnetic thin film deposited on a ferroelectric substrate: LSMO/BTO(001) and Fe,FeMn/PMN-PT(001). I focused on the magnetic response of the thin films to applied electric fields oriented perpendicular to the interface, and influencing the substrate. In both the chosen heterostructures the magnetic layers and ferroelectric substrates are all materials with high ordering temperature.
La misura della polarizzazione in spin di un fascio di elettroni fotoemessi da una superficie ferromagnetica permette di studiare in modo diretto la struttura elettronica determinata dall’interazione di scambio e quindi il momento magnetico di spin del sistema, caratterizzandone il comportamento magnetico. Da una parte lo sviluppo del campo della spintronica, dall’altra la richiesta sempre crescente di strumenti e dispositivi di immagazzinamento e trattamento dati ad alte prestazioni, marcano la necessità di esplorare le configurazioni degli stati elettronici e le loro eccitazioni.
The design and characterization of a HHG source conceived for Time and Angle Resolved PhotoElectron Spectroscopy (TR-ARPES) experiments are presented. The harmonics are selected through a grating monochromator with an innovative design able to provide XUV radiation for two distinct TR-ARPES setups.
PtTe2 is a novel transition-metal dichalcogenide hosting type-II Dirac fermions that displays application capabilities in optoelectronics and hydrogen evolution reaction. Here it is shown, by combining surface science experiments and density functional theory, that the pristine surface of PtTe2 is chemically inert toward the most common ambient gases (oxygen and water) and even in air. It is demonstrated that the creation of Te vacancies leads to the appearance of tellurium-oxide phases upon exposing defected PtTe2 surfaces to oxygen or ambient atmosphere, which is detrimental for the ambient stability of uncapped PtTe2-based devices. On the contrary, in PtTe2 surfaces modified by the joint presence of Te vacancies and substitutional carbon atoms, the stable adsorption of hydroxyl groups is observed, an essential step for water splitting and the water–gas shift reaction. These results thus pave the way toward the exploitation of this class of Dirac materials in catalysis.
The electric and nonvolatile control of the spin texture in semiconductors would represent a fundamental step toward novel electronic devices combining memory and computing functionalities. Recently, GeTe has been theoretically proposed as the father compound of a new class of materials, namely ferroelectric Rashba semiconductors. They display bulk bands with giant Rashba-like splitting due to the inversion symmetry breaking arising from the ferroelectric polarization, thus allowing for the ferroelectric control of the spin. Here, we provide the experimental demonstration of the correlation between ferroelectricity and spin texture. A surface-engineering strategy is used to set two opposite predefined uniform ferroelectric polarizations, inward and outward, as monitored by piezoresponse force microscopy. Spin and angular resolved photoemission experiments show that these GeTe(111) surfaces display opposite sense of circulation of spin in bulk Rashba bands. Furthermore, we demonstrate the crafting of nonvolatile ferroelectric patterns in GeTe films at the nanoscale by using the conductive tip of an atomic force microscope. Based on the intimate link between ferroelectric polarization and spin in GeTe, ferroelectric patterning paves the way to the investigation of devices with engineered spin configurations.
The knowledge of the picosecond dynamics of the energy level alignment between donor and acceptor materials in organic photovoltaic devices under working conditions is a challenge for fundamental material research. We measured by means of time-resolved Resonant X-ray Photoemission Spectroscopy (RPES) the energy level alignment in ZnPc/C60 films. We employed 800 nm femtosecond laser pulses to pump the system simulating sunlight excitation and X-rays from the synchrotron as a probe. We measured changes in the valence bands due to pump induced modifications of the interface dipole. Our measurements prove the feasibility of time-resolved RPES with high repetition rate sources.
This thesis completes my work as doctoral student of the Scuola di Dottorato in Fisica, Astrofisica e Fisica Applicata at the Università degli Studi di Milano that has been carried out, starting in November 4236, mostly at the Laboratorio TASC of IOM-CNR3 in the premises of the Elettra - Sincrotrone Trieste and FERMI@Elettra infrastructures4, in the framework of the NFFA and APE-beamline facilites5, as well as by accessing international large scale infrastructures and laboratories. The activity has addressed the development of experimental methodologies and novel instrumentation oriented to the study of the dynamical properties of highly correlated materials after high energy excitation. The science programme has been carried out by exploiting ultrafast femtosecond probes from the optical regime (Ti-Sa lasers, fibre laser oscillators) to the extreme UV-soft X rays at FERMI, to the picosecond hard X-rays from the SPring-: and Diamond synchrotron radiation source. The sample synthesis of correlated oxides and its characterization has been performed within the NFFA facility and APE-group collaboration in Trieste as well as the design and construction of the all new laser High Harmonic Generation beam line NFFA-SPRINT and its end station for time resolved vectorial electron spin polarimetry.
This thesis reports on the construction and commissioning tests of the novel experimental set-up needed for a long term research project, named ULTRASPIN, aiming at establishing time resolved spin-resolved photoemission measurements with ultra-short (10−14 s) photon pulses from Free Electron Laser beamlines or from table-top UV/Soft-X beamlines.
The ULTRASPIN project started in the summer 2013, building on competences and instrumentation in part available from the APE-beamline group of IOM-CNR at Elettra, and with the partial support of an European contract (EXSTASY-EXperimental STation for the Analysis of the Spin Dynamics, Grant agreement N.PIIF-GA-2012-326641) and related fellowship of a world-expert of Mott scattering.
I have been involved from the beginning in the final design, in the construction and commissioning of a novel stray-field free UHV apparatus for preparing and hosting atomically clean surfaces and for measuring the spin-polarization of the photo-emitted electrons with “single pulse” sensitivity down to the 10−14 s time scale, as well as in the standard high frequency spectroscopy mode. In the commissioning phase I have participated to test experiments on ULTRASPIN as well as to relevant experiments conducted in other apparatuses.