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.

The fabrication and integration of high-quality structures of Yttrium Iron Garnet (YIG) is critical for magnonics. Films with excellent properties are obtained only on single crystal Gadolinium Gallium Garnet (GGG) substrates using high-temperature processes. The subsequent realization of magnonic structures via lithography and etching is not straightforward as it requires a tight control of the edge roughness, to avoid magnon scattering, and planarization in case of multilayer devices. In this work a different approach is described based on local laser annealing of amorphous YIG films, avoiding the need for subjecting the entire sample to high thermal budgets and for physical etching. Starting from amorphous and paramagnetic YIG films grown by pulsed laser deposition at room temperature on GGG, a 405 nm laser is used for patterning arbitrary shaped ferrimagnetic structures by local crystallization. In thick films (160 nm) the laser induced surface corrugation prevents the propagation of spin-wave modes in patterned conduits. For thinner films (80 nm) coherent propagation is observed in 1.2 µm wide conduits displaying an attenuation length of 5 µm that is compatible with a damping coefficient of ≈5 × 10−3. Possible routes to achieve damping coefficients compatible with state-of-the art epitaxial YIG films are discussed.

AgCrSe2 exhibits remarkably high ionic conduction, an inversion symmetry-breaking structural transition, and is host to complex non-colinear magnetic orders. Despite its attractive physical and chemical properties and its potential for technological applications, studies of this compound to date are focused almost exclusively on bulk samples. Here, we report the growth of AgCrSe2 thin films via molecular beam epitaxy. Single-orientated epitaxial growth was confirmed by x-ray diffraction, while resonant photoemission spectroscopy measurements indicate a consistent electronic structure as compared to bulk single crystals. We further demonstrate significant flexibility of the grain morphology and cation stoichiometry of this compound via control of the growth parameters, paving the way for the targeted engineering of the electronic and chemical properties of AgCrSe2 in thin-film form.

The combination of the ability to absorb most of the solar radiation and simultaneously suppress infrared re-radiation allows selective solar absorbers (SSAs) to maximize solar energy to heat conversion, which is critical to several advanced applications. The intrinsic spectral selective materials are rare in nature and only a few demonstrated complete solar absorption. Typically, intrinsic materials exhibit high performances when integrated into complex multilayered solar absorber systems due to their limited spectral selectivity and solar absorption. In this study, we propose CoSbx (2 < x < 3) as a new exceptionally efficient SSA. Here we demonstrate that the low bandgap nature of CoSbx endows broadband solar absorption (0.96) over the solar spectral range and simultaneous low emissivity (0.18) in the mid-infrared region, resulting in a remarkable intrinsic spectral solar selectivity of 5.3. Under 1 sun illumination, the heat concentrates on the surface of the CoSbx thin film, and an impressive temperature of 101.7 °C is reached, demonstrating the highest value among reported intrinsic SSAs. Furthermore, the CoSbx was tested for solar water evaporation achieving an evaporation rate of 1.4 kg m−2 h−1. This study could expand the use of narrow bandgap semiconductors as efficient intrinsic SSAs with high surface temperatures in solar applications.

${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$ is a rare example of a layered ferrimagnet. It has recently been shown to host a colossal angular magnetoresistance as the spin orientation is rotated from the in- to out-of-plane direction, proposed to be underpinned by a topological nodal-line degeneracy in its electronic structure. Nonetheless, the origins of its ferrimagnetic structure remain controversial, while its experimental electronic structure, and the role of correlations in shaping this, are little explored to date. Here, we combine x-ray and photoemission-based spectroscopies with first-principles calculations to probe the elemental-selective electronic structure and magnetic order in ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$. Through these, we identify a marked Mn-Te hybridization, which weakens the electronic correlations and enhances the magnetic anisotropy. We demonstrate how this strengthens the magnetic frustration in ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$, which is key to stabilizing its ferrimagnetic order, and find a crucial role of both exchange interactions extending beyond nearest-neighbors and antisymmetric exchange in dictating its ordering temperature. Together, our results demonstrate a powerful methodology of using experimental electronic structure probes to constrain the parameter space for first-principles calculations of magnetic materials, and through this approach, reveal a pivotal role played by covalency in stabilizing the ferrimagnetic order in ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$.

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.

VO2 is one of the most studied vanadium oxides because it undergoes a reversible metal-insulator transition (MIT) upon heating with a critical temperature of around 340 K. One of the most overlooked aspects of VO2 is the band’s anisotropy in the metallic phase when the Fermi level is crossed by two bands: π* and d||. They are oriented perpendicularly in one respect to the other, hence generating anisotropy. One of the parameters tuning MIT properties is the unbalance of the electron population of π* and d|| bands that arise from their different energy position with respect to the Fermi level. In systems with reduced dimensionality, the electron population disproportion is different with respect to the bulk leading to a different anisotropy. Investigating such a system with a band-selective spectroscopic tool is mandatory. In this manuscript, we show the results of the investigation of a single crystalline 8 nm VO2/TiO2(101) film. We report on the effectiveness of linearly polarized resonant photoemission (ResPES) as a band-selective technique probing the intrinsic anisotropy of VO2.

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.

Single crystals of the hexagonal triangular lattice compound AgCrSe2 have been grown by chemical vapor transport. The crystals have been carefully characterized and studied by magnetic susceptibility, magnetization, specific heat, and thermal expansion. In addition, we used Cr-electron spin resonance and neutron diffraction to probe the Cr 3d3 magnetism microscopically. To obtain the electronic density of states, we employed x-ray absorption and resonant photoemission spectroscopy in combination with density functional theory calculations. Our studies evidence an anisotropic magnetic order below TN=32K. Susceptibility data in small fields of about 1 T reveal an antiferromagnetic (AFM) type of order for H⊥c, whereas for H∥c the data are reminiscent of a field-induced ferromagnetic (FM) structure. At low temperatures and for H⊥c, the field-dependent magnetization and AC susceptibility data evidence a metamagnetic transition at H+=5T, which is absent for H∥c. We assign this to a transition from a planar cycloidal spin structure at low fields to a planar fanlike arrangement above H+. A fully ferromagnetically polarized state is obtained above the saturation field of H⊥S=23.7T at 2 K with a magnetization of Ms=2.8μB/Cr. For H∥c, M(H) monotonically increases and saturates at the same Ms value at H∥S=25.1T at 4.2 K. Above TN, the magnetic susceptibility and specific heat indicate signatures of two dimensional (2D) frustration related to the presence of planar ferromagnetic and antiferromagnetic exchange interactions. We found a pronounced nearly isotropic maximum in both properties at about T∗=45K, which is a clear fingerprint of short range correlations and emergent spin fluctuations. Calculations based on a planar 2D Heisenberg model support our experimental findings and suggest a predominant FM exchange among nearest and AFM exchange among third-nearest neighbors. Only a minor contribution might be assigned to the antisymmetric Dzyaloshinskii-Moriya interaction possibly related to the noncentrosymmetric polar space group R3m. Due to these competing interactions, the magnetism in AgCrSe2, in contrast to the oxygen-based delafossites, can be tuned by relatively small, experimentally accessible magnetic fields, allowing us to establish the complete anisotropic magnetic H-T phase diagram in detail.

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.

Composite multiferroics containing ferroelectric and ferromagnetic components often have much larger magnetoelectric coupling compared to their single-phase counterparts. Doped or alloyed HfO2-based ferroelectrics may serve as a promising component in composite multiferroic structures potentially feasible for technological applications. Recently, a strong charge-mediated magnetoelectric coupling at the Ni/HfO2 interface has been predicted using density functional theory calculations. Here, we report on the experimental evidence of such magnetoelectric coupling at the Ni/Hf0.5Zr0.5O2(HZO) interface. Using a combination of operando XAS/XMCD and HAXPES/MCDAD techniques, we probe element-selectively the local magnetic properties at the Ni/HZO interface in functional Au/Co/Ni/HZO/W capacitors and demonstrate clear evidence of the ferroelectric polarization effect on the magnetic response of a nanometer-thick Ni marker layer. The observed magnetoelectric effect and the electronic band lineup of the Ni/HZO interface are interpreted based on the results of our theoretical modeling. It elucidates the critical role of an ultrathin NiO interlayer, which controls the sign of the magnetoelectric effect as well as provides a realistic band offset at the Ni/HZO interface, in agreement with the experiment. Our results hold promise for the use of ferroelectric HfO2-based composite multiferroics for the design of multifunctional devices compatible with modern semiconductor technology.

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.

Chirality and magnetism of molecules are two properties that in the last years raised notable interest for the development of novel molecular devices. Chiral helicenes combine these functionalities, and their nanostructuration is the first step toward developing new multifunctional devices. Here, we present a novel assembling strategy to deposit a sub‐monolayer of enantiopure thia[4]helicene radical cations on a pre‐functionalized Au(111) substrate permitting the persistence of both the paramagnetic character and chirality of these molecules at the nanoscale. In‐house characterizations demonstrated the retention of the chemical and paramagnetic properties after the deposition process. Furthermore, synchrotron‐based X‐ray natural circular dichroism confirmed that the handedness of the thia[4]helicene is preserved on the surface.

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.

We explored the properties of the quasi-binary Bi2Se3-Bi2S3 system over a wide compositional range. X-ray diffraction analysis demonstrates that rhombohedral crystals can be synthesized within the solid solution interval 0-22 mol% Bi2S3, while at 33 mol% Bi2S3 only orthorhombic crystals are obtained. Core level photoemission spectroscopy reveals the presence of Bi3+, Se2- and S2- species and the absence of metallic species, thus indicating that S incorporation into Bi2Se3 proceeds prevalently through the substitution of Se with S. Spin- and angle-resolved photoemission spectroscopy shows that topological surface states develop on the surfaces of the Bi2Se3-ySy (y <= 0.66) rhombohedral crystals, in close analogy with the prototypical case of Bi2Se3, while the orthorhombic crystals with higher S content turn out to be trivial semiconductors. Our results connect unambiguously the phase diagram and electronic properties of the Bi2Se3-Bi2S3 system.

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.

The study of ionic materials on nanometer scale is of great relevance for efficient miniaturized devices for energy applications. The epitaxial growth of thin films can be a valid route to tune the properties of the materials and thus obtain new degrees of freedom in materials design. High crystal quality SmxCe1-xO2-δ films are here reported at high doping level up to x=0.4, thanks to the good lat-tice matching with the (110) oriented NdGaO3 substrate. X-ray diffraction and transmission electron microscopy demonstrate the ordered structural quality and absence of Sm segregation at macroscopic and atomic level, respectively. Therefore, in epitaxial thin films the homogeneous doping can be obtained even with high dopant content not always approachable in bulk form, getting even an improvement of the structural properties. In situ spectroscopic measurements by x-ray photoemission and x-ray absorption show the O 2p band shift towards the Fermi level which can favor the oxygen exchange and vacancy formation on the surface when the Sm doping is increased to x=0.4. X-ray absorption spectroscopy also confirms the absence of ordered oxygen vacancy clusters and further reveals that the 5d eg and t2g states are well separated by the crystal field in the undistorted local structure even in the case of high doping level x=0.4.

We study the 2×2 charge density wave (CDW) in epitaxially-grown monolayer TiSe2. Our temperature-dependent angle-resolved photoemission spectroscopy measurements indicate a strong-coupling instability, but reveal how not all states couple equally to the symmetry-breaking distortion, with an electron pocket persisting to low temperature as a non-bonding state. We further show how the CDW order can be suppressed by a modest doping of around 0.06(2) electrons per Ti. Our results provide an opportunity for quantitative comparison with a realistic tight-binding model, which emphasises a crucial role of structural aspects of the phase transition in understanding the hybridisation in the ground state. Together, our work provides a comprehensive understanding of the phenomenology of the CDW in TiSe2 in the 2D limit.

Two-dimensional (2D) alloys represent a versatile platform that extends the properties of atomically thin transition-metal dichalcogenides. Here, using molecular beam epitaxy, we investigate the growth of 2D vanadium-molybdenum diselenide alloys, VxMo1–xSe2, on highly oriented pyrolytic graphite and unveil their structural, chemical, and electronic integrities via measurements by scanning tunneling microscopy/spectroscopy, synchrotron X-ray photoemission, and X-ray absorption spectroscopy (XAS). Essentially, we found a critical value of x = ∼0.44, below which phase separation occurs and above which a homogeneous metallic phase is favored. Another observation is an effective increase in the density of mirror twin boundaries of constituting MoSe2 in the low V concentration regime (x ≤ 0.05). Density functional theory calculations support our experimental results on the thermal stability of 2D VxMo1–xSe2 alloys and suggest an H phase of the homogeneous alloys with alternating parallel V and Mo strips randomly in-plane stacked. Element-specific XAS of the 2D alloys, which clearly indicates quenched atomic multiplets similar to the case of 2H-VSe2, provides strong evidence for the H phase of the 2D alloys. This work provides a comprehensive understanding of the thermal stability, chemical state, and electronic structure of 2D VxMo1–xSe2 alloys, useful for the future design of 2D electronic devices.

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.

The local atomic structure and the magnetic response of Co films intercalated between Graphene and Ir(111) were investigated combining polarized X-ray Absorption Spectroscopy at the Co K edge with Magneto-Optic Kerr Effect. The structural and magnetic evolution upon a 500 °C annealing was evaluated as a function of the film thickness. After the thermal treatment, our thick film (10 monolayers) presented a lower perpendicular magnetic anisotropy (PMA) as well as a reduced average structural disorder. On the other hand, in our thin film (5 monolayers), the annealing enhanced the perpendicular magnetic response and induced a local anisotropy by stretching the Co-Co bonds in the film plane and compressing those outside the plane. Our finding emphasizes the close relationship between the local structure of Co within the film and its magnetic properties.

We investigate the temperature-dependent electronic structure of the van der Waals ferromagnet, CrGeTe3. Using angle-resolved photoemission spectroscopy, we identify atomic- and orbital-specific band shifts upon cooling through TC. From these, together with x-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements, we identify the states created by a covalent bond between the Te 5p and the Cr eg orbitals as the primary driver of the ferromagnetic ordering in this system, while it is the Cr t2g states that carry the majority of the spin moment. The t2g states furthermore exhibit a marked bandwidth increase and a remarkable lifetime enhancement upon entering the ordered phase, pointing to a delicate interplay between localized and itinerant states in this family of layered ferromagnets.

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.

Magnetism in monolayer (ML) VSe2 has attracted broad interest in spintronics, while existing reports have not reached consensus. Using element-specific X-ray magnetic circular dichroism, a magnetic transition in ML VSe2 has been demonstrated at the contamination-free interface between Co and VSe2. Through interfacial hybridization with a Co atomic overlayer, a magnetic moment of about 0.4 μB per V atom in ML VSe2 is revealed, approaching values predicted by previous theoretical calculations. Promotion of the ferromagnetism in ML VSe2 is accompanied by its antiferromagnetic coupling to Co and a reduction in the spin moment of Co. In comparison to the absence of this interface-induced ferromagnetism at the Fe/ML MoSe2 interface, these findings at the Co/ML VSe2 interface provide clear proof that the ML VSe2, initially with magnetic disorder, is on the verge of magnetic transition.

Monolayer VSe2, featuring both charge density wave and magnetism phenomena, represents a unique van der Waals magnet in the family of metallic 2D transition‐metal dichalcogenides (2D‐TMDs). Herein, by means of in situ microscopy and spectroscopic techniques, including scanning tunneling microscopy/spectroscopy, synchrotron X‐ray and angle‐resolved photoemission, and X‐ray absorption, direct spectroscopic signatures are established, that identify the metallic 1T‐phase and vanadium 3d1 electronic configuration in monolayer VSe2 grown on graphite by molecular‐beam epitaxy. Element‐specific X‐ray magnetic circular dichroism, complemented with magnetic susceptibility measurements, further reveals monolayer VSe2 as a frustrated magnet, with its spins exhibiting subtle correlations, albeit in the absence of a long‐range magnetic order down to 2 K and up to a 7 T magnetic field. This observation is attributed to the relative stability of the ferromagnetic and antiferromagnetic ground states, arising from its atomic‐scale structural features, such as rotational disorders and edges. The results of this study extend the current understanding of metallic 2D‐TMDs in the search for exotic low‐dimensional quantum phenomena, and stimulate further theoretical and experimental studies on van der Waals monolayer magnets.

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.

This thesis contains a selection of the results on the shallow electron states of quantum materials that I obtained as doctoral student of the Scuola di Dottorato in Fisica, Astrofisica e Fisica Applicata at the Università degli Studi di Milano. I carried out my doctoral research activity mostly at the TASC-IOM CNR laboratory, in the framework of the NFFA and APE-beamline facilities (Elettra Sincrotrone Trieste), as well in dedicated sessions at the I2; beamline of the Diamond light source, Harwell Campus, UK. To access the electronic properties of materials I specialised myself in photoemission spectroscopy techniques. High quality samples are a prerequisite for any attempt to study quantum materials so that a major effort in my PhD project has been to master the growth of novel quantum materials by means of Pulsed Laser Deposition (PLD). Given that the PLD is integrated in the suite of UHV facilities attached in-situ to the APE beamline, I directly characterised the electronic properties of the PLD grown samples exploiting both the spectroscopic techniques available at the beamline (ARPES, X-ray photoemission and absorption spectroscopies: XPS and XAS), either ex-situ structural characterisation tools (X-ray diffraction –XRD– and X-ray reflectivity, XRR).

Le proprietà ottiche, elettroniche e magnetiche dei solidi e delle loro superfici dipendono dalla struttura degli stati elettronici entro alcuni eV dal livello di Fermi. I calcoli della struttura elettronica a bande sono efficaci solo nel caso di materiali a bassa interazione elettrone-elettrone (correlazione). L'esperimento e la guida necessaria per lo studio delle proprietà elettroniche dei solidi e delle loro superfici, ed in particolare la spettroscopia di fotoemissione (photoemission spectroscopy - PES) che si basa sulla misura dello spettro energetico degli elettroni emessi da un solido eccitato da un fascio di fotoni monocromatici di energia eccedente la funzione lavoro. La risoluzione dell'angolo di emissione (Angle-resolved photemission spectroscopy - ARPES) permette di avere informazioni sulla legge di dispersione E_n(k) dello stato elettronico iniziale, mentre la misura del grado di polarizzazione in spin del fascio di elettroni completa il set di numeri quantici, fornendo un dato molto importante per lo studio delle correlazioni elettroniche.

In this paper, we present the first publicly available human-annotated dataset of images obtained by the Scanning Electron Microscopy (SEM). A total of roughly 22,000 SEM images at the nanoscale are classified into 10 categories to form 4 labeled training sets, suited for image recognition tasks. The selected categories span the range of 0D objects such as particles, 1D nanowires and fibres, 2D films and coated surfaces as well as patterned surfaces, and 3D structures such as microelectromechanical system (MEMS) devices and pillars. Additional categories such as tips and biological are also included to expand the spectrum of possible images. A preliminary degree of hierarchy is introduced, by creating a subtree structure for the categories and populating them with the available images, wherever possible.

In order to enable the use of the prototypical 2D‐layered MoS2 for spintronics, its integration with ferromagnetic layers is mandatory. By employing interface‐sensitive 57Fe conversion electron Mössbauer spectroscopy (CEMS), hard X‐ray photoelectron spectroscopy (HAXPES), and transmission electron microscopy (TEM), the chemical, structural, and magnetic properties of the Fe/2D‐MoS2 interface are investigated. CEMS shows that out of the first 1 nm of Fe in direct contact with 2D‐MoS2, about half of the Fe atoms keeps the un‐perturbed Fe local environment, partly in regions where the original 2D‐layered structure of MoS2 is preserved as shown by TEM. The remaining reacting Fe atoms exclusively bond with Mo, with the majority of them being characterized by a ferromagnetic environment and the rest coordinating in a paramagnetic Fe‐Mo configuration. The preferential Fe bonding with Mo is corroborated by HAXPES analysis. The results provide detailed insight into the link between the bonding configuration and the interfacial magnetism at the Fe/2D‐MoS2 heterojunction.

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.

The superconducting properties of Sr1–xLaxCuO2 thin films are strongly affected by sample preparation procedures, including the annealing step, which are not always well controlled. We have studied the evolution of Cu L2,3 and O K edge x-ray absorption spectra (XAS) of Sr1–xLaxCuO2 thin films as a function of reducing annealing, both qualitatively and quantitatively. By using linearly polarized radiation, we are able to identify the signatures of the presence of apical oxygen in the as-grown sample and its gradual removal as a function of duration of 350 °C Ar annealing performed on the same sample. Even though the as-grown sample appears to be hole doped, we cannot identify the signature of the Zhang-Rice singlet in the O K XAS, and it is extremely unlikely that the interstitial excess oxygen can give rise to a superconducting or even a metallic ground state. XAS and x-ray linear dichroism analyses are, therefore, shown to be valuable tools to improving the control over the annealing process of electron doped superconductors.

Interfaces play a crucial role in the study of novel phenomena emerging at heterostructures comprising metals and functional oxides. For this reason, attention should be paid to the interface chemistry, which can favor the interdiffusion of atomic species and, under certain conditions, lead to the formation of radically different compounds with respect to the original constituents. In this work, we consider Cr/
BaTiO3 heterostructures grown on SrTiO3 (100) substrates. Chromium thin films (1–2 nm thickness) are deposited by molecular beam epitaxy on the
BaTiO3 layer, and subsequently annealed in vacuum at temperatures ranging from 473 to 773 K. A disordered metallic layer is detected for annealing temperatures up to 573 K, whereas, at higher temperatures, we observe a progressive oxidation of chromium, which we relate to the thermally activated migration of oxygen from the substrate. The chromium oxidation state is +3 and the film shows a defective rocksalt structure, which grows lattice matched on the underlying BaTiO3 layer. One out of every three atoms of chromium is missing, producing an uncommon tetragonal phase with Cr2O3 stoichiometry. Despite the structural difference with respect to the ordinary corundum α-Cr2O3 phase, we demonstrate both experimentally and theoretically that the electronic properties of the two phases are, to a large extent, equivalent.

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.

Interfaces between organic semiconductors and ferromagnetic metals offer intriguing opportunities in the rapidly developing field of organic spintronics. Understanding and controlling the spin-polarized electronic states at the interface is the key toward a reliable exploitation of this kind of systems. Here we propose an approach consisting in the insertion of a two-dimensional magnetic oxide layer at the interface with the aim of both increasing the reproducibility of the interface preparation and offering a way for a further fine control over the electronic and magnetic properties. We have inserted a two-dimensional Cr4O5 layer at the C60/Fe(001) interface and have characterized the corresponding morphological, electronic, and magnetic properties. Scanning tunneling microscopy and electron diffraction show that the film grows well-ordered both in the monolayer and multilayer regimes. Electron spectroscopies confirm that hybridization of the electronic states occurs at the interface. Finally, magnetic dichroism in X-ray absorption shows an unprecedented spin-polarization of the hybridized fullerene states. The latter result is discussed also in light of an ab initio theoretical analysis.

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.

The role of trivalent rare-earth dopants on the cerium oxidation state has been systematically studied by in situ photoemission spectroscopy with synchrotron radiation for 10 mol % rare-earth doped epitaxial ceria films. It was found that dopant rare-earths with smaller ionic radius foster the formation of Ce3+ by releasing the stress strength induced by the cation substitution. With a decrease of the dopant ionic radius from La3+ to Yb3+, the out-of-plane axis parameter of the crystal lattice decreases without introducing macroscopic defects. The high crystal quality of our films allowed us to comparatively study both the ionic conductivity and surface reactivity ruling out the influence of structural defects. The measured increase in the activation energy of films and their enhanced surface reactivity can be explained in terms of the dopant ionic radius effects on the Ce4+ → Ce3+ reduction as a result of lattice relaxation. Such findings open new perspectives in designing ceria-based materials with tailored properties by choosing suitable cation substitution.

TiO2 is commonly used as the active switching layer in resistive random access memory. The electrical characteristics of these devices are directly related to the fundamental conditions inside the TiO2 layer and at the interfaces between it and the surrounding electrodes. However, it is complex to disentangle the effects of film “bulk” properties and interface phenomena. The present work uses hard X-ray photoemission spectroscopy (HAXPES) at different excitation energies to distinguish between these regimes. Changes are found to affect the entire thin film, but the most dramatic effects are confined to an interface. These changes are connected to oxygen ions moving and redistributing within the film. Based on the HAXPES results, post-deposition annealing of the TiO2 thin film was investigated as an optimisation pathway in order to reach an ideal compromise between device resistivity and lifetime. The structural and chemical changes upon annealing are investigated using X-ray absorption spectroscopy and are further supported by a range of bulk and surface sensitive characterisation methods. In summary, it is shown that the management of oxygen content and interface quality is intrinsically important to device behavior and that careful annealing procedures are a powerful device optimisation technique.

We investigated the influence of surfaces in the formation of different crystal structures of a spin crossover compound, namely [Fe(L)2] (LH: (2-(pyrazol-1-yl)-6-(1H-tetrazol-5-yl)pyridine), which is a neutral compound thermally switchable around room temperature. We observed that the surface induces the formation of two different crystal structures, which exhibit opposite spin transitions, i.e. on heating them up to the transition temperature, one polymorph switches from high spin to low spin and the second polymorph switches irreversibly from low spin to high spin. We attributed this inversion to the presence of water molecules H-bonded to the complex tetrazolyl moieties in the crystals. Thin deposits were investigated by means of polarized optical microscopy, atomic force microscopy, X-ray diffraction, X-ray absorption spectroscopy and micro Raman spectroscopy; moreover the analysis of the Raman spectra and the interpretation of spin inversion were supported by DFT calculations.

Spin-crossover metal complexes are highly promising magnetic molecular switches for prospective molecule-based devices. The spin-crossover molecular photoswitches developed so far operate either at very low temperatures or in the liquid phase, which hinders practical applications. Herein, we present a molecular spin-crossover iron(II) complex that can be switched between paramagnetic high-spin and diamagnetic low-spin states with light at room temperature in the solid state. The reversible photoswitching is induced by alternating irradiation with ultraviolet and visible light and proceeds at the molecular level.