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 doping of metal oxides is an interesting route to increase catalyst activity and lower activation temperatures in H2 dissociation to replace Pt in catalysts for electrochemical devices. In this process, the roles of both the matrix and dopant cations are fundamental to understanding and designing more efficient catalysts. In this work, we have investigated the reduction process in pure and doped CeO2 films. We followed the oxidation states of Ce and dopants (Cu and Fe) during H2 exposure at ambient pressure by combining X-ray absorption spectroscopy and gas chromatography on 5 nm films in the temperature range of 300–620 K. We have observed that Cu doping (at concentrations of 5 and 14 at. %) promotes the ceria reduction, while the addition of Fe seems to have a limited impact on the oxide chemical reactivity only at low temperatures. Moreover, thanks to the chemical sensitivity of operando X-ray absorption spectroscopy, we were able to follow simultaneously the evolution of Ce and Cu oxidation states during the reaction, which has permitted to identify two distinct reduction processes taking place above and below 500 K. These measurements show that at low temperatures, the H2 dissociation takes place at the Cu1+ sites, thus explaining the higher reactivity of the Cu-doped samples. The described mechanism can help in the design of Pt-free catalysts with enhanced performances.
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.
This booklet collects the results of my work as a doctoral student of the Ph.D. School in Physics, Astrophysics and Applied Physics at Universit`a degli Studi di Milano, that has been carried out since November 2020 at Istituto Officina dei Materiali of Consiglio Nazionale delle Ricerche (IOM-CNR) and within the framework of Nanoscale Foundries and Fine Analysis (NFFA) consortium.
My experimental activity addressed the coupling of magnetic and acoustic degrees of freedom in transition-metal ferromagnetic systems. Within the NFFA-SPRINT laboratory, hosted in the premises of the facility FERMI@Elettra (Elettra-Sincrotrone Trieste), I developed a setup to perform optical Transient-Grating spectroscopy, and correlative time-resolved optical spectroscopies (time-resolved reflectivity and polarimetry). Via sub-picosecond optical pulses, acoustic and magnetic transients are impulsively generated: their intertwined evolution and decay are monitored via time-resolved optical probing.
In a first experiment, a Ni thin film was investigated via Transient-Grating spectroscopy. Acoustically-driven magnetization precession was observed at the condition of crossing of phononic and magnonic dispersions, at finite wavevector. With the aid of correlative ferromagnetic resonance measurements the boundary of applicability of the proposed experimental approach was established.
In a second experiment, time-resolved magneto-optical polarimetry was employed to investigate magneto-acoustic waves excited in a ferromagnetic nanostructured array. The details of the magnon-phonon mode crossing allowed to identify experimental features which qualify the degree of coherence in the coupling; a Hamiltonian model was proposed to account for the observations.
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.
Mn3Si2Te6 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 Mn3Si2Te6. 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 Mn3Si2Te6, 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 Mn3Si2Te6.
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.
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.
Given the urgency of achieving the forthcoming zero emission targets, the research of green fuels and efficient catalysts able to easily convert them in other valuable compounds is fundamental. The work presented in this thesis is focused on the application of an innovative spectroscopic technique, the operando Soft X-Rays NEXAFS spectroscopy, in order to investigate the surface reactivity of heterogeneous catalysts. In fact, it is well known the importance that operando characterizations have acquired in recent years, allowing to study a material at its working conditions. Since the technique requires the use of Synchrotron Radiation and a specific experimental setup, all the measurements reported in this thesis have been performed exploiting a home made reaction cell developed at the APE-HE beamline, at Elettra Synchrotron (Trieste). In this thesis work, we investigated the possibility of coupling the operando NEXAFS technique with other in situ spectroscopies, together with standard ex situ characterizations and computational simulations. This multitechnique approach allowed to extract the maximum potential of the technique, addressing its role as a key tool in the optic of speeding up the design of efficient heterogeneous catalysts.
The catalytic reactions investigated in this thesis are focused on methanol valorization, given its great potential in numerous applications related to the energy transition. In detail, we focused our first investigation on methanol production through the direct partial oxidation of methane, catalysed by a CeO2/CuO composite synthesized using a scalable and green milling process. We exploited the combination of in situ DRIFT and operando Soft X-Ray NEXAFS spectroscopies to monitor at the same time the electronic structure modifications occurring at the catalyst surface and the adsorbates evolution during the different reaction steps.
The operando analysis of the Cu L2,3 and Ce M4,5 edges during the catalyst thermal activation allowed us to detect a charge transfer from Ce3+ surface sites to Cu2+ atoms, resulting in the formation of reactive sites close to the CeO2/CuO interface. When the sample was exposed to CH4 at 250°C and at a pressure of 1 bar, a Cu2+ → Cu+ reduction was observed, indicating that the catalyst is able to activate the methane molecule. At the same time, DRIFT spectra shown the formation of methoxy and formate species, that are products of methane activation on the surface. Adding an oxidizing agent (O2), Cu+ sites were re-oxidized to Cu2+, together with the disappearing of the methoxy and formate related structures in the DRIFT spectra. The results indicated the reversibility of the chemical modifications occurring at the catalyst surface. During the operando NEXAFS experiment, the reaction products were monitored with an online micro-GC: the main products observed during the reaction were CO2, H2O, CH2O and CH3OH, indicating that total and partial oxidation of methane were occurring. As a comparison, an equivalent experiment has been conducted on a similar CeO2/CuO catalyst synthetized with a conventional impregnation method. In this case, no spectroscopic modification were observed with both NEXAFS and DRIFT techniques, confirming that the synthetic method used is crucial in creating specific active sites for methane activation and oxidation. The experimental results have been validated through DFT calculations, which confirmed that when CuO and CeO2 surfaces merge during the synthesis, a net charge transfer from Ce to Cu atoms occurs in proximity of the CeO2 − CuO interface. Another promising route to valorize methanol is represented by its catalytic decomposition to syngas mixture (H2 + CO), whose reaction mechanism was investigated in the second part of the thesis. Indeed, one major challenge for this reaction is related to the quest for stable, cost-effective, and selective catalysts operating below 400 °C. In the present study, we illustrate a surface reactivity study of a Ni3Sn4 catalyst working at 250 °C, by combining density functional theory (DFT), operando NEXAFS at ambient pressure, in situ XPS and high-resolution transmission electron microscopy (HR-TEM). For Ni3Sn4, we discovered that the catalytic reaction is driven by surface tin-oxide phases, able to protect the underlying Ni atoms from irreversible chemical modifications, increasing the catalyst durability. Moreover, exploiting the online micro-GC connected to the operando NEXAFS reaction cell and by comparing the results with a Ni3Sn2 compound, we found that Sn content plays a key role in enhancing the H2 selectivity, with respect to secondary products such as CO2. These findings open new perspectives for the engineering of scalable and low-cost catalysts for hydrogen production.
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.
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.
We grew Sr1-xLaxCuO2 thin films and SrCuO2/Sr0.9La0.1CuO2/SrCuO2 trilayers by reflection high-energy diffraction-calibrated layer-by-layer molecular beam epitaxy, to study their electrical transport properties as a function of the doping and thickness of the central Sr0.9La0.1CuO2 layer. For the trilayer samples, as already observed in underdoped SLCO films, the electrical resistivity versus temperature curves as a function of the central layer thickness show, for thicknesses thinner than 20 unit cells, sudden upturns in the low temperature range with the possibility for identifying, in the normal state, the T* and a T** temperatures, respectively, separating high-temperature linear behavior and low-temperature quadratic dependence. By plotting the T* and T** values as a function of TConset for both the thin films and the trilayers, the data fall on the same curves. This result suggests that, for the investigated trilayers, the superconducting critical temperature is the important parameter able to describe the normal state properties and that, in the limit of very thin central layers, such properties are mainly influenced by the modification of the energy band structure and not by interface-related disorder.
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.
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.
Perovskite-based heterostructures have recently gained remarkable interest, thanks to atomic-scale precision engineering. These systems are very susceptible to small variations of control parameters, such as two-dimensionality, strain, lattice polarizability, and doping. Focusing on the rare-earth nickelate diagram, LaNiO3 (LNO) catches the eye, being the only nickelate that does not undergo a metal-to-insulator transition (MIT). Therefore, the ground state of LNO has been studied in several theoretical and experimental papers. Here, we show by means of infrared spectroscopy that an MIT can be driven by dimensionality control in ultrathin LNO films when the number of unit cells drops to 2. Such a dimensionality tuning can eventually be tailored when a physically implemented monolayer in the ultrathin films is replaced by a digital single layer embedded in the Ruddlesden–Popper Lan+1NinO3n+1 series. We provide spectroscopic evidence that the dimensionality-induced MIT in Ruddlesden–Popper nickelates strongly resembles that of ultrathin LNO films. Our results can pave the way to the employment of Ruddlesden–Popper Lan+1NinO3n+1 to tune the electronic properties of LNO through dimensional transition without the need of physically changing the number of unit cells in thin films.
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.
Out-of-plane Ga2Se3 nanowires are grown by molecular beam epitaxy via Au-assisted heterovalent exchange reaction on GaAs substrates in the absence of Ga deposition. It is shown that at a suitable temperature around 560 degrees C the Audecorated GaAs substrate releases Ga atoms, which react with the incoming Se and feed the nanowire growth. The nanowire composition, crystal structure, and morphology are characterized by Raman spectroscopy and electron microscopy. The growth mechanism is investigated by X-ray photoelectron spectroscopy. We explore the growth parameter window and find an interesting effect of shortening of the nanowires after a certain maximum length. The nanowire growth is described within a diffusion transport model, which explains the nonmonotonic behavior of the nanowire length versus the growth parameters. Nanowire shortening is explained by the blocking of Ga supply from the GaAs substrate by thick, in-plane worm-like Ga2Se3 structures, which grow concomitantly with the nanowires, followed by backward diffusion of Ga atoms from the nanowires down to the substrate surface.
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 present thesis work has been performed within a new-born laboratory called Spin Polar-ization Research Instrument in the Nanoscale and Time domain (SPRINT laboratory), as apart of the research infrastructures circuit NFFA-Trieste (Nano Foundries and Fine Analysis -belonging to the wider NFFA-Europe circuit) and hosted in the experimental hall of the freeelectron laser FERMI@Elettra.The SPRINT laboratory rises as an answer to the urgent request of the scientific communityof extension of photoemission spectroscopies (PES), not only energy-, but possibly also angle-and spin-resolved, to the time domain in the sub-picosecond regime. The integration of a PESapparatus within a setup for stroboscopic measurements (that is in a pump-probe scheme) pavesthe way to time resolved study of the relaxation of optically populated electronic states, thusenabling the study the ultrafast dynamics of the excitations inside the materials, with greatbenefit from both the fundamental and the technological point of view.
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.
Transparent conductive oxides are a class of materials that combine high optical transparency with high electrical conductivity. This property makes them uniquely appealing as transparent conductive electrodes in solar cells and interesting for optoelectronic and infrared-plasmonic applications. One of the new challenges that researchers and engineers are facing is merging optical and electrical control in a single device for developing next-generation photovoltaic, optoelectronic devices and energy-efficient solid-state lighting. In this work, the authors investigated the possible variations in the dielectric properties of aluminum-doped ZnO (AZO) upon gating by means of spectroscopic ellipsometry (SE). The authors investigated the electrical-bias-dependent optical response of thin AZO films fabricated by magnetron sputtering within a parallel-plane capacitor configuration. The authors address the possibility to control their optical and electric performances by applying bias, monitoring the effect of charge injection/depletion in the AZO layer by means of in operando SE versus applied gate voltage.
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.
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 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.
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.
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.
Samaria-doped ceria (SDC) thin films are particularly important for energy and electronic applications such as microsolid oxide fuel cells, electrolyzers, sensors, and memristors. In this paper, we report a comparative study investigating ionic conductivity and surface reactions for well-grown epitaxial SDC films varying the samaria doping concentration. With increasing doping above 20 mol % of samaria, an enhancement in the defect association is observed by Raman spectroscopy. The role of such associated defects on the films̀ oxygen ion transport and exchange is investigated by electrochemical impedance spectroscopy and electrochemical strain microscopy (ESM). The measurements reveal that the ionic transport has a sharp maximum in ionic conductivity and drops in its activation energy down to 0.6 eV for 20 mol % doping. Increasing the doping concentration further up to 40 mol %, it raises the activation energy substantially by a factor of 2. We ascribe the sluggish transport kinetics to the “bulk” ionic-near ordering in case of the heavily doped epitaxial films. Analysis of the ESM first-order reversal curve measurements indicates that these associated defects may have a beneficial role by lowering the activation of the oxygen exchange “surface” reaction for heavily doped 40 mol % of samaria. In a model experiment, through a solid solution series of samaria doped ceria epitaxial films, we reveal that the occurrence of associated defects in the bulk affects the surface charging state of the SDC films to increase the exchange rates. The implication of these findings is the design of coatings with tuned oxygen surface exchange by controlling the bulk associated clusters for future electrocatalytic applications.
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.