4Hb-TaS2 is a superconductor that exhibits unique characteristics such as time-reversal symmetry breaking, hidden magnetic memory, and topological edge modes. It is a naturally occurring heterostructure comprising of alternating layers of 1H-TaS2 and 1T-TaS2. The former is a well-known superconductor, while the latter is a correlated insulator with a possible non- trivial magnetic ground state. In this study, we use angle resolved photoemission spectroscopy to investigate the normal state electronic structure of this unconventional superconductor. Our findings reveal that the band structure of 4Hb-TaS2 fundamentally differs from that of its constituent materials. Specifically, we observe a significant charge transfer from the 1T layers to the 1H layers that drives the 1T layers away from half-filling. In addition, we find a substantial reduction in inter-layer coupling in 4Hb-TaS2 compared to the coupling in 2H-TaS2 that results in a pronounced spin-valley locking within 4Hb-TaS2.
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 interplay of spin–orbit coupling and crystal symmetry can generate spin-polarized bands in materials only a few atomic layers thick, potentially leading to unprecedented physical properties. In the case of bilayer materials with global inversion symmetry, locally broken inversion symmetry can generate degenerate spin-polarized bands, in which the spins in each layer are oppositely polarized. Here, we demonstrate that the hidden spins in a Tl bilayer crystal are revealed by growing it on Ag(111) of sizable lattice mismatch, together with the appearance of a remarkable phenomenon unique to centrosymmetric hidden-spin bilayer crystals: a novel band splitting in both spin and space. The key to success in observing this novel splitting is that the interaction at the interface has just the right strength: it does not destroy the original wave functions of the Tl bilayer but is strong enough to induce an energy separation.
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.
Topological insulators are bulk insulators with metallic and fully spin-polarized surface states displaying Dirac-like band dispersion. Due to spin-momentum locking, these topological surface states (TSSs) have a predominant in-plane spin polarization in the bulk fundamental gap. Here, we show by spin-resolved photoemission spectroscopy that the TSS of a topological insulator interfaced with an antimonene bilayer exhibits nearly full out-of-plane spin polarization within the substrate gap. We connect this phenomenon to a symmetry-protected band crossing of the spin-polarized surface states. The nearly full out-of-plane spin polarization of the TSS occurs along a continuous path in the energy–momentum space, and the spin polarization within the gap can be reversibly tuned from nearly full out-of-plane to nearly full in-plane by electron doping. These findings pave the way to advanced spintronics applications that exploit the giant out-of-plane spin polarization of TSSs.
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.
Transition metal dichalcogenides exhibit many fascinating properties including superconductivity, magnetic orders, and charge density wave. The combination of these features with a non-trivial band topology opens the possibility of additional exotic states such as Majorana fermions and quantum anomalous Hall effect. Here, we report on photon-energy and polarization dependent spin-resolved angle-resolved photoemission spectroscopy experiments on single crystal 1T-VSe2, revealing an unexpected band inversion and emergent Dirac nodal arc with spin-momentum locking. Density functional theory calculations suggest a surface lattice strain could be the driving mechanism for the topologically nontrivial electronic structure of 1T-VSe2.
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.
We present CoTe2 as a type-II Dirac semimetal supporting Lorentz-symmetry violating Dirac fermions in the vicinity of the Fermi energy. By combining first-principles ab initio calculations with experimental angle-resolved photoemission spectroscopy results, we show CoTe2 hosts a pair of type-II Dirac fermions around 90 meV above the Fermi energy. In addition to the bulk Dirac fermions, we find several topological band inversions in bulk CoTe2, which gives rise to a ladder of spin-polarized surface states over a wide range of energies. In contrast to the surface states which typically display Rashba-type in-plane spin splitting, we find that CoTe2 hosts interesting out-of-plane spin polarization as well. Our work establishes CoTe2 as a potential candidate for the exploration of Dirac fermiology and applications in spintronic devices, infrared plasmonics, and ultrafast optoelectronics.
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.
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.
Here, we discuss the key features of electrocatalysis with mitrofanovite (Pt3Te4), a recently discovered mineral with superb performances in hydrogen evolution reaction. Mitrofanovite is a layered topological metal with spin-polarized topological surface states with potential applications for spintronics. However, mitrofanovite is also an exceptional platform for electrocatalysis, with costs of the electrodes suppressed by 47% owing to the partial replacement of Pt with Te. Remarkably, the Tafel slope in nanostructured mitrofanovite is just 33 mV/dec, while reduced mitrofanovite has the same Tafel slope (36 mV/dec) as state-of-the-art electrodes of pure Pt. Mitrofanovite also affords surface stability and robustness to CO poisoning. Accordingly, these findings pave the way for the advent of mitrofanovite for large-scale hydrogen production.
We report on a two-dimensional (2D) V1–xPtxSe2 alloy that exhibits ferromagnetic order and Rashba spin–orbit coupling. Although ferromagnetism is absent in 1T-VSe2 because of the competition with the charge density wave phase, we demonstrate theoretically and experimentally that the substitution of vanadium by platinum in VSe2 (10–50%) to form a homogeneous 2D alloy restores ferromagnetic order down to one monolayer of V0.65Pt0.35Se2. Moreover, the presence of platinum atoms gives rise to Rashba spin–orbit coupling in (V,Pt)Se2, providing an original platform to study the interplay between ferromagnetism and spin–orbit coupling in the 2D limit.
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.
TaSe3 is a layered van der Waals semimetal with several inverted band gaps throughout the entire Brillouin zone and nontrivial Z2 topological indices, which place it at the boundary between a strong and a weak topological phase. Our transport experiments reveal a quadratic nonsaturating magnetoresistance (MR) with values reaching 104% at 1.8 K and 14 T, whose origins have to be searched in the material's band structure. Here we combine angle-resolved photoelectron spectroscopy experiments, also with spin resolution, with ab initio calculations based on density functional theory in order to draw a connection between the Fermi surface topology and the measured transport properties. Simulations based on the calculated Fermi surface clarify that electron-hole compensation plays an important role for the observed MR in the bulk material. At the surface, the position of Fermi level differs, and it can be controlled by alkali metal deposition which accounts not only for the energy shift of the bands but it slightly modifies the dispersion of the valence and conduction bands. We propose that the observed band-gap renormalization might offer a route for engineering the topological phase in TaSe3, alternative to strain.
Topological materials are a promising platform for a wide range of next-generation technologies. In article number 2100063, Antonio Politano, Salvador Barraza-Lopez, Jin Hu and co-workers report a new topological material, SmSbTe, displaying a coexistence of magnetism, enhanced electronic correlations, and Dirac fermions, as illustrated in the cover image. This discovery suggests that SmSbTe represents an ideal platform for exotic quantum phenomena arising from the interplay between degrees of freedom. The manipulation of these phenomena would further pave a path for quantum material-based functional devices.
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.
By means of electrocatalytic tests, surface-science techniques and density functional theory, we unveil the physicochemical mechanisms ruling the electrocatalytic activity of recently discovered mitrofanovite (Pt3Te4) mineral. Mitrofanovite represents a very promising electrocatalyst candidate for energy-related applications, with a reduction of costs by 47% compared to pure Pt and superior robustness to CO poisoning. We show that Pt3Te4 is a weak topological metal with the Z2 invariant, exhibiting electrical conductivity (∼4 × 106 S/m) comparable with pure Pt. In hydrogen evolution reaction (HER), the electrode based on bulk Pt3Te4 shows a very small overpotential of 46 mV at 10 mA cm–2 and a Tafel slope of 36–49 mV dec–1 associated with the Volmer–Heyrovsky mechanism. The outstanding ambient stability of Pt3Te4 also provides durability of the electrode and long-term stability of its efficient catalytic performances.
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.
Dirac fermions play a central role in the study of topological phases, for they can generate a variety of exotic states, such as Weyl semimetals and topological insulators. The control and manipulation of Dirac fermions constitute a fundamental step toward the realization of novel concepts of electronic devices and quantum computation. By means of Angle-Resolved Photo-Emission Spectroscopy (ARPES) experiments and ab initio simulations, here, we show that Dirac states can be effectively tuned by doping a transition metal sulfide, BaNiS2, through Co/Ni substitution. The symmetry and chemical characteristics of this material, combined with the modification of the charge-transfer gap of BaCo1−xNixS2 across its phase diagram, lead to the formation of Dirac lines, whose position in k-space can be displaced along the Γ−M symmetry direction and their form reshaped. Not only does the doping x tailor the location and shape of the Dirac bands, but it also controls the metal-insulator transition in the same compound, making BaCo1−xNixS2 a model system to functionalize Dirac materials by varying the strength of electron correlations.
The ZrSiS family of compounds hosts various exotic quantum phenomena due to the presence of both topological nonsymmorphic Dirac fermions and nodal-line fermions. In this material family, the LnSbTe (Ln = lanthanide) compounds are particularly interesting owing to the intrinsic magnetism from magnetic Ln which leads to new properties and quantum states. In this work, the authors focus on the previously unexplored compound SmSbTe. The studies reveal a rare combination of a few functional properties in this material, including antiferromagnetism with possible magnetic frustration, electron correlation enhancement, and Dirac nodal-line fermions. These properties enable SmSbTe as a unique platform to explore exotic quantum phenomena and advanced functionalities arising from the interplay between magnetism, topology, and electronic correlations.
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.
Dirac semimetals are classified into different phases based on the types of Dirac fermions. Tuning the transition among different types of Dirac fermions in one system remains a challenge. Recently, KMgBi was predicted to be located at a critical state in which various types of Dirac fermions can be induced owing to the existence of a flatband. Here, we carried out systematic studies on the electronic structure of KMgBi single crystals by combining angle-resolve photoemission spectroscopy and scanning tunneling microscopy/spectroscopy. The flatband was clearly observed near the Fermi level. We also revealed a small bandgap of ∼20 meV between the flatband and the conduction band. These results demonstrate the critical states of KMgBi that transition among various types of Dirac fermions can be tuned in one system.
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.
In non-magnetic materials the combination of inversion symmetry breaking (ISB) and spin-orbit coupling (SOC) determines the spin polarization of the band structure. However, a local spin polarization can also arise in centrosymmetric crystals containing ISB subunits. This is namely the case for the nodal-line semimetal ZrSiTe where, by combining spin- and angle-resolved photoelectron spectroscopy with ab initio band structure calculations, we reveal a complex spin polarization. In the bulk, the valence and conduction bands exhibit opposite spin orientations in two spatially separated two-dimensional ZrTe sectors within the unit cell, yielding no net polarization. We also observe spin-polarized surface states that are well separated in energy and momentum from the bulk bands. A layer-by-layer analysis of the spin polarization allows us to unveil the complex evolution of the signal in the bulk states near the surface, thus bringing the intertwined nature of surface and bulk effects to the fore.
The advent of topological semimetals enables the exploitation of symmetry-protected
topological phenomena and quantized transport. Here, we present homogeneous rectifiers,
converting high-frequency electromagnetic energy into direct current, based on low-energy
Dirac fermions of topological semimetal-NiTe2, with state-of-the-art efficiency already in the
first implementation. Explicitly, these devices display room-temperature photosensitivity as
high as 251 mA W−1 at 0.3 THz in an unbiased mode, with a photocurrent anisotropy ratio of
22, originating from the interplay between the spin-polarized surface and bulk states. Device
performances in terms of broadband operation, high dynamic range, as well as their high
sensitivity, validate the immense potential and unique advantages associated to the control of
nonequilibrium gapless topological states via built-in electric field, electromagnetic polar-
ization and symmetry breaking in topological semimetals. These findings pave the way for the
exploitation of topological phase of matter for high-frequency operations in polarization-
sensitive sensing, communications and imaging.
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.
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.
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.
Trigonal tellurium, a small-gap semiconductor with pronounced magneto-electric and magneto-optical responses, is among the simplest realizations of a chiral crystal. We have studied by spin- and angle-resolved photoelectron spectroscopy its unconventional electronic structure and unique spin texture. We identify Kramers–Weyl, composite, and accordionlike Weyl fermions, so far only predicted by theory, and show that the spin polarization is parallel to the wave vector along the lines in k space connecting high-symmetry points. Our results clarify the symmetries that enforce such spin texture in a chiral crystal, thus bringing new insight in the formation of a spin vectorial field more complex than the previously proposed hedgehog configuration. Our findings thus pave the way to a classification scheme for these exotic spin textures and their search in chiral crystals.
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.
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 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.
Chiral crystal YbNi3Ga9 is known as an intermediate valence compound in which a strong hybridization between the 4f orbitals and the conduction band is present. The Co-substitution to YbNi3Ga9 works as a hole doping that reduces the Kondo temperature and enhances the effective mass of itinerant charge carriers. Using angle-resolved photoelectron spectroscopy, the complex band structure of Yb(Ni1−xCox)3Ga9 (x=0,0.1) is revealed. A Yb2+ 4f7/2 band and evidences of hybridization to valence bands are found near the Fermi level. Both YbNi3Ga9 and the Co-substituted compound exhibit double hexagonal Fermi surfaces centered at the Γ¯-point, surrounded by a large snowflake-like surface, and a triangular electron-like surface along the Γ¯M¯ direction. By changing the incident photon energy, the band dispersion along the c-axis and the barrel-shaped Fermi surface is observed.
Band inversions are key to stabilising a variety of novel electronic states in solids, from topological surface states to the formation of symmetry-protected three-dimensional Dirac and Weyl points and nodal-line semimetals. Here, we create a band inversion not of bulk states, but rather between manifolds of surface states. We realise this by aliovalent substitution of Nb for Zr and Sb for S in the ZrSiS family of nonsymmorphic semimetals. Using angle-resolved photoemission and density-functional theory, we show how two pairs of surface states, known from ZrSiS, are driven to intersect each other near the Fermi level in NbGeSb, and to develop pronounced spin splittings. We demonstrate how mirror symmetry leads to protected crossing points in the resulting spin-orbital entangled surface band structure, thereby stabilising surface state analogues of three-dimensional Weyl points. More generally, our observations suggest new opportunities for engineering topologically and symmetry-protected states via band inversions of surface states.
Implementation of in-situ and operando experimental set-ups for bridging the pressure gap in characterization techniques based on monitoring of photoelectron emission has made significant achievements at several beamlines at Elettra synchrotron facility. These set-ups are now operational and have been successfully used to address unsolved issues exploring events occurring at solid–gas, solid–liquid and solid-solid interfaces of functional materials. The sections in the article communicate the research opportunities offered by the current set-ups at APE, BACH, ESCAmicroscopy and Nanospectroscopy beamlines and outline the next steps to overcome the present limits.
Palladium ditelluride (PdTe2) is a novel transition‐metal dichalcogenide exhibiting type‐II Dirac fermions and topological superconductivity. To assess its potential in technology, its chemical and thermal stability is investigated by means of surface‐science techniques, complemented by density functional theory, with successive implementation in electronics, specifically in a millimeter‐wave receiver. While water adsorption is energetically unfavorable at room temperature, due to a differential Gibbs free energy of ≈+12 kJ mol−1, the presence of Te vacancies makes PdTe2 surfaces unstable toward surface oxidation with the emergence of a TeO2 skin, whose thickness remains sub‐nanometric even after one year in air. Correspondingly, the measured photocurrent of PdTe2‐based optoelectronic devices shows negligible changes (below 4%) in a timescale of one month, thus excluding the need of encapsulation in the nanofabrication process. Remarkably, the responsivity of a PdTe2‐based millimeter‐wave receiver is 13 and 21 times higher than similar devices based on black phosphorus and graphene in the same operational conditions, respectively. It is also discovered that pristine PdTe2 is thermally stable in a temperature range extending even above 500 K, thus paving the way toward PdTe2‐based high‐temperature electronics. Finally, it is shown that the TeO2 skin, formed upon air exposure, can be removed by thermal reduction via heating in vacuum.
The layered van der Waals antiferromagnet MnBi2Te4 has been predicted to combine the band ordering of archetypical topological insulators such as Bi2Te3 with the magnetism of Mn, making this material a viable candidate for the realization of various magnetic topological states. We have systematically investigated the surface electronic structure of MnBi2Te4(0001) single crystals by use of spin- and angle-resolved photoelectron spectroscopy experiments. In line with theoretical predictions, the results reveal a surface state in the bulk band gap and they provide evidence for the influence of exchange interaction and spin-orbit coupling on the surface electronic structure.
Electronic correlation is believed to play an important role in exotic phenomena such as insulator-metal transition, colossal magnetoresistance, and high-temperature superconductivity in correlated electron systems. Recently, it has been shown that electronic correlation may also be responsible for the formation of unconventional plasmons. Herewith, using a combination of angle-dependent spectroscopic ellipsometry, angle resolved photoemission spectroscopy, and Hall measurements, all as a function of temperature supported by first-principles calculations, the existence of low-loss high-energy correlated plasmons accompanied by spectral weight transfer, a fingerprint of electronic correlation, in topological insulator (Bi0.8Sb0.2)2Se3 is revealed. Upon cooling, the density of free charge carriers in the surface states decreases whereas that in the bulk states increases, and the recently reported correlated plasmons are key to explaining this phenomenon. Our result shows the importance of electronic correlation in determining correlated plasmons and opens an alternative path in engineering plasmonic-based topologically insulating devices.
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.
A proper understanding on the charge mobility in organic materials is one of the key factors to realize highly functionalized organic semiconductor devices. So far, however, although a number of studies have proposed the carrier transport mechanism of rubrene single crystal to be band-like, there are disagreements between the results reported in these papers. Here, we show that the actual dispersion widths of the electronic bands formed by the highest occupied molecular orbital are much smaller than those reported in the literature, and that the disagreements originate from the diffraction effect of photoelectron and the vibrations of molecules. The present result indicates that the electronic bands would not be the main channel for hole mobility in case of rubrene single crystal and the necessity to consider a more complex picture like molecular vibrations mediated carrier transport. These findings open an avenue for a thorough insight on how to realize organic semiconductor devices with high carrier mobility.
Whenever one is interested in making high temperature superconductor-based devices, the goodness of the sample surface in terms of structural and electrical properties is a strong issue. In fact, it is well known that the surface of high Tc superconducting samples is not bulk-representative, due to air contamination and to the possible presence of oxygen vacancies. In addition, the quality of the surface layer results to be crucial in surface sensitive measurements as in X-ray photoelectron and Angle-resolved photoemission spectroscopy. Recently, some studies have been dedicated to the realization of devices based on electron-doped cuprates, bilayers and nanowires, showing the actual possibility to realize good quality junctions by using these cuprates. In this work, we report on the fabrication of thin films of the electron-doped Nd2−xCexCuO4±δ compound and analyze the surface natural barrier of as-grown films by means of point contact spectroscopy measurements. Suitable treatments of samples in an ozone rich atmosphere have been developed in order to improve the surface quality of the films. Auger electron spectroscopy has been used to monitor the effectiveness of these treatments.
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.
The band inversions that generate the topologically non-trivial band gaps of topological insulators and the isolated Dirac touching points of three-dimensional Dirac semimetals generally arise from the crossings of electronic states derived from different orbital manifolds. Recently, the concept of single orbital-manifold band inversions occurring along high-symmetry lines has been demonstrated, stabilising multiple bulk and surface Dirac fermions. Here, we discuss the underlying ingredients necessary to achieve such phases, and discuss their existence within the family of transition metal dichalcogenides. We show how their three-dimensional band structures naturally produce only small k z projected band gaps, and demonstrate how these play a significant role in shaping the surface electronic structure of these materials. We demonstrate, through spin- and angle-resolved photoemission and density functional theory calculations, how the surface electronic structures of the group-X TMDs PtSe2 and PdTe2 are host to up to five distinct surface states, each with complex band dispersions and spin textures. Finally, we discuss how the origin of several recently-realised instances of topological phenomena in systems outside of the TMDs, including the iron-based superconductors, can be understood as a consequence of the same underlying mechanism driving k z -mediated band inversions in the TMDs.
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.
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).
I explored the properties of systems that were fabricated aiming to exploit enhanced multiferroic behavior and potentially useful functionalities at room temperature. The systems of choice for this thesis were two prototypical multiferroic heterostructures composed by a ferromagnetic thin film deposited on a ferroelectric substrate: LSMO/BTO(001) and Fe,FeMn/PMN-PT(001). I focused on the magnetic response of the thin films to applied electric fields oriented perpendicular to the interface, and influencing the substrate. In both the chosen heterostructures the magnetic layers and ferroelectric substrates are all materials with high ordering temperature.
By means of angle‐resolved photoemission spectroscopy measurements, the electronic band structure of the three‐dimensional PbBi4Te7 and PbBi6Te10 topological insulators is compared. The measurements clearly reveal coexisting topological and multiple Rashba‐like split states close to the Fermi level for both systems. The observed topological states derive from different surface terminations, as confirmed by scanning tunneling microscopy measurements, and are well‐described by the density functional theory simulations. Both the topological and the Rashba‐like states reveal a prevalent two‐dimensional character barely affected by air exposure. X‐ray and valence band photoemission measurements suggest Rashba‐like states stem from the van der Waals gap expansion, consistently with density functional theory calculations.
Topological insulators (TIs) with an inverted bulk band and a strong spin-orbit coupling exhibit gapless topological surface states (TSSs) protected by time-reversal symmetry. Helical spin textures driven by spin-momentum locking offer the opportunity to generate spin-polarized currents and therefore TIs are expected to be used for future spintronic applications. For practical applications TIs are urgently required that are operable at room temperature due to a wide bulk band gap as well as a distinct topological surface state that is robust to atmospheric exposure. Here we show two distinguishable TSSs originating from different terminations on PbBi4Te4S3 by using spin- and angle-resolved photoemission spectroscopy. We find that one TSS is persistently observed, while the other becomes invisible upon intentional oxygen exposure. The result signifies the presence of a protected TSS buried under the topmost surface. Our finding paves the way for realizing a topological spintronics device under atmospheric conditions.
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.
We report on the influence of spin-orbit coupling (SOC) in Fe-based superconductors via application of circularly polarized spin and angle-resolved photoemission spectroscopy. We combine this technique in representative members of both the Fe-pnictides (LiFeAs) and Fe-chalcogenides (FeSe) with tight-binding calculations to establish an ubiquitous modification of the electronic structure in these materials imbued by SOC. At low energy, the influence of SOC is found to be concentrated on the hole pockets, where the largest superconducting gaps are typically found. This effect varies substantively with the
kzdispersion, and in FeSe we find SOC to be comparable to the energy scale of orbital order. These results contest descriptions of superconductivity in these materials in terms of pure spin-singlet eigenstates, raising questions regarding the possible pairing mechanisms and role of SOC therein.
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.
We present a study on the growth and characterization of high-quality single-layer MoS2 with a single orientation, i.e. without the presence of mirror domains. This single orientation of the MoS2 layer is established by means of x-ray photoelectron diffraction. The high quality is evidenced by combining scanning tunneling microscopy with x-ray photoelectron spectroscopy measurements. Spin- and angle-resolved photoemission experiments performed on the sample revealed complete spin-polarization of the valence band states near the K and -K points of the Brillouin zone. These findings open up the possibility to exploit the spin and valley degrees of freedom for encoding and processing information in devices that are based on epitaxially grown materials.
The challenge of synthesizing graphene nanoribbons (GNRs) with atomic precision is currently being pursued along a one-way road, based on the synthesis of adequate molecular precursors that react in predefined ways through self-assembly processes. The synthetic options for GNR generation would multiply by adding a new direction to this readily successful approach, especially if both of them can be combined. We show here how GNR synthesis can be guided by an adequately nanotemplated substrate instead of by the traditionally designed reactants. The structural atomic precision, unachievable to date through top-down methods, is preserved by the self-assembly process. This new strategy’s proof-of-concept compares experiments using 4,4′′-dibromo-para-terphenyl as a molecular precursor on flat Au(111) and stepped Au(322) substrates. As opposed to the former, the periodic steps of the latter drive the selective synthesis of 6 atom-wide armchair GNRs, whose electronic properties have been further characterized in detail by scanning tunneling spectroscopy, angle resolved photoemission, and density functional theory calculations.
We study the low-energy surface electronic structure of the transition-metal dichalcogenide superconductor
PdTe2 by spin- and angle-resolved photoemission, scanning tunneling microscopy, and density-functional theory-based supercell calculations. Comparing PdTe2 with its sister compound PtSe2, we demonstrate how enhanced interlayer hopping in the Te-based material drives a band inversion within the antibonding p-orbital manifold well above the Fermi level. We show how this mediates spin-polarized topological surface states which form rich multivalley Fermi surfaces with complex spin textures. Scanning tunneling spectroscopy reveals type-II superconductivity at the surface, and moreover shows no evidence for an unconventional component of its superconducting order parameter, despite the presence of topological surface states.
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.
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.
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.
Carbon nanomaterials exhibit extraordinary mechanical and electronic properties desirable for future technologies. Beyond the popular sp2‐scaffolds, there is growing interest in their graphdiyne‐related counterparts incorporating both sp2 and sp bonding in a regular scheme. Herein, we introduce carbonitrile‐functionalized graphdiyne nanowires, as a novel conjugated, one‐dimensional (1D) carbon nanomaterial systematically combining the virtues of covalent coupling and supramolecular concepts that are fabricated by on‐surface synthesis. Specifically, a terphenylene backbone is extended with reactive terminal alkyne and polar carbonitrile (CN) moieties providing the required functionalities. It is demonstrated that the CN functionalization enables highly selective alkyne homocoupling forming polymer strands and gives rise to mutual lateral attraction entailing room‐temperature stable double‐stranded assemblies. By exploiting the templating effect of the vicinal Ag(455) surface, 40 nm long semiconducting nanowires are obtained and the first experimental assessment of their electronic band structure is achieved by angle‐resolved photoemission spectroscopy indicating an effective mass below 0.1m0 for the top of the highest occupied band. Via molecular manipulation it is showcased that the novel oligomer exhibits extreme mechanical flexibility and opens unexplored ways of information encoding in clearly distinguishable CN‐phenyl trans–cis species. Thus, conformational data storage with density of 0.36 bit nm−2 and temperature stability beyond 150 K comes in reach.
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.
Transition-metal dichalcogenides (TMDs) are renowned for their rich and varied bulk properties, while their single-layer variants have become one of the most prominent examples of two-dimensional materials beyond graphene. Their disparate ground states largely depend on transition metal d-electron-derived electronic states, on which the vast majority of attention has been concentrated to date. Here, we focus on the chalcogen-derived states. From density-functional theory calculations together with spin- and angle-resolved photoemission, we find that these generically host a co-existence of type-I and type-II three-dimensional bulk Dirac fermions as well as ladders of topological surface states and surface resonances. We demonstrate how these naturally arise within a single p-orbital manifold as a general consequence of a trigonal crystal field, and as such can be expected across a large number of compounds. Already, we demonstrate their existence in six separate TMDs, opening routes to tune, and ultimately exploit, their topological physics.
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.
The prediction of Weyl fermions in the low-temperature noncentrosymmetric
1T′ phase of MoTe2 still awaits clear experimental confirmation. Here, we report angle-resolved photoemission (ARPES) data and ab initio calculations that reveal a surface state arc dispersing between the valence and the conduction band, as expected for a Weyl semimetal. However, we find that the arc survives in the high-temperature centrosymmetric 1T'' phase. Therefore, a surface Fermi arc is not an unambiguous fingerprint of a topologically nontrivial phase. We have also investigated the surface state spin texture of the
1T′ phase by spin-resolved ARPES, and identified additional topologically trivial spin-split states within the projected band gap at higher binding energies.
Topological Weyl semimetal (TWS), a new state of quantum matter, has sparked enormous research interest recently. Possessing unique Weyl fermions in the bulk and Fermi arcs on the surface, TWSs offer a rare platform for realizing many exotic physical phenomena. TWSs can be classified into type-I that respect Lorentz symmetry and type-II that do not. Here, we directly visualize the electronic structure of MoTe2, a recently proposed type-II TWS. Using angle-resolved photoemission spectroscopy (ARPES), we unravel the unique surface Fermi arcs, in good agreement with our ab initio calculations that have nontrivial topological nature. Our work not only leads to new understandings of the unusual properties discovered in this family of compounds, but also allows for the further exploration of exotic properties and practical applications of type-II TWSs, as well as the interplay between superconductivity (MoTe2 was discovered to be superconducting recently) and their topological order.
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.
Topological insulators are a promising class of materials for applications in the field of spintronics. New perspectives in this field can arise from interfacing metal–organic molecules with the topological insulator spin-momentum locked surface states, which can be perturbed enhancing or suppressing spintronics-relevant properties such as spin coherence. Here we show results from an angle-resolved photemission spectroscopy (ARPES) and scanning tunnelling microscopy (STM) study of the prototypical cobalt phthalocyanine (CoPc)/Bi2Se3 interface. We demonstrate that that the hybrid interface can act on the topological protection of the surface and bury the Dirac cone below the first quintuple layer.