Polarized X-ray Absorption Spectroscopy Research Articles

Self-Assembled Monolayers of N-Heterocyclic Olefins on Au(111).

Self-assembled monolayers (SAMs) of N-heterocyclic olefins (NHOs) have been prepared on Au(111) and their thermal stability, adsorption geometry, and molecular order were characterized by X-ray photoelectron spectroscopy, polarized X-ray absorption spectroscopy, scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. The strong σ-bond character of NHO anchoring to Au induced high geometrical flexibility that enabled a flat-lying adsorption geometry via coordination to a gold adatom. The flat-lying adsorption geometry was utilized to further increase the surface interaction of the NHO monolayer by backbone functionalization with methyl groups that induced high thermal stability and a large impact on work-function values, which outperformed that of N-heterocyclic carbenes. STM measurements, supported by DFT modeling, identified that the NHOs were self-assembled in dimers, trimers, and tetramers constructed of two, three, and four complexes of NHO-Au-adatom. This self-assembly pattern was correlated to strong NHO-Au interactions and steric hindrance between adsorbates, demonstrating the crucial influence of the carbon-metal σ-bond on monolayer properties.

High-mobility spin-polarized two-dimensional electron gas at the interface of EuTiO3/SrTiO3 heterostructures

Spin polarization of two-dimensional electron gas (2DEG) at the interface of EuTiO3/SrTiO3(STO) heterostructures has been theoretical predicted and experimentally observed via x-ray magnetic circular dichroism and polarized x-ray absorption spectroscopy, which, however, is lack of magnetotransport evidence. Here, we report the fabrication of high-quality EuTiO3/STO heterostructures by depositing antiferromagnetic insulating EuTiO3 thin films onto STO substrates. Shubnikov-de Haas oscillation, Hall, and magnetoresistance (MR) measurements show that the interface is not only highly conducting, with electron mobility up to cm2V−1s−1 at 1.8 K, but also shows low-field hysteretic MR effects. MR of ∼9% is observed at 1.8 K and 20 Oe, which is one order of magnitude higher than those observed in other spin-polarized 2DEG oxide systems. Moreover, the heterostructures show ferromagnetic hysteresis loops. These results demonstrate that the high-mobility 2DEG is spin polarized, whose origin is attributed to the interfacial Ti3+-3d states due to oxygen deficiency and the exchange interactions between interfacial Eu spins and itinerant Ti-3d electrons.

Formation of oriented layered MoS2 from amorphous thin film revealed by polarized x-ray absorption spectroscopy

MoS2 is a prototypical two-dimensional van der Waals (vdW) solid, where covalently bonded S–Mo–S triplets are held together by weaker vdW forces. In this work, we have studied structural transformation from a three-dimensional amorphous phase of MoS2 into a layered vdW crystal using S L2,3 edge x-ray absorption near-edge structure (XANES) spectroscopy with in-plane and out-of-plane polarized x-ray beam. The crystallization process, which starts from an isotropic amorphous phase, is accompanied by the establishment of vdW interaction between covalently bonded layers, resulting in the anisotropic nature of the crystalline phase. We have disclosed that the preferential growth of MoS2 layers along the (200) Bragg reflection commences immediately from the amorphous phase with no intermediate crystal orientations. We have additionally identified a unique signature in the S L2,3 edge spectrum that is associated with vdW bonds and can be possibly used to determine sulfur-based single-layered and multi-layered transition metal dichalcogenides.

Low-energy electronic structure of perovskite and Ruddlesden-Popper semiconductors in the Ba-Zr-S system probed by bond-selective polarized x-ray absorption spectroscopy, infrared reflectivity, and Raman scattering

Chalcogenides in perovskite and the related layered Ruddlesden-Popper crystal structures (chalcogenide perovskites for brevity) are an exciting family of semiconductors but remain experimentally little studied. Chalcogenide perovskites share crystal structures and some physical properties with ionic compounds such as oxide and halide perovskites, but the metal-chalcogen bonds responsible for semiconducting behavior are substantially more covalent than in these more-studied perovskites. Here, we use complementary experimental and theoretical methods to study how the mixed ionic-covalent Zr-S bonds support the electronic structure and physical properties of perovskite ${\mathrm{BaZrS}}_{3}$ and Ruddlesden-Popper ${\mathrm{Ba}}_{3}{\mathrm{Zr}}_{2}{\mathrm{S}}_{7}$. We apply theoretical methods to assign features of experimentally measured x-ray absorption spectroscopy (XAS) to particular orbital transitions, enabling a clear physical interpretation of angle-dependent, polarized XAS data measured on single-crystal samples, and an atomistic view of the covalent bonding network that facilitates charge transport. Polarized Raman measurements identify signatures of crystalline anisotropy in ${\mathrm{Ba}}_{3}{\mathrm{Zr}}_{2}{\mathrm{S}}_{7}$ and enable the first assignments of mode symmetry in this material. Infrared reflectivity reveals electronic transport properties that augur well for the use of chalcogenide perovskites in optoelectronic and energy-conversion technologies.

Morphology of ultrathin lithium fluoride deposited on Ag(100): Dendrites versus islands

We have investigated lithium fluoride (LiF) in the monolayer range deposited by thermal sublimation onto the (100) surface of a silver single crystal in ultrahigh vacuum. Our study combines scanning tunneling microscopy, low-energy electron diffraction, and polarized x-ray absorption spectroscopy. The results reveal that LiF grows epitaxially on Ag(100), with a morphology strongly depending on the growth temperature: at room temperature, dendritic patterns are formed with branches along the [011] and $[0\overline{1}1]$ directions, exhibiting the same lattice orientation as the substrate. Conversely, at a substrate temperature of 500 K LiF forms square islands exhibiting a moir\'e pattern, with the islands' borders oriented along the [001] and [010] directions. While the lattice constant of the surface deposited samples is close to the one of bulk fcc LiF, the appearance of a strong x-ray linear dichroism for dendritic LiF points to a tetragonal distortion arising from the LiF-substrate interaction. The less prominent dichroism and the presence of the moir\'e pattern on the square islands both suggest the relaxation of the LiF grown on the hot Ag substrate.

Evidence of a thermally-induced microstructural anisotropy in Gr/Co/Ir(111) systems

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.

Local geometry around B atoms in B/Si(1 1 1) from polarized x-ray absorption spectroscopy

The arrangement of B atoms in a doped Si(1 1 1)-:B system was studied using a near-edge x-ray absorption fine structure (NEXAFS). Boron atoms were deposited via segregation from the bulk by flashing the sample repeatedly. The positions of B atoms are determined by comparing measured polarized (angle-dependent) NEXAFS spectra with spectra calculated for various structural models based on ab initio total energy calculations. It is found that most of boron atoms are located in sub-surface L positions, beneath a Si atom. However, depending on the preparation method a significant portion of B atoms may be located elsewhere. A possible location of these non-L-atoms is at the surface, next to those Si atoms which form the reconstruction.

Electronic and vibrational signatures of ruthenium vacancies in Sr2RuO4 thin films

The synthesis of stoichiometric ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ thin films has been a challenge because of the high volatility of ruthenium oxide precursors, which gives rise to ruthenium vacancies in the films. Ru vacancies greatly affect the transport properties and electronic phase behavior of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$, but their direct detection is difficult due to their atomic dimensions and low concentration. We applied polarized x-ray absorption spectroscopy at the oxygen $K$ edge and confocal Raman spectroscopy to ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ thin films synthesized under different conditions. The results show that these methods can serve as sensitive probes of the electronic and vibrational properties of Ru vacancies, respectively. The intensities of the vacancy-related spectroscopic features extracted from these measurements are well correlated with the transport properties of the films. The methodology introduced here can thus help one to understand and control the stoichiometry and transport properties in films of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ and other ruthenates.

Probing the Excited State of Methylcobalamin Using Polarized Time-Resolved X-ray Absorption Spectroscopy.

We use picosecond time-resolved polarized X-ray absorption near-edge structure (XANES) measurements to probe the structure of the long-lived photoexcited state of methylcobalamin (MeCbl) and the cob(II)alamin photoproduct formed following photoexcitation of adenosylcobalamin (AdoCbl, coenzyme B12). For MeCbl, we used 520 nm excitation and a time delay of 100 ps to avoid the formation of cob(II)alamin. We find only small spectral changes in the equatorial and axial directions, which we interpret as arising from small (<∼0.05 Å) changes in both the equatorial and axial distances. This confirms expectations based on prior UV-visible transient absorption measurements and theoretical simulations. We do not find evidence for the significant elongation of the Co-C bond reported by Subramanian [ J. Phys. Chem. Lett. 2018 , 9 , 1542 - 1546 ] following 400 nm excitation. For AdoCbl, we resolve the difference XANES contributions along three unique molecular axes by exciting with both 540 and 365 nm light, demonstrating that the spectral changes are predominantly polarized along the axial direction, consistent with the loss of axial ligation. These data suggest that the microsecond "recombination product" identified by Subramanian et al. is actually the cob(II)alamin photoproduct that is produced following bond homolysis of MeCbl with 400 nm excitation. Our results highlight the pronounced advantage of using polarization-selective transient X-ray absorption for isolating structural dynamics in systems undergoing atomic displacements that are strongly correlated to the exciting optical polarization.

Structural iron in smectites with different charge locations

The versatile structure of smectites can exhibit large variations in chemical compositions and cationic substitutions in different crystallographic sites, resulting in various locations of layer charge. Natural smectites can contain various amounts of structural iron, the chemical form of which can influence the reactivity of these minerals. The variety of Fe crystal chemistry in smectite was explored for eight natural smectites of distinct chemical compositions and charge locations, together with two synthetic ferric saponites used as reference compounds for tetrahedral Fe(III). All samples were identified as dioctahedral or trioctahedral smectite by X-ray diffraction and Fourier-transform infrared spectroscopy. The extent of [4]Al for [4]Si substitution was determined by 27Al and 29Si magic angle spinning nuclear magnetic resonance spectroscopy. The Fe local chemical environment was probed by polarized X-ray absorption spectroscopy. Only Fe(III) could be detected in all samples, with no evidence of cluster formation. The O shell at 1.86 Å in synthetic saponites suggests Fe insertion in tetrahedral sites, and the absence of detected octahedral Fe implies quantitative substitution of [4]Fe3+ for [4]Si4+. In natural smectites, Fe(III) is bound to six O atoms at ~ 2.00 Å, suggesting insertion in octahedral sites. This inference is also supported by the detection of in-plane Mg/Al/Fe atoms at ~ 3.05 Å and out-of-plane Si/Al atoms at ~ 3.25 Å. In one Fe-rich nontronite, the detection of an O subshell at ~ 1.88 Å suggests a concomitant insertion of Fe(III) in tetrahedral sites. Low numbers of octahedral neighbors were detected in natural saponite and hectorite, presumably because of the presence of vacancies and/or Li(I) in adjacent octahedral sites balancing the local charge excess originating from the substitution of Fe(III) for Mg(II). The substitution of [4]Fe3+ for [4]Si4+ can be readily obtained under defined conditions in the laboratory, but seems more rare in natural samples, or present in amounts below the detection limit of spectroscopic methods used in this study.

Influence of the pore diameter in Cu/Co/Cu antidots: A XANES study

Antidot materials, i.e., two-dimensional nanostructures with a periodic array of nanopores, are of great scientific interest due to their unique nanomagnetic properties and their potential application in storage devices. It is well known that physical properties of antidots are directly linked to the diameter of nanopores and their spacing, as well as to the morphology and the localizations of chemical species. However, due to their nanoscale size, their characterization remains challenging. Here, we present a detailed investigation of the morphology and presence of oxide species in antidots as a function of the pore diameter using polarized x-ray absorption spectroscopy. For this study we synthesized and characterized Cu(10 nm)/Co(12 nm)/Cu(10 nm) sputtered antidots, fabricated by the double-anodization technique assisted by atomic layer deposition. The pore size ranged from 20 to 80 nm, with a fixed interpore distance (105 nm). An unholed multilayer deposited on Si/${\mathrm{SiO}}_{2}$ was also investigated for comparison. We observed a clear correlation between the increase of pore diameter and the enhancement of oxide content from three different x-ray absorption near edge structure analysis methods. Polarized XAS allowed us to localize the CoO nanorings inside the pores. We propose that the CoO formation is directly related to the crescent shape of the multilayer deposit inside the pores. The coercivity of antidots is enhanced by increasing the magnetic atomic proportion in the periphery of nanopores. The structural observations were also used to develop a simple model in order to estimate the proportion of atoms inside the pores and on the top of the antidots as a function of the interpore distance, the hole diameter, and the penetration length of deposition inside the pores. This model can be easily used in the literature for estimating the atomic species deposited on antidots.

Orbital orientation mapping of V2O5 thin films

We report the effects of growth methods on the orbital orientation in vanadium pentoxide (V2O5) thin films, an important factor to consider when selecting growth techniques for highly selective catalysts and devices. Thermal evaporation and sol-gel methods were used to synthesize the V2O5 films. The surface morphology, roughness, and orientation of the films were characterized by atomic force microscopy and x-ray diffraction. Surface electronic properties and oxidation states were assessed by x-ray photoemission spectroscopy. Polarized x-ray absorption spectroscopy demonstrated that the thermally evaporated film [which was in the (001) orientation] exhibited greater anisotropy than the (100) oriented sol-gel film. The observed increase in anisotropy agrees well with computational findings which revealed that more vanadyl bonds are present at the surface of the thermally evaporated film than at the surface of the sol-gel film. The same computational study also found that the orientation of these bonds is more parallel to the film surface in the thermally evaporated film than in the sol-gel film. The data suggest that the method of growth may be used as a controlled variable to select key film characteristics for potential applications.

ChemInform Abstract: Interfacial Magnetism in Complex Oxide Heterostructures Probed by Neutrons and X‐Rays

AbstractReview: 117 refs.

Removal of Ca(2+) from the Oxygen-Evolving Complex in Photosystem II Has Minimal Effect on the Mn4O5 Core Structure: A Polarized Mn X-ray Absorption Spectroscopy Study.

Ca(2+)-depleted and Ca(2+)-reconstituted spinach photosystem II was studied using polarized X-ray absorption spectroscopy of oriented PS II preparations to investigate the structural and functional role of the Ca(2+) ion in the Mn4O5Ca cluster of the oxygen-evolving complex (OEC). Samples were prepared by low pH/citrate treatment as one-dimensionally ordered membrane layers and poised in the Ca(2+)-depleted S1 (S1') and S2 (S2') states, the S2'YZ(•) state, at which point the catalytic cycle of water oxidation is inhibited, and the Ca(2+)-reconstituted S1 state. Polarized Mn K-edge XANES and EXAFS spectra exhibit pronounced dichroism. Polarized EXAFS data of all states of Ca(2+)-depleted PS II investigated show only minor changes in distances and orientations of the Mn-Mn vectors compared to the Ca(2+)-containing OEC, which may be attributed to some loss of rigidity of the core structure. Thus, removal of the Ca(2+) ion does not lead to fundamental distortion or rearrangement of the tetranuclear Mn cluster, which indicates that the Ca(2+) ion in the OEC is not critical for structural maintenance of the cluster, at least in the S1 and S2 states, but fulfills a crucial catalytic function in the mechanism of the water oxidation reaction. On the basis of this structural information, reasons for the inhibitory effect of Ca(2+) removal are discussed, attributing to the Ca(2+) ion a fundamental role in organizing the surrounding (substrate) water framework and in proton-coupled electron transfer to YZ(•) (D1-Tyr161).

X-ray magnetic circular dichroism study of the magnetic anisotropy onTbMnO3

The magnetic anisotropy of $\mathrm{TbMn}{\mathrm{O}}_{3}$ was explored by means of polarized x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) measurements at the $\mathrm{Mn}\phantom{\rule{0.16em}{0ex}}{L}_{2,3}$ and $\mathrm{Tb}\phantom{\rule{0.16em}{0ex}}{M}_{4,5}$ edges as a function of temperature and magnetic-field intensity. The selective magnetometry measurements were compared with the macroscopic magnetic properties on single crystals. XMCD measurements at the $\mathrm{Tb}\phantom{\rule{0.16em}{0ex}}{M}_{4,5}$ edge as a function of the magnetic field reproduces quite well the macroscopic magnetic anisotropy at low temperatures with the Tb moments staying confined along their Ising axis within the $ab$ plane, whereas a weak XMCD signal is observed at the $\mathrm{Mn}\phantom{\rule{0.16em}{0ex}}{L}_{2,3}$ edge. These results point out that $\mathrm{T}{\mathrm{b}}^{3+}$ single-ion anisotropy is the only responsible for the magnetic anisotropy on this multiferroic compound at high magnetic fields. Moreover, we found $\mathrm{Mn}\phantom{\rule{0.16em}{0ex}}{L}_{2,3}$ XMCD measurements show that the cycloidal antiferromagnetic order is almost unaffected by the applied magnetic field at low temperatures under an applied magnetic field. Therefore, we discuss that this strong Ising nature of $\mathrm{T}{\mathrm{b}}^{3+}$ ions, through a magnetocrystalline coupling mediated by the oxygen atoms, must play an important role in the field-induced electric polarization flop and therefore in the magnetoelectric coupling on $\mathrm{TbMn}{\mathrm{O}}_{3}$.

Effect of pressure-driven local structural rearrangement on the superconducting properties ofFeSe0.5Te0.5

The pressure dependence of the superconducting transition temperature ${T}_{\mathrm{c}}$, the local structure, and the electronic properties of the iron chalcogenide superconductor ${\mathrm{FeSe}}_{0.5}{\mathrm{Te}}_{0.5}$ was investigated by means of SQUID magnetometry and site-selective polarized x-ray absorption spectroscopy at the iron and selenium $K$-edges. The measurements reveal a discontinuous decrease of the local bond distances by crossing a tetragonal to monoclinic structural phase transition at ${p}_{\mathrm{S}}\ensuremath{\sim}2.7$ GPa. The electronic structure, however, evolves evenly, but the superconducting properties are highly affected by changes in the local structure. These results underline the close connection of the lattice and the superconductivity in the iron-based superconductors.

Polarized X-ray Absorption Spectroscopy Observation of Electronic and Structural Changes of Chemical Vapor Deposition Graphene in Contact with Water

The interaction of chemical vapor deposition (CVD)-grown graphene films with water was studied by means of in situ angle-dependent X-ray absorption spectroscopy (XAS). We found that when the graphene layer is in contact with water there is a reduction in the π* peak intensity in the carbon K-edge absorption spectra, accompanied by an extension of the σ* peak to lower energies, which are indicative of chemical modifications of the graphene and a reduction in the number of unsaturated carbon bonds due to the covalent attachment of contaminant species. In addition to the chemical changes a decrease in the dichroic ratio measured by polarized XAS measurements was observed, which indicates an increase on the nanometer scale corrugation. These changes can strongly influence the electronic properties, mechanical robustness, and resistance to sloughing, as well as graphene reactivity.

Interplay of electronic and lattice degrees of freedom inA1−xFe2−ySe2superconductors under pressure

The local structure and electronic properties of Rb${}_{1\ensuremath{-}x}$Fe${}_{2\ensuremath{-}y}$Se${}_{2}$ are investigated by means of site selective polarized x-ray absorption spectroscopy at the iron and selenium $K$ edges as a function of pressure. A combination of dispersive geometry and a nanodiamond anvil pressure cell has permitted us to reveal a steplike decrease in the Fe-Se bond distance at $p\ensuremath{\simeq}11$ GPa and, at the same pressure, the local atomic structure of Rb${}_{1\ensuremath{-}x}$Fe${}_{2\ensuremath{-}y}$Se${}_{2}$ becomes more ordered. Thus, at this pressure the phase separation might be suppressed, which has a large influence on the electronic structure, as evidenced by changes in the Fe $K$-edge prepeak. The present results provide compelling evidence that the reemerging superconductivity in ${A}_{1\ensuremath{-}x}$Fe${}_{2\ensuremath{-}y}$Se${}_{2}$ is closely related to the transition to a locally more ordered state, which has a different electronic structure.

Local Lattice Distortion in Superconducting Cuprates Studied by XAS

In-plane local distortion in cuprates characterized by strong doping and temperature dependence is a signature of polaron (bipolaron). The nature of dynamic lattice response in relation to the onset of superconductivity was probed by polarized x-ray absorption spectroscopy (XAS), for pure and magnetic impurity-doped MxLa1.85Sr0.15Cu1-xO4 (M=Mn, Ni, Co, x<0.05) single crystals grown by a TSFZ method. The results confirm that the dynamical anti Jahn-Teller (AJT) distorted domains maximize at Tdmax~Tconset, which is described by the disappearance of the Cu-O(1) bond alternation upon the completion of phase coherence (onset of superconductivity). In contrast to the Ni and Co dopings which quickly suppress superconductivity by magnetic pair breaking, the Mn doping shows sustainable superconductivity i.e., a constant Tdmax and Tconset upto x=0.03 with the presence of dynamical AJT distortion. The results suggest that the dynamical AJT type distortion with either Q2 or pseudo JT symmetry extending over the basal plane could be a prerequisite for high temperature superconductivity.

Pseudogap-related charge dynamics in the layered nickelateR2−xSrxNiO4(x∼1)

Charge dynamics and its critical behavior are investigated near the metal-insulator transition of layered-nickelate R2-xSrxNiO4 (R=Nd, Eu). The polarized x-ray absorption spectroscopy experiment clearly shows the multi-orbital nature which enables the x2-y2-orbital-based checkerboard-type charge ordering or correlation to persist up to the critical doping region (x sim 1). In the barely metallic region proximate to the charge-ordered insulating phase, the nominal carrier density estimated from the Hall coefficient markedly decreases in accord with development of the pseudogap structure in the optical conductivity spectrum, while the effective mass is least enhanced. The present findings combined with the results of recent angle-resolved photoemission spectroscopy show that the pseudogap in the metal-insulator critical state evolves due to the checkerboard-type charge correlation to extinguish the coherent-motion carriers in a characteristic momentum (k)-dependent manner with lowering temperature.