Core-level Photoemission Study Research Articles

Review of actinide core-level photoemission

Photoelectron spectroscopy allows for the investigation of the electronic structure and chemical bonding of actinide elements and their compounds, providing insights into oxidation states, chemical environments, and electronic configurations. This knowledge can aid in comprehending reactivity, stability, and other properties of actinide materials, which is essential for ensuring safe handling, storage, and disposal in nuclear applications. We have reviewed a number of results in actinide core-level photoemission studies, with a particular focus on x-ray photoemission spectroscopy (XPS) techniques. Actinides, due to their inherent radioactivity, have not been as well studied with XPS as have other segments of the periodic table. Given the inherent safety concerns, equipment requirements, and short isotopic lifetimes associated with actinide research, we outline the strategies and precautions necessary for conducting successful and safe XPS experiments on these elements. Core-level photoemission can be a powerful proven tool for investigating the electronic structure, chemical bonding behaviors, and physical properties of actinides, providing valuable insights into an incredibly complex behavior of these systems. We highlight key findings from recent studies that demonstrate the potential of core-level photoemission in uncovering the unique properties of actinides and their compounds. Finally, we identify current knowledge gaps and future research directions that could enhance our understanding of actinide chemistry and physics.

Observation of the 4f ground-state symmetry in strongly correlated cubic Pr compounds probed by linearly polarized 3d core-level photoemission spectroscopy

We report the core-level photoemission study of cubic PrBe13, PrIr2Zn20 and PrB6 using linearly polarized hard X-ray. We have successfully observed linear dichroism (LD) in Pr3+ 3d core-level photoemission spectra reflecting the 4f ground-state symmetry for all the compounds. The observed LDs and polarization-dependent spectra are reproduced by ionic-model calculations including the full multiplet theory and the local crystalline electric field (CEF) splitting. We have confirmed that LDs in Pr3+ 3d core-level photoemission spectra are able to determine unambiguously the Pr3+ 4f ground-state symmetry.

Anomalous magnetic and spin glass behavior in Nb-substituted LaCo1−xNbxO3

We report the structural, magnetic, transport, and electronic properties of Nb-substituted ${\mathrm{LaCo}}_{1\ensuremath{-}x}{\mathrm{Nb}}_{x}{\mathrm{O}}_{3}$ ($x=0--0.2$). The Rietveld refinement of x-ray diffraction data demonstrate structural phase transitions from rhombohedral to orthorhombic and further to monoclinic with increasing the Nb concentration up to $x\ensuremath{\ge}0.2$. Interestingly, we observed dramatic changes in the magnetization ($M$) with increasing the Nb concentration, as the $M$ sharply increases below 10 K even at 2.5% substitution. Furthermore, ac susceptibility data show the spin glass behavior in $x=0.1$ sample. We find that the density of states near the Fermi level decreases and the activation energy increases, which results in the decreasing conductivity with higher Nb concentration. A significant shift in the peak position of ${A}_{2g}$ phonon mode has been observed using Raman spectroscopy, which indicates the change in the coupling due to the structural distortion with Nb substitution. The core-level photoemission study confirms that the Nb is present in $5+$ valence state. Our study reveals that the nonmagnetic ${\mathrm{Nb}}^{5+}$ (${d}^{0}$) substitution converts ${\mathrm{Co}}^{3+}$ ions to ${\mathrm{Co}}^{2+}$ and stabilizes both in the high-spin state. Our results suggest that structural and spin-state transitions as well as the difference in the ionic radii between ${\mathrm{Nb}}^{5+}$ and ${\mathrm{Co}}^{3+}$ are playing an important role in tuning the physical properties.

Surface structure and photoemission studies of nanocrystalline TiO2 layer/ITO coated glass interface

Room temperature growth of nanocrystalline TiO2 thin film was carried out by two-electrode cell. The film was characterized by surface probing techniques. Morphological studies revealed that film's grains are evenly distributed across substrate surface. Average height of grains distribution is below 64nm. Post-deposition annealing aided film's particles’ orientation in a structure that resulted in compact layer. Average crystallite size was estimated as 19.5nm. Quality and chemical states of film composition were observed by core level photoemission studies. From XPS studies, small shift (∼0.11eV) observed in binding energy position of Ti 2p3/2 corroborated oxidation states of titanium species found in TiO2 structure. Peak broadening and formation of InOSn linkage at oxygen valence band indicated interactions between substrate's atoms due to annealing. Apart from substrate's atoms interaction, chemical state of Ti profile remained stable implying no major chemical interaction between ITO atoms and film components. This study demonstrated TiO2 as a recipe for stable barrier layer capable of hindering charge trapping in nanostructured photonic devices.

Erratum: Angle-resolved and core-level photoemission study of interfacing the topological insulatorBi1.5Sb0.5Te1.7Se1.3with Ag, Nb, and Fe [Phys. Rev. B92, 075127 (2015)

Erratum: Angle-resolved and core-level photoemission study of interfacing the topological insulator<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Bi</mml:mi><mml:mrow><mml:mn>1.5</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi>Sb</mml:mi><mml:mrow><mml:mn>0.5</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi>Te</mml:mi><mml:mrow><mml:mn>1.7</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi>Se</mml:mi><mml:mrow><mml:mn>1.3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>with Ag, Nb, and Fe [Phys. Rev. B<b>92</b>, 075127 (2015)

Angle-resolved and core-level photoemission study of interfacing the topological insulatorBi1.5Sb0.5Te1.7Se1.3with Ag, Nb, and Fe

Interfaces between a bulk-insulating topological insulator (TI) and metallic adatoms have been studied using high-resolution, angle-resolved, and core-level photoemission. Fe, Nb, and Ag were evaporated onto ${\mathrm{Bi}}_{\mathbf{1}.\mathbf{5}}{\mathrm{Sb}}_{\mathbf{0}.\mathbf{5}}{\mathrm{Te}}_{\mathbf{1}.\mathbf{7}}{\mathrm{Se}}_{\mathbf{1}.\mathbf{3}}$ (BSTS) surfaces both at room temperature and 38 K. The coverage and temperature dependence of the adsorption and interfacial formation process have been investigated, highlighting the effects of the overlayer growth on the occupied electronic structure of the TI. For all coverages at room temperature and for those equivalent to less than 0.2 monolayer at low temperature all three metals lead to a downward shift of the TI bands with respect to the Fermi level. At room temperature Ag appears to intercalate efficiently into the van der Waals gap of BSTS, accompanied by low-level substitution for the Te/Se atoms of the termination layer of the crystal. This Te/Se substitution with silver increases significantly for low temperature adsorption, and can even dominate the electrostatic environment of the Bi/Sb atoms in the BSTS near-surface region. On the other hand, Fe and Nb evaporants remain close to the termination layer of the crystal. On room temperature deposition, they initially substitute isoelectronically for Bi as a function of coverage, before substituting for Te/Se atoms. For low temperature deposition, Fe and Nb are too immobile for substitution processes and show a behavior consistent with clustering on the surface. For both Ag and Fe/Nb, these differing adsorption pathways still lead to the qualitatively similar and remarkable behavior for low temperature deposition that the chemical potential first moves downward ($p$-type dopant behavior) and then upward ($n$-type behavior) on increasing coverage.

Core level photoemission studies on conducting polypyrrole polymer nanotubes showing switching transitions

Conducting polypyrrole (PPY) nanotubes is a classical model system for strongly correlated disordered materials showing intriguing switching transitions from low to high conductivity states at low temperature. This switching behaviour can be tuned by incorporating gold nanoclusters to form composite nanotubes (AuPPY). Here, we present core level electronic structure studies on PPY and AuPPY nanotubes with different diameters using hard X-ray photoemission spectroscopy at room temperature. The spectroscopic data provide information on the role of diameter as well as metal cluster incorporation for a modification of the electronic structure of this important class of nanotubes. Furthermore, electrical transport measurements were performed at low temperature to extract the change in localization length and doping level in these nanotubes. Based on these results, we explain the observed electrical transport behavior of these nanotubes as the interplay of disorder and carrier concentration.

Interfacial electronic structure of trimethyl-aluminum and water on an In0.20Ga0.80As(001)-4 × 2 surface: A high-resolution core-level photoemission study

In atomic-layer deposition (ALD), organoaluminum as trimethyl-aluminum (TMA) on a freshly molecular beam epitaxy grown In0.20Ga0.80As(001)-4 × 2 surface has three forms: Al bonding with 1, 2, and 3 methyl groups. The ALD method of using one pulse (0.1 s) of TMA plus one pulse (0.1 s) of water was ineffective in passivating the surface, as demonstrated by the increased intensity of the reacted surface components with high numbers of pulses. The intact TMA was physisorbed on the trough In atoms, while the methyl-deficient precursors are either bridged with two As atoms or on the row-edge As atoms. After the water pulse on the TMA-covered surface, a bond in the bridged As atoms was scissored off to produce the As-Al-CH3 and As-OH, and the physisorbed TMA was unaffected. TMA removed some row In atoms, which allowed the In and Ga atoms in the second layer to form OH bonds after the water exposure. The appearance of the As-OH bonds may cause interfacial defect density (Dit) a peak at the midgap.

High resolution synchrotron radiation based photoemission study of the in situ deposition of molecular sulphur on the atomically clean InGaAs surface

High resolution synchrotron radiation core level photoemission studies were performed on atomically clean 0.5 μm thick In0.53Ga0.47As (100) epilayers lattice matched to InP substrates following the removal of a 100 nm protective arsenic cap at 410 °C. Both n-type (Si doped 5 × 1017 cm−3) and p-type (Be doped 5 × 1017 cm−3) InGaAs samples were subsequently exposed in situ to molecular sulphur at room temperature, and the resulting changes in the surface chemical composition were recorded. The photoemission spectra indicate evidence of As-S, Ga-S, and In-S bond formation and the substitution of As in the near surface region by sulphur. Annealing to 400 °C results in the complete removal of the As-S bonding component with both Ga-S and In-S bonding configurations remaining. After the anneal, the Fermi level position for both n-type and p-type samples resides at the top of the bandgap indicating a near flat band condition for n-type and significant band bending on the p-type sample. The results of angle resolved photoemission measurements suggest that the sulphur has substituted arsenic in the near surface region resulting in both samples displaying n-type surface behaviour. Annealing to higher temperatures results in the loss of In from the surface without any significant change in the Ga, As, or S signals. Work function measurements on both doping types after sulphur deposition and anneal show similar behaviour displaying a value close to 6 eV which is indicative of the formation of a surface dipole layer related to the presence of sulphur on the surface.

Enhanced silicon oxidation on titanium-covered Si(001)

We report on a core level photoemission study of the formation of an ultrathinSiOx layer grown at the interface of a titanium-covered Si(001) surface. Oxygen exposure at roomtemperature induces a large chemical shift of the Si 2p state, predominantly assigned toSi4 + . The results indicatethat a SiO2 − δ layer, close tothe stoichiometry of SiO2, isformed below the TiOx film.The thickness of the SiO2 − δ layer is estimated to be ∼ 0.9 nm, corresponding to three to four oxide layers. Further chemical shiftcaused by annealing is attributed to the formation of titanium silicate (TiSixOy).

Depth dependence of itinerant character in Mn-substituted Sr3Ru2O7

We present a core-level photoemission study of Sr3(Ru1−xMnx)2O7, in which we monitor the evolution of the Ru-3d fine structure versus Mn substitution and probing depth. In both Ru-3d3/2 and -3d5/2 core levels, we observe a clear suppression of the metallic features, i.e. the screened peaks, implying a sharp transition from itinerant to localized character already at low Mn concentrations. A comparison of soft and hard x-ray photoemission, which enables tunable depth sensitivity, reveals that the degree of localized/metallic character for Ru at the surface is different from that in the bulk.

High-resolution core-level photoemission study of CF4-treated Gd2O3(Ga2O3) gate dielectric on Ge probed by synchrotron radiation

High-resolution core-level photoemission analysis using synchrotron radiation was used to investigate the superior electrical performance of aGa2O3(Gd2O3) gate dielectric on Ge(001) after CF4 treatment. Prior to the treatment, a thin germanate-like oxide layer that formed at the interface prevented Ge from diffusing to the surface. The Ge surface retained a small amount of buckled dimers from the as-grown sample. The buckled dimers were quickly removed by CF4 plasma treatment followed by an annealing process, resulting in a more uniform interface than that of the as-grown sample. The detailed interfacial electronic structure for the untreated and treated samples are presented.

Identification and thermal stability of the native oxides on InGaAs using synchrotron radiation based photoemission

A high resolution synchrotron radiation core level photoemission study of the native oxides on In0.53Ga0.47As was carried out in order to determine the various oxidation states present on the surface. The thermal stability of the oxidation states was also investigated by annealing the samples in vacuum at temperatures ranging from 150 to 450 °C. As well as the widely reported oxidation states, various arsenic, gallium, and indium oxides, along with mixed phase gallium arsenic and indium gallium oxides are identified. Elemental binary oxides have been identified as residing at the oxide substrate interface and could play an important role in understanding the growth of metal oxide dielectric layers on the InGaAs surface, due to their apparent chemical stability.

Origin of ordered two-dimensional structure ofSi(337)−4×1transformed fromSi(5 5 12)−2×1

Through combined studies using scanning tunneling microscopy (STM) and high-energy-resolution synchrotron photoemission, the origin of irreversible structural transformation from $\text{Si}(5\text{ }5\text{ }12)\text{\ensuremath{-}}2\ifmmode\times\else\texttimes\fi{}1$ to $\text{Si}(337)\text{\ensuremath{-}}4\ifmmode\times\else\texttimes\fi{}1$ with (113) facets has been investigated. From the C-dosed $\text{Si}(5\text{ }5\text{ }12)$ surface, it has been found by STM that the transformation from $(5\text{ }5\text{ }12)$ to (337) starts from a tetramer (T) row and a $\ensuremath{\pi}$-bonded $(\ensuremath{\pi})$ chain in the $(5\text{ }5\text{ }12)$ surface, and simultaneously the rest part of $(5\text{ }5\text{ }12)$ converts to T rows and $\ensuremath{\pi}$ chains, which will be transformed to (337) with additional Si atoms. By $\text{Si}\text{ }2p$ and $\text{C}\text{ }1s$ core-level photoemission studies using synchrotron radiation on the identical system, it has been confirmed that such an irreversible structural transformation is due to subsurface C atoms. If the (337) terrace is only composed of T rows and $\ensuremath{\pi}$ chains with a $2\ifmmode\times\else\texttimes\fi{}$ periodicity, the compressive stress exists so that the (337) terrace becomes unstable. However, subsurface C atoms release this compressive stress through breaking one-dimensional structures and induce a stable surface composed of $4\ifmmode\times\else\texttimes\fi{}1$ dotlike structures with additional Si atoms.

High resolution photoemission study of SiOx/Si(111) interface disruption following in situ HfO2 deposition

We report on an in situ high resolution core level photoemission study of the early stages of interface formation between an ultrathin SiOx layer (∼0.3 nm) grown on the atomically clean Si(111) surface and a HfO2 dielectric layer. Si 2p core level spectra acquired at 130 eV photon energy reveal evidence of a chemically shifted component on the lower binding energy side of the substrate peak which is attributed to interface defect states resulting from the incorporation of silicon atoms from the substrate into the interfacial oxide at room temperature. This evidence of Si/SiOx interface disruption would be expected to increase charge carrier scattering mechanisms in the silicon and contribute to the generally observed mobility degradation in high-k stacks with ultrathin silicon oxide interface layers.

In situphotoemission study ofNd1−xSrxMnO3epitaxial thin films

We have performed an in situ photoemission study of ${\text{Nd}}_{1\ensuremath{-}x}{\text{Sr}}_{x}{\text{MnO}}_{3}$ (NSMO) thin films grown on ${\text{SrTiO}}_{3}$ (001) substrates by laser molecular-beam epitaxy. The lattice constants of the thin films were relaxed from those of the substrates, and the transport properties were almost the same as those of bulk NSMO. From core-level photoemission studies, we found that the behavior of the chemical-potential shift was not much different from that of the bulk NSMO samples. In the valence-band spectra, finite intensity at the Fermi level was observed even in the insulating phase. The band dispersions of ${\text{Nd}}_{0.6}{\text{Sr}}_{0.4}{\text{MnO}}_{3}$ obtained by angle-resolved photoemission spectroscopy were almost the same as those of ${\text{La}}_{0.6}{\text{Sr}}_{0.4}{\text{MnO}}_{3}$ thin films. These results showed that NSMO is closely related to ${\text{La}}_{1\ensuremath{-}x}{\text{Sr}}_{x}{\text{MnO}}_{3}$ in terms of the band structure, except for a decrease in the coherent spectral weight.

High-resolution core-level photoemission study of dense Pb overlayers on Si(111)

Structure and bonding configuration of dense Pb overlayers on the Si(111) surface have been studied by low-energy-electron diffraction and high-resolution photoelectron spectroscopy using synchrotron radiation. Several representative phases in its devil's staircase phase diagram have been systematically investigated by varying the Pb coverage at 200--300 K. $\text{Pb}\text{ }5d$ photoelectron spectra indicate that there exist two distinct bonding configurations of Pb, which are interpreted as the hollow and on-top (T1) sites of the structure models proposed earlier. In case of surface Si atoms, mainly two different bonding environments are revealed by surface $\text{Si}\text{ }2p$ components for the low-density $\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ phase. These can be assigned to T1 and modified on-top $(\text{T}{1}^{\ensuremath{'}})$ sites surrounding hollow-site adatoms. As the coverage increases, the minority site T1 converts to $\text{T}{1}^{\ensuremath{'}}$ making the topmost Si layer have a unique bonding configuration. This behavior is also consistent with the structure models. The temperature-dependent study reveals that the $\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ phase undergoes a reversible phase transition into a $1\ifmmode\times\else\texttimes\fi{}1$ phase. This phase transition induces no significant change in Pb core levels but a marginal increase in the $\text{Si}\text{ }2p$ component for the $\text{T}{1}^{\ensuremath{'}}$ sites. We suggest a plausible scenario of the phase transition based on the structure model with 1.2 monolayer Pb and the active diffusion of hollow-site adatoms.

Doping evolution of the electronic structure in the single-layer cuprateBi2Sr2−xLaxCuO6+δ: Comparison with other single-layer cuprates

We have performed angle-resolved photoemission and core-level x-ray photoemission studies of the single-layer cuprate ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{La}}_{x}\mathrm{Cu}{\mathrm{O}}_{6+\ensuremath{\delta}}$ (Bi2201) and revealed the doping evolution of the electronic structure from the lightly doped to optimally doped regions. We have observed the formation of the dispersive quasiparticle band, evolution of the Fermi ``arc'' into the Fermi surface, and the shift of the chemical potential with hole doping as in other cuprates. The doping evolution in Bi2201 is similar to that in ${\mathrm{Ca}}_{2\ensuremath{-}x}{\mathrm{Na}}_{x}\mathrm{Cu}{\mathrm{O}}_{2}{\mathrm{Cl}}_{2}$ (Na-CCOC), where a rapid chemical potential shift toward the lower Hubbard band of the parent insulator has been observed, but is quite different from that in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ (LSCO), where the chemical potential does not shift, yet the dispersive band and the Fermi arc and/or surface are formed around the Fermi level already in the lightly doped region. The (underlying) Fermi surface shape and band dispersions are quantitatively analyzed using tight-binding fit, and the deduced next-nearest-neighbor hopping integral ${t}^{\ensuremath{'}}$ also confirms the similarity to Na-CCOC and the difference from LSCO.

High resolution spectroscopic and microscopic signatures of ordered growth of ferrous sulfate in SO2 assisted corrosion of Fe3O4(100)

The authors present a high-resolution core-level photoemission study of a Fe3O4(100) surface exposed to 50L (1L=10−6mbars) of H2O and 50L of SO2. S 2p core-level spectra reveal the presence of SO3 and SO4 species. An additional peak in the Fe 3p core-level spectrum shows that they bond with iron from the substrate. Complementary scanning tunneling microscopy of the same surface demonstrates formation of a long-range ordered sulfate locked in the (2×2)R45°-surface potential.

Core level photoemission study of nitrided hafnium silicate thin films

We present a synchrotron radiation and Al K α core level study of ultra-thin nitrided hafnium silicate thins films, potential candidate for a high κ oxide to replace SiO 2. Nitriding the as deposited, uniform silicate results in the formation of Si-N and Hf-N bonds, with a nitrogen profile near the silicate/SiO 2 interface. Controlled etching reveals that the nitridation has proceeded beyond the silicate into the SiO 2 bottom oxide, without degrading the sharp silicate/SiO 2 interface. The 1.2 nm Hf 0.4Si 0.6O 2 nitrided silicate on 0.6 nm SiO 2 has an estimated equivalent oxide thickness of 1.1 nm.