High-resolution Core-level Spectroscopy Research Articles

Thermal- and air- stability of the compositional variants of van der Waals Pt-Telluride thin films probed by high resolution photoemission spectroscopy

The Pt-Te compositional phase diagram consists of at least three different compositional line phases (PtTe2, Pt3Te4, and Pt2Te2) that can be described as layered van der Waals materials. This presents challenges in controlling the composition of ultrathin Pt-telluride 2D materials by physical vapor deposition methods. Here we show by temperature programmed synchrotron photoemission spectroscopy that the different phases have varying thermal stability in vacuum. This enables the synthesis of these materials by preparation of PtTe2 films at low growth temperatures and subsequent vacuum annealing to ∼ 350 °C for Pt3Te4, or ∼ 400 °C for Pt2Te2. Such prepared phases are characterized by high resolution core level spectroscopy to provide reference spectra for these materials. Moreover, the chemical stability of these materials was tested by exposure to oxygen and air. Even after prolonged air exposure only the surface Te layer was modified by oxygen chemical bonds that caused a 3-eV shift to higher binding energy of the Te-3d core levels. However, these oxygen species could be desorbed by vacuum annealing at 280 °C and pristine Pt-telluride samples can be re-established. This shows the excellent chemical stability of these materials, important for practical applications.

Three-dimensional angle-resolved photoemission study of bulk SiGe single crystals

The composition- and structure-dependent electronic band structure of SiGe alloys facilitates a rich variety of its application in various fields. In this study, we performed high-resolution core-level and angle-resolved photoemission spectroscopy (ARPES) of unstrained SiGe single crystals with the (001) surface. Thermally cleaned surfaces of the single crystals of SiGe alloys exhibited double domain (2 × 1) reconstruction, wherein the asymmetric dimer was composed of Ge atoms, and Si atoms were located below the subsurface region. The compositional dependence of three valence bands and their constant energy contours were clearly resolved by three-dimensional ARPES measurements using the high-intensity synchrotron radiation. The valence bands of unstrained SiGe alloys could be well described by the composition-based interpolation of the band parameters of pristine Si and Ge crystals.

Vibrational Fine Structure in C 1s High-Resolution Core-Level Spectra of CO Chemisorbed on Ir(111)

The vibrational features of carbon monoxide (CO) adsorbed on Ir(111) were studied by means of high-resolution core-level spectroscopy. By monitoring the Ir 4f7/2 core level as a function of exposure, we proved that the CO adsorbs on the surface always in on-top sites, in agreement with the results of vibrational spectroscopy techniques and density functional theory studies. The C 1s vibrational splittings measured for the p(√3 × √3)R30° (233.4 ± 0.5 meV) and c(4 × 2√3)rect (231.4 ± 0.4 meV) structures were in good agreement with the Z + 1 model. Despite the very small error bar of the measurements, it was not possibile to reveal any anharmonic contribution to the spectral lineshape. We speculate that the contribution of the unresolved vibrational mode of the frustrated translation or the effect of phonon-mediated interaction with the substrate can account for the observation of this outcome.

Epitaxial growth of massively parallel germanium nanoribbons by segregation through Ag(1 1 0) thin films on Ge(1 1 0)

We investigate the epitaxial growth of low-dimensional nanostructures formed by surface segregation of germanium on single crystalline Ag(110) thin films supported by a Ge(110) template. Detailed studies of the atomic geometries have been carried out using scanning tunneling microscopy, low-energy electron diffraction, Auger electron spectroscopy, and high-resolution synchrotron radiation core-level spectroscopy. At very low Ge coverages the segregated Ge atoms form two kinds of commensurate and incommensurate quantum structures after annealing. With increasing Ge coverages, narrow stripes, 0.5 nm in width and with lengths ranging from 5 nm to 50 nm, are formed. With further increase of the Ge atoms’ segregation on the surface, the narrow stripes transform into massively parallel one-dimensional Ge nanostructures with specific widths. From a combined analysis, these fascinating quantum structures are proposed to be zig-zag germanene nanoribbons, potentially with very exciting electronic, spintronic, and topological properties.

Precise chemical state analyses of ultrathin hafnium films deposited on clean Si(111)-7 × 7 surface using high-resolution core-level photoelectron spectroscopy

Herein, well-defined ultrathin hafnium films on Si(111) -7 × 7 [Hf/Si(111)] were prepared by using electron beam heating in an ultrahigh vacuum (UHV) chamber at a pressure below 1.1 × 10−8 Pa. High-resolution core-level photoelectron spectroscopy of the Si 2p1/2, 3/2 and Hf 4f5/2, 7/2 levels combined with a synchrotron radiation X-ray light source was employed to reveal the chemical states at the interface and surface of ultrathin Hf/Si(111) films. Ultrathin Hf layers grow on clean Si(111)-7 × 7 surfaces by a lever rule that the first phase is not consumed when subsequent phases form. Therefore, it was found that the surface and interface of Hf/Si(111) certainly contains three components, which can be ascribed as metallic Hf layers, Hf monosilicide (HfSi), and Si-rich Hf silicide just above the Si substrate (HfSi4). The metallic Hf layers were observed to comprise some diffused Si atoms. According to the results obtained from photoelectron spectroscopy and scanning electron microscopy, ultrathin Hf layers change into HfSi2 islands on a bare Si(111)-7 × 7 surface after annealing at 1073 K. Small dot-shaped islands of HfSi2 appeared after annealing a two-monolayer thick Hf sample; whereas, rectangular HfSi2 islands including metallic Hf species randomly appeared after annealing a six-monolayer thick Hf sample. The long axes of the rectangle islands expanded in a direction that connected the corner holes in a model with dimers, adatoms, and stacking-faults (DAS model) of a clean Si(111)-7 × 7 surface. Our precise chemical analyses will be very useful for preparing well-defined interfaces and surface structures for obtaining next generation 3D metal-oxide-semiconductor field effect transistors and ferroelectric devices.

Kagome-like silicene: A novel exotic form of two-dimensional epitaxial silicon

Since the discovery of graphene, intensive efforts have been made in search of novel two-dimensional (2D) materials. Decreasing the materials dimensionality to their ultimate thinness is a promising route to unveil new physical phenomena, and potentially improve the performance of devices. Among recent 2D materials, analogs of graphene, the group IV elements have attracted much attention for their unexpected and tunable physical properties. Depending on the growth conditions and substrates, several structures of silicene, germanene, and stanene can be formed. Here, we report the synthesis of a Kagome-like lattice of silicene on aluminum (111) substrates. We provide evidence of such an exotic 2D Si allotrope through scanning tunneling microscopy (STM) observations, high-resolution core-level (CL) and angle-resolved photoelectron spectroscopy (ARPES) measurements, along with Density Functional Theory calculations.

Effect of Ti doping on the crystallography, phase, surface/interface structure and optical band gap of Ga2O3 thin films

The effect of titanium (Ti) doping on the crystal structure, phase, surface/interface chemistry, microstructure and optical band gap of gallium oxide (Ga2O3) (GTO) films is reported. The Ti content was varied from 0 to ~ 5 at% in co-sputtering, using Ga2O3 ceramic and Ti metal targets, deposited GTO films produced. The sputtering power to the Ti target was varied in the range of 0–100 W, while keeping the sputtering power to Ga2O3 constant at 100 W, to produce GTO films with 0–5 at% Ti. The Ti-incorporation-induced effects were significant for the crystal structure, phase, surface/interface chemistry and morphology, which in turn induce changes in the band gap. The high-resolution core-level X-ray photoelectron spectroscopy (XPS) analyses confirm that the Ga ions exist as Ga3+ in both intrinsic Ga oxide and GTO films. However, XPS data reveal the formation of Ga2O3–TiO2 films with the presence of Ti4+ ions with increasing Ti sputtering power, i.e., higher Ti contents in GTO. Evidence for the formation of nanocrystalline Ga2O3–TiO2 films was also found in the structural analyses performed using electron microscopy and grazing incidence X-ray diffraction. Significant band gap reduction (Eg ~ 0.9 eV) occurs in GTO films with increasing Ti dopant concentration from 0 to 5 at%. A correlation between the Ti dopant concentration, surface/interface chemistry, microstructure and band gap of GTO films is established.

Layer-by-Layer Graphene Growth on β-SiC/Si(001).

The mechanism of few-layer graphene growth on the technologically relevant cubic-SiC/Si(001) substrate is uncovered using high-resolution core-level and angle-resolved photoelectron spectroscopy, low-energy electron microscopy, and microspot low-energy electron diffraction. The thickness of the graphitic overlayer supported on the silicon carbide substrate and related changes in the surface structure are precisely controlled by monitoring the progress of the surface graphitization in situ during high-temperature graphene synthesis, using a combination of microspectroscopic techniques. The experimental data reveal gradual changes in the preferential graphene lattice orientations at the initial stages of the few-layer graphene growth on SiC(001) and can act as reference data for controllable growth of single-, double-, and triple-layer graphene on silicon carbide substrates.

Peculiar bonding associated with atomic doping and hidden honeycombs in borophene

Engineering atomic-scale structures allows great manipulation of physical properties and chemical processes for advanced technology. We show that the B atoms deployed at the centers of honeycombs in boron sheets, borophene, behave as nearly perfect electron donors for filling the graphitic $\sigma$ bonding states without forming additional in-plane bonds by first-principles calculations. The dilute electron density distribution owing to the weak bonding surrounding the center atoms provides easier atomic-scale engineering and is highly tunable via in-plane strain, promising for practical applications, such as modulating the extraordinarily high thermal conductance that exceeds the reported value in graphene. The hidden honeycomb bonding structure suggests an unusual energy sequence of core electrons that has been verified by our high-resolution core-level photoelectron spectroscopy measurements. With the experimental and theoretical evidence, we demonstrate that borophene exhibits a peculiar bonding structure and is distinctive among two-dimensional materials.

Key role of rotated domains in oxygen intercalation at graphene on Ni(1 1 1)

Herein, we provide a complete description of the intercalation of oxygen at the strongly interacting graphene on Ni(1 1 1), highlighting the role of rotated graphene domains in triggering the intercalation. High-resolution core-level photoelectron spectroscopy provided a full characterization of the interface at each stage of the intercalation, revealing the formation of an oxide layer between graphene and the metal substrate. Angle-resolved photoemission spectroscopy measurements showed that the oxide decouples efficiently graphene from the substrate, restoring the Dirac cone and providing a slight n-doping. Photoelectron diffraction experiments revealed that graphene domains not aligned with the Ni substrate are the first to be intercalated with oxygen and are preferential regions under which the oxygen is retained during the deintercalation.

Modification of thermal and electronic properties of bilayer graphene by using slow Na+ ions

Bilayer graphene (BLG) has an extensive list of industrial applications in graphene-based nanodevices such as energy storage devices, flexible displays, and thermoelectric devices. By doping slow Na+ ions on Li-intercalated BLG, we find significantly improved thermal and electronic properties of BLG by using angle-resolved photoemission and high-resolution core level spectroscopy (HRCLS) with synchrotron photons. Our HRCLS data reveal that the adsorbed Na+ ions on a BLG produced by Li-intercalation through single layer graphene (SLG) spontaneously intercalate below the BLG, and substitute Li atoms to form Na-Si bonds at the SiC interface while preserving the same phase of BLG. This is in sharp contrast with no intercalation of Na+ ions on SLG though neutral Na atoms intercalate. The Na+-induced BLG is found to be stable upon heating up to T = 400 °C, but returns to SLG when heated at Td = 500 °C. The evolution of the π-bands upon doping the Na+ ions followed by thermal annealing shows that the carrier concentration of the π-band may be artificially controlled without damaging the Dirac nature of the π-electrons. The doubled desorption temperature from that (Td = 250 °C) of the Na-intercalated SLG together with the electronic stability of the Na+-intercalated BLG may find more practical and effective applications in advancing graphene-based thermoelectric devices and anode materials for rechargeable batteries.

Adsorption of hydrogen on stable and metastable Ir(100) surfaces

Using the combination of high resolution core level spectroscopy and density functional theory we present a detailed spectroscopic study for all clean and hydrogen covered phases of Ir(100). The results are complemented by an investigation of the hydrogen desorption process from various phases using temperature programmed desorption spectroscopy and scanning tunneling microscopy. In total, all experimentally determined core level shifts match very well with those predicted by density functional theory based on established structural models. In particular, we find for the (bridge site) adsorption on the unreconstructed 1×1 phase that the initial core level shift of surface Ir atoms is altered by +0.17eV for each Ir–H bond formed. In the submonolayer regime we find evidence for island formation at low temperatures. For the H-induced deconstructed 5×1-H phase we identify four different surface core level shifts with two of them being degenerate. Finally, for the reconstructed 5×1-hex phase also four surface components are identified, which undergo a rather rigid core level shift of +0.15eV upon hydrogen adsorption suggesting a similarly homogeneous charge transfer to all Ir surface atoms. Thermodesorption experiments for the 5×1-H phase reveal two different binding states for hydrogen independent of the total coverage. We conclude that the surface always separates into patches of fully covered deconstructed and uncovered reconstructed phases. We could also show by tunneling microscopy that with the desorption of the last hydrogen atom from the deconstructed unit cell the surface instantaneously reverts into the reconstructed state. Eventually, we could determine the saturation coverage upon molecular adsorption for all phases to be θmax1×1−H=1.0ML, θmax5×1−H=0.8ML, and θmax5×1−hex−H≥1.0ML.

High resolution core level spectroscopy of hydrogen-terminated (1 0 0) diamond

Synchrotron-based photoelectron spectroscopy experiments are presented that address a long standing inconsistency in the treatment of the C1s core level of hydrogen terminated (1 0 0) diamond. Through a comparison of surface and bulk sensitive measurements we show that there is a surface related core level component to lower binding energy of the bulk diamond component; this component has a chemical shift of eV which has been attributed to carbon atoms which are part of the hydrogen termination. Additionally, our results indicate that the asymmetry of the hydrogen terminated (1 0 0) diamond C1s core level is an intrinsic aspect of the bulk diamond peak which we have attributed to sub-surface carbon layers.

A nitride-based epitaxial surface layer formed by ammonia treatment of silicene-terminated ZrB2.

We present a method for the formation of an epitaxial surface layer involving B, N, and Si atoms on a ZrB2(0001) thin film on Si(111). It has the potential to be an insulating growth template for 2D semiconductors. The chemical reaction of NH3 molecules with the silicene-terminated ZrB2 surface was characterized by synchrotron-based, high-resolution core-level photoelectron spectroscopy and low-energy electron diffraction. In particular, the dissociative chemisorption of NH3 at 400 °C leads to surface nitridation, and subsequent annealing up to 830 °C results in a solid phase reaction with the ZrB2 subsurface layers. In this way, a new nitride-based epitaxial surface layer is formed with hexagonal symmetry and a single in-plane crystal orientation.

Combustion synthesis of Mg–Er ferrite nanoparticles: Cation distribution and structural, optical, and magnetic properties

Rare earth ion (Er3+)-doped magnesium ferrite nanoparticles of basic composition MgFe2−xErxO4 (x=0, 0.02, 0.04, and 0.06) were synthesized for the first time by a combustion method with use of glycine as fuel. The synthesized nanoparticles were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, UV-diffuse reflectance spectroscopy, and vibrating sample magnetometer analysis in order to study the structural, compositional, morphological, and magnetic changes with addition of dopant. The X-ray diffraction pattern revealed a single phase with cubic spinel structure, and from the Scherrer formula and the Williamson–Hall formula, the average grain sizes ranged from 35 to 56nm and from 31 to 54nm, respectively. The lattice parameter (a) increases with the increase of the Er3+ concentration x in the lattice. The cation distributions among the tetrahedral (A) and octahedral (B) sites of spinel-type Er-doped magnesium ferrites were also investigated. The Fourier transform infrared spectroscopy spectra of synthesized samples illustrate that the higher-frequency bands lying in the range from 550 to 620cm−1 and the lower-frequency bands lying in the range from 410 to 450cm−1 are associated with the asymmetric stretching modes of the AB2O4 type of spinel transition metal oxides. Information about the chemical elements and oxidation states of the samples was obtained from high-resolution core-level X-ray photoelectron spectroscopy spectra of Mg 1s, Fe 2p, Er 4d, and O 1s. Further information about the morphology of the nanoparticles was obtained by scanning electron microscopy. From UV-diffuse reflectance spectroscopy studies, the optical band gaps were found to range from 1.81 to 1.96eV. The magnetic hysteresis curves clearly indicate the soft ferromagnetic nature of the samples. Various magnetic properties such as saturation magnetization, coercivity, and remanent magnetization obtained from M–H loops were observed to increase with Er3+ substitution.

Carrier-Density Control of the SrTiO3 (001) Surface 2D Electron Gas studied by ARPES.

The origin of the 2D electron gas (2DEG)stabilized at the bare surface of SrTiO3 (001) is investigated. Using high-resolution angle-resolved photoemission and core-level spectroscopy, it is shown conclusively that this 2DEG arises from light-induced oxygen vacancies. The dominant mechanism driving vacancy formation is identified, allowing unprecedented control over the 2DEG carrier density.

On the feasibility of silicene encapsulation by AlN deposited using an atomic layer deposition process.

Since epitaxial silicene is not chemically inert under ambient conditions, its application in devices and the ex-situ characterization outside of ultrahigh vacuum environments require the use of an insulating capping layer. Here, we report on a study of the feasibility of encapsulating epitaxial silicene on ZrB2(0001) thin films grown on Si(111) substrates by aluminum nitride (AlN) deposited using trimethylaluminum (TMA) and ammonia (NH3) precursors. By in-situ high-resolution core-level photoelectron spectroscopy, the chemical modifications of the surface due to subsequent exposure to TMA and NH3 molecules, at temperatures of 300 °C and 400 °C, respectively, have been investigated. While an AlN-related layer can indeed be grown, silicene reacts strongly with both precursor molecules resulting in the formation of Si-C and Si-N bonds such that the use of these precursors does not allow for the protective AlN encapsulation that leaves the electronic properties of silicene intact.

Toward a Silver–Alumina Model System for NOx Reduction Catalysis

The growth and morphology of Ag deposited on NiAl(110) and on oxidized NiAl(110) have been investigated by a combination of scanning tunneling microscopy (STM) and high-resolution core-level spectroscopy (HRCLS). While the STM measurements reveal complete wetting and a bilayer growth on clean NiAl(110), Ag nanoparticles with a minimum size of 5 nm were obtained on the oxidized NiAl(110). The difference in Ag growth mode on clean and oxidized NiAl(110) is supported by Ag 3d HRCLS. The binding energy for Ag on clean NiAl(110) is the same as for bulk Ag, while the Ag 3d peak for particles on oxidized NiAl(110) shifts toward the bulk binding energy with increasing size. The adsorption properties at 100 K of CO and NO on oxidized NiAl(110) and on Ag particles on oxidized NiAl(110) were also investigated by probing the C 1s and N 1s core levels. In the case of oxidized NiAl(110), neither CO nor NO adsorbs. In the case of Ag particles on oxidized NiAl(110), CO does not adsorb, but a component at 397 eV is observed in the N 1s level after NO exposures. This component is tentatively assigned to silver nitride, suggesting NO dissociation in the presence of Ag particles on oxidized NiAl(110).

Role of resonant inelastic X-ray scattering in high-resolution core-level spectroscopy of actinide materials

This paper provides a brief overview of applications of advanced X-ray spectroscopic techniques that take advantage of the resonant inelastic X-ray scattering (RIXS) in the hard and tender X-ray range and have recently become available for studying the electronic structure of actinides. We focus here on the high-energy-resolution X-ray absorption near edge structure (XANES) and core-to-core and core-to-valence RIXS spectroscopies at the U L and M edges of uranium compounds. The spectral features are analyzed using a number of theoretical methods, such as the Anderson impurity model, density functional theory in the local density approximation with an added Coulomb interaction (LDA+U), and full multiple scattering (FEFF) and ab initio finite difference method near-edge structure (FDMNES) codes. In connection with presented results, the capabilities and limitations of the experimental techniques and theoretical methods are discussed.

CO Adsorption on Clean and Oxidized Pd(111)

The adsorption of CO on clean and oxidized Pd(111) surfaces has been investigated using a combination of high-resolution core level spectroscopy (HRCLS), reflection absorption infrared spectroscopy (RAIRS), and density functional theory (DFT) calculations. The HRCLS and RAIRS measurements reveal that CO adsorbs on Pd(111), Pd(5)O4 and PdO(101) at 100 +/- 10 K and that the CO coverage decreases with increasing oxidation state of Pd for the same CO exposures of 10 Langmuirs. Based on the DFT calculations, the CO layer on clean Pd(111) was found to include molecular adsorption in both hollow and bridge sites, whereas CO occupies a combination of bridge and atop sites on the Pd5O4 and PdO(101) surfaces.