High-resolution Core-level Photoelectron Spectroscopy Research Articles

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.

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.

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.

Alumina-Supported Array of Co Nanoparticles: Size-Dependent Oxidation Kinetics?

The kinetics of oxidation of ordered arrays of Co nanoparticles (NPs) supported on the ultrathin aluminum oxide (AlOx) on a Ni3Al(111) template surface has been investigated at 300 and 470 K and an oxygen pressure of 1 × 10–6 mbar by high-resolution core level photoelectron spectroscopy as a function of the particles’ mean diameter, which was varied between 3 and 7 nm. For both temperatures, the oxidation of the Co NPs starts at their surface, which leads at intermediate oxygen exposures to core–shell NP morphologies and ends with the complete oxidation of the NPs to CoO under the employed oxidation conditions. At 300 K the Co NP oxidation rate is fast and does not depend significantly on the particles size. In contrast, at 470 K an apparent NP size dependence is observed, which is caused by the thermally activated penetration of oxygen through the alumina film and the preferential oxidation of Al atoms at the Ni3Al(111)/AlOx interface. The changes in the Co and Al oxidation rates with the particle size at 470 K have been attributed to the variation of the surface morphology with the Co coverage.

Chemical vapor deposition of ordered TiOx nanostructures on Au(111)

The deposition of TiOx (x≤2) structures on Au(111) by chemical vapor deposition (CVD) in ultrahigh vacuum (UHV) has been investigated with high-resolution core level photoelectron spectroscopy (PES), low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Using titanium tetra-isopropoxide as single source precursor it is possible to form different TiOx phases on the surface after deposition: at low coverages, we observe large two-dimensional (2D) honeycomb-lattice Ti2O3 islands with a (2×2) registry with the substrate. Higher coverages are dominated by the formation of three-dimensional (3D) TiO2 structures. The TiO2 structures are atomically well ordered provided that the deposition temperature is high enough (500°C). The ordered structure exhibits a LEED pattern characteristic for a rectangular surface unit cell. By performing the deposition at different temperatures it is possible to tune the balance between the 2D and 3D phases: Growth at 500°C significantly favors the formation of 3D TiO2 islands as compared to growth at 200°C and 300°C.

Investigation of Corrosion-Inhibiting Aniline Oligomer Thin Films on Iron Using Photoelectron Spectroscopy

Polyaniline (PANI) is capable of inhibiting corrosion on iron by inducing the formation of a passive oxide film. The underlying mechanism however, is unknown. We have used photoelectron emission spectromicroscopy of thin films of a model PANI oligomer to investigate its interaction with the iron oxide film covering the iron surface. The oligomer chosen was a phenyl-capped aniline tetramer (PCAT). Thin undoped films of PCAT in its leucoemeraldine form were prepared by physical vapor deposition to obtain films from ∼1 A to over 10 nm thick. Films were investigated with a photoelectron emission microscope (PEEM) using synchrotron radiation to obtain spatially resolved valence band photoemission spectra. Analysis of PEEM results suggest that PCAT is capable of migrating several micrometers along the substrate surface and causes a decrease in substrate work function wherever present. High-resolution core level and valence band photoelectron spectroscopy using a laboratory-based photon source was used to charac...

A Surface Core Level Shift Study of Hydrogen-Induced Ordered Structures on Rh(110)

The interaction of hydrogen with Rh(110) was studied by means of high-resolution core level photoelectron spectroscopy, in combination with temperature-programmed desorption spectroscopy, low energy electron diffraction, and scanning tunneling microscopy measurements. By exploiting the H-induced Rh3d5/2 surface core level shift sensitivity to the local coordination environment, we propose here new structural models for the intermediate coverage 0.67 and 1.0 ML overlayers, while we confirm the previous results for the low coverage structures at 0.33 and 0.5 ML, and for the saturation phase at 2.0 ML. Surface core level shift values are interpreted on the basis of our previous data about the hydrogen adsorption on the (100) and (111) rhodium surfaces. The proposed structural models are supported by scanning tunneling images of the surface and provide a comprehensive description of all the available experimental data.

Yb-induced(2×3)and(2×4)reconstructions on Si(100) studied by first-principles calculations and high-resolution core-level photoelectron spectroscopy

We report here a combined theoretical and experimental study of $\text{Yb}/\text{Si}(100)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}3)$ and $\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}4)$ reconstructions by means of first-principles calculations and high-resolution core-level photoelectron spectroscopy. Energetically stable atomic structures are presented for these reconstructions. Yb atoms are found to occupy the cave sites in the structures, and the Si substrate is strongly rearranged due to Yb adsorption. It is shown that scanning tunneling microscopy images and surface core-level shifts (SCLSs) calculated for these atomic configurations agree with experimental data, giving further support to the models. In addition, by comparing our theoretical and experimental $\text{Si}\text{ }2p$ results, we discuss and interpret the atomic origins of SCLSs measured for the $\text{YbSi}(100)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}3)/(2\ifmmode\times\else\texttimes\fi{}4)$. Finally, the results presented are helpful in the analysis of $(2\ifmmode\times\else\texttimes\fi{}3)$ and $(2\ifmmode\times\else\texttimes\fi{}4)$ structures induced by other rare earth metals on Si(100).

Spectroscopic evidence for reversible hydrogen storage in unorderedMg5Ni1 thin films

Thin magnesium–nickel films grown on Mo(110) under ultra-high vacuum conditions werehydrogenated with atomic H and studied by high-resolution core-level photoelectronspectroscopy (PES) and thermal desorption spectroscopy (TDS). We show spectroscopicevidence for reversible hydrogen storage and clear evidence that no Mg segregation or desorptionoccurs when hydrogen is desorbed from the film. During the heat-up of the hydrogenatedMg5Ni1 film (0.7 K s−1) TDS shows that hydrogen desorption starts at 350 K and reaches its maximum at 420 K.Mg desorption starts at 450 K. The stability of the storage can be enhanced by cooling.

Understanding the2pcore-level spectra of manganese: Photoelectron spectroscopy experiments and Anderson impurity model calculations

Using high-resolution core-level photoelectron spectroscopy and modified Anderson impurity model calculations, we study the $\mathrm{Mn}\phantom{\rule{0.2em}{0ex}}2p$ spectrum of manganese metal and resolve the current debate about its spectral shape. An unusual satellite feature, $1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ from the main peak, is observed in the $\mathrm{Mn}\phantom{\rule{0.2em}{0ex}}2{p}_{3∕2}$ spectrum of a thick Mn layer grown on Al. It originates from intra-atomic multiplet effect related to Mn atoms with large local moment. The satellite decreases in intensity for thin Mn layers and for Al deposition on bulklike Mn because of enhanced $\mathrm{Mn}\phantom{\rule{0.2em}{0ex}}3d$ hybridization with Al $s,p$ bandlike states. The reason for the absence of a charge-transfer satellite is discussed.

High-resolution core-level photoemission study of Ge(1 1 1)2 × 1–Sb and Ge(1 1 1)(√3 × √3) R30°–Bi reconstructions

Using high-resolution core-level photoelectron spectroscopy with synchrotron radiation, we have studied Ge(1 1 1)2 × 1–Sb and Ge(1 1 1)(√3 × √3) R30°–Bi reconstructions formed at the metal coverage of 1 and 1/3 monolayers, respectively. The spectral line shape analysis shows that for the both surfaces, the Ge 3d emission includes one component, giving evidence for the bulk-terminated structure of the Ge substrate in these phases. The Bi 5d emission from the Ge(1 1 1)(√3 × √3) R30°–Bi surface reveals an intense component related to the Bi atoms at the T4 site in the √3 structure, in agreement with previous ab initio calculations. Another small Bi 5d component is assigned to Bi–Bi bondings in metallic Bi clusters. The Sb 4d spectra of the Ge(1 1 1)(2 × 1)–Sb surface can be reproduced well by a single component, suggesting that the Sb atoms in the Seiwatz chains have similar bonding configuration.

A high-resolution core-level study of Ni-catalyzed absorption and desorption of hydrogen in Mg-films

Ultra-thin magnesium and magnesium–nickel films grown on Mo(1 1 0) under ultra-high vacuum (UHV) conditions are hydrogenated at room temperature with atomic H and studied by high-resolution core-level photoelectron spectroscopy (PES). For the Mg-film a layer of hydride is formed at the Mg/Mo interface and a sub-mono layer of hydride is formed at the surface, while an intermediate region remained non-hydrogenated. For the Mg–Ni-film a thick layer of surface hydride was detected, showing that Ni enhances formation of Mg-hydride at the surface. During the heat-up of the hydrogenated Mg–Ni-film hydrogen desorption started at 400 K.

High-resolution core-level photoemission study of Eu-induced(3×2)∕(3×4)reconstruction on Ge(111)

We have investigated Eu-induced $\mathrm{Ge}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)∕(3\ifmmode\times\else\texttimes\fi{}4)$ reconstruction by high-resolution core-level photoelectron spectroscopy using synchrotron radiation and low-energy electron diffraction. Recent scanning tunneling microscopy (STM) observations [Phys. Rev. B 73, 125332 (2006)] revealed that the Ge arrangement of this reconstruction can be well described in terms of the honeycomb chain-channel (HCC) geometry proposed earlier for metal/$\mathrm{Si}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}1)$ and $\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)$ surfaces; the Eu atoms, however, were found to reside at two different adsorption sites in the $\mathrm{Eu}∕\mathrm{Ge}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)∕(3\ifmmode\times\else\texttimes\fi{}4)$ reconstruction, in contrast to the equivalent adsorption sites (e.g., $T4$) occupied in the case of Si. The present photoemission results provide further information about the atomic arrangement of $\mathrm{Eu}∕\mathrm{Ge}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)∕(3\ifmmode\times\else\texttimes\fi{}4)$. In particular, we show that the Ge $3d$ core-level data cannot be interpreted by the HCC structure with the Eu atoms adsorbed only on $T4$ sites, giving a spectroscopic support for the suggestions based on the earlier STM data. We consider here a modified HCC-based configuration for the $\mathrm{Eu}∕\mathrm{Ge}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)∕(3\ifmmode\times\else\texttimes\fi{}4)$ surface where the Eu atoms occupy two different sites in the empty channel between the neighboring Ge honeycomb chains. The atomic models are discussed in the context of the Ge $3d$ and Eu $4f$ data as well as the previous results available in the literature. Finally, we propose a structural model that allows us to account for the present photoemission and earlier STM findings.

First-principles study of lithium adsorption on Si(1 0 0)2 × 1 and Ge(1 0 0)2 × 1 surface at 1.0 monolayer coverage

First-principles calculations based on density functional theory have been carried out to determine the atomic structure and electronic properties of the Si(1 0 0)2 × 1–Li and Ge(1 0 0)2 × 1–Li adsorption systems at 1.0 monolayer (ML) coverage. For the Si(1 0 0)2 × 1–Li chemisorption system, the lowest energetic configuration is that one Li atom resides at the T3′ (which is displaced along the positive y-axis from the valley bridge (T3) site by 0.90 Å) and the other resides at pedestal (HH) site. For the Ge(1 0 0)2 × 1–Li chemisorption system, the most stable configuration is found to be that one Li atom resides at HH site and the other one will penetrate deeply into the substrate surface. Chemisorption of the lithium atoms is found to result in a minimum energy configuration characterized by symmetric dimers in agreement with the results of high-resolution core-level photoelectron spectroscopy. The adsorption systems show semiconducting surface characterization.