High-resolution Core-level Photoelectron 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.

The adsorption of silicon on an iridium surface ruling out silicene growth.

The adsorption of Si atoms on a metal surface might proceed through complex surface processes, whose rate is determined differently by factors such as temperature, Si coverage, and metal cohesive energy. Among other transition metals, iridium is a special case since the Ir(111) surface was reported first, in addition to Ag(111), as being suitable for the epitaxy of silicene monolayers. In this study we followed the adsorption of Si on the Ir(111) surface via high resolution core level photoelectron spectroscopy, starting from the clean metal surface up to a coverage exceeding one monolayer, in a temperature range between 300 and 670 K. Density functional theory calculations were carried out in order to evaluate the stability of the different Si adsorption configurations as a function of the coverage. Results indicate that, at low coverage, the Si adatoms tend to occupy the hollow Ir sites, although a small fraction of them penetrates the first Ir layer. Si penetration of the Ir surface can take place if the energy gained upon Si adsorption is used to displace the Ir surface atoms, rather then being dissipated differently. At a Si coverage of ∼1 monolayer, the Ir 4f spectrum indicates that not only the metal surface but also the layers underneath are perturbed. Our results point out that the Si/Ir(111) interface is unstable towards Si-Ir intermixing, in agreement with the silicide phase formation reported in the literature for the reverted interface.

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.

Non-local Effects on Oxygen-Induced Surface Core Level Shifts of Re(0001)

The electronic structure of clean and oxygen-covered Re(0001) was investigated by combining high energy resolution core level photoelectron spectroscopy and density functional theory. For the clean surface, we detect two distinct Re 4f7/2 components shifted by 95 meV, which we ascribe to photoemission from the bulk and the top layer of the crystal. Oxygen adsorption at room temperature leads to the appearance of new core level components, which we associate to non-equivalent Re atoms from the topmost layer. In addition to the linear relationship between oxygen-induced core level shifts and number of O–Re bonds, we find that the binding energy shift also depends on the number of oxygen atoms at second and third nearest neighbor positions. This non-local effect is discussed in terms of 5d band center shift and of charge transfer between the metallic substrate and the O atoms.

Evaluation of 18 Year-Service Lifetime of NR-SBR Lining

Our recent high resolution core level and valence level X-ray photoelectron spectra (XPS) of Natural-Styrene Butylene Rubber (NR/SBR) demonstrated much narrower linewidths than previously obtained on rubbers or other polymer coatings. This improvement is due to low-current, low-energy beam flood gun charge compensation on non-conductor rubbers. The different charging has compensated on the rough surface and in the bulk of sponge structure in depth 50 Å of rubber sample. Despite residual charging has occurred on the surface which is apparent in C1s and O1s shoulder on its right side of the peak shape. Carbon groups such as C-H, C-C, C=C cannot be easily differentiated by their principal C(1s) peak chemical shift. Carbon black fillers seem to behave quite differently in the rubber-curing reaction. The stability of vulcanized rubber with sulfur curing system was studied in components of C(1s), O(1s), S(2p) regions which represented appropriate estimation of changes of crosslinked with sulfide bonds on rubber surface. The connection between the Shore hardness of rubber and the presence of carbonyl and sulphonyl groups was apparent. A model has been proposed for the degradation of the crosslinking density of rubber after long term exposure.

Stress Conditions for Surface Analysis of Butyl Rubber Lining

An investigation has been conducted into the time-dependent chemical degradation of rubber linings produced from Isobutylene-Isoprene Rubber (IIR). These linings are used for the corrosion protection of steel reservoirs and pipelines within various chemical environments. High resolution core level and valence band X-Ray photoelectron spectra XPS of the degraded copolymer IIR have been obtained with the low-current, low-energy electron beam flood gun charge compensation. The components of region C(1s) and O(1s) have been distinguish in the individual peaks during testing. The Full Width of Half Maximum (FWHM) of curve-fitted components has appeared below 1.6 eV. All results of the C(1s) values have taken the stability of the carbon polymer chain, including the vulcanization agent, into consideration. Direct correspondence for carbon-oxygen systems between the O(2s) at ~27.5 to 24.1 eV and O(1s) at ~535.4 to 530.6 eV binding energies has been described. C(1s) loss peak has appeared at ~21 eV from the principal and it’s loss splitting for C-C and C-H bounds has been confirmed.

Surface core level shifts of clean and oxygen covered Ir(111)

We present the results of high resolution core level photoelectron spectroscopy employed to investigate the electronic structure of clean and oxygen covered Ir(111) surface. Ir 4f7/2 core level spectra are shown to be very sensitive to the local atomic environment. For the clean surface we detected two distinct components shifted by 550 meV, originated by surface and bulk atoms. The larger Gaussian width of the bulk component is explained as due to experimentally unresolved subsurface components. In order to determine the relevance of the phonon contribution we examined the thermal behaviour of the core level lineshape using the Hedin–Rosengren theory. From the phonon-induced spectral broadening we found the Debye temperature of bulk and surface atoms to be 298 and 181 K, respectively, which confirms the softening of the vibrational modes at the surface. Oxygen adsorption leads to the appearance of new surface core level components at −200 meV and +230 meV, which are interpreted as due to first-layer Ir atoms differently coordinated with oxygen. The coverage dependence of these components demonstrates that the oxygen saturation corresponds to 0.38 ML, in good agreement with recent density functional theory calculations.

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).

Characterization of leached layers on olivine and pyroxenes using high-resolution XPS and density functional calculations

High-resolution core level and valence band (VB) X-ray photoelectron spectra (XPS) of olivine [(Mg 0.87Fe 0.13) 2SiO 4], bronzite [(Mg 0.8Fe 0.2) 2Si 2O 6] and diopside [Ca(Mg 0.8Fe 0.2)Si 2O 6] were collected before and after leaching in pH ∼2 solutions with the Kratos magnetic confinement charge compensation system which minimizes differential charge broadening. The leached samples yield Si 2p, Mg 2p, Ca 2p and O 1s XPS spectral linewidths and lineshapes similar to those collected from the respective pristine samples prior to leaching. As with previous XPS studies on crushed samples, our broadscan XPS spectra show evidence for initial, preferential leaching of cations (i.e., Ca 2+ and Mg 2+) from the near-surface of these minerals. The O 1s spectra of leached olivine and pyroxenes show an additional peak due to OH −, which arises from H + exchange with near-surface cations (Ca 2+ and Mg 2+) via electrophilic attack of H + on the M–O–Si moiety to produce the H 2Mg(M1)SiO 4(surf) complex at olivine surfaces, and two complexes, H 2Mg(M1)Si 2O 6(surf) and H 4Si 2O 6(surf) at diopside and enstatite surfaces. The olivine and pyroxene surface complexes H 2Mg(M1)SiO 4(surf) and H 2Mg(M1)Si 2O 6(surf) have been proposed previously, but the second pyroxene surface complex H 4Si 2O 6(surf) has not. Two electrophilic reactions occur in both olivine and pyroxene. For olivine, the more rapid attacks the M2–O–Si moiety producing H 2Mg(M1)SiO 4(surf); while the second attacks the M1–O–Si moiety ultimately producing H 4SiO 4 which is released to solution. For pyroxenes, the first electrophilic reaction produces H 2Mg(M1)Si 2O 6(surf), while the second produces.H 4Si 2O 6(surf). These two reactions are followed by a nucleophilic attack of H 2O (or H 3O +) on Si of H 4Si 2O 6(surf). This reaction is responsible for rupture of the brigding oxygen bond of the Si–O–Si moiety and release of H 4SiO 4 to solution. The intensity of the OH − peak for the leached pyroxenes is about double the OH − intensity for the leached olivine, consistent with the equivalent of about a monolayer of the above surface complexes being formed in all three minerals. Valence band XPS spectra and density functional calculations demonstrate the remarkable insensitivity of the valence band to leaching of Ca 2+ and Mg 2+ from the surface layers. This insensitivity is due to a dearth of Ca and Mg valence electron density in the valence band: the Ca–O and Mg–O bonds are highly ionic, with metal-derived s orbital electrons taking on strong O 2p character. The valence band spectrum of leached olivine shows an additional very weak peak at about 13.5 eV, which is assigned to Si 3s valence orbitals in the surface complex H 2Mg(M1)SiO 4, as indicated by high quality density functional calculations on an olivine where Mg 2+ in M2 is replaced by 2H +. The intensity of this new peak is consistent with formation of the equivalent of a monolayer of the surface complex.