4f Spectra Research Articles

Ytterbium oxide formation at the graphene–SiC interface studied by photoemission

Synchrotron-based core level and angle resolved photoemission spectroscopy was used to study the formation of ytterbium (Yb) oxide at the graphene–SiC substrate interface. Oxide formation at the interface was accomplished in two steps, first intercalation of Yb into the interface region and then oxygen exposure while heating the sample at 260 °C to oxidize the Yb. After these processes, core level results revealed the formation of Yb oxide at the interface. The Yb 4f spectrum showed upon oxidation a clear valence change from Yb2+ to Yb3+. After oxidation the spectrum was dominated by emission from oxide related Yb3+ states and only a small contribution from silicide Yb2+ states remained. In addition, the very similar changes observed in the oxide related components identified in the Si 2p and Yb 4f spectra after oxidation and after subsequent heating suggested formation of a Si-Yb-O silicate at the interface. The electronic band structure of graphene around the K¯-point was upon Yb intercalation found to transform from a single π band to two π bands. After Yb oxide formation, an additional third π band was found to appear. These π bands showed different locations of the Dirac point (ED), i.e., two upper bands with ED around 0.4 eV and a lower band with ED at about 1.5 eV below the Fermi level. The appearance of three π-bands is attributed to a mixture of areas with Yb oxide and Yb silicide at the interface.

Ytterbium Intercalation of Epitaxial Graphene Grown on Si-Face SiC

The rare-earth metal, ytterbium (Yb), was deposited on graphene grown on Si-face SiC, kept at room temperature. Yb was not found to intercalate, destroy or dope the graphene layer before subsequent heating, unlike alkali metals such as Li and Na. Our photoemission results reveal that heating to 300?C promotes Yb intercalation into the graphene-substrate interface. Real-time low energy electron microscopy (LEEM) measurements indicated intercalation to start at a sample temperature of around 220oC. In the intercalation process, Yb penetrates through the graphene and buffer layer and forms bonds to the silicon in the topmost SiC substrate bilayer, as indicated by the shifted components observed in the C 1s, Si 2p, and Yb 4f spectra. The Yb intercalation decouples the buffer layer from the substrate and transforms it into another graphene layer as manifested by the absence of buffer layer spots in the μ-LEED pattern and the appearance of an additional π band in the ARPES spectra, respectively. Moreover, the observed shift of the Dirac point down from the Fermi level by 1.9 eV indicates electron doping of the graphene layer upon Yb intercalation. The Yb intercalated graphene sample was found to be thermodynamically stable up to temperatures around 700oC.

Effect of anions in Au complexes on doping and degradation of graphene

In this study, the role of anions in Au complexes was investigated as a dopant for graphene. Au(OH)3, Au2S, AuBr3, and AuCl3 were used as dopants. The average sheet resistance of the graphene decreased from 950 Ω sq−1 to 820, 600, 530, and 300 Ω sq−1 and the work function increased from 4.3 eV to 4.6, 4.8, 5.0 and 4.9 eV with Au(OH)3, Au2S, AuBr3, and AuCl3 dopants, respectively. However, thermal annealing of graphene increased the sheet resistance and decreased the work function. Furthermore, the G and 2D bands in the Raman spectra were recovered and the Au3+ peak in the Au 4f spectra decreased after thermal annealing, indicating the aggregation of metal particles. These results suggested that the degree of doping was related to the electronegativity of the anion in the Au complex and degradation of graphene was related to the bond strength between the Au cation and the counter anion. Therefore, anions with a high electronegativity and high bond strength are adequate for use as a p-type dopant in graphene.

Interface layer in hafnia/Si films as a function of ALD cycles

A systematic angle resolved x-ray photoelectron spectroscopy study of the structure of hafnia films grown on silicon with atomic layer deposition (ALD) is presented. The hafnium precursor employed was tetrakis dimethyl amino hafnium with water as the oxidant agent. The number of ALD cycles ranged from 3 to 25. The Hf 4f spectrum shows two components 0.47 eV apart, one associated with hafnia (17.6 eV) and the other (18.1 eV) with a hafnium silicate interface layer. The composition of the interface layer, HfxSi1-xOy, evolves continuously from silicon-rich (x = 0.1 for 3 ALD cycles) to hafnium rich (x = 0.75 for 25 ALD cycles). The binding energy difference between the Hf 4f components associated with the silicate and hafnia changes very little with the silicate composition. The binding energy of the Si4+ component, which is associated with the silicate, varies from 3.25 to 2.85 eV (referenced to the Si0+ component) with the number of ALD cycles. The oxygen stoichiometric coefficient of the silicate is close to the expected value of 2 for all the samples. However, the hafnia shows an excess of oxygen for some of the samples.

Platinum oxidation responsible for degradation of platinum–cobalt alloy cathode catalysts for polymer electrolyte fuel cells

Platinum oxidation of Pt–Co alloy catalysts for polymer electrolyte fuel cells was investigated for a series of Pt–Co alloy catalysts with different specification. The chemical state of platinum evaluated by soft X-ray photoemission spectroscopy was compared with the electrochemical properties to elucidate the origin of catalyst degradation. Increase in the particle size of Pt–Co alloy catalysts caused the decrease in the concentration of platinum hydroxide and improved the catalyst durability. Applying potential cycling below 1.0 V, only platinum hydroxide was observed, while platinum oxides, PtO and PtO2, appeared after potential cycling up to 1.2 V. The peak shift of Pt 4f spectra after the potential cycling implies that these platinum hydroxide and oxide are dissolved and deposited on another platinum catalyst in a reduced metallic state, which causes the catalyst degradation.

Evidence for an active oxygen species on Au/TiO 2(1 1 0) model catalysts during investigation with in situ X-ray photoelectron spectroscopy

The influence of oxygen (O 2) and carbon monoxide (CO) on Au nanoparticles supported on TiO 2(1 1 0) in the size range of 2–3 nm has been studied using X-ray photoelectron spectroscopy (XPS) and in situ (high pressure) XPS at 300 K for O 2 and/or CO pressures of 0.1–1 mbar. These experiments were aimed at revisiting Au 4f core level shifts as reported in the literature and most importantly, to establish the dependence of the core-level shifts on the knowledge that there exists a maximum in reactivity for CO oxidation. Two samples were prepared with a coverage corresponding to that maximum (Au coverage 0.14–0.2 ML, particle size estimated to ∼2–2.5 nm) while a third sample was expected to be less reactive (Au coverage 0.4 ML, particle size estimated to ∼3.3 nm). At elevated O 2 pressures, a new Au 4f component at higher binding energy (2.4–2.6 eV relative to the Au(0) bulk signal) evolved at all particle sizes. Its appearance was attributed to a radiation-induced activation of oxygen and simultaneous oxidation of gold. The activation was much more efficient on the ∼2–2.5 nm particles. The relative intensity of the oxide component depended strongly on O 2 pressure and, thus, on the equilibrium coverage of O 2. While not present in 0.1 mbar O 2 regardless of exposure time and particle size, it dominated the Au 4f spectrum of particles ∼2–2.5 nm in size at 1 mbar oxygen pressure. This pressure-dependent formation reconciles previously conflicting XPS data. Finally, the activated oxygen species were very reactive toward CO as manifested by the rapid disappearance of the new Au 4f component in a 1:1 mixture of CO and O 2. The rates of evolution and consumption of this component were found to depend on gold coverage (and thus, particle size) and were highest for the smaller particles.

Study of High-κ/In0.53Ga0.47As Interface by Hard X-ray Photoemission Spectroscopy

We have investigated the effect of La2O3 interlayer insertion on the thermal stability of a high-κ/In0.53Ga0.47As interface and the chemical bonding states at the high-κ/In0.53Ga0.47As interface by hard X-ray (hν= 7.94 keV) photoemission spectroscopy (HX-PES). The control of the oxide formation at the HfO2/In0.53Ga0.47As interface was tried by inserting La2O3, which has a large Gibbs free energy. Analyses of As 2p, Ga 2p, In 3d, Hf 3d, La 3d, and W 4f spectra show that the oxidation of In0.53Ga0.47As was suppressed by La2O3 interlayer insertion. We have also investigated the effect of surface treatment on the chemical bonding state of the In0.53Ga0.47As surface. (NH4)2S treatment can suppress the oxidation of the In0.53Ga0.47As surface.

Investigation of HfO2 high-k dielectrics electronic structure on SiO2/Si substrate by x-ray photoelectron spectroscopy

In this Letter, x-ray photoelectron spectroscopy (XPS) was used to investigate electronic structures of HfO2 with various thicknesses on SiO2/Si substrate prepared by atomic layer deposition with post deposition annealing of 600 °C, which is compatible with a gate last process in the fabrication of complementary metal oxide semiconductor. The hafnium silicate formed between HfO2 and SiO2 interface plays a key role in generating an internal electric field. This can be attributed to the presence of oxygen vacancies (Vo) at the interface. The XPS binding energy shifts of O 1s, Si 2p, and Hf 4f spectra with increasing HfO2 thicknesses can be explained by the presence of the internal field and differential charging effects. Electrical measurements of capacitor structures support the XPS result.

Sorption of Phenyl Mercury, Methyl Mercury, and Inorganic Mercury onto Chitosan and Barbital Immobilized Chitosan: Spectroscopic, Potentiometric, Kinetic, Equilibrium, and Selective Desorption Studies

The objective of the study was to elucidate the binding of Hg2+, methyl mercury, and phenyl mercury on chitosan and barbital-immobilized chitosan by potentiometric titration and XPS and FTIR spectroscopy. Adsorption experiments were conducted at room temperature (30 °C) to optimize the percentage removal of different forms of mercury, and the data were fitted to Langmuir and Freundlich adsorption isotherms. Speciation of inorganic, methyl, and phenyl mercury was achieved by sequentially eluting inorganic mercury using 0.1 N perchloric acid followed by methyl mercury using 0.015 N NaCl and finally phenyl mercury using 0.05 N EDTA. Potentiometric titration data and a pKa spectrum approach showed the presence of self-assembled amine/protonated amine, amine, and hydroxyl/amine functional groups. The total number of functional groups was found to be 158.05 mol·g−1 and 68.6 mol·g−1 for barbital-immobilized chitosan and chitosan, respectively, suggesting that a larger number of binding sites are available in barbital-immobilized chitosan for bonding to mercury. FTIR, pH, and XPS studies show that the possible functional groups in the metal binding include amino functional groups in the case of chitosan and amide groups in the case of barbital-immobilized chitosan. The peak areas of XPS Hg 4f spectra show greater adsorption for Hg using barbital-immobilized chitosan as adsorbent compared to chitosan.

XPS study of early stages of Al/Au interface formation

AbstractThe interface formation between Au polycrystalline substrate kept at temperature of liquid nitrogen and Al deposit and its evolution during sample thermal treatments have been investigated by means of XPS. The intermixing of the Al and Au atoms at the interface occurred readily already at temperature of liquid nitrogen and resulted in the appearance of well‐resolved components on the high‐binding‐energy side of the Au(4f) spectra assignable to AlAu intermetallic phase. The sample annealing resulted in the growth of the amount of intermetallic compound formed at liquid nitrogen temperature. The formation of the initial AlAu phase at the Al/Au interface in our experimental conditions can be explained on the basis of its thermodynamic stability. Copyright © 2009 John Wiley & Sons, Ltd.

Vertical alignment of liquid crystals on a fully oxidized HfO2 surface by ion bombardment

High-performance liquid crystals (LCs) driven at a 0.9 V threshold were demonstrated on very thin HfO2 films with vertical (homeotropic) alignment by ion bombardment. Atomic layer deposition was used to obtain LC orientation on ultrathin high-quality films of double-layer HfO2/Al2O3. X-ray photoelectron spectroscopy indicated that full oxidization of HfO2 film surfaces was induced by ion bombardment, shifting the Hf 4f spectra to lower binding energies. The increased intensities of the Hf 4f peaks after ion bombardment confirmed that nonstoichiometric HfOx was converted to the fully oxidized HfO2 surfaces.

A photoemission study of the ceria and Au-doped ceria/Cu(111) interfaces

The photoelectron spectroscopy (PES) study compares electronic states in three different ceria thin film surfaces prepared on a Cu(111) single-crystal surface by vapour deposition under different conditions: at 250 °C, at room temperature and finally Au-doped ceria film obtained by simultaneous deposition of Au and CeO 2 at room temperature (RT). Electronic properties of the layers and interaction of gold with CeO 2 were investigated using synchrotron-radiation-excited PES and resonant photoemission (Ce 4d → 4f transitions). We observed partial Ce 4+ → Ce 3+ reduction induced by the decrease of deposition temperature to RT instead of 250 °C and also by doping ceria with gold, accompanied by a 4f resonance enhancement of the Ce 3+ species. In the case of the Au-doped sample the surface reduction degree is stronger and can be explained by the possible creation of a new ionic Au + state observed in the Au 4f spectra.

Temperature‐dependent hard X‐ray photoemission spectra of ternary Tl compounds with high Seebeck coefficient

AbstractThe temperature dependence of the core‐level and valence‐band electronic structures of TlGaTe2 and TlInSe2 that exhibits high values of Seebeck coefficient has been studied by hard X‐ray photoemission spectroscopy over the temperature range 40–450 K. The relative peak position and peak width for Tl 4f, Ga 2p and Te 3d in TlGaTe2 are determined. It is shown that not only chemical shift defying the peak position but also electron‐phonon interaction responsible for temperature line‐broadening has rather peculiar temperature behaviour that reflects incommensurate phase transition. Thermoelectric power of TlGaTe2 is evaluated and found to be very close to that of TlInSe2. It is shown that Tl 4f spectra of both compounds also display close similarity (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic structures and Hall effect in low-doped La0.9Hf0.1MnO3 epitaxial films

The electronic structures of low-doped epitaxial La0.9Hf0.1MnO3 (LHMO) thin films are investigated by x-ray photoemission spectroscopy (XPS) for the first time. XPS spectra of core levels (La 3d, Hf 4f, O 1s, Mn 2p, and Mn 3s) are taken from the cleaned LHMO film surface. Hf 4f spectrum exhibits a typical binding energy (BE=2 eV) of Hf4+. The splitting energy of La 3d core-lever spectrum agrees with the previous reports of other doped LaMnO3 and suggests a trivalent state in the LHMO film. The calculated result of LHMO nominal composition, the shape of Mn 2p spectrum, the separated BE (111.1 eV) between O 1s and Mn 2p2/3 peaks and the splitting energy (6.0 eV) of Mn 3s all reveal that the LHMO compound is in a mixed valence state of Mn2+ and Mn3+, implying an electron-doped conduction mechanism. The magnetic-field dependence of Hall resistivity further confirmed that the carriers in LHMO are electrons.

High energy photoelectron spectroscopy of correlated electron systems and recoil effects in photoelectron emission

The bulk sensitivity of the high energy photoelectron spectroscopy (PES) in the soft and hard X-ray regions is gradually recognized to be essential for studying the electronic structures of strongly correlated electron systems with different surface and bulk electronic structures. With increasing hν, the kinetic energy (E K ) of photoelectrons is increased with realizing longer inelastic mean free path. Studies of three-dimensional genuine bulk band dispersions and bulk Fermiology are only possible by the soft X-ray angle resolved PES (ARPES). Examples are given for Nd 2 - x Ce x CuO 4 and CeRu 2 X 2 (X: Si, Ge). The temperature dependence of the Yb 4f spectra provides useful information on valence fluctuation and Kondo resonance. The applicability of the single impurity Anderson model (SIAM) and its limit is discussed in the examples of Yb 1 - x Lu x Al 3 and YbAl 3 . The importance of the bulk sensitivity is also demonstrated by the soft and hard X-ray PES for SmOs 4 Sb 12 . When E K becomes very large, the nucleus (ion) recoil effects become observable on the core photoelectron emission from light elements. The example is shown for Yb 7/8 Lu 1/8 B 12 and the influence of the recoil effects for the future high energy ARPES is briefly discussed.

XPS spectra of uranyl minerals and synthetic uranyl compounds. I: The U 4f spectrum

The occurrence and binding energies of the U 6+, U 5+ and U 4+ bands in the U 4f 7/2 peak of 19 uranyl minerals of different composition and structure were measured by XPS. The results suggest that these minerals can be divided into the following four groups: (1) Uranyl-hydroxy-hydrate compounds with no or monovalent interstitial cations; (2) Uranyl-hydroxy-hydrate minerals with divalent interstitial cations; (3) Uranyl-oxysalt minerals with ( TO n ) groups ( T = Si, P, and C) in which all equatorial O-atoms of the uranyl-polyhedra are shared with ( TO n ) groups; (4) Uranyl-oxysalt minerals with ( TO n ) groups ( T = S and Se), in which some equatorial O-atoms are shared only between uranyl polyhedra. The average binding energies of the U 6+and U 4+ bands shift to lower values with (1) incorporation of divalent cations and (2) increase in the Lewis basicity of the anion group bonded to U. The first observation is a consequence of an increase in the bond-valence transfer from the interstitial species (cations, H 2O) groups to the O-atoms of the uranyl-groups, which results in an electron transfer from O to U 6+. The second trend correlates with an increase in the covalency of the U O bonds with increase in Lewis basicity of the anion group, which results in a shift of the electron density from O to U. The presence of U 4+ on the surface of uranyl minerals can be detected by the shape of the U 4f 7/2 peak, and the occurrence of the U 5f peak and satellite peaks belonging to the U 4f 5/2 peak. The presence of U 4+ in some of the uranyl minerals and synthetics examined may be related to the conditions during their formation. A charge-balance mechanism is proposed for the incorporation of lower-valence U in the structure of uranyl minerals. Exposure of a Na-substituted metaschoepite crystal in air and to Ultra-High Vacuum results in dehydration of its surface structure associated with a shift of the U 6+ bands to higher binding energies. The latter observation indicates a shift in electron density from U to O, which must be related to structural changes inside the upper surface layers of Na-substituted metaschoepite.

Preparation and Evaluation of WO3-Bi2O3 Composite Thin Films

Tungsten oxide/bismuth oxide composite thin films prepared by multi target radio frequency (RF) reactive sputtering were investigated. X-ray diffraction measurements of the crystallinity of thin films prepared at room temperature revealed amorphous structure. Although the compositional ratio of composite thin films prepared at 50 W and 75 W of the RF power of the Bi target was almost the same, those prepared at 50 W were transparent, and had a much smaller optical absorption coefficient at 3.0 eV than those prepared at 75 W. Bi 4f spectra of these thin films were measured by X-ray photoelectron spectroscopy, and were analyzed by full width at half maximum of Bi 4f7/2 peaks. As a result, the transparency is thought to be caused by the change of bonding state of Bi-O. Electrochromic properties of the composite thin films were also measured in 0.1 M H2SO4. Coloration of composite thin films prepared at 20 W of the RF power of the Bi target changed reversibly from transparent to black and brown.

Electrical properties of AuN thin films

Gold nitride (AuN) is a recently synthesized component and is being studied for properties like optical, electrical and mechanical. Plasma-assisted physical vapor deposition (PAPVD) in pulsed arc system is used for deposition of AuN thin film. The system is formed of a reactor in which there are two faced electrodes, and a power-controlled system that performs the discharge systematically. Chemical analyses were realized by x-ray photoelectron spectroscopy (XPS) technique, and narrow N 1s and Au 4f spectra are shown using film stoichiometry, and I–V curves were obtained in two ways (substrate–film and film–substrate), to observe the electrical properties.

In Situ X-ray Photoelectron Spectroscopy of Catalytic Ammonia Oxidation over a Pt(533) Surface

The NH3 + O2 reaction on a Pt(533) surface has been studied in the 10−4 mbar range and close to 1 mbar pressure with in situ X-ray photoelectron spectroscopy using synchrotron radiation. The coverages of the various O- and N-containing surface species have been followed in T-cycling experiments with varying mixing ratios O2/NH3 and varying total pressure. In heating/cooling cycles hysteresis of ∼50−100 K width occur. Adsorbed NOad already decomposes at T > 350 K. Under stationary conditions, no adsorbed NO could be detected. At no time during the experiments were Pt bulk oxides formed. A shift in the surface core level component of the Pt 4f spectrum by more than 0.5 eV toward higher binding energy is attributed to Pt atoms of the (100) step edges which are coordinated to more than one oxygen atom similar to the model proposed by Wang et al. Phys. Rev. Lett. 2005, 95, 256102.

XPS studies of the intermediate valence state of Yb in (YbS) 1.25CrS 2

This paper presents clear evidences for valence fluctuation of Yb in (YbS) 1.25CrS 2. The X-ray photoelectron spectroscopy (XPS) spectra of Yb 4f, 5p, 4d and 4p have been measured. They reveal the multiplet structures of Yb 2+ and Yb 3+, indicating that it is a “homogeneous” mixed-valence misfit-layer compound like (EuS) 1.173NbS 2 while keeping non-metallic properties. Yb 4f, 4d and 4p spectra are deconvoluted into the components of Yb 2+ and Yb 3+, from the relative areal intensities of which an intermediate valence has been estimated to be 2.89 in average. This result indicates that about 1.11 electrons per chemical formula are transferred from a YbS layer. On the other hand, the Cr 2p spectrum shows the spin-orbit (s.o.)-split peaks of Cr 3+, implying that an electron per Cr is transferred to a CrS 2 layer. An apparent contradiction in the transferred electrons may arise from metal deficiency within a YbS layer.