Angle resolved photoemission study of the Ce/Pd(1 1 1) interface

X-ray and UV excitation angle resolved photoemission spectroscopy of ultra-thin films of cerium deposited on a Pd(1 1 1) single-crystal surface has been carried out. Both core level and valence band spectra show a formation of a surface alloy exhibiting d- and f-electron orbital hybridization. An azimuth and polar angle mapping of X-ray excited photoemission intensities results in a surface-geometrical structure information. Mapping of ultra-violet photoelectron intensities as a function of emission angles is used for a band mapping and an electronic structure determination. The d-, f-hybridization occurs even for 0.7 ML of Ce deposited at room temperature. The surface annealing at 260 °C enhances this behavior. A shift of Pd 4d-derived states to higher binding energy in the Ce-Pd systems is observed.     

Surface alloying and mixed valence in thin layers of Ce and Pd on Ru(0 0 0 1)

The Ce/Pd/Ru(0 0 0 1) system has been studied by photoelectron spectroscopy and low energy electron diffraction. The Pd overlayer thicknesses were in the range from one to two monolayers. The effective Ce overlayer thicknesses were in the range from 0.5 to 1.5 monolayers. The interfaces were studied for annealing temperatures from room temperature up to 1030 °C. A tendency of intermixing of Pd and Ce was observed already at room temperature. The estimated Ce valence and 4f-4d hybridization strength were found to be largest for the most Pd rich surfaces. The onset of desorption of Ce and Pd takes place at a temperature of 700 °C, which is considerably lower than the temperature of onset of desorption of Ce from the Ce-Pd system. This is argued to be due to a weakening of the substrate bonds when stronger Ce-Pd bonds form. Intermixing between the Ru substrate and Pd and Ce was not observed. The low value of the sample work function that was recorded throughout these studies shows that Ce was always present at the outermost surface layer.     

Methanol adsorption on a CeO2(1 1 1)/Cu(1 1 1) thin film model catalyst

Adsorption and desorption of methanol on a CeO₂(1 1 1)/Cu(1 1 1) thin film surface was investigated by XPS and soft X-ray synchrotron radiation PES. Resonance PES was used to determine the occupancy of the Ce 4f states with high sensitivity. Methanol adsorbed at 110 K formed adsorbate multilayers, which were partially desorbed at 140 K. Low temperature desorption was accompanied by formation of chemisorbed methoxy groups. Methanol strongly reduced cerium oxide by forming hydroxyl groups at first, which with increasing temperature was followed by creation of oxygen vacancies in the topmost cerium oxide layer due to water desorption. Dissociative methanol adsorption and creation of oxygen vacancies was observed as a Ce⁴⁺ → Ce³⁺ transition and an increase of the Ce 4f electronic state occupancy.     

Photoemission from the valence bands of Ce(1 1 1) on W(1 1 0)

We report a photoelectron spectroscopic study of the valence bands of epitaxial Ce(1 1 1) films grown on W(1 1 0) at room temperature. The evolution of γ → α → γ like phase transition of Ce is observed with increasing Ce coverage and the valence-band structures of γ-like Ce film are determined. The 4f and 5d photoemission cross sections in the photon energy region from 20 eV to 130 eV are presented and discussed. A 5d-like surface state and a 6s band bottom are identified.     

A study of barium ultra-thin films on the SrTiO3(1 0 0) surface by soft X-ray photoelectron spectroscopy

The growth and the electronic properties of Ba ultra-thin films (coverage ≤ 2 ML) on the SrTiO₃(1 0 0) surface was studied using synchrotron radiation facilities. The investigation was carried out mainly by soft X-ray photoelectron spectroscopy measurements at low core levels and the valence band region. The results show that the Ba overlayer develops in a layer-by-layer mode. The first two layers do not create any states in the band gap, thus leaving intact the insulating character of the substrate. This is due to the oxidation of Ba through transfer of O²⁻ anions from the substrate into the Ba overlayer. It is observed that the valency of the Ti atoms at the interface does not change. This is attributed to an outwards diffusion of O²⁻ anions from deeper layers promoted by substrate annealing. Within the coverage examined, no evidence for Ba metallization is found, contrary to what happens on metallic substrates. Comparing the results of this work with previous ones on similar metal–SrTiO₃(1 0 0) adsorption systems, it is concluded that a single factor such as charge transfer or thermodynamic stability is not always sufficient to determine the oxidation process of the adsorbate layer. Kinetics on surfaces as well as substrate doping and defects can also play a decisive role.     

Electronic structure of 1 ML NTCDA/Ag(1 1 1) studied by photoemission spectroscopy

We performed high-resolution photoemission experiments at different photon energies to investigate the valence band structure of 1 ML of 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTCDA) on Ag(1 1 1). Besides the known occupied molecular orbitals HOMO and HOMO − 1 we observe a new state close to the Fermi level, which results from the interaction between NTCDA and the substrate, partially filling the lowest unoccupied orbital of the free molecule (LUMO) in the monolayer system. By tuning the photon energy through the carbon K-edge, a resonance like change of the spectral intensity at the HOMO and HOMO − 1 energies is clearly revealed, which we use for an assignment of the individual spectral features to a predominant localization either at the naphthalene core or the anhydride group.     

Defects at the TiO2(1 0 0) surface probed by resonant photoelectron diffraction

We report photoelectron diffraction (PED) experiments of weakly sub-stoichiometric TiO₂(1 0 0) rutile surfaces. Apart from standard core-level PED from the Ti-2p3/2 line, we have studied valence band PED from the defect induced Ti-3d states in the insulating band gap. For maximum yield, the latter were resonantly excited at the Ti-2p absorption edge. The PED patterns have been analyzed within the forward scattering approximation as well as by comparison with simulated PED patterns obtained in multiple scattering calculations. The analysis shows that the defect induced Ti-3d charge is mainly located on the second layer Ti atoms.     

Angle-resolved and resonant photoemission study of the ZrO-like film on ZrC(1 0 0)

Angle-resolved photoemission spectroscopy (ARPES) and resonant photoemission spectroscopy (RPES) have been used to study the electronic structure of the ZrO-like film formed on a ZrC(1 0 0) surface. It is found that, in addition to the O 2p band observed at 6-8 eV, states exist at 0.15 and 0.75 eV around the point. ARPES measurements show that the states at 0.15 and 0.75 eV disperse towards the lower binding energy side and cross the Fermi level along the direction. RPES measurements show that the former state shows a resonant behavior characteristic of the intra-atomic Zr 4d resonance, suggesting that the state includes substantial contribution of Zr 4d orbitals. On the other hand, the latter state shows a resonant behavior similar to that of the O 2p state in ZrO. The resonance is thought to be caused through the inter-atomic deexcitation mechanism involving the emissions of O 2p electrons, and thus the latter state is ascribed to that mostly composed of the O 2p component.     

RAIRS studies of CO adsorption on Pd/CeO2−x(1 1 1)/Pt(1 1 1)

Reflection absorption infrared spectroscopy (RAIRS) has been used to investigate the adsorption of CO on CeO_2−x-supported Pd nanoparticles at room temperature. The results show that when CeO_2−x is initially grown on Pt(1 1 1), a small proportion of the surface remains as bare Pt sites. However, when Pd is deposited onto CeO_2−x/Pt(1 1 1), most of the Pd grows directly on top of the CeO_2−x(1 1 1). RAIR spectra of CO adsorption on 1 ML Pd/CeO_2−x/Pt(1 1 1) show a broad CO-Pd band, which is inconsistent with a single crystal Pd surface. However, the 5 ML and 10 ML Pd/CeO_2−x/Pt(1 1 1) spectra show vibrational bands consistent with the presence of Pd(1 1 1) and (1 0 0) faces, suggesting the growth of Pd nanostructures with well defined facets.     

ZnTe/GaAs(2 1 1)B heterojunction valence band discontinuity measured by X-ray photoelectron spectroscopy

Thin ZnTe layers were grown by molecular beam epitaxy on single crystal GaAs(2 1 1)B substrates. Reflection high energy electron diffraction monitored the deoxidation of substrate and entire growth process. Valence band offset was calculated with X-ray photoelectron spectroscopy. Also interface formation of the ZnTe/GaAs was studied. Analysis shows that interface is abrupt and calculated valance band offset is 0.25 ± 0.1 eV and indicates type I alignment. The experimental result agrees well with the theoretical predictions involving interface dipole effect as well as electron affinity rule.     

The formation of sharp NiO(1 0 0)-cobalt interfaces

An atomically sharp interface between an antiferromagnetic oxide and a ferromagnetic metal may be obtained by the deposition of an epitaxial oxide buffer nanolayer in between. The buffer layer consists of the oxide of the ferromagnetic metal. The concept has been demonstrated on the NiO(1 0 0)-Co system, where the inclusion of a 1-2 ML CoO(1 0 0) interlayer inhibits the interfacial redox reaction which takes place between NiO and Co metal in the absence of the buffer layer.     

Valence band and band gap photoemission study of (1 1 1) In2O3 epitaxial films under interactions with oxygen, water and carbon monoxide

Synchrotron radiation ultraviolet photoemission experiments at photon energies of 150 and 49 eV were performed on an epitaxial layer of (1 1 1) In₂O₃ with good crystallinity as established by a standard scanning probe and diffraction methods. Valence band (VB) and band gap photoemission spectra were monitored under separate oxygen, water and carbon monoxide exposures (100 L) at different activation temperatures within the range utilized for chemiresistive gas sensors (160-450 °C). Large changes in photoemission response within the whole VB were observed for all gases. Regular shifts of the valence band edge relative to the Fermi energy were found under gas exposures on two kinds of surface (partially reduced or partially oxidized), and are interpreted as changes of surface potential. Treatments in oxygen resulted in upward band bending (∼0.5 eV at T = 320 °C). Regardless of activation temperature, treatments in water resulted in downward band bending, but with small changes (<0.1 eV). Reduction properties of carbon monoxide were observed only at high temperatures of T ? 370 °C. At temperatures of 160 and 250 °C unusual “oxidizing” behavior of CO was observed with upward band bending of ∼0.7 eV (160 °C). Oxidizing and reducing effects of the gas interactions with the (1 1 1) In₂O₃ surface in all cases were accompanied by a corresponding behavior, i.e., a decrease or increase in photoemission response from so-called defect states in the band gap near the top of the valence band. The increases of photoemission within a band gap with maxima at binding energies (BE) of 0.4 (O₂-induced peak) and 1.0 eV (CO-induced peak) were, respectively, found for interactions with O₂ and CO for low temperatures (T = 160 and 250 °C). These responses were ascribed to acceptor-like electronic levels of O₂ and CO chemisorption states, respectively. A definite split of the top VB peak (BE ∼ 4.0 eV) was found under CO dosing at 160 °C. Established knowledge of the CO interaction with the (1 1 1) In₂O₃ surface explains earlier revealed acceptor-like behavior of In₂O₃ film conductivity during CO detection at operational temperatures lower than 250 °C through the formation of acceptor-like electronic levels of adsorbed CO molecules.     

Sn interaction with the CeO2(1 1 1) system: Bimetallic bonding and ceria reduction

A tin layer 0.8 nm thick was deposited onto the CeO₂(1 1 1) surface by molecular beam deposition at a temperature of 520 K. The interaction of tin with cerium oxide (ceria) was investigated by X-ray photoelectron spectroscopy (XPS), ultra-violet photoelectron spectroscopy (UPS) and resonant photoelectron spectroscopy (RPES). The strong tin-ceria interaction led to the formation of a homogeneous bulk Ce-Sn-O mixed oxide system. The bulk compound formation is accompanied by partial Ce⁴⁺ → Ce³⁺ reduction, observed as a giant 4f resonance enhancement of the Ce³⁺ species. CeO₂ and SnO₂ oxides were formed after oxygen treatment at 520 K. The study proved the existence of strong Ce-Sn interaction and charge transfer from Sn to the Ce-O complex that lead to a weakening of the cerium-oxygen bond, and consequently, to the formation of oxygen deficient active sites on the ceria surface. This behavior can be a key for understanding the higher catalytic activity of the SnO_x/CeO_x mixed oxide catalysts as compared with the individual pure oxides.     

Electronic exchanges between adsorbed Ni atoms and TiO2(1 1 0) surface evidenced by resonant photoemission

Nickel was deposited on stoichiometric TiO₂(1 1 0) surface in the 0.02–2.1 equivalent monolayer (eqML) range and analyzed by means of photoemission and resonant photoemission. In the case of very low coverage (lower than 0.1 eqML), deposited nickel reacts with the surface through an electronic transfer from nickel atoms towards titanium ions. This exchange caused the filling of unoccupied Ti3d states leading to the increase of a peak in the TiO₂ band gap. These states can be better characterized through resonant photoemission experiments at the Ti 3p → 3d absorption edge: for very low coverage, these states in the TiO₂ band gap have resonant behavior of Ti3d electrons rather than Ni3d ones, confirming the filling of Ti3d states and thus electron transfer between nickel and titanium. For coverage higher than 0.14 eqML, nickel peaks (both Ni3p core level and valence band) should be related to the presence of metallic nickel in small clusters.     

In situ oxidation study of MgO(1 0 0) supported Pd nanoparticles

We have investigated the oxidation behavior of Pd nanoparticles grown epitaxially on MgO(1 0 0) single crystal substrates. We find that the interaction of oxygen with octahedral Pd nanoparticles at 500 K can be subdivided in three stages: above 10⁻⁶ mbar O₂ pressure, the particles start to flatten; above 10⁻³ mbar, the particles begin to shrink laterally and to be less truncated at the corners. The formation of epitaxial bulk PdO sets in at oxygen pressures above 0.1 mbar, which is accompanied by a continuous shrinkage of the Pd particles. Our results point to a novel nanoparticle oxidation mechanism: the Pd particles act as dissociation centers for O₂ and serve at the same time as source for Pd atoms resulting in epitaxial PdO growth on MgO(1 0 0).     

The plasma oxidation of Pd(1 0 0)

The oxidation of Pd(1 0 0) by an oxygen plasma was characterized using X-ray photoelectron spectroscopy (XPS), low energy ion scattering spectroscopy (ISS), temperature programmed desorption (TPD), and low energy electron diffraction (LEED). The oxygen uptake followed a typical parabolic profile with oxygen coverages reaching 32 ML after 1 h in the plasma; a factor of 40 higher than could be achieved by dosing molecular oxidants in ultra high vacuum. Even after adsorbing 32 ML of oxygen, XPS revealed both metallic Pd and PdO in the surface region. The R27^o LEED pattern previously attributed to a surface oxide monolayer, slowly attenuated with oxygen coverage indicating that the PdO formed poorly ordered three dimensional clusters that slowly covered the ordered surface oxide. While XPS revealed the formation of bulk PdO, only small changes in the ISS spectra were observed once the surface oxide layer was completed. The leading edges of the O₂ TPD curves showed only small shifts with increasing oxygen coverage that could be explained in terms of the lower thermodynamic stability of small oxide clusters. The desorption curves, however, could not be adequately described as simple zero order decomposition of PdO. There has been an ongoing debate in the literature about the relative catalytic activities of PdO and oxygen phases on Pd, the results indicate that any differences in the reactivity between bulk PdO and surface oxides are not associated with differences in the density of exposed Pd atoms or the decomposition kinetics of these two phases.     

Oxygen adsorption on a Mo2C(0 0 0 1) surface: Angle-resolved photoemission study

Oxygen adsorption on a C-terminated α-Mo₂C(0 0 0 1) surface has been investigated with Auger electron spectroscopy, low-energy electron diffraction, and angle-resolved photoemission spectroscopy utilizing synchrotron radiation. It is found that the oxygen atoms adsorb on the Mo atoms in the second layer forming a (1 × 1) orthorhombic periodicity. The oxygen adsorption induces a peculiar state around the Fermi level, which is observed at 0.4 eV in the normal-emission spectra. ARPES measurements show that the state is a partially occupied metallic state. The photoionization cross section of the state shows a maximum at the photon energy of 56 eV, which is assigned as originating from the resonance of the Mo 4d photoemission involving Mo 4p → 4d photoexcitation.     

Methanol adsorption on Pd(1 1 0) and Ag/Pd(1 1 0) studied by high-resolution photoelectron spectroscopy

Adsorption of methanol on clean Pd(1 1 0) and on an alloyed Ag/Pd(1 1 0) surface has been studied by high-resolution photoelectron spectroscopy. On Pd(1 1 0) two different chemisorbed methanol species were observed for temperatures up to 200 K, with the one at lower binding energy remaining at low coverage. These species were attributed to methanol adsorbed in two different adsorption sites on the Pd(1 1 0) surface. As is well established for this system, heating to 250 K resulted in decomposition of methanol into CO. The adsorption and decomposition behaviour of methanol on the Ag/Pd(1 1 0) surface alloy formed by depositing Ag on Pd(1 1 0) at elevated temperature was similar to that of the pure Pd(1 1 0) surface. This suggests that the amount of Ag present in the Pd(1 1 0) surface in this study does not affect the decomposition behaviour of methanol as compared to pure Pd(1 1 0). Complementary density functional theory calculations also show little influence of Ag on the binding of methanol to Pd. These calculations predict an on-top adsorption site for low methanol coverages.     

Synchrotron photoemission studies of pentacene films on Cu(1 1 0)

We have investigated the nature of the interaction between pentacene and Cu(1 1 0) for films grown by two different methods, and the energy level alignment at the resulting pentacene/Cu(1 1 0) interface. The first film was grown in a stepwise fashion at room temperature while the second film was grown using an annealed monolayer to template the growth of the subsequent layers. Synchrotron based photoemission techniques have been used to compare the initial stages of the growth of the two films. A thorough examination of the growth of both films indicates a strong bond between the initial monolayer of pentacene and the Cu(1 1 0) substrate, in addition to a change in molecular orientation for subsequent layers after the completion of the initial monolayer. A comparison of the two films indicates identical film growth for both films with the film templated by the annealed monolayer exhibiting a more uniform growth mode for the film thicknesses investigated.     

Mixed water/OH structures on Pd(1 1 1)

Chemisorbed O and water react on Pd(1 1 1) at low temperatures to form a mixed OH/H₂O layer with a (√3 × √3)R30° registry. Reaction requires at least two water molecules to each O before the (2 × 2)O islands are consumed, the most stable OH/water structure being a (OH + H₂O) layer containing 0.67 ML of oxygen, formed by the reaction 3H₂O + O → 2(H₂O + OH). This structure is stabilised compared to pure water structures, decomposing at 190 K as OH recombines and water desorbs. The (√3 × √3)R30° − (OH + H₂O) phase cannot be formed by O/H reaction and is distinct from the (√3 × √3)R30° structure formed by O/H coadsorption below 200 K. Mixed OH/water structures do not react with coadsorbed H below 190 K on Pd(1 1 1), preventing this phase catalyzing the low temperature H₂/O₂ reaction which only occurs at higher temperatures.