Valence variations of Sm on polycrystalline Ag

Thin deposited films of Sm on a polycrystalline Ag are investigated by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and photoemission electron microscopy (PEEM). The Sm valence is mainly divalent for low Sm coverage, while the trivalent contribution to the XPS intensity increases considerably for higher coverage. For an Sm overlayer thicker than 4 Å, the average valence is estimated to be 2.65. The mixed valence in this system is concluded to be heterogeneous (all Sm atoms have integer and site-dependent valence). Alloy formation between Sm and Ag is observed upon annealing to temperatures between 400 and 550 °C. For these temperatures the change in average Sm valence is dependent on the initial Sm coverage deposited onto the Ag-foil. Systems with low initial coverage exhibit an increase in the average valence, while a decrease is observed for systems with coverage above 6 Å. For intermediate coverages around 3 Å an initial decrease in average valence is followed by a rapid increase for temperatures above 400 °C due to morphological changes in the surface layer.     

Coadsorption of samarium with oxygen on the molybdenum (2 1 1) surface

Samarium atoms exist in two different electronic configurations, namely, divalent (Sm²⁺) and trivalent (Sm³⁺). Up to now, those two electronic configurations have been connected with the existence of Sm atoms in different valence states. Recent theoretical calculations performed by Yakovkin [9] show that both electronic configurations have similar LDOS around the Fermi level, and as a result, should give a similar photoelectron emission in the valence band region, which was identified earlier as the emission from the Sm²⁺ state. The Sm³⁺ signal in photoelectron emission could originate from contaminations of Sm by other elements, e.g. oxygen. To check the influence of O on the electronic structure of Sm, the XPS experiments of coadsorption of O and Sm have been performed.     

Density of states and the problem of Sm valence

Density of states for Sm metal has been calculated in DFT/GGA approximation using the same treatment for f and s-d bands. It is concluded that the 4f electrons of Sm must be considered as valence electrons, not shallow core states. Therefore the 4f-5d transition cannot change the valence of Sm, which may be characterized (if necessary) by the sum of all electrons in the s-d-f valence bands. The possibility of alternative interpretation of UPS spectra of Sm adsorbed films is illustrated by calculations of DOS (density of states) for oxygen-contaminated Sm crystal.     

Coordination-dependent valence state of Sm adsorbed on Cu(1 0 0) and (1 1 0) studied by STM and XPS

Geometric and electronic structures of Sm adlayers on Cu(1 0 0) and (1 1 0) were studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). The present results, in addition to the previous results for Sm/Cu(1 1 1) [Y. Nakayama, H. Kondoh, T. Ohta, Surf. Sci. 552 (2004) 53], indicate that the valence state of Sm adsorbed on Cu surfaces is determined by Sm-Sm and Sm-Cu coordination numbers. We propose that the valence state of the adsorbed Sm atoms can be explained by a simple thermodynamic equation including the coordination numbers.     

The valence of Sm in a monolayer on Al(100)

Submonolayer and monolayer films of Sm deposited on an Al(100) crystal surface have been studied by LEED and X-ray photoemission spectroscopy. Layers deposited at — 100°C or RT and without subsequent annealing were disordered and composed of Sm in both divalent and trivalent forms. Annealed films showed a pseudo hexagonal structure and were predominantly trivalent. We conclude that a perfect monolayer of Sm on Al(100) is composed wholly of trivalent atoms.     

Samarium valence changes and reactive interdiffusion at the Si(111)-Sm interface

Synchrotron radiation photoemission studies show that the Si(111) 2×1-Sm interface has a complex morphology involving sequential formation stages with different Sm valence states. For metal coverages up to 2–3 Å, Sm atoms appear only in a divalent state. At higher coverages (up to 10–15 Å) the trivalent Sm configuration dominates and large chemical shifts of Si 2p and Sm 4f and 5p levels are observed. The Sm valence change is related to the onset of the reactive interdiffusion of Sm and Si.     

Effects of film growth conditions on many-body photoemission satellites in heavy rare earths

Using angle-resolved synchrotron-radiation photoemission, we have studied the many-body satellite spectra of gadolinium and dysprosium overlayers on several substrates, including Cu(100), Ni(111), Si(111) and W(110). In addition to a 6 eV valence band satellite previous reported, we report the observation of a constant-kinetic-energy (CKE) satellite. We propose a model that describes both the 6 eV satellite as well as the newly observed CKE satellite. Our model explains in a single picture the presence and absence of the 6 eV satellite for a variety of film growth conditions.     

Geometry of the valence transition induced surface reconstruction of Sm(0001)

We present a structural determination of the surface reconstruction of the Sm(0001) surface using surface x-ray diffraction, scanning tunneling microscopy, and ab initio calculations. The reconstruction is associated with a large (22%) expansion of the atomic radius for the top monolayer surface Sm atoms. The mechanism driving the surface reconstruction in Sm is unique among all elements and is connected to the strong correlations of the 4f electrons in Sm and the intermediate valence observed in certain Sm compounds. The atoms constituting the top monolayer of Sm(0001) have vastly different chemical properties compared to the layer underneath and behave as if they were an adsorbate of a different chemical species.     

The correlation between strain and electronic structure in monolayer thick films

We explote and empirical relationship between strain (and lattice constant) and the valence electronic structure for mercury, bromine and iron overlayers. These overlayers have a range of lattice constants. For the Hg overlayers, all share a common cubic crystallography. We generally observe that the smaller the overlayer lattice constant, the greater the energy separation betweend-bands (Hg and Fe) orp-bands (Br). These results have important implications in relating electronic structure to fundamental properties such as magnetism. In addition, the film thickness limits for pseudomorphic growth calculated from the bulk properties are consistent with the experimental studies of Hg pseudomorphic growth on Ag (100) at 90 K.     

Ad-atom interactions with III-V semiconductor surfaces

Photoemission techniques (core level, valence band and partial yield spectroscopies) using synchrotron radiation as the excitation source have been applied to study the changes in the surface electronic structure of the (110) cleavage face of III-V semiconductor surfaces as a function of different ad-atom coverages. In this paper we concentrate on Au overlayers on GaSb and in particular address the problem of the mechanism for Fermi level pinning and the formation of Schottky barrier heights. It appears that the Fermi level pinning is fully established at a small fraction of a monolayer coverage. Core level spectroscopy gives evidence for a strong metal-semiconductor interaction leading to decomposition of GaSb at the interface. The experimental results form the basis for proposing a new model for the Schottky barrier based on defect states at the interface.     

A precise first-principles study on the non-existence of ferromagnetism in V monolayer on the Nb(001) surface

We report the non-existence of ferromagnetism in V monolayer on the Nb(0 0 1) surface, based on the result obtained by a precise all-electron full-potential linearized augmented plane wave (FLAPW) method within the generalized gradient approximation (GGA). Precisely calculated total energy displays that the paramagnetic state is more stable than the ferromagnetic one. The calculated density of states reveals the microscopic origin of the non-existence of ferromagnetism in the V/Nb(0 0 1) system. The paramagnetic result of our precise calculation contradicts to those of the recent tight-binding calculation using Hubbard-type Hamiltonian by J. Khalifeh [J. Magn. Magn. Mater. 168 (1997) 25] that predicted magnetism of the V overlayers on the Nb(0 0 1) surface.     

Surface quasi-ordered nanostructures NbO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/Nb(110): investigation by surface analysis methods

The oxygen-induced surface structure on the Nb(110) face have been investigated by X-ray photoelectron spectroscopy and X-ray photoelectron diffraction. It is shown that the states (in terms of coordination and chemical bonding) of the niobium atoms belonging to surface oxide structures on Nb(110) are similar the metallic states in NbO. The thickness of the NbO _x layer was estimated to be 0.5 nm. Two nonequivalent chemical states of oxygen atoms on Nb(110) have been selected. It is suggested that the first state is the chemisorbed state of atomic oxygen on hexagonally packed surface areas of a Nb monolayer surface and the second state is the state of oxygen atoms belonging to NbO _x -like linear streaks formed on the Nb(110) face.     

Samarium(III) picrate tetraethylene glycol complex: Photoluminescence study and active material in monolayer electroluminescent

A mononuclear Sm(III) complex involving Pic and EO4 (where Pic=picrate anion and EO4=tetraethylene glycol) has been studied. It shows a bright-orange emission when used as active material in a monolayer electroluminescent device of ITO/EO4-Sm-Pic/Al. The crystal structure of the complex consists of [Sm(Pic)₂(H₂O)(EO4)]⁺ cation and [Pic]⁻ anion. The Sm(III) ion is coordinated with nine oxygen atoms from one EO4 ligand in a pentadentate mode, two Pic anions each in bidentate and monodentate modes, and one water molecule. Both the terminal alcohol groups of the acyclic EO4 ligand were involved in the O-H…O hydrogen bonding by infinite one-dimensional (1D) chain within a symmetry direction [0 1 0]. The photoluminescence (PL) spectrum of the thin film shows the typical spectral features of the Sm(III) ion (⁴G_5/2→⁶H_7/2 transitions). The root-mean-square (rms) of the roughness of thin film is 30.605 nm and indicates that the formation of the monolayer electroluminescent device is not uniform and retains a high crystallinity. Typical semiconductor current-voltage (I-V) property was also observed in this device with threshold and turn voltages of 2.8 and 6.2 V, respectively. The [Sm(Pic)₂(H₂O)(EO4)](Pic)·H₂O complex can be applied as a luminescent center in OLED for bright-orange emission.     

First-principles study on the incipient oxidization of Nb(110)

The incipient oxidization of Nb(110) has been investigated using the density functional theory method. We rationalize the well-known puzzle, i.e., Nb(110) is difficult to clean, by calculating the O dissolution, and the on-surface and subsurface adsorption at low concentration. It is found that the structure of on-surface O adsorption at 0.50 monolayer (ML) coverage has the largest binding energy and minimum work function, in agreement with experimental results. At 1.00 ML coverage, the inward diffusion of O atoms is promoted by O adatoms, attributed to the formation of a local electric field. Our theoretical results improve the understanding of the experiments showing that NbO(x) oxides on the surface can be formed and decomposed by treating samples at 1500-2000 K in vacuum. Furthermore, the thermodynamic analysis of the O/Nb(110) systems shows that bulk NbO is stable in vacuum, in agreement with the observed formation of NbO nanostructures on Nb(110).     

K promoted oxidation of Al and Ta

Core level and valence band photoemission have been used to study the response to oxygen exposure of potassium-precovered, polycrystalline aluminum and tantalum substrates. Deposition of potassium overlayers in the coverage regime from zero to one monolayer, results in an increase of the rate of oxygen uptake for both substrates. The increased oxygen uptake is, however, relatively stronger on aluminum. Correlated measurements of work function and oxygen uptake show that there is a correspondence between these two parameters for low K coverages, i.e., lower work function results in higher oxygen uptake. These results may be understood in terms of a model in which the alkali atoms are increasing the rate of dissociation of oxygen molecules at the surface, as well as lowering the barrier for oxygen penetration into the surface, through a reduction in the system work function.     

Electronic absorption spectra of Sm(II) and Yb(II) ions in a LiCl-KCl eutectic melt at 450 °C

We have measured the absorption spectra of Sm(II) and Yb(II) ions in LiCl-KCl eutectic at 450 °C. The UV spectra of Sm(II) and Yb(II) exhibited intense and broad peaks at around ∼260 nm, which is attributable to the 4f-5d transitions. With the aid of the UV-vis spectroscopic method, in-situ identification of spontaneous reduction of trivalent Sm and Yb ions to divalent ions was achieved.     

Surface valence changes in rare earths monitored by giant-resonance electron energy losses

Electron energy loss spectroscopy of 4d giant resonances in the rare earths is shown to be a sensitive monitor of valence changes in surface reactions with O₂ and S. Yb is found to be divalent on exposure to S but forms a trivalent oxide, while Gd is consistently trivalent. Eu forms a monosulfide, and an oxide of possibly mixed divalent and trivalent stoichiometry, while for Sm mixture of divalent and trivalent spectral features is consistently observed.     

Ultrathin film magnetic research at the Naval Research Laboratory

Scientists at the Naval Research Laboratory have been using molecular beam epitaxy techniques to grow a variety of high quality magnetic transition metal (TM) films and superlattices having TM thicknesses in the 1–10 monolayer regime. The magnetic and structural properties of these samples have been studied with a wide variety of standard and spin-sensitive techniques, both in vacuo and, when suitably protected, in air. The magnetic behavior of the materials is strongly modified as the TM layers become ultrathin. This work is illustrated with examples involving ultrathin Fe layers on Ag(001), Fe/Ag(001) superlattices containing ultrathin Fe layers, and bcc Co on GaAs(110).     

An oblique-incidence optical reflectivity difference and LEED study of rare-gas growth on a lattice-mismatched metal substrate

We studied the growth of Xe on Nb(110) from 33 K to 100 K using a combination of low-energy electron diffraction and an in situ oblique-incidence optical reflectivity difference technique. We found that a hexagonal close-packed Xe film grows after a transition layer of three monoatomic layers thick is formed. The first two monolayers, influenced by both the interaction with the Nb substrate and the Xe–Xe interaction, lack long-range order. The third monolayer forms a bulk-like hexagonal close-packed structure. Subsequently a bulk-phase Xe(111) film grows in step-flow mode from 54 K down to 40 K. At 40 K, we observed a brief crossover to a layer-by-layer mode. At 33 K the growth proceeds in a kinetically limited multilayer or a three-dimensional island mode. PACS 68.35.Fx; 68.35.Ja; 82.20.-w; 61.16.Ch     

Sm富勒烯的芯态光电子能谱

用X射线光电子能谱研究了Sm掺杂的固态C₆₀相衍变和芯电子态.在Sm_xC₆₀中Sm的含量x小于0.5时，样品是固溶相；在x=0.5和x=2.75之间的掺杂阶段，样品是这两个相的混合.在x=2.75和x=6之间的高掺杂阶段未观察到相分离的X射线光电子能谱证据.Sm 4f, 4d的实验数据表明Sm的价态是+2.二价Sm 3d_5/2芯态谱线存在两个子峰. 关键词： 60的Sm填隙化合物')" href="#">C₆₀的Sm填隙化合物 相结构 电子态 X射线光电子能谱