Cathodoluminescence of copper and nickel surfaces

Cathodoluminescence spectra of clean and oxidized Cu(100) and Ni(100) surfaces, using 60–1000 eV electron bombardment, has been measured. A broad peak at 310 nm has been observed for both clean Cu and Ni surfaces. This peak is attributed to radiative recombination of electrons from the point X'₄ with holes at the point X₅ of the energy band diagram. A narrow peak at 520 nm has been found to correlate with the growth of oxide on the Cu and Ni surfaces. This peak can be attributed to electronic transitions involving energy levels of the O^2- ions. The cathodoluminescence technique appears useful for studying the initial growth of metal oxides.     

Structures near the L2,3 core edges of clean and: Oxygen covered Si(111) studied with low-energy electrons

The L_2,3 core excitation spectra of clean and oxygen covered surfaces of Si(111), measured in low-energy electron energy loss Spectroscopy reproduce previous results obtained with highenergy electrons or synchrotron radiation. It is inferred that among the different transition channels, the matrix elements for the dipole term dominate. Spectra on oxidized surfaces indicate that oxygen adsorption on Si(111) induces the formation of SiO₄ structural units.     

Molecular binding states on clean and oxidized surfaces: SiO(g) + W(clean) and SiO(g) + W(oxidized)

The interactions between a molecular beam of SiO(g) and a clean and an oxidized tungsten surface were examined in the surface temperature range 600 to 1700 K by mass spectrometrically determined sticking probabilities, by flash desorption mass spectrometry (FDMS) and by Auger electron spectroscopy (AES). The sticking probability, S, of SiO has been determined as a function of coverage and of surface temperature for the clean and the oxidized tungsten surface. Over the temperature range studied and at zero coverage S = 1.0 and 0.88 for the clean and oxidized tungsten surfaces respectively. The results are consistent with both FDMS and AES. For coverage up to one monolayer there is one major adsorption state of SiO on the clean tungsten surface. FDMS shows that T_m = constant (T_m is the surface temperature at which the desorption rate is maximum) and that desorption from this state is described by a simple first order desorption process with activation energy, E_d = 85.3 kcal mole^?1 and pre-exponential factor, ν = 2.1 × 10¹⁴ sec^?1. AES shows that the 92 eV peak characteristic of silicon dominates. In contrast on the oxidized tungsten surface, T_m shifts to higher temperatures with increasing coverage. The data indicate a first order desorption process with a coverage dependent activation energy. At low coverage (θ ? 0.14) there is an adsorption state with E_d = 120 kcal mole^?1 and ν = 7.6 × 10¹⁹, while at θ = 1.0, E_d = 141 kcal mole^?1. This variation is interpreted as due to complex formation on the surface. AES shows that on oxidized tungsten, in contrast to clean tungsten, the dominant peaks occur at 64 and 78 eV, and these peaks are characteristic of higher oxidation states of silicon. Thus, it is concluded that SiO exists in different binding states on clean and oxidized tungsten surfaces.     

Characteristic electron energy loss structure for clean and oxidized chromium surfaces

AES and EELS spectra have been measured on clean and oxidized chromium surfaces. Auger peaks at 31.0 and 44.0 eV of the oxide are attributed to cross transitions between chromium and oxygen: {M_2,3(Cr)V(Cr)V(O)} and {M₁(Cr)L₁(O)V(Cr)} respectively. Core loss features are consistent with valence band structure with a newly observed loss peak at 41.1 eV for the oxidized surface being ascribed to a core exciton with binding energy E_b = 1.6 eV.     

Spectrum and nature of surface states

With the help of an approximative E(k) relation in the forbidden gap N(E) distributions for localized states are calculated. A potential fluctuation model gives energy distributions of states in the gap which exhibit features of surface state spectra or spectra of disordered systems. Binding and nonbinding states are coupled to pairs by the E(k) relation and characteristic properties of surface and interface state spectra like symmetry, acceptordonor character, minimum of the N(E) distribution may be described. Dangling bonds of preferential 111 character are the proper chemical picture. Approximative energy calculations show a preferential energy range for isolated interface states at a distance of 0.1–0.2 eV distance from the band edges. Comparison with experimental termspectra shows a good agreement with the treated model. Common characteristics of clean, ordinary and oxidized surfaces and the metal semiconductor contact are revealed.     

Spin-polarized metastable deexcitation spectroscopy study of iron films

A spin-polarized metastable deexcitation spectroscopy apparatus using a nozzle-skimmer pulsed discharge metastable atom source was developed. The oxygen adsorption dependence of the surface magnetism of thin iron films deposited on MgO(100) was investigated using this apparatus. The surface local density of states spilling towards the vacuum (SDOS) at around the Fermi energy, E_F, for the clean surface shows a negative polarization to the bulk. The oxygen derived SDOS for the lightly oxygen adsorbed surfaces (2–10 L) also shows a negative polarization while the SDOS at E_F changes its polarity to positive. The polarization of SDOS is not detected for the heavily oxidized surfaces (20–100 L).     

Electronic structure of gold nanoclusters on oxidized Ni(755) surface

The electronic energy structure of gold nanoclusters grown on oxidized single-crystal stepped surface Ni(755) is studied. It is shown that oxidation of the stepped Ni(755) surface results in the formation of a well-ordered continuous structure O(2 × 2) similar to that grown on a flat Ni(111) single-crystal surface. Evaporation of gold on such a surface leads to the formation of gold nanoclusters of a size determined by the size of the terraces on the Ni(755) surface. A comparison of the photoelectron spectra of the Au 4f _{5/2, 7/2} core levels in clusters grown on clean and oxidized Ni(755) surfaces reveals that the spectra obtained for a gold cluster system on an oxidized Ni(755) surface contain not only the spectral components characteristic of metallic gold but also additional components of Au^+δ. It is assumed that additional components for gold clusters on the oxidized Ni(755) surface originate from partial oxidation of gold atoms with the participation of defects inherent in the stepped relief of the nickel substrate.     

Surface photovoltage spectroscopy and surface piezoelectric effect in GaAs

“Real” (111) surfaces of n-type GaAs were investigated employing surface photovoltage spectroscopy and the surface piezoelectric effect. Surface states at the energy position E_c ? E_t ? 0.72 eV were found on both the Ga and the As surfaces. Both types of surfaces exhibited a barrier of about 0.55 V. No variations in the surface barrier or the energy position of the surface states were observed in various ambients at atmospheric pressure (dry air, wet air, ammonia and ozone). However, the capture cross-section of the surface states for electrons, as determined from the surface piezoelectric effect transients (of the order of 10^?13 cm²), was found to be sensitive to the ambient. It decreased in wet air and increased in ozone. This effect was more pronounced on the As than on the Ga surfaces. Additional surface states were found to be present in the energy region of 0.9 to 1.0 eV, below the bottom of the conduction band. However, their exact energy positions could not be determined due to interference caused by the carrier trapping of the surface states at E_c ? E_t ? 0.72 eV.     

Atomic structure of low-index and (11n) surfaces in ordered Cu3Au

By computing the surface energy at T = 0 K, we test the relative stability of various among possible atomic configurations of low-index and (11n) surfaces in the L1₂ compound Cu₃Au. The computations rely on two n-body phenomenological potentials and are performed using a classical energy minimization scheme. The energy of (100) and (110) surfaces is at a minimum for a mixed composition terminal layer, in agreement with available experimental results and previous calculations. For (113) and (115) surfaces the minimum energy is obtained when all (100) terraces are mixed. In this case, the height and width of steps and terraces are twice as large as those of a standard surface configuration.     

Strong-coupling planar diagrams,random surfaces,and the large-N phase transition in U(N) lattice gauge theories

The strong-coupling expansion of U(N) gauge theory on a D-dimensional lattice is reformulated in the limit N → ∞ through a set of diagrammatic rules directly for the free energy and Wilson loops. The strong-coupling planar diagrams are interpreted as surfaces embedded in the lattice. The large-N phase transition is related to the entropy of these surfaces. It is shown that the strong-coupling phase of the U(∞) gauge theory terminates with a phase transition of Gross-Witten type only in 2 and 3 dimensions. When D⩾4 the large-N singularity takes place in a metastable phase because of an earlier first-order transition to the weak-coupling phase of the theory.     

Size-dependent oxidation of Pdn (n ? 13) on alumina/NiAl(110): Correlation with Pd core level binding energies

Small Pd clusters Pd_n (n = 1, 4, 7, 10, 13) deposited on alumina/NiAl(110) at room temperature were examined by X-ray photoelectron spectroscopy (XPS), as-deposited and after exposure to O₂ at temperatures ranging from 100 to 500 K. After O₂ exposure at 100 K, the Pd clusters showed XPS shifts indicative of oxidation. The exception was Pd₄, which did not oxidize under any conditions. The inertness of Pd₄/alumina/NiAl(110) appears to be correlated with a significantly higher-than-expected Pd 3d binding energy, which we attribute to a particularly stable valence shell. None of the clusters examined oxidized during O₂ exposures at 300 K or above, but He⁺ scattering showed that oxygen was bound on the cluster surfaces. Upon heating, all the oxygen associated with these small clusters appeared to spill over and react with the alumina/NiAl(110) support.     

Configurational mixing and 4f-photoemission lineshapes in SmB6

Single crystal surfaces of the interconfiguration fluctuation material SmB₆ have been studied by X-ray photoemission spectroscopy. Comparison with data for LaB₆ allows a detailed analysis of the Sm 4f-spectrum. It yields a Sm²⁺/Sm³⁺ valence ratio of 0.4, a Coulomb correlation correlation energy oif 7 eV, and a 1 eV broadening of the Sm³⁺ 4f spectrum     

Observation of resonance recombination lines in electron excited auger spectra of Gd

Combined measurements of electron excited N_4,5 Auger spectra and photoelectron emission on clean and oxidized Gd lead to a distinction between Auger lines originating from 4d → continuum and 4d → 4? resonance excitations. Several Auger structures are identified as due to the direct recombination of 4d⁹4?⁸ states with the 4f and valence electrons. The shape of the most prominent Auger line for oxidized Gd agrees perfectly with the Fano profile of the 4? photoemission intensity.     

Double discontinuities of wide separation in the X-ray absorption spectra of transition elements in highly oxidized compounds

The paper consists of two parts. (1) The author studied the mechanism of the appearance of the double discontinuities of wide separation in the X-ray L₁ absorption spectrum of Mo in MoO₃, which were found in the recent research about the effects of chemical combination on various X-ray absorption limits, and that of Nb in Nb₂O₅. Then he arrived at the conclusion that the discontinuity of the higher energy corresponds to the electronic transition from L₁ to 5p, and that of the lower energy is attributed to that from L₁ to the 4d, 5s state. (2) The author applied the explanation to the double discontinuities which have been observed by many researchers in the K absorption spectra of 3d transition elements in highly oxidized compounds, and showed that these double absorption discontinuities are ascribed to the electronic transitions K?4p and K ? 3d, 4s.     

X-ray photoelectron spectroscopic studies on oxidation behavior of nickel and iron aluminides under oxygen atmosphere at low pressures

Oxidation behaviors of NiAl, Ni₃Al, and FeAl under oxygen atmosphere at low pressures were studied by X-ray photoelectron spectroscopy (XPS). Clean surfaces of these aluminides were prepared by fracturing in an ultra high vacuum, and then the fractured surfaces were oxidized by exposing to high-purity oxygen at pressures up to 1.3 Pa without exposing to air. The oxides formed on NiAl and FeAl surfaces were Al₂O₃, whereas the oxide on Ni₃Al was NiAl₂O₄. Aluminum, nickel, and iron on clean surfaces were oxidized even at a pressure of 1.3 × 10⁻⁶ Pa. The oxidation evolves with an increase in the pressure of oxygen, and further oxidation of aluminum occurs prior to that of nickel or iron. The oxidation behaviors under such oxygen atmosphere were similar to those of the aluminides oxidized in air, and these behaviors could be predicted from thermodynamic consideration.     

Hybrid-DFT study for the initial oxidation steps on silicon cluster surface

We performed a hybrid density functional theory calculation for the successive adsorption of nitrous oxide (N₂O) on Si(1 0 0)-Si₉H₁₂O_x (x = 0 and 1) cluster surfaces to elucidate N₂O decomposition and the subsequent surface oxidation processes. N₂O decomposed into N₂ and O fragments, and the latter fragment inserted into either surface-dimer bonds or back-bonds with similar activation barriers on both the clean and partially oxidized Si surfaces. The Si₉H₁₂ cluster surface was eventually oxidized to five distinct structures of Si₉H₁₂O₂.     

Thermal activation measurement of positron binding energies at surfaces

The formation of positronium by low-energy positrons incident on “clean” metal surfaces is thermally activated by increasing temperature. The activation energy E_a has been measured for a number of surfaces. E_a is understood as the energy required to form positronium (binding energy 1/2Ry) from positrons bound at the surface by an energy E_b, E_a = E_b+φ_-? 1/2Ry, where φ_- is the electron work function. Representative values of E_b derived are Al(100):3.03(5) eV; Al(110):2.92(4) eV; Cu(111):2.80(5) eV.     

Landau-Ginzburg theory of phase transitions in heated rotating nuclei

The Landau-Ginzburg theory is applied to heated rotating superfluid systems. It is demonstrated that the free energy F' is an even function of the order parameter Δ even when external fields are present. The energy surfaces F'(Δ) are calculated for various rotational frequencies and temperatures. These surfaces provide clear illustrations of first-order and second-order phase transitions, as well as a critical point.     

Thermal and photochemical reactivity of oxygen atoms on gold nanocluster surfaces

Reduction of oxidized gold nanoclusters by exposures to foreign gases and irradiation of UV photons has been investigated using X-ray photoelectron spectroscopy. Gold nanoclusters with narrow size distributions protected by alkanethiolate ligands were deposited on a TiO₂(1 1 0) surface with dip coating. Oxygen plasma etching was used for removal of alkanethiolate ligands and oxidization of gold clusters. The oxidized gold clusters were exposed to CO, C₂H₂, C₂H₄, H₂, and hydrogen atoms. Although, C₂H₄ and H₂ did not show any indications of reduction of oxidized gold clusters, CO, C₂H₂, and hydrogen atoms reduced the oxides on gold cluster surfaces. Among them, hydrogen atoms were most effective for reduction. Irradiation of UV photons around 400 nm could also reduce the oxidized gold clusters. The photochemical reduction mechanism was proposed as follows. The photo-reduction was initiated by electronic excitation of gold clusters and oxygen atoms activated reacted with carbon atoms at the surfaces of gold clusters. Carbon species were likely absorbed in gold clusters or remained at the boundaries between gold clusters when gold clusters agglomerated during oxygen plasma exposures. As the photochemical reduction progressed, carbon atoms segregated to the surfaces of gold clusters.     

Normal photoemission demonstration of two and three dimensionality of electron states in layer compounds

Energy spectra have been measured for photoelectrons emitted normal to the surfaces of GaSe and InSe. Some of the peaks display considerable energy dispersion ～ 1 eV as a function of photon energy indicating that the corresponding bands are three-dimensional. Two-dimensional bands with small energy dispersion (? 0.3 eV) are also observed. These features are related to the extent of Se p_z-character in calculated pseudopotential wave-functions.