Initial oxidation of the Mg(0001) surface,an electron spectroscopy study

The initial oxidation of Mg(0001) has been studied using AES (Auger electron spectroscopy), LEED (low energy electron diffraction), and EELS (electron energy loss spectroscopy). The oxidation proceeds through different stages; first oxygen atoms are incorporated to chemisorption sites below the top layer magnesium. This chemisorption phase is followed by the formation of an oxide layer. The oxide layer covers the Mg surface after an oxygen exposure of ～ 10 L O₂. After this exposure the bulk-like MgO formation slowly increases the oxide thickness. The oxide layer formed for exposures up to ≤ 10 L O₂ gives rise to a diffuse LEED pattern of the same symmetry as the original “clean” LEED pattern; the possibility of an epitaxial oxide formation at this stage is discussed.     

A LEED-AES study of the initial stages of oxidation of Fe (001)

LEED and AES have been used to study the structural changes and kinetics of the initial interaction between Fe(001) and oxygen at room temperature. The AES oxygen signal was quantified by using a two-dimensional oxide layer as a calibration point. This reproducible oxide layer was prepared by the high temperature reaction of H₂O at 10^?6 torr with Fe(001). The initial oxygen sticking coefficient was observed to be close to unity, which suggests that the chemisorption is non-activated and involves a mobile adsorption step. The rate of chemisorption decreased as (1-Θ) and exhibited a minimum at Θ = 0.5. LEED data indicate that the minimum value of the sticking coefficient corresponded to the completion of a c (2 × 2) surface structure. Upon additional exposure to oxygen, an increase in the sticking coefficient was observed in conjunction with the disappearance of the c (2 × 2) and a gradual fade out of all diffraction features. After mild heating, epitaxial FeO (001) and FeO (111) structures were observed. The simultaneous appearance of a shifted M_2,3M_4,5M_4,5 iron Auger transition with the increase in the sticking coefficient and the disappearance of the c (2 × 2) indicated that oxide nucleated on the surface after the complete formation of the c (2 × 2) structure. The relatively high sticking coefficient during the initial oxidation indicates that formation of a mobile adsorbed oxygen state precedes the formation of oxide.     

A LEED and AES study of the structure,debye temperature,and oxidation of the (111) crystal face of thorium

The structure, and reactivity towards O₂ and CO, of the (111) crystal face of a single crystal of high purity thorium metal was studied using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). After the sample was cleaned in vacuum by a combination of ion bombardment and annealing, a (1 × 1) LEED pattern characteristic of a (111) surface was obtained. Extended annealing of the cleaned sample at 1000 K produced a new LEED pattern characteristic of a (9 × 9) surface structure. A model of a reconstructed thorium surface is presented that generates the observed LEED pattern. When monolayer amounts of either O₂ or CO were adsorbed onto the crystal surface at 300 K, no ordered surface structures formed. Upon heating the sample following these exposures the (111) surface structure was restored accompanied by a reduction in the amount of surface carbon and oxygen. With continued exposure to either O₂ or CO and annealing, a new LEED pattern developed which was interpreted as resulting from the formation of thorium dioxide. Debye-Walter factor measurements were made by monitoring the intensity of a specularly reflected electron beam and indicated that the Debye temperature of the surface region is less than it is in bulk thorium. Consequently, the mean displacement of thorium atoms from their equilibrium positions was found to increase at the surface of the crystal. The presence of chemisorbed oxygen on the crystal surface affected the Debye temperature, raising it significantly.     

The effect of argon bombardment on the oxidation of Fe(110) by oxygen and water

The oxidation of Fe(110) by O₂ and H₂O vapour has been studied by AES and LEED at room temperature in well ordered and in Ar bombarded surfaces. Ion irradiation causes in both cases an enhancement of the rate of oxidation. For O₂ adsorption, it has been found that ion bombardment increases the rate of surface oxide growth, whereas in the case of H₂O adsorption, the effect of the irradiation consists in facilitating the penetration of oxygen into the crystal bulk.     

铁表面氧化的研究

用XPS,UPS和AES研究铁早期氧化过程,认为在温度不太高和氧分压不太大的条件下,多晶铁片与氧作用生成的化合物是Fe₃O₄而不是FeO和α-Fe₂O₃,也不是三种化合物的分层分布。在600℃以上高温下,氧分压近于1×10^-6Torr下,出现了铁表面上氧的热脱附过程而不是氧化过程。 关键词：     

Studies of molecular oxygen adsorbed on Cu surfaces

Molecular oxygen adsorbed on (110) and polycrystalline Cu surfaces has been investigated by UPS, XPS, AES, HREELS and LEED. Molecularly adsorbed O₂ on the (110) surface shows the characteristic three-peak He II spectrum due to π_g, π_u and σ_g orbitals, accompanied by an O-O stretching frequency at 660 cm⁻¹. On the polycrystalline Cu surface, adsorbed O₂ shows the three peak He II spectra with a considerably smaller separation between the π_u and σ_g band and two O-O stretching bands at 610 and 880 cm⁻¹. O₂ adsorbed on the Cu(110) surface gives rise to a (1 × 1) LEED pattern and characteristic K π¹π¹ transition in the Auger spectrum.     

AES studies of palladium films formed on copper and mica

Thin epitaxial films of palladium were grown on epitaxial copper films and cleaved mica in ultra high vacuum. The growth modes of these films were investigated by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), transmission electron microscopy (TEM), and TEM replica techniques. Layer by layer growth of Pd on Cu and mica was observed and inelastic mean free paths of Auger electrons for energies of 60 eV (Cu MMM) and 329 eV (Pd MNN) were calculated. These values were 5.7 and 6.9 Å respectively. The thermal stability of monocrystalline and polycrystalline Pd/Cu bilayer films at 483 K was also investigated by AES and TEM. It was found that Pd agglomerates on the Cu at this temperature to form a Stranski-Krastanov growth morphology. The agglomeration is much more rapid on polycrystalline films, suggesting that high surface diffusivity paths (grain boundaries and possibly other defects) enhance the surface diffusion of Pd on Cu.     

Oxygen chemisorption,surface oxidation,and the oxidation of carbon monoxide on cobalt (0001)

Oxygen chemisorbs on clean Co(0001) at 300 K with an initial sticking probability of ~0.3. The chemisorbed overlayer (which is very reactive towards CO) readily undergoes conversion to cobalt oxide, even at room temperature. This transformation is accelerated at higher temperatures, and the oxygen uptake rate falls as CoO growth proceeds. At a certain point, however, the uptake rate rises sharply, and this behaviour is ascribed to nucleation and growth ofCo₃O₄. This interpretation is consistent with the available Δφ, Auger, LEED, and reactivity data. Thus Δφ changes sign as lattice penetration by the ad sorbed oxygen takes place, and this is accompanied by a shift and broadening of the O(KLL) Auger signal. LEED indicates the epitaxial growth firstly of CoO(111) and then, at higher oxygen exposures, of Co₃O₄(111). At 300 K CO rapidly reduces the Co₃O₄ surface back to CoO, and the oxidation/reduction behaviour by O₂/CO appears to be completely reversible. Steady-state measurements yield a value of 19 ± 7 kJ mol^?1 for the activation energy to CO₂ production from CO + O₂. Earlier photoelectron spectroscopic studies by other authors are considered in the light of these results.     

Preparation and characterization of iron–molybdate thin films

Mixed Fe–Mo oxides are used in industrial catalytic processes of selective oxidation of methanol to formaldehyde. For better understanding of the structure-reactivity relationships of these catalysts we aim to prepare well-ordered iron–molybdate thin films as model catalysts. Here we have studied Mo deposition onto Fe₃O₄ (111) thin films produced on Pt(111) as a function of Mo coverage and annealing temperature using LEED, AES, STM and IRAS. At low temperatures, the iron oxide film is covered by Mo = O terminated molybdena nanoparticles. Upon oxidation at elevated temperatures (T > 900 K), Mo species migrate into the film and form new bonds with oxygen in the film. The resulting films maintain the crystal structure of Fe₃O₄, and the surface undergoes a (√3 × √3)R30° reconstruction. The structure is rationalized in terms of Fe substitution by Mo in the surface layers.     

Etude par spectroscopie de photoelectrons et d'electrons auger des etapes initiales de l'oxydation du chrome polycristallin

The joint application of various techniques of surface analysis (XPS, UPS, AES and work function measurements) allowed us to pinpoint the initial stages in the oxidation of polycrystalline chromium in the temperature range 180–720 K. At weak exposures ( < 10 L) the oxygen was dissociatively adsorbed without providing any information on the amount of oxygen involved or the sites of adsorption (chemisorption vs. physical interaction due to the roughness of the surface). At stronger exposures the oxide Cr₂ O₃ is observed to form, and this is covered by a layer of chemisorbed oxygen at ordinary temperatures and below. The maximum thickness of the oxide layer depends on the temperature, as would be expected from the logarithmic oxidation kinetics. The change in shape of the Auger peaks of chromium during oxidation has been interpreted in terms of interatomic transitions made via the 2p levels of oxygen.     

Oxygen induced surface segregation of Cu on the Au0.7Cu0.3(100) surface

The oxygen induced surface segregation of Cu on the Au_0.7Cu_0.3(100) surface was investigated by means of LEED and AES techniques. The dissociative adsorption of O₂ did not take place on this clean surface for a long time exposure at least up to 10⁴ L, and so the oxygen was forcibly introduced onto the surface through a pre-deposition of few a layers of Cu and its successive oxidation. The oxygen coverage was controlled by a heat treatment, which leads the system to a thermal equilibrium state. For the clean surface, the segregation of Au was clearly observed and the surface concentration of Au was estimated to be about 86%, greater than the bulk concentration of 70%. At low coverages below 0.16 ML, no remarkable oxygen induced segregation of Cu was observed. But, above 0.2 ML, the surface concentration of Cu was proportional to the oxygen coverage. The (2 × 4) LEED pattern was observed in a wide range of oxygen coverage. The maximum intensity of the (2 × 4) was observed at about 0.45 ML.     

Epitaxial growth of well-ordered ultra-thin Al2O3 film on NiAl (1 1 0) by a single-step oxidation

Well-ordered ultra-thin Al₂O₃ films were grown on NiAl (1 1 0) surface by exposing the sample at various oxygen absorption temperatures ranging from 570 to 1100 K at dose rates 6.6 × 10⁻⁵ and 6.6 × 10⁻⁶ Pa. From the results of low-energy electron diffraction (LEED), Auger electron spectrometer (AES) and X-ray photon spectroscopy (XPS) observations, it was revealed that oxidation mechanism above 770 K is different from well-known two-step process. At high temperature, oxidation and crystallization occurred simultaneously while in two-step process oxidation and crystallization occurred one after another. At high-temperature oxidation well-ordered crystalline oxide can be formed by a single-step without annealing. Well-ordered Al₂O₃ layer with thickness over 1 nm was obtained in oxygen absorption temperature 1070 K and a dose rate 6.6 × 10⁻⁶ Pa at 1200 L oxygen.     

Growth of ultra-thin cerium oxide layers on Cu(1 1 1)

The reactive vacuum deposition of CeO₂ on Cu(1 1 1) surface in oxygen atmosphere provides high quality epitaxial ceria overlayers. We report the growth characteristics of Ce oxide, the structures, and the temperature stability of the oxide phases as investigated by low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy. We find that Ce oxide on the Cu(1 1 1) grows initially in the form of islands giving sharp hexagonal LEED pattern of the CeO₂(1 1 1) structure corresponding to the (1.5 × 1.5) structure. The CeO₂-Cu(1 1 1) films exhibited mixed valence states and temperature dependent CeO₂-Ce₂O₃ transition above 900 K due to the vacuum annealing. The transition progressed more rapidly at the surface, probably by formation of oxygen vacancies.     

Autocatalytic partial reduction of FeO(1 1 1) and Fe3O4(1 1 1) films by atomic hydrogen

The interaction of atomic hydrogen with thin epitaxial FeO(1 1 1) and Fe₃O₄(1 1 1) films was studied by TDS, XPS and LEED. On the thin, one Fe-O bilayer thick FeO film, partial reduction occurs in two steps during exposure. It ends after removal of 1/4 monolayer (ML) of oxygen with a 2 × 2 pattern appearing in LEED. This FeO_0.75 film is passive against further reduction. The first reduction step saturates after removal of ∼0.2 ML and shows autocatalytic kinetics with the oxygen vacancies formed during reduction causing acceleration. The second step is also autocatalytic and is related with reduction to the final composition and an improvement of the 2 × 2 order. A structure model explaining the two-step reduction is proposed. On the thick Fe₃O₄ film, irregular desorption bursts of H₂O and H₂ were observed during exposure. Their occurrence appears to depend on the film quality and thus on surface order. Because of the healing of reduction-induced oxygen vacancies by exchange of oxygen or iron with the bulk, a change of the surface composition was not visible. The existence of partially reduced oxide phases resistant even to atomic hydrogen is relevant to the mechanism of dehydrogenation reactions using iron oxides as catalysts.     

The interaction of oxygen with gadolinium: UPS and XPS studies

The interaction of oxygen with evaporated Gd films at 300 K has been studied for the first time using AlK α XPS and Hel and Hell UPS. The characteristic changes in the Gd(6s5d) and O(2p) valence bands, Gd(4f), Gd(5p) and Gd(4d) core levels, and O(2s) and O(1s) core levels were studied. Evidence is presented for the initial formation of an intermediate oxidation state at low exposure (characterized by a new Gd valence band with a maximum in the DOS at ～ 2.5 ev below E_F and an ～ 0.6 eV shift in Gd(4f)) prior to formation of Gd₂O₃ where the Gd(6s5d) valence band disappears completely, as expected for Gd³⁺. In the higher exposure range there is little further increase in the oxide thickness, which is estimated as $\text{? 20}\text{A}\text{?}$, but there is a slow replacement of O by OH, as characterized by a second O(1s) feature at 532.3 eV and OH 1π and 3σ orbitals in UPS at ～ 6.7 and 11.5 eV. The interpretations are supported by parallel studies on bulk Gd₂O₃ and by Ar⁺ sputtering experiments. Comparisons are made to other rare earth oxidation studies.     

Growth of thin, crystalline oxide, nitride, and oxynitride films on NiAl and CoGa surfaces

At 300 K, an amorphous Al-oxide film is formed on NiAl(001) upon oxygen adsorption. Annealing of the oxygen-saturated NiAl(001) surface to 1200 K leads to the formation of thin well-ordered θ-Al₂O₃ films. At 300 K, and low-exposure oxygen atoms are chemisorbed on CoGa(001) on defects and on step edges of the terraces. For higher exposure up to saturation, the adsorption of oxygen leads to the formation of an amorphous Ga-oxide film. The EEL spectrum of the amorphous film exhibits two losses at ≈400 and 690 cm^-1. After annealing the amorphous Ga-oxide films to 550 K thin, well-ordered β-Ga₂O₃ films are formed on top of the CoGa(001) surface. The EEL spectrum of the β-Ga₂O₃ films show strong Fuchs-Kliewer (FK) modes at 305, 455, 645, and 785 cm^-1. The β-Ga₂O₃ films are well ordered and show (2×1) LEED pattern with two domains, oriented perpendicular to each other. The STM study confirms the two domains structure and allows the determination of the two-dimensional lattice parameters of β-Ga₂O₃. The vibrational properties and the structure of β-Ga₂O₃ on CoGa(001) and θ-Al₂O₃ on NiAl(001) are very similar. Ammonia adsorption at 80 K on NiAl(111) and NiAl(001) and subsequent thermal decomposition at elevated temperatures leads to the formation of AlN. Well-ordered and homogeneous AlN thin films can be prepared by several cycles of ammonia adsorption and annealing to 1250 K. The films render a distinct LEED pattern with hexagonal [AlN/NiAl(111)] or pseudo-twelve-fold [AlN/NiAl(001)] symmetry. The lattice constant of the grown AlN film is determined to be a_AlN= 3.11 Å. EEL spectra of AlN films show a FK phonon at 865 cm^-1. The electronic gap is determined to be E_g= 6.1±0.2 eV. GaN films are prepared by using the same procedure on the (001) and (111) surfaces of CoGa. The films are characterized by a FK phonon at 695 cm^-1 and an electronic band gap E_g= 3.5±0.2 eV. NO adsorption at 75 K on NiAl(001) and subsequent annealing to 1200 K leads to the formation of aluminium oxynitride (AlON). An oxygen to nitrogen atomic ratio of ≈2:1 was estimated from the analysis of AES spectra. The AlON films shows a distinct (2×1) LEED pattern and the EEL spectrum exhibits characteristic Fuchs-Kliewer modes. The energy gap is determined to be E_g= 6.6±0.2 eV. The structure of the AlON film is derived from that of θ-Al₂O₃ formed on NiAl(001). Received: 21 March 1997/Accepted: 12 August 1997     

Study of chemisorption on copper low-index and stepped surfaces

Adsorption of CHCl₃, O₂, and hydrocarbons has been studied on Cu(111) and stepped surfaces using LEED, AES, and UPS at room temperature. We find that ordered Cl overlayers form upon Cu(111), Cu[3(111) × (100)], and Cu[5(111) × (100)] surfaces upon exposure to CHCl₃. Exposure to O₂ results in rearrangement of the Cu[5(111) × (100)] surface to hill-and-valley regions with large (111) areas, whereas Cu[2(111) × (100)] is stable for the same exposure. The photoemission spectra show new energy levels due to C1 above and below the Cu d band region and a small splitting of the halogen p orbitals. Effects consistent with interaction with the Cu d band are observed. Similar effects are observed with oxygen adsorption. The initial rate of Cl or O₂ chemisorption as measured by photoemission is proportional to the density of steps on these surfaces. Apparently, structural effects play an important role in chemisorption on metals (such as copper) with low density of states at the Fermi energy.     

Oxidation of ultra-thin zinc films on Rh(100) surface

The oxidation of submonolayer zinc films on Rh(100) surface by O₂ gas has been studied using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM). With a zinc coverage of 0.8 ML, an atomically flat ultra-thin zinc oxide film formed at an oxygen partial pressure of 2 × 10^? 8 mbar and a temperature of 150 °C. The zinc oxide film showed a c(16 × 2) LEED pattern. The high resolution STM image of the zinc oxide film showed single dotted spots and double dotted spots arranged linearly and periodically along the [01¯1] direction. We propose an atomic arrangement model of the film accounting for the LEED pattern, the STM image, and the atomic arrangement of the bulk ZnO(0001) surface.     

高温超导薄膜中的涨落超导电性及其维度

对RBa₂Cu₃O_7-δ(R=Y,Gd)超导薄膜电阻温度关系的仔细测量,观察到在T_c附近有相当强的超导涨落现象,研究发现,多晶和外延的YBa₂Cu₃O_7-δ薄膜在T_c以上均呈现出2D涨落超导电性;而当温度趋向于T_c时,有—2D向3D的转变发生,这与Lawrence-Doniach理论很好地相符,实验中还发现,一 关键词：     

Interaction of NO with a Ni (111) surface

The interaction of NO with a Ni (111) surface was studied by means of LEED, AES, UPS and flash desorption spectroscopy. NO adsorbs with a high sticking probability and may form two ordered structures (c4 × 2 and hexagonal) from (undissociated) NO_ad. The mean adsorption energy is about 25 $\text{kcal}\text{mole}$. Dissociation of adsorbed NO starts already at ?120°C, but the activation energy for this process increases with increasing coverage (and even by the presence of preadsorbed oxygen) up to the value for the activation energy of NO desorption. The recombination of adsorbed nitrogen atoms and desorption of N₂ occurs around 600 °C with an activation energy of about 52 $\text{kcal}\text{mole}$. A chemisorbed oxygen layer converts upon further increase of the oxygen concentration into epitaxial NiO. A mixed layer consisting of N_ad + O_ad (after thermal decomposition of NO) exhibits a complex LEED pattern and can be stripped of adsorbed oxygen by reduction with H₂. This yields an N_ad overlayer exhibiting a 6 × 2 LEED pattern. A series of new maxima at ≈ ?2, ?8.8 and ?14.6 eV is observed in the UV photoelectron spectra from adsorbed NO which are identified with surface states derived from molecular orbitals of free NO. N_ad as well as O_ad causes a peak at ?5.6 eV which is derived from the 2p electrons of the adsorbate. The photoelectron spectrum from NiO agrees closely with a recent theoretical evaluation.