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Recently, the observations of spontaneous out-of-plane electric polarization and ferroelectric switching in WTe2 related devices with semimetal property a have generated considerable attentions. The polar structure of WTe2 was proposed to be vital, however there is still no consensus about the physical origin, which deserves detailed surface analysis. Here, by using angle-dependent X-ray photoelectron spectroscopy (ADXPS), we observed an obvious downward band bending in WTe2 after slightly surface oxidation, which can't be observed on the freshly cleaved WTe2. And it will disappear after fully oxidation, demonstrating strong correlation between band bending and surface oxidation. Our study suggests surface oxidation and resulted band bending in WTe2 should be taken into account in the WTe2-related devices, such as ferroelectric.

Oxygen chemisorption on β-Mo 2 C surface and its oxidation have been investigated by using the density functional theory with the periodic models. Two surfaces of (011) and (101) were chosen to perform the calculations and the most stable surface structures together with the energetics of oxygen stepwise adsorption were identified. Thermodynamic effect of temperature and reactant pressure on the chemisorption and surface oxidation was investigated. The results suggest that the (101) surface is more active than the (011) surface towards the oxygen adsorption. The (101) surface can be fully oxidized by O 2 at P O2 / P 0 of 10 − 21 –10 4 and temperature of 100–700 K. For the (011) surface with O 2 as the oxidant, the most stable structure is that with 1/2 ML or 7/8 ML oxygen coverage, depending on the temperature and P O2 / P 0 value. The increase of gaseous oxidant pressure or decrease of temperature can enhance the oxidation of β-Mo 2 C surface and lead a more negative reaction Gibbs free energy. High temperature and low oxidant pressure may hinder the surface oxidation process.

The initial stages of surface oxidation of Fe–17Cr (ferritic stainless steel) were investigated at 323 K by X-ray photoelectron spectroscopy (XPS) and inelastic electron background analysis. The results indicated the formation of a mixed iron–chromium oxide layer upon O2 exposure and the formation of a thin chromium oxyhydroxide layer upon H2O exposure. The oxidation of Fe did not occur in the latter case. Moreover, it was found that pre-exposing the Fe–17Cr surface to H2O significantly hinders subsequent oxidation by O2, thus providing a way to control the formation of nanoscale oxides on stainless steel materials. It was concluded that the formation of strongly bound hydroxyl species together with adsorbate-induced segregation of Cr severely limits the reaction between O2/H2O and Fe from the alloy.

In previous works a theoretical analysis was developed for the interactions of a pure liquid metal and an oxidizing atmosphere in both an inert gas carrier and a vacuum. In particular, the effects of oxidation reactions taking place in the gas phase and the evaporation of the condensed oxides were considered. In both cases the presence of oxidation phenomena contributed to maintaining interface cleanness and then strongly affected the shape of the boundary separating the oxidation and de-oxidation regimes. In this work we present a more general physico-mathematical analysis, which is extended to liquid binary alloys and their oxides. The theoretical results obtained indicate that the behaviour of the alloy towards oxygen tends to be similar to that of the less oxidizable component: so the existence of a volatile oxide can often prevent surface oxidation even if the corresponding metal is present in low percentages.

An UHV tribometer was used to make measurements of friction between the five-fold surfaces of two single-grain Al 70 Pd 21 Mn 9 quasicrystals. The surfaces of these quasicrystal samples were either truly clean or modified by controlled adsorption of desired species. The results of many (∼200) single-pass friction measurements using clean quasicrystal surfaces indicate that the average static friction coefficient is μ s =0.60±0.08. The friction coefficient of the perfectly clean quasicrystal surfaces is lower than those reported for the perfectly clean surfaces of many pure metals but is higher than those measured on quasicrystal surfaces exposed to air. Under shear the quasicrystal surfaces slide over one another without exhibiting stick-slip behavior or adhesion. This behavior differs from clean metal surfaces that often exhibit adhesion after being brought into contact in ultra-high vacuum. Studies were performed to determine the effects of surface oxidation, surface structure and surface composition on quasicrystal friction. During the early stages of oxidation the static friction coefficient decreased to a limit of μ s =0.35±0.05. More subtle changes to the quasicrystal surfaces such as adsorption of sulfur or formation of a thin Al–Pd–Mn alloy of CsCl structure produced no significant changes in the static friction coefficient.

The atomic and electronic structure of several low index surfaces of CaF 2 were calculated using the Hartree–Fock method, a slab model and the crystal -95 computer code. The calculated value of the (111) surface energy is 0.47 J m −2 . Two types of stepped surfaces have been considered, namely the (331) and (211) surface. These surfaces have 3.2 A high steps with different atomic configuration. The step energy was found to be different for these two types of steps. The structure of the CaF 2 (111)/CaO(111) interface was calculated using the same method. The O(2p) band of CaO was found 2.7 eV above the F(2p) band edge of CaF 2 .

Ultra-high vacuum scanning tunneling microscopy and depth profiling X-ray photoelectron spectroscopy were used to evaluate two methods for cleaning Si 1− x Ge x (001) films using ex-situ surface oxidation followed by in-situ oxide desorption at temperatures ≤ 1025°C. Dry ultra-violet ozone cleaning was found to be fast, simple, and highly effective for cleaning Si 1− x Ge x (001) surfaces with a wide range of Ge content as well as standard Si(001) surfaces, consistently yielding lower surface particulate densities than a standard wet chemical cleaning technique.

Evidence is presented for the involvement of surface oxidation phenomena in the photoluminescence response obtained on GaAs(110) surfaces. By using high intensity HeNe laser illumination on freshly cleaved (110) n-type GaAs surfaces, room temperature photoluminescence transients are observed having magnitudes determined by excitation intensity. The observed transients are in excess of the steady state photoluminescence signal on surfaces exposed to normal ambient air conditions. A model based on photodesorption and photoadsorption processes, is proposed to explain this phenomenon.

Multilayer surface oxidation of polycrystalline Pt wire is studied for the first time at moderate oxygen pressures using a high precision electrical resistance technique. The dependence of the oxidation rate on pressure, temperature, and time is investigated. Oxide films of up to 90 nm thickness are grown, and the temperature region for rapid dissociation of the oxide is identified. The dissociation pressure-temperature relationship for the oxide is determined in the region 450–600°C, yielding high precision values for the heat of decomposition and entropy change. A two zone model for the electrical resistance is developed and shown to agree qualitatively with the experimental results. The oxide is identified as the β phase of PtO 2 on the basis of its heat of decomposition, electron diffraction pattern, and acid solubility properties.

In order to examine the effect of surface sulfur on the oxidation of zirconium, we have tried to make a quantitative analysis of sulfur segregation on Zr and made a study of the oxidation of sulfur segregated Zr. Depth profiles of the S-segregated Zr surface determined after high temperature annealing in vacuum show that the saturated surface concentration of sulfur has an S/Zr atomic ratio of about 1 and the total amount of segregated S is about 1 monolayer. These values do not depend on the annealing time. The S-segregated surface is exposed to an oxygen atmosphere in a pressure range of 10 −6 −10 −5 Pa up to a total exposure of 9000 L at RT. Changes of the surface concentration of S, O and Zr are monitored by in situ AES analysis. Depth profiling of O, S and Zr for the oxidized specimen is also carried out. Surface oxidation is clearly delayed by segregated sulfur. This effect, however, is not so significant as for the SFe system, and three oxidation stages are clearly distinguished in the same manner as in the oxidation of clean Zr surfaces. This is because S is simultaneously oxidized and removed in the form of a volatile oxide such as SO or SO 2 .

Photooxidation results in an enhanced vaporization rate of As2S3 due to the high volatility of the reaction products, AS4O6 and free sulphur. Measurements of the light-enhanced vaporization (LEV) rate exhibit a strong dependence on the substrate material on which the As2S3 film is deposited. Measurements also show that the LEV rate is independent of film thickness, over a wide range of film thickness. A possible explanation is that the screening length of the substrate-As2S3 contact potential is unexpectedly long, (greater than 5 Mm). If so it is proposed that the unscreened substrate As2S3 contact potential affects a charge transfer rate-limiting step in the surface oxidation reaction through a Fermi level shift in the As2S3. Another possibility is the doping of the As2S3 film by impurities migrating from the substrate.

A clean Zr surface, prepared by several cycles of heating and Ar ion sputtering, is exposed to oxygen gas under 10−5–10−6 Pa at room temperature (RT), and surface oxidation behavior is examined by in-situ AES measurements. Subsequent depth profiling of the oxidized sample is carried out and the oxygen diffusion coefficient in α-Zr is evaluated. All AES peaks of Zr and O are modified with increasing oxygen exposure. The changes of the AES peaks show three stages of oxidation which are attributed to (1) oxygen solution in α-Zr, (2) nucleation and growth of ZrO2 on the surface and (3) growth of the ZrO2 layer. Above 9000 L, the surface is completely covered with ZrO2 and the present AES study shows no evidence of the appearance of suboxide suggested by Sen et al. and de Gonzalez et al. The depth profiling of the oxidized sample indicates coexistence of ZrO2 and α-Zr(O) with an oxygen content of around 30 at% over a depth of several nm without any clear-cut boundary of ZrO2 and α-Zr(O). The apparent oxygen diffusion coefficient at RT estimated using a simple model, 10−21 m2 s−1, is much larger than the extrapolated value, around 10−40 m2 s−1, from the literature at high temperatures.