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1.
The survival probability (SP) of metastable helium atoms (He1) during scattering from the clean, alkalated and oxygen-adsorbed Ni(1 1 0) surfaces has been examined in the kinetic energy range of 50–400 meV. The measurements were carried out using a time-of-flight technique and a pulsed-discharge type metastable helium atom source. The SP is nearly constant for a kinetic energy (Ekin) of 50–100 meV and decreases exponentially with the increase in Ekin at 100–400 meV. It has been shown that the SP at Ekin=100–400 meV depends on the repulsive part of the He1-surface interaction potential.  相似文献   

2.
The orientations of oxide nuclei during the oxidation of Cu(100), (110) and (111) surfaces have been examined by in situ transmission electron microscopy. Our results indicate that the epitaxial nucleation of oxide islands on these surfaces cannot be maintained for a whole range of oxygen gas pressure varying from 10? 5 Torr to 750 Torr. The critical oxygen gas pressure, pO2, leading to the transition from nucleating epitaxial to non-epitaxial oxide nuclei shows a dependence on the crystallographic orientations of the Cu substrates with pO2(100) > pO2(111) > pO2(110). By fitting the experimentally determined critical oxygen pressures to a kinetic model, we find that such dependence can be attributed to the effect of surface orientations of the Cu substrates on the oxygen surface adsorption and diffusion, which dominate the kinetic processes of oxide nucleation.  相似文献   

3.
Oxygen chemisorption on β-Mo2C 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 O2 at PO2/P0 of 10? 21–104 and temperature of 100–700 K. For the (011) surface with O2 as the oxidant, the most stable structure is that with 1/2 ML or 7/8 ML oxygen coverage, depending on the temperature and PO2/P0 value. The increase of gaseous oxidant pressure or decrease of temperature can enhance the oxidation of β-Mo2C surface and lead a more negative reaction Gibbs free energy. High temperature and low oxidant pressure may hinder the surface oxidation process.  相似文献   

4.
The properties of surface conductivity (SC) of impurity-non-doped CVD diamond (001) samples were studied by various methods of sheet-resistance (RS) measurement, Hall-effect measurement, XPS, UPS, SES, SR-PES, PEEM and 1D band simulation taking into account special emphases on deriving the information about the surface band diagram (SBD). The RS values in UHV conditions were determined after no-annealing or 200  300 °C annealing in UHV. C 1 s XPS profiles were measured in detail in bulk-sensitive and surface-sensitive modes of photoelectron detection. The energy positions of valence band top (EV) relative to the Fermi level (EF) in UHV conditions after no-annealing or 200  300 °C annealing in UHV were determined. One of the samples was subjected to SR-PES, PEEM measurements. The SBDs were simulated by a band simulator from the determined RS and EV ? EF values for three samples based on the two models of surface conductivity, namely, so-called surface transfer doping (STD) model and downward band bending with shallow acceptor (DBB/SA) model. For the DBB/SA model, there appeared downward bends of SBDs toward the surface at a depth range of ~ 1 nm. C 1 s XPS profiles were then simulated from the simulated SBDs. Comparison of simulated C 1 s XPS profiles to the experimental ones showed that DBB/SA model reproduces the C 1 s XPS profiles properly. PEEM observation of a sample can be explained by the SBD based on the DBB/SA model. Mechanism of SC of CVD diamonds is discussed on the basis of these findings. It is suggested that the STD model combined with SBD of DBB/SA model explains the surface conductivity change due to environmental changes in actual cases of CVD diamond SC with the presence of surface EF controlling defects.  相似文献   

5.
Periodic density functional theory has been utilized to investigate the structure and stability of monomeric HVOx species on anatase support. The three most stable surfaces of anatase were investigated, namely the (001), (100) and (101) surfaces. Unlike previous theoretical studies it was found that on the (001) surface vanadia species with five-coordinated vanadium atom are more stable than those with tetrahedrally coordinated vanadium atom. On the other hand, on the (100) and (101) surfaces, the vanadium atom in the vanadia species is still tetrahedrally coordinated. The stability of different VOx/TiO2 structures which are not fully dehydrated has been systematically studied and the results show that the vanadia species on the three surfaces follow an order of TiO2 (001) > TiO2 (100) > TiO2 (101). This can be understood from the acidity and basicity of the three anatase surfaces. The results suggest that monomeric VOx species may be better stabilized if the support exposes more (001) surfaces. Our analyses on electronic structure of the most stable VOx/TiO2 structure (D001) suggest that its bridging V–O–Ti oxygen atoms may have higher reactivity than the terminal vanadyl oxygen atoms.  相似文献   

6.
A study of porous surfaces having micropores significantly smaller than laser spot on the stainless steel 304L sample surface induced by a picosecond regenerative amplified laser, operating at 1064 nm, is presented. Variations in the interaction regime of picosecond laser pulses with stainless steel surfaces at peak irradiation fluences(Fpk=0.378–4.496 J/cm2) with scanning speeds(v=125–1000 μm/s) and scan line spacings(s=0–50 μm) have been observed and thoroughly investigated. It is observed that interactions within these parameters allows for the generation of well-defined structured surfaces. To investigate the formation mechanism of sub-focus micropores, the influence of key processing parameters has been analyzed using a pre-designed laser pulse scanning layout. Appearances of sub-focus ripples and micropores with the variation of laser peak fluence, scanning speed and scan line spacing have been observed. The dependencies of surface structures on these interaction parameters have been preliminarily verified. With the help of the experimental results obtained, interaction parameters for fabrication of large area homogeneous porous structures with the feature sizes in the range of 3–15 μm are determined.  相似文献   

7.
Density-functional calculations of molecular nitric oxide (NO) on defective (La,Sr)O (001) surfaces of (La,Sr)FeO3 ? δ using slab models are performed to elucidate the oxygen vacancy formation problem on the LaO (001) surface of LaFeO3 ? δ.From the estimation of the NO adsorption energy, NO adsorption is found on (La,Sr)O surfaces of (La0.83,Sr0.17)FeO3 ? δ with δ = 0 or 0.25.The absolute value of the NO adsorption energy shows a remarkable increase at oxygen vacancies in the top surface layer, where the nitrogen atoms of the adsorbed molecules are embedded in the first (La,Sr)O layer, because a bond with Fe in the second FeO2 layer is formed.Our data shows that Sr doping promotes formation of oxygen vacancies, which keep the NO adsorption ability high.Thus, we conclude that if Sr doping increases the number of oxygen vacancy sites by a charge compensation effect, NO adsorption on LaFeO3 is enhanced, which provides an explanation for several experimental observations.  相似文献   

8.
In this work, 150 nm thick polycrystalline BaTiO3 (BTO) films were deposited on Pt/TiO2/SiO2/Si substrate by ion beam assisted sputter deposition technique. The bias voltage dependent resistive switching (RS) and ferroelectric polarization characteristics of Au/BTO/Pt devices are investigated. The devices display the stable bipolar RS characteristics without an initial electroforming process. Fittings to current–voltage (I–V) curves suggest that low and high resistance states are governed, respectively, by filamentary model and trap controlled space charge limited conduction mechanism, where the oxygen vacancies act as traps. Presence of oxygen vacancies is evidenced from the photoluminescence spectrum. The devices also display P–V loops with remnant polarization (Pr) of 5.7 μC/cm2 and a coercive electric field (Ec) of 173.0 kV/cm. The coupling between the ferroelectric polarization and RS effect in BTO films is demonstrated.  相似文献   

9.
Lijun Xu  Ye Xu 《Surface science》2010,604(11-12):887-892
The adsorption and activation of methyl acetate (CH3COOCH3), one of the simplest carboxylic esters, on Pd(111) have been studied using self-consistent periodic density functional theory calculations. Methyl acetate adsorbs weakly through the carbonyl oxygen. Its activation occurs via dehydrogenation, instead of direct C–O bond dissociation, on clean Pd(111): It is much more difficult to dissociate the C–O bonds (Ea  2.0 eV for the carbonyl and acetate–methyl bonds; Ea = 1.0 eV for the acetyl–methoxy bond) than to dissociate the C–H bonds to produce enolate (CH2COOCH3; Ea = 0.74 eV) or methylene acetate (CH3COOCH2; Ea = 0.82 eV). The barriers for C–H and C–O bond dissociation are directly calculated for enolate and methylene acetate, and estimated for further dehydrogenated derivatives (CH3COOCH, CH2COOCH2, and CHCOOCH3) based on the Brønsted–Evans–Polanyi linear energy relations formed by the calculated steps. The enolate pathway leads to successive dehydrogenation to CCOOCH3, whereas methylene acetate readily dissociates to yield acetyl. The selectivity for dissociating the acyl–alkoxy C–O bond, which is desired for alcohol formation, is therefore fundamentally limited by the facility of dehydrogenation under vacuum/low-pressure conditions on Pd(111).  相似文献   

10.
Polycrystalline (Bi0.6K0.4) (Fe0.6Nb0.4)O3 material has been prepared using a mixed-oxide route at 950 °C. It was shown by XRD that at room temperature structure of the compound is of single-phase with hexagonal symmetry. Some electrical characteristics (impedance, modulus, conductivity etc.) were studied over a wide frequency (1 kHz–1 MHz) and temperature (25–500 °C) ranges. The Nyquist plot (i.e., imaginary vs real component of complex impedance) of the material exhibit the existence and magnitude of grain interior and grain boundary contributions in the complex electrical parameters of the material depending on frequency, input energy and temperature. The nature of frequency dependence of ac conductivity follows Joncher׳s power law, and dc conductivity follows the Arrhenius behavior. The appearance of PE hysteresis loop confirms the ferroelectric properties of the material with remnant polarization (2Pr) of 1.027 µC/cm2 and coercive field (2Ec) of 16.633 kV/cm. The material shows very weak ferromagnetism at room temperature with remnant magnetization (2Mr) of 0.035 emu/gm and coercive field (2Hc) of 0.211 kOe.  相似文献   

11.
S. Cohen  N. Shamir  M.H. Mintz  I. Jacob  S. Zalkind 《Surface science》2011,605(15-16):1589-1594
Auger-Electron-Spectroscopy (AES) and Direct-Recoils-Spectrometry (DRS) were applied to study the interaction of O2 with a polycrystalline gadolinium surface, in the temperature range 300–670 K and oxygen pressure up to 2 × 10? 6 Torr. It has been found that initial uptake of oxygen, at coverage measurable by the techniques used here, results in rapid oxide island formation. The subsurface is believed to be a mixture of oxide particles and oxygen dissolved in the Gd metal, the latter being the mobile species, even at relatively low temperatures.Enhanced inward diffusion of oxygen starts as early as 420 K and dictates the surface oxygen concentration and effective thickness of the forming oxide. The oxygen accumulation rate at the near-surface region, as measured by the O(KLL) AES signal intensity, goes through a maximum as a function of temperature at 420 K. This is a result of the combination of still efficient oxygen chemisorption that increases surface occupation and slow inward diffusion. The thickest oxide, ~ 1.7 nm, is formed at 300 K and its effective thickness was found to decrease with increasing temperature (due to oxygen dissolution into the metal bulk).Diffusion coefficients of the oxygen dissolution into the bulk were evaluated for various temperatures utilizing models for infinitely thin oxide layer and thick oxide layer, respectively. The best fit under our experimental procedure was obtained by the thick layer model, and the coefficients that were calculated are D0 = 2.2 × 10? 16m2s? 1 and Ea = 46kJ/mol.  相似文献   

12.
Reflection absorption infrared spectroscopy (RAIRS) and high resolution electron energy loss spectroscopy (HREELS) have been used to study the adsorption of oxygen on the (100) and (111) surfaces of lanthanum hexaboride. Exposure of the surface at temperatures of 95 K and above to O2 produces atomic oxygen on the surface and yields vibrational peaks in good agreement with those observed in previous HREELS studies. On the La-terminated (100) surface, RAIRS peaks correspond to vibrations of the boron lattice that gain intensity due to a decrease in screening of surface dipoles that accompanies oxygen adsorption. A sharp peak at ~ 734 cm?1 in the HREEL spectrum shows isotopic splitting with RAIRS into two components at 717 and 740 cm?1 with full widths at half maxima of only 12 cm?1. The sharpness of this mode is consistent with its interpretation as a surface phonon that is well separated from both the bulk phonons and other surface phonons of LaB6. On the boron-terminated LaB6(111) surface, broad and weak features are assigned to both vibrations of the boron lattice and of boron oxide. On the (100) surface, oxygen blocks the adsorption sites for CO, and adsorbed CO prevents the dissociative adsorption of O2.  相似文献   

13.
Effects of compressive stress on the ferroelectric properties of ceramics in PZT–PZN system were investigated. The ceramics with a formula (1−x)Pb(Zr1/2Ti1/2)O3xPb(Zn1/3Nb2/3)O3 or (1−x)PZT–(x)PZN (x = 0.1–0.5) were prepared by a conventional mixed-oxide method. The ferroelectric properties under the compressive stress of the PZT–PZN ceramics were observed at the stress levels up to 170 MPa using a compressometer in conjunction with a modified Sawyer–Tower circuit. It was found that with increasing compressive stress the area of the ferroelectric hysteresis (PE) loops, the saturation polarization (Psat), the remnant polarization (Pr), and the coercive field (Ec) decreased. These results were interpreted through the non-180° ferroelastic domain switching processes.  相似文献   

14.
X-ray photoelectron core-level and valence-band spectra for pristine and Ar+-ion irradiated (001) surfaces of KPb2Br5, K0.5Rb0.5Pb2Br5, and RbPb2Br5 single crystals grown by the Bridgman method have been measured and fundamental absorption edges of the ternary bromides have been recorded in the polarized light at 300 K and 80 K. The present X-ray photoelectron spectroscopy (XPS) results reveal high chemical stability of (001) surfaces of KxRb1?xPb2Br5 (x=0, 0.5, and 1.0) single crystals. Substitution of potassium for rubidium in KxRb1?xPb2Br5 does not cause any changes of binding energy values and shapes of the XPS constituent element core-level spectra. Measurements of the fundamental absorption edges indicate that band gap energy, Eg, increases by about 0.14 and 0.19 eV when temperature decreases from 300 K to 80 K in KPb2Br5 and RbPb2Br5, respectively. Furthermore, there is no dependence of the Eg value for KPb2Br5 upon the light polarization, whilst the band gap energy value for RbPb2Br5 is bigger by 0.03–0.05 eV in the case of E6c compared to those in the cases of E6a and E6b.  相似文献   

15.
The dielectric and pyroelectric responses of MgO-modified Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 ceramics were investigated near FR(LT)–FR(HT) phase transition. It was found that MgO additive reduced the FR(LT)–FR(HT) phase transition temperature from 41 °C to room temperature (24 °C). Superior room-temperature pyroelectric properties were obtained in the composition of 0.10 wt% MgO addition without DC bias. The largest pyroelectric coefficient, 65 × 10−8 C cm−2 K−1, was detected. Accordingly, the detectivity figures of merit Fd had maximum values of 20 × 10−5 Pa−1/2, and especially the voltage responsivity Fv = 0.91 m2C−1 is the highest value reported so far among all pyroelectric materials. It shows promising potential for application in uncooled pyroelectric infrared detector.  相似文献   

16.
《Current Applied Physics》2010,10(3):749-756
We report the synthesis, electrochemical and photovoltaic properties of triphenylamine based novel organic sensitizer, comprising donor, electron-conducting and anchoring group. Different triphenylamines were synthesized as donor unit and connected to cyano-acidic acid acceptor over vinyl group. In this work, we studied on two novel dyes (TPA2, TPA3) by attaching methyl to m-position and diphenylamine to p-position of the basic triphenylamine structure, respectively. All derivatives absorb at visible region of solar spectrum in the range of 350–632 nm. By introducing strong electron injection groups to triphenylamine ring, band gap becomes narrower while EHOMOELUMO levels are tuned. All current voltage (IV) measurements were done under 100 mW/cm2 light intensity and AM 1.5 conditions. TPA3 chromophore with the lowest band gap shows the best cell performance with an efficiency (η) of 4.12%, a short-circuit photocurrent density (Isc) of 8.07 mA/cm2, an open-circuit voltage (Voc) of 714 mV and a fill factor (FF) of 0.72.  相似文献   

17.
Continuous-time photoelectron spectroscopy (PES) and photon-exposure-dependent photon-stimulated desorption (PSD) were employed to investigate the monochromatic soft X-ray-induced dissociation of SF6 molecules adsorbed on Si(111)-7 × 7 at 30 K (SF6 dose = 3.4 × 1013 molecules/cm2, ~ 0.5 monolayer). The photon-induced evolution of adsorbed SF6 was monitored at photon energies of 98 and 120 eV [near the Si(2p) edge], and sequential valence-level PES spectra made it possible to deduce the photolysis cross section as a function of energy. It was found that the photolysis cross sections for 98 and 120 eV photons are ~ 2.7 × 10? 17 and ~ 3.7 × 10?17 cm2, respectively. The changes in the F? and F+ PSD ion yields were also measured during irradiation of 120 eV photons. The photon-exposure dependencies of the F? and F+ ion yields show the characteristics: (a) the dissociation of adsorbed SF6 molecules is ascribable to the substrate-mediated dissociations [dissociative attachment (DA) and dipolar dissociation (DD) induced by the photoelectrons emitting from the silicon substrate]; (b) at early stages of photolysis, the F? yield is mainly due to DA and DD of the adsorbed SF6 molecules, while at high photon exposure the F? formation by electron capture of the F+ ion is likely to be the dominant mechanism; (c) the F+ ion desorption is associated with the bond breaking of the surface SiF species; (d) the surface SiF is formed by reaction of the surface Si atom with the fluorine atom or F? ion produced by scission of S–F bond of SFn (n = 1–6) species.  相似文献   

18.
In the silicon wet etching process, the “pseudo-mask” formed by the hydrogen bubbles generated during the etching process is the reason causing high surface roughness and poor surface quality. Based upon the ultrasonic mechanical effect and wettability enhanced by isopropyl alcohol (IPA), ultrasonic agitation and IPA were used to improve surface quality of Si (1 1 1) crystal plane during silicon wet etching process. The surface roughness Rq is smaller than 15 nm when using ultrasonic agitation and Rq is smaller than 7 nm when using IPA. When the range of IPA concentration (mass fraction, wt%) is 5–20%, the ultrasonic frequency is 100 kHz and the ultrasound intensity is 30–50 W/L, the surface roughness Rq is smaller than 2 nm when combining ultrasonic agitation and IPA. The surface roughness Rq is equal to 1 nm when the mass fraction of IPA, ultrasound intensity and the ultrasonic frequency is 20%, 50 W and 100 kHz respectively. The experimental results indicated that the combination of ultrasonic agitation and IPA could obtain a lower surface roughness of Si (1 1 1) crystal plane in silicon wet etching process.  相似文献   

19.
Temperature-programmed-desorption (TPD) spectra and isothermal desorption rates of D2 molecules from a Si(100) surface have been calculated to reproduce experimental β1, A-TPD spectra and isothermal desorption rate curves. In the diffusion-promoted-desorption (DPD) mechanism, hydrogen desorption from the Si(100) (2 × 1) surfaces takes place via D atom diffusion from doubly-occupied Si dimers (DODs) to their adjacent unoccupied Si dimers (UODs). Taking a clustering interaction among DODs into consideration, coverages θDU of desorption sites consisting of a pair of a DOD and UOD are evaluated by a Monte Carlo (MC) method. The TPD spectra for the β1, A peak are obtained by numerically integrating the desorption rate equation R = νA exp(? Ed, A / kBT)θDU, where νA is the pre-exponential factor and Ed, A is the desorption barrier. The TPD spectra calculated for Ed, A = 1. 6 eV and νA = 2.7 × 109 /s are found to be in good agreement with the experimental TPD data for a wide coverage range from 0.01 to 0.74 ML. Namely, the deviation from first-order kinetics observed in the coverage dependent TPD spectra as well as in the isothermal desorption rate curves can be reproduced by the model simulations. This success in reproducing both the experimental TPD data and the very low desorption barrier validates the proposed DPD mechanism.  相似文献   

20.
《Solid State Ionics》2006,177(19-25):1725-1728
Apatite-type La10  xSi6  yAlyO27  3x/2  y/2 (x = 0–0.33; y = 0.5–1.5) exhibit predominant oxygen ionic conductivity in a wide range of oxygen partial pressures. The conductivity of silicates containing 26.50–26.75 oxygen atoms per formula unit is comparable to that of gadolinia-doped ceria at 770–870 K. The average thermal expansion coefficients are (8.7–10.8) × 10 6 K 1 at 373–1273 K. At temperatures above 1100 K, silicon oxide volatilization from the surface layers of apatite ceramics and a moderate degradation of the ionic transport with time are observed under reducing conditions, thus limiting the operation temperature of Si-containing solid electrolytes.  相似文献   

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