Adsorption of V on a hematite (0 0 0 1) surface and its oxidation: Submonolayer coverage

The adsorption of submonolayer V on an idealized model hematite (0 0 0 1) surface and subsequent oxidation under atomic O adsorption are studied by density functional theory. The preferred adsorption sites, adsorption energy and configuration changes due to V and O adsorption are investigated. It is found that in most cases V forms threefold bonds with surface O atoms, inducing a large geometry change at the hematite surface and near surface region and a bond stretch between surface Fe and O. The adsorption energy is mainly decided by interplay between adsorbed metal-surface oxygen bonding and adsorbed metal - subsurface metal interaction. The relative energy of subsequent O adsorption and geometry depends on the reformed V/hematite structure. Electronic properties such as projected densities of states and chemical state change upon V adsorption are studied through both periodic slab and embedded cluster localized orbital calculations; both strong vanadium-oxygen and vanadium-iron interactions are found. While V generally donates electrons to a hematite surface, causing nearby Fe to be partially reduced, the Fe and V oxidization state depends very much on the coverage and detailed adsorption configuration. When the V/hematite system is exposed to atomic O, V is further oxidized and surface/near surface Fe is re-oxidized. Our theoretical results are compared with X-ray surface standing wave and X-ray photoelectron spectroscopic measurements. The influence of d-electron correlation on the predicted structures is briefly discussed, making use of the DFT + U scheme.     

Effects of V/III ratio on the growth of a-plane GaN films

The non-polar a-plane GaN is grown on an r-plane sapphire substrate directly without a buffer layer by metal-organic chemical vapour deposition and the effects of V/III ratio growth conditions are investigated. Atomic force microscopy results show that triangular pits are formed at a relatively high V/III ratio, while a relatively low V/III ratio can enhance the lateral growth rate along the c-axis direction. The higher V/III ratio leads to a high density of pits in comparison with the lower V/III ratio. The surface morphology is improved greatly by using a low V/III ratio of 500 and the roughness mean square of the surface is only 3.9 nm. The high resolution X-ray diffraction characterized crystal structural results show that the rocking curve full width at half maximum along the m axis decreases from 0.757° to 0.720°, while along the c axis increases from 0.220° to 0.251° with the V/III increasing from 500 μmol/min to 2000 μmol/min, which indicates that a relatively low V/III ratio is conducible to the c-axis growth of a-plane GaN.     

Structure of V thin films on Al(100) using XPD,LEED, and LEIS

We have used X-ray photoelectron diffraction (XPD), low energy electron diffraction (LEED), and low energy ion scattering (LEIS) to determine the atomic structure of V thin films grown on the Al(100) single crystal surface. For V film thicknesses ranging from 0.75 ML–7 ML, the LEED patterns show no significant changes from the p(1 × 1) symmetry of a clean Al(100) surface, other than becoming more diffuse for higher V coverage. XPD and LEIS spectra indicate that during the initial deposition (1–3 ML) V atoms diffuse into the Al lattice, and tend to accumulate at the surface during subsequent deposition. Strain associated with the lattice mismatch for V and Al is likely relieved by the formation of a surface alloy during the first few ML's of V deposition. For 7 ML V coverage, XPD and LEED showed that an ordered V structure is stabilized on the Al(100) surface. Due to the close resemblance of the V and Al XPD polar scans, we conclude that V and Al are occupying similar lattice sites in tetragonally distorted bcc lattice, although the specific lattice location of the V and Al atoms is not clear.     

Photonic Electric-Field Sensor Utilizing an Asymmetric Ti:LiNbO3 Mach–Zehnder Interferometer with a Dipole Antenna

Abstract

The use of an asymmetric Ti:LiNbO₃ Mach–Zehnder interferometer with a dipole antenna to measure an electric-field strength is described in this article. The device has a small size of 46 × 7 × 1 mm and operates at a wavelength of 1.3 μm. The AC output characteristics show the modulation depth of ~75% at V_π voltage of ~5.3 V. The minimum detectable electric fields are ~0.28 V/m and ~0.646 V/m, corresponding to a dynamic range of about ~32 dB and ~26 dB at frequencies 20 MHz and 50 MHz, respectively. The sensors exhibit an almost linear response for the applied electric-field intensity from 0.298 V/m to 29.84 V/m.     

On the quality of the Nimbus 7 LIMS version 6 ozone for studies of the middle atmosphere

The Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS) radiance profile dataset of 1978/79 was reconditioned and reprocessed to Version 6 (V6) profiles of temperature and species that are improved significantly over those from Version 5 (V5). The LIMS V6 dataset was archived for public use in 2002. Improvements for its ozone include: (1) a more accurate accounting for instrument and spacecraft motion effects in the radiances, (2) the use of better spectroscopic line parameters for its ozone forward model, (3) retrievals of all its scans, (4) more accurate and compatible temperature versus pressure profiles (or T(p)) that are needed for the registration of the ozone radiances and for the removal of temperature effects from them, and (5) a better accounting for interfering species in the lower stratosphere. The retrieved V6 ozone profiles extend from near cloud top altitudes to about 80 km and from 64S to 84N latitude with better sampling along the orbit than for the V5 dataset. Calculated estimates of the single-profile precision and accuracy are provided; precision estimates based on the data themselves are of order 3% or better from 1 to 30 hPa. Estimates of total systematic error are hard to generalize because the separate sources of error may not all be of the same sign, and they depend somewhat on the atmospheric state. It is estimated that the accuracy of the V6 zonal mean ozone distribution is within ±9% from 50-10 hPa, improving to ±7% in the uppermost stratosphere. Simulation studies show that the LIMS T(p) retrievals are underestimating slightly the small amplitudes of the atmospheric temperature tides, which affect the retrieved day/night ozone differences. There are also small biases in the middle to lower stratosphere for the ascending versus descending node LIMS ozone, due principally to not accounting for the asymmetric weighting of its radiance within the tangent layer. The total accuracy for the LIMS ozone was assessed by comparing its daily zonal mean, daytime distributions against those from the Nimbus 7 SBUV Version 8 (V8) dataset for the same period. The LIMS V6 ozone agrees well with SBUV, except between 2 and 5 hPa where the LIMS ozone is greater. That bias is related to the differing vertical resolutions and forward models for the two experiments. The accuracy for LIMS V6 ozone in the lower stratosphere is improved over that reported for V5, as indicated by a small set of V6 comparisons with ECC ozonesonde profiles. Comparisons of diurnal, photochemical model calculations with the monthly-averaged, upper stratospheric ozone obtained with LIMS V6 indicate only a slight ozone deficit for the model at about 2 hPa. However, that deficit exhibits little to no seasonal variation and is in good agreement with similar model comparisons for a seasonal time series of ozone obtained with ground-based microwave instruments. Because the LIMS V6 ozone has improved accuracy and sampling versus that of V5 for the lower stratosphere it should now be possible to conduct quantitative studies of ozone transport and chemistry for the northern hemisphere, polar winter/spring of 1978/79—a time period when the catalytic loss of ozone due to reactive chlorine should not have been a major factor for the Arctic stratosphere.     

Bias temperature instability analysis on memory properties improved by hydrogen annealing treatment in Ti/HfOx /Pt capacitors

The influence of the hydrogen annealing treatment on the reliability of Ti/HfO_x /Pt capacitors is investigated by analyzing bias temperature instability (BTI). As compared to the N₂‐annealed sample, in the case of hydrogen‐annealed samples, both the set/reset voltages and currents are reduced from 6.5 V/–1.6 V to 3.8 V/–1.2 V and from 4 mA/170 nA to 800 µA/30 nA at 0.1 V, respectively. Of particular interest is the dramatic reduction in the set voltage variation from 3.3 V to 1.8 V. In addition, in BTI experiments, the current shift at the high resistance state (HRS) is reduced from 1.5 µA to 40 nA under a bias stress of –1 V/1000 s and from 40 µA to 0.5 µA under a temperature stress of 120 °C/1000 s. These results show that the hydrogen annealing treatment is very effective in improving the reliability of RRAM cells because it reduces the leakage current under bias temperature stress. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Perturbed γ−γ angular correlations: A spectroscopy for point defects in metals and alloys

Atomic defects which migrate and trap at impurity probe atoms can be labelled by the changes they induce in the hyperfine interactions of the probe nuclei. Many studies have been made using perturbed γ−γ angular correlations (PAC) and the¹¹¹In probe because of the excellent resolution of different sites. Identification of the bound states is the key problem in applying hyperfine interactions methods to point defects studies. In this study three structure-sensitive methods are applied to help identify the atomic structures of various multivacancy complexes in Pt and Au: (1) Quadrupole interaction parameters are compared with results of point-charge calculations of electric-field gradients for 20 structures containing 1–4 vacancies in the fcc lattice. (2) Hyperfine interactions induced by decorating vacancy complexes with hydrogen atoms are measured and interpreted with the assistance of point-charge calculations. (3) Transformations between complexes observed by annihilation of vacancies by mobile self-interstitials are used to test the consistency of the identifications. Using these methods in conjunction with analysis of the trapping behavior which occurs during annealing of damaged samples, structural models are presented for divacancy (2V), 3V and 4V complexes in Pt, and 3V and 4V complexes in Au. The activation temperatures of the 3V defect in Au and Pt are determined to be 162 K and 390 K, respectively, and activation temperatures of defects in Ni, Cu, Pt and Au are compared. For Pt, trapping of H at 1V and 2V complexes is observed to lead to small changes in the quadrupole interactions, consistent with well shielded protonic charges. However, trapping at 3V and 4V complexes leads to very large changes which we attribute to atomic restructuring to the defect complexes. Finally, the application of the same methodology to interpret recent experiments on NiAl, an ordered alloy, is described.     

Memory Effect of Metal--Insulator--Silicon Capacitors with SiO2/HfO2/Al2O3 Dielectrics

Charge trapping characteristics of the metal-insulator-silicon （MIS） capacitors with Si02/HfO2//A12O3 stacked dielectrics are investigated for memory applications＇. A capacitance-voltage hysteresis memory window as large as 7.3 V is achieved for the gate voltage sweeping of ±12 V, and a fiat-band voltage shift of 1.5 V is observed in terms of programming under 5 V and I ms. Furthermore, the time- and voltage-dependent charge trapping characteristics are also demonstrated, the former is related to charge trapping saturation and the latter is ascribed to variable tunnelling barriers for electron injecting and discharging under different voltages.     

Photochemical conversion and optical absorption spectra of V centers in coloured KI crystals

Abstract

The initial V₂ and V₃ bands observed after quenching of coloured KI crystals are found to be the superposition of absorption bands due to two kinds of V centres with ?110? and ?111? symmetries. The ratio of concentrations of the former to the latter V centers is estimated to be about 3:1. When a quenched crystal is irradiated at 19 K with V₃-light in the 270-nm region, V centers are bleached forming I_2-like centers. On the basis of the I₃ molecule-ion model bound to a cation vacancy, a review is given of recent work on the photochemical conversion of V centers at low temperatures.     

Structure and magnetism of the V/Ta(0 0 1) surface: A DFT calculation

Calculations using the density functional theory, norm-conserving pseudopotentials and the generalized gradient approximation are performed for vanadium (V) and tantalum (Ta) clean surfaces, and the V/Ta(0 0 1) system. Vanadium and Ta(0 0 1) clean surfaces are found to be nonmagnetic with large inward relaxations of 9.81% and 11.19%, respectively, in good agreement with experiment and other calculations. However, we obtained an appreciable magnetic moment of 2.24 μB for the V monolayer in the V/Ta(0 0 1) system. The inward relaxation is reduced to 6.01% for the V overlayer on Ta(0 0 1) as a result of its spin polarization. An induced magnetic moment of 0.41 μB is obtained on the Ta interfacial layer, which is found to be anti-ferromagnetically coupled with the V overlayer and the Ta layers below.     

The barrier-height inhomogeneity in identically prepared Ni/n-type 6H-SiC Schottky diodes

The effective barrier heights and ideality factors of identically fabricated Ni/n-type 6 H-SiC Schottky diodes (23 dots) have been calculated from their experimental forward bias current–voltage (I–V) and reverse bias capacitance–voltage (C–V) characteristics. A statistical study related to the experimental barrier heights (BHs) and ideality factors of the diodes has been made. The effective Schottky barrier heights (SBHs) and ideality factors obtained from the I–V and C–V characteristics have differed from diode to diode. The BHs obtained from the I–V characteristics varied from 0.85 to 1.03 eV, the ideality factors varied from 1.13 to 1.40 and the BHs from C^-2–V characteristics varied from 1.10 to 1.70 eV. The experimental BH and ideality factor distributions obtained from the I–V characteristics are fitted by a Gaussian function, and their mean values are found to be 0.92±0.04 eV and 1.29±0.08 eV, respectively. The lateral homogeneous SBH value of 1.16 eV for the Ni/n-type 6H-SiC diodes has been calculated from a linear extrapolation of the effective barrier heights to n_if=1.03. PACS 79.40.+z; 73.40.Sx; 73.30.+y; 71.20.Nr     

Elektroneninterferometer mit Feldemissionskathode zur Messung von Kontaktpotentialdifferenzen im Ultrahochvakuum

Two-beam electron-interferences have been applied to measure contact potential differences (CPD) between polycristalline metal surfaces. Highly coherent and intense electron waves are produced using a field emission source. The materials under investigation are evaporated in situ in a bakeable oilfree vacuumsystem with an ultimate pressure of 5 · 10^?10 Torr. Gold is used as a reference element, as its work function is almost unaffected by residual gas. The accuracy of this method is ± 0.01 V. CPD values of different films of the same material combination vary by ± 0.05 V. The CPD between gold and copper is 0.45 ± 0.05 V, and between gold and aluminium 1.10 ± 0.05 V, gold being negative. The CPD of the clean Al-surface increases under prolonged influence of residual gas by an amount of 0.85 ± 0.05 V. Adsorption measurements show that this increase is mainly due to water vapour and not to oxygen.     

Effect of Substrate Bias on Microstructures of Zirconia Thin Films Deposited by Cathodic Vacuum Arc

Zirconium oxide （Zr02） thin films are deposited at room temperature by cathodic arc at substrate biases of 0 V, -60 V and -120 V, respectively. The crystal structure, composition, morphology, and deposition rate of the as-deposited thin films are systematically investigated by x-ray diffraction, x-ray photoelectron spectroscopy （XPS） as well as scanning electron microscopy. The results show that the crystal structure, morphology and deposition rate of the films all are dependant on substrate bias. With the increase of bias voltage from 0 V to -120 V, the zirconium oxide thin film grown on silicon wafer first exhibits monoclinic lattice and tetragonal lattice, further evolves monoclinic phase with the preferred orientation along the （-111） and （-222） directions at -60 V and finally along nearly one observed preferred （002） direction under -120 V. In addition, the variations of morphology with bias voltage are correlated to changes of the film structure. The results of XPS demonstrate that Zr elements are almost oxidized completely in the films achieved under -120 V bias.     

Adsorption of hydrogen and carbon monoxide on Rh(1 1 1)/V surface alloys

A detailed study of the interaction of hydrogen and carbon monoxide with two different Rh(1 1 1)/V surface alloys (1/3 monolayer of V in the second atomic layer or 1/3 monolayer of V in form of islands on the surface) is presented in comparison to the clean Rh(1 1 1) surface. For hydrogen a decrease in the sticking coefficient is found for both alloy surfaces. The sticking coefficient of H₂ as a function of the translational energy is similar to the Rh(1 1 1) surface, showing a direct activated adsorption mechanism. For low translational energies hydrogen adsorption is dominated by dynamical steering on Rh(1 1 1) and by a precursor mechanism on the Rh(1 1 1)/V subsurface alloy. The H₂ TPD desorption peaks are shifted to lower temperatures on the alloy surfaces, caused by the downshift of the metal d-band due to V alloying. On all three surfaces the saturation coverage of hydrogen was measured, giving 1.2, 1.0 and 0.8 monolayer for Rh(1 1 1), the Rh(1 1 1)/V subsurface alloy and for the Rh(1 1 1)/V islands, respectively. For CO the sticking coefficients and the saturation coverages are basically the same on the Rh(1 1 1) and the alloy surfaces. There is an extrinsic precursor on the ordered CO (√3×√3) phase on the Rh(1 1 1) surface, but there is no evidence for such a precursor on the Rh(1 1 1)/V subsurface alloy. On the Rh(1 1 1)/V islands surface, the extrinsic precursor exists on the Rh(1 1 1) surface between the V islands. Apparently this precursor is only stable on the ordered CO layer on Rh(1 1 1).     

原子核力场之研究

本篇应用王淦昌所建议之两个核子间之相互位能即V=V(r)=-Ae^klr在r≥α时;及V=V(α)在r<α时。以求下列数点:(1)重子在正常情形时之波函数,(2)中子与质子间之扩散断面值,及(3)重子被γ射线击破之断面值。本篇所计算得之各断面值之结果与实验所得者,颇相近似。     

Mechanical properties of vanadium carbide and a ternary vanadium tungsten carbide

Ab initio total energy calculations are performed on non-stoichiometric vanadium carbide with supercells representing vacancy concentrations of V C_0.875 and V C_0.75. The V C_0.875 supercell retains a cubic symmetry whilst in the case of the V C_0.75 supercell C vacancies located in close proximity have the lowest energy configuration and the cubic lattice slightly distorts to a monoclinic symmetry. Using a stress–strain calculational procedure, the elastic constants of both the cubic and the monoclinic systems are deduced. In all cases C vacancies decrease the elastic moduli.A similar analysis is then applied to consider when W is incorporated into VC. In this case it is found that the elastic moduli increase with W content suggesting that a V-W-C alloy could have significant potential as a novel hard material.     

Anisotropy of the X-ray K-emission spectra of vanadium in KVO3 and NaVO3 single crystals

The results of an investigation of the angular dependence of the polarized X-ray V Kβ_1,3 and V Kβ″β₅ spectra from KVO₃ and NaVO₃ single crystals are discussed. Interpretation of these results on the basis of a group-theoretical analysis of the wavefunctions for a VO₄ cluster leads to the conclusion that V 4p—O 2p π interactions are predominant in forming the V Kβ₅ band for these two compounds.     

Combined Optical/MCD/ODMR investigations of photochromism in doubly-doped Bi 12 GeO 20

Electron paramagnetic resonance is detected optically via the change of magnetic circular dichroism under microwaves at 35 v GHz. The technique is applied to Bi 12 GeO 20 samples co-doped with vanadium and a second transition metal (Cr, Mn, Co, Cu). The optical and magnetic properties of several paramagnetic defects (V Ge 4+ and Cr Ge 4+ ) are directly correlated. The basic photochromic processes occuring in samples doped with V, Mn, and Mn+V are explained. The V Ge 4+/5+ level is positioned roughly 2.2 v eV above the valence band.     

Thermodynamics of high-pressure ice polymorphs: ices III and V

Thermodynamic properties of high-pressure ice polymorphs, ices III and V, are studied theoretically. The results of TIP4P molecular dynamics simulations in the NPT ensemble are used to calculate the temperature dependence of the specific volume of ices III and V at pressures 0.25 and 0.5 GPa, respectively. New P-V-T equations of state of ices III and V are derived using a method generalizing the one proposed by Fei et al. [J. Chem. Phys. 99 (1993) 5369], and new results concerning the equilibrium phase transitions ice III-water and ice V-water are presented.     

In Situ Self-Assembly of Quantum Dots at the Plasma Membrane Mediates Energy Transfer-Based Activation of Channelrhodopsin

The use of nanoparticle (NP) bioconjugates to control the activity of membrane ion channels has recently emerged as a new paradigm for the activation of electrically excitable cells. An NP-based strategy is reported for the specific activation of channelrhodopsin C1V1 (ChR-C1V1) expressed in the plasma membrane of HEK 293T/17 cells. Hydrophilic CdSe/ZnS core–shell semiconductor quantum dots (QDs) are self-assembled to the exofacial face of recombinantly expressed ChR-C1V1 by metal affinity-driven interaction of the QD ZnS shell with an N-terminal hexahistidine tag displayed on ChR-C1V1. This configuration enables the Förster resonance energy transfer (FRET)-based excitation and activation of the 11-cis-retinal moiety of ChR-C1V1 using the QD as a light harvesting transducer/energy donor. It is shown that the specific laser-induced opening of the ChR-C1V1 channel wherein the photoexcited QD (405 nm excitation, 530 nm emission) iteratively activates ChR-C1V1 channels as confirmed using the voltage-sensitive dye (VSD) bis-(1,3-diethylthiobarbituric acid)trimethine oxonol (DiSBAC ₂ (3)). In the absence of the QD transducer, excitation of ChR-C1V1-expressing cells at 405 nm results in no activation of ChR-C1V1. The results demonstrate the ability to controllably interface QDs with living cells for the activation of ChR membrane proteins and detail a new NP-bioconjugate hybrid system for the specific activation of ion channels.