Results 1 - 10 of 1264
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[en] The structural, electronic and chemical properties of thin films and solid surfaces have long attracted interest, both from a fundamental scientific standpoint, and from the strategic implications, a knowledge of such factors has, in a range of applied fields ranging from electronic devices, to catalysis and nanotechnology. In this context, this thesis explores surface properties on the atomic scale within a range of functional material systems. Variable temperature scanning tunnelling microscopy (VT-STM) studies of the oxidation of Si(111)7x7 surfaces, and the deposition of Au on the ultrathin silicon oxide films so formed, have been carried in situ under vacuum conditions. The extent of oxidation as a function of reaction conditions is further elucidated. STM observations indicate that gold grows as 3-D islands on the silicon oxide surface. Annealing of the Au deposited surface allows the gold particles to migrate onto step edges areas and undergo agglomeration. Prolonged annealing of the Au/SiO2/Si system brings about the Au-assisted decomposition of the silicon oxide layer, resulting in void formation and the migration of Au to the underlying Si substrate. Similar experimental methods were used to investigate growth of ultrathin layers CaF2 on Si(111)7x7. Two distinct surface phases were formed, depending on the growth flux involved. The deposition of Pt and Pd onto CaF2/Si surfaces was performed and both metals were found to grow as 3-D islands. Annealing studies of the Pd-deposited surface showed that the tendency for agglomeration depended sensitively on surface coverage, and 1-D metallic 'lines' were formed at step edges under certain circumstances. Needle sensor non-contact atomic force microscopy was employed to study alumina (0001) surfaces under ultra-high vacuum following different surface treatments. High quality alumina surfaces were obtained, allowing atomic resolution in this type of system to be obtained, using the AFM probe employed, and the mechanism of atomic imaging of the (√31 x √31) R±9 deg reconstruction is discussed. The electronic properties of thin-film diamond have been studied, with particular reference to the ultra-violet quantum photoyield (QPY). The results highlighted the dependence of the QPY on the diamond surface composition, with particular correlation with the concentration of oxygen. The observations are explained in terms of the electron affinity of the diamond surface, and its variation with surface composition. (author)
[en] Using the quantum tunneling theory, we investigate the spin-dependent transport properties of the ferromagnetic metal/Schottky barrier/semiconductor heterojunction under the influence of an external electric field. It is shown that increasing the electric field, similar to increasing the electron density in semiconductor, will result in a slight enhancement of spin injection in tunneling regime, and this enhancement is significantly weakened when the tunneling Schottky barrier becomes stronger. Temperature effect on spin injection is also discussed
[en] The charge pumping is investigated in a quasi-one-dimensional quantum wire within an external perpendicular magnetic field. A formula of the pumped current at finite frequency is derived by using scattering matrix method. It is found that the pumped current oscillates with the magnetic field due to Aharonov-Bohm effect and the oscillating period is only dependent on the effective area enclosed by two pumping potentials. At a weak magnetic field, the pumped current also exhibits an approximate magnetic field inversion symmetry
[en] Polyester thin films containing europium-substituted heteropolytungstate were obtained on quartz plate by the sol-gel method. The films exhibited the characteristic emission bands of the europium ion. The red to orange intensity ratio (R:O) of Eu3+ in the films increased as compared to the corresponding heteropolytungstate solids. The fluorescence lifetime of europium is shorter in the thin film than in the heteropolytungstate solid. The results indicated that the formation of europium-substituted heteropolytungstate/polyester thin film has great effect on the luminescence of europium-substituted heteropolytungstate
[en] Magnetic response of a superconductor depends on the thermodynamic stability of a vortex in the material. Here we show that the vortex stability has a close relation with the ratio of the magnetic field at the vortex core center to the thermodynamic critical field. This finding provides a new criterion for the classification of the superconductors according to their magnetic responses. - Highlights: • The intrinsic relation between vortex stability and the magnitude of the magnetic field at the vortex core center is revealed. • A new criterion for the classification of the superconductors is proposed. • Numerical solutions for the magnetic field at the vortex core center are presented
[en] The structural, electronic, and elastic properties of cubic HC(NH2)2PbI3 perovskite are investigated by density functional theory using the Tkatchenko–Scheffler pairwise dispersion scheme. Our relaxed lattice parameters are in agreement with experimental data. The hydrogen bonding between NH2 and I ions is found to have a crucial role in FAPbI3 stability. The first calculated band structure shows that HC(NH2)2PbI3 has a direct bandgap (1.02 eV) at R-point, lower than the bandgap (1.53 eV) of CH3NH3PbI3. The calculated density of states reveals that the strong hybridization of s(Pb)– p(I) orbital in valence band maximum plays an important role in the structural stability. The photo-generated effective electron mass and hole mass at R-point along the R–Γ and R–M directions are estimated to be smaller: and respectively, which are consistent with the values experimentally observed from long range photocarrier transport. The elastic properties are also investigated for the first time, which shows that HC(NH2)2PbI3 is mechanically stable and ductile and has weaker strength of the average chemical bond. This work sheds light on the understanding of applications of HC(NH2)2PbI3 as the perovskite in a planar-heterojunction solar cell light absorber fabricated on flexible polymer substrates. (paper)
[en] In this paper, recently introduced permutation entropy and sample entropy are further developed to the fractional cases, weighted fractional permutation entropy (WFPE) and fractional sample entropy (FSE). The fractional order generalization of information entropy is utilized in the above two complexity approaches, to detect the statistical characteristics of fractional order information in complex systems. The effectiveness analysis of proposed methods on the synthetic data and the real-world data reveals that tuning the fractional order allows a high sensitivity and more accurate characterization to the signal evolution, which is useful in describing the dynamics of complex systems. Moreover, the numerical research on nonlinear complexity behaviors is compared between the returns series of Potts financial model and the actual stock markets. And the empirical results confirm the feasibility of the proposed model. - Highlights: • Two new entropy approaches for estimation of nonlinear complexity are proposed for the financial market. • Effectiveness analysis of proposed methods is presented and their respective features are studied. • Empirical research of proposed analysis on seven world financial market indices. • Numerical simulation of Potts financial dynamics is preformed for nonlinear complexity behaviors.
[en] Hydrothermal process was successfully used to synthesize Fe3O4 powder using ferrous chloride (FeCl2) and diamine hydrate (H4N2·H2O) as starting materials by carefully controlling the reaction conditions. The as-prepared Fe3O4 sample was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM), and its magnetic properties were evaluated on a vibrating sample magnetometer (VSM). The nanoscale (40 nm) Fe3O4 powder obtained at 140 deg. C for 6 h possessed a saturation magnetization of 85.8 emu/g, a little lower than that of the correspondent bulk Fe3O4 (92 emu/g). It is suggested that the well-crystallized Fe3O4 grains formed under appropriate hydrothermal conditions should be responsible for the increased saturation magnetization in nanosized Fe3O4
[en] In an attempt to improve the forecasting accuracy of crude oil price fluctuations, a new neural network architecture is established in this work which combines Multilayer perception and ERNN (Elman recurrent neural networks) with stochastic time effective function. ERNN is a time-varying predictive control system and is developed with the ability to keep memory of recent events in order to predict future output. The stochastic time effective function represents that the recent information has a stronger effect for the investors than the old information. With the established model the empirical research has a good performance in testing the predictive effects on four different time series indices. Compared to other models, the present model is possible to evaluate data from 1990s to today with extreme accuracy and speedy. The applied CID (complexity invariant distance) analysis and multiscale CID analysis, are provided as the new useful measures to evaluate a better predicting ability of the proposed model than other traditional models. - Highlights: • A new forecasting model is developed by a random Elman recurrent neural network. • The forecasting accuracy of crude oil price fluctuations is improved by the model. • The forecasting results of the proposed model are more accurate than compared models. • Two new distance analysis methods are applied to confirm the predicting results.