We are growing ZnSe, ZnS and CdSe layers epitaxially on GaAs(001) substrates by atomic layer epitaxy and molecular beam epitaxy. The substrates are prepared by a H-plasma method in order to obtain a sharp interface between substrate and layer. The quality of our samples is controlled by reflection high energy diffraction and X-ray diffraction. Furthermore, the samples are characterized in situ by photoelectron spectroscopy. We observe resonant Zn 3d^{8} and Cd 4d^{8} satellites, which are used to check the layer quality. As a result, the valence band offsets of CdSe/ZnSe and ZnSe/CdSe were obtained. The values are ΔE_{v}(ZnSe/CdSe) = -(0.13 ± 0.07) eV and ΔE_{v}(CdSe/ZnSe) = -(0.13 ± 0.07) eV, which confirm the commutativity rule.
Positron lifetime spectroscopy was applied to investigate the thermal stability of nanocrystalline copper prepared by severe plastic torsion deformation. Positrons annihilating in as prepared specimens exhibited free positron component τ_{1} and two defect components τ_{2}=164 ps and τ_{3}=255 ps. Evolution of the lifetimes and relative intensities of all the three components with increasing annealing temperature during step-by-step isochronal annealing up to 630°C was studied. Behaviour of positrons in nanocrystalline copper could not be interpreted in the frame of conventional 3-state trapping model due to highly inhomogeneous defect distribution. Therefore a modified trapping model was developed and applied to explain the experimental results.
Recent investigations of ultra fine-grained metals (Cu, Fe, Ni) performed within a Prague-Rossendorf-Ufa collaboration will be reviewed. The specimens were prepared by severe plastic deformation: the high-pressure torsion and equal channel angular pressing. Positron annihilation spectroscopy was used as the main method including (i) the conventional lifetime and the Doppler broadening measurements with ^{22}Na and (ii) the slow-positron implantation spectroscopy with the Doppler broadening measurement. Other methods were also involved: transmission electron microscopy, X-ray diffraction, and microhardness. First, the mean grain size was determined and defects were identified in the as-deformed materials. Defects concentration and spatial distribution were studied in detail. Dislocations situated in distorted regions along grain boundaries, and a few-vacancy clusters distributed homogeneously inside dislocations-free grains, were observed in the ultra fine-grained Cu, Fe, and Ni. Subsequently, the thermal evolution of the ultra fine-grained structures during isochronal annealing was studied.
A detailed study of the bulk ultra fine grained pure copper and copper with Al_2O_3 particles was carried out in the present work. The specimens were prepared by the high-pressure torsion and their microstructure was investigated by positron lifetime spectroscopy combined with transmission electron microscopy and microhardness tests. Defects in the as-deformed materials were characterized and the thermal stability of the ultra fine grained microstructure was subsequently examined in annealing experiments. An addition of Al_2O_3 nanoparticles was found to improve significantly the thermal stability of the ultra fine grained structure, the optimum content of Al_2O_3 being ≈0.5 wt.%.
Electrical characteristics of the heterojunction fabricated by thermal deposition of copper phthalocyanine (CuPc) on an n-silicon substrate have been investigated. The frequency has significant effect on capacitance (C), conductance (G) and series resistance (R_{s}) interface states (D_{it}) of the junction. Measured capacitance and conductance were corrected for R_{s}. The conductance technique was used to measure the density of the interface states. This method revealed the value of the interface state density distribution for the Au/n-Si/CuPc/Au interfaces of the order of 10^{12} cm^{-2} eV^{-1}.
We present a new outlook at the study of metal-semiconductor interface formation. A resonant photoemission spectroscopy tuned to the Fe 3p-3d transition (56 eV) was used to investigate the changes after sequential deposition of Fe atoms on freshly cleaved Cd_{0.86}Fe_{0.14}Se crystal surface. In the first stages (0.6-4 ML) of Fe deposition the contribution of Fe 3d electrons to the valence band grows up markedly indicating the increase in Fe content in the Cd_{0.86}Fe_{0.14}Se crystal surface region. When the amount of Fe exceed 40 ML the resonant photoemission spectra became similar to the Fe metal with some contribution of the ternary crystal substrate.
The growth process of cobalt on Ru(0001) was characterized by photoelectron spectroscopy excited by X-ray and synchrotron radiation. The binding energy position and intensity of the Co 2p_{3/2} and Ru 3d_{5/2} core levels as well as the shape and structure of the valence band spectra corresponding to the different stages of the deposition were investigated. An observed small positive binding energy shift is a consequence of an increase in the cobalt adatoms coordination number. The core-level shift between bulk and surface Ru atoms is determined as -360 meV. Upon adsorption of cobalt, the interface peak appears with a shift of -(70÷80) meV relative to the bulk one. On the basis of unchanged energy positions and widths of the Ru-derived features of the valence band spectra, a weak interaction between cobalt and substrate is suggested. The measured valence band could be reproduced by superimposing the spectra of the pure elements.
The synchrotron radiation was used to apply tunable high energy X-ray photoemission spectroscopy for investigation of electronic structure of semiconductor nanostructure CdTe/Pb_{0.95}Eu_{0.05}Te/CdTe/GaAs(001) top part. The Pb_{0.95}Eu_{0.05}Te (6 nm thick) was buried under thin (22 nm) top layer of CdTe transparent for part of electrons photoemitted from Pb_{0.95}Eu_{0.05}Te buried layer. The top layer of CdTe was sputtered by Ar ion bombardment for surface cleaning and for leaving the thickness of CdTe more transparent for photoelectrons emitted from buried layer. For these thickness of the top layer the photoemission energy distribution curves corresponding to the valence band and core levels electrons of the buried layer atoms were measured with application of synchrotron radiation of energy hν = 3510 eV. The measured spectra corresponding to the buried layer atoms were observed in the valence band region and in the high binding energy region for core levels of Pb 4f, Pb 3d. The valence band contribution and core levels Cd 4d and Cd 3d were obtained mainly from top cover layer. Measured Te 4d, Te 3d and Te 4d spectra possess contribution as well from top cover layer as from the buried layer. The amount of Eu atoms was to small to be reasonable detected and presented in the paper.
Using photoemission spectroscopy, adsorption and reaction of CO_{2} on potassium modified Cu(110) were studied. In agreement with published results of thermally programmed desorption, apart from carbonate and carbon monoxide as the disproportionation reaction products, a linear CO_{2} molecule and a bent active CO_{2}(-) species were identified. The reaction paths are independent on the potassium precoverage but the number of CO_{3} molecules increases with the number of potassium adatoms. The presence of the CO_{2}(-), stable up to 200 K, suitable for the reactivity of the interface in respect of the methanol synthesis, could be confirmed in the complex valence band spectra by occurrence of the characteristic peak at binding energy of 6.8 eV.
We demonstrate experimentally that conductance steps can occur in nanowires formed at metal-semiconductor junctions, between a cobalt tip and a germanium surface revealing long-duration plateaus at reproducible levels. The high reproducibility of the conductance traces obtained leads to very sharp peaks in the conductance histogram suggesting formation of stable atomic configurations. We develop a new type of correlation analysis of the preferred conductance values that provide new type of information on a few-atomic-nanocontact formation dynamics.
Improved adsorption characteristics of a K predosed Cu(110) surface with a coverage corresponding to a point before the work function minimum have been confirmed. Analysis of the CO-induced orbitals in the photoelectron spectra of the CO/K/Cu(110) interface for low coverages of carbon monoxide adsorbed at 118 K has been done. Noticeable changes of the parameters of the orbitals with increasing CO coverage have been registered. Elongation of the C-O bond without dissociation of the molecule has been deduced from the energetic separation of 3.4 eV between the 4σ and the 1π orbital. A weakening of the CO-interface bond with coverage has been found on the base of decreasing 5σ-1π separation with increasing 4σ/5σ intensity ratio. Perpendicular orientation of CO molecules to the plane of the substrate surface has been concluded from analysis of the constant initial state spectra of the 4σ and 5σ/1π orbitals.
Despite the favourable strength and thermal stability, a disadvantage of the Mg-based alloys consists in a low ductility. Recently it has been demonstrated that ultra fine grained metals with grain size around 100 nm can be produced by high pressure torsion. A number of ultra fine grained metals exhibit favourable mechanical properties consisting in a combination of a very high strength and a significant ductility. For this reason, it is highly interesting to examine microstructure and physical properties of ultra fine grained Mg-based light alloys. Following this purpose, microstructure investigations and defect studies of ultra fine grained pure Mg and ultra fine grained Mg-10%Gd alloy prepared by high pressure torsion were performed in the present work using positron annihilation spectroscopy combined with X-ray diffraction, TEM observations, and microhardness measurements. Positrons are trapped at dislocations in Mg and Mg-10%Gd alloy deformed by high pressure torsion. A number of dislocations increases with the radial distance r from the centre to the margin of the sample. No microvoids (small vacancy clusters) were detected. Mg-10%Gd alloy deformed by high pressure torsion exhibits a homogeneous ultra fine grained structure with a grain size around 100 nm and high dislocations density. On the other hand, pure Mg deformed by high pressure torsion exhibits a binomial type of structure which consists of "deformed regions" with ultra fine grained structure and a high dislocation density and dislocation-free "recovered regions" with large grains. It indicates a dynamic recovery of microstructure during high pressure torsion processing.
Nanostructured W-Cu-Ni electrical contact materials to be used in low voltage vacuum switching contactors for nominal currents up to 630 A were developed successfully by hot isostatic pressing. W-Cu-Ni composite powder mixtures with copper content of 20 to 40 wt% and 1 wt% Ni were mechanically alloyed in Ar atmosphere by high-energy ball milling with a ratio of milling steel balls: powders mixtures of 8:1 and rotation speed of 400 rpm for 10 and 20 h. The effect of mechanical alloying on the sintering response of composite compacts was investigated. Also, the sintered contacts were characterized from the point of view of physical, microstructural, mechanical, and functional properties. The nanostructured electrical contacts presented very good sinterability and homogeneous structures with a maximum compactity degree of about 89%. The best W-Cu-Ni compositions with relative density of about 80%, chopping currents lower than 5 A, copper content lower than 40% as W-20Cu-1Ni (10 h of mechanical alloying and 20 h of mechanical alloying) and W-30%Cu-Ni (10 h of mechanical alloying) were selected to be used in vacuum contactors.
The CO/Pt/Ru(0001) interface has been re-examined, in great detail, by photoelectron spectroscopy of high resolution under UHV conditions. The Ru(0001) substrate has been modified by platinum at coverages less than corresponding to the one, saturated, Pt overlayer, with no Pt/Ru intermixing. The analysis of the extent to which different regions of the photoelectron spectrum allow a detailed characterization of the interface is presented. The CO adsorption displays interaction with two separate phases: the Pt(111) face and the Ru(0001) surface.
β-Ga_2O_3 nanorods were successfully fabricated through annealing Ga_2O_3/Mo films deposited on the Si (111) substrate by radio frequency magnetron sputtering technique. The morphology and structure of the as-synthesized nanorods were characterized by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and energy dispersive X-rays spectroscopy. The results show that the formed nanorods are single-crystalline Ga_2O_3 with monoclinic structure. The diameters of nanorods are 200 nm and lengths typically up to several micrometers. A photoluminescence spectrum at room temperature under excitation at 325 nm exhibits two strong blue-light peaks located at about 413.0 nm and 437.5 nm, attributed to the recombination of bound electron-hole exciton in β-Ga_2O_3 single crystal. The growth process of the β-Ga_2O_3 nanorods is probably dominated by conventional vapor-solid mechanism.
The SbSI/Sb_2S_3 single heterostructures as well as Sb_2S_3/SbSI/Sb_2S_3 and SbSI/Sb_2S_3/SbSI double heterostructures have been produced by applying CO_2 laser treatment of p-type SbSI single crystals. The current-voltage and transient characteristics of these heterostructures have been measured in temperatures below and above the SbSI single crystal Curie temperature (T_{c} = 293 K). The results have been fitted with appropriate theoretical formulae to determine the following types of the investigated heterojunctions: P-p SbSI/Sb_2S_3, p-P-p Sb_2S_3/SbSI/Sb_2S_3 and P-p-P SbSI/Sb_2S_3/SbSI. Influence of the illumination on electrical properties of SbSI/Sb_2S_3 single and double heterostructures has been reported. Fabricated new structures may be potentially applicable in electronics and optoelectronics as a new type of metal-ferroelectric-semiconductor devices.
The Mn/ZnO(0001) system was investigated by synchrotron radiation photoemission. The Mn/ZnO interface with 4 ML of manganese deposited onto the ZnO surface was annealed up to 500ºC. No Mn capping layer was found at the surface after annealing as was confirmed by scanning Auger spectroscopy experiment. We used a resonant photoemission to extract the Mn3d partial density of states in photoemission spectra. The Mn3d states contribute to the electronic structure of the system within 10 eV of the Fermi level. They show three features: a main peak at 3.8-4.5 eV, a valence structure at the top of the valence band (1-3 eV), and a broad satellite situated between 5.5 and 9 eV below E_F. The satellite/main branching ratio was determined to be 0.43, which is a fingerprint of strong hybridization between the Mn3d electrons and the valence band of the crystal. The hybridization effect in Zn_{1-x} Mn_xO surface alloy is comparable to Zn_{1-x}Mn_xS and much higher than in Zn_{1-x}Mn_xSe, Zn_{1-x}Mn_xTe, and Ga_{1-x}Mn_xAs semimagnetic compounds.
We present a method of measurement of the current-voltage (I-V) and conductance-voltage (G-V) characteristics of nanowires with quantum point contact formed at the Co-Ge_{99.99}Ga_{0.01} interface. The effect of the Fermi level pinning leads to the formation of an ohmic contact between Co and Ge_{99.99}Ga_{0.01}. On the measured characteristics, above the threshold value of voltage an exponential current growth is observed. Such effect could be useful in the production of the electronic nanodevices.
The general properties of the layered transition metal dichalcogenides and the possibility to modify these materials by intercalation are reviewed. Examples are given of experimental results obtained by using angle-resolved photoelectron spectroscopy and very-low-energy electron diffraction. The possibility to use layered semiconductors as model systems in studies of e.g. Schottky barriers and surface photovoltage is exemplified by the Rb/WSe_2 system. Attention is also paid to the use of van der Waals epitaxy in interface studies, and its possible practical applications. The potential of layered semiconductors like WSe_2 in solar cell applications is also mentioned.
As synchrotron radiation sources have been used for many experiments in the ultraviolet and X-ray regimes, the free-electron laser is an excellent source for a wide array of infrared-photon projects and applications. The free-electron laser delivers a beam of powerful tunable pulsed radiation which provides the opportunity for spatial and temporal localization of the energy delivered at any desired wavelength within the 2-10 μ regime. One application discussed employs the free-electron laser for spectroscopy as a probe of electronic and vibrational structures. Another application uses the free-electron laser beam as a tool for altering materials in a fundamentally new way.
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