Atomic structure of CaF2/MnF2-Si(1 1 1) superlattices from X-ray diffraction

X-ray reflectivity and non-specular crystal truncation rod scans have been used to determine the three-dimensional atomic structure of the buried CaF₂-Si(1 1 1) interface and ultrathin films of MnF₂ and CaF₂ within a superlattice. We show that ultrathin films of MnF₂, below a critical thickness of approximately four monolayers, are crystalline, pseudomorphic, and adopt the fluorite structure of CaF₂. High temperature deposition of the CaF₂ buffer layer produces a fully reacted, CaF₂-Si(1 1 1) type-B interface. The mature, “long” interface is shown to consist of a partially occupied layer of CaF bonded to the Si substrate, followed by a distorted CaF layer. Our atomistic, semi-kinematical scattering method extends the slab reflectivity method by providing in-plane structural information.     

Structural modifications of diamond like carbon films induced by MeV nitrogen ion irradiation

Diamond-like carbon (DLC) films were deposited on Si(1 0 0) substrates using plasma deposition technique. The deposited films were irradiated using 2 MeV N⁺ ions at fluences of 1×10¹⁴, 1×10¹⁵ and 5×10¹⁵ ions/cm². Samples have been characterized by using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). Analysis of Raman spectra shows a gradual shift of both D and G band peaks towards higher frequencies along with an increase of the intensity ratio, I(D)/I(G), with increasing ion fluence in irradiation. These results are consistent with an increase of sp² bonding. XPS results also show a monotonic increase of sp²/sp³ hybridization ratio with increasing ion fluence. Plan view TEM images show the formation of clusters in the irradiated DLC films. HRTEM micrographs from the samples irradiated at a fluence of 5×10¹⁵ ions/cm² show the lattice image with an average interplanar spacing of 0.34 nm, revealing that the clusters are graphite clusters. The crystallographic planes in these clusters are somewhat distorted compared to the perfect graphite structure.     

Aluminium adsorption on Ir(1 1 1) at a quarter monolayer coverage: A first-principles study

We present an ab initio density-functional study for aluminium adsorption on Ir(1 1 1) at high symmetry sites, namely, the fcc-, hcp-hollow, top and bridge sites. In each case, we calculate the atomic geometry, average binding energy, work function, and surface dipole moment at the coverage of 0.25 monolayer. We find the favourable structure to be Al at threefold hcp-hollow site, with a corresponding binding energy of 4.46 eV. We present and compare the electronic properties of the two lowest energy structures, i.e., at the threefold hollow sites and discuss the nature of the Al-Ir bond and binding site preference. In particular, we observe a large hybridization of Al-3s, 3p and Ir-5d states near Fermi level, forming an inter-metallic bonds. This results in a significant electron transfer from the Al atoms to the Ir(1 1 1) substrate, inducing an outward pointing surface dipole moment and a large decrease in the work function of 1.69 eV for Al in the hcp-hollow site. Compared to the fcc-hollow site, adsorption in the hcp-hollow site results in a lower density-of-states at the Fermi level, as well as a greater hybridization in the bonding states.     

Quantum size effects of Pb overlayers at high coverages

We have studied Pb thin films as a function of the thickness up to 60 monolayers (MLs) using ab initio first principles and model calculations. Magic heights corresponding to a modulated oscillatory pattern of the energy of Pb(1 1 1) films have been measured up to about 30 MLs. We demonstrate that this behaviour continues even for higher thickness due to an extra second modulation pattern in the energetics of the metal film as a function of the number of atomic layers. The origin of this second modulation is the nesting of two close values of the Fermi wavelength in the (1 1 1) direction.     

Co-induced nano-structures on Si(1 1 1) surface

The interaction of cobalt atoms with silicon (1 1 1) surface has been investigated by means of scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Besides the Co silicide islands, we have successfully distinguished two inequivalent Co-induced reconstructions on Si(1 1 1) surface. Our high-resolution STM images provide some structural properties of the two different derived phases. Both of the two phases seem to form islands with single domain. The new findings will help us to understand the early stage of Co silicide formations.     

Growth, coalescence, and electrical resistivity of thin Pt films grown by dc magnetron sputtering on SiO2

Ultra thin platinum films were grown by dc magnetron sputtering on thermally oxidized Si (1 0 0) substrates. The electrical resistance of the films was monitored in situ during growth. The coalescence thickness was determined for various growth temperatures and found to increase from 1.1 nm for films grown at room temperature to 3.3 nm for films grown at 400 °C. A continuous film was formed at a thickness of 2.9 nm at room temperature and 7.5 nm at 400 °C. The room temperature electrical resistivity decreases with increased growth temperature, while the in-plain grain size and the surface roughness, measured with a scanning tunneling microscope (STM), increase. Furthermore, the temperature dependence of the film electrical resistance was explored at various stages during growth.     

Nanoscale observations of the operational failure for phase-change-type nonvolatile memory devices using Ge2Sb2Te5 chalcogenide thin films

In this study, a phase-change memory device was fabricated and the origin of device failure mode was examined using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). Ge₂Sb₂Te₅ (GST) was used as the active phase-change material in the memory device and the active pore size was designed to be 0.5 m. After the programming signals of more than 2×10⁶ cycles were repeatedly applied to the device, the high-resistance memory state (reset) could not be rewritten and the cell resistance was fixed at the low-resistance state (set). Based on TEM and EDS studies, Sb excess and Ge deficiency in the device operating region had a strong effect on device reliability, especially under endurance-demanding conditions. An abnormal segregation and oxidation of Ge also was observed in the region between the device operating and inactive peripheral regions. To guarantee an data endurability of more than 1×10¹⁰ cycles of PRAM, it is very important to develop phase-change materials with more stable compositions and to reduce the current required for programming.     

Band alignments at SrZrO3/Ge(0 0 1) interface: Thermal annealing effects

High-κ dielectrics SrZrO₃ were prepared on Ge(0 0 1) substrate using pulse laser deposition, and band alignments and thermal annealing effects were studied with high resolution X-ray photoemission spectroscopy. Valence and conduction band offsets at this interface were measured to be 3.26 eV and 1.77 eV, respectively. Interfacial Ge oxide layers were found at the interface. After annealing at 600 °C, the interfacial Ge oxide layers were eliminated, and the valence band offset increased to 3.50 eV, but the amorphous SrZrO₃ became polycrystalline in the meantime.     

Influence of epitaxial strain on the terrace and inter-layer diffusions in metal epitaxy

The influence of epitaxial strain on the surface and inter-layer diffusions are investigated using molecular statics and transition state theory with several types of embedded atom method potentials for Ag and Ni. Quantitatively to analyze the competition of the surface and inter-layer diffusions in the instability to the multi-layer growth, Ehrlich-Schwoebel barrier and the attempt frequencies of the surface and inter-layer diffusions by both hopping and exchange mechanisms are considered simultaneously. The attempt frequencies of exchange mechanism are larger by order one than those of hopping mechanism and especially, the difference becomes more severe for the inter-layer diffusion. Considering both the attempt frequency and activation energy barrier shows that the layer-by-layer growth is enhanced by compressive strain for Ag(0 0 1), which is confirmed by the existing linear stability theory and kinetic Monte Carlo simulations.     

Elaboration of self-organized magnetic nanoparticles by selective cobalt silicidation

Self-organized magnetic nanoparticles are obtained through selective silicidation of cobalt using a silicon substrate pre-structured with tri-dimensional gold islands as template. On the step bunches array of a vicinal Si(1 1 1) surface, gold deposition results in the formation of nanodroplets aligned along the step bunches. A subsequent cobalt deposition is performed onto this gold islands-covered Si surface, with two silicidation processes investigated: reactive deposition (RD) and solid phase reaction (SPR). The cobalt is converted into a non-magnetic silicide film except where the surface is locally masked by the gold islands, giving rise to cobalt nanomagnets which can be capped by a gold layer. A scanning tunneling microscopy comparative study of RD and SPR processes demonstrates that the former induces strong surface morphology changes while the latter preserves the pristine islands. Magnetic measurements performed with alternating gradient force magnetometry at room temperature are used to demonstrate the presence of ferromagnetic cobalt nanoparticles on SPR-processed samples. These nanomagnets show a clear in-plane anisotropy behavior.     

A combinatorial sample-preparation robot system using the volumetric-weighing method

We have developed a new automatic sample-preparation robot system with use of the volumetric-weighing method. In this system, slurries, aqueous solutions, and other wet reagents are employed as starting materials and 64 (8×8) samples at the maximum are prepared on a library plate of 35 mm × 35 mm size. Volumetric-weighing and mixing of the starting materials and distributing reaction mixtures to the library plate are automatically performed by computer-controlled mechanisms with an easy-to-use programming software interface. While this robot is designed in terms of space saving and portability, it is able to equip with an atmosphere-controlled furnace to sinter the samples on the library plate. Typical preparation time for a library plate of 36 (6×6) samples is less than 40 min. This robot system is promising in enhancing throughput of wet-chemically synthesized materials researches.     

Anomalous effects in charging of Pd powders with high density hydrogen isotopes

A twin system for hydrogen absorption experiments has been constructed to replicate the phenomenon of heat and ⁴He generation by D₂ gas absorption in nano-sized Pd powders reported by Arata and Zhang, and to investigate the underlying physics. For Pd⋅Zr oxide nano-powders, anomalously large energies of hydrogen isotope absorption, 2.4±0.2 eV/D-atom and 1.8±0.4 eV/H-atom, as well as large loading ratio of D/Pd=1.1±0.0 and H/Pd=1.1±0.3, respectively, were observed in the phase of deuteride/hydride formation. The sample charged with D₂ also showed significantly positive output energy in the second phase after the deuteride formation.     

Fe monolayers on InAs(0 0 1): An in situ study of surface, interface and volume magnetic anisotropy

The magnetic anisotropy of epitaxial Fe films with thicknesses in the range of 2-142 monolayers (ML) grown on {4×2} reconstructed InAs(0 0 1) was investigated by in situ ferromagnetic resonance. The easy magnetization direction was found to be parallel to the -direction for Fe films below 4 ML, while it rotates by 45^° toward the -direction. It is observed that both surface-interface and volume contribution to the perpendicular anisotropy favor an easy axis perpendicular to the film plane. The cubic surface-interface anisotropy is relatively large with easy axes along -directions in contrast to the volume contribution which favors easy axes along the -directions. The volume contribution is found to be larger than the Fe bulk cubic anisotropy. A thickness independent uniaxial anisotropy has been found in films with a thickness of 2 up to 142 ML.     

Intense laser effects on donor impurity in a cylindrical single and vertically coupled quantum dots under combined effects of hydrostatic pressure and applied electric field

Using the effective mass and parabolic band approximations and a variational procedure we have calculated the combined effects of intense laser radiation, hydrostatic pressure, and applied electric field on shallow-donor impurity confined in cylindrical-shaped single and double GaAs-Ga_1−xAl_xAs QD. Several impurity positions and inputs of the heterostructure dimensions, hydrostatic pressure, and applied electric field have been considered. The laser effects have been introduced by a perturbative scheme in which the Coulomb and the barrier potentials are modified to obtain dressed potentials. Our findings suggest that (1) for on-center impurities in single QD the binding energy is a decreasing function of the dressing parameter and for small dot dimensions of the structures (lengths and radius) the binding energy is more sensitive to the dressing parameter, (2) the binding energy is an increasing/decreasing function of the hydrostatic pressure/applied electric field, (3) the effects of the intense laser field and applied electric field on the binding energy are dominant over the hydrostatic pressure effects, (4) in vertically coupled QD the binding energy for donor impurity located in the barrier region is smaller than for impurities in the well regions and can be strongly modified by the laser radiation, and finally (5) in asymmetrical double QD heterostructures the binding energy as a function of the impurity positions follows a similar behavior to the observed for the amplitude of probability of the noncorrelated electron wave function.     

Metallic nanosieves formed by ultra-short-pulse laser ablation

The formation of free-standing gold nanosieves by ablation with ultra-short laser pulses is demonstrated. Macroscopic areas are generated fast and efficiently by the application of a parallel production technique. The technique is based on a lens array formed by self-assembling quartz microspheres on a thin metal foil. The evaporated foils have a final thickness of 400 nm, and the hole spacing is set by the diameter of the microspheres (∼7 m) while the pore size is ∼700 nm. The characteristic spacing of the generated hole structure is verified by an optical diffraction technique.     

Stabilization of the pentagonal surface of the icosahedral AlPdMn quasicrystal by controlled Si absorption

The Debye-temperature of the pentagonal surface of the icosahedral AlPdMn quasicrystal (QC) is measured by means of low-energy electron diffraction after the absorption of different amounts of Si. We observe an increase of the surface Debye-temperature from 300±7 K for the freshly prepared surface to 330±7 K after the absorption of 60-Å Si. Because the quasicrystalline order persists at the surface in spite of the diffusion of Si into the substrate, we suggest that the diffusion is dominated by a vacancy-mediated process.     

Evidence for shape coexistence at medium spin in Rb

Four previously known rotational bands in ⁷⁶Rb have been extended to moderate spins using the Gammasphere and Microball γ ray and charged particle detector arrays and the ⁴⁰Ca(⁴⁰Ca, 3pn) reaction at a beam energy of 165 MeV. The properties of two of the negative-parity bands can only readily be interpreted in terms of the highly successful Cranked Nilsson-Strutinsky model calculations if they have the same configuration in terms of the number of g9/2 particles, but they result from different nuclear shapes (one near-oblate and the other near-prolate). These data appear to constitute a unique example of shape coexisting structures at medium spins.     

Pressure induced anisotropy of electrical conductivity in polycrystalline molybdenum disulfide

Anisotropic specimens of MoS₂ are obtained by pressing the microcrystalline powder into special die. This inelastic compression results in a rearrangement of the disulfide micro platelets observed by atomic force microscopy and reflected in the macroscopic anisotropy in electrical conductivity in these samples. The conductivity measured parallel and perpendicular to the direction of applied pressure exhibits an anisotropy factor of ∼10 at 1 GPa. This behaviour of the conductivity as a function of applied pressure is explained as the result of the simultaneous influence of a rearrangement of the micro platelets in the solid and the change of the inter-grain distances.     

The static and dynamic properties of PuCoGa5 and NpCoGa5 investigated by neutron scattering experiments

Neutron scattering results on single crystals shed light on the static and dynamic properties of the superconductor () PuCoGa₅ and its isostructural but antiferromagnetic () homologue NpCoGa₅. By polarized neutron diffraction the magnetization density in the paramagnetic state of both compounds has been inferred. The microscopic magnetization of NpCoGa₅ is consistent with the orbital and spin components of a localized Np³⁺ magnetic moment. In the case of PuCoGa₅ the microscopic magnetization is small, temperature-independent and cannot be described as a localized Pu³⁺ magnetic moment. For NpCoGa₅ a dynamic magnetic signal has been observed by three-axis spectroscopy in the antiferromagnetically ordered state. The magnetic signal is strongest at the antiferromagnetic zone center and an energy transfer of about 5 meV. Magnetic fluctuations persist at this position in the paramagnetic state. The dynamic response is similar to the dynamic response observed in other actinide intermetallic compounds. This supports the possibility that magnetic fluctuations could also be present in the paramagnetic superconductor PuCoGa₅.     

Soft x-ray PHA measurement of the long pulse discharge in HT-7 tokamak

By analyzing the soft x-ray energy spectrum measured by the soft x-ray pulse height analysis (PHA) system, the electron temperature (Te) and the effective charge number (Zeff) of the ultra-long pulse discharge driven by lower hybrid wave (LHW) were obtained in the HT-7 tokamak. Moreover, the information of medium-Z impurities such as Ti, Cr, Fe, and Ni intrinsic to HT-7 tokamak can also be inspected. The accuracy of the electron temperature derived from the soft x-ray energy spectrum measurements is verified by comparing with the temperature measured by the Thomson scattering system for various plasmas and electron cyclotron emission diagnostic system for ohmic plasmas. The bulk electron temperature of about 1 keV and Zeff≈2 were achieved for long pulse plasma. The appreciable Kα lines of Ti, Cr, Fe and Ni metallic impurities released from the antennas of radio frequency wave and/or the first wall and Ar injected into plasma can be observed, and they kept stable during the long duration discharges. As a result, the longest pulse discharge with relatively high temperature of Te(0)∼1 keV, and ne(0)∼0.5×¹⁰¹⁹ m−3 has been achieved with a duration of 400 s in the HT-7 experimental campaign in 2008.