The composition-dependent mechanical properties of Ge/Si core–shell nanowires

The Stillinger–Weber potential is used to study the composition-dependent Young's modulus for Ge-core/Si-shell and Si-core/Ge-shell nanowires. Here, the composition is defined as a ratio of the number of atoms of the core to the number of atoms of a core–shell nanowire. For each concerned Ge-core/Si-shell nanowire with a specified diameter, we find that its Young's modulus increases to a maximal value and then decreases as the composition increases. Whereas Young's modulus of the Si-core/Ge-shell nanowires increase nonlinearly in a wide compositional range. Our calculations reveal that these observed trends of Young's modulus of core–shell nanowires are essentially attributed to the different components of the cores and the shells, as well as the different strains in the interfaces between the cores and the shells.     

Shellwise Mackay transformation in iron nanoclusters

Structure and magnetism of iron clusters with up to 641 atoms have been investigated by means of density functional theory calculations including full geometric optimizations. Body-centered cubic (bcc) isomers are found to be lowest in energy when the clusters contain more than about 100 atoms. In addition, another stable conformation has been identified for magic-number clusters, which lies well within the range of thermal energies as compared to the bcc isomers. Its structure is characterized by a close-packed particle core and an icosahedral surface, while intermediate shells are partially transformed along the Mackay path between icosahedral and cuboctahedral geometry. The gradual transformation results in a favorable bcc environment for the subsurface atoms. For Fe55, the shellwise Mackay-transformed morphology is a promising candidate for the ground state.     

A Two-Parameter Model for the Metal–Semiconductor Phase Transition in a One-Dimensional Atomic System

A model is developed for a metal–semiconductor phase transition and a concurrent structural phase transformation in a one-dimensional system of atoms, each having one external electron. The interatomic interaction involves two components. One component is associated with atom shells, while another (of covalent nature) is due to overlap of the electron wave functions of neighboring atoms. It is found experimentally that the phase transition gives rise to the formation of a band gap in the electron spectrum and to the development of static phonon modes. These effects are accounted for by the numerical model under review. The results obtained are used to interpret the experimental data on the metal–semiconductor phase transition in VO₂.     

高压退火对碳纳米管微结构转变的影响

利用高分辨电子显微镜研究了在高压退火条件下碳纳米管的微结构变化规律，发现在5.5GPa下，770℃退火后即发生了明显的结构改变；在950℃退火25min后发现形成了类碳纳米洋葱和纳米碳条带结构.碳纳米管转变成类碳纳米洋葱可以分成几个步骤，首先碳纳米管破裂，然后形成碳纳米球，最后形成类碳纳米洋葱结构.讨论了高压和退火温度对碳纳米管结构变化的影响. 关键词：     

Continuum modelling of spherical and spheroidal carbon onions

Carbon nanostructures are of considerable interest owing to their unique mechanical and electronic properties. Experimentally, a wide variety of different shapes are obtained, including both spherical and spheroidal carbon onions. A spheroid is an ellipsoid with two major axes equal and the term onion refers to a multi-layered composite structure. Assuming structures of either concentric spherical or ellipsoidal fullerenes comprising n layers, this paper examines the interaction energy between adjacent shells for both spherical and spheroidal carbon onions. The Lennard-Jones potential together with the continuum approximation is employed to determine the equilibrium spacing between two adjacent shells. We also determine analytical formulae for the potential energy which may be expressed either in terms of hypergeometric or Legendre functions. We find that the equilibrium spacing between shells decreases for shells further out from the inner core owing to the decreasing curvature of the outer shells of a concentric structure.     

Analytic inner shell internal conversion coefficients for screened Coulomb potentials

An analytic method is presented for evalution of internal conversion coefficients (ICC) in inner shells of neutral or ionized atoms. Analytic formulas for ICC are given which include electron screening and static nuclear size effects. Predictions have also been obtained from these formulas for point Coulomb ICC or for screening effects included only in bound state normalization. Very good agreement with full numerical calculations is found, except in those cases where the formation region of the ICC is not well inside the atom. While the discussion in this paper concerns mediumZ elements (30<Z<80) and not too low energy, the method can be extended to other situations. The method is suited also for ICC of the inner shells of ionized atoms, even for very high degree of ionization.     

Formation of Mn3O4/C core-shell nanowires and a new Mn-O phase by electron beam irradiation

Carbon shells have been formed on Mn₃O₄ nanowires under electron beam irradiation in a transmission electron microscope. The microstructure is characterized by transmission electron microscopy (TEM) imaging, electron energy-loss spectroscopy and energy-filtered imaging. Detailed TEM analysis confirms that the outer shell is composed of amorphous carbon with a thickness of 5–20 nm while the core is single crystalline Mn₃O₄. Electron beam irradiation induces the formation of the carbon shells. A new manganese oxide phase is also found by the transformation of Mn₃O₄ under the attack of the electron beam. PACS 61.82.Rx; 81.07.-b; 68.37.Lp; 79.20.Rf; 61.80.Lj     

Transformation mechanism of n-butyl terminated Si nanoparticles embedded into Si1−xCx nanocomposites mixed with Si nanoparticles and C atoms

Bright-field transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) images, and fast-Fourier transformed electron-diffraction patterns showed that n-butyl terminated Si nanoparticles were aggregated. The formation of Si_1−xC_x nanocomposites was mixed with Si nanoparticles and C atoms embedded in a SiO₂ layer due to the diffusion of C atoms from n-butyl termination shells into aggregated Si nanoparticles. Atomic force microscopy (AFM) images showed that the Si_1−xC_x nanocomposites mixed with Si nanoparticles and C atoms existed in almost all regions of the SiO₂ layer. The formation mechanism of Si nanoparticles and the transformation mechanism of n-butyl terminated Si nanoparticles embedded into Si_1−xC_x nanocomposites mixed with Si nanoparticles and C atoms are described on the basis of the TEM, HRTEM, and AFM results. These results can help to improve the understanding of the formation mechanism of Si nanoparticles.     

Polarization bremsstrahlung in α decay

A mechanism of formation of electromagnetic radiation that accompanies α decay and is associated with the emission of photons by electrons of atomic shells due to the scattering of α particles by these atoms (polarization bremsstrahlung) is proposed. It is shown that, when the photon energy is no higher than the energy of K electrons of an atom, polarization bremsstrahlung makes a significant contribution to the bremsstrahlung in α decay.     

Computation of vector coupling coefficients for atoms with one and two open shells in the Hartree–Fock approximation

A new way to introduce and calculate vector coupling coefficients (VCCs) is proposed for computations in a Hartree–Fock approximation of atoms with one and two open shells. In the case of atoms with one open shell (Roothaan method), the obtained formulas for VCCs are applicable to the calculation of all terms of such atoms without exception. The VCCs are also calculated for a number of configurations of atoms with two open shells (Huzinaga method). The correctness of the obtained VCCs is confirmed by numerous calculations of energies and dipole polarizabilities for atoms with one and two open shells.     

Compton scattering of an X-ray photon by an open-shell atom

A nonrelativistic quantum theory for the nonresonant Compton scattering of an X-ray photon by a free many-electron atom with an open shell in the ground state has been constructed in the single-configuration Hartree-Fock approximation outside the impulse approximation widely used in the literature. The transition to an atom with closed shells reproduces the results obtained previously in [6, 7]. The results of a test calculation for atoms with open (Ti, Fe) and closed (Zn) 3d core shells are presented. The effects of the radial relaxation of one-electron states in the field of core vacancies have been taken into account. The results of the calculation agree well with the experimental results [15, 16]. It has been established that the results of the impulse approximation in the investigated X-ray photon energy ranges disagree with those of our theory not only quantitatively but also qualitatively. In particular, the impulse approximation near the elastic (Thomson and Rayleigh) scattering line leads to a gross overestimation of the contributions from the deep atomic shells involved in the inelastic photon scattering only virtually to the scattering probability. The presented theory is general in character and its applicability to a particular element of the Mendeleev table with an open core shell or to a many-electron atomic ion is limited only by the requirement that the nonrelativistic Hartree-Fock approximation be properly used in describing the scattering-state wave functions.     

Physical cluster mechanics: Statics and energy surfaces for monatomic systems

The potential energy surfaces for clusters of some three to sixty atoms under Lennard-Jones forces have been systematically explored using numerical optimization techniques. In searching for minimum-energy configurations three particularly compact non-lattice growth schemes emerge showing tetrahedral, pentagonal (D_{5 ^h} ) and icosahedral symmetry respectively. All these systems were found to be appreciably more stable than microcrystallites based on the face-centred cubic structure while certain lattice elements were shown to be metastable for small numbers of atoms.

Some qualitative conclusions are then drawn concerning the occurrence of saddle-points for delocalized motion and the contribution of these to the generation of configurational entropy in small clusters. A crucial feature of the energy surfaces examined is the breakdown of strict local symmetry in compact clusters of more than two ‘shells’ of atoms and the possibility of delocalized motion of surface atoms around a solid-like core. These and other qualitative thresholds are pointed out as possible manifestations of entropy which could contribute to the existence of a ‘critical nucleus’ for discrete clusters with a role similar to that played in liquid-drop theories.     

Comments on the self-diffusivity data reported by Kitchlew and Rao

The method of a previous paper is applied to the numerical calculation of hole states of free atoms, in which one or more electrons have been removed from inner shells. The results are used in the interpretation of experimental data from ESCA and Auger spectroscopy, and in an analysis of the ‘equivalent core’ model.     

Dynamics and control of shock shells in the coulomb explosion of very large deuterium clusters

The explosion dynamics of very large (approximately 10(6)-10(7) atoms) deuterium clusters irradiated by ultraintense laser pulses (I approximately 10(18) W/cm(2)) is analyzed self-consistently with one-to-one three-dimensional and two-dimensional fully relativistic particle-in-cell simulations. Small-scale shock shells in the expanding ion cloud are observed. A technique to induce the formation of large shock shells inside a single cluster, increasing the probability of intracluster nuclear reactions, is proposed and demonstrated.     

Theorie der spontanen Magnetisierung kleiner ferromagnetischer Teilchen

With a modified conception of the Néel sublattices a molecular field theory is developed for small ferromagnetic particles. They are treated as a system of neighboring shells. These are supposed to consist each of atoms with equal average properties and are coupled to the next neighbors by an (exchange-) interaction. To simplify the numerical calculations the model was further modified by taking together all shells of the interior to form a core. Calculations of the temperature dependence of the spontaneous magnetization and of Curie-temperatures have been done for some particle forms and sizes of the f.c.c., b.c.c. and hexagonal lattices. The results are discussed and compared to experimental data of other authors from small particles of nickel, iron and cobalt.     

Shell histogram equalization of color images

Histogram equalization (HE) is an effective technique for image enhancement. In this study, we devised a new technique called shell histogram equalization for color images. The technique is a dimensionality reduction method, which transforms 3-D space enhancement to 1-D shell enhancement. First, the 3-D RGB color space is decomposed into L (L = 256) RGB shells, which are similar to a quarter sphere shells or a quarter onion squamae. Then, HE is implemented on shells, and makes the shells coincide with the distribution of the iso-luminance-planes in the RGB cube. After analyzing the computational complexity of the proposed method, comparison experiments are carried out and validated by subjective and objective assessments. The experimental results show that the method provides better enhancement for underexposed and high dynamic range images, and the computational time of the method is much lower.     

Electron delocalization in magnesium clusters grown in supercold helium droplets

The formation of bare clusters from highly reactive metals can be achieved very effectively by the pickup of atoms into superfluid helium droplets. We report on the experimental observation of electronic shells in small magnesium clusters produced by this method. Mass spectra taken under various ionization conditions show steps and outstanding peaks, as well as pronounced minima. The abundance distribution suggests a transition to full electron delocalization which is complete at about 20 atoms. A so-far-not-reported electron reorganization is observed, leading to a novel shell structure.     

Influence of the promotion of terms on the emission of auger electrons upon collisions of multicharged ions with a metal surface

Upon the collision of multicharged ions with a metal surface there occurs the formation of a dipole, generated from an ion and an induced charge in the metal. Strong rearrangement of electron shells takes place in the dipole field, which leads to the “promotion” of terms generated from states with a principal quantum number n = 2, 3. When the promoted terms cross with the levels of metal atoms, an electron is captured into these states. This effect makes it possible to explain the observed Auger electron spectra.     

Melting behavior of large disordered sodium clusters

The melting-like transition in disordered sodium clusters Na₉₂ and Na₁₄₂ is studied by performing density functional constant-energy molecular dynamics simulations. The orbital-free version of the density functional formalism is used. In Na₁₄₂ the atoms are distributed in two distinct shells (surface and inner shells) and this cluster melts in two steps: the first one, at ≈130 K, is characterized by the development of a high intrashell atomic mobility, and the second, homogeneous melting at ≈270 K, involves diffusive motion of all the atoms across the whole cluster volume. On the contrary, the melting of Na₉₂ proceeds smoothly over a very broad temperature interval, without any abrupt change in the thermal or structural indicators. The occurrence of two steps in the melting transition is suggested to be related to the existence of a grouping of the atoms in radial shells, even if those shells present a large degree of internal disorder. It then appears that a cluster can be considered fully amorphous (totally disordered) only when there are no radial regions of low atomic density separating shells. The isomer of Na₉₂ studied here fulfills this criterion and its thermal behavior can be considered as representative of that expected for fully amorphous clusters. Disordered Na₁₄₂, on the other hand, that has a discernible structure of an inner and a surface shell, should be considered as not fully disordered. The thermal behavior of these two clusters is also compared to that of icosahedral (totally ordered) sodium clusters of the same sizes. Received 5 February 2001 and Received in final form 21 May 2001