Two-stage formation model and helicity of gold nanowires

A model for the formation of helical multishell gold nanowires is proposed and is confirmed with quantum mechanical molecular dynamics simulations. The model can explain the magic number of the helical gold nanowires in the multishell structure. The reconstruction from ideal nonhelical to realistic helical nanowires consists of two stages: dissociations of atoms on the outermost shell from atoms on the inner shell and slip deformations of atom rows generating (111)-like structure on the outermost shell. The elementary processes are governed by competition between energy loss and gain by s and d electrons together with the width of the d band. The possibility for the helical nanowires of platinum, silver, and copper is discussed.     

Electronic structure of bimetallic Ni–Rh nanowires

The electronic structure of a bare Rh(553) surface and of a Ni-decorated Rh(553) surface has been investigated by angle-resolved UV photoelectron spectroscopy and density functional theory calculations. The self-assembly of Ni adatoms leads to the decoration of the steps of the Rh(553) surface with monoatomic Ni rows under suitable kinetic conditions, thus forming a regular array of pseudomorphic bimetallic Ni–Rh nanowires. The electronic structure of the clean Rh(553) surface has been compared to the one of the flat Rh(111) surface, and additional surface states localized at the step edges due to the lower coordination of the step atoms have been detected. The Ni wires are weakly hybridized with the Rh substrate states and are characterized by only weakly dispersing states. This leads to a strong narrowing of the d-band, which is argued to be the origin of the observed high chemical reactivity of the Ni–Rh nanowires.     

Models for the heavy rare earth metals and (rare earth) Fe2 compounds involving 5d and 6s electrons

A model involving 5d electrons is introduced to explain the differences between the observed saturation moments in the heavy rare earth metals and those of the corresponding tripositive ions. Atomic 5d states, whose energies are determined by 4f–5d and spin-orbit interactions, are assumed to be broadened into partly overlapping bands with individual widths of the order of 1 eV. The 5d electrons produce negative contributions to the hyperfine fields but positive or near zero contributions to the magnetic moments. It is postulated that the 5d electrons are transferred from the rare earth ions to those of the iron in the (Rare Earth) Fe₂ compounds. This leads to increases in the magnetic hyperfine fields because the negative 5d contributions are lost, but in detailed application of the model increases in the 6s contributions also play a large part. Published energy level and wave function analyses for atomic Gd, Tb, Dy, Er and Tm are used in order to apply the theory to these materials.     

Metallic beta-phase silicon nanowires: Structure and electronic properties

Electronic band structure and energetic stability of two types of 〈110〉 and 〈001〉 oriented silicon nanowires in β-Sn phase with the surface terminated by hydrogen atoms were studied using density functional theory. It was found that β-Sn nanowires are metastable with zero band gap against to nanowires in diamond phase. The relative energy of the studied wires tends to the energy of the bulk silicon crystal in β-Sn phase.     

A reexamination of theI.S. results for the cluster compound Au55(PPh3)12Cl6

The observed lowI.S. values [1] for the four distinct gold sites in the cluster compound Au₅₅(PPh₃)₁₂Cl₆ have been explained in terms of a decrease of the 6s electron density at the nucleus, due either to a lattice expansion or to a delocalization of 6s electrons over the ligand shell surrounding the gold core. Recent EXAFS measurements [2] indicate a single average distance between the gold atoms, about 3.5% less than for bulk gold. This not only excludes the lattice expansion hypothesis, it effectively increases the density of 6s electrons, making an explanation for the observedI.S. even more difficult. For an understanding of theI.S. values it is necessary to reconsider the probable occupation of the 5d orbitals within the framework provided by the XPS surface atom core level shifts developed by Citrin and Wertheim [3]. Partial confirmation has been found in preliminary XPS results [5]. The consequences for thef-factor and specific heat results [1] will also be examined.     

197Au Mössbauer effect studies on ternary alloys of gold with main-group metals

¹⁹⁷Au Mössbauer effect studies in the ternary gold alloys Li₂AuX (X = Ga, In, Tl; Ge, Sn, Pb; Bi) and Li₂Au_2?xIn_x (1.0 ? x ? 1.75), all of which crystallize in the cubic NaTl structure, have been performed at 4.2 K. The isomer shifts derived from the single-line spectra have been correlated with the average Allred-Rochow electronegativity of the first three coordination spheres around the gold atoms, normalized to the number of outer electrons and corrected for the distance from the gold atom. The isomer shifts have also been correlated with structural data from X-ray diffraction studies. Results of Dirac-Fock atomic structure calculations have been taken into account in discussing the possible valence electron configurations of gold. It is suggested that substantial 5d-6s mixing occurs with nearly matching charge compensation and additional 5d depletion through an interaction of the 5d band of gold with host orbitals of proper symmetry. A net charge flow off the gold sites appears in every case.     

Structural,electronic and magnetic properties of the 3d transition metal atoms adsorbed on boron nitride nanotubes

Adsorption configurations for a series of transition metal (TM) 3d atoms adsorbed on the zigzag (8, 0) BNNT at five different sites have been investigated using the first-principles PAW potential within DFT under GGA. The most stable adsorption sites are different for different TM atoms. Partially filled 3d metals V, Cr and Mn can bind strongly with zigzag (8, 0) BNNT, and Sc, Ti, Co and Ni can be chemically adsorbed on the (8, 0) BNNT. The binding between the Fe or Cu atom and the BNNT is only marginal. One unusual case is Zn. Its zero binding energy independent of the adsorption sites implies it can only physically adsorbed on the BNNT mainly stemmed from the van de Waals interaction. Electronic structure analyses show that: (1) for each TM atom adsorbed at five different sites, the total DOS curves of both majority and minority spins make a slightly relative shift along the energy axis, and for each site the total DOS of the minority spin shifts slightly in high energy direction with respect to that of the majority spin lead to a exchange splitting, except fully filled 3d metals Cu and Zn; (2) total DOS curves of both the majority and minority spins for the adsorbed systems shift to the lower energy region compared with that of the pristine (8, 0) BNNT. And the smaller 3d electrons number of the TM atom, the larger shift to the lower energy region of its DOS curves; (3) for V-, Mn- and Fe-adsorbed (8, 0) BNNT, only one type of electrons (either majority spin or minority spin) passes through the Fermi level implies these adsorbed systems are all half-metals.     

Computer simulations in the study of gold nanowires: the effect of impurities

Suspended gold nanowires have recently been made in an ultra-high vacuum ambient and were imaged by electron microscopy. Two puzzles were presented: one atom thick wires are produced and some of the atomic distances between these atoms before their breaking were too large. Simulations using realistic molecular dynamics method were able to unveil some processes to explain the mechanisms of formation, evolution, and breaking of these atomically thin Au nanowires under stress. The calculations showed how defects induce the formation of constrictions that eventually will form the one-atom chains. Atomically thin chains, five atoms long were obtained, before breaking. The results were in excellent agreement with experimental results except for the large Au-Au distances. In fact no theoretical calculation of pure gold nanowires have been able to produce such large distances. Light impurities that cannot be imaged in these experiments may be responsible for these large Au-Au distances. Using ab initio total energy calculations based on the density functional theory, we have studied the effect of H, C, O, N, B, S, CH, CH₂, and H₂ impurities on the nanowire’s electronic and structural properties, in particular how they affect the rupture of the nanowire. We find that the impurities tend to locally increase the nanowire’s strength, in such a way that its rupture always occurs at an Au-Au bond and never at an Au-X bond (X being an impurity). In particular, oxygen seems to form very stable bonds that may be used to pull longer Au chains. Regarding the observed large Au-Au bond lengths, it was found, based on quasi-static calculations, that the best candidate to explain the large distances is H. However, some particular experimental conditions may lead to different results. PACS 68.65.-k; 68.37.Lp; 71.15.Pd     

Electronic contribution to electric field gradient in dilute alloys

A simple relation for calculating the electronic contribution to the electric field gradient in dilute alloys of transition metals is reported and is compared with the conduction electron charge shift model. The dependence of the field gradient on thes andd electrons and the difference in radii between the host and the probe atoms is considered for calculating the field gradient. It is found that thed electrons are the major contributors to the field gradient.     

Calculation of the Knight shift in palladium

The direct and the exchange core polarization (ECP) contributions of the conduction electrons to the Knight shift of palladium are evaluated. To obtain the wave functions for the conduction electrons and the partial densities of states at the Fermi surface a KKR energy band calculation was performed. The contributions of the core electrons to the Knight shift were determined by using the moment perturbation method (MP). Electron-electron interactions are taken into account by individual enhancement factors for thed ands electrons. The agreement between the theoretical results and the available experimental data is quite satisfactory.     

Strong blue emission from ZnSe nanowires grown at low temperature

We report optimized photoluminescence of ZnSe nanowires grown by molecular beam epitaxy, obtained by lowering the growth temperature down to 300 °C. The low‐temperature growth method has been developed using Si(111) and GaAs(111)B substrates. On the latter, vertical oriented blue‐emitting nanowires have been obtained. The growth mecha‐ nism is discussed with the help of in‐situ and ex‐situ electronic and structural measurements. We also report strong blue luminescence from ZnSe nanowires grown on ITO‐coated glasses, demonstrating that ZnSe nanowires are optimal candidates for transparent optoelectronics. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study of mechanisms of formation of X-ray emission bands in crystals by the density functional method: The Mg <Emphasis Type="Italic">L</Emphasis><Subscript>2,3</Subscript> bands in metal and in MgO

Using the density functional method in the pseudopotential approximation, we calculate the probabilities of X-ray emission transitions and shapes of the L _2,3 X-ray emission bands of Mg in crystals of metallic magnesium and monoxide MgO. We have paid special attention to the study of mechanisms by which these bands were formed; therefore, along with the total intensities, we determine the partial s- and d-contributions. In addition, the intensities of the L _2,3 bands of Mg have been separated into contributions from direct (intra-atomic) transitions and charge-transfer crossover transitions. We show that an unexpectedly large contribution of partial d-transitions in magnesium oxide is caused by crossover transitions of electrons of valence states of ligands to the core 2p-level of Mg, i.e., charge-transfer transitions. In metallic magnesium, the contribution of these transitions proved to be insignificant. An appreciable contribution of crossover transitions has also been revealed in s-contributions of X-ray emission bands. To estimate the calculation accuracy, we have compared the shapes of theoretical spectra with the shapes of experimental L _2,3 X-ray emission bands of Mg.     

Saturation magnetic moments,magnetic hyperfine fields and electric field gradients at nuclei in the heavy rare earth metals

The contributions of 4f, 5d and 6s electrons to the saturation magnetic moments and magnetic hyperfine fields in the heavy rare earth metals are calculated using the model described in the previous paper. It is found that 4f shell moments are reduced from their free ion values by amounts varying from 0.05µ _B in Gd to several tenths of a Bohr magneton in Tb and Dy, in qualitative agreement with a recent published analysis of neutron diffraction results in Tb, but that the calculated total saturation moments in Tb and Dy are slightly larger than commonly accepted experimental values. After 6s contributions to magnetic hyperfine fields are determined by fitting observations in Gd, the predicted differences between the fields for metals and those for free ions are such that the estimated uncertainty ranges of the theoretical values overlap the experimental ranges. The 5d contribution in the model is negative, varying from about –40 kOe in Tb to –200 kOe in Er. Electric field gradients are also analysed. Observed results can be fitted if the average effective Sternheimer screening factorR _d ^* for 5d electrons in the metals satisfies (1 —R _d ^* )0.7.     

钴原子及其团簇在Rh(111)和Pd(111)表面的扫描隧道显微学研究

利用扫描隧道显微镜和扫描隧道谱(STM/STS)及单原子操纵,系统研究了单个钴原子(Co) 及其团簇在Rh (111)和Pd (111)两种表面的吸附和自旋电子输运性质. 发现单个Co原子在Rh (111)上有两种不同的稳定吸附位,分别对应于hcp和fcc空位, 他们的高度明显不同,在针尖的操纵下单个Co原子可以在两种吸附位之间相互转化. 在这两种吸附位的单个Co原子的STS谱的费米面附近都存在很显著的峰形结构, 经分析认为Rh (111)表面单个Co原子处于混价区,因此这一峰结构是d轨道共振 和近藤共振共同作用的结果.对于Rh (111)表面上的Co原子二聚体和三聚体, 其费米面附近没有观测到显著的峰,这可能是由于原子间磁交换相互作用 和原子间轨道杂化引起的体系态密度改变所共同导致.与Rh (111)表面不同, 在Pd (111)表面吸附的单个Co原子则表现出均一的高度.并且对于Pd (111)表面所有 单个Co原子及其二聚体和三聚体,在其STS谱的费米面附近均未探测到显著的电子结构, 表明Co原子吸附于Pd (111)表面具有与Rh (111)表面上不同的原子-衬底相互作用与自旋电子输运性质.     

Processes of multiple scattering in the formation of M EELFS spectra of 3d metals

The processes of multiple scattering of a secondary electron from a local atomic structure and their contributions to the EELFS spectra have been considered. For the M EELFS spectra of 3d metals (i.e., the EELFS spectra behind the M edges of energy loss), the contributions of the processes provided by multiple processes of electron-impact excitation of 3d valence electrons have been analyzed.     

Temperature and quantum effects in the stability of pure and doped gold nanowires

We investigate, using ab initio molecular dynamics simulations, the effect of temperature on the stability of pure, C and H doped, atomically thin gold nanowires. We show how thermal fluctuations lead to local instabilities that result in the rupture of the nanowires, and how appropriate impurities may help to stabilize these wires. We also show that when light impurity atoms, such as hydrogen, are incorporated in the Au nanowires, quantum effects will affect the nuclear dynamical evolution.     

面心立方单晶镍纳米线稳定性及磁性的第一性原理计算

本文采用第一性原理方法,研究了轴向为低指数晶向的面心立方(fcc)单晶镍纳米线的稳定性和磁性.计算表明,[110] 是fcc镍纳米线最容易出现的取向,[111] 取向次之,而 [001] 取向则很难出现,这一结果与实验事实符合.镍纳米线按照原子位置和磁性强弱的不同,可以分成简单的芯-壳结构,在纳米线芯部,原子的磁矩大小与块体基本一致.在纳米线表面,镍原子的磁矩比芯部原子有所增加.表面原子磁矩与轴向的取向相关,[110] 为轴向的纳米线表面原子磁矩最低,而[001] 为轴向的纳米线表面原子磁矩最高. 关键词： 镍 纳米线 第一性原理 原子磁矩     

Self-interstitial configuration in molybdenum studied by modified analytical embedded atom method

The stability of various atomic configurations containing a self-interstitial atom (SIA) in a model representing Mo has been investigated using the modified analytical embedded atom method (MAEAM). The lattice relaxations are treated with the molecular dynamics (MD) simulation at absolute zero of temperature. Six relatively stable self-interstitial configurations and formation energies have been described and calculated. The results indicate that the [111] dumbbell interstitial S₁₁₁ has the lowest formation energy, and in ascending order, the sequence of the configurations is predicted to be S₁₁₁, C, S₁₁₀, T, S₀₀₁ and O. From relaxed displacement field up to the fifth-NN atoms of six configurations, we know that the relaxed displacements depend not only on separation distances of the NN atoms from the defect centre but also strongly on the direction of the connected line between the NN atoms and the defect centre. The equilibrium distances between two nearest atoms in the core of the S₁₁₁, C, S₁₁₀, T, S₀₀₁ and O configurations are 0.72a, 0.72a, 0.71a, 0.72a, 0.70a and 0.70a, respectively.      

IR transmission and reflection spectra of structures with ZnTe and ZnTe/ZnMgTe quantum wires

The IR transmission and reflection spectra of ensembles of one-layer (ZnTe) and two-layer (ZnTe/ZnMgTe) quantum wires have been measured in s- and p-polarized light. The optical parameters of the structures and the material/vacuum volume ratio have been found by dispersion analysis. The frequencies of IR-active modes have been calculated using a quasi-electrostatic approximation and models of prolate ellipsoid and two-layer cylinders. The spectra of ensembles of ZnTe nanowires differ little from the spectrum of bulk ZnTe, exception for some softening of the LO-like mode. The spectra of two-layer ZnTe/ZnMgTe nanowires contain interface modes along with the TO-like mode. The calculated and measured data agree well.