Ab initio modelling of dopants in diamond nanowires: Ii

In this study an analysis is presented of the bonding and structural properties of dehydrogenated and hydrogenated doped cylindrical diamond nanowires calculated using the Vienna Ab Initio Simulation Package, employing density functional theory within the generalized-gradient approximation. The dopants studied here have been inserted substitutionally, equidistant along the axis of an infinite (periodic) diamond nanowire. These dopants include aluminium, phosphorus, oxygen and sulphur. The doped nanowires have then been re-relaxed, and properties compared with previously calculated results for undoped, boron-doped and nitrogen-doped structures. Structural properties of relaxed nanowires considered here include an examination bonding via the electron charge density, with the aim of providing a better understanding of the effects of dopants on the stability of diamond nanostructures and nanodevices.     

Ab initio modelling of band states in doped diamond

Presented in this study is an analysis of the electronic properties of doped diamond calculated using the Vienna ab initio simulation package, employing density functional theory within the generalized-gradient approximation. The dopants studied here have been inserted substitutionally into a 64-atom diamond supercell and include the single-electron acceptors boron and aluminium, the single-electron donors nitrogen and phosphorus and the double-electron donors oxygen and sulphur. Co-doping of diamond with sulphur and boron has also been briefly examined. The doped supercells have been relaxed, followed by calculation of electronic properties from the electronic density of states such as the indirect bandgap E _g, the valence bandwidth and an examination of the acceptor and donor states in the bandgap. It is anticipated that this study will provide a useful comparison of the third- and fourth-row donors and acceptors in diamond.     

From nanodiamond to diamond nanowires: structural properties affected by dimension

Although production of nanowires from various materials is proving very successful, the development of diamond nanowires has been slow. However, a significant amount of successful research has been conducted regarding zero-dimensional nanodiamond crystals, which may offer a basis for the development of one-dimensional diamond nanostructures. Observations of the structural transitions between nanodiamonds into carbon onions inevitably lead to questions as to whether a similar transformation occurs in one dimension and, if so, how it may be avoided. Presented here are ab initio investigations of dehydrogenated nanodiamond crystals and analogous diamond nanowires, to examine how the additional dimension effects structural properties.     

Electronic and Structural High Pressure Properties of CuGaS 2 Chalcopyrite Semiconductor

We have investigated high-pressure structural properties and ab-initio band structure calculations of the ternary CuGaS 2 by single crystal X-ray diffraction up to 8 GPa. The single crystal X-ray diffraction experiments were performed in a Merrill-Bassett diamond anvil cell. The analysis of the X-ray data makes possible the accurate determination of the atomic position of the unit cell under pressure. The structural parameter u and the Cu-S and Ga-S bond lengths have been deduced. The results of the electronics band structure calculation using a first-principles pseudo-potential method and the local density approximation (LDA) are reported. The pressure derivatives of the energy gap are calculated and the values are in reasonable good agreement with the experimental ones.     

Reactions and stresses in polycrystalline diamond-metal and diamond-carbide compacts

Abstract

The kind of bonding phase has a significant influence on the microstructure and mechanical properties of diamond compacts. Microstructural studies of diamond with 5% wt. Ti and 5%wt. Tic (and also 30%wt. Tic) were carried out with a Transmission Electron Microscope. The TEM microstructural observations show differences between the metal and metal carbide bonding phase in diamond compacts. The mismatch of thermal expansion coefficients between diamond and the bonding metal or the compound induces significant internal stresses and may generate micro-cracks in polycrystalline diamond compacts. Twins and dislocations are the important details of microstructures in diamond crystals after HPHT sintering. They can appear as a result of residual stress relaxation. Results of measurements of residual stresses on a diamond compact surface by means of the “sin²ψ X-ray diffraction method are reported.     

Modulation Mechanism of Transition Elements Fe and Mn on the Interface Bonding Performance of Bronze/Diamond Composites

The modulation mechanism of iron (Fe) and manganese (Mn) in transition-metal elements on the interface bonding and mechanical properties of bronze (Cu₃Sn)-based/diamond composites is investigated through first-principles calculations. Transition-elements-doping scenarios are investigated employing six-layer slab models. It is revealed that the doping of Fe or Mn can make the Cu₃Sn/diamond interface more stable, which effectively improves the wettability of the Cu₃Sn/diamond interface based on the calculation results and analysis of interface energy, differential charge density model, and density of states. However, co-doping with both Fe and Mn weakens the wettability of the Cu₃Sn/diamond interface. Finally, wettability tests and microstructure characterizations demonstrate that the doping of Fe and Mn represents an effective approach to controlling the interface bonding performance of bronze/diamond composites.     

Schottky contacts in germanium nanowire network devices synthesized from nickel seeds

This paper presents reliable process to the synthesis of germanium nanowires by the vapor–liquid–solid method using nickel as an alternative catalyst to gold, the most commonly used metal, without toxic gas precursors. The structural study showed single-crystalline germanium nanowires with diamond structure, lengths of tens of microns and diameters smaller than 40 nm. The reduced dimensions of the nanowires led to phonons localization effect, with correlation lengths of the same order of the nanowires diameters. Additionally, the analysis of electronic properties of metal-nanowire-metal devices indicated the presence of Schottky barriers, whose values depend linearly on temperature. This linear dependence was assigned to the tunneling process through an insulator layer (mostly GeO_x) at the metal-semiconductor interface. These results point to the existence of another channel for electrons transference from metal to semiconductor being very significant to electronic devices fabrication.     

Theoretical Study of the Structural,Electronic, Chemical Bonding and Optical Properties of the A2<Subscript>1</Subscript><Emphasis Type="Italic">am</Emphasis> Orthorhombic SrBi<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>9</Subscript>

The structural parameters, density of states, electronic band structure, charge density, and optical properties of orthorhombic SrBi₂Ta₂O₉ have been investigated using the plane-wave ultrasoft pseudopotential technique based on the first-principle density functional theory (DFT). The calculated structural parameters were in agreement with the previous theoretical and experimental data. The band structure showed an indirect (S to Γ) band gap with 2.071 eV. The chemical bonding along with population analysis has been studied. The complex dielectric function, refractive index, and extinction coefficient were calculated to understand the optical properties of this compound, which showed an optical anisotropy in the components of polarization directions (100), (010), and (001).     

Gold under high pressure

Abstract

First principle predictions for the equation of state of gold using solid and liquid state theories are compared up to combined pressures and temperatures of 600 GPa and 17 000 K with static diamond anvil cell compression, ultrasonic measurements and shock Hugoniot data which include a recent laser driven shock Hugoniot points at 600 GPa. Excellent agreement between theoretical and experimental data is observed. The theoretically estimated 300 K isotherm agrees to within 2 GPa with the isotherm that has been measured to 70 GPa using the diamond anvil cell. The structural energy estimates show that the normal f.c.c. phase remains stable under pressure. The estimate of the shock Hugoniot temperature of gold at 600 GPa based on a liquid state model is consistent with the measurements of laser induced shock luminescence, which in fact provides an experimental determination of the temperature of gold above its Hugoniot melting point. The powerful means provided by theory in the prediction of material properties of gold at ultra high pressures and temperatures is significant because gold is an efficient converter of laser energy into soft X-rays and is a potential candidate as a standard for high pressure, high temperature work.     

The formation of a diamond layer on a carbide substrate during diamond interaction with Si,WC and Co

Abstract

A diamond layer was formed on a carbide substrate in an irregular temperature field at high pressures (HP). A gradient scheme of HP cell set-up has been developed, which provides for a simultaneous impregnation of opposite planes of a diamond layer by components that differ in melting temperature. The cell temperature field has been calculated and physico-mechanical properties of the obtained composite material have been studied.     

Structural,electronic and magnetic properties of hcp Fe,Co and Ni nanowires encapsulated in zigzag carbon nanotubes

The structural, electronic and magnetic properties of hcp transition metal (TM = Fe, Co or Ni) nanowires TM₄ encapsulated inside zigzag nanotubes C(m, 0) (m = 7, 8, 9, 10, 11 or 12), along with TM _n (n = 4, 10 or 13) encapsulated inside C(12, 0), have been systematically investigated using the first-principle calculations. The results show that the TM nanowires can be inserted inside a variety of zigzag carbon nanotubes (CNTs) exothermically, except from the systems TM₄@(7, 0) and TM₁₃@(12, 0) which are endothermic. The charge is transferred from TM nanowires to CNTs, and the transferred charge increases with decreasing CNT diameter or increasing nanowire thickness. The magnetic moments of hybrid systems are smaller than those of the freestanding TM nanowires, especially for the atoms on the outermost shell of the nanowires. The magnetic moment per TM atom of TM/CNT system increases with increasing CNT diameter or decreasing nanowire thickness. Both the density of states and spin charge density analysis show that the spin polarization and the magnetic moments of all hybrid systems mainly originate from the TM nanowires, implying these systems can be applied in magnetic data storage devices.     

金刚石延(111)面生长的第一性原理研究

利用密度泛函理论(DFT)对碳原子在镍(111)表面吸附结构进行了计算，得到了吸附能以及态密度 (density of state, DOS)分布，分析了吸附在镍(111)面的碳原子和金刚石(111)面的碳原子的分波态密度(PDOS)，结果表明吸附在镍表面的碳原子具有与金刚石表面碳原子相类似的电子结构特点，即两者都存在孤对的和成键的sp³杂化电子,进而发现吸附在镍表面的碳原子极易与金刚石表面相互作用形成稳定的类金刚石几何结构. 关键词： 密度泛函理论 化学吸附 电子结构 金刚石生长     

Preparation/Property Relationships for a Nitrate Amperometric Sensor: Effect of Nanowires Polymerization Parameters

Nitrate amperometric sensors based on polypyrrole (PPy) nanowire modified electrodes were developed by electropolymerization of pyrrole with a template‐free method. Polymerization parameters of PPy nanowires were changed to improve the amperometric response to NO₃ ^?. The experimental results show that the polymerization parameters such as concentration of pyrrole, concentration of electrolyte, acidity of polymerization solution, and the kinds and concentration of dopants have significant effects on the morphologies of the nanowires and the electroreduction current density of NO₃ ^?. The PPy nanowires prepared in a solution containing NO₃ ^? have an obvious “memory effect” for NO₃ ^?. The determination sensitivity and detection limit may be varied with the change of modification parameters. The PPy nanowire modified electrode prepared under a certain polymerization condition has good electrocatalytic effect toward electroreduction of NO₃ ^? and has good linearity between the electroreduction current density and the concentration of NO₃ ^?. The sensitivity and detection limit are 606.54 mA/M cm² and 9.98×10^?6 M, respectively.     

Synthesis,phase composition,and magnetic properties of iron nanowires prepared in the pores of polymer track-etched membranes

Arrays of iron nanowires prepared by the method of galvanic filling of polymer track-etched membrane pores (matrix synthesis) under different electrolysis modes and electrolyte temperatures have been studied. The conditions of the synthesis have been analyzed. The optimal composition and electrolyte temperature have been found. The phase composition and magnetic properties of nanowires have been studied using the methods of electron microscopy, X-ray diffraction, elemental energy-dispersive microanalysis, and Mössbauer spectroscopy. The average nanowire diameter is 100–200 nm. The length varies from 6 to 10 μm. The surface density is ～10⁸ cm^?2 at the average distance to each other of about 1 μm. It has been established that the basis of nanowires is formed by the metal iron nanocomposite that manifests the magnetic properties of bulk α-Fe. It has been found that the preferred orientation of the magnetization inside the iron nanowires arises for an array prepared at a potential of ?750 mV.     

Synthesis and field emission properties of GaN nanowires

Gallium nitride (GaN) nanowires grown on nickel-coated n-type Si (1 0 0) substrates have been synthesized using chemical vapor deposition (CVD), and the field emission properties of GaN nanowires have been studied. The results show that (1) the grown GaN nanowires, which have diameters in the range of 50-100 nm and lengths of several micrometers, are uniformly distributed on Si substrates. The characteristics of the grown GaN nanowires have been investigated using X-ray diffraction (XRD) and transmission electron microscopy (TEM), and through these investigations it was found that the GaN nanowires are of a good crystalline quality (2) When the emission current density is 100 μA/cm², the necessary electric field is an open electric field of around 9.1 V/μm (at room temperature). The field enhancement factor is ∼730. The field emission properties of GaN nanowires films are related both to the surface roughness and the density of the nanowires in the film.     

Thermal conductive performance of deposited amorphous carbon materials by molecular dynamics simulation

A series of diamond-like carbon (DLC) films with different microstructure were prepared by depositing carbon atoms on diamond surface with incident energy ranging from 1 to 100 eV. The thermal conductivity of the deposited films and the Kapitza resistance between the film and the diamond substrate were investigated. Results show that the average density, the average fraction of sp³ bonding and the thermal conductivity of the DLC films increase first, reaching a maximum around 20–40 eV before decreasing, while the Kapitza resistance decreases gradually with increased deposition energy. The analysis suggests that the thermal resistance of the interface layer is in the order of 10^?10 m²K/W, which is not ignorable when measuring the thermal conductivity of the deposited film especially when the thickness of the DLC film is not large enough. The fraction of sp³ bonding in the DLC film decreases gradually normal to the diamond surface. However, the thermal conductivity of the film in normal direction is not affected obviously by this kind of structural variation but depends linearly on the average fraction of sp³ bonding in the entire film. The dependence of the thermal conductivity on the fraction of sp³ bonding was analysed by the phonon theory.     

Structural phase transition in FeBO under pressure

Using the density functional theory the structural and magnetic properties of iron borate under high pressure have been studied. At about P = 22.7 GPa a first order phase transition to the phase described by the same space group Rc has been found. The phase transition is accompanied by a 9% volume change of the unit cell, a four times decrease of the magnetic moment on Fe, an increase of the charge density at Fe, and a disappearance of the energy gap in the electronic density of states. Received 21 September 2001 and Received in final form 6 January 2002 Published online 6 June 2002     

Theoretical predictions of the structural and thermodynamic properties of MgZn2 Laves phase under high pressure

The structural and thermodynamic properties of MgZn₂ Laves phase under hydrostatic pressure have been investigated by using a first-principles method based on the density functional theory within the generalized gradient approximation. The calculated equilibrium structural parameters are consistent with the previous experimental and theoretical data. Especially, we study the pressure dependence of the elastic constants, polycrystalline elastic moduli, Poisson’s ratio, elastic anisotropy, and theoretical Vickers hardness of MgZn₂. It is found that the pressure plays a significant role in the elastic properties of MgZn₂ due to the variations of inter-atomic distance. In addition, the density of states and Mulliken analysis are performed to reveal the bonding characteristics of MgZn₂. It is observed that the total density of states exhibits a certain offset with the increase of external pressure. Finally, the dependences of thermodynamic properties on pressure and temperature of MgZn₂ Laves phase have been also successfully predicted and analyzed within the quasi-harmonic Debye model for the first time.     

Effect of doping Zn atom on the structural stability,mechanical and thermodynamic properties of AlLi phase in Mg–Li alloys from first-principles calculations

ABSTRACT

This work uses first-principles total energy calculations on the basis of density functional theory to predict the structural stability, mechanical and thermodynamic properties of Zn atom doped AlLi phase in Mg–Li–Al–Zn alloy. The values of the equilibrium lattice parameters and the formation of enthalpy are highly consistent with the experimental and previous calculations results available. Negative enthalpies of formation ΔH are predicted for all AlLi phase doped concentrations which have positive consequences for its structural stability. The elastic modulus is deduced by Voigt–Reuss–Hill arithmetic approximation. The bulk modulus of the Al–Li–Zn compounds increases as the doping concentrations increase, which are larger than the value of the AlLi phase. In particular, the stability and mechanical anisotropy of the Al–Li–Zn compounds are discussed. The charge density cloud map is drawn to reveal the bonding characteristics of four compounds. The changes in thermodynamic properties are derived by the phonon frequencies within the quasi-harmonic approximation.