Dielectric permittivity of ultrathin PbTiO3 nanowires from first principles

We propose an efficient method to compute the dielectric permittivity of nanostructures by combining first principles density functional perturbation theory with effective medium theory. Specifically, ultrathin axially symmetric ferroelectric PbTiO₃ nanowires are considered. As established previously by Pilania and Ramprasad (Phys Rev B 82:155442, 2010), (4 × 4) PbO-terminated nanowire and (4 × 4) TiO₂-terminated nanowire display, respectively, a uniform axial and a vortex polarization in their ground state configurations (the latter with a non-zero axial toroidal moment). Both nanowires, regardless of the lateral surface termination, display a significantly larger dielectric constant value along the axial direction, and diminished values along the off-axis directions, as compared to the corresponding bulk values. Our results further suggest that the nanowires with unconventional vortex-type polarization states are expected to have an increased dielectric response as compared to those with conventional uniform axial polarization. The method proposed here is quite general and readily extendable to other zero-, one-, and two-dimensional nanostructures.     

应力条件下ZnO电子结构的第一性原理研究

计算了外压调制下ZnO体系电子结构特性,分析了外压对ZnO电子结构的影响.所有计算都是基于密度泛函理论(DFT)框架下的第一性原理平面波超软赝势方法.结果表明:随着压力的逐渐增大,Zn-O键长缩短,价带与导带分别向低能和高能方向漂移,带内各峰发生小的劈裂,带隙Eg明显展宽,Zn的3d电子与O的2p电子杂化增强.     

Mechanical properties of ZnO nanowires under different loading modes

A systematic experimental and theoretical investigation of the elastic and failure properties of ZnO nanowires (NWs) under different loading modes has been carried out. In situ scanning electron microscopy (SEM) tension and buckling tests on single ZnO NWs along the polar direction [0001] were conducted. Both tensile modulus (from tension) and bending modulus (from buckling) were found to increase as the NW diameter decreased from 80 to 20 nm. The bending modulus increased more rapidly than the tensile modulus, which demonstrates that the elasticity size effects in ZnO NWs are mainly due to surface stiffening. Two models based on continuum mechanics were able to fit the experimental data very well. The tension experiments showed that fracture strain and strength of ZnO NWs increased as the NW diameter decreased. The excellent resilience of ZnO NWs is advantageous for their applications in nanoscale actuation, sensing, and energy conversion.      

Two-dimensional cadmium selenide electronic and optical properties: first principles studies

Structural, electronic and optical properties of two-dimensional (2D) cadmium selenide (CdSe) structures with \(2\times 2\) periodicities are investigated. First principles total energy calculations are performed within the periodic density functional theory. Initially, the structural properties are determined using the local density approximation as implemented in the PWscf code of quantum ESPRESSO package. To investigate the electronic properties, the GW method is applied to determine the energy gap within the plasmon pole and the random phase approximations. Optical properties are investigated to determine the dielectric constant and the Bethe–Salpeter theory is used to calculate the exciton binding energies. Zinc blende and wurtzite phases are considered to calculate the bulk energy gaps, which are compared to the experimental values, finding good agreement. The 2D structure exhibits an energy gap larger than that of the bulk, indicating the effects of reduction in dimensionality; these changes can be attributed to the dangling bonds that are present in the 2D layer.     

Anisotropic and temperature effects on mechanical properties of copper nanowires under tensile loading

Atomistic simulations are used to investigate the mechanical properties of copper nanowires (NWs) along 〈1 0 0〉, 〈1 1 0〉 and 〈1 1 1〉 crystallographic orientations under tensile loading at different temperatures. The inter-atomic interactions are represented by employing embedded-atom potential. To identify the defects evolution and deformation mechanism, a centrosymmetry parameter is defined and implemented in the self-developed program. The simulations show that Cu NWs in different crystallographic orientations behave differently in elongation deformations. The stress–strain responses are followed by a particular discussion on yield mechanism of NWs from the standpoint of dislocation moving. Generally, the study on the incipient plastic deformation will be helpful to further understanding of the mechanical properties of nanomaterials. In addition, the Young’s modulus decreased linearly with the increase of temperature. The crystal structure is less stable at elevated temperatures.     

Dielectric properties of organosilicons from first principles

Density functional perturbation theory calculations have been performed to determine the dielectric constant of Si “doped” polyethylene (PE). Substitution of C atoms in PE by Si ranging from 0 to 100% has been considered. Both the electronic and ionic contributions to the dielectric constant increase with increasing Si content. These increases are attributed, respectively, to enhanced σ conjugation and increased IR vibrational intensity of modes involving Si containing bonds (owing to their softness and polarity).     

Atomic and electronic properties of realizable size single-crystal GaN nanotubes by first principles

We studied the diameter and wall thickness dependent atomic and electronic properties of practical size single-crystal GaN nanotubes using first principle calculations. Single-crystal GaN nanotubes are similar to the hexagonal GaN nanowires, grown in the [0001] direction with [10-10] facets, except there is an axial hexagonal void in them. We first demonstrated that the atomic and electronic properties of these tubes are mainly determined by the thickness of their wurtzite walls; and their diameters have negligible effects. Then, considering the individual walls of GaN nanotubes in two-dimensional slab calculations we examine the bond distances, formation energy, band gap, effective electron mass and the evolution of electronic density of the states as a function of thickness for unsaturated and hydrogen-saturated slabs of GaN. Calculations revealed that the unsaturated dangling bonds at the surfaces induce defect states in the band gap region of unsaturated tubes. Therefore, regardless of diameter and wall thickness, their band gaps are always smaller than that of the bulk GaN. However, the band gaps of the hydrogen-saturated tubes are found to be amplified with respect to bulk GaN. The amplification in the band gaps as a function of wall thickness in the range of 5.6-16.9 A and 16.9-28.1 A scales with a factor of 1/d(0.9281) and 1/d(1.769), respectively. Our results show that, regardless of diameter, hydrogen saturated single-crystal GaN tubes with the wall thickness as small as 28.1 A would be stable and they would have a noticeably larger band gap with respect to the band gap of bulk GaN.     

First principles study of electronic properties of gallium nitride nanowires grown along different crystal directions

The electronic properties of hydrogen-saturated GaN nanowires with different orientations and sizes are investigated using first principles calculations, and three types of nanowires oriented along the [0 0 1], [1 1 0] and [1 −1 0] crystal directions are considered. The electronic properties of nanowires in all three directions are extremely similar. All the hydrogen-saturated GaN nanowires show semiconducting behavior with a direct band gap larger than that of bulk wurtzite GaN. Quantum confinement leads to a decrease in the band gap of the nanowires with increasing nanowire size. The [0 0 1]-oriented nanowires with hexagonal cross sections are energetically more favorable than the [1 0 0]- and [1 −1 0]-oriented nanowires with triangular cross sections.     

Vertically aligned nanowires from boron-doped diamond

Vertically aligned diamond nanowires with controlled geometrical properties like length and distance between wires were fabricated by use of nanodiamond particles as a hard mask and by use of reactive ion etching. The surface structure, electronic properties, and electrochemical functionalization of diamond nanowires were characterized by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) as well as electrochemical techniques. AFM and STM experiments show that diamond nanowire etched for 10 s have wire-typed structures with 3-10 nm in length and with typically 11 nm spacing in between. The electrode active area of diamond nanowires is enhanced by a factor of 2. The functionalization of nanowire tips with nitrophenyl molecules is characterized by STM on clean and on nitrophenyl molecule-modified diamond nanowires. Tip-modified diamond nanowires are promising with respect to biosensor applications where controlled biomolecule bonding is required to improve chemical stability and sensing significantly.     

Mechanical properties of ultrahigh-strength gold nanowires

Nanowires have attracted considerable interest as nanoscale interconnects and as the active components of both electronic and electromechanical devices. Nanomechanical measurements are a challenge, but remain key to the development and processing of novel nanowire-based devices. Here, we report a general method to measure the spectrum of nanowire mechanical properties based on nanowire bending under the lateral load from an atomic force microscope tip. We find that for Au nanowires, Young's modulus is essentially independent of diameter, whereas the yield strength is largest for the smallest diameter wires, with strengths up to 100 times that of bulk materials, and substantially larger than that reported for bulk nanocrystalline metals (BNMs). In contrast to BNMs, nanowire plasticity is characterized by strain-hardening, demonstrating that dislocation motion and pile-up is still operative down to diameters of 40 nm. Possible origins for the different mechanical properties of nanowires and BNMs are discussed.     

聚碳酸酯中应变率压缩力学特性研究

利用高速液压伺服试验机开展聚碳酸酯中应变率压缩实验,同时开展低、高应变率压缩的对比实验,验证了中应变率实验的有效性.基于不同温度下各应变率的压缩真实应力-应变曲线,获得了聚碳酸酯在中应变率压缩下的力学特征.结果表明:聚碳酸酯在中应变率下的压缩经历了弹性形变、屈服、应变软化和应变硬化四个阶段,材料力学行为的应变率和温度相关性表现为:提高应变率或降低温度,压缩屈服强度和屈服应变均增大;反之,升高温度或降低应变率,材料的应变软化更为显著.基于实验结果,构建了可描述聚碳酸酯中应变率压缩过程力学特征的 ZWT 非线性黏弹本构模型,该模型可为透明件结构设计与鸟撞仿真提供有力支撑.     

Mechanical properties of diamond thin films

Abstract

The mechanical properties of diamond films deposited via hot filament chemical vapour deposition have been determined using a range of techniques, and related to the composition and morphology of the diamond films as determined by laser Raman spectroscopy. As the quality of the film increases, its hardness (as determined by the volume law of mixtures hardness model) also increases until it is larger than values often reported for polycrystalline bulk material, a consequence of the very small grain size in the films. Coating adhesion, as determined from indentation adhesion tests, also appears to improve with coating quality. Variations in the behaviour of the friction coefficient between diamond films and diamond and steel counterfaces are less well defined, but it appears that the surface morphology of the film is important in dictating the behaviour rather than the quality of the diamond. These results are discussed in the context of the potential use of diamond coatings in tribological applications.

MST/1695     

Mechanical properties and failure characteristics of a recycled CFRP under tensile and cyclic loading

An examination has been made of the mechanical and failure properties of a recycled short carbon fiber reinforced plastic (rCFRP). The rCFRP samples were fabricated by the following process: the CFRP, consisting of epoxy resin with carbon fiber, is ground before mixing with acrylonitrile butadiene styrene resin with different weight fractions of CFRP. The tensile strength (σ_UTS) increased with increasing CFRP content, but dropped considerably for the sample with higher fiber content. From in situ measurement of localized failure in rCFRP, it appeared that material failure occurs even if a low tensile stress of 30% σ_UTS is applied. The localized damage was related to the pull-out (or debonding) of the fibers from the matrix. The fatigue strength increased with increasing the content of the recycled carbon fiber even for the samples with low tensile strength. This was attributed to the low crack driving force arising from severe crack closure. Details of the crack growth behavior were discussed using various crack growth models proposed in previous studies.     

Mechanical properties and electronic structures of one BN nanotube under radial compression

The Tersoff-potential based MD (molecular dynamics) method was used to simulate the radial compression of one (10,0) BN nanotube, and its compressive properties was compared with those of one (10,0) carbon nanotube. The semi-empirical PM3 QC (Quantum chemistry) method was adopted to calculate the electronic structures of the compressed BN-tube, and the effect of the radial compression on the electronic structures of the BN-tube was discussed. It is shown that (i) BN-tube has comparable radial compressive stiffness to carbontube, but lower energy-absorbing, load-support and deformation-support capabilities, and (ii) with the increase of compressive strain, the HOMO energy of the BN-tube increases, the LUMO energy and the LUMO-HOMO energy-gap decrease, and its chemical activity and conductance increase.     

Mechanical and electronic properties of diborides of transition 3d-5d metals from first principles: Toward search of novel ultra-incompressible and superhard materials

The appreciable progress has been achieved currently in the development of design principles, synthesis, investigations and predictions of various groups of ultra-incompressible and superhard materials. One of the recently proposed families of these promising materials is represented by the diborides of heavy 4d and 5d metals. Along with experiments, the theoretical ab initio methods, which involve no a priori assumptions about the electronic structure and intra-atomic interactions, are very effective approaches in the determination and prediction of structural, mechanical, magnetic, optical, dielectric and superconducting properties of such materials.This paper offers a review of the recent advances in theoretical understanding and predictions of the mechanical properties (including elastic (reversible) deformations, which are related to compressibility, as well as the effects of plastic (irreversible) deformations, which are related to hardness of materials) as obtained by means of ab initio calculations - for a broad family of metal diborides MB₂ (AlB₂-like diborides and the recently discovered diborides of heavy 4d and 5d metals with “puckered” boron sheets) and their relations to electronic, cohesive and bonding characteristics of these materials.     

Thermal and tensile properties of Si/Ge core-shell and superlattice nanowires

The Stillinger-Weber potential-based MD (Molecular dynamics) method is used to simulate the heating-up and axial tension of Si/Ge core-shell and superlattice nanowires; according to the simulative results, the differences in their thermal and mechanical properties are discussed. The results show the following: (1) The Si/Ge superlattice nanowire is more thermally stable than the core-shell one, and their melting points are 1160 and 1320 K, respectively. (2) The Si/Ge core-shell nanowire has higher elastic module than the super-lattice one. (3) Under tension, the super-lattice nanowire has better antideformation capability than the core-shell one but has comparative antiloading capability.     

Thermal and tensile properties of Si/Ge core-shell and superlattice nanowires

The Stillinger-Weber potential-based MD (Molecular dynamics) method is used to simulate the heating-up and axial tension of Si/Ge core-shell and superlattice nanowires; according to the simulative results, the differences in their thermal and mechanical properties are discussed. The results show the following: (1) The Si/Ge superlattice nanowire is more thermally stable than the core-shell one, and their melting points are 1160 and 1320 K, respectively. (2) The Si/Ge core-shell nanowire has higher elastic module than the super-lattice one. (3) Under tension, the super-lattice nanowire has better antideformation capability than the core-shell one but has comparative antiloading capability.     

How strain controls electronic linewidth in single beta-phase polyfluorene nanowires

Low-temperature single-molecule fluorescence spectroscopy reveals pure, virtually defect-free chains of the one-dimensional crystalline beta-phase of polyfluorene. The likelihood of beta-phase formation is shown to correlate directly with the initial shape of the polymer chain, with extended chains preferentially forming this planarized phase. Planarized chains, characterized by a distinct spectroscopic signature can, however, exhibit substantial bending within the plane. This bending results in a strong increase in the elementary transition linewidth of the conjugated segment. The transition linewidth provides a lower limit to the electronic dephasing time of the excited state of >3 ps at 5 K. Remarkably, bending does not appear to disrupt the pi-electron conjugation so that the emission from a single bent beta-phase chromophore is not necessarily linearly polarized as is generally assumed.     

Voltage generation from individual BaTiO(3) nanowires under periodic tensile mechanical load

Direct tensile mechanical loading of an individual single-crystal BaTiO(3) nanowire was realized to reveal the direct piezoelectric effect in the nanowire. Periodic voltage generation from the nanowire was produced by a periodically varying tensile mechanical strain applied with a precision mechanical testing stage. The measured voltage generation from the nanowire was found to be directly proportional to the applied strain rate and was successfully modeled through the consideration of an equivalent circuit for a piezoelectric nanowire under low-frequency operation. The study, besides demonstrating a controlled experimental method for the study of direct piezoelectric effect in nanostructures, implies also the use of such perovskite piezoelectric nanowires for efficient energy-harvesting applications.