Perfect Nanowires Research Articles

Atomistic mechanism effects on the growth of GaAsBi and GaAs nanowires

Kinetic Monte Carlo (KMC) simulations were developed to describe all the kinetic processes occurring during epitaxial growth on vicinal surfaces and to interpret the formation of one-dimensional structures on vicinal surface. Moreover, we know that whatever the considered mechanism, it allows the attachment of deposited atoms in the step edge of the substrate surface. Among these mechanisms that can be studied as adjustable parameters by KMC, are the Ehrlich-Schwoebel (ES) barrier, and inverse (iES), have been considered in this work and shown to play a key role in the formation of perfect nanowires (NWs) on substrate surface. We also assessed the effect of the terrace width of substrate surface on the quality of NWs. The results show that the GaAsBi NWs have a better quality than the GaAs NWs. This work provides experimentalists with optimum growth parameters and atomistic mechanisms governing the creation of a uniform distribution of wires on vicinal surfaces.

Deformation behavior of Cu nanowire with axial stacking fault

The aim of the present investigation is to study the effect of axial stacking fault on the tensile and compressive deformation behavior of Cu nanowires using molecular dynamics (MD) simulations. Cylindrical nanowires with an axial orientation of 〈110〉 and 〈112〉 containing a longitudinal stacking fault on {111} plane have been used for this study. 〈110〉 nanowire exhibited elastic and yield point asymmetries with respect to loading type, while the 〈112〉 nanowire has not shown any asymmetry. Under tensile loading, the presence of axial stacking fault in 〈110〉 nanowire introduced reorientation from 〈110〉 to 〈100〉 axial orientation, which was otherwise absent in its perfect nanowire (without stacking fault). The results also revealed that the ductility or strain to failure in 〈110〉 nanowire has been enhanced by the presence of axial stacking fault. In contrast to tensile loading, the influence of stacking fault was insignificant during compressive loading, except for a slight increases in yield stress (strengthening) of faulted 〈110〉 nanowire compared to its perfect nanowire. In addition to these, various microstructural features have been presented in this paper. At larger size, reduction in ductility and absence of reorientation has been noticed in faulted 〈110〉 nanowire whereas other deformation mechanisms remain same in both 〈110〉 and 〈112〉 nanowires. However, with increase in size, ductility in perfect & faulted 〈112〉 nanowire has been increased. The conclusions derived from this study may contribute towards atomistic understanding of the effect of an axial stacking fault in FCC nanowires.

Thermal effects on transverse domain wall dynamics in magnetic nanowires

Magnetic domain walls are proposed as data carriers in future spintronic devices, whose reliability depends on a complete understanding of the domain wall motion. Applications based on an accurate positioning of domain walls are inevitably influenced by thermal fluctuations. In this letter, we present a micromagnetic study of the thermal effects on this motion. As spin-polarized currents are the most used driving mechanism for domain walls, we have included this in our analysis. Our results show that at finite temperatures, the domain wall velocity has a drift and diffusion component, which are in excellent agreement with the theoretical values obtained from a generalized 1D model. The drift and diffusion component are independent of each other in perfect nanowires, and the mean square displacement scales linearly with time and temperature.

Role of B diffusion in the interfacial Dzyaloshinskii-Moriya interaction inTa/Co20Fe60B20/MgOnanowires

We report current-induced domain wall motion (CIDWM) in Ta\Co20Fe60B20\MgO nanowires. Domain walls are observed to move against the electron flow when no magnetic field is applied, while a field along the nanowires strongly affects the domain wall motion direction and velocity. A symmetric effect is observed for up-down and down-up domain walls. This indicates the presence of right-handed domain walls, due to a Dzyaloshinskii-Moriya interaction (DMI) with a DMI coefficient D=+0.06 mJ/m2. The positive DMI coefficient is interpreted to be a consequence of boron diffusion into the tantalum buffer layer during annealing. In a Pt\Co68Fe22B10\MgO nanowire CIDWM along the electron flow was observed, corroborating this interpretation. The experimental results are compared to 1D-model simulations including the effects of pinning. This advanced modelling allows us to reproduce the experiment outcomes and reliably extract a spin-Hall angle {\theta}SH=-0.11 for Ta in the nanowires, showing the importance of an analysis that goes beyond the currently used model for perfect nanowires.

Reply to “Comment on ‘Dynamic Processes of Resistive Switching in Metallic Filament-Based Organic Memory Devices’”

J. Phys. Chem. C 2012, 116 (33), 17955−17959. DOI: 10.1021/jp305482c. J. Phys. Chem. C 2013, 10.1021/jp4014306. I our recent publication, we attributed the bipolar resistive switching phenomenon in the Cu/poly(3-hexylthiophene): [6,6]-phenyl C61-butyric acid methyl ester/indium−tin oxide (Cu/P3HT:PCBM/ITO in short) structure to the Cu filament, which was proposed to grow from the anode (Cu) to the cathode (ITO) and to rupture initially near the cathode (ITO). The same switching kinetics has also been demonstrated directly by the use of in situ transmission electron microscope (TEM) in Ag/α-Si/Pt, Cu(Ag)/ZrO2/Pt, 3 and Au/ZnO/Au sandwich structures. The switching kinetics in our work is completely opposite to that in memory cells based on traditional solid electrolytes (e.g., Cu-doped GeTe) with extremely high cation mobility. This contradictory situation somehow verifies that the filament growth mode is profoundly dependent on cation mobility in the storage media. Moreover, in our very recent work, Ag filaments have been confirmed to nucleate initially at the middle region (neither near the anode nor near the cathode) in the Ag/poly(3,4ethylene-dioxythiophene):poly(styrenesulfonate)/Pt planar device, further supporting the theory that the filament growth mode is dependent on cation mobility. Valov and Waser made a comment, in which they declared that switching kinetics in our work is violating the laws of physical chemistry and electrochemistry. In their opinion, if the Cu filament (irrespective of its dimensions) is mechanically (and therefore electrically) connected to the Cu electrode, only dissolution (growth) of the Cu filament is possible in the case that the Cu electrode is positively (negatively) charged. First of all, we strongly agree with Valov and Waser about the basic laws of physical chemistry and electrochemistry described in the comment. Unfortunately, they ignore the core issue in our paper that the organic material adopted has very low cation mobility, which is in sharp contrast to traditional solid electrolytes with extremely high cation mobility. Figure 1 shows a more detailed schematic of the Cu filament growth mode. According to this figure, the Cu ions originate initially from anode dissolution and then move toward the cathode under an external electric field (Figure 1a,b). However, due to very low mobility, the ions can only migrate an extremely short distance before becoming reduced by the oncoming electrons (Figure 1c). The precipitated Cu atoms in the vicinity of the anode share almost the same electrostatic potential as the anode, acting as an extension of the anode. Subsequently, the extended anode tip will be dissolved again and reduced nearby, depicted in Figure 1d,e, thus resulting in the phenomenon that the Cu filament grows from the anode to the cathode. This process means that the growth of metallic filaments is not a simple ′′growth′′ mode. Instead, it includes growth, dissolution, and the repetition of these two steps. Such a growth mode is reasonable from the viewpoint of kinetics and can not be explained only by the classic laws of physical chemistry and electrochemistry. Our concept is also supported by ref 2 in which the filaments in storage medium are composed of a string of particles, instead of perfect nanowires. We admit that Figure 5b in our recent paper is just a brief illustration of Figure 1 in this response. To directly observe the Cu filament, we fabricated a planar Cu/P3HT:PCBM/Cu device via a two-step ultraviolet lithography process followed by spin-coating. Figure 2a shows the schematic of the planar device and measurement configuration. A scanning electron microscope (SEM) image of the pristine planar device is displayed in Figure 2b. In this figure, one can see that the distance between these two Cu electrodes is about 200 nm. Under positive voltage sweep, a sharp current jump appears at ∼6.3 V (Figure 2c), indicating

MD INVESTIGATIONS FOR MECHANICAL PROPERTIES OF COPPER NANOWIRES WITH AND WITHOUT SURFACE DEFECTS

Based on the molecular dynamics (MD) method, the single-crystalline copper nanowire with different surface defects is investigated through tension simulation. For comparison, the MD tension simulations of perfect nanowire are first carried out under different temperatures, strain rates, and sizes. It has concluded that the surface–volume ratio significantly affects the mechanical properties of nanowire. The surface defects on nanowires are then systematically studied in considering different defect orientation and distribution. It is found that the Young's modulus is the insensitive of surface defects. However, the yield strength and yield point show a significant decrease due to the different defects. Different defects are observed to serve as a dislocation source.

Numerical Exploration of the Defect’s Effect on Mechanical Properties of Nanowires under Torsion

Molecular dynamics (MD) simulations have been carried out to investigate the defect’s effect on the mechanical properties of single-crystal copper nanowire with different surface defects, under torsion deformation. The torsional rigidity is found insensitive to the surface defects and the critical angle appears an obvious decrease due to the surface defects, the largest decrease is found for the nanowire with surface horizon defect. The deformation mechanism appears different degrees of influence due to surface defects. The surface defects play a role of dislocation sources. Comparing with single intrinsic stacking faults formation for the perfect nanowire, much affluent deformation processes have been activated because of surface defects, for instance, we find the twins formation for the nanowire with a surface 45odefect.

Tensile and compressive mechanical behavior of twinned silicon carbide nanowires

Molecular dynamics simulations with the Tersoff potential were used to study the response of twinned SiC nanowires under tensile and compressive strain. The critical strain of the twinned nanowires can be enhanced by twin stacking faults, and their critical strains are larger than those of perfect nanowires with the same diameters. Under axial tensile strain, the bonds of the nanowires are stretched just before failure. The failure behavior is found to depend on the twin segment thickness and the diameter of the nanowires. An atomic chain is observed for thin nanowires with small twin segment thickness under tension strain. Under axial compressive strain, the collapse of twinned SiC nanowires exhibits two different failure modes, depending on the length and diameter of the nanowires, i.e., shell buckling for short nanowires and columnar buckling for longer nanowires.

The Electronic Structure and Transmission Characteristics of Disordered AlGaAs Nanowires

Perfect nanowires may be studied from both the bandstructure and transmission perspectives, and relating features in one set of curves to those in another often yields much insight into their behavior. For random-alloy nanowires, however, only transmission characteristics and virtual-crystal approximation (VCA) bands have been available. This is a serious shortcoming since the VCA cannot properly capture disorder at the primitive cell level: those bulk properties which it can satisfactorily reproduce arise from spatially extended states and measurements which average out primitive cell-level fluctuations. Here we address this deficiency by projecting approximate bands out of supercell states for Al0.15Ga0.85As random alloy nanowires. The resulting bands correspond to the transmission characteristics very closely, unlike the VCA bands, which cannot explain important transmission features. Using both bandstructure and transmission results, we are better able to explain the operation of these nanowires

Stress and dislocations at cross-sectional heterojunctions in a cylindrical nanowire

The limit of crystal lattice coherency of a cross-sectional heteroepitaxial junction in a nanowire is calculated in terms of the critical nanowire radius R c, based on a general calculation of elastic stresses in a long cylindrical rod. R c is derived from the kinetics of a possible misfit dislocation which can slip in the heterointerface and rest in an energetic minimum, if it occurs at all, regardless of whether it is of zero total energy as assumed in the literature. A close comparison is made with the known models for the critical radius of a dislocation half-loop and the critical thickness h c of a heteroepitaxial film, where all models are refined by including the energy of the slip step formed or accidentally annihilated. For a symmetrical, abrupt heterojunction, we obtain R c as a lower and, therefore, quite safe limit, about five times larger than h c of a comparable thin film. An even larger R c is found for junctions of finite transition width instead of abrupt transitions. It is estimated that nucleating the dislocations in a perfect nanowire is difficult, in agrement with experimental reports of dislocation-free nanowires with R well above the theoretical R c.