Bending Of Nanowires Research Articles

Surface stress effects on the bending properties of fcc metal nanowires

The major purpose of this work is to investigate surface stress effects on the bending behavior and properties of $⟨100⟩/{100}$ gold nanowires with both fixed/fixed and fixed/free boundary conditions. The results are obtained through utilization of the recently developed surface Cauchy-Born model, which captures surface stress effects on the elastic properties of nanostructures through a three-dimensional, nonlinear finite element formulation. There are several interesting findings in the present work. First, we quantify the stress and displacement fields that result in the nanowires due to bending deformation. In doing so, we find that regardless of boundary condition, the stresses that are present in the nanowires due to deformation induced by surface stresses prior to any applied bending deformation dominate any stresses that are generated by the bending deformation unless very large $(\ensuremath{\approx}5%)$ bending strains are applied. In contrast, when the stresses and displacements induced by surface stresses prior to bending are subtracted from the stress and displacement fields of the bent nanowires, we find that the bending stresses and displacements do match the solutions expected from bulk continuum beam theory, but only within the nanowire bulk, and not at the nanowire surfaces. Second, we find that the deformation induced by surface stresses also has a significant impact on the nanowire Young's modulus that is extracted from the bending simulations, where a strong boundary-condition dependence is also found. By comparing all results to those that would be obtained using various linear surface-elastic theories, we demonstrate that a nonlinear, finite deformation formulation that captures changes in both bulk- and surface-elastic properties resulting from surface stress-induced deformation is critical to reproducing the experimentally observed boundary-condition dependence in Young's modulus of metal nanowires. Furthermore, we demonstrate that linear surface-elastic theories based solely on the surface energy erroneously predict an increase in Young's modulus with decreasing nanowire size regardless of boundary condition. In contrast, while the linear surface-elastic theories based upon the Gurtin and Murdoch formalism can theoretically predict elastic softening with decreasing size, we demonstrate that, regardless of boundary condition, the stiffening due to the surface stress dominates the softening due to the surface stiffness for the range of nanowire geometries considered in the present work. Finally, we determine that the nanowire Young's modulus is essentially identical when calculated via either bending or resonance for both boundary conditions, indicating that surface effects have a similar impact on the elastic properties of nanowires for both loading conditions.

Curved nanowire structures and exciton binding energies

Growth of quantum-confined semiconductor structures is a complicated process thatmay lead to imperfect and complex shapes as well as geometrical nonuniformitieswhen comparing a large number of intended identical structures. On the otherhand, the possibility of tuning the shape and size of nanostructures allows forextra optimization degrees when considering electronic and optical propertiesin various applications. This calls for a better understanding of size and shapeeffects. In the present work, we express the one-band Schrödinger equation in curvedcoordinates convenient for determining eigenstates of curved quantum-wire andquantum-dash structures with large aspect ratios. Firstly, we use this formulation tosolve the problem of single-electron and single-hole states in curved nanowires.Secondly, exciton states for the curved quantum-wire Hamiltonian problem are foundby expanding exciton eigenstates on a product of single-particle eigenstates. Asimple result is found for the Coulomb matrix elements of an arbitrarily curvedstructure as long as the radius-of-curvature is much larger than the cross-sectionaldimensions. We use this general result to compute the groundstate exciton bindingenergy of a bent nanowire as a function of the bending radius-of-curvature. Itis demonstrated that the groundstate exciton binding energy increases by40 meV as the radius-of-curvature changes from20 to2 nm while keeping the total length (and volume) of the nanowire constant.

The finite element absolute nodal coordinate formulation incorporated with surface stress effect to model elastic bending nanowires in large deformation

Surface stress was incorporated into the finite element absolute nodal coordinate formulation in order to model elastic bending of nanowires in large deformation. The absolute nodal coordinate formulation is a numerical method to model bending structures in large deformation. The generalized Young-Laplace equation was employed to model the surface stress effect on bending nanowires. Effects from surface stress and large deformation on static bending nanowires are presented and discussed. The results calculated with the absolute nodal coordinate formulation incorporated with surface stress show that the surface stress effect makes the bending nanowires behave like softer or stiffer materials depending on the boundary condition. The surface stress effect diminishes as the dimensions of the bending structures increase beyond the nanoscale. The developed algorithm is consistent with the classical absolute nodal coordinate formulation at the macroscale.

Photoconducting response on bending of individual ZnO nanowires

The bending effect of individual zinc oxide (ZnO) nanowires on photoconducting behavior has been investigated by an in situtransmission electron microscopy (TEM) method. By increasing the nanowire bending, the photocurrent of ZnO nanowire under ultraviolet illumination drops dramatically and the photoresponse time becomes much shorter. A possible mechanism has been proposed and discussed. The improved photo response performance by bending ZnO nanowires could be of significance for their optoelectronics and sensor applications.

Surface stress effect on bending resonance of nanowires with different boundary conditions

The influence of surface stress on the resonance frequencies of bending nanowires was studied by incorporating the generalized Young–Laplace equation into Euler–Bernoulli beam theory. Theoretical solutions are presented for three different boundary conditions. The overall Young’s modulus was used to study the surface stress influenced mechanical behavior of bending nanowires and a comparison was made for the overall Young’s modulus calculated from nanowires in resonance and static bending. It was found that the overall Young’s modulus can be simply related to a nondimensional surface effect factor via empirical formulae.

Torsion and bending periodic boundary conditions for modeling the intrinsic strength of nanowires

We present a unified approach for atomistic modeling of torsion and bending of nanowires that is free from artificial end effects. Torsional and bending periodic boundary conditions (t-PBC and b-PBC) are formulated by generalizing the conventional periodic boundary conditions (PBC) to cylindrical coordinates. The approach is simpler than the more general objective molecular dynamics formulation because we focus on the special cases of torsion and bending. A simple implementation of these boundary conditions is presented and correctly conserves linear and angular momenta. We also derive the virial expressions for the average torque and bending moment under these boundary conditions that are analogous to the virial expression for the average stress in PBC. The method is demonstrated by molecular dynamics simulation of Si nanowires under torsion and bending, which exhibit several modes of failure depending on their diameters.

“Nanofingers” Based on Binary Gold–Polypyrrole Nanowires

Round the bend: Reversible bending of conducting-polymer–gold nanowires is achieved by template synthesis and a selective coating process that leaves only one side of the polymer segment exposed. The bending motion of the resulting “nanofingers” can be tailored by adjusting the applied potential (see figure). This represents the smallest bending arm reported to date and the first example of electrochemically induced nanowire bending.

Surface Effect on the Elastic Behavior of Static Bending Nanowires

The surface effect from surface stress and surface elasticity on the elastic behavior of nanowires in static bending is incorporated into Euler-Bernoulli beam theory via the Young-Laplace equation. Explicit solutions are presented to study the dependence of the surface effect on the overall Young's modulus of nanowires for three different boundary conditions: cantilever, simply supported, and fixed-fixed. The solutions indicate that the cantilever nanowires behave as softer materials when deflected while the other structures behave like stiffer materials as the nanowire cross-sectional size decreases for positive surface stresses. These solutions agree with size dependent nanowire overall Young's moduli observed from static bending tests by other researchers. This study also discusses possible reasons for variations of nanowire overall Young's moduli observed.

In situ probing electrical response on bending of ZnO nanowires inside transmission electron microscope

In situ electrical transport measurements on individual bent ZnO nanowires have been performed inside a high-resolution transmission electron microscope, where the crystal structures of ZnO nanowires were simultaneously imaged. A series of consecutively recorded current-voltage (I-V) curves along with an increase in nanowire bending show the striking effect of bending on their electrical behavior. The bending-induced changes of resistivity, electron concentration, and carrier mobility of ZnO nanowires have been retrieved based on the experimental I-V data, which suggests the applications of ZnO nanowires as nanoelectromechanical sensors.

InN nanorods and nanowires grown on different substrates

AbstractTransmission Electron Microscopy was applied to study InN nanorods grown by non‐catalytic, template‐free hydride metal‐organic vapor phase epitaxy (solid‐vapor mechanism) and nanowires grown by a vapor‐liquid‐solid growth (VLS) mechanism in a MOVPE reactor. The nanorods and nanowires were grown on different planes of Al2O3, (0002) GaN/ Al2O3, and (001) Si. The majority of nanorods have a high structural perfection and exhibit abrupt side walls, but some of them show formation of the planar defects. Depend‐ ing on the substrate used, the size, shape and tips of the nanorods vary from hexagonal to pencil‐like. Nanowires, however have very corrugated surfaces along their side‐walls and tips, and depending on the V/III flow ratio these nanowires can have either a constant diameter or taper‐off. Their length can extend up to 100 μm. Bending and branching of nanowires was observed, but this phenomenon was never observed for nanorods. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Analysis of vertical alignment and bending of crystalline α-Fe2O3 nanowires using normal and grazing incidence x-ray diffraction intensities

Owing to the increasing importance of orderly placed nanostructures, the following study was conducted on oriented and disoriented nanowires. Use of grazing incidence x-ray diffraction (GIXRD) and normal x-ray diffraction (XRD) as possible tools to assess their alignment is discussed. The property of crystalline nanowires to grow in a preferred direction over a substrate is made use of, in which deviation in diffraction intensities is measured on a group of oriented and disoriented nanowires. We find that the difference is sufficiently large to predict the spread in orientations away from a reference direction. The observed intensity variation is explained using a modeled view of diffraction geometry with different nanowire orientations. An alignment index Anw has been calculated using (110) and (300) peak intensities in XRD and GIXRD measurements. The values were found to be related to the extent of vertical alignment as observed using scanning electron microscopy (SEM). The difference in diffraction geometry and penetration depth between the two techniques is found to give complementary information on the upper and lower portions of the nanowires, respectively. Analysis of the diffraction patterns also shows that bending of these nanowires occurs preferentially about the c axis as compared to bending in other directions. In the present study samples with crystalline α-Fe2O3 nanowires grown on pure Fe substrate by controlled thermal oxidation route have been investigated using GIXRD, XRD, SEM and atomic force microscopy. Morphology of the nanowires is discussed briefly.

Electromechanical and electro-optical properties of nanowires

In an external (static of high-frequency) electric field, image forces cause a torque moment to act on semiconducting or metallic nanowires. As a result, a system of nanowires which are suspended in a dielectric liquid acquire characteristics, typical of those of nematic liquid crystals, such as field-induced optical anisotropy. For freestanding nanowires prepared on a substrate, an applied electric field is shown to be capable of causing nanowire bending, which can potentially provide a means of nanoactuation. The bending of such nanowires in a high-frequency electric field is comprised of a superposition of static bending and induced mechanical oscillations, whose amplitude depends on the resonant mechanical vibration characteristics of the nanowires.

AlN nanorings

Aluminum nitride nanorings with hexangular crystal structure were first synthesized by evaporating of aluminum alloy in ammonia/nitrogen atmosphere at 1100 °C. The products were characterized by transmission electron microscopy, scanning electron microscopy, FT-IR absorption and Raman scattering. Nanorings with the diameter of about 3 μm are formed by bending nanowires. HRTEM and electron diffraction studies revealed that nanorings are single crystal and the c-axis of the AlN crystal directs along the radial direction of the nanoring. The electrostatic polar charge model is employed in the explanation of the AlN nanoring formation.

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.

Magnetotransport properties of bent ferromagnetic nanowires

Magnetotransport measurements were performed on individual multisegmented Pt–Ni–Pt nanowires fabricated by electrochemical deposition in nanoporous alumina templates. The nanowires were removed from the template, and precipitated onto substrates from liquid suspension. The Pt end segments provide an oxide-free interface to the magnetic central segment of interest. Centrifugation prior to precipitation induces sharp bends in the nanowires. The angular dependence of the magnetoresistance of both straight and bent nanowires was used to observe domain switching. The magnetic response of straight nanowires is well described by the curling model of domain reversal. In the case of the bent nanowires, the general behavior of each individual straight segment is also consistent with this model, but evidence for interactions between the segments is also observed.