Nanobridge Structure Research Articles

High power factor n-type Ag2Se/SWCNTs hybrid film for flexible thermoelectric generator

Flexible thermoelectric (TE) devices can generate electricity by harvesting low-grade heat from curved surfaces, which has great potential in the energy supply of flexible electronic and wearable sensing. Extensive research has focused on flexible TE materials based on conductive polymers due to their lightweight, but it is a huge challenge to obtain high performance and regulate conductivity type comparable to inorganic TE materials. Here, we report n-type Ag2Se/single-walled carbon nanotubes hybrid film with a high power factor ∼1030.7 μW m−1 K−2 and excellent flexibility at room temperature. Excellent mechanical and TE properties can be attributed to its network and nano-bridge structure. A TE prototype composed of six leg generates a voltage and a maximum output power of 24.8 mV and 725 nW respectively at a temperature gradient of 30 K. Our results provide insight into the design and fabrication of high-performance flexible hybrid films.

In Situ Liquid Cell TEM Reveals Bridge-Induced Contact and Fusion of Au Nanocrystals in Aqueous Solution.

Duringnanoparticle coalescence in aqueous solution, dehydration and initial contact of particles are critically important but poorly understood processes. In this work, we used in situ liquid-cell transmission electron microscopy to directly visualize the coalescence process of Au nanocrystals. It is found that the Au atomic nanobridge forms between adjacent nanocrystals that are separated by a ∼0.5 nm hydration layer. The nanobridge structure first induces initial contact of Au nanocrystals over their hydration layers and then surface diffusion and grain boundary migration to rearrange into a single nanocrystal. Classical density functional theory calculations and ab initio molecular dynamics simulations suggest that the formation of the nanobridge can be attributed to the accumulation of auric ions and a higher local supersaturation in the gap, which can promote dehydration, contact, and fusion of Au nanocrystals. The discovery of this multistep process advances our understanding of the nanoparticle coalescence mechanism in aqueous solutions.

A controllable way to measure the interfacial strength between carbon nanotube and polymer using a nanobridge structure

The interfacial strength between an embedded carbon nanotube (CNT) and a polymer matrix is a direct indicator of the interfacial load transfer which significantly impacts the effective mechanical properties of CNT/polymer nanocomposites. However, unavoidable kinks or entanglement of the CNTs in the polymer may complicate the characterization of the interface as measured through nanopullout experiments. In the current work a CNT/polymer nanobridge structure in a suspended polymer film is developed to allow the observation of the embedded portion of the CNT after the pullout process. The nanobridge structure was created on the basis of phase separation of a polymer blend, self-assembly of the CNTs in liquid flow, and ductile fracture of the polymer under thermal and solvent treatment. This structure accommodates a nanomechanical interfacial characterization method where a transverse force applied on the suspended CNT using an atomic force microscope results in an axial pullout force, which is obtained by the force-distance curves and a force balance relation. The high resolution of the AFM in the loading process and the ability to observe the embedded portion of the CNT eliminate uncertainties of the nanopullout process and provide a more controllable approach for measuring the interfacial strength in CNT/polymer nanocomposite systems.

Modeling the coupled effects of surface layer and size effect on the static and dynamic instability of narrow nano-bridge structure

For modeling the electromechanical behavior of nano-bridge structures with slender narrow-width beam elements, not only the simultaneous effects of surface layer and size dependency should be taken into account but also corrected force models should be considered. In this paper, the instability of a narrow-width nano-bridge is studied based on strain gradient theory and Gurtin–Murdoch surface elasticity. The mid-plane stretching is incorporated in the governing equation as well as corrected force distribution. Using Rayleigh–Ritz method, a parametric analysis is conducted to examine the impacts of surface layer, size dependence, dispersion forces and structural damping on static and dynamic instability voltage of the nano-bridge.

Nonlinear beam formulation incorporating surface energy and size effect: application in nano-bridges

A nonlinear beam formulation is presented based on the Gurtin-Murdoch surface elasticity and the modified couple stress theory. The developed model theoretically takes into account coupled effects of the energy of surface layer and microstructures sizedependency. The mid-plane stretching of a beam is incorporated using von-Karman nonlinear strains. Hamilton’s principle is used to determine the nonlinear governing equation of motion and the corresponding boundary conditions. As a case study, pull-in instability of an electromechanical nano-bridge structure is studied using the proposed formulation. The nonlinear governing equation is solved by the analytical reduced order method (ROM) as well as the numerical solution. Effects of various parameters including surface layer, size dependency, dispersion forces, and structural damping on the pullin parameters of the nano-bridges are discussed. Comparison of the results with the literature reveals capability of the present model in demonstrating the impact of nanoscale phenomena on the pull-in threshold of the nano-bridges.

English

In the present paper, the size and strain rate effects on ultra-thin /{100} Cu nanowires at an initial temperature of 10 K have been discussed. Extensive molecular dynamics (MD) simulations have been performed using Embedded atom method (EAM) to investigate the structural behaviours and properties under high strain rate. Velocity-Verlet algorithm has been used to solve the equation of motions. Two different thermal loading cases have been considered: (i) Isothermal loading, in which Nose-Hoover thermostat is used to maintain the constant system temperature, and (ii) Adiabatic loading, i.e., without any thermostat. Five different wire cross-sections were considered ranging from 0.723 x 0.723 nm(2) to 2.169 x 2.169 nm(2) The strain rates used in the present study were 1 x 10(9) s(-1), 1 x 10(8) s(-1), and 1 x 10(7) s(-1). The effect of strain rate on the mechanical properties of copper nanowires was analysed, which shows that elastic properties are independent of thermal loading for a given strain rate and cross-sectional dimension of nanowire. It showed a decreasing yield stress and yield strain with decreasing strain rate for a given cross- section. Also, a decreasing yield stress and increasing yield strain were observed for a given strain rate with increasing cross-sectional area. Elastic modulus was found to be similar to 100 GPa, which was independent of processing temperature, strain rate, and size for a given initial temperature. Reorientation of /{100} square cross-sectional copper nanowire into a series of stable ultra-thin Pentagon copper nanobridge structures with dia of similar to 1 nm at 10 K was observed under high strain rate tensile loading. The effect of isothermal and adiabatic loading on the formation of such pentagonal nanobridge structure has been discussed.

On-chip fabrication of ZnO-nanowire gas sensor with high gas sensitivity

ZnO-nanowire gas sensors were fabricated by a selective growth of nanowires on patterned Au catalysts thus forming nanowire air bridges or ‘ nanobridges’ between two Pt pillar electrodes. The gas sensing properties of nanobridge gas sensors were demonstrated using a diluted NO 2. The response, as a function of temperature, was highest at 225 °C and was linearly increased with the concentration of NO 2 in the range of 0.5–3 ppm and then showed a sign of saturation. Our sensor showed higher response compared with different types of sensors including ZnO nanocrystals, Sn- and In-doped ZnO thin film, or ZnO nanowires. The enhanced response was attributed to the additional modulation of the sensor resistance due to potential barrier at nanowire/nanowire junctions as well as the surface depletion region of each nanowire. Also nanobridge structure enabled fast recovery behavior because desorbed gas molecules can be easily swept away from the surface of ZnO nanowire without re-adsorption.

Coupled effect of size, strain rate, and temperature on the shape memory of a pentagonalCu nanowire

A body-centered pentagonal nanobridge structure with lattice constantsc = 2.35 and a = 2.53 Å has been observed under high strain rate tensile loading on an initially constrained Cu nanowire at various temperatures. Extensive molecular dynamics (MD) simulations have beenperformed using the embedded atom method (EAM) for cross-sectional dimensions ranging from0.723 × 0.723 to2.169 × 2.169 nm2, temperature ranging from 10 to 600 K, and strain rates of109–107 s−1. Formations of such pentagonal nanowire are observed for a temperature range 200–600 Kfor particular cross-sectional dimensions and strain rates. A large inelastic deformation of∼50% is obtained under both isothermal loading and adiabatic loading. With very high degree ofrepeatability of such pentagonal nanowire formation, the present findings indicate that theinteresting stability property and high strength of elongated nanowires have variouspotential applications in nanomechanical and nanoelectronic devices. Further,we demonstrate a novel thermomechanical unloading mechanism by which it ispossible to impart recovery from a pentagonal nanowire following a hysteresisloop: .

Formation of stable ultra-thin pentagon Cu nanowires under high strain rateloading

Molecular dynamics (MD) simulations of Cu nanowires at 10 K with varying cross-sectional areas ranging from0.3615 × 0.3615 nm2 to2.169 × 2.169 nm2 have been performed using the embedded atom method (EAM) to investigate theirstructural behaviors and properties at high strain rate. Our studies reported in this papershow the reorientation of square cross-sectional Cu nanowires into a series of stableultra-thin pentagon Cu nanobridge structures with diameter of∼1 nm under a high strain rate tensile loading. The strain rates used for the present studies range from1 × 109 to0.5 × 107 s−1. The pentagonal multi-shell nanobridge structure is observed for cross-sectional dimensions<1.5 nm.From these results we anticipate the application of pentagonal Cu nanowires even with diameters of∼1 nm in nano-electronic devices. A much larger plastic deformation is observed in thepentagonal multi-shell nanobridge structure as compared to structures that do not formsuch a nanobridge. It indicates that the pentagonal nanobridge is stable. The effect ofstrain rate on the mechanical properties of Cu nanowires is also analyzed and shows adecreasing yield stress and yield strain with decreasing strain rate for a given cross-section.Also, a decreasing yield stress and decreasing yield strain are observed for a givenstrain rate with increasing cross-sectional area. The elastic modulus is found to be∼100 GPa and is independent of strain rate effect and independent of size effect for a giventemperature.

Study on spin dependent tunneling and Coulomb blockade in granular systems with restricted tunneling paths

We describe the microfabrication process of a granular nanobridge structure consisting of electrodes separated by a nanometer-sized gap in which a thin insulating Co–Al–O granular film is filled, and discuss the observed Coulomb blockade with a clear threshold voltage ( V th) and the enhanced tunnel magnetoresistance of about 30% at a voltage slightly above V th. Recent results of our scanning tunneling microscopy studies to clarify the topographic features and single electron tunneling phenomena on the surface of Co–Al–O granular films are also presented.