Effects of strain on the carrier mobility in silicon nanowires

We investigate electron and hole mobilities in strained silicon nanowires (Si NWs) within an atomistic tight-binding framework. We show that the carrier mobilities in Si NWs are very responsive to strain and can be enhanced or reduced by a factor >2 (up to 5×) for moderate strains in the ± 2% range. The effects of strain on the transport properties are, however, very dependent on the orientation of the nanowires. Stretched 100 Si NWs are found to be the best compromise for the transport of both electrons and holes in ≈10 nm diameter Si NWs. Our results demonstrate that strain engineering can be used as a very efficient booster for NW technologies and that due care must be given to process-induced strains in NW devices to achieve reproducible performances.     

Mapping the "forbidden" transverse-optical phonon in single strained silicon (100) nanowire

The accurate manipulation of strain in silicon nanowires can unveil new fundamental properties and enable novel or enhanced functionalities. To exploit these potentialities, it is essential to overcome major challenges at the fabrication and characterization levels. With this perspective, we have investigated the strain behavior in nanowires fabricated by patterning and etching of 15 nm thick tensile strained silicon (100) membranes. To this end, we have developed a method to excite the "forbidden" transverse-optical (TO) phonons in single tensile strained silicon nanowires using high-resolution polarized Raman spectroscopy. Detecting this phonon is critical for precise analysis of strain in nanoscale systems. The intensity of the measured Raman spectra is analyzed based on three-dimensional field distribution of radial, azimuthal, and linear polarizations focused by a high numerical aperture lens. The effects of sample geometry on the sensitivity of TO measurement are addressed. A significantly higher sensitivity is demonstrated for nanowires as compared to thin layers. In-plane and out-of-plane strain profiles in single nanowires are obtained through the simultaneous probe of local TO and longitudinal-optical (LO) phonons. New insights into strained nanowires mechanical properties are inferred from the measured strain profiles.     

Structural and optical characterization of strained free-standing InP nanowires

The structural and optical properties of high-quality crystalline strained InP nanowires are reported in this article. The nanowires were produced by the vapor-liquid-solid growth method in a chemical-beam epitaxy reactor, using 20 nm gold nanoparticles as catalysts. Polarization-resolved photoluminescence experiments were carried out to study the optical properties of the InP nanowires. These experiments revealed a large blue shift of 74 meV of the first electron-to-heavy hole optical transition in the nanowires, which cannot be solely explained by quantum size effects. The blue shift is mainly attributed to the presence of biaxial compressive strain in the inward radial direction of the InP nanowires. High-resolution transmission electron microscopy Electron and selected area electron diffraction experiments show that the nanowires have high crystal quality and grow along a [001] axes. These experiments also confirmed the presence of 1.8% compressive radial strain and 2% tensile longitudinal strain in the nanowires. A simple theoretical model including both quantum confinement and strain effects consistently describes the actual energy position of the InP nanowires optical emission.     

Size-Dependent Bandgap Modulation of ZnO Nanowires by Tensile Strain

We quantified the size-dependent energy bandgap modulation of ZnO nanowires under tensile strain by an in situ measurement system combining a uniaxial tensile setup with a cathodoluminescence spectroscope. The maximal strain and corresponding bandgap variation increased by decreasing the size of the nanowires. The adjustable bandgap for the 100 nm nanowire caused by a strain of 7.3% reached approximately 110 meV, which is nearly double the value of 59 meV for the 760 nm nanowire with a strain of 1.7%. A two-step linear feature involving bandgap reduction caused by straining and a corresponding critical strain was identified in ZnO nanowires with diameters less than 300 nm. The critical strain moved toward the high strain level with shrunken nanowires. The distinct size effect of strained nanowires on the bandgap variation reveals a competition between core-dominated and surface-dominated bandgap modulations. These results could facilitate potential applications involving nanowire-based optoelectronic devices and band-strain engineering.     

PMN-PT nanowires with a very high piezoelectric constant

A profound way to increase the output voltage (or power) of the piezoelectric nanogenerators is to utilize a material with higher piezoelectric constants. Here we report the synthesis of novel piezoelectric 0.72Pb(Mg(1/3)Nb(2/3))O(3)-0.28PbTiO(3) (PMN-PT) nanowires using a hydrothermal process. The unpoled single-crystal PMN-PT nanowires show a piezoelectric constant (d(33)) up to 381 pm/V, with an average value of 373 ± 5 pm/V. This is about 15 times higher than the maximum reported value of 1-D ZnO nanostructures and 3 times higher than the largest reported value of 1-D PZT nanostructures. These PMN-PT nanostructures are of good potential being used as the fundamental building block for higher power nanogenerators, high sensitivity nanosensors, and large strain nanoactuators.     

Molecular dynamics evaluation of strain rate and size effects on mechanical properties of FCC nickel nanowires

Based on molecular dynamics method, an atomistic simulation scheme for damage evolution and failure process of nickel nanowires is presented, in which the inter-atomic interactions are represented by employing the modified embedded atom potential. Extremely high strain rate effect on the mechanical properties of nickel nanowires with different cross-sectional sizes is investigated. The stress–strain curves of nickel nanowires at different strain rates subjected to uniaxial tension are simulated. The elastic modulus, yield strength and fracture strength of nanowires at different loading cases are obtained, and the effect of strain rate on these mechanical properties is analyzed. The numerical results show that the stress–strain curve of metallic nanowires under tensile loading has the trend identical to that of routine polycrystalline metals, and the yield strain of nanowires is independent of the strain rate and cross-sectional size. Based on the simulation results, a set of quantitative prediction formulas are obtained to describe the strain rate sensitivity of nickel nanowires on the mechanical properties, and the resulting formulas of the Young’s modulus, yield strength and fracture strength of nickel nanowires exhibit a linear relation with respect to the logarithm of strain rate. Furthermore, some comprehensive correlation equations revealing both the strain rate and size effects on mechanical properties of nickel nanowire are proposed through the numerical fitting and regression analysis, and the mechanical behaviors observed in this study are consistent with those from the experimental and available numerical results.     

In Situ Electron Microscopy Four‐Point Electromechanical Characterization of Freestanding Metallic and Semiconducting Nanowires

Electromechanical coupling is a topic of current interest in nanostructures, such as metallic and semiconducting nanowires, for a variety of electronic and energy applications. As a result, the determination of structure‐property relations that dictate the electromechanical coupling requires the development of experimental tools to perform accurate metrology. Here, a novel micro‐electro‐mechanical system (MEMS) that allows integrated four‐point, uniaxial, electromechanical measurements of freestanding nanostructures in‐situ electron microscopy, is reported. Coupled mechanical and electrical measurements are carried out for penta‐twinned silver nanowires, their resistance is identified as a function of strain, and it is shown that resistance variations are the result of nanowire dimensional changes. Furthermore, in situ SEM piezoresistive measurements on n‐type, [111]‐oriented silicon nanowires up to unprecedented levels of ～7% strain are demonstrated. The piezoresistance coefficients are found to be similar to bulk values. For both metallic and semiconducting nanowires, variations of the contact resistance as strain is applied are observed. These variations must be considered in the interpretation of future two‐point electromechanical measurements.     

Protection of strain gauges for long-term stability

A strain gauge encapsulation and protection system is described which ensures the long term stability of resistance strain gauges. Protected gauges have been subjected to normal outdoor conditions for a period of 2 years together with over 40 temperature cycles to 60°C and 215 strain cycles to 800 micro-strain. Zero shift has not exceeded 40 microstrain.     

Enhanced Cu₂S/CdS coaxial nanowire solar cells by piezo-phototronic effect

Nanowire solar cells are promising candidates for powering nanosystems and flexible electronics. The strain in the nanowires, introduced during growth, device fabrication and/or application, is an important issue for piezoelectric semiconductor (like CdS, ZnO, and CdTe) based photovoltaic. In this work, we demonstrate the first largely enhanced performance of n-CdS/p-Cu(2)S coaxial nanowire photovoltaic (PV) devices using the piezo-phototronics effect when the PV device is subjected to an external strain. Piezo-phototronics effect could control the electron-hole pair generation, transport, separation, and/or recombination, thus enhanced the performance of the PV devices by as high as 70%. This effect offers a new concept for improving solar energy conversation efficiency by designing the orientation of the nanowires and the strain to be purposely introduced in the packaging of the solar cells. This study shed light on the enhanced flexible solar cells for applications in self-powered technology, environmental monitoring, and even defensive technology.     

X-ray diffraction line profile analysis for defect study in Zr-2.5% Nb material

The microstructure characterization by X-ray line profile analysis is possible for determination of dislocation density, micro-strain within grains due to dislocation and average coherent domain size (subgrain) within the grain. This study presents the X-ray diffraction peaks shape analysis and their broadening with different thermal treatments in Zr-2.5% Nb pressure tube material. The peak shape is analysed using Fourier transformation and information about coherent domain size, micro-strain and dislocation density could be obtained from the Fourier coefficients of the peak. Analysis of broadening of the peaks by integral breadth method also gives the coherent domain size, dislocation density and micro-strain present in the material. The results from the X-ray techniques are comparable to those obtained from direct observation of transmission electron microscopy. The measured yield strength increases with dislocation density. An empirical relationship is obtained for the yield strength from the dislocation density of the material. The measured strength is in agreement with the one calculated from dislocation density.     

Bending-induced conductance increase in individual semiconductor nanowires and nanobelts

Reliable ohmic contacts were established in order to study the strain sensitivity of nanowires and nanobelts. Significant conductance increases of up to 113% were observed on bending individual ZnO nanowires or CdS nanobelts. This bending strain-induced conductance enhancement was confirmed by a variety of bending measurements, such as using different manipulating tips (silicon, glass or tungsten) to bend the nanowires or nanobelts, and is explained by bending-induced effective tensile strain based on the principle of the piezoresistance effect. This article is published with open access at Springerlink.com     

Comparison of nondestructive microfailure evaluation of fiber-optic Bragg grating and acoustic emission piezoelectric sensors using fragmentation test

Nondestructive evaluation of microfailure mechanisms in two-diameter SiC fibers/epoxy composites is investigated using a directly embedded fiber-optic sensor attached with an acoustic emission piezoelectric (AE-PZT) sensor. Interfacial shear strength by fragmentation test, and optical failure observation inside microcomposite can contribute to analyze two sensors quantitatively. Although fiber Bragg grating (FBG) sensor exhibits sudden wavelength shift due to plastic deformation by larger diameter SiC fiber breakage, AE-PZT monitors much more precise microfailure process, such as the fiber break or matrix cracking. Since the FBG sensor can measure the strain at only a single point, whether it can detect a fiber break in single-fiber composite specimen depends on its proximity to the failure location. In addition, micro-strain measurement at one single point may not provide enough information on the whole microfailure process including multiple fiber breakage and matrix crack. It can be considered that FBG sensor can be somewhat effective in measuring the continuous micro-strain change due to the internal disturbance such as resin curing, whereas AE-PZT sensor can be effective in detecting the microfailure by elastic wave propagation through the composite materials.     

Direct atomic-scale imaging about the mechanisms of ultralarge bent straining in Si nanowires

To safely and reliably use nanowires (NWs) for exploring new functions for different nanodevices, the mechanical properties and structural evolution of the nanowires under external stress become highly important. Large strain (up to 14%) bending experiments of Si NWs were conducted in a high-resolution transmission electron microscope at atomic resolution. The direct dynamic atomic-scale observations revealed that partial and full dislocation nucleation, motion, escape, and interaction were responsible for absorbing the ultralarge strain of up to 14% in bent Si nanowires. The prevalent full dislocation movement and interactions induced the formation of Lomer lock dislocations in the Si NWs. Finally, in contrast to the unlock process of Lomer dislocations that can happen in metallic materials, we revealed that the continuous straining on the Lomer dislocations induced a crystal-amorphous (c-a) transition in Si NWs. Our results provide direct explanation about the ultralarge straining ability of Si at the nanometer scale.     

Molecular dynamics simulation of mechanical properties of Ni–Al nanowires

We employed molecular dynamics simulations to study mechanical properties of Ni–Al nanowires by calculating the stress–strain response of the wires under various loading conditions. For this purpose, nanowires were subjected to uniaxial strain at different strain rates and temperatures using embedded atom model potential. The behaviour of the wires at lower and higher strain rates was investigated, and the yield and rupture strain values and also Young’s Modulus were obtained which are essential factors for the ductility of the wires. This work indicates that how the stress–strain response of the nanowires are affected by varying strain rates and temperatures.     

Strain and shape of epitaxial InAs/InP nanowire superlattice measured by grazing incidence X-ray techniques

Quantitative structural information about epitaxial arrays of nanowires are reported for a InAs/InP longitudinal heterostructure grown by chemical beam epitaxy on an InAs (111)B substrate. Grazing incidence X-ray diffraction allows the separation of the nanowire contribution from the substrate overgrowth and gives averaged information about crystallographic phases, epitaxial relationships (with orientation distribution), and strain. In-plane strain inhomogeneities, intrinsic to the nanowires geometry, are measured and compared to atomistic simulations. Small-angle X-ray scattering evidences the hexagonal symmetry of the nanowire cross-section and provides a rough estimate of size fluctuations.     

Surface-induced structural transformation in nanowires

One of the unique features of nanomaterials is that they have large surface-to-volume atom ratios compared to bulk materials. The intrinsic compressive stress along the nanowire axis can be as large as tens of GPa, and spontaneous reorientation or phase transformation may occur in order for the nanowires to return to the low-energy state. Upon tensile loading, the nanowires can revert back to the original high-energy orientation or phase without introducing any defects. Two mechanisms are mainly involved in the deformation: (1) twinning/detwinning and (2) stress-induced martensitic phase transformation (MT)/inverse MT. Generally, this surface-induced behavior can only occur at a temperature higher than the critical temperature, T_c, due to the energy barrier for structural transformation. As a result, ordinary nanoscale metals can exhibit pseudo-elasticity and shape memory effects previously only observed from special alloys such as nickel titanium (NiTi). These nanowires have the predicted recoverable strain on the order of 40%–70% which is much larger than that of bulk NiTi (5%–10%), but have extremely low energy dissipation (2% for W nanowires, for example). Surface-induced structural transformation has been observed from fcc, bcc, and hcp single-element metal nanowires, intermetallic alloy nanowires, multilayered and core-shell composite nanowires, and even oxide and nitride compound semiconductor nanowires. This unique phenomenon enables the design of novel and flexible nanoelectromechanical systems (NEMS) having potential applications in nanomanipulators, energy storage, sensors, switches, and so on. We will review the breakthrough and development in this field in the past ten years, mainly focusing on the physical mechanisms and dominant factors governing this spontaneous structural transition. Future developments will also be discussed.     

Linear-superelastic metals by controlled strain release via nanoscale concentration-gradient engineering

The elastic strain limit of most metals are less than 0.2% except for whiskers or freestanding nanowires whose elastic strain limit could reach 4–7%. Ferroelastic metals such as shape memory alloys (SMAs) do exhibit giant recoverable strains (up to ∼13%). However, the strong non-linear pseudo-elasticity of SMAs leads to mechanical instability. By taking advantage of the strong composition-dependent critical stress for stress-induced martensitic transformation (MT) in NiTi SMA, this work demonstrates a novel design approach to achieve linear-superelasticity (∼4.6%) and ultralow modulus (8.7 GPa) of a NiTi single crystal. These unprecedented properties are realized through precisely controlling strain release during the MT via nanoscale concentration-gradient engineering. The computer simulation results and theoretical analyses reveal that the stress–strain behavior of NiTi and other SMAs can be regulated effectively by fine-tuning the concentration gradient. This may open a new avenue for the design of next generation ferroelastic materials.     

Ultralarge Bending Strain and Fracture‐Resistance Investigation of Tungsten Carbide Nanowires

Hard tungsten carbide (WC) with brittle behavior is frequently applied for mechanical purposes. Here, ultralarge elastic bending deformation is reported in defect‐rare WC [0001] nanowires; the tested bending strain reaches a maximum of 20% ± 3.33%, which challenges the traditional understanding of this material. The lattice analysis indicates that the dislocations are confined to the inner part of the WC nanowires. First, the high Peierls–Nabarro barrier hinders the movement of the locally formed dislocations, which causes rapid dislocation aggregation and hinders long‐range glide, resulting in a dense distribution of the dislocation network. In this case, the loading is dispersed along multiple points, which is then balanced by the complex internal mechanical field. In the compressive part, the possible dislocations predominantly emerge in the (0001) plane and mainly slip along the axial direction. The disordered shell first forms at the tensile side and prevents the generation of nanocracks at the surface. The novel lattice kinetics make WC nanowires capable of substantial bending strain resistance. Analytical results of the force–displacement (F–d) curves based on the double‐clamped beam model exhibit an obvious nonlinear elastic characteristic, which originates fundamentally from the lattice anharmonicity under moderate stress.     

检测薄膜压电形变的双光束探测干涉仪的设计

为了检测压电薄膜的在电场作用下的微小形变，同时避免基底弯典效应的影响，设计了一种双光束探测干涉仪，通过反馈控制和锁相检测技术，实现了高稳定度、高分辨力的目标，最小可探测形变可达到0.001nm。系统采用计算机控制测量和数据处理，使测量更加佰、准确。     

Piezoresistance of top-down suspended Si nanowires

Measurements of the gauge factor of suspended, top-down silicon nanowires are presented. The nanowires are fabricated with a CMOS compatible process and with doping concentrations ranging from 2 × 10(20) down to 5 × 10(17) cm(-3). The extracted gauge factors are compared with results on identical non-suspended nanowires and with state-of-the-art results. An increase of the gauge factor after suspension is demonstrated. For the low doped nanowires a value of 235 is measured. Particular attention was paid throughout the experiments to distinguishing real resistance change due to strain modulation from resistance fluctuations due to charge trapping. Furthermore, a numerical model correlating surface charge density with the gauge factor is presented. Comparison of the simulations with experimental measurements shows the validity of this approach. These results contribute to a deeper understanding of the piezoresistive effect in Si nanowires.