The Young's modulus and fracture strength of silicon nanowires with diameters between 15 and 60 nm and lengths between 1.5 and 4.3 µm were measured. The nanowires, grown by the vapor-liquid-solid process, were subjected to tensile tests in situ inside a scanning electron microscope. The Young's modulus decreased while the fracture strength increased up to 12.2 GPa, as the nanowire diameter decreased. The fracture strength also increased with the decrease of the side surface area; the increase rate for the chemically synthesized silicon nanowires was found to be much higher than that for the microfabricated silicon thin films. Repeated loading and unloading during tensile tests demonstrated that the nanowires are linear elastic until fracture without appreciable plasticity.Silicon (Si) nanowires (NWs) are one of the key building blocks for nanoelectronic and nanoelectromechanical devices. 1 They exhibit excellent mechanical, 2,3 electrical, 4 and optical 5 properties, in addition to interesting multifunctional properties such as piezoresistivity 6 and thermoelectricity. [7][8][9] As such, Si NWs have been used in a broad range of applications including nanoelectronics, 10-12 nanosensors, 13 nanoresonators, 14 light-emitting diodes, 15 and thermoelectric energy scavengers. 7,8 The operation and reliability of these nanodevices depend on the mechanical properties of Si NWs, which are expected to be different from their bulk counterparts due to their increasing surface-to-volume ratio.Existing techniques for measuring the mechanics of individual NWs include observing the vibration (or resonance) of cantilevered NWs inside a transmission or scanning electron microscope (TEM/SEM), [16][17][18] measuring the lateral bending of suspended NWs with an atomic force microscope (AFM), 3,19-21 measuring uniaxial tension of suspended NWs in SEM or TEM, 2,22-26 and nanoindentation of NWs on a substate. 27 Available experimental results on Si NWs exhibit significant scatter including the following: (1) some reported a decrease in Young's modulus with decreasing size, 2,9,24,28 while others showed an opposite trend; 20,21 (2) the reported strength values of vapor-liquid-solid (VLS) grown Si NWs ranged from 500 MPa to 12 GPa; 3,28 (3) Han et al. 2 observed pronounced plastic deformation of Si NWs by in situ TEM tensile tests at room temperature, while Gordon et al. 21 reported linear elastic behavior followed by brittle fracture using AFM bending tests.Moreover the experimental data show large discrepancy with the simulation results. 29 For instance, the experimentally measured Young's moduli started deviating from the bulk value at diameters of about 200 nm; 28 conversely, computational studies using both density functional theory (DFT) and classical molecular dynamics (MD) indicated that the transition diameter for Young's modulus of Si NWs is less than 10 nm. [30][31][32] The experimentally observed plasticity at room temperature occurred for Si NWs with diameter less than 60 nm, while MD simulations 33 predicted a simi...

This paper reports the first quantitative measurement of a full spectrum of mechanical properties of five-fold twinned silver (Ag) nanowires (NWs) including Young's modulus, yield strength and ultimate tensile strength. In situ tensile testing of Ag NWs with diameters between 34 and 130 nm was carried out inside a scanning electron microscope (SEM). Young's modulus, yield strength and ultimate tensile strength all increased as the NW diameter decreased. The maximum yield strength in our tests was found to be 2.64 GPa, which is about 50 times the bulk value and close to the theoretical value of Ag in the <110> orientation. The size effect in the yield strength is attributed to the increase in the Young's modulus. Yield strain scales reasonably well with the NW surface area, which reveals that yielding of Ag NWs is due to dislocation nucleation from surface sources. Pronounced strain hardening was observed for most NWs in our study. The strain hardening, which has not previously been reported for NWs, is mainly attributed to the presence of internal twin boundaries. KEYWORDSfive-fold twin, size effect, Young's modulus, yield strength, ultimate tensile strength, strain hardening

There has been relatively little study on time-dependent mechanical properties of nanowires, in spite of their importance for the design, fabrication and operation of nanoscale devices. Here we report a dislocation-mediated, time-dependent and fully reversible plastic behaviour in penta-twinned silver nanowires. In situ tensile experiments inside scanning and transmission electron microscopes show that penta-twinned silver nanowires undergo stress relaxation on loading and complete plastic strain recovery on unloading, while the same experiments on single-crystalline silver nanowires do not exhibit such a behaviour. Molecular dynamics simulations reveal that the observed behaviour in penta-twinned nanowires originates from the surface nucleation, propagation and retraction of partial dislocations. More specifically, vacancies reduce dislocation nucleation barrier, facilitating stress relaxation, while the twin boundaries and their intrinsic stress field promote retraction of partial dislocations, resulting in full strain recovery.

This paper reports quantitative mechanical characterization of silicon carbide (SiC) nanowires (NWs) via in situ tensile tests inside scanning electron microscopy using a microelectromechanical system. The NWs are synthesized using the vapor-liquid-solid process with growth direction of ⟨111⟩. They consist of three types of structures, pure face-centered cubic (3C) structure, 3C structure with an inclined stacking fault (SF), and highly defective structure, in a periodic fashion along the NW length. The SiC NWs are found to deform linear elastically until brittle fracture. Their fracture origin is identified in the 3C structures with inclined SFs, rather than the highly defective structures. The fracture strength increases as the NW diameter decreases from 45 to 17 nm, approaching the theoretical strength of 3C SiC. The size effect on fracture strength of SiC NWs is attributed to the size-dependent defect density rather than the surface effect that is dominant for single crystalline NWs.