Growth Conditions Control the Elastic and Electrical Properties of ZnO Nanowires.

Great efforts have been made to synthesize ZnO nanowires (NWs) as building blocks for a broad range of applications because of their unique mechanical and mechanoelectrical properties. However, little attention has been paid to the correlation between the NWs synthesis condition and these properties. Here we demonstrate that by slightly adjusting the NW growth conditions, the cross-sectional shape of the NWs can be tuned from hexagonal to circular. Room temperature photoluminescence spectra suggested that NWs with cylindrical geometry have a higher density of point defects. In situ transmission electron microscopy (TEM) uniaxial tensile-electrical coupling tests revealed that for similar diameter, the Young's modulus and electrical resistivity of hexagonal NWs is always larger than that of cylindrical NWs, whereas the piezoresistive coefficient of cylindrical NWs is generally higher. With decreasing diameter, the Young's modulus and the resistivity of NWs increase, whereas their piezoresistive coefficient decreases, regardless of the sample geometry. Our findings shed new light on understanding and advancing the performance of ZnO-NW-based devices through optimizing the synthesis conditions of the NWs.

A synchrotron study of microstructure gradient in laser additively formed epitaxial Ni-based superalloy.

Laser additive forming is considered to be one of the promising techniques to repair single crystal Ni-based superalloy parts to extend their life and reduce the cost. Preservation of the single crystalline nature and prevention of thermal mechanical failure are two of the most essential issues for the application of this technique. Here we employ synchrotron X-ray microdiffraction to evaluate the quality in terms of crystal orientation and defect distribution of a Ni-based superalloy DZ125L directly formed by a laser additive process rooted from a single crystalline substrate of the same material. We show that a disorientation gradient caused by a high density of geometrically necessary dislocations and resultant subgrains exists in the interfacial region between the epitaxial and stray grains. This creates a potential relationship of stray grain formation and defect accumulation. The observation offers new directions on the study of performance control and reliability of the laser additive manufactured superalloys.

Elastic properties of GaN nanowires: revealing the influence of planar defects on young's modulus at nanoscale.

The elastic properties of gallium nitride (GaN) nanowires with different structures were investigated by in situ electron microscopy in this work. The electric-field-induced resonance method was utilized to reveal that the single crystalline GaN nanowires, along [120] direction, had the similar Young's modulus as the bulk value at the diameter ranging 92-110 nm. Meanwhile, the elastic behavior of the obtuse-angle twin (OT) GaN nanowires was disclosed both by the in situ SEM resonance technique and in situ transmission electron microscopy tensile test for the first time. Our results showed that the average Young's modulus of these OT nanowires was greatly decreased to about 66 GPa and indicated no size dependence at the diameter ranging 98-171 nm. A quantitative explanation for this phenomenon is proposed based on the rules of mixtures in classical mechanics. It is revealed that the elastic modulus of one-dimensional nanomaterials is dependent on the relative orientations and the volume fractions of the planar defects.