In situ observation of size-scale effects on the mechanical properties of ZnO nanowires

Abstract

In this investigation, the size-scale in mechanical properties of individual [0001] ZnOnanowires and the correlation with atomic-scale arrangements were explored viain situ high-resolution transmission electron microscopy (TEM) equipped withatomic force microscopy (AFM) and nanoindentation (NI) systems. The Young’smodulus was determined to be size-scale-dependent for nanowires with diameter,d, in the range of40 nm ≤ d ≤ 110 nm, and reachedthe maximum of ∼ 249 GPa for d = 40 nm. However, this phenomenon was not observed for nanowires in the range of200 nm ≤ d ≤ 400 nm, where an average constant Young’s modulus of ∼ 147.3 GPa was detected, close to the modulus value of bulk ZnO. A size-scale dependencein the failure of nanowires was also observed. The thick ZnO nanowires (d ≥ 200 nm) were brittle, whilethe thin nanowires (d ≤ 110 nm) were highly flexible. The diameter effect and enhanced Young’s modulus observed inthin ZnO nanowires are due to the combined effects of surface relaxation andlong-range interactions present in ionic crystals, which leads to much stiffer surfacesthan bulk wires. The brittle failure in thicker ZnO wires was initiated from theoutermost layer, where the maximum tensile stress operates and propagates alongthe (0001) planes. After a number of loading and unloading cycles, the highlycompressed region of the thinner nanowires was transformed from a crystalline to anamorphous phase, and the region near the neutral zone was converted into a mixture ofdisordered atomic planes and bent lattice fringes as revealed by high-resolution images.