Novel Phase Transformation in ZnO Nanowires under Tensile Loading

We predict a previously unknown phase transformation from wurtzite to a graphitelike ($P{6}_3/mmc$) hexagonal structure in $[01\overline{1}0]$-oriented ZnO nanowires under uniaxial tensile loading. Molecular dynamics simulations and first principles calculations show that this structure corresponds to a distinct minimum on the enthalpy surfaces of ZnO for such loading conditions. This transformation is reversible with a low level of hysteretic dissipation of $0.16\mathrm{J}/{\mathrm{m}}^3$ and, along with elastic stretching, endows the nanowires with the ability to recover pseudoelastic strains up to 15%.

Figure 1

(a) Wurtzite (WZ) and newly discovered hexagonal (HX) crystal structures, (b) nanowire with HX and WZ phases [transformation in progress under tensile loading (point $C$ in Fig. 2 with a strain of 5.9%)]; and (c) charge density plots on the $(11\overline{2}0)$ planes of WZ, HX, and the layered structure (LY) reported in Refs. 9, 10.

Figure 2

Tensile stress-strain response of a $21.22\times 18.95\AA$ nanowire at 100 K during loading-unloading. The hysteresis loop is relatively small.

Figure 3

Enthalpy surface maps from DFT calculations for uniaxial tensile stress of (a) ${\sigma }_b=7\mathrm{GPa}$, (b) ${\sigma }_b=10\mathrm{GPa}$ and (c) ${\sigma }_b=13\mathrm{GPa}$ along the $b$ axis and uniaxial compressive stress of (d) ${\sigma }_c=-6\mathrm{GPa}$ along the $c$ axis.