Mechanical, Electrical, and Crystallographic Property Dynamics of Bent and Strained Ge/Si Core-Shell Nanowires As Revealed by in situ Transmission Electron Microscopy.

Research on electromechanical properties of semiconducting nanowires, including plastic behavior of Si nanowires and superb carrier mobility of Ge and Ge/Si core-shell nanowires, has attracted increasing attention. However, to date, there have been no direct experimental studies on crystallography dynamics and its relation to electrical and mechanical properties of Ge/Si core-shell nanowires. In this Letter, we in parallel investigated the crystallography changes and electrical and mechanical behaviors of Ge/Si core-shell nanowires under their deformation in a transmission electron microscope (TEM). The core-shell Ge/Si nanowires were bent and strained in tension to high limits. The nanowire Young's moduli were measured to be up to ∼191 GPa, and tensile strength was in a range of 3-8 GPa. Using high-resolution imaging, we confirmed that under large bending strains, Si shells had irregularly changed to the polycrystalline/amorphous state, whereas Ge cores kept single crystal status with the local lattice strains on the compressed side. The nanowires revealed cyclically changed electronic properties and had decent mechanical robustness. Electron diffraction patterns obtained from  in situ TEM, paired with theoretical simulations, implied that nonequilibrium phases of polycrystalline/amorphous Si and β-Sn Ge appearing during the deformations may explain the regarded mechanical robustness and varying conductivities under straining. Finally, atomistic simulations of Ge/Si nanowires showed the pronounced changes in their electronic structure during bending and the appearance of a conductive channel in compressed regions which might also be responsible for the increased conductivity seen in bent nanowires.