Ultrathin Ni nanowires embedded in SrTiO3 : Vertical epitaxy, strain relaxation mechanisms, and solid-state amorphization

Abstract

Strain is a key parameter affecting the physical properties of heterostructured thin films and nanosized objects. Generally, the design of application-optimized hybrid structures requires good structural compatibility between the involved phases. However, when controlled appropriately, lattice mismatch can turn from a detrimental to a beneficial property, enabling further functionality tuning. Due to their large heterointerface, nanocolumnar composites are an ideal test bed for such strain engineering approaches, but coupling mechanisms at vertical interfaces are still poorly understood. In the present paper, we therefore present a detailed analysis of ultrathin Ni nanowires, with diameters between 1.7 nm and 5.3 nm, vertically epitaxied in a ${\mathrm{SrTiO}}_{3}$/${\mathrm{SrTiO}}_{3}$(001) matrix. Using a combination of x-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and x-ray absorption spectroscopy (XAS) measurements, we unveil peculiar structural features of this hybrid system. We show that the axial deformation of the nanowires depends on their diameter and that their radial strain differs sensitively from the value expected when considering the Poisson ratio. We also provide evidence for the existence of a relaxation mechanism consisting in a slight tilting of crystallographic nanowire domains which reduces the misfit at the Ni-${\mathrm{SrTiO}}_{3}$ heterointerface. This, in turn, induces significant structural disorder and results in a successive amorphization of the metallic phase upon diameter reduction of the nanowires.