Probing the three-dimensional strain inhomogeneity and equilibrium elastic properties of single crystal Ni nanowires

Abstract

We employ three dimensional x -ray coherent diffraction imaging to map the lattice strain distribution, and to probe the elastic properties of a single crystalline Ni (001) nanowire grown vertically on an amorphous Si02 Si substrate. The reconstructed density maps show that with increasing wire width, the equilibrium compressive stress in the core region decreases sharply while the surface tensile strain increases, and gradually trends to a nonzero constant. We use the retrieved projection of lattice distortion to predict the Young's Modulus of the wire based on the elasticity theory.Due to their high surface-to-volume ratio, transition metal nanostructures such as nanowires (NWs) could potentially be used in a broad range of applications in catalysis, sensors, batteries, fuel cells, and magnetic devices. [1-4] Most of these devices require NWs with welldefined size, shape, and spatial ordering. In addition, economical routes to mass production of NWs are desirable for practical applications. However, structural morphology of the NWs is strongly affected by the complex interplay between several growth parameters, such as temperature of the substrate, vertical and lateral material transfer, growth rate, in-plane mobility of ad-atoms, etc. In order to understand resulting morphology of NWs it is therefore vital to develop characterization tools that can nondestructively probe the three dimensional (3D) structural and mechanical properties of these NWs with nanoscale resolution.In this letter we study the structure, shape, 3D lattice distortion, and the electron density distribution in single crystal Ni NWs with the aid of synchrotron-based coherent x-ray diffraction (CXD) imaging [5,6]. The NWs were grown using thermal chemical vapor deposition (CVD) on a Si substrate that has been oxidized resulting in a 500-nm SiO 2 amorphous coating layer. CVD at 650 o C yields densely populated coverage of verticallyoriented single-crystal NWs depicted in Fig. 1(b) with well-defined orthogonal and smooth facets. [7,8] The NWs are grown in the [001] crystallographic orientation with widths ranging from 50 nm to about 300 nm and lengths of up to 5 µm and average coverage density of between 0.1 and 0.3 NW/µm 2 . CXD in the Bragg geometry has been shown to be a powerful characterization technique for imaging of local nanoscale lattice distortions, commonly described by deformation fields, u(r) within a small crystal. [5,6] CXD allows us to image not only the overall shape of the nanostructure in 3D, but also the projection of the crystal lattice displacement field on to the Q vector of the measured Bragg spot. The lattice distortion represents the * efohtung@physics.ucsd.edu imaginary component of object density and manifests itself as local asymmetry of the measured Bragg diffraction pattern. Upon an inversion of coherent x-ray diffraction pattern, the resulting real-space image of the object will be complex valued ρ (r) = ρ G hkl (r) exp [−iG hkl · u (r)], where the amplitude ρ G hkl represents the density of the object and the...

Displacement field of a screw dislocation in a〈011〉Cu nanowire: An atomistic study

“…At that time, only the diffraction pattern caused by the isotropic displacement u ∞,i z = bθ/2π was known from Wilson calculations. 29,30 Nowadays, it becomes possible to perform coherent x-ray diffraction on a single nanowire 6,7 and from the atomistic simulations to get a lot of information on the predicted diffractogram. 15 The aim of this section is to investigate the effect on the 044 reflection of each contribution (i.e., anisotropy, dislocation core, surface effects, image effects, twist) to the total u z field caused by an axial screw dislocation in a [011] circular copper nanowire.…”

“…5 Other studies based on sophisticated x-ray diffraction techniques are currently carried out to resolve the displacement field in these nanosystems. [6][7][8] One significant advance would be, for instance, to identify a single screw dislocation in a single nanowire in order to correlate the influence of this defect to the mechanical properties of the nanowire.…”

Bragg coherent diffractive imaging of strain at the nanoscale

“…This method of exploiting coherence to the probe strain inside small crystals has been developed for over a decade now. [14][15][16][17]38,47 A major breakthrough was achieved by the group of Robinson that demonstrated inversion of three-dimensional diffraction near the Bragg point into a real-space volume of the gold nanocrystal. 15 Figure 6 shows slices of the reconstructed particle made more than a decade ago.…”

“…As an example, we show results from the imaging of an individual nickel nanowire (NW). 47 The nanowires were grown on the Si substrate using a chemical vapor deposition method at 650°C. The crystallinity of the nanowires was confirmed with electron diffraction, while SEM micrographs provided knowledge on the shape.…”

Imaging Local Polarization in Ferroelectric Thin Films by Coherent X-Ray Bragg Projection Ptychography

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