Three-dimensional nanostructure determination based on scanning electron nanodiffraction

Nanocrystalline materials in general feature a large density of surfaces, interphase and grain boundaries, and a large surface/volume ratio, which have attracted tremendous interest for their unique mechanical, chemical and electronic properties. For example, nanocrystalline metals and alloys exhibit improved hardness, enhanced strength and often reduced ductility. The structure of nanocrystalline materials is determined by the constitutive phases, composition, three-dimensional (3D) grain morphology, orientation and distribution, which can only be obtained from a 3D structure determination, which is an outstanding challenge in crystallography. A diffraction based technique is developed here for the determination of 3D nanostructures. The technique employs high resolution and low-dose scanning electron nanodiffraction (SEND) to acquire diffraction patterns in the 3D reciprocal space, with the help of a special sample holder for large angle rotation. Grains are identified in the 3D real space based on the crystal orientation and the reconstructed dark-field images from the recorded diffraction patterns. Various algorithms are discussed in terms of their capabilities of processing the diffraction information recorded in the big 3D-SEND data set. Applications to the determination of nanocrystalline TiN thin-film structure show that the 3D morphology of the columnar TiN grains of tens of nm in diameter can be reconstructed using an algebraic iterative algorithm under the specified prior conditions, together with their crystallographic orientations. The principles can be extended to multi-phase nanocrystalline materials as well. Thus, the 3D-SEND technique provides an effective and adaptive way to determine 3D nanostructures