Deriving the three-dimensional structure of ZnO nanowires/nanobelts by scanning transmission electron microscope tomography

Abstract

Characterizing the three-dimensional (3D) shape of a nanostructure by conventional imaging techniques in scanning electron microscopy and transmission electron microscopy can be limited or complicated by various factors, such as two-dimensional (2D) projection, diffraction contrast and unsure orientation of the nanostructure with respect to the electron beam direction. In this paper, in conjunction with electron diffraction and imaging, the 3D morphologies of ZnO nanowires and nanobelts synthesized via vapor deposition were reconstructed by electron tomography in a scanning transmission electron microscope (STEM). The cross-sections of these one-dimensional (1D) nanostructures include triangle, hexagonal, and rectangle shapes. By combining the reconstructed shape with the crystalline information supplied by electron diffraction patterns recorded from the same nanowire/nanobelt, the growth direction and its exposed surfaces were uniquely identified. In total, three different growth directions were confirmed. These directions are 〈0001〉, 〈2 $$ \bar 1 $$ $$ \bar 1 $$ 0〉 and 〈2 $$ \bar 1 $$ $$ \bar 1 $$ 3〉 corresponding to 〈001〉, 〈100〉 and 〈101〉 orientations in three-index notation. The 〈0001〉 growth nanowires show triangle or hexagonal cross-sections, with exposed {01 $$ \bar 1 $$ 0} side surfaces. The dominant surfaces of the 〈2 $$ \bar 1 $$ $$ \bar 1 $$ 0〉 growth nanobelt are ±(0001) planes. Both hexagonal and rectangle cross-sections were observed in the 〈2 $$ \bar 1 $$ $$ \bar 1 $$ 3〉 growth ZnO nanostructures. Their surfaces include the {01 $$ \bar 1 $$ 0}, { $$ \bar 1 $$ 101} and { $$ \bar 2 $$ 112} planes. The nanobelts with a large aspect ratio of ∼10 normally grow along the 〈2 $$ \bar 1 $$ $$ \bar 1 $$ 0〉 direction, while nanobelts with small aspect ratio grow along 〈2 $$ \bar 1 $$ $$ \bar 1 $$ 3〉 growth direction. The approach and methodology demonstrated here can be extended to any nanostructures that can be crystalline, polycrystalline or even amorphous.