Doping and planar defects in the formation of single-crystal ZnO nanorings

We have recently reported the growth of freestanding, single-crystal, seamless nanorings of zinc oxide via a spontaneous self-coiling process during the growth of polar-nanobelts [X.Y. Kong et al., Science 303, 1348 (2004)]. The nanoring is made by coaxial and uniradius loop-by-loop winding of a fine ZnO nanobelt. An important fact is that each and every nanoring is made of a nanobelt that contains basal-plane planar defects, which are suggested to be important for leading the fastest growth of the nanobelt as well as lowering its elastic deformation energy. In this paper, high-resolution transmission electron microscopy is applied to investigate the nature of the planar defects in the nanobelts and in nanorings. The planar defects were initiated and formed by single-layer segregation of the doping element, such as indium, which was introduced in the growth process. The accumulation of impurity ions forms two vicinal $\mathrm{In}\mathrm{O}$ octahedral layers parallel to the basal plane. They form ``head-to-head'' and ``tail-to-tail'' polar-inversion domain boundaries. For a nanobelt that self-coils into a nanoring, we found that the head-to-head and tail-to-tail polar-inversion domain boundaries are paired, thus, the polarity of the nanobelt is unchanged. Therefore, our data support the proposed model [X.Y. Kong et al., Science 303, 1348 (2004)] that the nanoring is initiated by circularly folding a nanobelt due to long-range electrostatic interaction between the surface polar charges on the two sides, and a loop-by-loop winding of the nanobelt forms a complete ring.