“In this paper, we report, for the very first time, the success of synthesizing structurally controlled piezoelectric and ferroelectric ZnO nanobelts of sizes 10-60 nm in width and 5-20 nm in thickness.”

In parallel to semiconductor nanostructures, piezoelectric- and ferroelectric-based smart materials are equally important, because they are the transducers and actuators for nano-scale machines and devices. But there had been no report about the success of synthesizing piezoelectric one-dimensional nanostructures. In this paper, we report, for the very first time, the success of synthesizing structurally controlled piezoelectric and ferroelectric ZnO nanobelts of sizes 10-60 nm in width and 5-20 nm in thickness. The most exciting result is the formation of single crystalline ZnO helical nanosprings due to spontaneous polarization. Our manuscript is the first paper that analyzes such structures that will have major scientific and technological impacts on the field.

Major results:

1. Nanobelts, exhibiting piezoelectric effect and spontaneous polarization, are synthesized for the very first time. The wurtzite-structured nanobelt grows along the a-axis, with its top and bottom surfaces the polar (0001) c-plane. Owing to the positive and negative ionic charges on the zinc- and oxygen-terminated basal planes, respectively, a spontaneous polarization normal to the nanobelt surface is induced. This will lead to a whole new field in nanowire/nanobelt research.
2. Helical nanosprings/nanocoils, formed by rolling up single crystalline nanobelts, are reported for the very first time. The mechanism for the helical growth is suggested to be a consequence of minimizing the total energy contributed by spontaneous polarization and elasticity. Our theoretical model gives an excellent explanation to the experimental observation. This is a new mechanism for forming the helical nanostructures. The helical structure will have a wide range of impacts both in scientific research and technological applications.
3. The growth of polar-facets-dominated nanobelt surfaces is a major step towards the development of piezoelectric and possibly, ferroelectric, one-dimensional nanostructures. Since the ZnO (2 0) plane has a lower surface energy, lower than that of either (0001) or (01 0), a fast growth of nanobelt structure that is dominated by the (0001) polar surface is energetically unfavorable. But the success of the controlled growth of (0001) plane-dominated nanobelts shows that controlling growth kinetics and experimental conditions can overcome the barrier placed by surface energy in nanostructure growth, thus, opening a new channel for the growth of structurally controlled nanobelts of technological importance.

Nanowire and nanotube-based materials have been demonstrated as building blocks for nanocircuits, nanosystems, and nano-optoelectronics. Synthesis, characterization, and applications of oxide nanostructures are the major research directions of my group. Quasi-one-dimensional nanostructures—so called nanobelts or nanoribbons—have been successfully synthesized for the semiconducting oxides of zinc, tin, indium, cadmium, and gallium, by simply evaporating the desired commercial metal oxide powders at high temperatures in our lab. [1]. The Science paper [1] we published in 2001 is the most-cited paper in the field of chemistry from the period 2002-2003. The belt-like morphology appears to be a unique and common structural characteristic for the family of semiconducting oxides with cations of different valence states and materials of distinct crystallographic structures.

Using the technique demonstrated for measuring the mechanical properties of carbon nanotubes based on in situ transmission electron microscopy [2,3], the bending modulus of the oxide nanobelts and the work function at the tip have been measured. Field-effect transistors [4] and ultra-sensitive nano-size gas sensors [5], nanoresonators, and nanocantilevers [6], have also been fabricated based on individual nanobelts. Thermal conductivity of a nanobelt has also been measured. Very recently, nanobelts, nanorings, and nanosprings that exhibit piezoelectric properties have been synthesized, which are potential candidates for nano-scale traducers, actuators, and sensors [7, 8, 9, 10]. This presentation will focus on our recent progress in the controlled growth, nano-scale property measurements and nano-size device fabrication using oxide nanostructures that are semiconducting and piezoelectric.