Single-crystal nanocastles of ZnO

doi:10.1016/j.cplett.2006.04.013

Chemical Physics Letters

Volume 424, Issues 1–3, 12 June 2006, Pages 86-90

https://doi.org/10.1016/j.cplett.2006.04.013 Get rights and content

Abstract

As a result of switching the growth from hexagonal disks to nanorods/nanobelts, single-crystalline ‘nanocastles’ of ZnO have been synthesized. This new structure was constructed by two basic parts: a sheet-like hexagonal crystal base and vertical nanorods/nanobelts surrounding the base as the ‘walls’ of the castle. Moreover, for the first time, ZnO nanorods with a triangular and diamond cross-sections were found growing on top of the nanocastles. The whole nanocastle structure was enclosed by the (0 0 0 1) and {01 $\bar{1}$ 0} facets. This configuration is a new member in the family of ZnO nanostructures. The in situ morphology switching mechanism would be an useful technique for fabricating high quality heterostructures or interconnects at nanometer scale.

Graphical abstract

Nano-“castles” of ZnO have been synthesized by controlling growth kinetics. This new structure was constructed by two basic parts: a sheet-like hexagonal crystal base and vertical nanorods/nanobelts surrounding the base as the “walls” of the castle. ZnO nanorods with a triangular and diamond cross-sections were found growing on top of the nanocastles. This configuration is a new member in the family of ZnO nanostructures.

Introduction

Zinc oxide is one of the most important functional materials [1]. With a direct bandgap of 3.37 eV, a high electron-hole binding energy of 60 meV and non-centrosymmetric crystal structure, ZnO has been widely used as optoelectronics [2], piezoelectric devices and sensors [3]. Due to its hexagonal Wurtzite structure and polar crystal surfaces, ZnO exhibits a large family of nanostructures [4]. In addition to the conventional nanowire and nanobelt [5] structures, many interesting morphologies have been discovered recently, such as nanocages [6], nanocombs [7], nanorings [8], nanosprings [9], [10], nanobows [11], and nanodisks [12], [13]. Those structures not only provided valuable models in understanding crystal growth mechanisms in nanometer scale, but also exhibited high potential for fabricating novel nano electronic and optical devices with enhanced performance.

Different morphologies were generally induced by different vapor concentration. It has been found that changing the ZnO vapor concentration during the growth can switch the growth morphology from nanowire to curved nanobelts [14], thus, forming a nanobow structure. In this Letter, by quickly cooling down the furnace, we have successfully switched the growth from hexagonal disks to nanorods/nanobelts, resulting in the formation of a ‘nanocastle’, a new structural configuration for ZnO. The morphology is basically composed of two parts: quasi-two-dimensional (2D) hexagonal crystal base and quasi-one-dimensional (1D) nanostructures surrounding the base as the ‘walls’ of the nanocastle. The two different structures appeared to grow consecutively and formed a single-crystal structure.

Section snippets

Experiments

The nanocastles were fabricated through a carbon-thermal evaporation process. In a typical experiment, 0.3 g of high purity ZnO powders were mixed with 0.3 g of graphite powders as the source materials and the mixture was loaded into a quartz boat. The substrate was a 5 × 5 mm²c-plane orientated sapphire coated with a thin-layer of single-crystal c-plane GaN. A droplet of 15 nm gold colloidal solution (purchased from Ted Pella Inc.) was dispersed onto the substrate as deposition seeds and the

Morphologies of the nanocastles

The as-synthesized products were first examined by SEM. Fig. 1a is a low magnification SEM image showing the scattered distribution of the nanocastles owing to the low concentration of the gold nanoparticle solution. Dictated by the sixfold symmetry of the Wurtzite ZnO crystal, all of the nanocastles exhibited a hexagonal-like shape. The perfect lattice match between GaN and ZnO made most of the nanocastles orientated perpendicular to the substrate. The size of the nanocastle varied from 2 μm to

Conclusion

In this Letter, a new nanocastle morphology was found for the family of ZnO nanostructures. It is formed by two different basic structures: a 2D hexagonal crystal base and 1D nanorods/nanobelts surrounding the base as the ‘walls’ of the castle. These two different structures seamlessly grew together and formed a single-crystal. The switching of morphology was induced by changing the vapor concentration during the growth. Moreover, ZnO nanorods were first found with a triangular and diamond

Acknowledgement

Thanks to support from NSF, the NASA Vehicle Systems Program and Department of Defense Research and Engineering (DDR&E), and the Defense Advanced Research Projects Agency.

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Single-crystal nanocastles of ZnO

Abstract

Graphical abstract

Introduction

Section snippets

Experiments

Morphologies of the nanocastles

Conclusion

Acknowledgement

Mater. Today

J. Appl. Phys.

Science

Appl. Phys. Lett.

Science

J. Am. Chem. Soc.

Phys. Rev. Lett.