Fern leaves

Zinc Oxide (ZnO) is a well known multifunctional material that has been studied since the 1960s due to its importance in sensors, actuators, and transducers. ZnO nanostructures are of great interest due to their application in field effect transistors, nanolasers, and many other optoelectronic and nanoelectronic devices. What's more, the behavior of ZnO on the nanoscale is distinct from the bulk, thanks to the quantum confinement of charge carriers within the nanostructures.

ZnO is a wide band gap (3.37 eV) semiconductor which lends itself to applications in short wavelength light emission [1], nanolasers, and optoelectronic devices [2]. Besides UV emission from direct band edge transitions, visible emission has also been reported as a consequence of the existence of various defect states in the ZnO crystals [3]. Thus it is also an excellent material for generating white light. The system possesses the wurtzite crystal structure, and so lacks a centre of symmetry; this results in the piezoelectric nature of the material. This property is being used to realize piezoelectric energy generation [4].

There are several processes, such as the vapor-liquid-solid method, the physical and chemical vapor deposition method, sputtering, atomic layer deposition, and wet chemical and hydrothermal methods that have been used to fabricate varieties of ZnO structures [5]. Each process has its own advantages and limitations. The growth mechanism is different under different fabrication processes; however, even when using the same process, the growth mechanism differs under different experimental conditions. It is therefore critical that we understand how ZnO grows.

Among the various reported structures of ZnO, dendritic morphology is of considerable interest to researchers due to the connections with fractal growth phenomena and current crystallography research, and because of the possible applications in micro- and nanoscale devices. This type of dendritic growth pattern is also found in nature, and even in animal physiology. Thus it is important that we understand dendritic growth, and the connection to various natural/biological processes.

The image featured on this month's cover is a field emission scanning electron microscope (FE-SEM) image of electrochemically grown ZnO. The structure was grown using the two electrode based electrochemical process at a constant DC voltage of 8 V, using zinc acetate solution as the electrolyte. The sample was coated with gold in a gold plasma deposition chamber, and the image was taken using a Zeiss FE-SEM at an operating voltage of 5 kV after the sample was mounted on a carbon tape holder.

The structure consists of (leaf-like) platelets connected by a main stem: similar to the branch of natural fern. The length of the platelets is ∼50–100 μm, with the structure being highly symmetric about the main stem. The angle between the stem and the leaf is ∼45–55°. The fixed angle between the leaflets and the stem indicates that the growth is diffusion controlled (globally) but locally accomplished through oriented attachment. This type of oriented attachment growth occurs due to dislocations that occur during the growth of the crystals. The angle between the (101) and (002) planes is about ∼45°. Therefore the formation of the fern-like structure comes from the preferential growth of the (101) and (002) planes. As the growth rates of various planes are different, the length of the main stem and the branches are also different. However, the angle between the planes is not exactly 45°, which means that the growth of the leaves is significantly affected by the diffusion of the ZnO precursor rather than the direction of crystal growth. The growth is highly dependent on the applied voltage, electrolyte concentration, and deposition time. By conducting further study on the growth dynamics on this type of structure it will be possible to obtain more information on the mechanisms behind fractal-dendritic growth.

Further Reading
[1] Wong et al. Appl Phys Lett, 74 (1999), p. 2939
[2] Samanta et al. Physica E, 41 (2009), p. 664
[3] Samanta et al. Sci Adv Mater, 3 (2011), p. 107
[4] Yang et al. Appl Phys Lett, 94 (2009), p. 022905
[5] Ozgur et al. J Appl Phys, 98 (2005), p. 041301