ZnO nano-flowers

Nanomaterials are an important class of material in view of developing technologies as well as basic studies. Zinc oxide (ZnO) is one of the most versatile and well-studied materials within the context of nanoscience and technology. ZnO (a II–VI compound semiconductor) has been a promising material in the development of exciton-based optoelectronic devices such as light-emitting diodes (LEDs) [1] and photovoltaic cells [2] due to its direct band-gap of 3.3 eV at room temperature and large exciton binding energy of 60 meV. It has attracted significant research efforts and thanks to its unique properties and suitability and versatility in applications such as piezoelectric devices, ultraviolet (UV) light emitters, transparent electronics, chemical sensors and spin electronics. For example, transparent thin film transistors (TFTs) using ZnO as an active channel have achieved a much higher field effect mobility than amorphous silicon TFTs [3] where these transistors can be extensively used for display applications. ZnO has been recommended to be a more favorable UV emitting phosphor than GaN due to its larger exciton binding energy (60 meV). This results in a reduced UV lasing threshold, yielding higher UV emitting efficiency at room temperature. Surface acoustic wave filters made using ZnO films [4] are currently being used for video and radio frequency circuits. Piezoelectric ZnO thin films have been created for use in ultrasonic transducer arrays operating at 100 MHz. Bulk and thin films of ZnO have demonstrated a high sensitivity for toxic gases. Based on these outstanding physical properties and the motivation of device miniaturization, great effort has been focused on nano-ZnO synthesis, characterization and device applications.

An assortment of ZnO nanostructures, such as nanowires, nanorings, nanotubes, and nano-tetrapods has been successfully grown via a variety of methods including chemical vapor deposition, thermal evaporation, electro-deposition, etc. But these techniques require high temperatures or sophisticated technologies, which are time consuming and expensive. Most nanowires of ZnO are grown by a template or seed method, which requires a few days and can be non-reproducible. At Department of Physics, at the Indian Institute of Technology, Indore we have developed a simple and economical technique; a so-called soft solution growth method, to grow a variety of ZnO nanomaterials including nano-balls, nano-flowers, nano-rods, etc. at about ~70 °C and free from a template or seeds and neither auto-clove or microwaves assisted. All these kind of shapes of ZnO nanomaterials can be grown on flexible plastic sheets, transparency sheets, glasses or substrates, etc. (depending on the desired application) in about 60 min, making this technique more commercially viable for industrial applications [5].

The image on this issue's cover is an Field effect scanning electron microscope (FE-SEM) image of the one of the ZnO morphologies produced using the aforementioned technique developed for ZnO nanomaterial synthesized on glass substrates. Different layers of nano-rods can be grown with tunable dimensions. The typical diameter of the nano-rods grown in the form of a nano-flower is about 40–50 nm and the lengths and diameter can be tuned by adjusting the growth time and temperature of the starting solution. The starting solution was prepared by dissolving salts of zinc in double distilled water. The H⁺ concentration was controlled by using an ammonia based solution. Once the solution reaches the desired temperature, the substrates can be dipped vertically and held in place for a few minutes. It is our belief that this technique will save time and money, and will help to revolutionize the field of nanotechnology.

Acknowledgements

This work was supported by the Department of Science and Technology, India via a prestigious ‘Ramanujan Fellowship’ (SR/S2/RJN-121/2012) awarded to the author. The author is thankful to Prof. Pradeep Mathur, Director, IIT Indore, for encouraging the research and providing the necessary facilities. The author thanks Dr. Kalubarme for technical support.

Further reading

[1] Y.S. Choi et al., IEEE Transactions on Electron Devices, 57 (1) (2010), p. 26
[2] S. Flickyngerova et al., Journal of Electrical Engineering, 61 (5) (2010), p. 291
[3] E.M.C. Fortunato et al., Advanced Materials, 17 (5) (2005), p. 590
[4] Q.J. Wang, C. Pflügl, W.F. Andress, D. Ham, F. Capasso, Journal of Vacuum Science & Technology B, 26 (6) (2008), p. 1848
[5] P.M. Shirage, unpublished work.