Hydrothermal growth of ultra-thin and high aspect ratio ZnO nanowires

Zinc oxide (ZnO) is a non-toxic, biocompatible material with unique physico-chemical properties that can be synthesized into a wide variety of 0D, 1D, 2D or 3D structures. It is widely used in extremely varied fields, including energy (solar cells, piezoelectric nanogenerators), optoelectronics (lasers, LEDs, photo-detectors), microelectronics, chemical or biological sensors, photocatalysis, power supply and sun creams.

Due to its crystal structure and very different surface energies from one crystal face to the other, nanowire (NW) growth is easy and rapid along the c-axis. Thus, ZnO NWs can be synthesized using a wide range of techniques that can be divided into two main categories: vapour phase synthesis and liquid phase synthesis. Although it has certain advantages, vapour phase synthesis has two major disadvantages: it operates at high temperatures and it is expensive. In general, liquid phase synthesis is very attractive compared to vapour phase synthesis because it operates at low temperature, does not require complex equipment, is low cost and easily adaptable on a large scale. It is this method that the French researchers from LMGP (http://www.lmgp.grenoble-inp.fr/) have chosen to use in this work at Univ. Grenoble Alpes. More precisely they implemented hydrothermal synthesis at ambient pressure from the aqueous mixture zinc nitrate/HMTA on silicon substrates coated with a layer of ZnO. This process is one of the most widely used in the literature because it gives good quality and reproducible ZnO NWs.

However, the morphology of NWs remains difficult to control. In particular, obtaining fine NWs (diameter < 40 nm) with high aspect ratio (> 50) is a recurrent problem and it is the subject of numerous research since such characteristics are essential for many applications. Fabricating ZnO nanowires with such small diameter and controllable length at low cost, with a facile process, is extremely interesting from an application point of view as such NWs are conducting, flexible, biocompatible and highly sensitive to their environment. Moreover, they are more robust than thin films in aggressive environments.

The main problem lies in the fact that lateral growth of NWs is not fully inhibited. As a consequence, an increase in length is always accompanied by a diameter increase, so that high aspect ratios are difficult to obtain. By inhibiting separately the lateral growth rate, researchers of LMGP developed a reproducible and robust hydrothermal procedure for growing ultra-thin as well as high aspect ratio ZnO nanowires without the use of additives. Their research work is published in journal, Applied Surface Science.

By working with very low reactant concentrations, the authors were able to inhibit the lateral growth whereas the nanowire length was controlled over the growth duration through a bath renewal enabling to overcome the precursor depletion. With their experimental conditions, Thomas Demes, Céline Ternon and their co-workers managed to grow remarkable NWs with fixed diameter at 20-25 nm and controllable length up to 10µm. The resulting aspect ratio (length over diameter) is as high as 500 for 10µm long NWs and it increases with length.

Moreover, the authors were able to propose a model based on kinetic and thermodynamic considerations to explain the final geometry. First of all, surface energy is favorable to the growth of nanoparticles at nucleation sites. Then, from a certain diameter (20-25nm), the surface energy switches to nanowire geometry, the low concentration of reagents allowing inhibition of lateral growth.

Finally, the authors assessed the functionality of such NWs through the fabrication of simple electrical devices. First, the authors assembled the NWs into the very interesting structure of nanowire networks, also called nanonets, and then deposited metallic electrodes through a shadow mask. Thus, the authors were able to demonstrate the capacity of hydrothermally grown ultra-thin ZnO NWs to transport electrons over a long distance of approximately 60µm.

The authors consider with optimism that these first electrical results are highly promising for future integration into functional devices, particularly for gas sensing.