Aspect ratio improvement of ZnO nanowires grown in liquid phase by using step-by-step sequential growth

Abstract

ZnO is a semiconductor with a direct band gap (3.37 eV) and a large exciton binding energy (60 meV) at room temperature. ZnO nanowires have shown advantages in the sensing field and optoelectronics due to their very large surface-to-volume ratio, high crystallinity and simple preparation methods. As compared to gas phase synthesis, that is widely used for growing ZnO nanowires, liquid phase synthesis is known for being a very powerful and versatile technique due to its low cost, flexibility, facility and environmentally benign processes, and when using homogeneous nucleation, it paves the way for large-scale volume production of nanowires. The main drawback of the liquid phase synthesis is however the difficulty in renewing or changing the precursors, which is used in gas phase in order to grow nanowires with an extremely high aspect ratio as well as heterostructures. Here, we demonstrate a growth regime for ZnO nanowires, in the liquid phase and with homogeneous nucleation, where the step-by-step addition of precursors leads to an increase in the nanowire length without a significant increase in their width, thus supporting a mechanism of cycle growth. This process strongly contrasts with the direct increase in the concentration of precursors which leads to strong increases in both length and diameter. The involved mechanism is explained as a competition between the capping effect of Cl⁻ ions and the buffer effect of hexamethylenetetramine. The process shows a direct increase in the nanowire aspect ratio as a function of the number of cycles, which is further enhanced by using an additional capping agent (polyethylene glycol).