Printed ZnO nanoparticle seed layers to grow ZnO nanowires on flexible substrates

Zinc oxide (ZnO) nanowires (NWs) are excellent candidates for the fabrication of energy harvesters, mechanical sensors, piezotronic and piezo-phototronic devices thanks to the interplay between piezoelectric and semiconducting properties. The growth of ZnO NWs on flexible substrates would further broaden their possible applications. However, such a growth requires low temperature synthesis to prevent any damage to the flexible polymer. Another difficulty lies in the fact that the deposition of patterned ceramic thin films on flexible substrates is challenging, especially under vacuum free conditions. In this framework, printing technologies like inkjet and gravure printing have a noteworthy potential since they allow to deposit thin films onto flexible substrates and offer several other advantages like cost efficiency, use of low temperatures, vacuum-free processing, high throughput and the possibility of patterning during the deposition process. In this work, we report the chemical bath deposition (CBD) growth of high quality ZnO NWs on polyethylene terephthalate (PET) starting from inkjet printed seed layer constituted of ZnO nanoparticles and a comparison with that obtained with seed layer deposited by gravure technology. Using Piezoresponse Force Microscopy (PFM), we observed that the Zn-polar domains are homogeneously distributed at the top surface of the grown ZnO NWs. This work demonstrates the key benefit of the printing techniques over conventional methods (e.g. Atomic Layer Deposition, ALD) to deposit seed layers at low temperature on flexible substrates. This opens the possibility of manufacturing completely vacuum-free solution-based flexible piezoelectric devices.