Material Jetting of Carbon Nano Onions for Printed Electronics.

As an additive manufacturing process, material jetting techniques allow to selectively deposit droplets of materials in liquid or powder form through a small-diameter aperture, such as a nozzle of a print head. For the fabrication of printed electronics, a variety of inks and dispersions of functional materials can be deposited by drop-on-demand printing on rigid and flexible substrates. In this work, zero-dimensional multi-layer shell-structured fullerene material, also known as carbon nano-onion (CNO) or onion-like carbon (OLC), is printed on polyethylene terephthalate (PET) substrates using drop-on-demand inkjet printing. CNOs are produced using a low-cost flame synthesis technique and characterized by electron microscopy, Raman, X-ray photoelectron spectroscopy, and specific surface area and pore size measurements. The produced CNO material has an average diameter of ~33 nm, pore diameter in the range ~2-40 nm and a specific surface area of 232 m2.g-1. The CNO dispersions in ethanol have a reduced viscosity (~1.2 mPa.s) and are compatible with commercial piezoelectric inkjet heads. The jetting parameters are optimized to avoid satellite drops and to obtain a reduced drop volume (52 pL), resulting in optimal resolution (220 µm) and line continuity. A multi-step process is implemented without inter-layer curing and a fine control over the CNO layer thickness is achieved (~180 nm-thick layer after 10 printing passes). The printed CNO structures show an electrical resistivity of ~600 Ω.m, a high negative temperature coefficient of resistance (-4.35x10-2 ºC-1) and a marked dependency on relative humidity (-1.29x10-2 RH%-1). The high sensitivity to temperature and humidity, combined to the large specific area of the CNOs, make this material and the corresponding ink a viable prospect for inkjet-printed technologies, such as environmental and gas sensors.