Formulating Conductive Inks of Nanowires Built from Silver Nanoparticle Precursors

Silver nanowires (AgNWs) have FCC crystal structure and, at small diameters of 30nm, also show 4H- coexistence. They are 1D five-fold twinned nanostructures having pentagonal cross-sections and high aspect ratios with long lengths (1-100 microns) relative to their small diameters (10-100 nanometers). The length & diameter of the nanostructure is known to impact the electrical, mechanical, and optical properties of their incorporated devices; however, precise control of AgNW dimensions remains unsolved. Silver is highly conductive and AgNWs show promise for their integration into flexible and stretchable electronics. Furthermore, recent dermal assessments have verified that healthy skin provides an adequate barrier against AgNWs, thus protecting underlying cells from cytotoxic effects and enabling their use at bioelectronic interfaces. Aerosol jet printing (AJP) enables high-resolution, automated, and scalable deposition of a variety of materials (including silver nanospheres); however, after deposition onto a substrate, most thin films are only conductive after attempts to sinter the materials and reduce non-conducting gaps; this requires harsh chemical or thermal treatment regardless of the printing method. Previous studies have shown that overlapping AgNW materials can exhibit conductivity at low voltages without sintering, making them a promising candidate for direct printing onto human skin.<br/><br/>In this work, we study the synthesis, stabilization, and printing of AgNWs, analyzing the many challenges related to undesired ink effects (irreversible aggregation, poor dispersion, and non-uniform length distributions of the AgNWs) and undesired electronic performance (wide print lines, significant overspray, and low conductivity without sintering). Also, the literature contains many different “recipes” and so we have taken a reductionist reconstitutionist approach to develop a printing methodology which begins with the synthesis and characterization of Ag nanoparticles (AgNPs). For example, starting with just Ag in the ethylene glycol (both solvent and reducing agent) we discovered that 80nm Ag nanoparticles are formed even at room temperature (not reported before). We are therefore developing a novel two-step synthetic procedure for AgNWs to control of the crystal growth and measure it using DLS, SEM, and an optical microscopy. Adding the polymer, Polyvinylpyrrolidone (PVP), highlighted its role as both reducing agent, capping agent and stabilizer against particle aggregation. It is expected that higher resolution will be achieved by formulating AgNW inks with this approach including with dispersants such as 0.1% HPMC and other additives. Employing these dispersant agents and varying AgNW aspect ratio could lead to the deposition of highly conductive thin films without biologically damaging post processing, paving a path forward for direct and conformal printing of electronic components onto human skin.