Strategic Diamine Selection for Improved Crystallinity in Particle-Free Iron Reactive Inks

Particle-free reactive inks enable precise deposition of functional materials through chemical reactions that precipitate metal at low temperatures for applications in printed electronics, magnetic devices, and millimeter-scale structural components. Strategic selection of organometallic precursors and ink components is critical for achieving both ink stability and decomposition into high-quality metal. Previous work with triiron dodecacarbonyl dissolved in octylamine demonstrated low-temperature iron printing but produced fully amorphous material with less than 20 at.% iron. This work systematically investigates linear aliphatic diamines as alternative complexing agents to decrease contamination and improve the crystallinity of iron printed from reactive inks. Three diamines with increasing alkyl chain lengths (ethylenediamine, 1,3-diaminopropane, and 1,4-diaminobutane) were evaluated by characterizing decomposition products using X-ray diffraction with quantitative phase analysis, Mössbauer effect spectroscopy, and elemental analysis. Results showed that 1,3-diaminopropane produced iron with the greatest crystallinity and metallic iron content. The superior performance of 1,3-diaminopropane was attributed to the formation of an iron salt comprised of a tris(diaminopropane)iron(II) cation and triiron undecacarbonyl anion that undergoes a single-step thermal decomposition. This salt was isolated and dissolved in N -methyl-2-pyrrolidone to formulate a shelf-stable, particle-free reactive ink. Printed material from this ink showed 47% crystallinity and 53.0 at.% iron content, which is a significant improvement over the fully amorphous and only 20 at.% iron films obtained from the triiron dodecacarbonyl dissolved in octylamine inks previously reported. The ability to tune atomic structure through amine selection enables spatial control over material properties within the same material system. As a demonstration, alternating crystalline and amorphous iron layers were printed within a single composite material and verified using transmission electron microscopy with phase mapping. Overall, this work establishes that strategic complexing agent selection in iron carbonyl reactive inks provides tunable control over atomic structure and contamination levels in printed iron materials.