Hydrophilic Sparse Ionic Monolayer-Protected Metal Nanoparticles: Highly Concentrated Nano-Au and Nano-Ag "Inks" that can be Sintered to Near-Bulk Conductivity at 150°C

Here, monolayer-protected gold and silver nanoparticles with extremely high solvent dispersibility (over 200 mg mL(-1) in water and glycols) and low coalescence temperature (approximately 150 degrees C, measured by the percolation transition temperature T-p) are developed, to reach conductivities better than 1 X 10(5) S cm(-1). These materials are suitable for inkjet and other forms of printing on substrates that may be solvent- and/or temperature-sensitive, such as for plastic electronics, and as bus lines for solar and lighting panels. This is achieved using a new concept of the sparse ionic protection monolayer. The metal nanoparticles; are initially protected by a two-component mixed ligand shell comprising an omega-functionalized ionic ligand and a labile ligand. These are selectively desorbed to give a sparse shell of the to-ionic ligands of ca. 25% coverage. Through a systematic study of different monolayer-protected Au nanoparticles using FTIR spectroscopy, supported by XPS and DSC, it is shown that T-p is not determined by thermodynamic size melting or by surface area effects, as previously thought, but by the temperature when ca. 80% of the dense monolayer is eliminated. Therefore, T-p depends on the thermal stability and packing density of the shell, rather than the size of the metal core, while the solubility characteristics depend strongly on the exposed terminal group.