Robust low-sintering-temperature metal nanoinks suitable for plastic electronics
31 Jan 2010. NUS scientists at the Organic Nano Device Laboratory (ONDL) have together produced robust nanometal “inks” that can sinter to the continuous metal film and reach near bulk conductivity at 1500C. This is amongst the lowest temperature reported so far for stable nanometal dispersions which can be kept for over six months under ambient conditions.
In a paper published in Advanced Functional Materials (Bibin et al, Advanced Functional Materials 20 (2010) 296.), the scientists have reported that they successfully developed monolayer-protected gold and silver particles in the nanometer-size regime that can simultaneously show high dispersability in solvents (> 200 mg/mL in water and the glycols) and low coalescence temperature (approximately 1500C) as measured by the percolation transition temperature Tp.
“This makes it possible now to print these nanometal inks in the ambient on substrates that are compatible with plastic electronics, and to convert these subsequently to the conductive lines through a low-temperature heat treatment step,” said Loke-Yuen Wong, who has developed inkjet printing at the ONDL. Previously, nanometal inks needed temperatures well in excess of 2000C to reach the sufficiently conductive state, which is not generally compatible with plastic electronics. Furthermore it often requires organic solvents which also introduce constraints to the deposition sequence.
“The present breakthrough came about as the result of an ‘accidental’ discovery of the concept of ‘sparse protection shell’, said Prof Lay-Lay Chua of the Department of Chemistry and a principal investigator of the project. “This built on the “mixed functionalisation” concept reported by us three years ago in Nature Materials. Now by selective desorption of the labile component of the ligand shells, we showed it is possible to bring Tp down while keeping dispersability high.”
Futhermore the scientists found that the Tp is not related to the thermodynamic size-dependent melting point as widely believed, but the temperature at which the shell becomes approximately 80% desorbed. "This opens new approaches in the development and applications of other functional nanoparticles,: said Prof Chua.