Nanometal inks for plastic electronics applications

Abstract

In order to use metal nanoparticle (NP) films in printable electronics applications, they have to be annealed to eliminate the protective monolayer ligand shell or displace the polymeric dispersant to effect coalescence to a nanocrystalline ¿bulk-like¿ continuous film. We demonstrate here a new concept of sparse monolayer protection of these metal NPs by w-ionic functionalised alkylthiol ligands that offer both high water and alcohol solubility; and low coalescence temperature (Tp ¿ 145ºC for Au and 155ºC for Ag, which is the lowest reported sintering temperature to date) at the same time. This allows, for example, both Au and Ag NP films to be coalesced to continuous metallic films with conductivity near half the value of the bulk metal after only brief annealing (few min) at this temperature, far shorter than that required for commercial systems. This method is, therefore, compatible with commercially available plastic substrates that are sensitive to temperature and organic solvents. Detailed study by Fourier transform infrared spectroscopy reveals that the coalescence of metal nano-cores is not due to thermodynamic size effects on melting as widely speculated in the literature, but dominated by desorption of protective ligands. This therefore allows the possibility to control the insulator-to-metal transformation without restriction on NP size.

In chapter 1, we summarize about the present nanometal ink technologies and their potential applications in plastic electronics circuits, which forms the background for this thesis work.

In chapter 2, we describe several aspects of metal nanoparticle (NP) systems and their design considerations such as optimal core diameter, summary of the known methods of production / synthesis, insulator to metal transformation temperature and epigrammatic picture of optical properties of these nanometals.

In chapter 3, we describe the evidence of sparse ionic monolayer ligand shell assembly on Au NP inks and controlled preparation methodology of sparse monolayer protected metal NPs by detailed analysis of Fourier transform infrared spectroscopy (FTIR), together with X-ray photoelectron spectroscopy, which shows the sparse monolayer protected NP systems have only 25% monolayer density of conventional NPs.

In chapter 4, we demonstrate the Tp value of 145ºC for sparse monolayer protected Au NP fims from the electrical conductivity measurements. The generality of this principle is further demonstrated by showing the similar Tp transformation in Ag NP films. We discuss the ligand shell transformation of metal NP films with respect to anneal temperature using FTIR spectroscopy. Effects of ligand shell structure on the Tp of metal NP films are discussed in detail.

In chapter 5, we address an aspect of core-core coalescence on monolayer protected metal NP films. We show that, contrary to frequent assertion in the literatures, the coalescence of metal NPs are not due to the thermodynamic size effect and surface melting. The evidence came from the invariance of Tp in NPs of different core diameter and same ligand shell protection. The coalescence is dominated by the ligand desorption process, immediately followed by the cold-neck formation between the adjacent nanocores due to surface diffusion of atoms. We further discuss this picture with differential scanning calorimetry of metal NPs, which shows the clear evidence of desorption of ligand shell at Tp.