| Metal nanoparticles (NPs) can be sintered at low temperatures and are considered essential precursor materials for printing conductive patterns of flexible electronics. Using uniform surfactant-stabilized silver NPs as a model system, we have investigated the process of metal nanoparticle film (NPF) sintering through systematic measurements of the loss of organic components of NPFs using forward recoil spectrometry, the shrinkage of film thickness using neutron and x-ray reflectivity, the evolution of particle morphology using electron microscopy, and the variation of sheet resistance using 4-point probe. As surfactant molecules evaporate, NPFs become thinner, denser and conductive metallic films. Both the residual surfactant and the film thickness decrease exponentially with sintering time, indicative of activation processes. The discrepancy between the two quantities implies the porosity of sintered films. Morphology and electric resistance data show that NPFs sinter through two consecutive percolation processes. Initially, as surfactants evaporate and NPs fuse to form bigger ones, electrons tunnel between adjacent particles resulting in the first percolating network. At the longer sintering time, inter-particle necking occurs, forming low resistance passages connecting ever ripening NPs. Increasing numbers of necking passages lead to the formation of second percolating network, consequently the sheet resistance of NPFs drops dramatically. Sintering for even longer time gradually increases the necking cross-section area hence further reduces the sheet resistance. The electrical-mechanical performance of sintered metal NPFs has been evaluated through in situ electrical resistance measurements on NPFs undergoing uniaxial stretching. Ag NPFs on polyethylene terephthalate (PET) remain conductive upon elongating up to 300% strain or 2% amplitude cyclic loading of up to 104 cycles. Microstructure analysis of MPFs stretched to various degree shows stages of morphology development, which is due to the interplay between of the deformation of NPFs and the PET substrate facilitated by a unique adhesion mechanism. Atomic force microscopy on interfaces between Ag NPFs and PET after removing NPFs reveals distributed pinning of NPFs into the plastic substrate. The applications of sintered metal NPFs have been demonstrated in a number of inkjet-printed functional devices and show great promises of NPFs in device applications. |
