Surface enhanced Raman scattering and microwave absorption in silver nanoparticle inks

Abstract

Metallic nanoparticle inks - colloidal suspensions of silver or gold nanoparticles in water or other organic solvents - can be sintered at relatively low temperatures (70 - 200°C). With appropriate thermal treatment the sintering can be controlled to fabricate nanoparticle substrates with a distribution of clusters sizes and interparticle distances. Such substrates exhibit relatively high (108 - 109) surface enhanced Raman scattering (SERS) amplification factors (AFs). The high AFs in such substrates arise from several mechanisms. The 'dimers' - two nanoparticles separated by a nanometersize gap - are known to produce amplification of the local electric field orders of magnitude larger than at the surface of an isolated single nanoparticle due to surface plasmon resonance. Furthermore, the lack of translational symmetry in the clusters leads to localizations of electromagnetic excitations to very small regions that can create SERS hot spots. Here we report that microwave absorption (~ 10 GHz) as a function of thermal annealing in dry-drop substrates can be used to monitor the sintering process in metallic nanoparticle inks. The predominant contribution to microwave absorption comes from electrically resistive weak links that are formed between nanoparticles as a result of the thermal treatment. Just before the creation of these weak links, such nanoparticle pairs are also the ones that make a major contribution to the SERS AFs. This leads to a correlation between the observed microwave absorption and the SERS signal intensities. We also present a simple model that describes the microwave absorption as a function of the isothermal annealing treatment.