Capillary-condensed water in nonporous nanoparticle films evaluated by impedance analysis for nanoparticle devices.

We study capillary-condensed water in nonporous nanoparticle films and evaluate its effect on impedance analysis for practical nanoparticle devices. Nanoparticle-based electronic/optoelectronic devices have been intensively studied for applications in ambient air. Non-sintered nanoparticle films have porous structures and a vapor phase of water molecules condenses in nanopores between nanoparticles at a lower vapor pressure. This condensed water hinders intrinsic electrical properties of nanoparticle films. To clarify the effects of capillary-condensed water between nanoparticles on impedance, we electrically investigate 50 nm and 10 nm nonporous silica nanoparticle films. In a 50 nm nanoparticle film, an impedance hysteresis is observed at higher than 80% of relative humidity (RH). On the contrary, a larger impedance hysteresis appears in a 10 nm nanoparticle film at higher than 50% RH. We evaluate critical pore sizes in 50 nm and 10 nm nanoparticle films as 5-10 and 1.6 nm, respectively. These values correspond to the critical size of nanopores where adjacent menisci between nanoparticles coincide as following the Kelvin equation. This condensation of gas/vapor molecules will be an important issue for developments of practical devices using nanoparticle films.