Nanocrystalline silver is deposited using a modified Tollen’s reaction coating 1 μm, 530nm, or 260nm colloidal silica spheres. Functionalization and surface decoration are not required. Studies of silver films on flat substrates are providing fundamental characterization of the differences between evaporated and chemically deposited films. This analysis provides a foundation for spherical film studies. Properties being characterized include s urface morphology and roughness, thickness measurements, crystallinity, and resistivity measurements.

The fundamental properties of noble metals alter from their bulk values when patterned as nanoparticles, nanowires and thin films. The electrical resistance is increased for nanoscale metals due to the limiting of the electron mean free path (emfp) by the structure size. For silver, the emfp is about 50nm, that is to say that the average distance traveled by an electron before it scatters off of phonons in the metals crystal structure is 50nm. In films less than 50nm the distance an electron can travel is limited to the thickness of the film which increases scattering at the film boundary. Scattering off of the crystallite boundaries in polycrystalline films also increase the resistance. This elevated resistance makes the use of silver as a sensor feasible. We have found that the resistivity of our silver films is much greater that that of evaporated films. Even those as thick at 150nm have elevated resistance due to their rough, polycrystalline structure.

Sensing based on a change in resistivity in response to specific gases, including humidity and ammonia will be tested using rough silver films. If a gas is adsorbed it may either contribute or sequester an e- in the nano-metal network. This change in electron density changes the the resistance which signals the presence of the gas. We expect spherical films to be the most effective structure for sensing because the texture and porosity of our films provides a much larger surface area for molecules to attach to.