The Role of Surface Charging in Electrospray Printing of Thin Films*

We report on the use of electrospray printing to create thin continuous multiscale films from nanoparticle aggregates. Nanomaterials are delivered to the target substrate in a dry state with an excess electric charge. The charge accumulates on the target and its decay rate plays a significant role in the film formation. Films printed on conductive substrates are thick and exhibit a periodic island structure, while films on insulating substrates are thickness-limited and more uniform. We leveraged the surface charging from electrospray to direct the assembly of nanoparticles using electrostatic focusing. This was achieved by patterning a photoresist layer on the target substrate. Charge accumulates on this layer and electrostatic forces direct the emitted material to the exposed parts of the substrate, which created high aspect ratio porous structures when the target substrate was silicon. The deposit growth was inhibited on glass substrates. To quantify the accumulated electric charge and its decay rate, we measured the spray and image charging for nylon and glass substrates. A scaling analysis revealed the importance of the processing conditions in determining the maximum accumulated charge and its decay rate.