Multi-layer Self Limiting Electrospray Deposition using Secondary Voltage Bias

The self-limiting regime of electrospray deposition (SLED) has proven to be a viable alternative to other coating methods, such as dip coating, when considering complicated architectures. This is accomplished by the accumulation of charge in the deposit, which reshapes the driving electric field and thus redirects the spray to uncoated regions of the film. There are, however, drawbacks to this approach. First, SLED coatings, being self-limiting, cannot be applied in multiple layers. Second, the thickness of SLED coatings is determined by the material and spray parameters, and tends to be in the range of 2-6 microns when fully densified, which prevents the use of nanoscale coatings, such as those possible with atomic layer deposition (ALD) or spin coating. Our recent manuscript circumvented this latter limitation by applying a secondary bias to the spray target, which serves to both reduce the SLED thickness and also increase the coating uniformity by making the charged droplets more responsive to the driving electric field and less dominated by droplet momentum. This control has the additional implication of being dynamically adjustable—the bias on the target can be decreased either gradually or stepwise during the spray process. Here, we demonstrate that the stepwise removal of bias can, in turn, result in the stepwise increase of the SLED thickness. This should overcome the first limitation by instead allowing for multilayer coatings that fit within the envelope of the SLED thickness. This encompasses many nanoarchitectures currently deposited through complex, multistep spin coating and liftoff, such as diodes, capacitors, and batteries. This would be provided that appropriate SLED-compatible inks could be formulated to act as the functional layers of these devices at sufficient performance. The results demonstrate how SLED can be utilized for specific multi-layer coating applications by achieving the desired thicknesses through the manipulation of the applied secondary voltage bias.