MULTI-LAYER SELF-LIMITING ELECTROSPRAY DEPOSITION USING 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, for complex 3D architectures. This is accomplished by the accumulation of charge in the deposit that reshapes the driving electric field, thus redirecting 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 µm when fully densified, preventing nanoscale coatings such as those possible by 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, provided that appropriate SLED-compatible inks could be formulated to act as the functional layers of these devices at sufficient performance. This said, SLED multilayer devices could be deposited on non-planar surfaces and in much fewer steps than liftoff (i.e., one spray per layer followed by one debinding/annealing step). In this work, we demonstrate, (i) tri-layer polystyrene stacks with bias-controlled thickness, (ii) uniform tri-layer coatings on a complex geometry, (iii) mechanically robust nanocomposite using thermal capillary rise infiltration (CaRI) of electrosprayed bilayer films, and (iv) a working capacitor on a heat sink, providing the concept of multilayer SLED devices on non-planar surfaces.