Laser-Assisted Zone Crystallization in Superscalable, Contactless Wet Coating of Ultrathin Films for Flexible Piezoelectrics and Electronics

Fabrication of elastic piezoelectric generators, polymer-based pressure or vibration sensors, as well as flexible thin-film-based electronic devices, represent examples of a growing number of material technologies that require large-scale thin functional films deposition. There are several technologically-proven deposition solutions available such as roll-to-roll (roto)gravure/doctor blade coating, spray coating, or inkjet printing. Application of a particular deposition method depends on surface properties and deposited material. Quite often, however, implementation of a particular thin film technology is limited by scalability of the deposition method. The scalability is important, since deposition method have a key influence on e.g. wetting dynamics, solidification, crystallization and other factors critically affecting e.g. crystal size and orientation. The other challenge today is coating films on wavy, dimpled or other non-flat surfaces. Contact methods, such as listed above, typically fail in film deposition on non-flat surfaces.<br/>In our report, we demonstrate a contactless wet film deposition method that addresses the scalability issues and enables wet, ink-based fabrication of films on flat and curved surfaces. In our approach we used specifically tuned laser beam to induce thermal gradients of substrate surface free energy and ink viscosity. The gradients induce local, internal flow in the ink causing formation of an ink bulge close to the laser spot. The bulge can be moved on the surface by moving the laser-heated spot over the substrate. The moving ink bulge is followed by a local solvent evaporation that causes deposition of a film. The footprint profile of the liquid bulge can be controlled by the laser focus. Using the line-focused laser enables deposition of the solid film in the way analogical to the doctor blade, but without any physical contact of the coating tool with the solution – the laser beam acts as a coating blade. The length of the focal line defines the coated area. Because of the working principle, we call the method laser-assisted zone crystallization (LAZEC).<br/>We tested the LAZEC in fabrication of polymer and molecular films with thickness ranging from 3 to 100 nm and specific electronic properties. X-ray diffraction revealed high degrees of structural order and spatial orientation of the films. The ordered nature of the sample stems from, inter alia, controlled thermal conditions of the solidification process. Structure control in the LAZEC permits e.g. fabrication of organic semiconductor films revealing highly anisotropic charge transport properties. In our study we have also tested fabrication of piezoelectric polymer films. Imparting piezoelectricity to a polymer requires uniaxially, homogeneously and permanently oriented dipole moments in specific crystal polymorphs. Since the LAZEC inherently enables control over crystallization thermal conditions, it also permits a wide range control over a polymorphism. A control over the orientation of dipole moments was achieved by using external electric field. Using poly(vinylidene fluoride) (PVDF) and its copolymers as examples we demonstrate deposition of 100-nm thin piezoelectric films revealing d₃₃ piezoelectric moduli comparable to their conventionally fabricated free standing film counterparts. During the fabrication of the piezoelectric films we used external electric films to pole the samples. Noteworthy, the poling in LAZEC deposition required electric field several orders of magnitude weaker than the field used in conventional techniques (e.g. corona poling) and films required no mechanical treatment after the deposition. Voltages lower than 1 kV were enough to render PVDF film piezoelectric. In addition to the above experiments the presentation is aimed at showcasing deposition of functional organic films on curved parts of optical lenses.<br/><br/>The work was supported by National Science Centre Poland (NCN) through the grant UMO-2017/25/B/ST5/02869