Piezoactive dense diphenylalanine thin films via solid-phase crystallization

Piezoactive biomaterials are currently in the forefront of the worldwide research due to the multitude of applications ranging from implantable biosensors to biocompatible energy harvesters. Among them, biomolecular piezoelectrics based on amino acids and dipeptides (as exemplified by diphenylalanine, FF) are the most studied. Major problem is an inability to control the self-assembly process to produce dense films with controlled orientation and thickness. To overcome this, we propose a novel method of the formation of crystalline piezoactive FF films via solid phase crystallization directly from the amorphous phase. The process starts from the spin-coating of FF monomers in an organic solution. These layers are then exposed to a controlled humidity that triggers nucleation and growth of highly oriented piezoactive areas (domains). The crystallization process proceeds without changing the morphology and results in dense films with controlled thickness. Large ferroelectric-like domains possess uniform piezoresponse of about 30 pm/V with the in-plane polarization. The growth kinetics is controlled by the temperature and humidity, suggesting that fully in-plane oriented films can be obtained. It is hypothetized that the solid-phase crystallization can be applied to other bioorganic piezoelectrics and thus open an avenue for further use of these materials in implantable piezotronics and beyond. (c) 2021 Elsevier Ltd. All rights reserved.