Investigation of morphology and piezoelectric properties of films based on mixed langmuir monolayers of polyvinylidene fluoride and graphene oxide by atomic-force microscopy

In this work, using the Langmuir-Blodgett technology, thin piezoactive films based on polyvinylidene fluoride (PVDF), its copolymer with polytetrafluoroethylene (P(VDF:TFE)), and their mixtures with dispersions of graphene oxide (GO) particles were fabricated. It was found that the addition of GO particles to the working solutions of PVDF and P(VDF:TFE) affects the formation process and the surface properties of the monolayers. The formed monolayers were transferred onto solid substrates of highly oriented pyrolytic graphite (HOPG). Their morphology and piezoelectric properties were investigated by piezoresponse force microscopy (PFM). It was established that films based on P(VDF-TFE) possess a significantly lower roughness of 10 nm compared to PVDF films with a roughness of 47 nm. The main structural difference between PVDF and P(VDF:TFE) films is the presence of grains with diameters of up to 150 nm in the PVDF films. The presence of GO particles in the PVDF:GO film led to a decrease in grain size and an increase in the coating uniformity. For the film based on P(VDF:TFE) and graphene oxide particles, a slight increase in roughness was characteristic, due to the formation of individual non-piezoactive aggregates of GO particles. Thermal treatment of the films promoted an increase in the uniformity of their surface morphology and led to a growth in the local piezoelectric response for all systems, except for the P(VDF:TFE):GO film. Quantitative analysis of the piezoelectric hysteresis loops allowed for the estimation of the local effective piezoelectric coefficient d33. The highest d33 value, reaching 10.1 pm/V after annealing, was possessed by the film of P(VDF:TFE) composition. The introduction of GO particles into the PVDF matrix led to an increase in the d33 value from 0.6 pm/V to 3.4 pm/V. At the same time, the introduction of GO into the P(VDF:TFE) matrix resulted in a decrease in d33 from 6.6 pm/V to 3.0 pm/V. Such behavior of the d33 value may be associated with different types of incorporation of GO particles into the PVDF and P(VDF:TFE) polymer matrices.