Thickness‐Dependent High‐Temperature Piezo‐ and Ferro‐Electricity in a Fluorenone‐Based Molecular Crystal

Organic polar crystalline materials, featuring the merits of lightweight, flexibility, and low fabrication costs, are emerging as promising alternatives for inorganic ferroelectrics, but so far, they are not competitive. The main reasons are the moderate polar properties of such materials and the fact that the temperature of the phase transition from polar to nonpolar states (Curie point) is typically located near room temperature. The organic molecular crystal of the fluorenone derivative 2,7-diphenyl-9H-fluorene-9-one (DPFO) is demonstrated to feature robust high-temperature piezo- and ferro-electric properties, with a relatively high local piezoelectric coefficient (d(33)) of approximate to 120 pm V-1. The origin of the strong piezoelectricity is attributed to the presence of intrinsic domain structures in the DPFO microfiber crystals, originating from intramolecular co-operation between the central fluorenone backbone and the external phenyl rings that are found stable up to 423 K. Moreover, this intramolecular co-operation and the corresponding polar properties are found to depend on the thickness of the DPFO microfiber, resulting in a change from ferroelectric (<0.5 mu m) to piezoelectric (>= 0.5 mu m) behavior. Considering the low cost and flexible production of such fluorenone-based organic lead-free ferroelectrics, this is a very promising strategy toward technological applications in electromechanical actuators, sensors, energy harvesters, and non-volatile memory cells.