The piezoelectric and dielectric properties of flexible, nanoporous, self-assembled boron nitride nanotube thin films

Boron nitride nanotubes (BNNTs) are one-dimensional dielectric and piezoelectric nanomaterials with non-cytotoxic properties and superb chemical and thermal stabilities. Regarding practical applications, a flexible, nanoporous, self-assembled film is one of the most low-cost usable structures. Here, we reveal a unique nanocatalyst-assisted ink spray coating method to synthesize 4.69-eV optical bandgap NTs and an easy mechanical peeling method to obtain an attractive nanoporous freestanding NT thin film with high flexibility and 91.45% porosity. Importantly, the piezoelectric d₃₃ coefficient of the 90-nm-radius NT is 41.12 pm V⁻¹, more than triple the value reported previously, due to the inner bamboo structure, and this coefficient is a function of radius. The nanoporous bamboo-type NT film shows excellent dielectric properties, with a relation dielectric constant of 5.15 and a dielectric constant of 9.78 for the solid type (ignoring air), which is more than triple the value of the cylindrical type. Moreover, the capacitance loss and electrical conductance versus frequency have great potential for a dielectric layer under high-frequency capacitance. Hence, bamboo-type BNNTs and their films can reasonably be widely applied in flexible electronic devices, high-precision piezoelectric sensors, and intelligent bioelectronic devices in harsh environments.