We demonstrated that ultraviolet Raman spectroscopy is an effective technique to measure the transition temperature ( T _c ) in ferroelectric ultrathin films and superlattices. We showed that one-unit-cell-thick BaTiO ₃ layers in BaTiO ₃ /SrTiO ₃ superlattices are not only ferroelectric (with T _c as high as 250 kelvin) but also polarize the quantum paraelectric SrTiO ₃ layers adjacent to them. T _c was tuned by ∼500 kelvin by varying the thicknesses of the BaTiO ₃ and SrTiO ₃ layers, revealing the essential roles of electrical and mechanical boundary conditions for nanoscale ferroelectricity.

Ab initio calculations of the spontaneous polarization and piezoelectric properties of boron nitride nanotubes show that they are excellent piezoelectric systems with response values larger than those of piezoelectric polymers. The intrinsic chiral symmetry of the nanotubes induces an exact cancellation of the total spontaneous polarization in ideal, isolated nanotubes of arbitrary indices. Breaking of this symmetry by intertube interaction or elastic deformations induces spontaneous polarization comparable to those of wurtzite semiconductors.

A combined experimental and computational investigation of coupling between polarization and epitaxial strain in highly polar ferroelectric ${\mathrm{PbZr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_3$ (PZT) thin films is reported. A comparison of the properties of relaxed (tetragonality $c/a\approx 1.05$) and highly strained ($c/a\approx 1.09$) epitaxial films shows that polarization, while being amongst the highest reported for PZT or ${\mathrm{PbTiO}}_3$ in either film or bulk forms $P_r\approx 82\mu \mathrm{C}/{\mathrm{cm}}^2$), is almost independent of the epitaxial strain. We attribute this behavior to a suppressed sensitivity of the $A$-site cations to epitaxial strain in these Pb-based perovskites, where the ferroelectric displacements are already large, contrary to the case of less polar perovskites, such as ${\mathrm{BaTiO}}_3$. In the latter case, the $A$-site cation (Ba) and equatorial oxygen displacements can lead to substantial polarization increases.

Highly piezoelectric and pyroelectric phases of boron-nitrogen-based polymers have been designed from first principles. They offer excellent electrical and structural properties, with up to 100% improvement in the piezoelectic response and an enhanced thermal stability with respect to polyvinylidene fluoride (PVDF). Since methods for their synthesis are readily available, these polymers are extremely promising for numerous technological applications, rivaling the properties of ferroelectric ceramics and superseding PVDF-based materials in high-performance devices.

Composition-dependent structural and polar properties of epitaxial short-period CaTiO3∕-SrTiO3∕BaTiO3 superlattices grown on a SrTiO3 substrate are investigated with first-principles density-functional theory computational techniques. Polarization enhancement with respect to bulk tetragonal BaTiO3 is found for two- and three-component superlattices with a BaTiO3 concentration of more than 30%. Individual BaTiO3 layer thickness is identified as an important factor governing the polarization improvement. In addition, the degree of inversion-symmetry breaking in three-component superlattices can be controlled by varying the thicknesses of the component layers. The flexibility allowed within this large family of structures makes them highly suitable for various applications in modern nanoelectromechanical devices.