Robust Ferroelectricity in Chalcogenides Down to 2 nm Particles

ABSTRACT

Nano‐ferroelectric materials are essential for advancing miniaturized energy converters, optoelectronics, and nonvolatile memory devices. However, the depolarizing fields of size‐reduced nanoparticles will weaken or even eliminate the ferroelectric spontaneous polarizations. Here, we report that nano‐ferroelectric SnSe has been induced by the local structure, with the polar AA stacking order triggering robust ferroelectricity down to 2 nm particles. To date, it is the smallest size of ferroelectric material among previously reported nanoparticles. The ferroelectricity was verified by scanning transmission electron microscopy, second‐harmonic generation, and piezoelectric force microscopy. Further local structural investigations employing atomic pair distribution functions (PDF) and extended X‐ray absorption fine structure (EXAFS) have demonstrated that the non‐ferroelectric SnSe experiences a compressive strain of 8.5% at 2 nm. This strain induces a structural transformation from an AB‐stacked Pnma phase to an AA‐stacked Pmn2 ₁ configuration. Density functional theory calculations confirm that the polar Pmn2 ₁ structure provides a large ferroelectric polarization. Our work first breaks the critical size limit in nanoparticles, provides a structural manipulation strategy to discover ferroelectrics and improve ferroelectric polarization, and offers promising ferroelectric materials for novel non‐volatile memory devices and optoelectronic applications.