High thermal robustness of molecularly thin perovskite nanosheets and implications for superior dielectric properties

A systematic study has been conducted to examine the thermal stability of layer-by-layer assembled films of perovskite-type nanosheets, (Ca ₂Nb₃O₁₀ ^-)_n (n = 1-10), which exhibit superior dielectric and insulating properties. In-plane and out-of-plane X-ray diffraction data as well as observations by atomic force microscopy and transmission electron microscopy indicated the high thermal robustness of the nanosheet films. In a monolayer film with an extremely small thickness of ∼2 nm, the nanosheet was stable up to 800°C, the temperature above which segregation into CaNb₂O₆ and Ca₂Nb₂O₇ began. The critical temperature moderately decreased as the film thickness, or the number of nanosheet layers, increased, and reached 700°C for seven- and 10-layer films, which is comparable to the phase transformation temperature for a bulk phase of the protonic layered oxide of HCa₂Nb₃O₁₀·1. 5H₂O as a precursor of the nanosheet. This thermal stabilization of perovskite-type nanosheets should be associated with restricted nucleation and crystal growth peculiar to such ultrathin 2D bound systems. The stable high-k dielectric response (ε_r = 210) and highly insulating nature (J < 10^-7 A cm^-2) remained substantially unchanged even after the nanosheet film was annealed up to 600°C. This study demonstrates the high thermal stability of 2D perovskite-type niobate nanosheets in terms of structure and dielectric properties, which suggests promising potential for future high-k devices operable over a wide temperature range.