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, (Ca2Nb3O10-)(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 similar to 2 nm, the nanosheet was stable up to 800 degrees 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 degrees 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 center dot 1.5H(2)O 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 (epsilon(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 degrees 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.