We report a size dependence of switchable polarization in PbTiO3 nanoislands with heights of 2–7 nm and widths of 40–80 nm. An atomic force microscopy (AFM) system that was capable of probing a switching charge from a single PbTiO3 nanoisland using a conductive AFM tip as a top electrode has been developed. Using the AFM system, the switching charge as small as 10 fC was detected within a tolerance of ±4 fC. The switchable polarization (ΔP) over 150 µC/cm2 was observed for the nanoislands as small as 5 nm in height, and ΔP rapidly decreased with the height below 5 nm. Comparing the height or thickness dependence of ΔP and stable domain states between the nanoislands and ultrathin films, it was found that the polarization in the nanoislands can be switched by external fields more easily than that in the ultrathin films. The decrease in ΔP of the nanoislands with the height below 5 nm can be attributed to the intrinsic size effects.

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