We have investigated the stability of nano-amorphous region of Ge2Sb2Te5 (GST), fabricated by Electron Beam Lithography (EBL), dry etching, and ion implantation. Nano-structures, less than 100 nm in diameter and 20 nm thick, were either embedded in a crystalline environment or just isolated. We have observed nano-structure crystallization by in situ Transmission Electron Microscopy (TEM) in the 75°C-150°C temperature range. Re-crystallization of amorphous dots embedded in a crystalline region (either in the cubic or hexagonal phase) occurs by the movement of the interface at relatively low temperature (about 90°C). Instead, in the isolated structures the transition occurs at about 145°C by nucleation and growth. These results might be of relevance for the data retention of sub-50nm devices. Indeed, the more stable amorphous phase in self-standing regions indicates the better retention properties of isolated cells with respect to the traditional mushroom cell configuration.