(color online) In-situ STEM imaging of Pt₃Co nanocrystals under oxygen environment and elevated temperature. (a) STEM-HAADF image of a Pt₃Co nanocrystal after annealing and (b) the corresponding intensity line profiles. (c) False-colored STEM-BF image showing the formation of CoO layers on the Pt₃Co {111} surfaces and the corresponding schematic of Co atom arrangement along the CoO zone axis (inset). ((d)–(g)) Sequential STEM images showing the diffusion and formation of a Pt atomic layer on the {100} surface of an oxidized Pt₃Co particle.^[70]