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We report a colloidal synthesis approach to CsPbBr₃ nanoplatelets (NPLs). The nucleation and growth of the platelets, which takes place at room temperature, is triggered by the injection of acetone in a mixture of precursors that would remain unreactive otherwise. The low growth temperature enables the control of the plate thickness, which can be precisely tuned from 3 to 5 monolayers. The strong two-dimensional confinement of the carriers at such small vertical sizes is responsible for a narrow PL, strong excitonic absorption, and a blue shift of the optical band gap by more than 0.47 eV compared to that of bulk CsPbBr₃. We also show that the composition of the NPLs can be varied all the way to CsPbBr₃ or CsPbI₃ by anion exchange, with preservation of the size and shape of the starting particles. The blue fluorescent CsPbCl₃ NPLs represent a new member of the scarcely populated group of blue-emitting …