Phase‐Stable CsPbI3 Nanocrystals: The Reaction Temperature Matters

High temperature colloidal synthesis for obtaining thermal, colloidal and phase‐stable CsPbI₃ nanocrystals with near‐unity quantum yield is reported. While standard perovskite synthesis reactions were carried out at 160 °C (below 200 °C), increase of another ≈100 °C enabled the alkylammonium ions to passivate the surface firmly and prevented the nanocrystals from phase transformation. This did not require any inert atmosphere storage, use of heteroatoms, specially designed ligands, or the ice cooling protocol. Either at high temperature in reaction flask or in the crude mixture or purified dispersed solution; these nanocrystals were observed stable and retained the original emission. Different spectroscopic analyses were carried out and details of the surface binding of alkyl ammonium ligands in place of surface Cs in the crystal lattice were investigated. As CsPbI₃ is one of the most demanding optical materials, bringing stability by proper surface functionalization without use of secondary additives would indeed help in wide spreading of their applications.