Preparation of cesium oleate solution. Cesium oleate was prepared following a previously reported method. 37 0.768 g (4 mmol) of cesium acetate were loaded in a 50 ml three-neck flask together with 7.56 ml of OA and 18.72 ml of ODE. The solution was degassed through 3 cycles under argon and vacuum and heated at 110 °C for 30 minutes under argon. Afterwards, the solution was left under vacuum for 3 hours at 110 C. The as-prepared solution was stored under nitrogen. Ti 1-x Sn x X 6 NCs (X = I, Br). Synthesis of Cs 2 SnX 6 . Cs 2 SnX 6 (X = I, Br) NCs were prepared following our previously published procedure. For the synthesis of Cs 2 SnI 6 NCs, 112.8 mg of Sn(IV) acetate (0.31 mmol) and 60 mg of DMOP were weighed in the glovebox and loaded in a 50 ml three-neck flask together with 4 ml of the cesium oleate solution (0.62 mmol), 2 ml of ODE and 2 ml of OA. The solution was degassed through 3 cycles of argon and vacuum and heated at 110 C for 35 minutes under vacuum. At this point, 273µL of TMSI (1.86 mmol) was injected under argon at 140 C and the reaction was carried out for 20 seconds and then quenched by diluting the solution in 10 ml of toluene. The synthesis of Cs 2 SnBr 6 NCs was carried out following the above procedure, except that the Cs:Sn:X precursors ratio used was 2:2:9 instead of 2:1:6, which means that 225.6 mg (0.62 mmol) of Sn acetate and 380 µL (2.8 mmol) of TMSBr were used in the synthesis.