Washing of the NCs. In the first step of the washing procedure, 5 ml of methylacetate was added as antisolvent to 5 ml of all the NC solutions except for the case of Cs 2 TiI 6 , which does not need the addition of an antisolvent to precipitate during centrifugation and that decomposes into a whitish product in case of addition of it. The solutions were centrifuged at 5800 rpm for 10 minutes. Afterwards, the supernatant was discarded and the precipitate was redispersed in 4 ml of toluene. The solution was centrifuged again at 5800 rpm for 10 minutes and the supernatant again discarded while the precipitate redispersed in 3 ml of toluene. Finally, the solution was centrifuged at 1400 rpm for 4 minutes to remove possible aggregates and the supernatant was filtered using a 0.22 µm PTFE filter. The solutions were stored under inert atmosphere.