We report unprecedented phase stability of cubic CsPbBr 3 quantum dots in ambient air obtained by using Br 2 as halide precursor. Mechanistic investigation reveals the decisive role of temperaturecontrolled in situ generated, oleylammonium halide species from molecular halogen and amine for the long term stability and emission tunability of CsPbX 3 (X ¼ Br, I) nanocrystals.High photoluminescence quantum yield (PL QY), narrow emission linewidth, tunable band gap, large diffusion lengths and low exciton binding energies are some of the key attributes of all-inorganic caesium lead halide perovskite nanocrystals (LHP NCs) i.e., CsPbX 3 , X ¼ I, Br, This novel class of NCs has been shown to be highly "defect tolerant", i.e. defect states are either shallow or localized in the valence or the conduction band. 1,2 Unlike conventional semiconductor NCs, the rigorous passivation of their surface via formation of core/shell structures or other methods is not required to achieve high QY. These LHP NCs are promising building blocks for light emitting diode, 3,4 solar cell, 5,6 laser, 7 photocatalysis 8 and detector. 9 Despite the recent surge of studies on CsPbX 3 perovskite NCs, a persisting drawback is their poor phase stability in ambient air. For example, cubic (a) "black" phase CsPbI 3 (E g ¼ 1.73 eV) perovskite NCs undergo rapid phase transformation to nonluminescent orthorhombic (d) "yellow" phase in ambient condition ( Fig. S1 †) leading to undesired changes of the band gap, optical and electrical properties. 6,10,11 Similarly, cubic (a) CsPbBr 3 (E g ¼ 2.25 eV) is unstable at ambient condition. 12 For successful integration of these materials into devices, the issue of long-term phase stability must thus be addressed. 6,13 Most of the reported strategies involve the use of additives such as halide salt, 12 phosphinic acid, 14 ammonium halide, 11 2,2 0 -iminodibenzoic acid 15 sulphides and metal ions 16 and polymers 5 or via special post-synthetic purication step. 6,10 Herein, we report the rst synthesis of highly stable, cubic a-CsPbBr 3 perovskite NCs using Br 2 as an independent halide precursor. In a typical synthesis, lead acetate is dissolved in 1-octadecene in the presence of oleyl amine (OAm) and oleic acid (OA). To this solution, Br 2 (warning: handle the liquid Br 2 in fume hood, Br 2 vapors are toxic) and caesium oleate solutions (both dissolved in ODE) are sequentially added. Phase stability and emission colour tunability are achieved by controlling the reaction temperature (75-200 C) and amount of Br 2 (0.6-1.2 mmol) under air-free synthetic condition (cf. ESI; Experimental section †). The "three-precursor" nature 16-18 of our synthetic scheme allows for independent tuning of the amount of the individual elements viz., Cs + , Pb 2+ and X À ions and in turn, allows for the precise control over the surface chemistry. 17 Highly crystalline, monodisperse 7.62 AE 1.0 nm sized cubic a-CsPbBr 3 NCs (Fig. 1a) were synthesised under optimized conditions using Cs : Pb : Br 2 ratio of 1 : 1 : 6 at 200 C. Highresol...

Aqueous-stable, Cd- and Pb- free colloidal quantum dots with fluorescence properties in the second near-infrared region (NIR-II, 1000-1400) are highly desirable for non-invasive deep-tissue optical imaging and biosensing. The low...

The metal salts of fatty acid (M-FA) are the most widely used metal precursors to colloidal semiconductor nanocrystals (NCs). They play a key role in controlling the composition, shape, and size of semiconductor NCs, and their purity is essential for attaining impeccable batch-to-batch reproducibility in the optical and electrical properties of the NCs. Herein, we report a novel, one-pot synthesis of a library of highly pure M-FAs at near-quantitative yields (up to 91%) using 1,8-diazabicyclo[5.4.0]­undec-7-ene or the related nonionic/noncoordinating base as an inexpensive and ecofriendly catalyst in a green solvent medium. The method is highly general and scalable with vast academic and industrial potential. As a practical application, we also demonstrate the use of these high-quality M-FAs in the synthesis of the spectrum of colloidal semiconductor NCs (III–V, II–VI, IV–VI, I–VI, I–III–VI, and perovskite) having absorption/emission in visible to the near-infrared region.