Alkyl Phosphonic Acids Deliver CsPbBr₃ Nanocrystals with High Photoluminescence Quantum Yield and Truncated Octahedron Shape

Abstract

We devised a colloidal approach for the synthesis of CsPbBr 3 nanocrystals (NCs) in which the only ligands employed are alkyl phosphonic acids. Compared to more traditional syntheses of CsPbBr 3 NCs, the present scheme delivers NCs with the following distinctive features: (i) The NCs do not have cubic but truncated octahedron shape enclosed by Pb-terminated facets. This is a consequence of the strong binding affinity of the phosphonate groups toward Pb 2+ ions. (ii) The NCs have near unity photoluminescence quantum yields (PLQYs), with no need of postsynthesis treatments, indicating that alkyl phosphonic acids are effectively preventing the formation of surface traps. (iii) Unlike NCs coated with alkylammonium or carboxylate ligands, the PLQY of phosphonate coated NCs remains constant upon dilution, suggesting that the ligands are tightly bound to the surface.

Stable and Size Tunable CsPbBr3 Nanocrystals Synthesized with Oleylphosphonic Acid

“…Various studies have demonstrated that a simple variation of the local pH, which can cause protonation/deprotonation of carboxylate/oleylammonium ions, or even a dilution of the NC suspension, which decreases the chemical potential of the free ligands with respect to that of bound ones, can lead to ligands detachment, which is deleterious for both colloidal stability and PL. 9 , 12 Consequently, these types of ligands are weakly bound to the NCs surface, and a highly dynamic equilibrium exists between free and bound ligands. 9 , 13 − 15 In order to optimize the surface passivation of LHP NCs, various alternative surfactants have been investigated, with zwitterionic molecules, quaternary alkyl ammonium ions, and sulfonic and alkyl phosphonic acids being the most promising ones.…”

“… 9 , 13 − 15 In order to optimize the surface passivation of LHP NCs, various alternative surfactants have been investigated, with zwitterionic molecules, quaternary alkyl ammonium ions, and sulfonic and alkyl phosphonic acids being the most promising ones. 3 , 9 , 12 , 16 , 17 The improved stability in these cases arises from the fact that either they cannot be deprotonated (zwitterionic molecules, quaternary alkylammonium ions), or they have a strong affinity toward surface Pb 2+ ions (sulfonic and phosphonic acids). As a result, all these ligands yield LHP NCs with higher stability and near-unity PL quantum yield (QY).…”

Lead Halide Perovskite Nanocrystals: Room Temperature Syntheses toward Commercial Viability

“…Further interesting binding chemistry with phosphonic acids was reported by Zhang et al [76] They discovered that the conditions of the hot-injection method drove a condensation reaction, forming phosphonic acid anhydrides from phosphonic acids. These phosphonic acid anhydride species could then form bidentate bonds to the CsPbBr 3 NC surface (Figure 11a), presumably because the distance between adjacent hydroxyl groups on the anhydride was similar to the spacing between lead sites of the NC.…”

“…Typically, short esters have been most successful. Depending on the ligand system employed, butyl acetate, [44] ethyl acetate, [35,41,43,[71][72][73][74][75][76][77] mixtures of ethyl and methyl acetate, [78,79] and methyl acetate alone, [67,[80][81][82][83] have been employed. Figure 6 shows a screening study of 36 potential antisolvents, it concurred on the efficacy of short esters, but indicated that diethylene glycol dimethyl ether (diglyme) was the most effective.…”

“…The use of phosphonic acid ligands for CsPbBr 3 NC passivation: a) phosphonic acids and phosphonic acid anhydrides facilitated near-unity PLQY, absorbance, and PL spectra are shown with HRTEM image in the inset and TEM image in background (adapted with permission. [76] Copyright 2019, American Chemical Society), and b) the proposed interligand hydrogen bonding motif formed between adjacent octylphosphonate species on the NC surface (adapted with permission. [73] Copyright 2019, Royal Society of Chemistry).…”