Essentially Trap-Free CsPbBr₃ Colloidal Nanocrystals by Postsynthetic Thiocyanate Surface Treatment

Abstract

We demonstrate postsynthetic modification of CsPbBr nanocrystals by a thiocyanate salt treatment. This treatment improves the quantum yield of both freshly synthesized (PLQY ≈ 90%) and aged nanocrystals (PLQY ≈ 70%) to within measurement error (2-3%) of unity, while simultaneously maintaining the shape, size, and colloidal stability. Additionally, the luminescence decay kinetics transform from multiexponential decays typical of nanocrystalline semiconductors with a distribution of trap sites, to a monoexponential decay, typical of single energy level emitters. Thiocyanate only needs to access a limited number of CsPbBr nanocrystal surface sites, likely representing under-coordinated lead atoms on the surface, in order to have this effect.

Surface ligand modification of cesium lead bromide nanocrystals for improved light-emitting performance

“…25 Meanwhile, the loss of the oleylammonium halide surface species generates under-coordinated Pb atoms as electron-trapped surface sites. 26 These issues limit the further application of these NCs.…”

Highly Luminescent Lead Halide Perovskite Quantum Dots in Hierarchical CaF2 Matrices with Enhanced Stability as Phosphors for White Light‐Emitting Diodes

“…[6] Although trichromatic sources could give good CRI values, obtaining the green emissive phosphors with the merits including high photoluminescence quantum yield (PL QY) and low cost is still a challenge. [12][13][14][15][16][17][18][19][20][21] The high quantum efficiency, high color purity, and facile preparation make them as promising candidates for LED or backlight downconverters for LCDs. Recently, cesium lead halide perovskite quantum dots (PQDs) CsPbX 3 (X = Cl, Br, I) with outstanding optical performances are the focus of considerable research efforts.…”

Emission Recovery and Stability Enhancement of Inorganic Perovskite Quantum Dots

“…Such aged PQDs without fluorescence are regarded useless for any optoelectronic applications. In a recent report, 21 Alivisatos and colleagues used thiocyanate salts (NaSCN and NH 4 SCN) to treat the aged CsPbBr 3 PQDs. This treatment improved the PLQY of the aged PQDs from 63 to 100%.…”

“…35,36 However, quite a few publications argued the influence of surface defects on the PL multicomponent decay kinetics of PQDs. 10,14,15 In Alivisatos’s work, 21 the PLQY improvement after treatment was attributed to the removal of excess lead from the surface of CsPbBr 3 PQDs. TOP was also reported to remove the excess Se on the surface of CdSe QDs and made the QDs activated.…”

“…38–42 On the basis of these facts, we rationalize that in our case TOP effectively promoted some ions to migrate on the PQD surface to repair existing surface defects for better emissions. 21 This ion migration should also increase the various stabilities of PQDs when they have perfect surface conditions. Considering the very fast interparticle halogen ion exchanges among CsPbBr 3 and CsPbI 3 PQD particles in solution, 43–45 our assumption for quick ion migration on the particle surface is reasonable.…”