Surface Science and Colloidal Stability of Double-Perovskite Cs₂AgBiBr₆ Nanocrystals and Their Superlattices

Abstract

Capping ligand bonding and the thermal and colloidal stability of Cs 2 AgBiBr 6 nanocrystals were studied. Oleylamine and oleic acid bonding to Cs 2 AgBiBr 6 nanocrystals was studied with 1 H nuclear magnetic resonance and nuclear Overhauser effect spectroscopy. Both molecules are present in the ionic metathesis synthesis reaction, but only oleylamine remains bound to the nanocrystals after purification. Oleic acid is not a capping ligand; however, the synthesis requires it, and its concentration determines the yield of the reaction while oleylamine primarily affects the uniformity of the sample. Substitution of oleic acid in the reaction with diisooctylphosphinic acid still yielded nanocrystals with similar size, cuboidal shape, uniformity, cubic double-perovskite crystal structure, and optical properties. Nanocrystals were assembled into superlattices and heated in air. Grazing incidence smallangle and wide-angle X-ray scattering showed that the nanocrystals sinter at 250 °C, but the crystal structure and preferred crystal orientation on the substrate does not change. When nanocrystals were dispersed in hexane and exposed to light, they precipitated within 24 h. Scanning electron microscopy showed that the aggregated nanocrystals still retained their initial size and shape and had not coalesced.

Colloidal Bi-Doped Cs2Ag1–xNaxInCl6 Nanocrystals: Undercoordinated Surface Cl Ions Limit their Light Emission Efficiency

“…These findings, which are substantially different from what observed in Cs 2 AgBiBr 6 NCs (where akylammonium ions only have been detected), 14 point to a complex ligand shell in which ligands are homogeneously intermixed on the surface of perovskite NCs.…”

“… 33 − 36 While our knowledge of the surface passivation of Pb-based perovskite NCs is reaching maturity, we know much less on the ligand shell of double perovskite NCs. 14 , 37 − 41 To date, only the ligand shell of Cs 2 AgBiBr 6 NCs has been examined by Zhang et al, who concluded that the only species passivating the surface are alkylammonium ions. 14 …”

“… 14 , 37 − 41 To date, only the ligand shell of Cs 2 AgBiBr 6 NCs has been examined by Zhang et al, who concluded that the only species passivating the surface are alkylammonium ions. 14 …”

Synthesis of TlBr and Tl2AgBr3 Nanocrystals

“…This capping ligand layer thickness is slightly smaller than the 1.5–1.8 nm length of a buckled C 17 hydrocarbon chain . Ligand layers with similar thickness have also been reported for oleylamine and oleic acid‐capped double perovskite nanocrystals in superlattices . The GIWAXS pattern in Figure D is indexed to TlBr with CsCl crystal structure.…”

“…When the reaction temperature exceeded 140 °C, additional TlBr nanocrystals also formed (see Supporting Information Figure S3). The need for short reaction times is similar to other metal halides, such as CsPbI 3 or Cs 2 AgBiBr 6 . Perhaps rapid quenching of nanocrystal growth reactions is more generally necessary for perovskite and related metal halide nanocrystals with tertiary or quaternary composition than binary compounds .…”