Surface Chemistry of Lead Halide Perovskite Colloidal Nanocrystals

Abstract

ConspectusThe surface chemistry of lead halide perovskite nanocrystals (NCs) plays a major role in dictating their colloidal and structural stability as well as governing their optical properties. A deep understanding of the nature of the ligand shell, ligand-NC, and ligand-solvent interactions is therefore of utmost importance. Our recent studies have revealed that such knowledge can be harnessed following a multidisciplinary approach comprising chemical, structural, and spectroscopic analyses coupled with atomistic modeling. We show that specific surface terminations can be produced only by employing flexible and versatile syntheses that enable to work under desired conditions. In this Account, we first describe our studies aimed at synthesizing CsPbBr₃ NCs with various surface terminations. These include CsPbBr₃ NCs prepared under Br- and oleylamine-rich conditions, which feature a ligand shell composed of alkylammonium-Br species and a photoluminescence quantum yield (PLQY) of ∼90%. On the other hand, taking advantage of the inability of secondary amines to bind to the perovskite NCs surface, we could prepare cuboidal CsPbBr₃ NCs bearing a Cs-oleate surface termination and a PLQY of 70% by employing oleic acid and secondary alkylamines. In the quest to identify ligands that can bind more strongly than oleates or primary alkylammonium ions to the surface of NCs already in the synthesis step, we used phosphonic acids as the sole ligands in the CsPbBr₃ NCs synthesis, which yielded NCs with a truncated octahedron shape, high PLQY (∼100%), and a PbBr₂-terminated surface passivated by hydrogen phosphonates and phosphonic acid anhydride. The surface chemistry and the stability of perovskite NCs were investigated via ad-hoc postsynthesis treatments. We found, for example, that reacting oleylammonium-Br-terminated NCs with stoichiometric amounts of neutral primary alkylamines (or their conjugated acids) led to a partial replacement of oleylammonium ions with new alkylammonium ions (following a deprotonation/protonation mechanism), which resulted in a boost of the PLQY (up to 100%) and of the NCs' colloidal stability. Similar results in terms of optical properties were achieved by treating Cs-oleate-terminated NCs with alkylammonium-carboxylate or quaternary ammonium-Br (namely, didodecyldimethylammonium-Br, DDA-Br) couples. Interestingly, when the native NCs are ligand exchanged with DDA-Br, the ligand shell is then composed of species not bearing any proton. This, in turn, enabled us to study the interaction of such NCs with a variety of ligands under completely aprotic conditions wherein these DDA-Br-capped NCs were basically inert. The only exceptions were carboxylic, phosphonic, and sulfonic acids that were capable of stripping surface DDA-Br couples. As a note, most studies on CsPbBr₃ NCs to date have focused primarily on choosing ligands with specific anchoring groups rather than on tuning the length and type of alkyl chains, as this is time-consuming and requires a large number of syntheses. Our recent developments in the computational chemistry of colloidal NCs are expected to provide a pivotal role in this direction since they can be integrated with machine learning models to investigate with greater details the ligand-NC, ligand-ligand, and ligand-solvent interactions and ultimately find optimal candidates through the prediction of surfactant properties using high-throughput data sets.

Figure 1

Schematic representation of a) the LHP crystal structure; b) typical defects present in the LHP lattice; c) LHP electronic structure at the band edges; d) variation of the highest occupied molecular orbital (HOMO) composition as a function of extent of removal of surface-bound ion couples. b,c) Reproduced with permission from ref (8). Copyright 2018 Springer Nature. d) Adapted with permission from ref (1). Copyright 2018 American Chemical Society.

Scheme 1. Schematic Representation of a CsPbBr₃ NC and Its Surface Termination in Which Ligands Occupy Lattice Sites

Figure 2

Schematic representation of a) the classical hot-injection approach and c) the one relying on benzoyl-bromide. b) Range of products that can be observed with the classical approach when varying the relative amounts of Olam and OA and the reaction temperature. Reproduced with permission from ref (21). Copyright 2018 American Chemical Society. d) TEM picture and optical properties of CsPbBr₃ NCs synthesized with the benzoyl-halide approach under Br- and Olam-rich conditions. Reproduced with permission from ref (2). Copyright 2018 American Chemical Society.

Figure 3

a) Synthesis scheme of CsPbBr₃ NCs employing secondary alkylamines. b) TEM images of CsPbBr₃ NCs prepared by employing either dihexylamine (DHA) or dioctadecylamine (DOA). Side and planar views of the binding of c) primary and d) secondary amines in the CsPbBr₃ lattice. The structural relaxation was carried out at the DFT level of theory. In the case of secondary amines, the anchoring groups are closer to each other than those in primary amines, indicating a larger steric hindrance. Adapted with permission from ref (20). Copyright 2018 American Chemical Society.

Figure 4

a) High-resolution TEM image of a PA-based CsPbBr₃ NC showing a truncated octahedron shape. b) Absorbance and PL emission of CsPbBr₃ NCs of different sizes made with oleylphosphonic acid. c) PL intensity variation as a function of PA-based NCs concentration in toluene. d) Relaxed CsPbBr₃ NC model of 3.0 nm diameter computed at the DFT level of theory in which methylphosphonate (MPA) ligands have been visually excluded (left) or included (right). (e) Projected density of states (PDOS) on each atom and ligand type of the NC model. A delocalized and trap-free molecular orbital plot of the HOMO state is also shown. f) ³¹P NMR spectrum of PA-based LHP NCs in toluene. g) Schematic representation of the surface termination of PA-based NCs. a,c–e) Reprinted with permission from ref (3). Copyright 2019 American Chemical Society. b,f) Reprinted with permission from ref (32). Copyright 2020 American Chemical Society.

Figure 5

a) Schematic representation of the ligand exchange between oleylammonium-Br CsPbBr₃ NCs and octylamine. b) Absorption and c) PL emission variation upon the addition of stoichiometric amounts of octylamine. d) Schematic representation of the ligand exchange between Cs-oleate CsPbBr₃ NCs and octylammonium-carboxylates salts. e) Absorption and PL emission variation upon the addition of different salts. Adapted with permission from ref (18). Copyright 2019 American Chemical Society.

Figure 6

Ligand exchange between Cs-oleate CsPbBr₃ NCs and DDA-Br: a) ¹H NMR spectra before and after the exchange; b) absorption and PL emission of starting and product NCs; c) TEM picture of DDA-Br capped NCs; d) schematic representation of the resulting surface termination; e) stability of the nanocrystals upon storage in air; f) relaxed structure of a 2.4 nm CsPbBr₃ NC model passivated with tetramethylammonium-bromide computed at the DFT level of theory; and g) PDOS on each atom and ligand type. The band gap is free of traps. Adapted with permission from ref (4). Copyright 2019 American Chemical Society.

Scheme 2. Exposure of DDA-Br NCs to Exogenous Molecules under Completely Aprotic Conditions