Long-chain saturated hydrocarbons and alkoxysilanes are ligands that are commonly used to passivate perovskite quantum dots (PQDs) to enhance their stability and optical properties. However, the insulating nature of these capping ligands creates an electronic energy barrier and impedes interparticle electronic coupling, thereby limiting device applications. One strategy to solve this problem is the use of short conductive aromatic ligands that allow delocalization of the electronic wave function from the PQDs, which, in turn, facilitates charge transport between PQDs by lowering the energy barrier. This is demonstrated with methylammonium lead bromide (MAPbBr 3 ) QDs prepared using benzylamine (BZA) and benzoic acid (BA) capping ligands. Optimized BZA-BA-MAPbBr 3 QDs are highly stable and show very high photoluminescence (PL) quantum yield (QY) (86%). More importantly, the BZA-BA-MAPbBr 3 QD film exhibits higher conductivity and carrier lifetime and more efficient charge extraction compared to PQDs with insulating ligands, as indicated by electrochemical measurements and transient photocurrent and photovoltage spectroscopy.

Magic sized clusters (MSCs), small nanoparticles with a single size or narrow size distribution, possess unique chemical and physical properties. CH3NH3PbBr3 perovskite MSCs (PMSCs) were synthesized using butylamine (BTYA) and valeric acid (VA) as well as 3,3-diphenylpropylamine (DPPA) and VA as capping ligands in an excess amount. The PMSCs were characterized using UV–vis and photoluminescence (PL) spectroscopy. For BTYA-VA PMSCs, there is an excitonic absorption peak at 424 nm and PL peak at 434 nm, while the DPPA-VA PMSCs exhibit an absorption peak at 428 nm and PL peak at 451 nm. The size and optical properties of the PMSCs are found to be dependent on the ligands used for passivation. Additionally, in a series dilution of DPPA-VA PMSCs in toluene, it was discovered that different sized perovskite quantum dots (PQDs) can be synthesized with tunable PL emission with peaks ranging between 451 and 525 nm. This phenomenon was not observed for BTYA-VA PMSCs. This contrast indicates the unique interaction between the DPPA ligand and the toluene solvent for the dilution-induced PQD formation. These findings provide new insights into the mechanism of growth and stability of PMSCs that are useful for generating and understanding the formation of larger nanostructures such as QDs.

Recent progress has been made on the synthesis and characterization of metal halide perovskite magic-sized clusters (PMSCs) with ABX3 composition (A=CH3NH3+ or Cs+, B=Pb2+, and X=Cl−, Br-, or I-). However, their mechanism of growth and structure is still not well understood. In our effort to understand their structure and growth, we discovered that a new species can be formed without the CH3NH3+ component, which we name as molecular clusters (MCs). Specifically, CH3NH3PbBr3 PMSCs, with a characteristic absorption peak at 424 nm, are synthesized using PbBr2 and CH3NH3Br as precursors and butylamine (BTYA) and valeric acid (VA) as ligands, while MCs, with an absorption peak at 402 nm, are synthesized using solely PbBr2 and BTYA, without CH3NH3Br. Interestingly, PMSCs are converted spontaneously overtime into MCs. An isosbestic point in their electronic absorption spectra indicates a direct interplay between the PMSCs and MCs. Therefore, we suggest that the MCs are precursors to the PMSCs. From spectroscopic and extended X-ray absorption fine structure (EXAFS) results, we propose some tentative structural models for the MCs. The discovery of the MCs is critical to understanding the growth of PMSCs as well as larger perovskite quantum dots (PQDs) or nanocrystals (PNCs).

Metal halide perovskites, such as methylammonium lead halide (CH 3 NH 3 PbBr 3 ), have recently attracted tremendous attention due to their outstanding properties and promising applications. Here, CH 3 NH 3 PbBr 3 perovskite magic-sized clusters (PMSCs) and perovskite quantum dots (PQDs) are synthesized by surface passivation with various organic acids and amines through a ligand-assisted reprecipitation process (LARP). Their optical properties are investigated with ultraviolet−visible (UV−vis) absorption and photoluminescence (PL) spectroscopy. The structures and optical properties of the PMSCs and PQDs can be controlled by varying the ligand/precursor ratio and type as well as the concentration of ligands. Specifically, a high ligand/precursor ratio and excessive amines favor PMSCs, whereas excessive acids lead to the generation of PQDs over PMSCs. As a result, we can tune between PQDs and PMSCs by changing the amine/acid ligand ratio. Energy-dispersive X-ray (EDS) elemental mapping confirms the presence of the constituent elements of the perovskite, namely, Pb and Br. With the help of Fourier transform infrared (FTIR) results, a model is proposed to explain the mechanism of formation of both PQDs and PMSCs as well as their interconversion in terms of the ligands passivating the surface defect sites. This study provides important insights into the fundamental growth mechanisms of perovskite nanostructures, especially in terms of the interplay between the perovskite precursors and passivating ligands. The work has significant implications in applications of metal halide perovskites in photonics and energy conversion.

In this work, we report a polar-solvent-free, low-cost, and environmentally friendly method for synthesizing highly stable all-inorganic CsPbBr 3 nanocrystals (NCs). Their shape and size are controlled by adjusting key parameters in the synthesis, such as capping ligands as well as ultrasound power and radiation time. The CsPbBr 3 NCs are tunable from three-dimensional to two-dimensional and finally to 0-dimensional, by controlling the ratios and types of capping ligands. The CsPbBr 3 NCs exhibit optical properties dependent on their shape and size, with photoluminescence quantum yield as high as 83%. Moreover, the role of nonpolar solvents in improving the stability of the CsPbBr 3 NCs and the mechanism for the tunable growth of CsPbBr 3 NCs are studied and discussed. Controlled shape and size of perovskite NCs are desired for applications, including solar cells, lasers, light-emitting diodes, and solar concentrators.