Magic-sized clusters (MSCs) of semiconductor are typically defined as specific molecular-scale arrangements of atoms that exhibit enhanced stability. They often grow in discrete jumps, creating a series of crystallites, without the appearance of intermediate sizes. However, despite their long history, the mechanism behind their special stability and growth remains poorly understood. It is particularly difficult to explain experiments that have shown discrete evolution of MSCs to larger sizes well beyond the "cluster" regime and into the size range of colloidal quantum dots. Here, we study the growth of MSCs, including these larger magic-sized CdSe nanocrystals, to unravel the underlying growth mechanism. We first introduce a synthetic protocol that yields a series of nine magic-sized nanocrystals of increasing size. By investigating these crystallites, we obtain important clues about the mechanism. We then develop a microscopic model that uses classical nucleation theory to determine kinetic barriers and simulate the growth. We show that magic-sized nanocrystals are consistent with a series of zinc-blende crystallites that grow layer by layer under surface-reaction-limited conditions. They have a tetrahedral shape, which is preserved when a monolayer is added to any of its four identical facets, leading to a series of discrete nanocrystals with special stability. Our analysis also identifies strong similarities with the growth of semiconductor nanoplatelets, which we then exploit to further increase the size range of our magic-sized nanocrystals. Although we focus here on CdSe, these results reveal a fundamental growth mechanism that can provide a different approach to nearly monodisperse nanocrystals.

There has been a tremendous amount of interest in developing highefficiency light-emitting diodes (LEDs) based on colloidal nanocrystals (NCs) of hybrid lead halide perovskites. Here, we systematically investigate the ligand effects on EL characteristics by tuning the hydrophobicity of primary alkylamine ligands used in NC synthesis. By increasing the ligand hydrophobicity, we find (i) a reduced NC size that induces a higher degree of quantum confinement, (ii) a shortened exciton lifetime that increases the photoluminescence quantum yield, (iii) a lowering of refractive index that increases the light outcoupling efficiency, and (iv) an increased thin-film resistivity. Accordingly, ligand engineering allows us to demonstrate high-performance green LEDs exhibiting a maximum external quantum efficiency up to 16.2%. The device operational lifetime, defined by the time lasted when the device luminance reduces to 85% of its initial value, LT85, reaches 243 min at an initial luminance of 516 cd m −2 .

Miniaturized photonic sources based on semiconducting two-dimensional (2D) materials offer new technological opportunities beyond the modern III-V platforms. For example, the quantum-confined 2D electronic structure aligns the exciton transition dipole moment parallel to the surface plane, thereby outcoupling more light to air which gives rise to highefficiency quantum optics and electroluminescent devices. It requires scalable materials and processes to create the decoupled multi-quantum-well superlattices, in which individual 2D material layers are isolated by atomically thin quantum barriers. Here, we report decoupled multi-quantum-well superlattices comprised of the colloidal quantum wells of lead halide perovskites, with unprecedentedly ultrathin quantum barriers that screen interlayer interactions within the range of 6.5 Å. Crystallographic and 2D k-space spectroscopic analysis reveals that the transition dipole moment orientation of bright excitons in the superlattices is predominantly in-plane and independent of stacking layer and quantum barrier thickness, confirming interlayer decoupling.

Colloidal metal halide perovskite (MHP) nanocrystals (NCs) are an emerging class of fluorescent quantum dots (QDs) for next-generation optoelectronics. A great hurdle hindering practical applications, however, is their high lead content, where most attempts addressing the challenge in the literature compromised the material's optical performance or colloidal stability. Here, we present a postsynthetic approach that stabilizes the lead-reduced MHP NCs through high-entropy alloying. Upon doping the NCs with multiple elements in considerably high concentrations, the resulting high-entropy perovskite (HEP) NCs remain to possess excellent colloidal stability and narrowband emission, with even higher photoluminescence (PL) quantum yields, η PL , and shorter fluorescence lifetimes, τ PL . The formation of multiple phases containing mixed interstitial and doping phases is suggested by X-ray crystallography. Importantly, the crystalline phases with higher degrees of lattice expansion and lattice contraction can be stabilized upon high-entropy alloying. We show that the lead content can be approximately reduced by up to 55% upon high-entropy alloying. The findings reported here make one big step closer to the commercialization of perovskite NCs.

Colloidal nanocrystals (NCs) of lead halide perovskites have generated considerable interest in demonstrating their optoelectronic devices, such as light emitting diodes (LEDs), because of their tunable optical bandgap, narrow spectral...

Perovskite quantum dots (QDs) are emerging as one of the most promising candidates for the monochromatic light‐emitting diodes (LEDs) approaching the Rec. 2020 color gamut due to their extremely narrow emission bandwidth. Another important aspect are the high photoluminescence quantum yield (PLQY) values that can be obtained either in solution or thin films making these materials as promising candidates for optoelectronic applications such as LEDs or solar cells. Considerable research efforts in chemistry, chemical engineering, solid‐state physics, and material sciences have been made in the past years. The new opportunity in the field of semiconductor quantum dots is still in the beginning phase and is expected to remain active in the following years. Here, in this invited commentary, we briefly discuss and summarize the opportunities and challenges in both fundamental and technological aspects, based on our work and the recent work in this field.

Magic-sized clusters (MSCs) of semiconductor are typically defined as specific molecular-scale arrangements of atoms that exhibit enhanced stability. They often grow in discrete jumps, creating a series of crystallites, without the appearance of intermediate sizes. However, despite their long history, the mechanism behind their special stability and growth remains poorly understood. This is particularly true considering experiments that have shown discrete evolution of MSCs to sizes well beyond the “cluster” regime and into the size range of colloidal quantum dots. Here, we study the growth of these larger magic-sized CdSe nanocrystals to unravel the underlying growth mechanism. We first introduce a synthetic protocol that yields a series of nine magic-sized nanocrystals of increasing size. By investigating these crystallites, we obtain important clues about the mechanism. We then develop a microscopic model that uses classical nucleation theory to determine kinetic barriers and simulate the growth. We show that magic-sized nanocrystals are consistent with a series of zinc-blende crystallites that grow layer by layer under surface-reaction-limited conditions. They have a tetrahedral shape, which is preserved when a monolayer is added to any of its four identical facets, leading to a series of discrete nanocrystals with special stability. Our analysis also identifies strong similarities with the growth of semiconductor nanoplatelets, which we then exploit to increase further the size range of our magic-sized nanocrystals. Although we focus here on CdSe, these results reveal a fundamental growth mechanism that can provide a different approach to nearly monodisperse nanocrystals.

Magic-sized clusters (MSCs) of semiconductor are typically defined as specific molecular-scale arrangements of atoms that exhibit enhanced stability. They often grow in discrete jumps, creating a series of crystallites, without the appearance of intermediate sizes. However, despite their long history, the mechanism behind their special stability and growth remains poorly understood. This is particularly true considering experiments that have shown discrete evolution of MSCs to sizes well beyond the “cluster” regime and into the size range of colloidal quantum dots. Here, we study the growth of these larger magic-sized CdSe nanocrystals to unravel the underlying growth mechanism. We first introduce a synthetic protocol that yields a series of nine magic-sized nanocrystals of increasing size. By investigating these crystallites, we obtain important clues about the mechanism. We then develop a microscopic model that uses classical nucleation theory to determine kinetic barriers and simulate the growth. We show that magic-sized nanocrystals are consistent with a series of zinc-blende crystallites that grow layer by layer under surface-reaction-limited conditions. They have a tetrahedral shape, which is preserved when a monolayer is added to any of its four identical facets, leading to a series of discrete nanocrystals with special stability. Our analysis also identifies strong similarities with the growth of semiconductor nanoplatelets, which we then exploit to increase further the size range of our magic-sized nanocrystals. Although we focus here on CdSe, these results reveal a fundamental growth mechanism that can provide a different approach to nearly monodisperse nanocrystals.