Colloidal perovskite nanoplatelets are a promising class of semiconductor nanomaterials-exhibiting bright luminescence, tunable and spectrally narrow absorption and emission features, strongly confined excitonic states, and facile colloidal synthesis. Here, we demonstrate the high degree of spectral tunability achievable through variation of the cation, metal, and halide composition as well as nanoplatelet thickness. We synthesize nanoplatelets of the form L2[ABX3]n-1BX4, where L is an organic ligand (octylammonium, butylammonium), A is a monovalent metal or organic molecular cation (cesium, methylammonium, formamidinium), B is a divalent metal cation (lead, tin), X is a halide anion (chloride, bromide, iodide), and n-1 is the number of unit cells in thickness. We show that variation of n, B, and X leads to large changes in the absorption and emission energy, while variation of the A cation leads to only subtle changes but can significantly impact the nanoplatelet stability and photoluminescence quantum yield (with values over 20%). Furthermore, mixed halide nanoplatelets exhibit continuous spectral tunability over a 1.5 eV spectral range, from 2.2 to 3.7 eV. The nanoplatelets have relatively large lateral dimensions (100 nm to 1 μm), which promote self-assembly into stacked superlattice structures-the periodicity of which can be adjusted based on the nanoplatelet surface ligand length. These results demonstrate the versatility of colloidal perovskite nanoplatelets as a material platform, with tunability extending from the deep-UV, across the visible, into the near-IR. In particular, the tin-containing nanoplatelets represent a significant addition to the small but increasingly important family of lead- and cadmium-free colloidal semiconductors.

Despite their technological importance, lead sulfide (PbS) nanocrystals have lagged behind nanocrystals of cadmium selenide (CdSe) and lead selenide (PbSe) in terms of size and energy homogeneity. Here, we show that the ratio of lead to sulfur precursor available during nucleation is a critical parameter affecting subsequent growth and monodispersity of PbS nanocrystal ensembles. Applying this knowledge, we synthesize highly monodisperse (size dispersity <5%) PbS nanocrystals over a wide range of sizes (exciton energies from 0.70 to 1.25 eV, or 1000-1800 nm) without the use of size-selective precipitations. This degree of monodispersity results in absorption peak half width at half max (HWHM) values as small as 20 meV, indicating an ensemble that is close to the homogeneous limit. Photoluminescence emission is correspondingly narrow and exhibits small Stokes shifts and quantum efficiencies of 30-60%. The nanocrystals readily self-assemble into ordered superlattices and exhibit exceptional air stability over several months.

On solvent evaporation, non-interacting monodisperse colloidal particles self-assemble into a close-packed superlattice. Although the initial and final states can be readily characterized, little is known about the dynamic transformation from colloid to superlattice. Here, by using in situ grazing-incidence X-ray scattering, we tracked the self-assembly of lead sulfide nanocrystals in real time. Following the first appearance of an ordered arrangement, the superlattice underwent uniaxial contraction and collective rotation as it approached its final body-centred cubic structure. The nanocrystals became crystallographically aligned early in the overall self-assembly process, showing that nanocrystal ordering occurs on a faster timescale than superlattice densification. Our findings demonstrate that synchrotron X-ray scattering is a viable method for studying self-assembly in its native environment, with ample time resolution to extract kinetic rates and observe intermediate configurations. The method could be used for real-time direction of self-assembly processes and to better understand the forces governing self-organization of soft materials.

Metal halide perovskites are a family of semiconductor materials with exciting properties such as long charge carrier diffusion lengths, ease of synthesis and composition tunability, and remarkable defect tolerance. Recently, methods have been developed to synthesize metal halide perovskites in the form of colloidal nanosheetsor nanoplateletswhich are only a few unit cells in thickness and, as a result, experience the effects of strong dielectric and quantum confinement. This leads to narrow and blue-shifted absorption/emission as compared to the bulk stateallowing lead bromide and lead iodide nanoplatelets to cover the entire visible range. In contrast to bulk crystals, nanoplatelets exhibit strongly excitonic properties and enhanced radiative recombination. In this article we present an overview of colloidal perovskite nanoplatelets: how they are made, what their capabilities are, why 2D is beneficial, and where these materials are headed. We draw analogues to solid phase layered perovskites, cadmium selenide nanoplatelets, and 2D transition metal dichalcogenides to emphasize some of the most promising attributes of 2D materials such as their penchant for directional emission, fast/directional energy transfer, strong exciton binding energy, and reduced dielectric screening effects. We discuss the interesting physics present in these materials, remaining stability issues, and the future applications for nanoplatelets in LEDs, photovoltaics, photodetectors, and lasers.

Colloidal quantum dots (QDs) are promising materials for use in solar cells, light-emitting diodes, lasers, and photodetectors, but the mechanism and length of exciton transport in QD materials is not well understood. We use time-resolved optical microscopy to spatially visualize exciton transport in CdSe/ZnCdS core/shell QD assemblies. We find that the exciton diffusion length, which exceeds 30 nm in some cases, can be tuned by adjusting the inorganic shell thickness and organic ligand length, offering a powerful strategy for controlling exciton movement. Moreover, we show experimentally and through kinetic Monte Carlo simulations that exciton diffusion in QD solids does not occur by a random-walk process; instead, energetic disorder within the inhomogeneously broadened ensemble causes the exciton diffusivity to decrease over time. These findings reveal new insights into exciton dynamics in disordered systems and demonstrate the flexibility of QD materials for photonic and optoelectronic applications.

Using liquid-liquid interfacial assembly, we control the deposition of CdSe nanoplatelets into face-down or edge-up configurations. Controlled assembly, combined with back focal plane imaging, enabled unambiguous determination of the transition dipole orientation. The transition dipole moment of the emissive band-edge exciton in CdSe nanoplatelets was found to be isotropically oriented within the plane of the nanoplatelet with no measurable out-of-plane component and no preference for the long- or short-axis of the nanoplatelet. Importantly, CdSe nanoplatelet films in the face-down configuration exhibited unity dipole orientation within the plane of the film, which could improve the external efficiency of nanoplatelet LEDs, lasers, photodetectors, and photovoltaic cells beyond that which is possible with isotropic emitters. We also show that the two self-assembled configurations have different Förster energy transfer rates, as a result of different dipole orientation and internanoplatelet distance.

Controlling the interparticle spacing in quantum dot (QD) thin films is the most readily accessible way to control transport rates between neighboring QDs and a critical component of device optimization. Here, we use X-ray scattering measurements to accurately measure the interparticle spacing in films of highly monodisperse lead sulfide (PbS) QDs that have undergone a variety of device-relevant ligand exchanges. We tabulate these values for use in simulations and data analysis. We find that monothiol and dithiol ligand species typically result in interparticle spacing values that are equal to the length of a single monothiol or dithiol ligand. Additionally, we find that spin-coating a thick film of QDs followed by a long-duration ligand exchange results in a more complete ligand exchange than spin-coating many thin layers with short-duration ligand exchanges in between. The former method also preserves a remarkable degree of the long-range ordering that was present in the film prior to ligand exchange. These results shed light on ways to produce highly-ordered QD solids with compact and functional ligands, which could lead to enhanced interdot coupling and transport phenomena.

Organic−inorganic perovskites have been shown to have excellent optoelectronic properties. Further, layered perovskites have been demonstrated, utilizing quantum confinement to achieve emission blueshifted from the bulk band gap. Here, we tune this blue-shift to build LEDs that span the visible spectrum. We demonstrate that electroluminescence from red-shifted layers dominates emission from mixed-thickness devices and that the addition of excess ligand is necessary to drive emission toward blue-shifted layers. By tuning the thickness of the layers, we build LEDs with blue emission utilizing the lead bromide system and orange emission utilizing the lead iodide system. Finally, we demonstrate that these materials suffer reversible degradation under an applied electric field. The spectrally narrow emission, combined with the favorable electronic properties of perovskite materials and access to shorter emission wavelengths through quantum confinement, demonstrates the promise of these materials as a new platform for low-cost, high-performance light-emitting devices.