Maryna I. Bodnarchuk

Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut

Metal halides perovskites, such as hybrid organic–inorganic CH3NH3PbI3, are newcomer optoelectronic materials that have attracted enormous attention as solution-deposited absorbing layers in solar cells with power conversion efficiencies reaching 20%. Herein we demonstrate a new avenue for halide perovskites by designing highly luminescent perovskite-based colloidal quantum dot materials. We have synthesized monodisperse colloidal nanocubes (4–15 nm edge lengths) of fully inorganic cesium lead halide perovskites (CsPbX3, X = Cl, Br, and I or mixed halide systems Cl/Br and Br/I) using inexpensive commercial precursors. Through compositional modulations and quantum size-effects, the bandgap energies and emission spectra are readily tunable over the entire visible spectral region of 410–700 nm. The photoluminescence of CsPbX3 nanocrystals is characterized by narrow emission line-widths of 12–42 nm, wide color gamut covering up to 140% of the NTSC color standard, high quantum yields of up to 90%, and radiative lifetimes in the range of 1–29 ns. The compelling combination of enhanced optical properties and chemical robustness makes CsPbX3 nanocrystals appealing for optoelectronic applications, particularly for blue and green spectral regions (410–530 nm), where typical metal chalcogenide-based quantum dots suffer from photodegradation.

Fast Anion-Exchange in Highly Luminescent Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, I)

Postsynthetic chemical transformations of colloidal nanocrystals, such as ion-exchange reactions, provide an avenue to compositional fine-tuning or to otherwise inaccessible materials and morphologies. While cation-exchange is facile and commonplace, anion-exchange reactions have not received substantial deployment. Here we report fast, low-temperature, deliberately partial, or complete anion-exchange in highly luminescent semiconductor nanocrystals of cesium lead halide perovskites (CsPbX3, X = Cl, Br, I). By adjusting the halide ratios in the colloidal nanocrystal solution, the bright photoluminescence can be tuned over the entire visible spectral region (410–700 nm) while maintaining high quantum yields of 20–80% and narrow emission line widths of 10–40 nm (from blue to red). Furthermore, fast internanocrystal anion-exchange is demonstrated, leading to uniform CsPb(Cl/Br)3 or CsPb(Br/I)3 compositions simply by mixing CsPbCl3, CsPbBr3, and CsPbI3 nanocrystals in appropriate ratios.

Low-threshold amplified spontaneous emission and lasing from colloidal nanocrystals of caesium lead halide perovskites

Metal halide semiconductors with perovskite crystal structures have recently emerged as highly promising optoelectronic materials. Despite the recent surge of reports on microcrystalline, thin-film and bulk single-crystalline metal halides, very little is known about the photophysics of metal halides in the form of uniform, size-tunable nanocrystals. Here we report low-threshold amplified spontaneous emission and lasing from ∼10 nm monodisperse colloidal nanocrystals of caesium lead halide perovskites CsPbX3 (X=Cl, Br or I, or mixed Cl/Br and Br/I systems). We find that room-temperature optical amplification can be obtained in the entire visible spectral range (440–700 nm) with low pump thresholds down to 5±1 μJ cm−2 and high values of modal net gain of at least 450±30 cm−1. Two kinds of lasing modes are successfully observed: whispering-gallery-mode lasing using silica microspheres as high-finesse resonators, conformally coated with CsPbX3 nanocrystals and random lasing in films of CsPbX3 nanocrystals.

Quasicrystalline order in self-assembled binary nanoparticle superlattices

The discovery of quasicrystals in 1984 changed our view of ordered solids as periodic structures and introduced new long-range-ordered phases lacking any translational symmetry. Quasicrystals permit symmetry operations forbidden in classical crystallography, for example five-, eight-, ten- and 12-fold rotations, yet have sharp diffraction peaks. Intermetallic compounds have been observed to form both metastable and energetically stabilized quasicrystals; quasicrystalline order has also been reported for the tantalum telluride phase with an approximate Ta1.6Te composition. Later, quasicrystals were discovered in soft matter, namely supramolecular structures of organic dendrimers and tri-block copolymers, and micrometre-sized colloidal spheres have been arranged into quasicrystalline arrays by using intense laser beams that create quasi-periodic optical standing-wave patterns. Here we show that colloidal inorganic nanoparticles can self-assemble into binary aperiodic superlattices. We observe formation of assemblies with dodecagonal quasicrystalline order in different binary nanoparticle systems: 13.4-nm Fe2O3 and 5-nm Au nanocrystals, 12.6-nm Fe3O4 and 4.7-nm Au nanocrystals, and 9-nm PbS and 3-nm Pd nanocrystals. Such compositional flexibility indicates that the formation of quasicrystalline nanoparticle assemblies does not require a unique combination of interparticle interactions, but is a general sphere-packing phenomenon governed by the entropy and simple interparticle potentials. We also find that dodecagonal quasicrystalline superlattices can form low-defect interfaces with ordinary crystalline binary superlattices, using fragments of (33.42) Archimedean tiling as the ‘wetting layer’ between the periodic and aperiodic phases.

Monodisperse and Inorganically Capped Sn and Sn/SnO2 Nanocrystals for High-Performance Li-Ion Battery Anodes

We report a facile synthesis of highly monodisperse colloidal Sn and Sn/SnO2 nanocrystals with mean sizes tunable over the range 9-23 nm and size distributions below 10%. For testing the utility of Sn/SnO2 nanocrystals as an active anode material in Li-ion batteries, a simple ligand-exchange procedure using inorganic capping ligands was applied to facilitate electronic connectivity within the components of the nanocrystalline electrode. Electrochemical measurements demonstrated that 10 nm Sn/SnO2 nanocrystals enable high Li insertion/removal cycling stability, in striking contrast to commercial 100-150 nm powders of Sn and SnO2. In particular, reversible Li-storage capacities above 700 mA h g(-1) were obtained after 100 cycles of deep charging (0.005-2 V) at a relatively high current of 1000 mA h g(-1).

Expanding the chemical versatility of colloidal nanocrystals capped with molecular metal chalcogenide ligands.

We developed different strategies toward the synthesis of colloidal nanocrystals stabilized with molecular metal chalcogenide complexes (MCCs). Negatively charged MCCs, such as SnS(4)(4-), Sn(2)S(6)(4-), SnTe(4)(4-), AsS(3)(3-), MoS(4)(2-), can quantitatively replace the organic ligands at the nanocrystal surface and stabilize nanocrystal solutions in different polar media. We showed that all-inorganic nanocrystals composed of metals, semiconductors, or magnetic materials and capped with various MCC ligands can be synthesized using convenient and inexpensive chemicals and environmentally benign solvents such as water, formamide, or dimethylsulfoxide. The development of mild synthetic routes was found to be crucial for the design of highly luminescent all-inorganic nanocrystals, such as CdSe/ZnS and PbS capped with Sn(2)S(6)(4-) MCCs, respectively. We also prepared conductive and luminescent layer-by-layer assemblies from inorganically capped colloidal nanocrystals and polyelectrolytes. In close-packed films of 5-nm Au nanocrystals stabilized with Na(2)Sn(2)S(6) we observed very high electrical conductivities (>1000 S cm(-1)).

Energetic and Entropic Contributions to Self-Assembly of Binary Nanocrystal Superlattices: Temperature as the Structure-Directing Factor

We studied the effect of temperature on self-assembly of monodisperse colloidal nanocrystals into single-component and binary superlattices. Temperature, which serves as a weighting factor for the internal energy (U) and entropy (S) contributions to the Helmholtz free energy F = U - TS, allows tailoring relative weights of the interparticle interactions and free-volume entropy during the formation of nanocrystal superlattices. Temperature also provides a convenient tool for directing self-assembly of nanocrystals toward desired superlattice structures. We found that temperature strongly affects the structures of binary superlattices self-assembled from the mixtures of CdSe + PbS nanocrystals and PbSe + Pd nanocrystals. In the former case, small Hamaker constants for CdSe and PbS nanocrystals led to a relatively simple phase diagram, including only high-density NaZn(13)-, AlB(2)-, and NaCl-type binary superlattices. In contrast, binary superlattices self-assembled at different temperatures from PbSe and Pd nanocrystals showed a number of low-density complex phases stabilized by strong local van der Waals interactions between Pd nanocrystals. The structural diversity of nanoparticle superlattices is shown to be a result of the cooperative effect of the entropy-driven crystallization and the interparticle interactions. Both DeltaU and TDeltaS terms associated with the superlattice formation should be of the same order of magnitude, with |DeltaU| < |TDeltaS| for the assembly of CdSe and PbS nanocrystals and |DeltaU| > |TDeltaS| for the PbSe and Pd nanocrystals.

Lead Halide Perovskites and Other Metal Halide Complexes As Inorganic Capping Ligands for Colloidal Nanocrystals

Lead halide perovskites (CH3NH3PbX3, where X = I, Br) and other metal halide complexes (MXn, where M = Pb, Cd, In, Zn, Fe, Bi, Sb) have been studied as inorganic capping ligands for colloidal nanocrystals. We present the methodology for the surface functionalization via ligand-exchange reactions and the effect on the optical properties of IV–VI, II–VI, and III–V semiconductor nanocrystals. In particular, we show that the Lewis acid–base properties of the solvents, in addition to the solvent dielectric constant, must be properly adjusted for successful ligand exchange and colloidal stability. High luminescence quantum efficiencies of 20–30% for near-infrared emitting CH3NH3PbI3-functionalized PbS nanocrystals and 50–65% for red-emitting CH3NH3CdBr3- and (NH4)2ZnCl4-capped CdSe/CdS nanocrystals point to highly efficient electronic passivation of the nanocrystal surface.

Single Cesium Lead Halide Perovskite Nanocrystals at Low Temperature: Fast Single-Photon Emission, Reduced Blinking, and Exciton Fine Structure

Metal-halide semiconductors with perovskite crystal structure are attractive due to their facile solution processability, and have recently been harnessed very successfully for high-efficiency photovoltaics and bright light sources. Here, we show that at low temperature single colloidal cesium lead halide (CsPbX3, where X = Cl/Br) nanocrystals exhibit stable, narrow-band emission with suppressed blinking and small spectral diffusion. Photon antibunching demonstrates unambiguously nonclassical single-photon emission with radiative decay on the order of 250 ps, representing a significant acceleration compared to other common quantum emitters. High-resolution spectroscopy provides insight into the complex nature of the emission process such as the fine structure and charged exciton dynamics.

Monodisperse Formamidinium Lead Bromide Nanocrystals with Bright and Stable Green Photoluminescence

Bright green emitters with adjustable photoluminescence (PL) maxima in the range of 530–535 nm and full-width at half-maxima (fwhm) of <25 nm are particularly desirable for applications in television displays and related technologies. Toward this goal, we have developed a facile synthesis of highly monodisperse, cubic-shaped formamidinium lead bromide nanocrystals (FAPbBr3 NCs) with perovskite crystal structure, tunable PL in the range of 470–540 nm by adjusting the nanocrystal size (5–12 nm), high quantum yield (QY) of up to 85% and PL fwhm of <22 nm. High QYs are also retained in films of FAPbBr3 NCs. In addition, these films exhibit low thresholds of 14 ± 2 μJ cm–2 for amplified spontaneous emission.