Dmitry Baranov

Three-dimensional nanoarchitected materials provide tunable confined environments for organizing colloidal nanocrystals. However, little is known about how nanocrystals organize inside them across extended areas. Here, we map the infilling of CsPbBr3 nanocrystals within nanoarchitected SU-8 sheets of around 30 μm thickness with a characteristic pore size of ∼500 nm across millimeter-scale areas. Nanocrystals dispersed in hexane or toluene were deposited into these uncoated and TiO2-coated nanoarchitected materials. Scanning small-angle X-ray scattering (scanning SAXS) resolved nanometer-scale packing across the material, while microphotoluminescence (micro-PL) mapped the corresponding optical response. Hexane produced comparatively uniform nanocrystal-nanocrystal spacing with broad SAXS features, indicating weak structural order. In contrast, toluene produced narrower diffraction features and, in the TiO2-coated material, a radial gradient in nanocrystal-nanocrystal spacing. A correlated PL shift supported the assignment of this gradient to nanocrystal size segregation. Measurements of the uncoated material further showed that wall chemistry modifies the spatial nanocrystal packing pattern. By enabling millimeter-scale mapping of nanometer-scale organization, this approach reveals how solvent choice influences nanocrystal packing and highlights the importance of multimodal mapping for understanding complex nanomaterials for emerging applications.

We have studied halide exchange reactions in large colloidal nanocrystals of lead halide perovskites, with sizes (∼40–60 nm) that are intermediate between those of typical nanocrystals synthesized and studied by the community (5–10 nm) and those of microcrystals or thin films. This size regime still allows an investigation of the nanocrystals with techniques used for colloidal nanocrystals while enabling the observation of phenomena such as phase segregation, generation of planar defects, strain accumulation and relaxation, which are harder to identify in smaller nanocrystals, also given their high instability under the electron beam. We compare nanocubes with hollow nanocubes, and highlight how the presence of the hollow region can influence structural and morphological changes during the reactions. Notably, we report core–shell epitaxial CsPbI3/CsPbCl3 heterostructures by partial I– to Cl– exchange on CsPbI3 nanocrystals. Transient absorption spectroscopy provides evidence consistent with electronic coupling and exciton transfer between the Cl-rich shell and I-rich core in these heterostructures, evidenced by the accelerated bleach decay of the CsPbCl3 component and the delayed population of the CsPbI3 band edge.

Colloidal nanocrystals are complex objects, comprising a solid inorganic core and a soft shell of organic ligands, both of which contribute to defining their properties. Obtaining an atomically precise description of both components remains an open challenge. Yet, these hybrid building blocks can self-assemble into supercrystals sufficiently large to be isolated and analyzed individually. It may therefore come as a surprise that single-supercrystal X-ray diffraction is not routinely used to investigate the structure of their constituent nanocrystals. Here, we propose a roadmap for establishing supercrystallography as the science of obtaining structural information about colloidal building blocks through diffraction experiments performed on their supercrystals. We review existing examples of single-supercrystal X-ray diffraction, examine the opportunities that may emerge from extending crystallographic methods to colloidal supercrystals, and discuss the main challenges that must be overcome to realize this vision. Drawing inspiration from the crystallography of nanoclusters and proteins, systems that share deep similarities with colloidal nanocrystals, we outline a possible path connecting state-of-the-art experiments to complete atomistic models through innovative data-analysis strategies. Finally, we consider how supercrystallography could complement existing characterization techniques, and develop into an accessible framework for investigating the structure of colloidal nanocrystals and their surface chemistry.

Chiral surface ligands can impart optical activity to all-inorganic perovskite nanocrystals, but how this affects chiroptical properties of their assemblies remains unclear. Here we show that self-assembly of CsPbBr3 nanocubes synthesized with a small amount of added chiral ligand N-benzylcinchonidinium bromide (BCD-Br) results in measurable circularly polarized luminescence (CPL). BCD-Br-treated nanocrystals show weak circular dichroism but no detectable CPL in colloidal dispersion. Upon assembly into CsPbBr3 and mixed halide superlattices, they exhibit CPL with a luminescence dissymmetry factor, glum, of (5 ± 2) × 10–3, whereas untreated controls remain near the noise level. Polarization-resolved transient absorption (TA) reveals an extended spin lifetime of hundreds of picoseconds in BCD-Br-treated nanocrystals, while linear-response time-dependent density functional theory (LR-TDDFT) calculations suggest that inter-nanocrystal coupling can enhance the chiroptical response. These results support the idea that ordered nanocrystal assembly can convert weak ligand-induced chirality into an emergent chiroptical response in perovskite nanocrystal solids.

Semiconductor heterostructures that combine components with different dimensionalities provide an interesting avenue to manipulate the physical properties of the resulting material. Two‐dimensional lead halide perovskites crystallize as flat microcrystals and have efficient in‐plane exciton mobility, while perovskite nanocrystals are efficient emitters with a tunable bandgap that can self‐assemble into microscopic superlattices. However, combining such intricate architectures into heterostructures has been challenging due to the mismatch in solubility and difficult transfer procedures. Here, we realize heterostructures where CsPbBr ₃ nanocrystal superlattices are deterministically grown along the faces of PEA ₂ PbBr ₄ 2D layered perovskite microcrystals. The growth can either be limited to the lateral faces of the microcrystals and result in core–crown epitaxial heterostructures, or extended to the vertical direction leading to core–shell‐like structures. We demonstrate that these heterostructures can be employed as efficient light‐harvesting systems. In fact, energy can be transferred from the 2D microcrystal domain to the superlattices, enabling switching between linear and nonlinear carrier recombination regimes by tuning the excitation fluence. Moreover, by exploiting the lifetime shortening of CsPbBr ₃ nanocrystal emission upon sample cooling, we ensure that energy transfer occurs after the biexcitonic and single‐excitonic decays of the nanocrystals, effectively extending the radiative life time of the superlattices.

Methylammonium lead iodide (MAPbI ₃ ) is a prototypical soft‐lattice semiconductor whose orthorhombic and tetragonal polymorphs can coexist and interconvert under optical excitation or changes in temperature. The mixed‐phase regime attracts attention because of the carrier transfer between phases, which enables optoelectronic functionalities such as optical gain. Here, we investigate carrier dynamics in mixed‐phase MAPbI ₃ thin films by comparing laser‐induced phase coexistence at 77 K using time‐resolved photoluminescence with thermally‐induced phase coexistence at 148 K using transient absorption (TA) spectroscopy. By using low‐repetition‐rate TA to mitigate cumulative heating and avoid thermally induced phase conversion, we resolve the distinct spectral fingerprints of the orthorhombic and tetragonal phases and identify two carrier‐transfer timescales of 36 and 539 ps, attributed to interfacial and bulk‐mediated transfer, respectively. Despite carrier funneling in the mixed phase and high carrier densities, these conditions do not inherently lead to observable optical gain in the studied films.

The David Jonas Festschrift is a collection of 67 contributions that cover the latest developments and applications in multidimensional and ultrafast spectroscopies, spanning a wide range of topics. The editorial provides background and summarizes the contributions to the special issue across the areas of experimental advances, theory and computation, proteins and biological systems, nanomaterials, molecules, liquids, and soft matter.

Colloidal superlattices are widely studied for the collective properties emerging from nanocrystal interactions. This focus has shaped the description of superlattices as ordered aggregates of particles that remain structurally unaffected by self-assembly, and whose value lies mainly in the emergent properties they can express. In direct analogy with structural biology, where protein crystals are not the end goal but rather the starting point of scientific inquiry, we instead propose superlattices as tools to characterize nanocrystals and their environment. Indeed, CsPbBr ₃ superlattices display striking similarities with protein crystals, sharing comparable unit cell size, structural complexity, and crystallization dynamics. Like proteins, CsPbBr ₃ nanocrystals display a measurable contraction of their atomic structure upon self-assembly, which we attribute to a restriction of vibrational freedom by analogy with well-documented protein crystallization dynamics. Like some proteins, CsPbBr ₃ nanocrystals reconcile their orthorhombic structure with the cubic symmetry of the superlattice by adopting different orientations without disrupting structural coherence, a mechanism known as rotational order-disorder. Like proteins, nanocrystals self-assemble by retaining a shell of molecules – the ligands – that preserves a chemical environment similar to solution. Inspired by the molecular dynamics studies on water behavior in protein crystals, we exploit this property and the precise geometric information from X-ray diffraction to simulate the behavior of surface ligands, finding that a significant fraction of these molecules may remain free in the interparticle space, where they form micelles resembling the interstitial water networks found in protein crystals. In a final parallel, we show that single-superlattice X-ray diffraction encodes structural information at both the atomic and nanoparticle scales, which can be extracted by adapting strategies from protein crystallography. Together, this evidence prompts a redefinition of CsPbBr ₃ superlattices as hybrid organic-inorganic crystals, and highlights structural biology as a source of ideas for studying nanomaterials and engineering their properties. Following this lead, we anticipate that crystallography on supercrystals – or supercrystallography – will provide atomic-resolution insight into elusive features such as surface structure, ligand binding motifs, and the presence of strain fields across the particle core.

Ordered assemblies of colloidal semiconductor nanocrystals have traditionally been studied for energy transfer and electronic transport arising from wavefunction overlap between neighboring nanocrystals. In this perspective, we instead focus on light‐coupled nanocrystal solids engineered primarily for light emission. These materials include ordered superlattices and compositionally doped assemblies, spanning a continuum from optically dense solids in which all nanocrystals are emissive to optically diluted solids where emitters are spatially separated by transparent filler nanocrystals. By tuning emitter density and interparticle spacing relative to the emission wavelength, light‐coupled nanocrystal solids enable access to optical regimes ranging from collective phenomena such as superfluorescence and superradiance to single‐nanocrystal and single‐photon emission. Rooted in lead halide perovskite and cadmium chalcogenide nanocrystal superlattices, this concept can be extended to heavy‐metal‐free nanocrystals and provides a materials platform for optical information processing and quantum technologies.

Supercrystals of lead-halide perovskite nanocrystals combine the semiconducting properties of bulk perovskites with quantum confinement effects and extend them to the macroscopic scale. Supercrystals assembled via a two-layer phase diffusion process using an acetonitrile antisolvent were recently shown to be unusually robust. We investigate how the acetonitrile-assisted self-assembly process influences surface chemistry, the atomic lattice of nanocrystals, and the structure of the supercrystal. Using quantitative NMR spectroscopy, nanofocused X-ray diffraction, and optical spectroscopy, we show that a reduced density of the ligand shell caused by the exposure to acetonitrile in the assembly underlies the mechanical robustness of these supercrystals. Ligand stripping further drives a highly size-selecting lateral growth of the supercrystal and induces anisotropic relaxation of the nanocrystal atomic lattice while preserving the electronic coupling and robust light-emitting properties of the assembly. That enables the mechanical manipulation of supercrystals such as stacking, thereby opening new avenues for integration into optoelectronic devices.