The high surface-to-volume ratio in nanocrystals (NCs) enables surface energy effects that stabilize phases that are otherwise unstable in a bulk state. Double perovskites (DP) containing lithium show exactly this effect. Bulk Cs2LiInCl6 adopts a triclinic structure; however, we show here that the cubic phase can be stabilized in a nanocrystalline form at a wide range of lithium-sodium alloyed compositions. Density functional theory (DFT) calculations support this finding. Although bulk formation energy favors the triclinic structure for Li-rich compositions, the cubic phase becomes stable when the surface energy of the {100} facet is significant due to the large surface-to-volume ratio in nanocrystals. The importance of this Li-Na alloying is seen in the change in the physical properties, apparent in the controllable blue shift of the emission with increased Li content. This advantageous effect, which is also observed for Li-cation exchange in presynthesized colloidal nanocrystals, is overshadowed by a competing phase that we identify as an orthorhombic hydrate phase, Cs2InCl5·H2O. We characterize its emergence and propose strategies to mitigate its impact.

Self-healing is rarely observed in semiconductors, where structural distortions typically result in an irreversible performance loss. Halide perovskites defy this paradigm, exhibiting spontaneous recovery of optoelectronic properties even at room temperature, yet the underlying mechanisms remain poorly understood. Here, we subject CsPbBr3 single crystals to facet-oriented focused ion beam (FIB) milling to induce localized mechanical damage and directly track the subsequent healing dynamics. By selectively exposing different crystallographic orientations, we correlate structural reconstruction with photoluminescence recovery. Milling aligned with low-index surfaces enables complete recovery, often with enhanced emission compared to that of the pristine surface, whereas milling across facets, along effectively higher-index crystal planes, leads to permanent emission quenching. The differences arise due to the facet-dependent stabilization and higher formation energies of Br interstitials for higher-index surfaces, a hypothesis that is supported by DFT modeling. Our work establishes facet-oriented FIB milling as a versatile approach for systematically probing self-healing processes in functional materials.

Halide perovskites possess an intrinsically dynamic structure that strongly influences their electro-optical performance and stability. Understanding how a microscopic phenomenon affects a global physical property is critical for better using these materials. Here, we suggest a new mechanism that explains synchronized electron-hole radiative recombination that extends over 10 & micro;m well beyond the current understanding. The physical observable we follow is photoluminescence intermittencies in vapor grown, all-inorganic halide perovskite crystals. This blinking effect is synchronized across distances well beyond the electron diffusion length, contradicting the widely accepted theory that assumes a "supertrap" involvement in photoluminescence intermittencies in halide perovskites. Such theories limit the range of synchronization of electron-hole radiative recombination by an order of magnitude to what we measure. We show beyond doubt a clear connection between the appearance of blinking and Pb-rich growth conditions. Thus, the new framework puts the focus on hole diffusion to explain micron scale synchronization in halide perovskites.

Breaking crystal symmetry is essential for engineering emissive double perovskite metal halides. The goal is to overcome their inherently indirect and disallowed optical transitions. Here we introduce a synthesis for silver - Cs2AgInCl6 two-dimensional hybrid nanoplate products that break the symmetry in two ways, their shape and their heterointerfaces. A comparative study between Cs2AgInCl6 nanocubes and nanoplates is presented to emphasize the difference in optical properties. A modified colloidal synthesis for Cs2AgInCl6 yields high-quality nanoplates with small lateral dimensions very different from the symmetric cubes. Each nanoplate is decorated with metallic silver nanoparticles, with diameters on the scale of the thickness of the perovskite nanoplate, forming significant heterointerfaces that further break symmetry. The Cs2AgInCl6 two-dimensional nanoplates also demonstrate facile transformation into larger crystalline nanosheets once deposited on substrates. We thus highlight those nanoplates as potential building blocks for assemblies of functional surfaces.

Enhancing the efficiency and stability of lead halide perovskite devices is crucial to their practical application. Previous treatments with thiocyanate (SCN-) have demonstrated significant improvements in the photoluminescence quantum yield (PLQY) and stability of CsPbBr3 nanocrystals (NCs), but the underlying mechanisms remain partially unresolved. Addressing the challenge of low SCN- solubility in traditional nonpolar solvents, our study introduces a urea-ammonium thiocyanate (UAT)-based ionic liquid surface treatment. This method facilitates a higher SCN- loading by creating a liquid-liquid interface that is compatible with the organic colloidal suspension, preventing NC degradation, and achieving near-unity PLQY. Utilizing transmission electron microscopy techniques, we present atomic resolution evidence that thiocyanate-treated surfaces are rich in sulfur and display structural dilation of the lattice spacing of 3%. This supports that thiocyanate acts as a pseudohalide and binds to Pb cations on the NC surfaces. As a result, the treated NCs show enhanced stability against ionic substitution while maintaining the perovskite structure intact. Our findings provide conclusive evidence that the primary mechanism of performance enhancement is the passivation of surface traps attributed to bromide vacancies rather than the scavenging of excess lead cation. This surface treatment method slows ion migration, a prominent challenge in photovoltaics, offering a significant advancement in the development of perovskite-based devices.

Radiation detection is being revolutionized by integrating photonic elements into scintillators. In this study, a scalable and cost-effective method is proposed to achieve tuneable emission enhancement across the visible spectrum by colloidal self-assembly of photonic crystals on scintillator surfaces. This concept is demonstrated for Eu3+/Tb3+-doped Gd and Ta oxides. Widely available and affordable colloidal nanospheres of SiO2 or polymethyl methacrylate are self-assembled on these scintillators. The size of the nanospheres is carefully optimized to match the desired emission lines of Eu3+/Tb3+. The result is homogeneous and closely-packed structures with clear photonic bandgap in the visible range. Under X-ray excitation, the scintillators covered with the photonic layers exhibit enhanced light extraction in the direction perpendicular to the surface, compared to isotropic emission in the bare scintillator. Such scintillation directionality, when optically matched with a proper detector, will result in higher efficiency of the overall detection system. Moreover, X-ray imaging demonstrates an enhancement of 25% in system resolution of the scintillator supplemented with the photonic layer compared to unmodified scintillators. The proposed method is scintillator- and nanosphere-agnostic, thus offering a promising versatile approach for directing the scintillation light toward a photodetector and increasing detection system performance, including high-resolution imaging applications. The integration of photonic elements into scintillators is transforming high-energy detection. A scalable, tunable, and cost-effective method to improve light output from highly-scattering scintillators is suggested. This is achieved using colloidal self-assembly of photonic structures on scintillator surfaces. Enhanced light extraction and directionality are shown, along with a 25% resolution increase in X-ray imaging. image

Functionalization of perovskite nanocrystal surfaces with thiocyanate anions presents a transformative approach to enhancing stability and photoluminescence quantum yield (PLQY) through surface defect passivation. This study investigates the role of thiocyanate ligands in modifying the optoelectronic properties of CsPbBr3 nanocrystals. We employed ultrafast two-dimensional infrared spectroscopy to investigate the nature of the dynamic interaction of thiocyanate ligands with nanocrystal surfaces, providing insights into the mechanisms underlying the observed increase in PLQY and stability. Our analysis reveals that the thiocyanate ligands efficiently passivate the surface defects, thereby enhancing the PLQY and the stability of the treated nanocrystals. The spectroscopic evidence supports a model where thiocyanate binds to under-coordinated lead atoms, contributing to a stable nanocrystal surface with enhanced optoelectronic performance. This ligand-induced passivation mechanism advances our understanding of surface chemistry's role in optimizing nanomaterials for solar cell and LED applications.

Layered 2D perovskites are solution‐processed quantum‐wells. Their effective band‐gap is determined via the inorganic perovskite layer thickness and exciton quantum confinement effects. Alternatively, by changing the organic moieties, one can tune the dielectric constant and distance between the monolayers modifying the excitonic interactions. In colloidal perovskites, a dynamic equilibrium exists between the free organic moieties in the solution and the surface of the nanocrystal. Colloidal synthesis is used to make single monolayer L 2 PbBr 4 platelets and assemble these into layered 2D stacks. In the experiment, L is an alkylamine surface ligand whose length (4‐18 carbons) determines the interlayer distances between the quantum‐wells. The dynamic equilibrium of ligand mixtures in solution and perovskite surfaces leads to optimal mixing of the molecules. During the self‐assembly of monolayers, the distance between the inorganic layers is thus engineered. The interlayer distance is proportional to the average ligand mixture length. This results in controlled interactions between the 2D‐excitons, enabling red‐shifted absorption and emission and extended lifetimes for longer alkyl chains. Using entropic mixing of ligands for the engineering of 2D excitonic interactions is therefore demonstrated. Formation of layered 2D perovskites from colloidal building blocks allows intermixing of dissimilar materials opening possibilities for new heterostructures and junctions.

Collective optical properties can emerge from an ordered ensemble of emitters due to interactions between the individual units. Superlattices of halide perovskite nanocrystals exhibit collective light emission, influenced by dipole-dipole interactions between simultaneously excited nanocrystals. This coupling changes both the emission energy and rate compared to the emission of uncoupled nanocrystals. We demonstrate how quantum confinement governs the nature of the coupling between the nanocrystals in the ensemble. The extent of confinement is modified by controlling the nanocrystal size or by compositional control over the Bohr radius. In superlattices made of weakly confined nanocrystals, the collective emission is red-shifted with a faster emission rate, showing the key characteristics of superfluorescence. In contrast, the collective emission of stronger quantum-confined nanocrystals is blue-shifted with a slower emission rate. Both types of collective emission exhibit correlative multiphoton emission bursts, showing distinct photon bunching emission statistics. The quantum confinement changes the preferred alignment of transition dipoles within the nanocrystal and switches the relative dipole orientation between neighbors, resulting in opposite collective optical behaviors. Our results extend these collective effects to relatively high temperatures and provide a better understanding of exciton interactions and collective emission phenomena at the solid state.

Advancements in ferroic materials center upon understanding domain structures and their boundaries. This study innovates by directing the growth of lead halide perovskite microcrystals from the gas phase onto pre-chosen substrates, thereby inducing an interfacial strain that prompts ferroelasticity-driven structural transformation. A method of ferroelastic engineering in vapor-grown perovskite heterostructures is thus unveiled, revealing ordered domain patterns of bright crystals with contrasting electro-optical properties. In the case of the frustrated halide perovskite crystals, the strain is relaxed by the formation of alternating crystallographic twin domains. In the demonstrated orthorhombic system, these domains alternate between (110) and (002) orientation, with a (112) domain boundary. Perovskite ferroelastic effect with facet stability is correlated, attributed to the exposure of lower surface energy terminations, promoting the material's stability and efficiency. This findings result from direct electrical measurements, which pave the way for the tailored design of ferroic materials, optimizing their optoelectronic characteristics for enhanced device performance. This work demonstrates the connection between ferroelasticity, surface stability, and the electro-optical performance of lead halide perovskite microcrystals. Ferroelastic engineering of vapor-grown perovskite heterostructures results in patterns of ordered crystal domains. The alternating domains that are measured here presents strikingly contrasting local conductivity and photoluminescence. These results are important for the performance and stability of future electro-optical devices. image

Heterostructures in nanoparticles challenge our common understanding of interfaces due to quantum confinement and size effects, giving rise to synergistic properties. An alternating heterostructure in which multiple and reoccurring interfaces appear in a single nanocrystal is hypothesized to accentuate such properties. We present a colloidal synthesis for perovskite layered heterostructure nanoparticles with a (PbBr2)2(AMTP)2PbBr4 composition. By varying the synthetic parameters, such as synthesis temperature, solvent, and selection of precursors, we control particle size, shape, and product priority. The structures are validated by X-ray and electron diffraction techniques. The heterostructure nanoparticles' main optical feature is a broad emission peak, showing the same range of wavelengths compared to the bulk sample.

Lead-free perovskite nanocrystals are of interest due to their nontoxicity and potential application in the display industry. However, engineering their optical properties is nontrivial and demands an understanding of emission from both self-trapped and free excitons. Here, we focus on tuning silver-based double perovskite nanocrystals' optical properties via two iso-valent dopants, Bi and Sb. The photoluminescence quantum yield of the intrinsic Cs2Ag1-yNayInCl6 perovskite increased dramatically upon doping. However, the two dopants affect the optical properties very differently. We hypothesize that the differences arise from their differences in electronic level contributions and ionic sizes. This hypothesis is validated through absorption and temperature dependence photoluminescence measurements, namely, by employing the Huang-Rhys factor, which indicates the coupling of the exciton to the lattice environment. The larger ionic size of Bi also plays a role in inducing significant microstraining verified via synchrotron measurements. These differences make Bi more sensitive to doping concentration over antimony which displays brighter emission (QY similar to 40%). Such understanding is important for engineering optical properties in double perovskites, especially in light of recent achievements in boosting the photoluminescence quantum yield.

Double perovskites are considered for future photovoltaic and electro‐optic applications as a toxic‐free alternative to lead halide perovskites. Alas, due to the lower efficiency of lead‐free devices, material properties need to improve to compete. In this work, the self‐healing and annealing of crystal voids is reported. Experiments are conducted on nanocrystals and in situ a transmission electron microscopy (TEM) microscope. The setup enables creation of crystal voids and to monitor their dynamics in real time. Void trajectories and velocities are calculated for TEM videos. An inaccessible, protected volume for migration near the nanocrystal outer surface is discovered, confining the migration of voids to inner crystal parts. Once surface passivation in the form of organic ligands is removed, void dynamics changes, to enable annealing of the voids and self‐healing of the crystal. It is determined that surface ligand protection against void migration is extending several atomic layers below the crystal surface. Modeling based on these results predict equilibrium positions for the voids, which are discovered in the data. The study suggests that tuning of organic ligand density influences structural stability and crystal defect tolerance in double perovskites. Engineering surfaces with inherent self‐healing properties may increase efficiencies in future devices based on these materials.

Lead-free double perovskites are studied as an optional replacement to lead halide perovskites in optoelectronic applications. Recently, double-perovskite materials in which two divalent lead cations are replaced with an Ag+ and a trivalent cation have been demonstrated. The presence of a reactive silver cation and observations of metallic silver nanodecorations raised concerns regarding the stability and applicability of these materials. To better understand the nucleation and crystal growth of lead-free double perovskites, we explore the origin and role that metallic silver nanoparticles (NPs) play in the Ag-based Pb-free double-perovskite nanocrystal (NC) systems such as Cs2AgInCl6, Cs2AgSbCl6, Cs2AgBiCl6, and Cs2AgBiBr6. With major focus on Cs2AgInCl6 NCs, we show evidence supporting growth of the NCs through heterogeneous nucleation on preexisting metallic silver seeds. The silver seeds nucleate prior to injection of halide through reduction of the Ag+ ion by the aminic ligand. The presence of preexisting silver NPs is supported by a localized surface plasmon resonance (LSPR). The injection of halide precursor into the reaction mixture step initiates a fast nucleation and growth of the perovskite NC on the silver seed. The change in the dielectric medium at the interface of the silver NP results in a quantifiable red shift of the LSPR peak. In addition, we demonstrate charge transfer from the perovskite to the silver NP through photoinduced electrochemical Ostwald ripening of the silver NPs via UV irradiation. The ripened perovskite–metal hybrid nanocrystal exhibits modified optical properties in the form of quenched emission and enhanced plasmonic absorption. Future development of Ag-based double-perovskite NC applications depends on the ability to control Ag+ reduction at all synthetic stages. This understanding is critical for delivering stability and functionality for silver-based lead-free perovskite nanocrystals.