Sadegh Yazdi

Developing a detailed understanding of ternary nanoparticle (TNP) formation is essential for their optimized rational synthesis and development of synthetic routes for new TNPs. Herein, we explore the reaction of Cu3–xP and dibenzyl diselenide (Bn2Se2) to form colloidal Cu3PSe4 TNPs. Temperature-resolved X-ray scattering (XRD and PDF), electron microscopy (TEM and STEM), and spectroscopy (EDS, EELS, XPS, and MAS NMR) reveal that Cu3–xP reacts by surface coordination of Se leading to fragmentation followed by rearrangement to Cu–Se binary phases, during which all obvious crystalline P-containing phases disappear via XRD. However, partially oxidized P in solid phases was observed using STEM-EDS and XPS, in which P is found to preform P–Se bonds prior to Cu3PSe4 formation. Using a combination of 31P MAS NMR and PDF analysis obtained from synchrotron total scattering data, P–Se bonds in [PSe4]3– tetrahedral building blocks were identified within intermediate Cu–Se phases containing P cation substitution (PCu), denoted (Cu,P)–Se, that assemble into Cu3PSe4. We hypothesize that these intermediate compounds with their substoichiometric, vacancy-rich structures and significant Cu disorder are important for accessing Cu3PSe4─offering a new insight into complex TNP syntheses. We summarize our findings by writing plausible pseudoelementary steps (PESteps) in which the Cu3–xP precursor converts to smaller fragments of Cu–Se phases containing P en route to the final Cu3PSe4 product. Additional interesting aspects of this system include the use of Bn2Se2 as a readily monitorable probe for the reaction and the Se–P bond formation that facilitates Cu–P bond cleavage in an overall 8-electron redox reaction involving P3– and 4 Se0. The results obtained lay the groundwork for future mechanistic investigations, notably kinetics studies working from the PESteps aimed ultimately at the rational design and synthesis of complex ternary pnictogen chalcogenide nanoparticles.

Nanocrystals (NCs) serve as versatile building blocks for the creation of functional materials with NC self-assembly offering opportunities to enable novel material properties. Here, we demonstrate that PbS NCs functionalized with strongly negatively charged metal chalcogenide complex (MCC) ligands, such as Sn₂S₆^4- and AsS₄^3-, can self-assemble into all-inorganic superlattices with both long-range superlattice translational and atomic-lattice orientational order. Structural characterizations reveal that the NCs adopt an unexpected edge-to-edge alignment, and numerical simulation clarifies that orientational order is thermodynamically stabilized by many-body ion correlations originating from the dense electrolyte. Furthermore, we show that the superlattices of Sn₂S₆^4--functionalized PbS NCs can be fully disassembled back into the colloidal state, which is highly unusual for orientationally attached superlattices with atomic-lattice alignment. The reversible oriented attachment of NCs, enabling their dynamic assembly and disassembly into effectively single-crystalline superstructures, offers a pathway toward designing reconfigurable materials with adaptive and controllable electronic and optoelectronic properties.

We explore silane-coated formamidinium lead bromide (FAPbBr₃) quantum dots (QDs) as single photon emitters and compare them to FAPbBr₃ QDs passivated with a phosphoethylammonium derivative (PEAC₈C₁₂), which represents current state-of-the-art ligand passivation. We compare properties including single-photon purity (g⁽²⁾(τ)), line width, blinking, and photostability. We find that at room temperature, these silane-coated dots perform comparably to PEAC₈C₁₂-passivated dots, while exhibiting improvements in photostability. However, we find that at 4 K, silane-coated FAPbBr₃ QDs perform worse than the PEAC₈C₁₂-passivated samples, exhibiting faster blue-shifting and photobleaching under illumination. Analysis of fluorescence lifetime intensity distributions from the photon-counting data indicates increased efficiency of fast nonradiative processes in the silane-coated QDs at 4 K. We propose a trion-related degradation pathway at low temperatures that is consistent with the observed kinetics and estimate that at 4 K with 6.1 μJ/cm², 472 nm excitation the silane-coated QDs build up double the trion population of their PEAC₈C₁₂-passivated counterparts.

Cation exchange in molten salts has emerged as a method to synthesize colloidal III-V quantum dots with tunable composition and excellent optoelectronic properties. However, the atomistic details of how cation exchange occurs and the elemental distribution in the products remain elusive. In this work, we use scanning transmission electron microscopy to reveal the chemical and structural evolution of In_1-xGa_xP_1-yAs_y nanocrystals made by cation exchange of InP_1-yAs_y nanocrystals in a gallium-containing molten salt. The starting material and In_1-xGa_xP_1-yAs_y nanocrystals at multiple time points in the reaction were characterized by aberration-corrected HAADF-STEM and STEM-EDS. HAADF-STEM reveals subtle changes in size and shape, but the tetrahedral morphology and zinc-blende crystal structure are maintained throughout the reaction. We overcome the technical challenges of beam damage and low signals in STEM-EDS through a combination of experimental optimization and image processing to yield elemental composition maps representative of individual nanocrystals. This analysis revealed that the particles have Ga-rich surfaces at the early reaction stage. As the reaction proceeds, Ga permeates the lattice, but the elemental distribution remains graded with In enrichment in the core even after 16 h. To investigate the origin of the observed elemental distributions, we use finite element analysis to model the cation exchange as a diffusion-limited process, which suggests that cation exchange becomes progressively more difficult as In atoms are replaced by Ga. We discuss the implication of the graded composition on optical properties. More broadly, the image processing techniques developed here could be applied to other compositionally complex nanocrystals.

Nanowires are promising structures for next‐generation photonic devices due to their superior structural, optical, and electronic properties compared to thin films. In this study, unexpected electrostatic potential wells across the non‐polar m ‐plane and at the core/shell interface in n‐type GaN core/shell nanowires, grown via metal‐organic vapor phase epitaxy, are reported. Using advanced electron microscopy, including off‐axis electron holography, electrostatic potential distributions are mapped and shallow quantum wells are identified at the core/shell interface and core center. High‐resolution transmission electron microscopy ruled out planar and line defects, implicating point defects as their source. Valence electron energy loss spectroscopy revealed localized bandgap narrowing due to strain from concentrated point defects. Hyperspectral cathodoluminescence linked lower potential in the core to C _N defects, while the absence of related luminescence at the core/shell interface suggests V _Ga O _N defect complexes as plausible causes. These findings highlight the critical role of point defects in GaN nanowires, with significant implications for device performance.

The morphology of quantum dots plays an important role in governing their photophysics. Here, we explore the photoluminescence of spheroidal CsPbBr₃ quantum dots synthesized via the room-temperature trioctylphosphine oxide/PbBr₂ method. Despite photoluminescence quantum yields nearing 100%, these spheroidal quantum dots exhibit an elongated red photoluminescence tail not observed in typical cubic quantum dots synthesized via hot injection. We explore the origins of this elongated red tail through structural and optical characterization including small-angle X-ray scattering, transmission electron microscopy and time-resolved, steady-state, and single quantum dot photoluminescence. From these measurements we conclude that the red tail originates from emissive traps. We show that treating spheroidal quantum dots with phenethylammonium bromide decreases the line shape asymmetry and increases passivation-consistent with emissive traps due to polar facets.

Colloidal quantum dots, with their size-tunable optoelectronic properties and scalable synthesis, enable applications in which inexpensive high-performance semiconductors are needed. Synthesis science breakthroughs have been key to the realization of quantum dot technologies, but important group III–group V semiconductors, including colloidal gallium arsenide (GaAs), still cannot be synthesized with existing approaches. The high-temperature molten salt colloidal synthesis introduced in this work enables the preparation of previously intractable colloidal materials. We directly nucleated and grew colloidal quantum dots in molten inorganic salts by harnessing molten salt redox chemistry and using surfactant additives for nanocrystal shape control. Synthesis temperatures above 425°C are critical for realizing photoluminescent GaAs quantum dots, which emphasizes the importance of high temperatures enabled by molten salt solvents. We generalize the methodology and demonstrate nearly a dozen III-V solid-solution nanocrystal compositions that have not been previously reported.