Justin C. Ondry

Molten indium halide salts enable high-temperature and oxygen-free processing and production of III–V nanocrystals, but their redox chemistry can also induce nanocrystal decomposition. Here we show that redox potential of indium halide melts controls the chemical stability of InP, InAs, and InSb nanocrystals. Raman probes of halide speciation and powder XRD of recovered solids reveal that iodide melts, such as KInI4, can oxidize indium pnictide nanocrystals, coupling salt reduction to oxidative pnictide loss, whereas bromide melts, such as KInBr4, resist reduction and stabilize InP and InAs nanocrystals. This redox-tuned stability enables composition-preserving annealing that smooths (111) facets, converting round or tetrapod-like InP and InAs particles into highly faceted tetrahedra, and promotes atomically aligned self-assembly and preferential substrate orientation. Controlled ripening in KInBr4 yields larger III–V colloids inaccessible by synthesis conducted in conventional organic solvents. Furthermore, combining molten-salt annealing with monolayer ZnS growth by colloidal atomic layer deposition (c-ALD) produces significant photoluminescence enhancement, underscoring the chemical benefits of redox-tuned molten salt solvents for the synthesis of novel III–V nanocrystals. Finally, we demonstrate a strategy to enhance the reactivity of reduced indium halide salts through halide coordination with a strong Lewis acid, enabling direct synthesis of InP and InAs nanocrystals. Reaction environments with highly reactive reduced indium salt engineered in molten salts also reveal that under certain reaction conditions, unprecedented 2D InP nanoplatelets can be synthesized.

Colloidal quantum dots (QDs) are being actively explored for QD light-emitting diode (QD-LED) displays with high color purity and wide color gamut, high brightness, and low-cost, solution-based manufacturing. Heavy metal-free InP QDs have enabled red- and green-emitting QD LEDs with excellent device characteristics. However, the performance of blue QD-LEDs, required for completing the red, green, and blue (RGB) full color gamut, is lagging behind, which prompts searches for novel blue-emitting QDs. Here, we report the synthesis, computational modeling, and structural and spectroscopic studies of heavy metal-free blue-emitting core-shell QDs featuring 2.1 nm In1–xGaxP cores, colloidally synthesized in a molten inorganic salt, epitaxially coated with ZnS shells, and optimized surface treatments. The computational study suggests that the introduction of a ZnS shell induces mixing between core and shell electronic states, enhancing the direct-gap character of In1–xGaxP/ZnS QDs. The resulting In1–xGaxP/ZnS QDs show blue band-edge photo- and electroluminescence that can be fine-tuned around the technologically important region of 470 nm. This material holds strong promise for filling the blue gap in full-gamut QD-LED displays.

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.

Colloidal quantum dots offer tunable optical properties for optoelectronic applications, but the synthesis of high-quality III–V quantum dots (QDs) other than indium pnictides, and particularly GaAs, has remained elusive due to synthetic challenges. Colloidal GaAs QDs have been reported very recently and have shown only weak band-edge photoluminescence. Here, we demonstrate a large-scale synthesis of colloidal GaAs QDs in molten salts, followed by a high-temperature surface treatment with K₂S, which removes native oxide and enables uniform zinc chalcogenide shell growth on GaAs. These QDs demonstrate bright band-edge photoluminescence and electroluminescence in QD LED devices. Low-temperature spectroscopy reveals a well-resolved exciton fine structure with distinct bright-state splitting and temperature-independent decay dynamics from 4 to 100 K. These results establish a practical pathway for the preparation of high-quality colloidal GaAs QDs from molten salts with potential applications in quantum technologies and optoelectronics.

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.

Colloidal quantum dots (QDs) have seen expanded applications in optoelectronics from visible to near-infrared (NIR) wavelengths. However, the options for QDs with bright and size-tunable emission in the NIR region are mostly limited to heavy-metal-based (Pb, Cd, Hg) semiconductors. Here, Ga-rich In_1-XGa_XAs QDs with zinc chalcogenide shells are demonstrated as candidates for NIR emitters. Based on new developments in inorganic molten salt chemistry, we synthesized colloidal In_1-XGa_XAs QDs containing up to 85% gallium with high crystallinity based on Raman and XRD analyses. Zinc selenide and sulfide shells with different morphologies were grown on In_1-XGa_XAs QDs by controlling the Zn precursor chemistry. Despite the nominally reduced lattice mismatch in the In_1-XGa_XAs/ZnSe heterostructure, a ZnS shell was found to be a much more effective passivation material: In_1-XGa_XAs/ZnS core-shell QDs show a photoluminescence quantum yield (PL QY) of about 30% versus 11% for ZnSe shells. Based on the analysis of PL decay and transient absorption (TA) dynamics, we surmise that electron trapping is the major reason for efficiency loss, providing a clear heterostructure design principle for realizing efficient NIR-emitting In_1-XGa_XAs QDs.

Quantum dots leverage quantum confinement to modify the electronic structure of materials, separating electronic transitions from the composition of the corresponding bulk material. With ternary quantum dots, the composition may be varied continuously so that both composition and size may be used to tune the bandgap. As composition influences electron-phonon coupling which in turn governs relaxation dynamics, the composition of ternary quantum dots may be adjusted to change dynamics. Here, we show that exciton-phonon coupling and phonon-assisted exciton relaxation dynamics remain strongly correlated to material composition in ternary In_0.62Ga_0.38P/ZnS and In_0.35Ga_0.65P/ZnS quantum dots using both experimental two-dimensional electronic spectroscopy measurements and quantum dynamical simulations. Theoretical calculations show that alloyed In_1-xGa_xP quantum dots have more complex exciton level structure than parent InP quantum dots. We identify a slower hot exciton cooling rate in In_0.62Ga_0.38P/ZnS, attributed to the presence of ‘energy-retaining’ valley exciton states with strong exciton-phonon coupling. Experimental quantum beating maps reveal a more localized quantum beat pattern for In_0.35Ga_0.65P/ZnS quantum dots, which may relate to the increased number of ‘dim’ exciton levels with reduced spacings. These findings highlight that exciton relaxation dynamics and exciton-phonon coupling in an alloyed In_1-xGa_xP quantum dot system are composition-dependent.

Gallium pnictides, such as GaAs and GaP, are among the most widely used semiconductors for electronic, optoelectronic, and photonic applications. However, solution syntheses of gallium pnictide nanomaterials are less developed than those of many other colloidal semiconductors, including indium pnictides, II-VI and IV-VI compounds, and lead halide perovskites. In this work, we demonstrate that the Wells dehalosilylation reaction can be carried out in molten inorganic salt solvents to synthesize colloidal GaAs, GaP, and GaP_1-xAs_x nanocrystals. We demonstrate that discrete colloidal nanocrystals can be nucleated and grown in a molten salt with control over their size and composition. Additionally, we found that reaction temperatures above 400 °C are crucial for annealing structural defects in GaAs nanocrystals. We also highlight the utility of the as-synthesized GaP nanocrystals by showing that GaP can be solution-processed into high-refractive-index coatings and patterned by direct optical lithography with micron resolution. Finally, we demonstrate that dehalosilylation reactions in molten salts can be generalized to synthesize indium pnictide (Pn = As, P) and ternary (In_1-xGa_xAs and In_1-xGa_xP) quantum dots.

Electrostatically stabilized nanocrystals (NCs) and, in particular, quantum dots (QDs) hold promise for forming strongly coupled superlattices due to their compact and electronically conductive surface ligands. However, studies of the colloidal dispersion and interparticle interactions of electrostatically stabilized sub-10 nm NCs have been limited, hindering the optimization of their colloidal stability and self-assembly. In this study, we employed small-angle X-ray scattering (SAXS) experiments to investigate the interparticle interactions and arrangement of PbS QDs with thiostannate ligands (PbS-Sn2S6 4-) in polar solvents. The study reveals significant deviations from the ideal solution behavior in electrostatically stabilized QD dispersions. Our results demonstrate that PbS-Sn2S6 4- QDs exhibit long-range interactions within the solvent, in contrast to the short-range steric repulsion characteristic of PbS QDs with oleate ligands (PbS-OA). Introducing highly charged multivalent electrolytes screens electrostatic interactions between charged QDs, reducing the length scale of the repulsive interactions. Furthermore, we calculated the second virial (B 2) coefficients from SAXS data, providing insights into how surface chemistry, solvent, and size influence pair potentials. Finally, we explore the influence of long-range interparticle interactions of PbS-Sn2S6 4- QDs on the morphology of films produced by drying or spin-coating colloidal solutions. The long-range repulsive term of PbS-Sn2S6 4- QDs promotes the formation of amorphous films, and screening the electrostatic repulsion by the addition of an electrolyte enables the formation of crystalline domains. These findings highlight the critical role of NC-NC interactions in tailoring the properties of functional materials made of colloidal NCs.

Symmetry control is essential for realizing unconventional properties, such as ferroelectricity, nonlinear optical responses, and complex topological order, thus it holds promise for the design of emerging quantum and photonic systems. Nevertheless, fast and reversible control of symmetry in materials remains a challenge, especially for nanoscale systems. Here, reversible symmetry changes are unveiled in colloidal lead chalcogenide quantum dots on picosecond timescales. Using a combination of ultrafast electron diffraction and total X‐ray scattering, in conjunction with atomic‐scale structural modeling and first‐principles calculations, it is revealed that symmetry‐broken lead sulfide quantum dots restore to a centrosymmetric phase upon photoexcitation. The symmetry restoration is driven by photoexcited electronic carriers, which suppress lead off‐centering for about 100 ps. Furthermore, the change in symmetry is closely correlated with the electronic properties, and the bandgap transiently red‐shifts in the symmetry‐restored quantum dots. Overall, this study elucidates reversible symmetry changes in colloidal quantum dots, and more broadly defines a new methodology to optically control symmetry in nanoscale systems on ultrafast timescales.