arXiv subjects

Zehan Mi

Publications and source records attributed to Zehan Mi.

3 recordsLinked to original sources

arXiv2026

Universal effect of ammonia pressure on synthesis of colloidal metal nitrides in molten salts

Metal nitrides represent a large class of materials with extensive applications in optoelectronics, energy, and healthcare technologies. For example, GaN and related nitride semiconductors are key materials for solid-state lighting and high-power electronics, TiN and other early transition metal nitrides (TMNs) are widely used in wear-resistant alloys, tool coatings, catalysts and medical implants. Strong metal-nitrogen bonds grant nitrides structural rigidity as well as chemical and thermal stability. However, the covalency of metal-nitrogen bonds necessitates high temperatures to synthesize crystalline metal nitrides. Common synthetic routes include high-temperature solid-state nitridation, crystal growth in supercritical ammonia, molecular-beam epitaxy (MBE), reactive sputtering, and chemical vapor deposition (CVD). The solution synthesis of colloidal nitride nanocrystals (NCs) is rare and particularly challenging because commonly used solvents and surfactants decompose at temperatures far below those required for crystallization of most metal nitrides. Here we report a general approach to solution synthesis of colloidal metal nitride NCs by reacting metal halides and ammonia dissolved in molten inorganic salts at elevated pressures. Successful syntheses of colloidal TiN, VN, GaN, NbN, Mo2N, Ta3N5, W2N, as well as ternary Ti1-xVxN NCs, are demonstrated. These NCs expand the scope of solution-processable technologically important materials.

cond-mat.mtrl-sci↗

arXiv2026

Atomic Alignment in PbS Nanocrystal Superlattices with Compact Inorganic Ligands via Reversible Oriented Attachment of Nanocrystals

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 $Sn_2S_6^{4-}$ and $AsS_4^{3-}$, 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 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 $Sn_2S_6^{4-}$-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.

cond-mat.mtrl-sci↗

arXiv2024

Colloidal dispersions of sterically and electrostatically stabilized PbS quantum dots: the effect of stabilization mechanism on structure factors, second virial coefficients, and film-forming properties

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 on the colloidal dispersion and interparticle interactions of electrostatically stabilized sub-10 nm NCs have been limited, hindering the optimization of 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-Sn2S64-) in polar solvents. The study reveals significant deviations from ideal solution behavior in electrostatically stabilized QD dispersions. Our results demonstrate that PbS-Sn2S64- 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 calculate the second virial (B2) 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-Sn2S64- QDs on the morphology of films produced by drying or spin-coating colloidal solutions. The long-range repulsive term of PbS-Sn2S64- QDs promotes the formation of amorphous films, and screening the electrostatic repulsion by addition of an electrolyte enables the formation of a crystalline film. These findings highlight the critical role of NC-NC interactions in tailoring the properties of functional nanomaterials.

cond-mat.soft↗