Classical Force Field Parameters for InP and InAs Quantum Dots with Various Surface Passivations.

Classical molecular dynamics (MD) simulations on realistic colloidal quantum dot (QD) systems are often hampered by missing force field (FF) parameters for an accurate description of the QD-ligand interface. However, such calculations are of major interest, specifically for studying the surface chemistry of colloidal nanocrystals. In this work, we have utilized a previously published stochastic optimization algorithm to obtain FF parameters for InP and InAs QDs capped by Cl, amine, carboxylate, and thiolate ligands. Our FF parameters are interfaced with well-established FFs for organic molecules, allowing for the simulation of InP and InAs QDs with a broad range of organic ligands in explicit apolar solvents. The quality of our FF parameters was assessed by comparing properties of the classical MD simulations with ab initio MD simulations and experimental and theoretical values from the literature.

Acid-Base Mediated Ligand Exchange on Near-Infrared Absorbing, Indium-Based III-V Colloidal Quantum Dots.

Colloidal quantum dots (QDs) made from In-based III-V semiconductors are emerging as a printable infrared material. However, the formulation of infrared inks and the formation of electrically conductive QD coatings is hampered by a limited understanding of the surface chemistry of In-based QDs. In this work, we present a case study on the surface termination of IR active III-V QDs absorbing at 1220 nm that were synthesized by reducing a mixture of indium halides and an aminoarsine by an aminophosphine in oleylamine. We find that this recently established synthesis method yields In(As,P) QDs with minor phosphorus admixing and a surface terminated by a mixture of oleylamine and chloride. Exposing these QDs to protic surface-active compounds RXH, such as fatty acids or alkanethiols, initiates a ligand exchange reaction involving the binding of the conjugate base RX- and the desorption of 1 equiv of alkylammonium chloride. Using density functional theory simulations, we confirm that the formation of the alkylammonium chloride salt can provide the energy needed to drive such acid/base mediated ligand exchange reactions, even for weak organic acids such as alkanethiols. We conclude that the unique surface termination of In(As,P) QDs, consisting of a mixture of L-type and X-type ligands and acid/base mediated ligand exchange, can form a general model for In-based III-V QDs synthesized using indium halides and aminopnictogens.

Extended Nucleation and Superfocusing in Colloidal Semiconductor Nanocrystal Synthesis.

Hot-injection synthesis is renowned for producing semiconductor nanocolloids with superb size dispersions. Burst nucleation and diffusion-controlled size focusing during growth have been invoked to rationalize this characteristic yet experimental evidence supporting the pertinence of these concepts is scant. By monitoring a CdSe synthesis in-situ with X-ray scattering, we find that nucleation is an extended event that coincides with growth during 15-20% of the reaction time. Moreover, we show that size focusing outpaces predictions of diffusion-limited growth. This observation indicates that nanocrystal growth is dictated by the surface reactivity, which drops sharply for larger nanocrystals. Kinetic reaction simulations confirm that this so-called superfocusing can lengthen the nucleation period and promote size focusing. The finding that narrow size dispersions can emerge from the counteracting effects of extended nucleation and reaction-limited size focusing ushers in an evidence-based perspective that turns hot injection into a rational scheme to produce monodisperse semiconductor nanocolloids.

Aging of the reaction mixture as a tool to modulate the crystallite size of UiO-66 into the low nanometer range.

Nanosized UiO-66 with an unprecedented crystallite size of 10 nm was synthesized by exploiting controlled aging of stock solutions of Zr(4+) in N,N-dimethylformamide in the presence of variable amounts of water and acetic acid prior to the addition of the ligand. The yield of the synthesis is not affected, affording high conversion of the starting reagents into the product.

On-surface derivatisation of aromatic molecules on graphene: the importance of packing density.

An efficient, high-throughput method for the formation of densely packed molecular films on graphene is reported. The films exhibit high stability and remain intact during a subsequent derivatisation reaction, offering a versatile route for the non-covalent functionalisation of graphene.

Understanding and optimising the packing density of perylene bisimide layers on CVD-grown graphene.

The non-covalent functionalisation of graphene is an attractive strategy to alter the surface chemistry of graphene without damaging its superior electrical and mechanical properties. Using the facile method of aqueous-phase functionalisation on large-scale CVD-grown graphene, we investigated the formation of different packing densities in self-assembled monolayers (SAMs) of perylene bisimide derivatives and related this to the amount of substrate contamination. We were able to directly observe wet-chemically deposited SAMs in scanning tunnelling microscopy (STM) on transferred CVD graphene and revealed that the densely packed perylene ad-layers adsorb with the conjugated π-system of the core perpendicular to the graphene substrate. This elucidation of the non-covalent functionalisation of graphene has major implications on controlling its surface chemistry and opens new pathways for adaptable functionalisation in ambient conditions and on the large scale.