Resolving the Growth of 3D Colloidal Nanoparticle Superlattices by Real-Time Small-Angle X‑ray Scattering

The kinetics and intricate interactions governing the growth of 3D single nanoparticle (NP) superlattices (SLs, SNSLs) and binary NP SLs (BNSLs) in solution are understood by combining controlled solvent evaporation and <i>in situ</i>, real-time small-angle X-ray scattering (SAXS). For the iron oxide (magnetite) NP SLs studied here, the larger the NP, the farther apart are the NPs when the SNSLs begin to precipitate and the closer they are after ordering. This is explained by a model of NP assembly using van der Waals interactions between magnetite cores in hydrocarbons with a ∼21 zJ Hamaker constant. When forming BNSLs of two different sized NPs, the NPs that are in excess of that needed to achieve the final BNSL stoichiometry are expelled during the BNSL formation, and these expelled NPs can form SNSLs. The long-range ordering of these SNSLs and the BNSLs can occur faster than the NP expulsion

Resolving the Growth of 3D Colloidal Nanoparticle Superlattices by Real-Time Small-Angle X‑ray Scattering

The kinetics and intricate interactions governing the growth of 3D single nanoparticle (NP) superlattices (SLs, SNSLs) and binary NP SLs (BNSLs) in solution are understood by combining controlled solvent evaporation and <i>in situ</i>, real-time small-angle X-ray scattering (SAXS). For the iron oxide (magnetite) NP SLs studied here, the larger the NP, the farther apart are the NPs when the SNSLs begin to precipitate and the closer they are after ordering. This is explained by a model of NP assembly using van der Waals interactions between magnetite cores in hydrocarbons with a ∼21 zJ Hamaker constant. When forming BNSLs of two different sized NPs, the NPs that are in excess of that needed to achieve the final BNSL stoichiometry are expelled during the BNSL formation, and these expelled NPs can form SNSLs. The long-range ordering of these SNSLs and the BNSLs can occur faster than the NP expulsion

Resolving the Growth of 3D Colloidal Nanoparticle Superlattices by Real-Time Small-Angle X‑ray Scattering

The kinetics and intricate interactions governing the growth of 3D single nanoparticle (NP) superlattices (SLs, SNSLs) and binary NP SLs (BNSLs) in solution are understood by combining controlled solvent evaporation and <i>in situ</i>, real-time small-angle X-ray scattering (SAXS). For the iron oxide (magnetite) NP SLs studied here, the larger the NP, the farther apart are the NPs when the SNSLs begin to precipitate and the closer they are after ordering. This is explained by a model of NP assembly using van der Waals interactions between magnetite cores in hydrocarbons with a ∼21 zJ Hamaker constant. When forming BNSLs of two different sized NPs, the NPs that are in excess of that needed to achieve the final BNSL stoichiometry are expelled during the BNSL formation, and these expelled NPs can form SNSLs. The long-range ordering of these SNSLs and the BNSLs can occur faster than the NP expulsion

Small Angle X‑ray Scattering of Iron Oxide Nanoparticle Monolayers Formed on a Liquid Surface

In situ small-angle X-ray scattering (SAXS) is used to show that iron oxide nanoparticles (NPs) of a range of sizes form hexagonally ordered monolayers (MLs) on a diethylene glycol liquid surface, after drop-casting the NPs in hexane and subsequent hexane evaporation. The formation of the ordered NP ML is followed in real time by SAXS when using a heptane solvent. During drying, the NPs remain in the hexane or heptane layer, and an ordered structure is not formed then. After drying, the NPs are farther apart than expected from only van der Waals attraction between the NP cores and Brownian motion considerations, which suggests the importance of ligand attraction in binding the NPs

Passivation of CdSe Quantum Dots by Graphene and MoS₂ Monolayer Encapsulation

The encapsulation of a monolayer of CdSe quantum dots (QDs) by one-to-three layer graphene and MoS₂ sheets protects the QDs from oxidation. Photoluminescence (PL) from the QD cores shows a much slower decrease in core diameter over time due to slower oxidation in regions where the QDs are covered by van der Waals (vdW) layers than in those where they are not, for chips stored both in the dark and in the presence of light. PL mapping shows that the CdSe QDs under the central part of the vdW sheet age slower than those near its edges, because oxidation of the covered QDs is limited by transport of oxygen from the edges of the vdW sheets and not transport across the vdW layers. The transport of oxygen to the covered QDs is analyzed by coupling the PL results and a model of QD core oxidation