Revealing Driving Forces in Quantum Dot Supercrystal Assembly

Abstract

The assembly of semiconductor nanoparticles, quantum dots (QDs), into dense crystalline nanostructures holds great promise for future optoelectronic devices. However, knowledge of the sub-nanometer scale driving forces underlying the kinetic processes of nucleation, growth, and final densification during QD assembly remains poor. Emulsion-templated assembly has recently been shown to provide good control over the bulk condensation of QDs into highly ordered 3D supercrystals. Here, emulsion-templated assembly is combined with in situ small-angle X-ray scattering to obtain direct insight into the nanoscale interactions underlying the nucleation, growth, and densification of QD supercrystals. At the point of supercrystal nucleation, nanoparticles undergo a hard-sphere-like crystallization into a hexagonal-close-packed lattice, slowly transforming into a face-centered-cubic lattice. The ligands play a crucial role in balancing steric repulsion against attractive van der Waals forces to mediate the initial equilibrium assembly, but cause the QDs to be progressively destabilized upon densification. The rich detail of this kinetic study elucidates the assembly and thermodynamic properties that define QD supercrystal fabrication approaching single-crystal quality, paving the way toward their use in optoelectronic devices.

Simultaneous Photonic and Excitonic Coupling in Spherical Quantum Dot Supercrystals

“…This coupling is still negligible in the smallest supraparticles, likely because of their structural disorder. Instead, larger FCC SCs of OA-capped QDs feature 12 nearest neighbors with an average surface-to-surface distance of ≈2 nm, 27 increasing the probability of coupling events. The decrease in interdot distance due to the exchange to shorter TG ligands further enhances this coupling probability, resulting in a free biexciton system.…”

“…While a multitude of techniques have been developed to assemble nanocrystals into two- and three-dimensional superstructures, 22 − 26 the emulsion template is particularly valuable as it allows the formation of structurally ordered supraparticles, or SCs, with grain sizes >600 nm by imposing exceptionally slow rates of solvent evaporation of the order of ∼1 μL/min, 27 therefore allowing the QDs to reach their minimum free energy configurations. 16 , 27 − 29 The diffraction pattern measured from a dispersion of these SCs features sharp reflections revealing a face-centered cubic (FCC) structure with an average crystalline grain size of ξ ≈ 276 nm as extracted from the Scherrer equation, corresponding to QDs per grain on average, where l = 1 nm is the thickness of the ligand shell 27 and ϕ FCC = 0.74 is the volume fraction of a FCC crystal at close packing ( Figure 1 b). These results are further confirmed by transmission electron microscopy, showing spherical SCs with diameters comparable to the extracted grain size ( Figure 1 c,d), while for smaller diameters the structure appears disordered ( Figure S11 ).…”

Quantum Dot Solar Cells: Small Beginnings Have Large Impacts

“…Figure 6b shows that regardless of device configuration, bandgap, or research group (with the exception of 2 devices) the practical limit for the V OC is~1/2 the bandgap (black line). The disorder in the energy landscape caused by film formation (e.g., QD necking, random packing) that leads to energy tailing and mid-gap states could be mitigated by further improvements in film formation of superlattices (long-range ordering of QDs) [243][244][245][246][247][248][249][250]. In summary, we described the processes that occur when a QD absorbs a photon and how these energy flow pathways influence quantum dot solar cells.…”

Highly Stable Perovskite Supercrystals via Oil-in-Oil Templating

“… 20 For the CdSe NCs in oil-in-water emulsions, we observed that their assembly is determined by effective hard-sphere behavior, while the late stages are accompanied by an additional attractive component upon compression of the ligand shell due to the receding interface of the evaporating droplets. 11 We speculate that, in analogy, the perovskite NC assembly may be dominated by an effective hard (rounded) cube behavior. 21 − 23 …”