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International audience; Colloidal semiconductor nanocrystals (NCs) are believed to strongly absorb resonant light; this property is key to ongoing applications of NCs as phosphor in light-emitting diodes (LEDs) for lighting, in displays, and photodetectors. However, it has been realized that NCs in a low dielectric medium suffer from a weak permeation of the electromagnetic field in the solid due to formation of depolarization fields, strongly reducing light absorption, as quantified by the local-field factor. For low-dimensional semiconductors, such as one-dimensional (1D) rods and two-dimensional (2D) superlattices, this depolarization field depends critically on the electromagnetic field direction versus the long direction of the material. Here we present atomistic calculations of the screening of the electromagnetic field for individual NCs and technologically important architectures, i.e., 1D and 2D superlattices of epitaxially connected NCs. We also investigate the combined effects of epitaxial connections and excitonic interactions in the particular case of dimers of NCs. Even if it can be improved by a matrix of high dielectric constant, the local-field factor of individual NCs is very low but can increase more than an order of magnitude in NC chains or NC superlattices for an in-line or in-plane electromagnetic field. The calculations of the optical absorptance agree with recent experimental results obtained on superlattices of PbSe and CdSe superlattices. The reduced screening of the electric field in the case of strong epitaxial connections between the NCs was recently invoked to explain that the absorptance of a 2D superlattice approaches the absorptance quantum (πα, where α is the fine structure constant) observed with semiconductor quantum wells [P.T. Prins et al., Nano Lett. 21, 9426 (2021)]. We conclude that atomically coherent superlattices have a strong light absorption and emission, even in the NC monolayer limit, and are primary candidates for polarization-selective photodetectors and LEDs.

Three quantum dot luminescent solar concentrators (QDLSCs) are constructed to assess their performance in an outdoor environment over an entire year. The QDLSCs have a (Formula presented.) PMMA-Kraton-PMMA sandwich structure with either InP/ZnSe/ZnS, (Formula presented.), or CdSe/CdS/ZnS core/shell quantum dots incorporated in the Kraton interlayer. Furthermore, two reference LSCs are included: one using Lumogen F Red 305 as the luminophore and one without a luminophore in the Kraton layer. The power conversion efficiency is assessed for a cloudy and a sunny day, showing the influence of diffuse and direct irradiance. Moreover, the influence of mounting orientation and direct irradiance is analyzed for individual solar strips attached to the sides. Long-term results show an efficiency increase of (Formula presented.) and InP/ZnSe/ZnS QDLSC while the CdSe/CdS/ZnS QDLSCs and the Lumogen LSC show a pronounced drop in efficiency in the first 3 months. Photodegradation studies under continuous white light exposure for 420 h are performed on smaller pieces cut from the QDLSCs before their assembly outdoors and show similar trends to those observed in the 1 year outdoor study. Future research will focus on the postmortem analysis of the QDLSCs and increasing the efficiencies.

Semiconductor superstructures made from assembled and epitaxially connected colloidal nanocrystals (NCs) hold promise for crystalline solids with atomic and nanoscale periodicity, whereby the band structure can be tuned by the geometry. The formation of especially the honeycomb superstructure on a liquid substrate is far from understood and suffers from weak replicability. Here, we introduce 1,4-butanediol as an unreactive substrate component, which is mixed with reactive ethylene glycol to tune for optimal reactivity. It shows us that the honeycomb superlattice has a NC precursor state before oriented attachment occurs, in the form of a self-assembled hexagonal bilayer. We propose that the difference between the formation of the square or honeycomb superstructure occurs during the self-assembly phase. To form a honeycomb superstructure, it is crucial to stabilize the hexagonal bilayer in the presence of solvent-mediated repulsion. In contrast, a square superstructure benefits from the contraction of a hexagonal monolayer due to the absence of a solvent. A second experiment shows the very last stage of the process, where the increasing alignment of NCs is quantified using selected-area electron diffraction (SAED). The combination of transmission electron microscopy (TEM), SAED, and tomography used in these experiments shows that the (100)/(100) facet-to-facet attraction is the main driving force for NC alignment and attachment. These findings are validated by coarse-grained molecular dynamic simulations, where we show that an optimal NC repulsion is crucial to create the honeycomb superstructure.

The growth of two-dimensional platelets of the CdX family (X = S, Se, or Te) in an organic solvent requires the presence of both long- and short-chain ligands. This results in nanoplatelets of atomically precise thickness and long-chain ligand-stabilized Cd top and bottom surfaces. The platelets show a bright and spectrally pure luminescence. Despite the enormous interest in CdX platelets for optoelectronics, the growth mechanism is not fully understood. Riedinger et al. studied the reaction without a solvent and showed the favorable role for short-chain carboxylates for growth in two dimensions. Their model, based on the total energy of island nucleation, shows favored side facet growth versus growth on the top and bottom surfaces. However, several aspects of the synthesis under realistic conditions are not yet understood: Why are both short- and long-chain ligands required to obtain platelets? Why does the synthesis result in both isotropic nanocrystals and platelets? At which stage of the reaction is there bifurcation between isotropic and 2D growth? Here, we report an in situ study of the CdSe nanoplatelet reaction under practical synthesis conditions. We show that without short-chain ligands, both isotropic and mini-nanoplatelets form in the early stage of the process. However, most remaining precursors are consumed in isotropic growth. Addition of acetate induces a dramatic shift toward nearly exclusive 2D growth of already existing mini-nanoplatelets. Hence, although myristate stabilizes mini-nanoplatelets, mature nanoplatelets only grow by a subtle interplay between myristate and acetate, the latter catalyzes fast lateral growth of the side facets of the mini-nanoplatelets.

Conspectus Intuitively, chemists see crystals grow atom-by-atom or molecule-by-molecule, very much like a mason builds a wall, brick by brick. It is much more difficult to grasp that small crystals can meet each other in a liquid or at an interface, start to align their crystal lattices and then grow together to form one single crystal. In analogy, that looks more like prefab building. Yet, this is what happens in many occasions and can, with reason, be considered as an alternative mechanism of crystal growth. Oriented attachment is the process in which crystalline colloidal particles align their atomic lattices and grow together into a single crystal. Hence, two aligned crystals become one larger crystal by epitaxy of two specific facets, one of each crystal. If we simply consider the system of two crystals, the unifying attachment reduces the surface energy and results in an overall lower (free) energy of the system. Oriented attachment often occurs with massive numbers of crystals dispersed in a liquid phase, a sol or crystal suspension. In that case, oriented attachment lowers the total free energy of the crystal suspension, predominantly by removal of the nanocrystal/liquid interface area. Accordingly, we should start by considering colloidal suspensions with crystals as the dispersed phase, i.e., “sols”, and discuss the reasons for their thermodynamic (meta)stability and how this stability can be lowered such that oriented attachment can occur as a spontaneous thermodynamic process. Oriented attachment is a process observed both for charge-stabilized crystals in polar solvents and for ligand capped nanocrystal suspensions in nonpolar solvents. In this last system different facets can develop a very different reactivity for oriented attachment. Due to this facet selectivity, crystalline structures with very specific geometries can be grown in one, two, or three dimensions; controlled oriented attachment suddenly becomes a tool for material scientists to grow architectures that cannot be reached by any other means. We will review the work performed with PbSe and CdSe nanocrystals. The entire process, i.e., the assembly of nanocrystals, atomic alignment, and unification by attachment, is a very complex and intriguing process. Researchers have succeeded in monitoring these different steps with in situ wave scattering methods and real-space (S)TEM studies. At the same time coarse-grained molecular dynamics simulations have been used to further study the forces involved in self-assembly and attachment at an interface. We will briefly come back to some of these results in the last sections of this review.

The optical absorptance of a single graphene layer over a wide range of wavelengths is known to be remarkably constant at the universal value pi alpha where alpha is the fine structure constant. Using atomistic tight-binding calculations, we show that the absorptance spectra of nanometer-thin layers (quantum wells) of group-IV, III-V, II-VI, or IV-VI semiconductors are characterized by marked plateaus at integer values of pi alpha, in the absence of excitonic effects. In the case of InAs, the results obtained are in excellent agreement with the currently available experimental data. By revisiting experimental data on semiconductor superlattices, we show that pi alpha is also a metric of their absorption when normalized to a single period. We conclude that the pi alpha quantization is universal in semiconductor quantum wells provided that excitonic effects are weak and is therefore not specific to the zero-gap graphene case. The physical origin of this universality and its limits are discussed using analytical models that capture the main underlying physics of the lowest optical transitions in III-V and II-VI semiconductor quantum wells. These models show that the absorptance is ruled by pi alpha independent of the material characteristics because of the presence of a dominant term in the Hamiltonian, linear in the wave vector k (similar to V center dot k), which couples the conduction band to the valence bands. However, the prefactor in front of pi alpha is not unity as in graphene due to the different nature of the electronic states. In particular, the spin-orbit coupling plays an important role in bringing the absorptance plateaus closer to integer values of pi alpha. The case of IV-VI semiconductor (PbSe) quantum wells characterized by a rocksalt lattice and multivalley physics is very similar to that of graphene, with the specification that a "massful gap" is formed around the Dirac points.