Supercrystals (SCs) offer the opportunity to integrate nanoparticles into current technologies without losing their unique and designable properties. In the past two decades, much research has been conducted, allowing the synthesis of differently shaped nanoparticles of various materials. Employing those building units, several methods have been developed enabling the preparation of an increasing number of different superstructures. In this review, an overview is given of the large versatility of surfactant molecules used for SC preparation. While SCs with uncharged organic ligands are by far the largest group, the use of charged or uncommon ligands allows the preparation of unique SCs and superlattices. Additionally, the influence of the ligands on the self‐assembly and properties of the resulting SCs is highlighted.

Much effort has been put into the characterization of the final superstructures and the investigation of the NP assembly by many groups. We know from these studies that the particles can arrange into fcc, bcc, or hcp superlattices showing a long-range ordering of the primary building units. [8] These supracrystals show a high symmetry and well-defined facets yielding octahedra, [7] hexagonal plates, [6] five-armed stars, [9] and more complex twinned structures. [10] Furthermore, there are a lot of investigations dealing with the self-assembly of nanoparticles and the corresponding mechanism. Different models have been developed ranging from a hard sphere model, [11,12] where the NPs are assumed to be spherical objects of similar size, to soft sphere models, where the kind of ligand determines the self-assembly. [13,14] Also the driving force and the assembly probability on steps, holes, or edges have been calculated. [15] Nevertheless, the formation process itself, which takes place in solution, has yet to be fully understood.In this study, we achieved insight into the formation process in solution by investigating the morphology of the resulting supracrystals. Due to the large variety of reported morphologies, we decided to examine one of the most frequently observed symmetrical crystal shapes, which will be called trigonal supracrystal (see Figure 1) in the following. Additionally, we surveyed the influences of different preparation parameters on the resulting superstructures.Trigonal Ag supracrystals have been prepared via gas-phase destabilization techniques (details can be found in the Supporting Information). On the basis of high-resolution scanning electron microscopy (HRSEM), small angle X-ray scattering (SAXS), and transmission electron microscopy (TEM) measurements, the concept of substrate-affected growth is introduced to explain the formation of trigonal shaped supracrystals yielding the size of the supracrystals, which are formed in solution. We will show that the self-assembly can be influenced by the preparation parameters such as concentration, temperature, NP size, and size distribution leading to a control of the size of the supracrystals formed in solution. In order to evaluate the concept of substrate-affected growth, the investigations have been extended toward Au.From the literature, we can conclude that trigonal supracrystals occurred, if there is on the one hand a fcc arrangement of the NPs and on the other hand a plane (substrate) surface. The Formation and Morphology of Nanoparticle SupracrystalsDanny Haubold, Annett Reichhelm, Alexander Weiz, Lars Borchardt, Christoph Ziegler, Lydia Bahrig, Stefan Kaskel, Michael Ruck, and Alexander Eychmüller* Supracrystals are highly symmetrical ordered superstructures built up from nanoparticles (NPs) via self-assembly. While the NP assembly has been intensively investigated, the formation mechanism is still not understood. To shed some light onto the formation mechanism, one of the most common supracrystal morphologies, the trigonal structures, ...

We report on a novel and simple approach to surface ligand design of CdSe-based nanocrystals (NCs) with biocompatible, heterobifunctional polyethylene glycol (PEG) molecules. This method provides high transfer yields of the NCs into aqueous media with preservation of the narrow and symmetric emission bands of the initial organic-capped NCs regardless of their interior crystal structure and surface chemistry. The PEG-functionalized NCs show small sizes, high photoluminescence quantum yields of up to 75%, as well as impressive optical and colloidal stability. This universal approach is applied to different fluorescent nanomaterials (CdSe/CdS, CdSe/CdSCdZnS, and CdSe/CdS/ZnS), extending the great potential of organic-capped NCs for biological applications.

We report on a near-infrared (NIR)-emitting semiconductor nanocrystal (NC) heterostructure with tunable optical properties. This heterostructure is built of two different semiconductor sections in the same NC combining various absorption/emission properties of each constituent part. In our approach, PbSe NCs of different sizes are grown on the tips of CdSe tetrapods (TPs) serving as seeds. In these structures, the CdSe TP acts as an efficient light-harvesting antenna, which can transfer the energy to the PbSe NCs heterogeneously grown on the tips. In turn, the PbSe domains receive the transferred energy from CdSe and perform as NIR light emitters. By adjusting the reaction parameters, we achieved precise control of the optical properties of these PbSe-tipped CdSe TPs, resulting in the fine-tuning of their emission maxima over a wide range from 1000 nm to more than 2000 nm. The obtained results demonstrate the potential and functionality of these heterostructures as novel NIR emitting nanomaterials, which makes them promising candidates to be used in the field of optoelectronics, for example, in NIR light-emitting diodes.

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/pssr.202000282.