The latest advances in mesocrystal formation and non-classical crystallization of pre-synthesised nanoparticles have been reviewed with the focus on providing a fuller description of a number of complex systems and their properties and applications through examination of the crystallisation mechanisms at work. Two main crystallization principles have been identified; classical crystallization and particle based aggregation modes of non-classical pathways. To understand the non-classical pathways classical crystallization and its basics are introduced before non-classical pathways, such as oriented attachment and mesocrystal formation, are examined. In particular, the various destabilization mechanisms as applied to the pre-synthesized building blocks in order to form mesocrystalline materials as well as the interparticular influences providing the driving forces are analyzed and compared to the mechanisms at work within classical crystallization. Furthermore, the new properties of the mesocrystalline materials that derive from the collective properties of the nanoparticular building units, and their applications potential are presented.It is shown that this new class of materials has the potential to impact in a number of important areas such as sensor applications, energy conversion, photonic crystals as well as for energy storage, optoelectronics and heterogeneous catalysis or photocatalysis.
Bright emitters with photoluminescence in the spectral region of 800-1600 nm are increasingly important as optical reporters for molecular imaging, sensing, and telecommunication and as active components in electrooptical and photovoltaic devices. Their rational design is directly linked to suitable methods for the characterization of their signal-relevant properties, especially their photoluminescence quantum yield (Φ(f)). Aiming at the development of bright semiconductor nanocrystals with emission >1000 nm, we designed a new NIR/IR integrating sphere setup for the wavelength region of 600-1600 nm. We assessed the performance of this setup by acquiring the corrected emission spectra and Φ(f) of the organic dyes Itrybe, IR140, and IR26 and several infrared (IR)-emissive Cd(1-x)Hg(x)Te and PbS semiconductor nanocrystals and comparing them to data obtained with two independently calibrated fluorescence instruments absolutely or relative to previously evaluated reference dyes. Our results highlight special challenges of photoluminescence studies in the IR ranging from solvent absorption to the lack of spectral and intensity standards together with quantum dot-specific challenges like photobrightening and photodarkening and the size-dependent air stability and photostability of differently sized oleate-capped PbS colloids. These effects can be representative of lead chalcogenides. Moreover, we redetermined the Φ(f) of IR26, the most frequently used IR reference dye, to 1.1 × 10(-3) in 1,2-dichloroethane DCE with a thorough sample reabsorption and solvent absorption correction. Our results indicate the need for a critical reevaluation of Φ(f) values of IR-emissive nanomaterials and offer guidelines for improved Φ(f) measurements.
Make it connected! 2D close-packed layers of inorganic nanoparticles are interconnected by organic fibrils of oleic acid as clearly visualized by electron holography. These fibrils can be mineralised by PbS to transform an organic-inorganic framework to a completely interconnected inorganic semiconducting 2D array.
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, ...
The relationship between nanoparticle geometry and their two dimensional assembly is investigated in order to provide insights into the three dimensional arrangement of mesocrystals. The crystal structure of the nanoparticles and their homogeneity are investigated during structure formation on the mesoscale whereby effects such as fibrillation have been observed.
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