Mesocrystals, which originally was a term to designate superstructures of nanocrystals with a common crystallographic orientation, have now evolved to a materials concept. The discovery that many biominerals are mesocrystals generated a large research interest, and it was suggested that mesocrystals result in better mechanical performance and optical properties compared to single crystalline structures. Mesocrystalline biominerals are mainly found in spines or shells, which have to be mechanically optimized for protection or as a load-bearing skeleton. Important examples include red coral and sea urchin spine as well as bones. Mesocrystals can also be formed from purely synthetic components. Biomimetic mineralization and assembly have been used to produce mesocrystals, sometimes with complex hierarchical structures. Important examples include the fluorapatite mesocrystals with gelatin as the structural matrix, and mesocrystalline calcite spicules with impressive strength and flexibility that could be synthesized using silicatein protein fibers as template for calcium carbonate deposition. Self-assembly of nanocrystals can also result in mesocrystals if the nanocrystals have a well-defined size and shape and the assembly conditions are tuned to allow the nanoparticles to align crystallographically. Mesocrystals formed by assembly of monodisperse metallic, semiconducting, and magnetic nanocrystals are a type of colloidal crystal with a well-defined structure on both the atomic and mesoscopic length scale.Mesocrystals typically are hybrid materials between crystalline nanoparticles and interspacing amorphous organic or inorganic layers. This structure allows to combine disparate materials like hard but brittle nanocrystals with a soft and ductile amorphous material, enabling a mechanically optimized structural design as realized in the sea urchin spicule. Furthermore, mesocrystals can combine the properties of individual nanocrystals like the optical quantum size effect, surface plasmon resonance, and size dependent magnetic properties with a mesostructure and morphology tailored for specific applications. Indeed, mesocrystals composed of crystallographically aligned polyhedral or rodlike nanocrystals with anisotropic properties can be materials with strongly directional properties and novel collective emergent properties. An additional advantage of mesocrystals is that they can combine the properties of nanoparticles with a structure on the micro- or macroscale allowing for much easier handling.

Ferdinand Bernauer proposed in his monograph, "Gedrillte" Kristalle (1929), that a great number of simple, crystalline substances grow from solution or from the melt as polycrystalline spherulites with helically twisting radii that give rise to distinct bull's-eye patterns of concentric optical bands between crossed polarizers. The idea that many common molecular crystals can be induced to grow as mesoscale helices is a remarkable proposition poorly grounded in theories of polycrystalline pattern formation. Recent reinvestigation of one of the systems Bernauer described revealed that rhythmic precipitation in the absence of helical twisting accounted for modulated optical properties [Gunn, E. et al. J. Am. Chem. Soc. 2006, 128, 14234-14235]. Herein, the Bernauer hypothesis is re-examined in detail for three substances described in "Gedrillte" Kristalle, potassium dichromate, hippuric acid, and tetraphenyl lead, using contemporary methods of analysis not available to Bernauer, including micro-focus X-ray diffraction, electron microscopy, and Mueller matrix imaging polarimetry. Potassium dichromate is shown to fall in the class of rhythmic precipitates of undistorted crystallites, while hippuric acid spherulites are well described as helical fibrils. Tetraphenyl lead spherulites grow by twisting and rhythmic precipitation. The behavior of tetraphenyl lead is likely typical of many substances in "Gedrillte" Kristalle. Rhythmic precipitation and helical twisting often coexist, complicating optical analyses and presenting Bernauer with difficulties in the characterization and classification of the objects of his interest.

The ability to prepare nanostructured and/or nanocomposite materials on a large scale by simple and controllable routes still remains a challenge in chemistry and material science. By employing well-established synthetic strategies, nanoparticles with different sizes, shapes and compositions can be readily produced. The high tendency for self-aggregation and selforganization of these nanosized building blocks, which are surface-stabilized by organic molecules, into superstructures has been used to create 2D and 3D assemblies. [1][2][3][4][5][6][7][8][9] In the last decade, a series of 3D colloidal superlattices composed of different nanoparticles (Ag, Au, PbS, PbSe, CeO 2 , FePt, CoPt 3 , Fe 3 O 4 , PbSe/Au, Fe 2 O 3 /PbSe, PbSe/Pd, CdSe/PbSe, etc.) has been reported. [10][11][12][13][14][15][16][17][18][19][20] Particularly, PbS-organic nanoparticles of different shapes (including cubes, octahedra, truncated octahedra, rhombicuboctahedra, etc.) can be easily synthesized on a large scale, and attracted much attention [15,16,21,22] because of a variety of possible applications. [23][24][25][26][27] Therefore, this kind of system provides a chance to study diverse packing arrangements (e.g., fcc, bcc, etc.) and specific orientational ordering of nanoparticles. [11,13,15,16,28,29] Experimental observations [16,[29][30][31][32][33][34] suggest that nanoparticles within a colloidal crystal tend to arrange in such a way that the optimal packing efficiency is achieved (principle of maximum space filling [35] ). In a relatively common case of truncated octahedrally shaped nanoparticles, the available experimental data [4,5,16,[29][30][31][32][33]36] allow to rationalize the formation of a particular type of the superlattice array (depending on the degree of coverage of nanocrystals by organic molecules) by considering four phenomenological models: A) Rigid, anisotropically shaped space-filling nanoparticles (inorganic part) without or with a tiny shell of organic molecules; B) Hard spheres with a comparatively large anisotropic core (inorganic part) covered by a relatively thin shell of organic molecules; C) Hard spheres with a comparatively small anisotropic core (inorganic part) and a thick shell of organic molecules; D) Soft and easily deformable spheres with a small anisotropic core (inorganic part) and an even thicker shell of organic molecules.For each case (A-D) the type of superlattice packing (translational order [37] ) and orientational ordering of nanoparticles within the superlattice array is significantly different. In the simplest case (A), the more or less pure, inorganic, truncated octahedrally shaped nanoparticles assemble into a bcc superlattice [33] with 100 % packing efficiency and strong orientational relationship (crystallographic directions of nanocrystals are coaxial with those of the superlattice). [35,37] In case of models B and C, by increasing the degree of coverage of anisotropic (inorganic) nanoparticles by organic molecules, their faces are continuously smoothed, thereby introducing a certa...

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.

Spontaneous twisting of single crystals is a common growth induced deformation. But as twisted crystals thicken they can untwist. restoring a straight form. The mechanics of this process was studied for vapor grown needle-like crystals of N-benzoylglycine (hippuric acid) and N-(2-thienylcarbonyl)glycine, and analyzed by phenomenological models. The elastic stress at the crystal tip undergoes plastic relaxation leading to the twisting deformations. As the crystal grows and thickens it partially untwists showing linear increases of the twist period with crystal thickness. Such behavior was simulated with a model that assumes the constant density of defects in successive growth layers. However. transmission electron microscopy does not reveal any dislocations or other extended defects typically associated with plastic deformation. Published data on other materials show the linear dependencies of pitch on thickness suggesting comparable untwisting mechanisms for different materials.