Patterning symmetry in the rational design of colloidal crystals

Colloidal particles have the right size to form ordered structures with periodicities comparable to the wavelength of visible light. The tantalizing colours of precious opals and the colour of some species of birds are examples of polycrystalline colloidal structures found in nature. Driven by the demands of several emergent technologies, efforts have been made to develop efficient, self-assembly-based methodologies for generating colloidal single crystals with well-defined morphologies. Somewhat unfortunately, these efforts are often frustrated by the formation of structures lacking long-range order. Here we show that the rational design of patch shape and symmetry can drive patchy colloids to crystallize in a single, selected morphology by structurally eliminating undesired polymorphs. We provide a proof of this concept through the numerical investigation of triblock Janus colloids. One particular choice of patch symmetry yields, via spontaneous crystallization, a pure tetrastack lattice, a structure with attractive photonic properties, whereas another one results in a colloidal clathrate-like structure, in both cases without any interfering polymorphs

The early development of inorganic chlathrates

In this chapter the authors relate the discovery of the first inorganic clathrates, Na8Si46 and NaxSi136 (3 ≤ x ≤ 11), whose cage-like structures were determined by comparison with those of the two most classical gas and liquid clathrate hydrates. The main characteristics of clathrate compounds are recalled and a brief review of clathrate hydrates is given. The different polyhedral cages and their arrangements in the so-called type I structure (Na8Si46) and type II structure (NaxSi136) are described in details. The synthesis, composition and structure of other inorganic clathrates of silicon, germanium and tin with potassium, rubidium and cesium as guest atoms are reported. The crystal structure (type I or type II) and corresponding composition is closely related to the size of the guest alkali atoms. The formation of the characteristic polyhedral cages with a majority of pentagonal faces is discussed, and results from the arrangement of all the tetrahedrons in eclipsed position. The relation between clathrate structures and those of clathrasils (silica-based clathrates), Frank-Kasper alloys and fullerene forms of carbon is also discussed. The first measurements of the physical properties of inorganic clathrates are reviewed, including electrical conductivity, thermal properties, high pressure behavior, NMR and ESR investigations. The ability for the silicon, germanium and tin host lattices to form non-stoichiometric and mixed frameworks with elements of neighboring groups is briefly described, giving rise to a large variety of new inorganic clathrates with ionic guest-host interactions and semiconducting properties

The greening of the McGill Paleoclimate Model. Part II: Simulation of Holocene millennial-scale natural climate changes

Various proxy data reveal that in many regions of the Northern Hemisphere (NH), the middle Holocene ( 6 kyr BP) was warmer than the early Holocene ( 8 kyr BP) as well as the later Holocene, up to the end of the pre-industrial period ( 1800 AD). This pattern of warming and then cooling in the NH represents the response of the climate system to changes in orbital forcing, vegetation cover and the Laurentide Ice Sheet ( LIS) during the Holocene. In an attempt to better understand these changes in the climate system, the McGill Paleoclimate Model (MPM) has been coupled to the dynamic global vegetation model known as VECODE ( see Part I of this two-part paper), and a number of sensitivity experiments have been performed with the "green'' MPM. The model results illustrate the following: ( 1) the orbital forcing together with the vegetation - albedo feedback result in the gradual cooling of global SAT from about 6 kyr BP to the end of the pre-industrial period; ( 2) the disappearance of the LIS over the period 8 - 6 kyr BP, associated with vegetation - albedo feedback, allows the global SAT to increase and reach its maximum at around 6 kyr BP; ( 3) the northern limit of the boreal forest moves northward during the period 8 - 6.4 kyr BP due to the LIS retreat; ( 4) during the period 6.4 - 0 kyr BP, the northern limit of the boreal forest moves southward about 120 km in response to the decreasing summer insolation in the NH; and ( 5) the desertification of northern Africa during the period 8 - 2.6 kyr BP is mainly explained by the decreasing summer monsoon precipitation

Aridification of the Sahara desert caused by Tethys Sea shrinkage during the Late Miocene

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