Micrometre-sized colloidal particles can be viewed as large atoms with tailorable size, shape and interactions. These building blocks can assemble into extremely rich structures and phases, in which the thermal motions of particles can be directly imaged and tracked using optical microscopy. Hence, colloidal particles are excellent model systems for studying phase transitions, especially for poorly understood kinetic and non-equilibrium microscale processes. Advances in colloid fabrication, assembly and computer simulations have opened up numerous possibilities for such research. In this Review, we describe recent progress in the study of colloidal crystals composed of tunable isotropic spheres, anisotropic particles and active particles. We focus on advances in crystallization, melting and solidsolid transitions, and highlight challenges and future perspectives in phase-transition studies within colloidal crystals.Micrometre-sized colloidal particles can assemble into numerous structures and phases (FIG.1), and are excellent model systems for the study of phase transitions [1-6] because their thermal motions can be directly visualized by optical video microscopy [7] and tracked by image processing [8]. By contrast, the small spatial and timescales of atomic motions make the microscopic kinetics during phase transitions difficult to resolve.Phase transitions are ubiquitous in nature and industry, and have an important role in materials science, statistical physics, cosmology, biophysics, chemistry and earth science. They depend strongly on dimensionality, surface properties, defects, heating and cooling rates, and external fields. Phase transitions can be classified into first-, secondand infinite-order (in which the free energy is an infinitely differentiable exponential function such as the Koster-litzThouless (KT) transition). The equilibrium behaviour of continuous phase transitions, including second-and infinite-order transitions, can be well described theoretically; however, the fundamental theory to describe first-order phase transitions is lacking. The kinetics of first-order and continuous phase transitions are difficult to predict [9].Colloidal particles can be viewed as large atoms with tailorable size, shape and interactions [6]. Although colloids and atoms differ in some aspects (BOX 1), their phase transitions share many similarities; for example, they both follow classical nucleation theory (CNT) at weak supersaturations [10] and deviate from CNT at strong supersaturations [11]. Hence, colloids can provide general information about phase transitions [4][5][6]. Besides being important as micrometre-scale analogues of atoms, colloids are interesting in their own right. In the past two decades, colloidal model systems have provided vital insights into the microscopic kinetics of melting, crystallization, glass transitions and solid solid transitions. Recent breakthroughs in colloidal fabrication, assembly and computer simulations open new opportunities for phase-transition studies. Comparison between ...

A highZTof up to 0.6 at 773 K with improved mid-temperature thermoelectric performance is achieved for textured SnSe polycrystals without deliberate doping.

Crystal surfaces typically melt into a thin liquid layer at temperatures slightly below the melting point of the crystal. Such surface premelting is prevalent in all classes of solids and is important in a variety of metallurgical, geological and meteorological phenomena. Premelting has been studied using X-ray diffraction and differential scanning calorimetry, but the lack of single-particle resolution makes it hard to elucidate the underlying mechanisms. Colloids are good model systems for studying phase transitions because the thermal motions of individual micrometre-sized particles can be tracked directly using optical microscopy. Here we use colloidal spheres with tunable attractions to form equilibrium crystal-vapour interfaces, and study their surface premelting behaviour at the single-particle level. We find that monolayer colloidal crystals exhibit incomplete premelting at their perimeter, with a constant liquid-layer thickness. In contrast, two- and three-layer crystals exhibit conventional complete melting, with the thickness of the surface liquid diverging as the melting point is approached. The microstructures of the surface liquids differ in certain aspects from what would be predicted by conventional premelting theories. Incomplete premelting in the monolayer crystals is triggered by a bulk isostructural solid-solid transition and truncated by a mechanical instability that separately induces homogeneous melting within the bulk. This finding is in contrast to the conventional assumption that two-dimensional crystals melt heterogeneously from their free surfaces (that is, at the solid-vapour interface). The unexpected bulk melting that we observe for the monolayer crystals is accompanied by the formation of grain boundaries, which supports a previously proposed grain-boundary-mediated two-dimensional melting theory. The observed interplay between surface premelting, bulk melting and solid-solid transitions challenges existing theories of surface premelting and two-dimensional melting.

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Stripes on a plane: A set of highly ordered microscopic stripes (purple; see scheme) were produced over a large area by using controlled evaporative self-assembly in a cylinder-on-Si geometry of conjugated homopolymers or all-conjugated diblock copolymer (P3BHT). The crystallinity of the as-prepared assemblies of P3BHT was greatly improved following chloroform vapor annealing, resulting in a fourfold increase in electrical conductivity.

We present the fabrication and characterization of nanoscale electrical interconnect test structures constructed from aligned single-wall carbon nanotubes using a template-based fluidic assembly process. This CMOS-friendly process enables the formation of highly aligned parallel nanotube interconnect structures on SiO(2)/Si substrates of widths and lengths that are limited only by lithographical limits and, hence, can be easily integrated onto existing Si-based platforms. These structures can withstand current densities of approximately 10(7) A.cm(-2), comparable or better than copper at similar dimensions. Both the nanotube alignment and failure current density improve with decreasing structure width. In addition, we present a novel Pt nanocluster decoration method that drastically decreases the resistivity of the test structures. Ab initio density functional theory calculations indicate that the increase in conductivity of the nanotubes is caused by an increase in conduction channels close to their Fermi levels due to the platinum nanocluster decoration, with a possible conversion of the semiconducting single-wall carbon nanotubes into metallic ones. These results reflect a huge step toward the proposed replacement of copper-based interconnects with carbon nanotubes at gigascale integration levels.

A phase-field variational implicit-solvent approach is developed for the solvation of charged molecules. The starting point of such an approach is the representation of a solute-solvent interface by a phase field that takes one value in the solute region and another in the solvent region, with a smooth transition from one to the other on a small transition layer. The minimization of an effective free-energy functional of all possible phase fields determines the equilibrium conformations and free energies of an underlying molecular system. All the surface energy, the solute-solvent van der Waals interaction, and the electrostatic interaction are coupled together self-consistently through a phase field. The surface energy results from the minimization of a double-well potential and the gradient of a field. The electrostatic interaction is described by the Coulomb-field approximation. Accurate and efficient methods are designed and implemented to numerically relax an underlying charged molecular system. Applications to single ions, a two-plate system, and a two-domain protein reveal that the new theory and methods can capture capillary evaporation in hydrophobic confinement and corresponding multiple equilibrium states as found in molecular dynamics simulations. Comparisons of the phase-field and the original sharp-interface variational approaches are discussed.

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