Because the Roman god Janus was usually represented with two heads placed back to back, the term Janus is used for the description of particles whose surfaces of both hemispheres are different from a chemical point of view. So, they could be used as building blocks for supraparticular assemblies, as dual-functionalized devices, as particular surfactants if one hemisphere is hydrophilic and the other hydrophobic, etc. If they could allow the segregation of negative charges on one hemisphere and positive charges on the other one, they would display a giant dipole moment allowing their remote positioning by rotation in an electric field as a function of field polarity. This review deals with the great and imaginative efforts which were devoted to the synthesis of Janus particles in the last fifteen years. A special emphasis is made on scalable techniques and on those which apply to the preparation of Janus particles in the nanometer range. Specific properties and applications of Janus particles are discussed.

Over the last three decades, photonic crystals (PhCs) have attracted intense interests thanks to their broad potential applications in optics and photonics. Generally, these structures can be fabricated via either “top-down” lithographic or “bottom-up” self-assembly approaches. The self-assembly approaches have attracted particular attention due to their low cost, simple fabrication processes, relative convenience of scaling up, and the ease of creating complex structures with nanometer precision. The self-assembled colloidal crystals (CCs), which are good candidates for PhCs, have offered unprecedented opportunities for photonics, optics, optoelectronics, sensing, energy harvesting, environmental remediation, pigments, and many other applications. The creation of high-quality CCs and their mass fabrication over large areas are the critical limiting factors for real-world applications. This paper reviews the state-of-the-art techniques in the self-assembly of colloidal particles for the fabrication of large-area high-quality CCs and CCs with unique symmetries. The first part of this review summarizes the types of defects commonly encountered in the fabrication process and their effects on the optical properties of the resultant CCs. Next, the mechanisms of the formation of cracks/defects are discussed, and a range of versatile fabrication methods to create large-area crack/defect-free two-dimensional and three-dimensional CCs are described. Meanwhile, we also shed light on both the advantages and limitations of these advanced approaches developed to fabricate high-quality CCs. The self-assembly routes and achievements in the fabrication of CCs with the ability to open a complete photonic bandgap, such as cubic diamond and pyrochlore structure CCs, are discussed as well. Then emerging applications of large-area high-quality CCs and unique photonic structures enabled by the advanced self-assembly methods are illustrated. At the end of this review, we outlook the future approaches in the fabrication of perfect CCs and highlight their novel real-world applications.

The concept of colloidal molecules was first evoked by van Blaaderen in 2003 for describing small non-spherical colloids made of the aggregation of a small number of particles. He predicted original properties to the complex assemblies of such colloids, in particular in optics. This critical review deals with the different strategies reported for creating robust clusters of spherical particles which could mimic the space-filling models of simple conventional molecules. These routes concern either the controlled clustering of preformed colloids directed by coalescence, physical routes, chemical routes, or 2-D/3-D geometrical confinement, or strategies starting from a single colloid which is decorated by satellite colloids by taking advantage of controlled phase separation or nucleation and growth phenomena. These routes are compared from the viewpoint of the accessible shapes, their tunability and scalability (146 references).

This review describes the latest advances in the synthesis and assembly of specific colloids such as the colloidal molecules as defined by van Blaaderen in 2003 and the patchy particles imagined a few years later. The two concepts are closely related because some may serve as precursors of others and vice versa. To best mimic the molecular structures, it is necessary to introduce the notions of directed binding and valence which result in the concept of patches arranged on the particle surface according to the conventional repulsion figures. The assembly of patchy particles has made it possible to reconstitute molecules and macromolecules of simple geometry. But the existence of extended assemblies of larger dimensions has been demonstrated mostly by simulation and it struggles experimentally with the purity of the batches of building blocks.