Colloidal Nanocrystal Compounds: Colloidal nanocrystals have properties that fall between those of the individual atoms but also differ from bulk due to confinement effects. They can thus be thought of as analogs of atoms, and, like atoms, there is a desire to bond together neighboring particles, which will also affect their properties. During synthesis, organic ligands are used to prevent the colloidal nanocrystals from growing too large or agglomerating, but these ligands result in poor interparticle coupling and communication. Kovalenko et al. (p. 1417 ) show that chalcogenide complexes (compounds based on S, Se, or Te) can effectively link together neighboring particles. Upon gentle heating, the ligands can be converted from insulating to semiconducting without altering the chemistry of the nanocrystals.

Nanoparticle lattices and surfaces: The challenge of resolving the details of the surfaces or assemblies of colloidal semiconductor nanoparticles can be overcome if several characterization methods are used (see the Perspective by Boles and Talapin). Boneschanscher et al. examined honeycomb superlattices of lead selenide nanocrystals formed by the bonding of crystal faces using several methods, including high-resolution electron microscopy and tomography. The structure had octahedral symmetry with the nanocrystals distorted through “necking”: the expansion of the contact points between the nanocrystals. Zherebetskyy et al. used a combination of theoretical calculations and spectroscopic methods to study the surface layer of lead sulfide nanocrystals synthesized in water. In addition to the oleic acid groups that capped the nanocrystals, hydroxyl groups were present as well. Science , this issue p. 1377 , p. 1380 ; see also p. 1340