Crystalline Sheets, Courtesy Of A Cosolvent

By adding a chloroalkane cosolvent to the standard procedure for preparing lead sulfide nanocrystals, chemists have shifted the shape the crystals take from spheres to ultrathin sheets (Science 2010, 329, 550). The quality of the resulting sheets, which are several hundred nanometers across but only a few nanometers thick, is so high that the material can be integrated into a photodetector without any further processing. The research comes from a group led by Horst Weller of the University of Hamburg, in Germany. According to the researchers, the chlorinated solvent encourages two-dimensional sheet growth over 3-D particle formation by slowing down the PbS growth rate during primary nanocrystal formation. This process, in turn, exposes highly reactive facets on the growing crystal surface that act as fusing points for other nanocrystals. A layer of oleic acid—an organic ligand used in the nanoparticle synthesis—forms on the sheet’s surfaces, driving the 2-D growth. This so-called oriented attachment of nanocrystal building blocks “is one of the most promising approaches in nanotechnology,” the researchers note.