n the past few years, the field of nanocrystal synthesis has advanced greatly, but the challenge now is to develop strategies for making low-dimensional nanostructures which are much more complex. With the exception of papers #5 and #9, all those in the current Hot Ten are about nanoscale structures, but it is paper #2 which looks set to revolutionize their synthesis. This is the work of Yadong Li and colleagues at the University of Tsinghua and the National Center for Nanoscience and Nanotechnology, both in Beijing.

Paper #2 reports a new method of fabricating nanocrystals that form at the interfaces of a mixture of liquid, solid, and aqueous solution. The process, referred to as LSS, is simple in terms of procedure and chemicals, but sophisticated in terms of what it produces. Typical of the LSS method is the formation of uniform noble metal quantum dots. These grow from a combination of an aqueous solution of a noble metal salt, a long-chain fatty acid, the salt of that acid, plus ethanol. For example, heating a solution of silver nitrate, linoleic acid, sodium linoleate, and ethanol at 90 degrees Celsius for 10 hours gave masses of uniform silver nanocrystals of dimension 6.1 nm, as shown by transmission electron microscopy (TEM). The size of the crystals can be tuned by varying the temperature and the chain length of the fatty acid.

The reactants need to be added to the reaction vessel in a specific order: first the aqueous solution of the noble metal salt, followed by sodium linoleate or sodium stearate, then the fatty acid itself, and finally the ethanol. Three phases form. These are the solid sodium fatty acid salt, an ethanol solution of the fatty acid, and a water-ethanol solution of the noble metal salt. Movement of noble metal ions then occurs between the phases as the mixture is heated, and reduction occurs. Nanocrystals are formed, and on to their surface are absorbed long-chain fatty acids giving them a hydrophobic skin, and they fall to the bottom of the container.

Li has shown that the LSS process can generate semiconducting, fluorescent, magnetic, and dielectric nanocrystals. Band-gap semiconductors, such as cadmium sulfide and cadmium selenide, can be prepared by this method. For example, the CdSe nanocrystals were formed using an aqueous solution of cadmium selenite (CdSeO₃) with heating at 180 Celsius. Nanocrystals of metal oxides, namely those of titanium, copper, zirconium, and tin, and of metal sulfides, namely silver, zinc, lead, and manganese, were also produced.

Paper #2 describes the fabrication of rare-earth fluorescent nanocrystals such as YbF₃ and LaF₃ which TEM shows to be spherical in structure (see X. Wang, et al., Inorganic Chemistry, 45[17]: 6661-5, 2006). Conducting polymer nanocrystals made from polypyrrole (PPy) and optoelectronic nanocrystals of copper phthalocyanine are also described and illustrated in paper #2. The new nanocrystals can easily be dispersed in nonpolar solvents such as cyclohexane and chloroform and can give homogeneous colloidal solutions which remain stable for months. From these solutions it is easy to produce monolayer films on surfaces by dripping or dip-coating.