"In 2003 our teams at IBM and Columbia University reported the first binary superlattice assembled from semiconductor (PbSe) and magnetic (Fe2O3) nanoparticles," Dmitri Talapin, formerly of IBM but who now works at Lawrence Berkeley National Laboratory, told nanotechweb.org. "Since that time we learned how to grow about twenty binary superlattice structures using all possible combinations of about fifteen different materials."

The team made the superlattices by placing a substrate in a colloidal solution of two types of nanoparticles. Evaporating the solvent in a low-pressure chamber enabled the nanoparticles to self-assemble into the ordered structures. The superlattice constituents included nanoparticles of gold, lead selenide (PbSe), palladium, lead sulphide, iron oxide, and silver, as well as triangular nanoplates of lanthanum fluoride. The resulting superlattices had a range of crystal structures.

The scientists were able to direct the self-assembly process by tuning the charge state of the nanoparticles. They achieved this by adding carboxylic acids, tri-n-octylphosphine oxide (TOPO) or dodecylamine to solutions of the nanoparticles. For example, adding oleic acid to PbSe nanocrystals converted some neutral and negatively charged nanocrystals into positively charged ones. Adding TOPO, on the other hand, increased the population of negatively charged PbSe nanocrystals. And adding oleic acid to gold nanoparticles caused most of the nanoparticles to become negatively charged.

"By gently directing the self-assembly process we were able to produce a large family of novel materials, varying combinations of the building blocks and packing them into different structures," said Talapin. "This is one of the key challenges of nanoscience and nanotechnology - to produce novel materials and generate novel properties by engineering material composition at the nanometre scales and by employing natural self-assembly phenomena."

The researchers were also able to control self-assembly of the lattices by tailoring the shape of the nanoparticles and using different proportions of the two nanoparticle constituents.

According to the scientists, combining two or more materials in a superlattice enables a modular approach to the design of materials. These "metamaterials" can then both combine useful attributes of the constituent building blocks and generate entirely new properties as a result of intermixing of the components.

"In our binary superlattices we can combine semiconductors, metals, magnetic, ferroelectric, dielectric and other materials," said Talapin. "For example, binary superlattices of magnetic and semiconducting nanoparticles are promising for magneto-optic data storage and spintronic devices, and superlattices built of two different semiconductors can be employed for a new generation of solar cells and thermoelectric devices. Finally, binary superlattices can be a tool for designing novel efficient catalysts with a precise arrangement of catalytic centres."

Now Talapin and colleagues are investigating the optical, electronic, thermoelectric, catalytic and other physical and chemical properties of binary nanoparticle superlattices. They also plan to extend the family of binary superlattices by creating novel structures and combining different materials. Finally, they are researching assemblies of three or more different nanocomponents.

The researchers reported their work in Nature.