Colloidal metal nanocrystals are up-and-coming technologically important materials that might be ideal in various electronics, optoelectronics and thermoelectric applications. These materials can be easily processed in solution and benefit from injection or extraction of charge carriers to improve their carrier mobility. As-synthesized nanocrystals therefore require a post-treatment to remove the original long chain in these structures to make strong coupling between adjacent nanocrystals easier.

A team led by Dmitri Talapin recently found that inorganic molecular metal chalcogenide ligands such as Sn2S64- and In2Se42- could replace the bulky organic ligands normally used here. The researchers also discovered that simpler chalcogenide ions, like S2-, Se2- and Te2-, and other small charged ions, such as SCN-, worked just as well too.

"All-inorganic” nanocrystal solids

Spurred on by these findings, the same team has now been looking at making even better “all-inorganic” nanocrystal solids with higher charge carrier mobilities. Until now, most research in this field focused on chalcogenides (or anions binding to the nanocrystal surface via VIA elements) but Talapin and colleagues wanted to see if non-chalcogenide-based inorganic ligands, such as the halides (Cl-, Br-, I-) would work as well.

Halides are sturdy ligands and the concept is not really that new, says Talapin. Indeed halide ions were already being used to make AgI colloids more stable over a century ago. More recent work has also confirmed that Br- can replace bulky organic ligands on lead chalcogenide nanocrystals when used as a co-surfactant. The Br- passivates the surface of these nanocrystals and the material has much better photovoltaic properties as a result.

Enhancing charge transport between nanocrystals

The Chicago-Argonne researchers have now studied simple halides, such as NH4I, NH4Br, NH4Cl and even NaCl and KBr as inorganic ligands for colloidal nanocrystals. Similar to other such ligands, these halide ions bind to the electrophilic nanocrystal surface and stabilize the crystals electrostatically in solution. The halide ligands are rather simple and short, and are stable in air and humid atmospheres, as well as being less toxic than chalcogen-based ligands. And, because they are so short and compact, they enhance charge transport between nanocrystals – as seen in the high mobility of 12 cm2/Vs in thin films made of iodide-capped CdSe nanocrystals. They also passivate surface dangling bonds and remove electronic traps from the nanocrystal surface.

The team also studied pseudohalides (like N3- and CN-) and the azide ligand (N3-), which works especially well with III-V nanocrystals such as InAs and InP. “N3- is also the first example of a group VA ligand, which means that we might now be able to construct all III-V nanocrystal solids with III-V group nanocrystals,” says Talapin. “This would be an important milestone in bottom-up materials design.”

Lead halide-based halometallates (like CH3NH3PbI3, which have a perovskite structure) could also be good surface ligands for PbCh nanocrystals, especially PbTe crystals, he adds.

More extensive toolbox of robust ligands

“Our work enriches the family of inorganic ligands for colloidal nanocrystals and makes designing these materials easier,” he tells nanotechweb.org. “These newcomers also provide us with a more extensive toolbox of robust ligands to redesign the surface chemistry of nanocrystals.”

According to the researchers, who describe their work in ACS Nano, halide-, pseudohalide- and perovskite-capped nanocrystals might be good for making field-effect transistors and in photovoltaics and thermoelectrics. The team says that it will now be looking at making devices from its nanostructures and measuring their performance. “We will also be fabricating the 'all III-V' nanocrystal solids mentioned above based on azide ligands and III-V nanocrystals (which are very important in optoelectronics applications),” says Talapin.