Using the scattering of high-energy x-rays produced at the Stanford Synchrotron Radiation Lightsource (SSRL), researchers have tracked the colloidal growth and self-assembly of palladium nanoparticles into high-quality superlattices. These results, published recently in Nature, provide key insights into the self-assembly of nanoparticles, which could be used to form complex materials for catalysis.

Christopher Tassone, staff scientist at SSRL and corresponding author on the Nature article, described the results as serendipitous. His research team originally set out to characterize the growth of individual nanoparticles using in situ small angle x-ray scattering (SAXS). His co-author and colleague, Matteo Cargnello of Stanford University, says, “These nanocrystals started to form, were attracted to each other, and arranged themselves into ordered structures.” Within seconds, the research team observed the formation of nearly perfect supercrystals composed of identically sized nanoparticles.

Even more interesting, the individual nanoparticles continued to grow after they had assembled into the superlattice without changing the symmetry and structure of the colloidal crystal. This continued growth after self-assembly has caught the eye of many researchers in the field, including Sharon Glotzer of the University of Michigan. “Without the in situ monitoring of the process, most researchers would have assumed that first the nanoparticles assembled in solution, and only when the nanoparticles’ growth was finished, would they then assemble into a crystal superlattice,” says Glotzer, who was not affiliated with this study. The coherent growth of all the constituent nanoparticles, Tassone explains, means that reagents are able to pass through the superlattice unhindered.  

To understand why the nanoparticles were forming superlattices, Jian Qin of Stanford University developed a theoretical framework to describe the interactions driving the self-assembly process. Qin says, “The assembly process can be rationalized by considering the delicate balance between the van der Waals attraction and the steric repulsion [between ligands].” This threshold size, according to Qin, is material-dependent; different materials need to reach different critical sizes before self-assembly can occur.

In addition to palladium, the research team also investigated other metallic (iron) and semiconductor (lead telluride) nanoparticles. All of them formed supercrystals once the individual particles grew to the critical size needed for the van der Waals forces to produce sufficient attraction, demonstrating that this phenomenon might be universal. After the superstructure formed, the van der Waals forces only became stronger as the nanoparticles continued to grow, making the final superlattices extremely stable.

Future applications of this research are numerous, because the forces driving self-assembly are universal. These results show that many materials that can be grown past their critical size will self-assemble into a superlattice. Cargnello sees this as a way to develop new materials for magnetic storage, solar cells, optoelectronics, or catalysis. To aid these efforts, Qin wants to expand his model to include factors that more explicitly take into account the kinetic growth of the nanoparticles.

The particles’ increase in size after self-assembly could also be manipulated. By adding a different reagent after the supercrystal has formed, the complexity of the constituent nanoparticles can be increased. “Once the superlattice is formed, we can use that as a way to template bimetallic or intermetallic nanocrystal systems,” Tassone says.

Glotzer is also interested in observing what happens to the symmetry of the larger superstructure if the particles expand anisotropically, such as rod-shaped particles that elongate along just one direction. “The key to this is to identify what advantages the nanoparticles’ continued growth after superlattice formation holds,” Glotzer says.

Read the abstract in Nature.