Gold nanoparticles self-assemble into pinwheel superlattices

SOPHIA CHEN: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics in materials research. My name is Sophia Chen. Jiahui Li designs structures out of tiny gold pyramids. They’re so small; they’re microscopic.

JIAHUI LI: We are using nanoparticles from 40 nanometer to 100 nanometers in length.

SOPHIA CHEN: Li, a materials science graduate student at the University of Illinois Urbana-Champaign, recently demonstrated how to assemble tens of thousands of these minute gold nanoparticles into a repetitive pattern known as a superlattice. The pattern in her superlattice had a special geometric property known as chirality. You may know chirality better as handedness, because your hands exhibit chirality. Put your hands out. If you place your left hand over your right hand, both palms down, it’s impossible to make your fingers on one hand line up with the fingers of the other hand. Chirality is this broken symmetry, where an object cannot be superimposed on its mirror image through rotation or translation. Lots of phenomena in nature exhibit chirality, Li says.

JIAHUI LI: At the smaller scale, the DNA, the helical structure, is actually a chiral structure. And at the largest scale, the galaxy itself and if you imagine it rotating in this case, it is also a chiral structure.

SOPHIA CHEN: Li’s nanoparticle structure takes the shape of a pinwheel. So what’s the point of this structure? The thing is, due to its chirality, different types of light interact with the pinwheel differently. Specifically, a light wave has this property known as polarization, which describes the direction that the light wave’s electromagnetic field oscillates. The polarization direction might be changing in time, too. For example, you can imagine a light wave spiraling through space, where its electromagnetic field direction rotates clockwise or counterclockwise. When that light hits Li’s material, different wavelengths might be transmitted depending on whether the light’s polarization is rotating clockwise or counterclockwise. Li says this property could be useful for filtering light in optical applications.

JIAHUI LI: You can use it as lenses.

SOPHIA CHEN: To make the material, Li and her team place the gold nanoparticles in a solution with a surfactant, and then apply that solution to a substrate and let the solution evaporate. By controlling the rate of evaporation, they can tune the interactions between the nanoparticles. The nanoparticles self-assemble into a pinwheel because they are attracted to each other via van der Waals forces. They can tune this attraction with the level of surfactant, which creates a repellent force between the nanoparticles. The substrate also plays a role in the self-assembly. They found that silicon as a substrate enabled the formation of superlattices with the largest area, up to 37 microns squared. Li says that previously, other researchers had formed chiral superlattices by building them on DNA structures. Their structure differs because they don’t use DNA or any sort of frame to make the structure.

JIAHUI LI: It's the first time that people were using only the nanoparticles to form the chiral superlattice.

SOPHIA CHEN: In future work, Li says they would like to develop more methods to engineer this material. For example, they’re pursuing a 3D-printer like device which uses the gold nanoparticle solution as ink, rather than evaporating it from a substrate. She’s also excited for the possibilities of creating other types of chiral superlattices by tuning the attraction and repulsion between nanoparticles.

JIAHUI LI: Since we can engineer this structure, by manipulating the interaction that we have rich design space of the structure that we can control how chiral we want it to be.

SOPHIA CHEN: This work was published in a recent issue of Nature. My name is Sophia Chen from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on twitter, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.