Chemists discover mechanism in controlled growth of tetrahedron-shaped nanoparticles

Illustration showing the transformation of a golden seed into a crystallized, asymmetrical nanoparticle. These images were taken at Rice University using a technique called liquid cell transmission electron microscope. Credit: Jones Research Group/Rice University
Nature loves symmetry. Take a look at your own hands. Nature can sometimes produce asymmetrical things and the reasons for this aren't always obvious.

Matthew Jones, a Rice University chemist, and his team were searching for answers to questions about useful nanoparticles. They now have one.

Jones, postdoctoral researcher Muhua SUN, and Zhihua Cheng, graduate student, has published a new study that shows how symmetry breaks during particle growth to form pyramid-shaped nanocrystals of gold tetrahedron tetrahedrons.

Small fluctuations in a system's development can determine its fate. This is called symmetry breaking. This applies to crystal growth from nanoscale seeds that start with a symmetrical, atomic lattice.

Rice researchers demonstrated that balancing the thermodynamic and kinetic forces can be used to tilt the growth of particles in the desired direction. This discovery opens the door to using asymmetrical particles as building blocks for metamaterials.

This study was published in the American Chemical Society journal ACS Nano. It is based on work supported by Jones' Packard Fellowship in 2018, which was granted to him in order to pursue research in liquid cell transmission electron microscope (TEM).

Jones's lab developed a technique that allows researchers to observe single metal nanoparticles in liquid using a window large enough for electrons to pass. Transmission electron microscopes are used in high vacuum to evaporate liquids.

Researchers noted that nanoparticles with tetrahedron shapes are often produced by other processes. However, it has been difficult to make them in the laboratory.

Jones stated that if a particle is one crystal, it inherits the lattice's symmetry. Crystals are highly symmetrical, such as cubes, rhombic dodecahedrons and octahedrons. There are also strange outliers that people notice that have lower symmetry than their parent lattice.

This is Jones' first study to demonstrate the effectiveness of the liquid cell technique. They were able to watch the growth of fluid containing precursors and ligands flow through the cell, which allowed them to pinpoint the problem and redirect the symmetry to the final nanoparticle product.

The key was the speed of growth and the conditions in which gold atoms attached themselves to particles at their edges and tips rather than to thermodynamically preferred faces.

Jones stated that "now that we can screen a variety of conditions, it was possible to see a spectrum including kinetic growth and equilibrium." Protrusions grow quickly and kinetic growth is fast. It's difficult to control. In equilibrium, growth slows down and the system does what it needs to, which is maintain symmetry.

"But liquid cell TEM enabled us to change one variable and see the behavior at the middle. We could see strange symmetry breaking off and a well-defined tetrahedron particles come out. We concluded that this was a balance of kinetic and equilibrium factors.

Jones stated that understanding fundamental balance should be applicable to many other situations.

He stated that liquid cell TEM is a valuable tool to observe and analyze dynamic chemical processes. This could eliminate a lot of trial-and-error in the synthesis and production of particles for biomedicine or nanophotonics.

He said, "There's nothing quite as satisfying as being able to see the entire thing unfold." This is what the technique does. It doesn't require you to shoot photons at something, then do lots of analysis to interpret them. It's all about watching the process unfold. "Seeing is believing."

More information: Muhua Sun, Understanding Symmetry Breaking At the Single-Particle Level via Growth of Tetrahedron Shaped Nanocrystals From Higher-Symmetry Prcursors, ACS Nano (2021). Information from the Journal: ACS Nano Muhua Sun et. al. Understanding Symmetry Breaking at Single-Particle Level via Growth of TetrahedronShaped Nanocrystals From Higher-Symmetry. (2021). DOI: 10.1021/acsnano.1c04056