PNNL

In relationships and in materials, the rule that opposites attract applies. If nature builds material from ions -- such as the salt crust at the edge of the Dead Sea -- negative and positive ions work together. Led by Dr. Jonas Warneke, a visiting fellow at DOE's Pacific Northwest National Laboratory (PNNL), a team explored what happens if negative ions alone accumulate on surfaces. The result? A never-before-seen liquid-like material. The material is the result of soft landing ions on a surface and the adsorption of molecules from the surroundings. When exposed to air, the thin liquid layer crumbles. The resulting droplets scatter light, creating colorful displays.

"It's a new kind of material that's never been produced before -- because there was no way to produce it until the technology became available at PNNL," said Warneke, who has a Feodor Lynen Fellowship from the Alexander von Humboldt Foundation in Germany.

Why It Matters: Today's materials won't solve tomorrow's problems. More than likely, the needed materials won't be discovered via costly trial-and-error research. Rather, scientists will use new predictive approaches for material design. Such design requires detailed knowledge of the chemistry and physics of materials as well as their molecular-level interactions. This study offers insights into how negatively charged ions (anions) may be used to create materials with desirable properties.

"We can make subtle changes at the molecular level to the ions that result in exceptionally large changes at the macroscopic level," said Dr. Grant Johnson, a physical chemist at PNNL. 

Summary: To create the liquid-like material by accumulating negatively charged ions (anions) on surfaces, the international team used unique ion soft landing capabilities. The technique lets researchers place a quadrillion ions, which retain their negative charge (2^-) state, on a circular spot on a surface. The ions, selected for these experiments, contained twelve boron and twelve halide atoms (X) of one type, such as chlorine, iodine, or fluorine (B₁₂X₁₂^2-).

The first step of generating the materials was landing anions without their positive partners or solvent onto a surface. The researchers found that neutral molecules from the gas phase join with the ions on the surface. "It's similar to having a static charge," said Dr. Julia Laskin, Purdue University. "A charged surface will attract something, like dust. Here, instead of dust, the surface attracts molecules out of the gas phase."

The team developed methods to control the type of molecules that accumulate with the soft landed anions. In addition to the anions, the neutral molecules offer an additional parameter to tune the material's properties.

Having created layers of material by this method, the team was surprised when they removed it from vacuum and exposed it to air. The material self-organized. "When it comes in contact with air, the film crumbles into liquid droplets," said Laskin.

This self-organization process is known as dewetting -- a process often used for controlled structure formation in technologically relevant materials. Think of a leaf in the rain. The water hits the leaf's surface and shortly starts to bead up, forming droplets. The same thing happens to these layers created by anion landing due to changes in their composition when exposed to air. Understanding why the layer rearranges itself required intense analysis as the material was complex and unprecedented in nature.

"We used nearly every surface characterization technique in EMSL," said Johnson. The computational resources along with the soft landing capabilities used by the team are also located in EMSL, Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility sponsored by the Office of Biological and Environmental Research.

Exactly how the droplets organized depended on the halide in the anion and the amount of time that passed. "It looks beautiful when it starts to crumble and scatter light," said Warneke.

The complex and sensitive evolution of the material can be traced back to its frustrated nature. "The material is hungry for everything that can stabilize it -- it wants more positive ions!" said Warneke.

Having created this unusual material and started to understand some of its fundamental phenomena, the team continues to study its properties and possibilities for applications for the controlled formation of thin films.

Acknowledgments

Sponsors: Feodor Lynen Fellowship from the Alexander von Humboldt Foundation in Germany and Pacific Northwest National Laboratory through the Alternate Sponsored Fellow Program (J.W.); Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences, Separation Science (G.E.J. and J.L.); Environmental Molecular Sciences Laboratory science theme award (M.M., S.R., S.C., M.E., E.A., R.Y., and N.W.)

Facilities: Environmental Molecular Sciences Laboratory (surface characterization), a Department of Energy Office of Science user facility sponsored by the Office of Biological and Environmental Research; Pacific Northwest National Laboratory Institutional Computing (density functional theory calculations)

Reference: J. Warneke, M.E. McBriarty, S.L. Riechers, S. China, M.H. Engelhard, E. Apra, R.P. Young, N.M. Washton, C. Jenne, G.E. Johnson, and J. Laskin, "Self-organizing layers from complex molecular anions." Nature Communications 9, 1889 (2018). [DOI: 10.1038/s41467-018-04228-2]

Related Links:

Pacific Northwest National Laboratory research highlight: Towards Controlled Preparation of Complex Architectures Using High-Intensity Beams of Mass-Selected Ions 

Pacific Northwest National Laboratory research highlight: The Hard Facts About Soft Landing Ions