Researchers at Sun Yat-sen University in China demonstrated in the journal Science Advances what they claim is the first material that simultaneously has both plasmonic-like and all-dielectric properties when exposed to sunlight.

The key to achieving this combination is the use of tellurium (Te) nanoparticles, which have unique optical duality, according to G. W. Yang, professor at Sun Yat-sen University and co-author of the research.

By dispersing these nanoparticles into water, the water evaporation rate is improved by a factor of three under solar radiation. This makes it possible to increase the water temperature from 29 degrees to 85 degrees Celsius within 100 seconds.

“The Te nanoparticles perform like a plasmonic nanoparticle when it is smaller than 120 nanometers (nm) and then as a high-index all-dielectric nanoparticle when those nanoparticles are larger than 120 nm,” said Yang.

The Te nanoparticles are able to achieve this duality because they have a wide size distribution (from 10 to 300 nm). This enhanced absorption can cover the whole solar radiation spectrum.

Another property of the Te nanoparticle is that when it is excited by sunlight, the excitation energy is transferred entirely to the carriers (electrons and holes). This pushes the carriers out of equilibrium and into special states of momentum with higher temperatures.

Yang explains that as the system evolves toward equilibrium, these carriers relax. As the carriers scatter, it leads to a phenomenon known as Coulomb thermalization, which forms a hot gas of thermalized carriers that couple with phonons and transfer their excess energy to the lattice. This results in the efficient heating of the Te nanoparticles.

For this approach to work for commercial desalination, Yang acknowledges that the current method of producing the Te nanoparticles with nanosecond laser ablation in liquid is limited. “Now, we are trying to prepare the Te nanoparticles by other methods,” he added.

But because the Te nanoparticles have a unique optical duality, Yang envisions other applications for the technology. “We want to apply them in sensors or nanoantennas,” he said.