#king abdullah university of science and technology

Improving the sensitivity of light sensors or the efficiency of solar cells requires fine-tuning of light capturing. KAUST researchers have used complex geometry to develop tiny shell-shaped coverings that can increase the efficiency and speed of photodetectors.

Many optical-cavity designs have been investigated to seek efficiencies of light: either by trapping the electromagnetic wave or by confining light to the active region of the device to increase absorption. Most employ simple micrometer- or nanometer-scale spheres in which the light propagates around in circles on the inside of the surface, known as a whispering gallery mode.

Former KAUST scientist Der-Hsien Lien, now a postdoctoral researcher at the University of California, Berkeley, and his colleagues from China, Australia and the U.S. demonstrate that a more complex geometry comprising convex nanoscale shells improves the performance of photodetectors by increasing the speed at which they operate and enabling them to detect light from all directions.

Surface effects play an important role in the operation of some devices, explains KAUST principal investigator, Jr-Hau He. Nanomaterials offer a way to improve performance because of their high surface-to-volume ratio. “However, although nanomaterials have greater sensitivity in light detection compared to the bulk, the light–matter interactions are weaker because they are thinner,” describes He. “To improve this, we design structures for trapping light.”

Hydrogen gas, an important synthetic feedstock, is poised to play a key role in renewable energy technology; however, its credentials are undermined because most is currently sourced from fossil fuels, such as natural gas. A KAUST team has now found a more sustainable route to hydrogen fuel production using chaotic, light-trapping materials that mimic natural photosynthetic water splitting.

The complex enzymes inside plants are impractical to manufacture, so researchers have developed photocatalysts that employ high-energy, hot electrons to cleave water molecules into hydrogen and oxygen gas. Recently, nanostructured metals that convert solar electrons into intense, wave-like plasmon resonances have attracted interest for hydrogen production. The high-speed metal plasmons help transfer carriers to catalytic sites before they relax and reduce catalytic efficiency.

Getting metal nanoparticles to respond to the entire broadband spectrum of visible light is challenging. “Plasmonic systems have specific geometries that trap light only at characteristic frequencies,” explains Andrea Fratalocchi, who led the research. “Some approaches try to combine multiple nanostructures to soak up more colors, but these absorptions take place at different spatial locations so the sun’s energy is not harvested very efficiently.”

By combining multiple nanomaterials into a single structure, scientists can create hybrid materials that incorporate the best properties of each component and outperform any single substance. A controlled method for making triple-layered hollow nanostructures has now been developed at KAUST. The hybrid structures consist of a conductive organic core sandwiched between layers of electrocatalytically active metals: their potential uses range from better battery electrodes to renewable fuel production.

Although several methods exist to create two-layer materials, making three-layered structures has proven much more difficult, says Peng Wang from the Water Desalination and Reuse Center who co-led the current research with Professor Yu Han, member of the Advanced Membranes and Porous Materials Center at KAUST. The researchers developed a new, dual-template approach, explains Sifei Zhuo, a postdoctoral member of Wang’s team.

The researchers grew their hybrid nanomaterial directly on carbon paper—a mat of electrically conductive carbon fibers. They first produced a bristling forest of nickel cobalt hydroxyl carbonate (NiCoHC) nanowires onto the surface of each carbon fiber (image 1). Each tiny inorganic bristle was coated with an organic layer called hydrogen substituted graphdiyne (HsGDY) (image 2).

A hybrid material that could lead to cheaper and more effective methane storage has been created by a globally prominent research team at King Abdullah University of Science and Technology (KAUST), Saudi Arabia, with collaborators at the University of Crete, Greece.

Natural gas, which is almost 95 percent methane, is a good candidate for replacing gasoline and coal. It can provide the same amount of energy as these fossil fuels, while releasing much less of the greenhouse gas carbon dioxide and the toxic pollutants carbon monoxide, nitrogen oxides and sulfur oxides. Methane is more environmentally friendly in several ways, but its widespread adoption for powering vehicles and other local and mobile applications is limited by shortcomings of existing storage and transport technologies.

Professor Mohamed Eddaoudi of KAUST’s Advanced Membranes and Porous Materials Research Center leads a wide range of research projects involving metal-organic frameworks, or MOFs. These hybrid materials contain single metal ions or metal clusters held together by carbon-based ‘organic’ chemical groups known as linkers. Rearranging different linker and inorganic molecular building blocks allows scientists to fine-tune the size and chemical properties of the pore system in MOFs to perform useful functions. These include highly selective gas adsorption and catalysis.