“We are made of star stuff,” declared astronomer Carl Sagan in the science-themed television series Cosmos: A Personal Voyage. He wasn’t just waxing lyrical—scientists have used a technique called spectroscopy to reveal that elements such as carbon, hydrogen, nitrogen and oxygen, which are all essential to life, were indeed created by earlier generations of stars.
Spectroscopy typically involves shining a beam of electromagnetic radiation on an object and observing how it responds to the radiation. The spectral response reveals information about the object’s structure and properties. In certain materials, typically metals with free electrons, exposure to radiation can cause these collective charges to oscillate in what is known as plasmons, which affect the optical properties of the material. Non-metals that exhibit plasmon resonances under ultraviolet (UV) radiation, however, are rare.
Leveraging on nanomaterials, a group of scientists led by Joel Yang, a Senior Scientist at A*STAR’s Institute of Materials Research and Engineering (IMRE), is looking to develop miniaturized UV spectral filters based on the interband plasmonics of silicon nanostructures. The team, including collaborators in China and Denmark, first tested silicon nanodisks and nanoholes, both in isolation and in pairs.
“Our experiments showed that silicon, despite being a non-metal, exhibits plasmonic resonances like a metal, specifically in the UV spectrum,” explained Zhaogang Dong, the study’s lead author, adding that this has implications for UV spectral filters and paves the way for plasmon-enhanced silicon photodetectors.
The researchers also explored the potential improvement in plasmonic resonances if silicon and aluminum were combined. “Both silicon and aluminum are complementary metal-oxide-semiconductor-compatible materials. In addition, aluminum is a known ‘good’ plasmonic metal in the UV spectrum, making it a useful benchmark for comparing silicon’s plasmonic properties to industry standards,” Dong said.
To test whether silicon and aluminum function synergistically, the researchers ran computer-aided simulations of silicon-aluminum nanoantennae. They then fabricated the nanoantennae to experimentally record the extent of plasmonic resonance under UV light exposure, with the aim of validating the results from their simulations.
Their results indicate that, on its own, silicon exhibits plasmonic resonance comparable to that of aluminum. Furthermore, the researchers noted that hybrid silicon-aluminum nanostructures could provide improved resonances for UV-related applications such as examining the geometric properties of molecules, or splitting water with UV light to generate hydrogen fuel.
“Importantly, the hybrid silicon-aluminum nanostructures could be potentially used for creating miniaturized spectrometers in the UV region,” suggested Dong. “We would like to explore nanostructured silicon for UV opto-electronic devices that rely on this localized plasmon resonance characteristic,” he concluded.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE).