When electrons are confined into very small spaces, they can exhibit unusual electrical, optical and magnetic behaviour. From confining electrons in two-dimensional atomic sheet graphene to restricting electrons even further to achieve one-dimen­sionality, this broad line of research is transforming the landscape of funda­mental research and technological advances in physics, chemistry, energy harvesting, information and beyond. Now, an inter­national team led by Aalto University researchers has now found that fibrous red phosphorous, when electrons are confined in its one-dimen­sional sub-units, can show large optical responses: the material shows strong photo­luminescence under light irra­diation.

Red phosphorous, like graphene, belongs to a unique group of one-dimen­sional van der Waals (1D vdW) materials. A 1D vdW material is a radically new type of material that was discovered only in 2017. Until now, research on 1vdW materials has focused on electrical properties. The team uncovered the optical properties of 1D vdW fibrous red phosphorous through measurements like photo­luminescence spectro­scopy, where they shone laser light on the samples and measured the colour and brightness of the light emitted back. The findings show the 1D vdW material demonstrates giant anisotropic linear and non-linear optical responses – in other words, the optical responses strongly depend on the orienta­tion of the fibrous phos­phorous crystal – as well as emission intensity, which relates to the number of photons emitted during a specific time.

“The way it responded in the experiments makes 1D vdW fibrous red phosphorus a really exciting material. For example, it shows both giant anisotropic linear and non-linear responses as well as emission intensity, which is striking,” says Luojun Du, a post­doctoral researcher at Aalto University. The material’s photo­luminescence also struck the researchers by surprise. The team compared the photo­luminescence of fibrous red phosphorous with monolayer molybdenum disulfide (MoS₂), which is well-known for its strong photo­luminescence, and found that the intensity of the photo­luminescence was more than 40 times more intense, making it ultra-bright – albeit very briefly.

“The strong photo­luminescence of fibrous red phosphorus is unexpected. In fact, we initially expected that the photo­luminescence of fibrous red phosphorous would be only weak. Based on theoretical calculations, this effect shouldn’t actually be strong so we’re now doing more experiments to clarify the origin of its after-glow,” says Du. “I believe that one-dimensional van der Waals materials like fibrous red phosphorous show real promise for displays and other appli­cations, which rely on materials that demonstrate exactly the behaviours we’ve seen in this study. The spectrum of its aniso­tropic optical response also seems to be very wide if we compare it with responses from conven­tional materials,’”says group leader Zhipei Sun. (Source: Aalto U.)

Reference: L. Du et al.: Giant anisotropic photonics in the 1D van der Waals semiconductor fibrous red phosphorus, Nat. Commun. 12, 4822 (2021); DOI: 10.1038/s41467-021-25104-6

Link: Photonics group, Dept. of Electronics and Nanoengineering, Aalto University, Espoo, Finland