Pakistani Origin Scientists Discover Ultra-Narrow Linewidth Photo-Emission in Polymorphic Selenium Nanoflakes 

In a recent study, a team of Pakistani-origin scientists from USA, Turkey, and China demonstrate that selenium nanoflakes (SeNFs), fabricated via a unique hot-pressing strategy, exhibit ultra-narrow linewidth photo-emitters, especially at room temperature. The study has been recently highlighted by Wiley and published as the inside front cover of the journal “small” (

Historically, the hot-press machine has been typically used to densify polymers and mold other plastics, said by Dr. Naveed Hussain, the lead author of the manuscript.  For the first time, he has developed and optimized the hot-pressing strategy to mechanically squash nanoparticles and mold them into a variety of quasi-two-dimensional materials, using the simple hot-pressing machine. He has been able to demonstrate successful synthesis of 2D flakes based on a range of metals, semi-metals, metalloids, and polymers, and the results have been published in top-tier journals such as ACS Nano, Nano Energy, Small, Advanced optical materials etc. 

Initially thought to be well-suited only for syntheses, Dr. Naveed and his project PI Dr. Khurram Shahzad also discovered that the hot-pressing method inheritably introduces some built-in lattice strains in as-synthesized 2D materials during their Raman spectroscopic investigations. Furthermore, it was also observed that the method has the additional capability of invoking localized polymorphic phase transition with in micro-sized and ultrathin flakes. 

This is what they report in their recent study, where a process-induced crystal symmetry breaking in SeNFs results in a localized polymorphic phase transition. He further added that this transition from native trigonal to orthorhombic phase causes degenerate bright excitons in orthorhombic Se, that serve as photoemitters with extremely narrow linewidth. Ultra-narrow linewidth photoemitters which form the basis of solid-state photonics and quantum communication are usually reported at extremely low temperatures, making them practically unfavourable. The work is also significant because ultranarrow linewidth photo-emitters in selenium flakes are obtained at room-temperature, making them practical/technologically favourable considering the increasing demand of much lacking single-photon sources in 2D materials, their study reports quasi-2D selenium crystals as a promising material platform for next-generation quantum communication.

Previously reported PL linewidths in 2D materials. A comparison of FWHM (Γ) values of previously reported 2D materials systems and the polymorphic SeNFs, as a function of their emission energy. Green ellipse and triangle represent linewidth (Γ) of type-I and type-II photo-emitters in SeNFs, respectively. The inset shows the coherence time (ps) of X1, X2, and X3 photo-emitters and their corresponding FWHM and PL peak energy.

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