Researchers at the physics department have recently published work elucidating the luminescence pathway responsible for the bright ionizing-radiation-induced optical response of LiF nanoparticles at room temperature. Such material, synthetized by PhD student/Postdoc Camilla L. Nielsen, has been proven to store spatially resolved information of the amount of energy deposited by an ionizing particle, known as the dose, and exhibits the curious property of forming holes dressed as molecules in the most simple crystalline array.

For further insight, please join Camilla’s PhD defense scheduled for Friday 3^rd of November at 13:00 in the Physics Auditorium.

Luminescence phenomena in the lightest and widest band-gap dielectric, lithium fluoride (LiF), is understood to arise from the radiative decay of a complex bound state of an electron-hole pair called a self-trapped exciton. Electronic excitation, created in the material by the passing of ionizing radiation, relaxes and forms self-trapped holes, which can eventually recombine with the electrons and emit light. The holes are self-trapping almost immediately after ionizing radiation has interacted with the crystal, and they are very interesting quasiparticles enabling covalent bond formation between neighboring fluorine anions in a lattice otherwise purely held together by ionic bonds, i.e. Li⁺ and F^- are ions with full electronic shells. In the bulk case, such self-trapped holes forming F₂^- molecules are unstable at room temperature and they proceed to hop or diffuse in the lattice at temperatures above 100 K, hindering the luminescence performance at room temperature.

In the work recently published in Physical Review Materials, AU researchers Camilla L. Nielsen, B. Julsgaard, P. Balling and R. M. Turtos, together with colleagues from Risø National Laboratory, proceed to demonstrate that copper doping enables the stabilization of such self-trapped holes at room temperature in LiF nanoparticles and this allows trapping of a fraction of the hole population, which remains in the crystalline lattice with a lifetime of around 100 days. Such stabilization and trapping mechanisms are behind the luminescence phenomenon exhibited by the nanoparticles after ionizing radiation has interacted, where the material is interrogated with visible light and information is retrieved in the form of UV photons decaying from the triplet state of a self-trapped exciton.

This mechanism called optical stimulated luminescence allows monitoring the radiation dose in a nearly tissue-equivalent material and shows much potential in the verification of dose delivered during radiotherapy treatments in all three dimensions. 

Publication:

The origin of room-temperature self-trapped-exciton emission in LiF nanoparticles,

Camilla L. Nielsen, Pavao Andričević, Brian Julsgaard, Mayank Jain, Peter Balling, and Rosana M. Turtos, Phys. Rev. Materials 7, 106001, 2023.

https://journals.aps.org/prmaterials/abstract/10.1103/PhysRevMaterials.7.106001