User:Thomsnoeren/Persistent luminescence

Persistent luminescence (also known as persistent fluorescence or afterglow luminescence) refers to long-timescale emission of light upon excitation by UV or visible light. After the excitation source is removed, light is emitted over a period of seconds up to hours, making it a significantly slower process than either fluorescence or phosphorescence. Due to their unique luminescence properties, persistent luminescence materials are commonly used as glow-in-the-dark materials. In recent years, applications in in-vivo bioimaging have also been a subject of study.

Mechanism
Light at a wavelength specific to the emission center excites an electron from the valence band or dopant ground state to the conduction band, leaving a hole in the valence band or ground state of the material (1). The electron can then freely move through the conduction band and either radiatively recombine with the hole (fast - not shown), or it can be captured by an electron trap state with an energy that lies within the band gap of the material, just below the conduction band (2). If the energy difference between the bottom of the conduction band and the trap state is small enough, the electron can be thermally excited back into the conduction band following Boltzmann statistics (3). On the other hand, for persistent luminescence detrapping has to be slow, meaning that the trap state cannot be too shallow or the electrons would escape at too high of a rate. Detrapping of the electron is followed by electron transfer to the excited state of a secondary dopant ion (3), which can then either be recaptured by the trap state (inverse of 3), or radiatively decay to the ground state of the secondary dopant, resulting in light emission (4). The distinguishing feature of this process that gives rise to long luminescence lifetimes is the ability of electron traps near the conduction band edge to effectively “hold” the electrons and release them slowly, producing a slow trickle of emissive relaxation.