Phosphorescence | EPFL Graph Search

La phosphorescence est le phénomène observé lorsqu'une matière continue à émettre de la lumière après avoir été éclairée. Le terme signifie approximativement illuminer comme le phosphore. Le phosphore blanc donne en effet de la lumière dans le noir, mais dans cette matière ce sont des réactions d'oxydation (chimiluminescence) qui en sont la cause. Phosphorescence et fluorescence sont deux formes différentes de luminescence. Le phénomène de phosphorescence proprement dit est dû, lui, à une autre réaction : il s'agit d'une suite de pertes d'énergie par des électrons qui ont été excités et qui retournent à des niveaux d'énergie plus bas. Le fait que cela se passe lentement relève du domaine de la mécanique quantique. Des matières phosphorescentes comme les aluminates de terres rares sont utilisées pour peindre les aiguilles de certains réveils ou montres, ainsi que dans la fabrication de jouets lumineux. Différences entre fluorescence et phosphorescence vignette|upright=1.5

Manganese doped ZnS nanoparticles

Yvonne Axmann

Nanostructured materials can be defined as those materials whose structural elements - clusters, crystallites or molecules - have dimensions in the 1-100 nm range. The explosion in both academic and industrial interest over the past 20 years arises from the remarkable variations in fundamental electrical, optical and magnetic properties that occur as one progresses from an "infinitely extended" solid to a particle of material consisting of a countable number of atoms. In this work the attention was laid on the optical properties of Mn2+ doped ZnS nanoparticles with respect to a possible application as bio sensor in medical and biological analytics. A synthesis for this type of particles has been developed and the nanocrystals obtained have been investigated in terms of their composition, particle size and optical properties. The particles synthesized were stabilized with the amino acid L-cysteine and showed the typical ZnS:Mn emission spectrum with its orange emission due to the 4T1 –> 6A1 transition within the d-orbitals of the Mn2+ at 585 nm and its blue emission due to recombination over the ZnS band gap from shallow electron traps at 380 nm. The onset of the absorption was blue shifted from 340 nm (bulk) to 310 nm, indicating a quantum size effect. The particle sizes and particle size distributions were determined with methods such as transmission electron microscopy, photon correlation spectroscopy, analytical ultra centrifugation and field flow fractionation. The particle size has also been calculated from the shift in the band gap with the help of the effective mass approximation. The limits of the applied techniques are discussed and the particle size distribution determined to go from 3-20 nm with a first maximum at about 5 nm and a second maximum at about 17 nm. The particles' luminescence quantum yields have been determined between 1.5-2 %. They could be enhanced by a factor of three by coating the particles with a SiO2 shell. The quantum yields showed a strong dependence on the dispersion concentration and Mn2+ content. These effects have been investigated with quantum yield measurements at different temperatures and Mn2+ contents, and lifetime measurements. The lifetime measurements showed decay times in the ns region for the blue and in the μs-ms region for the orange emission, indicating a florescence mechanism for the ZnS and a phosphorescence mechanism for the Mn2+ radiation.

EPFL2004

Thiazole Boron Difluoride Dyes with Large Stokes Shift, Solid State Emission and Room-Temperature Phosphorescence

Shuo Tong, Wei Wang, Jieping Zhu

The small Stokes shift and weak emission in the solid state are two main shortcomings associated with the boron-dipyrromethene (BODIPY) family of dyes. This study presents the design, synthesis and luminescent properties of boron difluoro complexes of 2-aryl-5-alkylamino-4-alkylaminocarbonylthiazoles. These dyes display Stokes shifts (Delta lambda, 77-101 nm) with quantum yields (phi(FL)) up to 64.9 and 34.7 % in toluene solution and in solid state, respectively. Some of these compounds exhibit dual fluorescence and room-temperature phosphorescence (RTP) emission properties with modulable phosphorescence quantum yields (phi(PL)) and lifetime (tau(p) up to 251 mu s). The presence of intramolecular H-bonds and negligible pi-pi stacking revealed by X-ray crystal structure might account for the observed large Stokes shift and significant solid-state emission of these fluorophores, while the enhanced spin-orbit coupling (SOC) of iodine and the self-assembly driven by halogen bonding, pi-pi and C-H center dot center dot center dot pi interactions could be responsible for the observed RTP of iodine containing phosphors.

WILEY-V C H VERLAG GMBH2022

On the Response Speed of Narrowband Organic Optical Upconversion Devices

Surendra Babu Anantharaman, Matthias Diethelm, Roland Hany, Wei-Hsu Hu, Frank Nüesch

Organic upconversion devices (OUCs) consist of an organic infrared photodetector and an organic visible light-emitting diode (OLED), connected in series. OUCs convert photons from the infrared to the visible and are of use in applications such as process control or imaging. Many applications require a fast OUC response speed, namely the ability to accurately detect in the visible a rapidly changing infrared signal. Here, high image-contrast, narrowband OUCs are reported that convert near-infrared (NIR) light at 980 and 976 nm with a full-width at half maximum of 130 nm into visible light. Transient photocurrent measurements show that the response speed decreases when lowering the NIR light intensity. This is contrary to conventional organic photodetectors that show the opposite speed-versus-light trend. It is further found that the response speed increases (when using a phosphorescent OLED) or decreases (for a fluorescent OLED) when increasing the driving voltage. To understand these surprising results, an analysis by numerical simulation is conducted. Results show that the response speed behavior is primarily determined by the electron mobility in the OLED. It is proposed that the low electron drift velocity in the emitter layer sets a fundamental limit to the response speed of OUCs.

WILEY-V C H VERLAG GMBH2022

Manganese doped ZnS nanoparticles

Yvonne Axmann

EPFL2004