Dr. Max Gmelch Profile

Dr. Max Gmelch

at TU Dresden

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Publications (4)

Proceedings Article | 30 May 2022 Presentation

Continuous-wave readable and subsecond programmable luminescent tags based on organic room temperature phosphorescence

Tim Achenbach, Max Gmelch, Heidi Thomas, Sebastian Reineke

Proceedings Volume PC12149, PC121490T (2022) https://doi.org/10.1117/12.2621557

KEYWORDS: Oxygen, Phosphorescence, Ultraviolet radiation, Image enhancement, Polymethylmethacrylate, Luminescence, Infrared imaging, Image processing, Coating, Switching

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By using the room temperature phosphorescence of organic materials, transparent and flexible tags with resolutions of up to 700 dpi and an activation time of 120 ± 20 ms can be achieved. Any pattern can be written into the film by selectively illuminating the film with ultraviolet light through a shadow mask. The inscribed information can then be erased again with the aid of a heat source, and the overall cycle can be repeated many times (> 40 tested). The functional layer of these devices consists of organic biluminescent emitters doped into a poly(methyl methacrylate) (PMMA) host matrix covered with an oxygen barrier layer. By wet processing under ambient conditions, molecular oxygen is present in the emission layer, hence quenching the phosphorescence effectively. During this quenching process, highly reactive singlet oxygen is generated, which can chemically bond to the surrounding environment. Therefore, the molecular oxygen concentration decreases in the illuminated areas. The decreasing oxygen quenching is at some point outcompeted by the radiative rate of the emitter, enabling locally resolved phosphorescence in shape of the used mask. The oxygen permeability of the barrier layer is temperature dependent and increases with rising temperature. This enables an oxygen refilling of the functional layer by heating the device and thus increases the quenching rate to a value prior to the activation process. After a short cooling phase, the film can be rewritten. Under continuous-wave (cw) illumination, any luminescence that is independent of the oxygen concentration, be it fluorescence or phosphorescence with a high radiative rate, limits the contrast between activated and oxygenated areas. Therefore, different material systems are developed and tested, showing reduced fluorescence but maintaining the oxygen quenching ability of the phosphorescence. The emitter 4,4‘-dithianthrene-1-yl-benzophenone (BP-2TA), with a phosphorescence lifetime of 30 ms and a high phosphorescence-to-fluorescence ratio of about 20, fulfills these requirements best and thus enables high-contrast tags that can also be read under cw illumination.

Proceedings Article | 1 April 2020 Presentation

Rewritable and flexible high-contrast tags using switchable organic room temperature phosphorescence (Conference Presentation)

Tim Achenbach, Max Gmelch, Heidi Thomas, Felix Fries, Sebastian Reineke

Proceedings Volume 11365, 113650B (2020) https://doi.org/10.1117/12.2560708

KEYWORDS: Phosphorescence, Oxygen, Ultraviolet radiation, Image enhancement, Polymethylmethacrylate, Coating, Luminescence, Quenching (fluorescence), Image processing, Infrared imaging

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By utilizing the room temperature phosphorescence (RTP) of organic materials, transparent and flexible optical tags with high resolutions up to 700 dpi are realized. Through masked ultraviolet (UV) illumination or laser ray writing, any phosphorescent pattern can be printed into the transparent device. With the help of infrared (IR) light, it is possible to fully erase the imprint again. This cycle is shown to be repeatable at least 40 times. The functional layer of these devices consists of different organic biluminescent emitters doped into a polymethylmethacrylate (PMMA) host matrix covered with an oxygen barrier layer. However, due to the sample preparation in ambient conditions, molecular oxygen is still present in the emissive layer. This leads to a full quenching of the phosphorescence and the generation of excited singlet oxygen due to triplet-triplet interactions involving the oxygen triplet groundstate. The singlet oxygen has a high chemical reactivity and thus is able to form a bond with the surrounding materials. Consequently, the molecular oxygen concentration decreases in the illuminated areas. The concurrently decreasing oxygen-quenching rate is at some point outcompeted by the radiative rate of the emitter, enabling locally resolved phosphorescence. This emission resembles the intended pattern. The oxygen permeability of the barrier layer is temperature dependent and increases with rising temperature, which can be realized using a hotplate or IR illumination. This enables an oxygen refilling of the functional layer and therefore increases the quenching rate to a value prior to the activation process, leading to the vanishing of the phosphorescent image. After a short cooling phase, new information can be printed into the device. In continuous wave illumination, fluorescence may limit the contrast between activated areas and those still containing oxygen. Hence, different material systems showing reduced fluorescence without losing the oxygen quenching ability of the phosphorescence, are developed and tested.

Proceedings Article | 10 March 2020 Presentation

The importance of statistics for photoluminescence quantum yield measurements (Conference Presentation)

Felix Fries, Heidi Thomas, Max Gmelch, Tim Achenbach, Sebastian Reineke

Proceedings Volume 11277, 1127708 (2020) https://doi.org/10.1117/12.2540033

KEYWORDS: Luminescence, Quantum efficiency, Error analysis, Photons, Statistical analysis, Materials science, Classification systems, Data analysis, Lead, Solid state physics

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Proceedings Article | 10 March 2020 Presentation

Rewritable luminescent tags using room-temperature phosphorescence (RTP) (Conference Presentation)

Max Gmelch, Heidi Thomas, Felix Fries, Tim Achenbach, Sebastian Reineke

Proceedings Volume 11277, 1127707 (2020) https://doi.org/10.1117/12.2541863

KEYWORDS: Oxygen, Phosphorescence, Ultraviolet radiation, Coating, Data storage, Infrared radiation

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