Thesis defence of Brian DUSOLLE

Development of a supercritical millifluidics route for the synthesis of III-N colloidal nanocrystals applied to self-emissive QLED devices

« Quantum-dots », semi-conductor nanocrystals capable of emitting monochromatic light, lead to the development of a new technology: « quantum-dot light emitting devices » or QLED. These electroluminescent sources which aim at replacing current systems such as OLED, still suffer from commonly used materials’ toxicity and a difficulty to obtain all wavelength of the visible spectrum.

In this project, pressurized millifluidic reactors are employed for the continuous synthesis of GaN and InGaN quantum-dots via supercitical solvothermal routes. Indeed, these biocompatible materials whose emission can be tuned in the whole visible range as well as near UV and infrared, represent an alternative of choice, but their synthesis remain challenging in particular via solvothermal ways. We hereby explore the reaction between gallium cupferronates and hexamethyldisilazane in supercritical conditions. Following the formation of agregated, colloidally unstable particles using methanol and hexane as co-solvants, a new route is developped using oleylamine as a solubilization intermediate in hexane or toluene. Consequently, non-agegated soluble particles are obtained, whose size can be controlled through the reaction conditions such as precursor concentration. A bandgap-to-size dependance very close to the theoretical model is observed, although an energy level is identified in the bandgap leading to the quasi-extinction of the excitonic emission and the appearance of a wide defect fluorescence band. Our novel route is also adapted for the synthesis of InGaN solid solutions in the whole composition range. Indium inclusion is confirmed by chemical and cristallographic analyses, and the bandgap measured for 30 at.% indium is coherent with a joint effect of size and doping.

The particles are then deposited by spin-coating and integrated into multi-layered LED structures allowing direct charge injection to induce electroluminescence. After development of a model device using CdSe, our GaN quantum-dots are used as the emissive layer. Despite low quantum yields, a voltage induced emission is obtained whose origin can be attributed to our materials. This represents a proof of concept leading the way to GaN and InGaN based QLED fabrication, provided optimization of our materials’ synthesis.

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