Research Projects

Conventional light-emitting diodes which convert electrical signals to light signals have been developed using AlGaAs or organic materials. Both have their inherent limitation on the cost or stability of the material for blue and green emissions. Solution processable semiconductors such as halide perovskites are well-known for covering the emission in blue and green regions. However, the halide diffusion under electric bias leads to perovskite degradation. This calls for a search for new materials which are robust and thin. 2D excitonic semiconductors namely transition metal dichalcogenide (TMDC - WS₂) monolayer show electroluminescence in the red part (650 nm) of the visible region.¹,² There are not many 2D excitonic semiconductors that can emit in blue and green regions, except hBN which still shows low EQE due to defect-state dominated emission.³ TMDC monolayer placed in an optical cavity undergoes strong light-matter interaction to form exciton-polaritons, which are half-light, half-matter states. This alters the intrinsic density of states, thereby to tune the emission. However, adding electrodes inside the cavity is highly challenging. The PI has shown exciton-polariton emission at room-temperature from both TMDC superlattice,⁹ and 2D halide perovskites¹⁰,¹¹ in the absence of an external cavity. Further, blue emission from a new class of material called metal-organic chalcogenolates (MOCHAs) crystals was demonstrated by the PI.¹² Although MOCHAs are emissive in the blue region with narrow linewidth, the photoluminescence quantum yield is less than 1%. Also, tuning the MOCHA composition can lead to green emission. The objectives of this proposal are to grow multilayer MOCHA films using a chemical vapor deposition route for the first time to form exciton-polariton states. Further, our hypothesis is that the low emission from MOCHA can be due to defects in the inorganic layer such as chalcogen vacancies or antisite defects. Using the atom probe tomography and ⁷⁷Se NMR techniques, we will identify the atomic-level defects that lead to exciton quenching in MOCHA and increase the quantum yield. Optical studies to correlate the structural modification will be performed. Further, the device stack to obtain narrow emission will be optically simulated and experimentally realized by transferring 2D materials using a wet-transfer route or exfoliation route. The electrically-driven exciton-polariton emission from multilayered MOCHA to achieve narrow linewidth emission will be evaluated. Further, we will verify, if strong coupling reduces the linewidth and enhances the brightness of the LEDs (electrical pumping) as reported from optical pumping. The research outcome will demonstrate blue and green LEDs from 2D materials. Realizing these narrow emitters in the blue and green region will unleash the full potential of new class of 2D materials for light emission technology as they are proven for scaling devices in industrial settings.