Towards electrically injected colloidal semiconductor lasers

Colloidal semiconductor nanocrystals are widely adapted in lighting, sensing, and lasing applications. Their advantages of excellent solution processability, large absorption cross-sections, narrow full-width-half-maximum, flexible wavelength tunability, and reduced Auger effects make them promising...

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Bibliographic Details
Main Author: Thung, Yi Tian
Other Authors: Hilmi Volkan Demir
Format: Thesis-Doctor of Philosophy
Language:English
Published: Nanyang Technological University 2025
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Online Access:https://hdl.handle.net/10356/182185
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Institution: Nanyang Technological University
Language: English
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Summary:Colloidal semiconductor nanocrystals are widely adapted in lighting, sensing, and lasing applications. Their advantages of excellent solution processability, large absorption cross-sections, narrow full-width-half-maximum, flexible wavelength tunability, and reduced Auger effects make them promising gain media for next-generation lasing. They present a cost-effective alternative to traditional epitaxial laser diodes, plagued by limitations of optoelectronic application compatibility and fabrication complexities. Despite successful demonstrations of optically pumped lasers, electrically injected colloidal semiconductor lasers saw limited progress. Overcoming this requires careful selection of nanocrystals as gain media, a resonant optical cavity supporting low lasing thresholds and high optical gain, and an architecture suitable for high electrical injection currents to achieve population inversion for lasing. In addition, spectral tunability in nanocrystal emission is critical to their feasibility as an alternative gain medium. To this end, this thesis explores employing two-dimensional CdSe-based nanoplatelets (NPLs) as an alternative gain medium, investigating their electrical injection and lasing applications, and demonstrating optically pumped NPL lasing with potential for electrically injected laser development. Effective charge injection into CdSe NPLs is crucial to enable electrically injected NPL lasers. Previous efforts focused on heterostructure NPLs emitting in the yellow to red range, limiting spectral tunability. Addressing this gap, I explored electrical charge injection into Br--capped CdSe NPLs using an inverted p-i-n device architecture, resulting in narrow electroluminescence at 530 nm. The findings emphasize the significance of an appropriate device architecture for balanced charge injection, crucial for radiative recombination in the NPL gain medium—a key factor in achieving electrically injected NPL lasers. This successful demonstration of EL at 530 nm, unprecedented in NPLs, enhances their spectral tunability, positioning them as alternatives to traditional epitaxial laser diodes. Secondly, NPLs must demonstrate superior gain performance and a low lasing threshold to serve as an alternative gain medium. To showcase NPLs' exceptional lasing capabilities, I integrated core/alloyed-shell CdSe/CdZn1-xSx NPLs via kinetically driven liquid-liquid interface self-assembly deposition into silica microspheres. This resulted in nanosecond laser-pumped whispering-gallery-mode (WGM) microlasers with ultrathin NPL coatings on the microspheres' surface, enhancing optical confinement and minimizing optical loss. The fabricated microlasers exhibited stable lasing at room temperature with a record-breaking quality (Q-) factor of 13000, a low lasing threshold of 27.67 μJ cm-2, and stable single-mode lasing with a Q-factor of 7340 via evanescent field coupling between an NPL-coated microsphere and a thin uncoated microfiber in a 2D-3D microcavity configuration. This study underscores the core/alloyed-shell NPLs' suitability for outstanding lasing demonstrations and the potential of the novel self-assembly method for integrating colloidal nanocrystals onto complex 3D microstructures for next-generation miniaturized colloidal optoelectronic and photonic applications. Thirdly, leveraging on the solution processability and colloidal stability of NPLs is key to unlocking their potential for practical lasing applications. Core/hybrid-shell CdSe/CdS@Cd1−xZnxS NPLs are incorporated into a soft-matter polymer using capillary immersion techniques. The resulting microfiber lasers demonstrated WGM lasing with a low threshold of 14.8 μJ cm-2 and a single-mode Q-factor of 5500. The flexibility of these NPL-based polymer microfiber lasers allowed for the creation of polygonal self-coupling microresonators with a Q-factor of 3500, surpassing contemporary 1D resonant cavities by an order of magnitude. Further manipulation of the NPL-based polymer enabled the formation of an interconnected fiber network capable of mode selection and modulation. These breakthroughs underscore the ability of colloidal NPLs to be seamlessly integrated into practical lasing applications, overcoming previous limitations with epitaxially grown laser solid microcavities. Lastly, pursuing electrically injected colloidal NPLs lasers, I sandwiched core/hybrid-shell CdSe/CdS@Cd1−xZnxS NPLs as a gain material between two double Bragg reflectors (DBRs) to fabricate a vertical cavity surface-emitting lasers (VCSELs). The NPL-VCSEL demonstrated a low lasing threshold of 21.6 W cm-2 with a single-mode Q-factor of 11800 under continuous-wave (cw) excitation, with good agreement between the cw-pumped lasing spectra and the ps pulse-pumped lasing spectra. to highlighting the NPL-VCSEL’s robustness under different optical pumping regimes, I demonstrated the NPL-VCSEL’s performance under continuous wave optical pump, affirming the suitability of the colloidal NPLs as the preferred gain medium to realize the desired electrically injected colloidal semiconductor lasers. In conclusion, this thesis provides a deep understanding of colloidal two-dimensional NPLs’ lasing properties, their novel miniaturized lasing applications, providing insights on working towards an electrically injected colloidal NPL laser with currently achieved outcomes and knowledge.