Quantum Optoelectronics

Abstract:

We demonstrate that semiconductor quantum dots can be excited efficiently in a resonant three-photon process, whilst resonant two-photon excitation is highly suppressed. Time-dependent Floquet theory is used to quantify the strength of the multi-photon processes and model the experimental results. The efficiency of these transitions can be drawn directly from parity considerations in the electron and hole wavefunctions in semiconductor quantum dots. Finally, we exploit this technique to probe intrinsic properties of InGaN quantum dots. In contrast to non-resonant excitation, slow relaxation of charge carriers is avoided which allows us to measure directly the radiative lifetime of the lowest energy exciton states. Since the emission energy is detuned far from the resonant driving laser field, polarization filtering is not required and emission with a greater degree of linear polarization is observed compared to non-resonant excitation.

Abstract:

The main challenges in the optical and morphological tuning of highly stable orange-emitting Mn-doped perovskite include achieving uniform dopant distribution, maintaining structural integrity under varying environmental conditions, and optimizing luminescent efficiency while minimizing non-radiative recombination pathways. This study presents a novel, one-step, water-induced ultrafast synthesis strategy for obtaining Mn-doped mixed-halide perovskites at room temperature. This technique offers morphological control by varying the amount of water-based precursor, allowing the tuning of resulting nanostructures to produce nanoplatelets, nanocubes, or nanowires. In the growth mechanism, Mn2+ dopants affect the crystal structure by promoting stable growth and uniform doping at higher concentrations, while water improves ion dispersion, reaction kinetics, and passivation, facilitating optimal crystal growth and the formation of desired nanostructure morphologies. The synthesized Mn:CsPbBr3−xClx NCs form a highly stable colloidal solution with approximately 100 % emission stability for up to one year under ambient conditions and retain 98.9 % of its photoluminescence after aging at 85 °C for 200 h. We also explore the PL mechanism in Mn:CsPbBr3-xClx NCs, where temperature-dependent PL analysis reveals energy transfer from CsPbBr3-xClx exciton states to Mn2+-doped levels, enhancing PL intensity, with both exciton and Mn2+ emissions exhibiting a blue shift as the temperature increased from 6 K to 300 K, attributed to lattice expansion and electron–phonon interactions. A warm white light emission is achieved with excellent stability and an exceptionally wide color gamut coverage. The proposed strategy has the potential to enable large-scale synthesis and fabrication of highly stable perovskite devices for high-quality display and lighting applications.

Abstract:

Lead halide perovskite nanocrystals exhibit excellent optoelectronic properties, yet simultaneous observation of bright singlet and triplet exciton emissions under identical conditions has remained elusive. This limitation hinders optimization of quantum efficiency in light-emitting devices. Here, we provide the direct spectroscopic evidence for coexisting bright singlet and triplet excitons in CsPbBr3 nanocrystals, overcoming the conventional 25 % spin-statistical efficiency ceiling. Using polarization-resolved, spatially resolved, and time-resolved micro-photoluminescence at 7 K, we resolve three sharp triplet fine-structure components (T1, T2, T3) with energy separations of 1–3 meV and linear polarization >85 %, coexisting with broad singlet emission. The triplet emissions display distinct polarization axes, nonlinear intensity scaling, and nanosecond lifetimes, confirming their assignment as Rashba-split bright triplet states. Spatial mapping reveals that these emissions arise from structurally pristine domains with exciton diffusion lengths exceeding 9 μm. Time-resolved measurements show concurrent fast and slow decay components, consistent with singlet-to-triplet intersystem crossing followed by radiative triplet recombination. Our findings establish a comprehensive picture of exciton spin dynamics in perovskite nanocrystals and open new avenues for spin-engineered photonic devices. This work lays the foundation for next-generation LEDs, lasers, and quantum light sources that leverage both singlet and triplet radiative channels to exceed traditional efficiency limits. While these findings are demonstrated at cryogenic temperatures, they highlight essential spin-related mechanisms that could be harnessed for room-temperature operation through enhanced Rashba coupling, dielectric engineering, or compositional tuning.

Abstract:

Achieving stable and efficient circularly polarized luminescence (CPL) from achiral perovskite nanocrystals (PNCs) remains a major challenge in the development of advanced chiroptical materials. Herein, the syntheses of a total of nine compounds, including full‐color colloidal polymer‐capped PNC composites are reported based on organic‐inorganic hybrid perovskites and inorganic 2D nanosheets (NSs) using phenacyl halide as a single organic source of halide precursor. While the initial PNCs exhibit low photoluminescence quantum yield (PL QY) and poor stability, a previously unexplored surface absorption/ion exchange strategy employing 2D‐ZrH2P2O8 NSs significantly enhances both optical properties and long‐term stability, e.g., the FAPbBr3@ZrH2P2O8 (FA = formamidinium) composite exhibits a significantly enhanced PL QY of 88.57%, compared to 30.9% for the pristine counterparts, owing to the protective effect of the robust 2D ZrH2P2O8 network that enhances stability under ambient conditions. Crucially, embedding these stabilized PNCs into a chiral polymer matrix induces distinct mirror‐image strong CPL signals both in solution and solid‐state. This rare dual‐phase CPL activity arises from the conformational adaptability of the chiral polymer, which imparts chirality to the achiral PNCs via both covalent and non‐covalent interactions. These findings present a versatile strategy for producing robust, CPL‐active stable perovskite materials across the visible spectrum for next‐generation chiroptoelectronic devices.

Abstract:

Weakly bonded native ligands severely degrade the performance of perovskite quantum dot (PeQD) light-emitting diodes (LEDs). While conventional approaches can be used to strengthen ligand binding, they fail to achieve complete ligand exchange, leaving residual ligands that promote degradation. Herein, we present a dual-functional fluorinated benzyl phosphonic acid (F-BPA) ligand that modulates the acidity and enhances the binding affinity between the phosphonate groups of F-BPA and the perovskite surface compared to BPA due to a significant redistribution of the electrostatic potential of the molecule induced by fluorination. The F-BPA treatment facilitates effective ligand exchange and obtains well-passivated CsPbI3 PeQDs with improved stability under thermal, light, and polar solvent stress. Red-emissive LEDs achieved a maximum external quantum efficiency of 24.0% with improved device stability (half-lifetime of 1,020 min at 100 cd m–2). This study demonstrates a dual-functional ligand strategy and opens a new pathway toward PeQDs for next-generation display technologies.