Emission States Variation of Single Graphene Quantum Dots

Heteroatom Doping Effects on the Exciton Behavior in Carbonized Polymer Dots

Carbonized polymer dots (CPDs) are attractive optoelectronic materials for a variety of applications, but their complex structures hinder a full understanding of their photophysical and excited-state properties. This work illustrates that heteroatom doping (N-doped, N,S-doped, and N,S,F-doped CPDs) significantly affects the exciton behavior of CPDs. It reveals unconventional photoluminescence (PL) blinking with multilevel intensity fluctuations, which depend on the doping type and excitation power density. The multilevel PL blinking is attributed to the formation of charged excitons induced by heteroatom doping, which resembles the characteristic behavior of semiconductor quantum dots. Additionally, nonradiative recombination enhances photothermal conversion efficiency, enabling effective photothermal therapy with significant cytotoxicity against cancer cells. Our results provide compelling evidence that CPDs possess quantum properties similar to those of semiconductors instead of emissive chromophores on the surface, while also highlighting their multifunctionality as photoluminescent probes and photothermal agents.

Electron-phonon coupling-assisted universal red luminescence of o-phenylenediamine-based carbon dots

Due to the complex core-shell structure and variety of surface functional groups, the photoluminescence (PL) mechanism of carbon dots (CDs) remain unclear. o-Phenylenediamine (oPD), as one of the most common precursors for preparing red emissive CDs, has been extensively studied. Interestingly, most of the red emission CDs based on oPD have similar PL emission characteristics. Herein, we prepared six different oPD-based CDs and found that they had almost the same PL emission and absorption spectra after purification. Structural and spectral characterization indicated that they had similar carbon core structures but different surface polymer shells. Furthermore, single-molecule PL spectroscopy confirmed that the multi-modal emission of those CDs originated from the transitions of different vibrational energy levels of the same PL center in the carbon core. In addition, the phenomenon of "spectral splitting" of single-particle CDs was observed at low temperature, which confirmed these oPD-based CDs were unique materials with properties of both organic molecules and quantum dots. Finally, theoretical calculations revealed their potential polymerization mode and carbon core structure. Moreover, we proposed the PL mechanism of red-emitting CDs based on oPD precursors; that is, the carbon core regulates the PL emission, and the polymer shell regulates the PL intensity. Our work resolves the controversy on the PL mechanism of oPD-based red CDs. These findings provide a general guide for the mechanism exploration and structural analysis of other types of CDs.

Two-Dimensional Self-Assembly of Boric Acid-Functionalized Graphene Quantum Dots: Tunable and Superior Optical Properties for Efficient Eco-Friendly Luminescent Solar Concentrators

Carbon-based nanomaterials hold promise for eco-friendly alternatives to heavy-metal-containing quantum dots (QDs) in optoelectronic applications. Here, boric acid-functionalized graphene quantum dots (B-GQDs) were prepared using bottom-up molecular fusion based on nitrated pyrenes and boric acid. Such B-GQDs with crystalline graphitic structures and hydrogen-bonding functionalities would be suitable model systems for unraveling the photoluminescence (PL) mechanism, while serving as versatile building blocks for supramolecular self-assembly. Unlike conventional GQDs with multiple emissive states, the B-GQDs exhibited excitation-wavelength-independent, vibronic-coupled excitonic emission. Interestingly, their PL spectra can be tuned without largely sacrificing the quantum yield (QY) due to two-dimensional self-assembly. In addition, such B-GQDs in a polystyrene matrix possessed an ultrahigh QY (∼90%) and large exciton binding energy (∼300 meV). Benefiting from broadband absorption, ultrahigh QY, and long-wavelength emission, efficient laminated luminescent solar concentrators (100 × 100 × 6.3 mm³) were fabricated, yielding a high power conversion efficiency (1.4%).

Rational Building of Nonblinking Carbon Dots via Charged State Recovery

Carbon dots (CDs), as emerging luminescent nanomaterials, possess excellent but complex properties, and thus, they have attracted immense attention for their applications. Further practical application of CDs has been hindered by their limited photostability and photoluminescence intermittency. In this study, we demonstrated that an antioxidant (Trolox) can dramatically enhance the photostability and minimize the photoblinking of CDs without affecting their native spectral characteristics. Significant photoluminescence enhancement and stabilization were observed with the addition of Trolox in ensemble level. Meanwhile, strikingly stable emissions from individual CDs have been observed in the presence of Trolox in single-particle-level experiments. Our observations revealed that the charged state of CDs can be effectively recovered to a neutral state by Trolox via electron transfer. These results prove that the combination of antioxidants and CDs is a powerful means to improve their fluorescence robustness, which is crucial for applications that demand long-lived, nonblinking emission.

Continuously Graded Quantum Dots: Synthesis, Applications in Quantum Dot Light-Emitting Diodes, and Perspectives

Colloidal quantum dot (QD) light-emitting diodes (QLEDs) hold the promise of next-generation displays and illumination owing to their excellent color saturation, high efficiency, and solution processability. For achieving high-performance light-emitting diodes (LEDs), engineering the fine compositions and structures of QDs is of paramount importance and attracts tremendous research interest. The recently developed continuously graded QDs (cg-QDs) with gradually altered nanocompositions and electronic band structures present the most advanced example in this area. In this Perspective, we summarize the current progress in LEDs based on cg-QDs, mainly concentrating on their synthesis and advantages in addressing the great challenges in QLEDs, like efficiency roll-off at high current densities, short operation lifetimes at high brightness, and low brightness near the voltage around the bandgap. In addition, we propose accessible approaches exploiting the cutting-edge mechanisms and techniques to further optimize and improve the performance of QLEDs.