High solid-state photoluminescence quantum yield of carbon-dot-derived molecular fluorophores for light-emitting devices.

Several recent studies of carbon dots (CDs) synthesized by bottom-up methods under mild conditions have reported the presence of organic molecular fluorophores in CD dispersions. These fluorophores have a tendency to aggregate, and their properties strongly depend on whether they are present in the form of discrete molecules or aggregates. The aggregation becomes more prominent in the solid state, which motivates the study of the properties of the fluorophores associated with CDs in the solid state. Here, we report the solid-state characterization of N4,N11-dimethyldibenzo[a,h]phenazine-4,11-diamine (BPD) - a molecular fluorophore that forms CDs. Discrete BPD molecules show excitation-wavelength-independent photoluminescence (PL) emission in the green wavelength region at ∼520 nm. However, additional blue PL is also observed due to aggregation, making the PL emission significantly broad. For detailed studies, BPD is mixed in different solid matrices, and it is observed that the PL quantum yield (PLQY) of BPD films strongly depends on the concentration of BPD in the solid matrices. Increasing the concentration of BPD results in a considerable decrease in the PLQY. The PLQY of the films with an optimum concentration of BPD is 75.9% and 40.2% in polymethyl methacrylate and polystyrene, respectively. At higher concentrations, these PLQY values decrease to ∼11%. The significant decrease in the PLQY is ascribed to reabsorption and nonradiative exciton decay that is facilitated by BPD aggregation at higher concentrations. Finally, light-emitting devices (LEDs) were fabricated with almost pure white emission color, having CIE (International Commission on Illumination) coordinates of (0.35, 0.37) using BPD in the color-converting layer of blue-pumped LEDs. The device shows a luminous efficiency 3.8 lm W-1 and luminance of 43 331 cd m-2.

Self-Sealing Carbon Patterns by One-Step Direct Laser Writing and Their Use in Multifunctional Wearable Sensors.

A combined photothermal simulation and experimental study leads to a novel internal reflection-assisted direct laser writing carbonization method (IR-DLWc), which enables in situ fabrication of carbon features/patterns that are self-sealed in the interior of a thin polyimide (PI) film in one step without additional packaging procedures. With this new method, carbon line patterns that are fully contained in a 50 µm PI film are fabricated, characterized, and evaluated for their electrical and piezoresistive performance. The self-sealing character of the carbon features created by IR-DLWc imparts them unprecedented mechanical stability/robustness as compared to those fabricated by the conventional DLWc method. Upon applying a double-writing scheme and strain-engineering treatment, the IR-DLWc-created carbon lines show significantly improved piezoresistive sensitivity with a gauge factor evaluated to be 428 in tension and 107 in compression. The high piezoresistive sensitivity, excellent dynamic response, reasonably good durability, self-sealing character, and compliant nature of the IR-DLWc generated carbon patterns make them suitable for a variety of wearable sensing applications. In this work, we demonstrated their use as a tactile sensor for sensing contact force; a functional bandage for monitoring physiological activities like swallowing, pulsing, and breathing; and a glove sensing system for finger gesture recognition.