Emissive carbon dots (CDs) that are synthesized from biomass can be highly sustainable, but the number of reported biomass-derived CDs that emit in the ultraviolet (UV) range is small. Moreover, current commercial UV-emitting materials rely heavily on the use of non-sustainable resources, such as rare metals, heavy metals, and petroleum chemicals. This yields that the development of efficient biomass-derived UV-CDs is desired. Here, we report on the hydrothermal conversion of a common green-tea extract (Polyphenon 60) into UV-CDs, which feature a photoluminescence (PL) peak wavelength of 384 nm, a full width at half maximum of 72 nm, and a photoluminescence quantum yield (PLQY) of 17% in water. By shifting to a lower-polarity solvent of 3-phenoxyanisole, the PLQY is strongly enhanced to 81%, and the PL peak blue-shifts to 370 nm, while the maximum solubility is lowered. These observations support the notion that the UV-CDs feature aggregation-induced emission and that they are endowed with hydrophilic surface groups. Moreover, the findings of excitation-wavelength-independent PL and a nanosecond-level short emission lifetime reveal that it is a single distinct fluorophore that produces the UV emission. We finally report preliminary results that the UV-CDs exhibit potential for inhibiting the proliferation of cancer cells.

Perovskite quantum dots (PeQDs) endowed with capping ligands exhibit impressive optoelectronic properties and enable for cost-efficient solution processing and exciting application opportunities. We synthesize and characterize three different PeQDs with the same cubic CsPbBr₃ core, but which are distinguished by the ligand composition and density. PeQD-1 features a binary didodecyldimethylammonium bromide (DDAB) and octanoic acid capping ligand system, with a high surface density of 1.53 nm⁻², whereas PeQD-2 and PeQD-3 are coated by solely DDAB at a gradually lower surface density. We show that PeQD-1 endowed with highest ligand density features the highest dispersibility in toluene of 150 g/L, the highest photoluminescence quantum yield of 95% in dilute solution and 59% in a neat film, and the largest core-to-core spacing in neat thin films. We further establish that ions are released from the core of PeQD-1 when it is exposed to an electric field, although it comprises a dense coating of one capping ligand per four surface core atoms. We finally exploit these combined findings to the development of a light-emitting electrochemical cell (LEC), where the active layer is composed solely of solution-processed pure PeQDs, without additional electrolytes. In this device, the ion release is utilized as an advantage for the electrochemical doping process and efficient emissive operation of the LEC.

Light-emitting electrochemical cells (LECs) can be fabricated with cost-efficient printing and coating methods, but a current drawback is that the LEC emitter is commonly either a rare-metal complex or an expensive-to-synthesize conjugated polymer. Here, we address this issue through the pioneering employment of metal-free and facile-to-synthesize carbon nanodots (CNDs) as the emitter in functional LEC devices. Circular-shaped (average diameter = 4.4 nm) and hydrophilic CNDs, which exhibit narrow cyan photoluminescence (peak = 485 nm, full width at half maximum = 30 nm) with a high quantum yield of 77% in dilute ethanol solution, were synthesized with a catalyst-free, one-step solvothermal process using low-cost and benign phloroglucinol as the sole starting material. The propensity of the planar CNDs to form emission-quenching aggregates in the solid state was inhibited by the inclusion of a compatible 2,7-bis(diphenylphosphoryl)-9,9’-spirobifluorene host compound, and we demonstrate that such pristine host-guest CND-LECs turn on to a peak luminance of 118 cd·m⁻² within 5 s during constant current-density driving at 77 mA·cm⁻².