Blue-light-excitable broadband yellow-emitting CaGd2HfSc(AlO4)3:Ce3+ garnet phosphors for white light-emitting diode devices with improved color rendering index

The current commercial white light-emitting diodes (LEDs) are generally based on the combination of blue LED chips and Y₃Al₅O₁₂:Ce³⁺ yellow phosphors. However, because of the lack of red component, such white LED devices exhibit cool white-light emissions with low color rendering index (Ra < 75, R9 < 0). Therefore, it is urgent to discover new blue-light-excitable yellow-emitting phosphors with enhanced red emissions for fabricating high color-quality white LEDs. In the present work, we demonstrate a novel broadband yellow-emitting CaGd₂HfScAl₃O₁₂:Ce³⁺ garnet phosphor for blue-light-excited white LEDs with improved color rendering index. The as-prepared CaGd₂HfScAl₃O₁₂:Ce³⁺ garnet phosphor possesses a cubic structure with Ia$\overline{3}$d space group, and the unit cell parameters of the representative CaGd₂HfScAl₃O₁₂:2%Ce³⁺ phosphor are a = b = c = 12.450 Å, α = β = γ = 90°, and V = 1,929.59(4) ų. Impressively, we find that the CaGd₂HfScAl₃O₁₂:Ce³⁺ garnet phosphor shows an intense absorption band in the 300–500 nm wavelength range with a maximum at 452 nm owing to the 4f→5d transition of Ce³⁺ ions. On 452 nm excitation, the optimal CaGd₂HfScAl₃O₁₂:2%Ce³⁺ sample exhibits a broad asymmetric yellow emission band in the wavelength range of 470–750 nm with peak at 564 nm and full width at half maximum of 151 nm. The Commission Internationale de l’Eclairage chromaticity coordinates and internal quantum efficiency of the CaGd₂HfScAl₃O₁₂:2%Ce³⁺ sample are (0.4485, 0.5157) and 30.4%, respectively. Finally, a white LED device is fabricated by combing a 450 nm blue LED chip with commercial Y₃Al₅O₁₂:Ce³⁺ yellow-emitting phosphor, which generates white light with low color rendering index (CRI; Ra = 74.7, R9 = ?12.7) and high correlated color temperature (CCT = 6,554 K) under the 60 mA driving current. In sharp contrast, another white LED device, which is made by coating our as-prepared CaGd₂HfScAl₃O₁₂:2%Ce³⁺ yellow-emitting phosphors onto the surface of a 450 nm blue LED chip, produces white-light emission with high CRI value (Ra = 84.5, R9 = 26.3) and relatively low CCT of 5,649 K. This work reveals that the newly discovered broadband yellow-emitting CaGd₂HfScAl₃O₁₂:Ce³⁺ phosphors can serve as a potential color converter in high-color-quality phosphor-converted white LEDs.     

Red-emitting,self-oxidizing carbon dots for the preparation of white LEDs with super-high color rendering index

Carbon dots(CDs), as a kind of carbon nanomaterials, have attracted widespread attention due to their unique structure and excellent optical properties, and they are low-cost, environmentally friendly and biocompatible. However, the development of near-infrared(NIR) emission CDs remains a challenge. In this study, we successfully prepared CDs with a maximum emission of714 nm using citric acid as the carbon source, thiourea and ammonium fluoride as the dopant source, and N,N-dimethylformamide as the solvent. The quantum yield(QY) is as high as 22.64%. Interestingly, the prepared CDs self-oxidize in the presence of oxygen, resulting in a blue shift of their emission. Therefore, they can be used to prepare white light-emitting diodes(WLEDs) without adding other fluorescent substances. Notably, the work presented herein constitutes the first report of WLEDs preparation from single CDs.     

Determination of Binding Constants for Cyclodextrin Complexes with Alkanols by the 1H NMR Measurements of Longitudinal Relaxation Time Using Tetramethylammonium Chloride as an Internal Reference

The longitudinal relaxation times (T₁)of ¹H NMR signals for a variety of alkanols in D₂Omarkedly decreased with increasing concentrations (c)of - and -cyclodextrins (CD). Tetramethylammoniumchloride (TMA) used as an internal reference wasavailable for evaluating an effect of solutionviscosity on relaxation rates R(=1/T₁), since TMAshowed no appreciable interaction with CD. Changes inthe ratio of R for alkanol protons to R for TMAprotons with c were analyzed by the curve-fittingmethod to give K_a. These K_a values agreedwell with those obtained by the analysis of changesin , indicating that T₁ measurement is available forthe determination of K_a for CD complexes. 2D ROESYspectra provided definite information on the molecularstructures of CD complexes with alkanols.