Editor's recommendation: In this paper, a blue and green double-peak emission carbon quantum dot was prepared, and the luminescence mechanism of the prepared carbon quantum dot was explored through fluorescence and structural characterization. The resulting carbon quantum dots were mixed with PMMA to solve their agglomeration problem, and combined with 365 nm UV-LEDs to fabricate solid-state WLEDs.
Carbonquantum dots (CQDs) are a kind of green and environmentally friendly fluorescent material , with excellent optical properties, chemical stability and good biocompatibility, which have attracted extensive attention of researchers. , bioluminescence imaging, solid-state light-emitting devices (LEDs) and other fields have huge application potential. At present, there are two main methods for fabricating solid-state LEDs using CQDs. One is to use CQDs as the electroluminescent material of the emission layer to recombine the injected electrons and holes to prepare the quantum dot light-emitting device (QLED). There is still a lot of room for improvement in efficiency. The other is to use CQDs as light-converting phosphors to replace traditional semiconductor quantum dots and rare-earth fluorescent materials to construct solid-state LEDs. Since previous reports mainly focused on CQDs with single emission peak, it is necessary to combine with CQDs with different emission colors or other luminescent materials to prepare solid-state white LED (WLED). Due to the agglomeration of CQDs, the emission spectrum of in solid state of CQDs has a red-shift phenomenon compared with the emission spectrum of in solution. Therefore, how to develop white light-emitting CQDs and overcome their agglomeration is a challenge.
Recently, the research group of Chen Xifang from Southwest University of Science and Technology used 1,3-dihydroxynaphthalene, hydrochloric acid and anhydrous ethanol as raw materials, and synthesized white fluorescent CQDs with graphite core structure by one-step hydrothermal method .The related paper was published in APPLIED PHYSICS LETTERS titled "Blue and green double band luminescent carbonquantum dots: Synthesis, origin of photoluminescence, and application in white light-emitting devices".
Paper link:
https://aip.scitation.org/doi/full/10.1063/5.0046495
In this work, the author utilizes a simple one-step hydrothermal method with 1,3-dihydroxynaphthalene, hydrochloric acid and anhydrous Ethanol was used as a reactant to synthesize CQDs. The obtained CQDs have an average particle size of 2.7 nm, are easily soluble in absolute ethanol, emit white fluorescence under UV irradiation, and exhibit blue and green dual-band luminescence.
Fig. 1 (a) TEM image of CQDs; (b) size distribution map; (c) PL spectrum in absolute ethanol The existence of oxygen bonds (C–O–C), ester groups [C(=O)O], these functional groups can also be found on the surface of C8 CQDs and SiC QDs prepared earlier by our group, which can further deduce Like C8 CQDs and SiC QDs, CQDs luminescence originates from oxygen-related fluorophores on the surface of the graphite core, including C(=O)OH, C–OC, and C(=O)O.
Fig. 2 FTIR spectra of CQDs and CQDs-PMMA
Fig. 3 High-resolution XPS spectra of (a) C 1s and (b) O 1s.
Figure 4 (a) UV-Vis absorption spectrum of CQDs and (b) PLE spectrum of CQDs The green emission band in the spectrum should originate from the C(=O)O functional group.The other two observed absorptions at 306 and 368 nm in the PLE spectrum belong to the 282 and 336 nm absorption bands, respectively, where the 306 nm excitation contributes to the blue emission and the 368 nm excitation contributes to the blue and green emission , from which it is deduced that the excitation band at 306 nm corresponds to the emission at 405 nm, while the excitation band at 368 nm corresponds to the emission at 420, 433 and 460 nm. Combined with the PL spectra, the emission at 405 nm should be attributed to the C(=O)OH surface functional group, while the emission at 420, 433, and 460 nm were all derived from the C–OC–C surface functional group, consistent with the FTIR and XPS results.
The origin of multiple excitation/emission bands can be further understood through time-resolved spectroscopy. Each PL decay spectrum conforms to the rule of a single or double exponential function. Four fluorescence lifetimes for blue emission and three fluorescence lifetimes for green emission are obtained from the fitted values. Fluorescence lifetimes, 3.6 ns were assigned to the C(=O)OH surface functional group corresponding to the 405 nm emission, based on the relative amplitudes (R) of the components; 8.2 ns, 4.1 ns and 2.6 ns, due to the source Surface defect states from the C-O-C group, which contribute to the emission at 420, 433 and 460 nm; for the green emission band, the short lifetimes of 4.5 ns, 4.2 ns and 4.0 ns should be due to the photoexcited electrons occupying the C (=O ) O surface defect energy levels, consistent with previous analyses.
Table 1 Two-component lifetime (τ) and relative amplitude (R) of time-resolved fluorescence spectral fitting By mixing anhydrous ethanol solution of CQDs with PMMA acetone solution, CQDs can be effectively separated and cut surface structure without obvious change, and CQDs/PMMA is coated on the lampshade of 365nm commercial LED to prepare solid-state WLED.It can be seen in the EL spectra under different operating currents that the emission peaks of solid-state CQDs are located at 399, 420, 437 and 558 nm, which are slightly shifted compared to the spectra of solutions, proving that the principle of similar solubility in solvents is the process of forming solid-state films This method can be extended to solve the agglomeration problem of other quantum dots. Under the working current of 5 mA, the International Committee on Illumination (CIE) chromaticity coordinates (0.3122, 0.3429) of the prepared WLED are close to pure white light (0.33, 0.33), and the color temperature (CCT) is 6428 K, suitable for use as cool white light source for indoor and outdoor lighting.
Figure 5 (a) EL spectrum and (b) CIE chromaticity coordinates and CCT based on solid-state WLEDs with CQDs-PMMA.
In summary, we synthesized fluorescent CQDs with graphitic core structures from 1,3-dihydroxynaphthalene, hydrochloric acid, and anhydrous ethanol by a one-step hydrothermal method. The obtained CQDs exhibited blue and green dual-band emission in anhydrous ethanol solution, and showed white fluorescence under UV light irradiation. PL spectroscopy, surface structure characterization, UV-Vis absorption spectroscopy , PLE and PL decay spectroscopy together reveal that the light source of CQDs is from carbon-oxygen-related surface fluorophores. The CQDs-based WLEDs are cool-white light sources with CIE coordinates of (0.3122, 0.3429) and CCT of 6428 K. (Text: Wu Wenze)
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