A Facile Approach to Solid-State White Emissive Carbon Dots and Their Application in UV-Excitable and Single-Component-Based White LEDs

Abstract

Large-scale applications of conventional rare-earth phosphors in white light-emitting diodes (W-LEDs) are restricted by the non-renewable raw material sources and high energy consumption during the production process. Recently, carbon dots (CDs) have been proposed as promising alternatives to rare-earth phosphors and present bright prospects in white lighting. However, the use of CDs in W-LEDs still has two major obstacles, i.e., solid-state quenching and lack of single-component white emissive products. In this work, a facile, rapid, and scalable method for the preparation of solid-state white emissive CDs (W-CDs) is reported via microwave-irradiation heating of L-aspartic acid (AA) in the presence of ammonia. The W-CDs exhibit blue photoluminescence (PL) in dilute aqueous dispersion and their emission spectra gradually broaden (emerging new emissions at orange-yellow regions) with concentration increases. Interestingly, the W-CDs powder displays a very broad PL spectrum covering nearly the whole visible-light region under ultraviolet (UV) excitation, which is responsible for the observed white emission. Further studies revealed that the self-quenching-resistance feature of the W-CDs is probably due to a covering of polymer-like structures on their surface, thus avoiding the close contact of nanoparticles with each other. PL emission of the W-CDs is reasonably ascribed to a cross-linked enhanced effect (CEE) of the sub-fluorophores contained in the material (e.g., –NH2 and C=O). Finally, applications of the W-CDs in fabricating single-component-based W-LEDs using commercially available UV chips were attempted and shown to exhibit satisfactory performances including high white light-emitting purity, high color rendering index (CRI), and tunable correlated color temperature (CCT), thus rendering great promise for W-CDs in the field of white lighting.

Visible-Light-Excited Room Temperature Phosphorescent Carbon Dots

“…The surface groups and chemical compositions of the AA-CDs are followed and identified by Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS). As seen in Figure 3d (red line), the FT-IR spectrum of AA-CDs shows obvious absorption peaks at 3432, 3273, 2939, 1712, 1659, 1400 and 1168 cm −1 , which are attributed to the stretching vibrations of O-H, N-H, -CH2-, C=O, C=C, C-N and C-O bonds, respectively [41,48,49]. These FT-IR analyses could be further confirmed by the XPS characterizations.…”

“…The HR XPS of O 1s spectrum was deconvoluted into three peaks at 530.9 eV, 532.6 eV and 533.7 eV, corresponding to -OH, C=O and N-C=O, respectively (Figure 3h) [59,62]. shows obvious absorption peaks at 3432, 3273, 2939, 1712, 1659, 1400 and 1168 cm −1 , which are attributed to the stretching vibrations of O-H, N-H, -CH2-, C=O, C=C, C-N and C-O bonds, respectively [41,48,49]. These FT-IR analyses could be further confirmed by the XPS characterizations.…”

“…Please note that the concentrations of AA and ammonia are critical for the successful preparation of AA-CDs. For example, solid-state white emissive CDs could be prepared by a similar procedure if lower concentrations of AA and ammonia were being used [48].…”

“…As shown in Figure 1a, L-aspartic acid (AA) was taken as the carbon precursor, and the AA-CDs can be obtained by microwave-irradiation heating AA in the presence of ammonia water (see Materials and Methods section for details). Notably, other kinds of natural amino acids do not produce RTP CDs by the same reaction conditions; this is because that AA could form crosslinked polymer-like structures under high temperatures and thus self-immobilize fluorophores in the AA-CDs [48,49]. A dilute aqueous dispersion of the purified AA-CDs exbibits common blue fluorescence (FL), like many other reported CDs, under the irradiation of a UV lamp (365 nm) ( Figure 1b).…”

Preparation and Biomedical Applications of Multicolor Carbon Dots: Recent Advances and Future Challenges

“…The PL QY of the W‐CDs powder is still relatively low in contrast to the traditional rare‐earth phosphors. [ 179 ] Hu et al attained H‐CDs with reversible two‐switch‐mode luminescence (blue dissolved fluorescence and red AIE) from an eco‐friendly, low‐cost one‐pot solvothermal treatment. The hydrophobicity stems from the abundance of pyridinic and epoxy groups on their symmetrical heterocyclic surfaces.…”