Red-emissive solid-state carbon nanoparticles (CNPs) with a hollow sphere structure for white light-emitting diodes (WLEDs) were designed and synthesized by molecular self-assembly and microwave pyrolysis. Highly ordered graphite-like structures for CNPs were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, and ultraviolet-visible (UV-vis) spectroscopy. The emission mechanism of the red-emissive solid-state CNPs was investigated in detail by steady-state and time-resolved photoluminescence (PL) spectroscopy. The as-prepared CNPs showed a red emission band centered at 620 nm with excitation wavelength independence, indicating uniform size of sp2 carbon domains in the CNPs. The CNPs also had a PL quantum yield (QY) of 17% under 380 nm excitation. Significantly, the PL QY of the organosilane-functionalized CNPs was 47%, which is the highest value recorded for red-emissive solid-state carbon-based materials under UV-light excitation. More importantly, the red-emissive CNPs exhibited a PL QY of 25% after storage in air for 12 months, indicating their excellent stability. The red-emissive CNP powders were used as environmentally friendly and low-cost phosphors on a commercial 460 nm blue GaN-based chip, and a pure white light with CIE coordinates of (0.35, 0.36) was achieved. The experimental results indicated that the red-emissive CNP phosphors have potential applications in WLEDs.
Keywords: carbon nanoparticles, white light-emitting diodes, red emission; self-assembly, excitation wavelength-independent photoluminescence.