Photoluminescent carbon quantum dots as a directly film-forming phosphor towards white LEDs

Photoluminescent organosilane-functionalized carbon quantum dots (CQDs), 3.0-3.5 nm in diameter, were synthesized via a facile hydrothermal method using citric acid monohydrate as a precursor and N-(3-(trimethoxysilyl) propyl) ethylenediamine as a coordinating and passivation agent. The optical properties of the as-obtained CQDs were investigated in detail. The CQD aqueous solution emits bright blue-white light under ultraviolet (UV) illumination with a quantum yield of 57.3% and high red-green-blue (RGB) spectral composition of 60.1%, and in particular the CQDs exhibit excitation-independent photoluminescence. The CQDs have a narrow size distribution around 3.1 nm and good film-forming ability through simple heat-treatment. By virtue of these excellent optical characteristics and good film-forming ability, a white light-emitting device (LED) was fabricated by combining a UV-LED chip with a single CQD phosphor film, which exhibited cool white light with a CIE coordinate of (0.31, 0.36), a color rendering index of 84 and a correlated color temperature of 6282 K. In addition, the white LED exhibits good optical stability under various working currents and for different working time intervals. Moreover, the interaction between the carbogenic core and surface groups was discussed using the DMol(3) program based on density functional theory. This research suggests the great potential of CQDs for solid-state lighting systems and reveals the effect of the surface state on the photoluminescent mechanism of CQDs.