Synthesis And Luminescent Properties Of Single Phase Carbon Quantum Dot Phosphors For Light Emitting Diodes

Nowadays, light emitting diode(LED) is considered to be a novel alternative to traditional lighting technology as a new generation of solid-state illumination source owing to its energy saving, high energy efficiency, long lifetime, and small volume. Unlike rare-earth and semiconductor quantum dots used as LED conversion phosphors, carbon quantum dots(CQDs), with wide emission spectrum, high optical stability, low cost, and low toxicity, are promising phosphors for application in LEDs. At present, quantum yield(QY) and red-green-blue(RGB) spectral component ratio of CQD phosphors for LEDs are low, which results in decreased light efficiency and color rendering index. The main idea of this dissertation is to fabricate single-phase CQD fluorescent material with high QY and RGB ratio. Luminescent CQDs were synthesized by using glucose, ascorbic acid, salicylic acid, and thiosalicylic acid as carbon source, polyethylene glycol 200(PEG200), ethylenediamine, and p-phenylenediamine as additives, separately. The micromorphology, surface structure and optical properties of CQDs were analyzed. CQDs as a single light converter were applied to yellow and white LED. The main research contents are as follows:First, single-phase CQDs, with the QY of 3.5% and RGB ratio of 63.5%, were prepared by a one-step hydrothermal method using glucose as carbon source and PEG200 as additive. The effects of glucose concentration, reaction time, and modification way on fluorescent properties of CQDs were explored. The optimized reaction conditions are as follows: glucose(40 m L, 5 mmol/L) and PEG200(5 m L) were engaged in one-step hydrothermal reaction at 180℃ for 6 h. The resulted CQDs with average diameter of 4 nm emit blue fluorescence. There are hydroxyl, carboxyl and other O-rich groups on the surface of CQDs. The photoluminescence wavelength of the CQDs was excitation-dependent and p H-independent. The single-phase CQDs based LED device exhibited cool white light with color coordinates of(0.32, 0.37) and corresponding color temperature(CCT) of 5584 K.Second, nitrogen-doped CQDs(N-CQDs), with the QY of 12.1% and RGB ratio of 78.4%, were prepared by a one-step hydrothermal method using ascorbic acid as carbon source and ethylenediamine as additive. The surface structure of CQDs could be changed by doping nitrogen atoms to enhance QY and RGB ratio of CQDs.The effects of reaction temperature and additive kinds(ethylenediamine, p-phenylenediamine) on fluorescent properties of N-CQDs were investigated. The optimized reaction conditions are as follows: ascorbic acid as carbon source and ethylenediamine as additive were engaged in one-step hydrothermal reaction at 90℃ for 2 h. The obtained N-CQDs have the average diameter of 6.2 nm, and emit green fluorescence. Hydroxyl, carboxyl and amide groups appear on the surface of N-CQDs. It seems that the introduced nitrogen atoms would enhance the QY of CQDs. They also have excitation- and p H-independent emission behavior. The single-phase N-CQDs based LED device emitted yellow light with color coordinates of(0.48, 0.45) and CCT of 3705 K.Third, N-CQDs, with the QY of 45.7% and RGB ratio of 70.8%, were prepared by a one-step hydrothermal method using salicylic acid containing aromatic ring as carbon source and ethylenediamine as additive. The aromatic structure can strengthen the π conjugate system of the surface of N-CQDs to improve QY of N-CQDs. The effects of reaction temperature and molar ratio of salicylic acid to ethylenediamine on fluorescent properties of N-CQDs were explored. The optimized reaction conditions are as follows: N-CQDs with the highest QY(45.7%) were synthesized with reaction temperature 200℃ and molar ratio of salicylic acid to ethylenediamine 1:3. The improvement of QY is due to doped-nitrogen atoms and conjugate aromatic ring. As-prepared N-CQDs exhibit green fluorescence with average diameter of 10.5 nm. There are hydroxyl and amide groups on the surface of N-CQDs. The N-CQDs possessed excitation-dependent emission. The single-phase N-CQDs based LED exhibited yellow-white light with color coordinates of(0.41, 0.41) and CCT of 4305 K.Final, nitrogen and sulphur co-doped CQDs(NS-CQDs), with QY of 51.4% and RGB ratio of 67.4%, were prepared by a one-step hydrothermal method using thiosalicylic acid containing aromatic ring and sulfur functional groups as carbon source and ethylenediamine as additive. Sulphur atoms as electron donor can enhance the π conjugate system of the surface of CQDs, leading to improved QY of CQDs. The effects of reaction temperature and molar ratio of thiosalicylic acid to ethylenediamine on fluorescent properties of NS-CQDs were investigated. The optimized reaction conditions are as follows: NS-CQDs with the highest QY(51.4%) were prepared with reaction temperature 200 ℃ and molar ratio of thiosalicylic acid to ethylenediamine 1:3. The synergism between N, S doping atoms and conjugate aromatic ring leads to enhanced QY of NS-CQDs. The bule photoluminescent NS-CQDs have the average diameter of 2.4 nm. The surface of NS-CQDs contains hydroxyl and amide groups. The photoluminescence wavelength of the CQDs was excitation-independent. The single-phase NS-CQDs based LED emitted cool white light with color coordinates of(0.34, 0.38) and CCT of 5227 K.In summary, the introduction of carbon source with aromatic ring and nitrogen, sulfur atoms is helpful to improve the QY of CQDs. With the introduction of nitrogen atom, carbon source with aromatic ring and sulfur atom, RGB ratio increases first and then decreases. RGB ratio is not only affected by energy level structure of CQDs, but also by the size of CQDs. Therefore, RGB ratio could be further improved by regulating size and structure of CQDs to produce different colors of LED.