| As a new type of fluorescent carbon nanomaterials, carbon dots have received much attentions because of their excellent biocompatibility, good fluorescent performance, simple synthesis process, low cost, etc., which have high potential in applications such as the biological fluorescence labeling, medical sensors and light emitting devices.The inner structure of fluorescent carbon dots is usually graphite crystal structure, thus the properties of the electronic transmission and transition are different from the semiconductor quantum dots, and that is why they have different fluorescence behavior. There is no unified theory to explain the fluorescent mechanism of the carbon dots. Surface level fluorescent theory is considered to be one of the most scientific, thus it is of great significance to analyze the internal crystal structure and surface groups. At the same time, as the representative of the new generation of green nanomaterials, the carbon dots can be synthesized by many approaches with a variety of raw materials, which are not environmental-friendly and need much improvement.This dissertation demonstrates that fluorescent carbon dots with wide color gamut are synthesized through hydrothermal and pyrolysis methods with green and environmental-friendly precursor ammonia to modulate, as well as their structure and fluorescent mechanism. What is more, white LED excited by ultraviolet are also fabricated by applying highly fluorescent carbon dots and rare earth phosphors as color conversion layers, which shows high color rendering index. The main work is listed as follows.1. Water-soluble ammonium hydroxide-modulated fluorescent carbon dots have been synthesized through hydrothermal method with citric acid as the only carbon source and ammonium hydroxide as reaction assistant. The as-synthesized carbon dots exhibit excellent crystallinity and monodispersity, wide color gamut and high quantum yield. HRTEM images show that the carbon dots have good monodispersity with average diameter of 3 nm. The measurements of space distances and included angles of 2-dimensional lattices reveal the crystal structure of graphite 2H. FTIR and XPS were applied to investigate the surface function groups. Comparative experiments show that N-groups play a vital role in fluorescent enhancement. Meanwhile, electron transition band models of C-N bonds and surface defects absorption and emission are built according to first principle to provide a reasonable explanation of the fluorescent mechanism of carbon dots.2. Carbon dots with good crystallinity and monodispersity have been prepared through hydrothermal approach by using carbamide or citric acid & carbamide as the raw materials. This approach have advantages of low cost, short reaction time and simple experiment equipment. The structure of the carbon dots prepared by citric acid & carbamide has been deeply analyzed. The result shows that the interplanar spacings, interfacial angles are identical with the PDF#89-8487&75-1621&89-7213 (graphite 2H sructure). The optical properties have also been explored and the carbon dots exhibited red-shift with the increase of excitation wavelength between 380 nm and 390 nm.3. The R-G-B three-band WLEDs have been successfully fabricated with the blue-emission CDs and rare earth luminescent materials (RELM) (green-emission SrSi2O2N2:Eu powders and red- emission Sr2Si5N8:Eu powders) as color conversion layers and a 360 nm UV chip as excitation light source. The as-prepared WLEDs emitte bright white light with CIE coordinate of (0.33,0.37), a CRI up to 95 (R9=97) and a color temperature (Tc) of 5447K under 100 mA driven current, which have excellent optical property and broadened carbon dots application range. |
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