Preparation Of Fluorescent Carbon Nanoparticles And Application Of White Light Conversion

In recent years, carbon nanomaterials have attracted a lot of attentions due to their unique and innovative properties. Among them, water-soluble carbon nanoparticles(CNPs) are a new class of fluorescent nanomaterials and can be applied in biomedical imaging, photocatalysis, heavy metal ions detection, white light conversion and other fields. Especially due to their no toxicity, low cost, stable fluorescence and broad photoluminescence, CNPs have potential applications as alternatives to traditional rare-earth metal phosphors. In this paper, we use different precursors and synthesize fluorescent CNPs in hydrothermal processes. Furthermore, we explore the synthesis mechanism of CNPs during the heating. We also investigate their applications in phosphor-converted white light emitting diodes(LEDs). In this thesis, the specific work includes following sections:1. Water-soluble CNPs were synthesized in a hydrothermal process using hexamethylenetetramine as precursor. Transmission electron microscopy(TEM) measurements reveal hollow carbon nanospheres(HCNs) with a narrow size distribution of 260-300 nm in diameter. By UV-vis spectra, Fourier transform infrared(FTIR) spectra, and photoluminescence(PL) spectra tests, we investigate their optical properties. CNPs solutions exhibit strong yellow-green emission under ultraviolet light excitation and possess typical excitation-dependent PL behavior as CNPs. The luminescence quantum yield of CNPs was determined to be 35%. We coat a blue LED with CNPs layer which plays a role as color-converted phosphors. The LED shows bright white light. The white emission is the combination of blue light from the blue LED and yellow light from the CNPs layer. In this part, we develop a novel method to synthesize template-less HCNs without the need of high-temperature step of the removal of template and their application for white LEDs.2. Using hexamethylenetetramine/glucose mixture as precursor, CNPs were synthesized in a hydrothermal process. The mixed glucose was found to facilitate the decomposition of hexamethylenetetramine, which leads to rapid and low-temperature growth of CNPs. We explore the mechanism and find the polymerization reaction between HMT and glucose plays a role in the rapid decomposition of HMT. CNPs are composed of ~2nm carbon dots. We further investigate their optical properties. The quantum yield of the obtained CNPs is as high as 58.3%. We coat onto a ultraviolet LED with CNPs. Broad yellow emission was achieved, indicating the synthesized CNPs can be used in fluorescent lamps as yellow emission phosphors and are capable of replacing conventional rare-earth phosphors. CNPs were also coated onto blue light emitting diodes, which emitted warm white light with a color temperature closer to the sunlight.3. Nitrogen-doped CNPs(N-CNPs) were synthesized in a hydrothermal process using glucose as carbon source and ammonia as nitrogen source, respectively. Ammonia was found to facilitate the decomposition of glucose and decrease the synthesis temperature to be as low as 90℃. We explore the mechanism. The optical properties were investigated. The luminescence quantum yield of N-CNPs was determined to be 10.3%.By FTIR spectra and X-ray photoelectron spectroscopy(XPS) tests, we focus on the elemental composition and functional groups on N-CNPs. Nitrogen carbon atomic percentages were 7.7 at. %. Most importantly, the N-CNPs coating on near ultraviolet LEDs(365-410nm) demonstrates bright white light emissions. In this part, we develop a simple method to synthesize N-CNPs at a low synthesis temperature(below boiling point of water). Furthermore, N-CNPs were used as direct white light-emitting phosphors for white light emission with ultraviolet excitation.4.Water-soluble carbon sheet-like nanoparticles deposited on Si can be used in Si-based electronic circuit, and the related research is beneficial for the applications of Si-based nanoscale electronics, nanoelectromechanical systems, as well as nano- and bio-sensors. The traditional chemical approaches of growing carbon nanosheets(CNSs) are incompatible with semiconductor processes, and template synthesis needs removing templates during post-processing. CNSs directly grown on Si substrates can obviously facilitate the subsequent processing of nanoelectronics products. Liquid Ga was deposited on Si substrate by electron-beam evaporation. Carbon thin film was subsequently deposited on Ga droplets. Submicron-scale Ga droplets were generated from the non-wetting nature. The carbon films supported on Ga droplets were annealed in tube furnace. After annealing, Raman spectra and atomic force microscopy(AFM) measurements demonstrate that CNSs were synthesized on Si surface with the graphitizing catalyst of Ga droplets on amorphous carbon. In this part, we propose a simple and transfer-free route to synthesize CNSs on Si substrates, which facilitates the subsequent processing of Si-based nanoelectronics products.This study shows that water-soluble CNPs can be synthesized in a hydrothermal process with a low synthesis temperature(below boiling point of water). The obtained CNPs processes excellent optical properties and can serve as promising non-toxic and environment-friendly phosphors for white light emission.