The Fabrication And Photoluminescent Properties Of Carbon Quantum Dots And Its Composite Films

Carbon materials are abundant in nature. However, the macro carbon is not suitable for fluorescent material. With the progress and development of science and technology, many kinds of fluorescent carbon nano materials arise at the historic moment. Due to the low toxicity of carbon materials, they are more and more get the attention of the researchers, especially the fluorescent carbon quantum dots (CQDs) which appeared in recent years. It not only has the common advantages of other carbon materials, and has many other benefits, such as simple preparation, excellent biocompatibility, luminescent stability and tunable wavelength, and so on. Thus, it has important application value in many research fields, for instance, bioimaging, biosensing, chemical sensing, optoelectronic devices and photocatalytic. It is expected to bring a new development space for luminous materials, photoelectric device and biomedical sciences. Although CQDs have excellent application prospect, there are still many problems needed to resolve, such as, the origins of their photoluminsence and the controllability, multiparticle problem in carbon quantum dots and how to preparation the solid CQDs with efficiency and controllable fluorescence, and so on. It is these problems limiting theirs effective application in many fileds. Aiming to solving these problems and on the premise of full investigation, our main focus is to discuss the PL origins of fluorescent CQDs and to solve the condensation induced fluorescence quenching, as well as to realize intense and tunable emission from solid CQDs. The obtained research contents and results are as follows:1. The CQDs are prepared by a top-down method, refluxing wooden actived carbon in nitric acid. The PL spectra show that the blue emission independent of the excitation wavelength as well as the long-wavelength emission (LW emission) depending on the excitation wavelength, and the intensity ratio of maximum intensity of LW emission and blue emission increases with the increase of the concentration of nitric acid. Spectroscopy analyses combined with morphology characterization indicate that the LW emission of CQDs originated from quantum confinement effect. The surface structures of the CQDs were characterized by Fourier transformation infrared absorption spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). It was found that the oxygen content and surface structure changed a little with the increase of nitrate concentration when prepared the CQDs. In addition, the electron paramagnetic resonance (EPR) signal intensity was decreased with the increase of the concentration of nitric acid, which is consistent with the intensity changes of the blue emission, indicating the decrease of irregular carbon rings or carbon defects. According to our research about the photoluminescence of carbon materials before, we believe that the blue emission of the CQDs should be originated from the topological defects. At the same time, combined with the microscopic structure of the activated carbon, we present the formation mechanism of the CQDs from refluxing activated carbon in nitric acid.2. Carriers doping to CQDs is realized by adding HC1 (H+) to the CQDs aqueous solution, and the dependence of the two emissions (the bule emission and LW emission) on pH is investigated in detail. The experimental results demonstrate that the peak position of the LW emission does not change with pH; however, the blue emission becomes more asymmetrical, and obvious shoulder peaks emerge as the pH increases. According to the second chapter, we put forward the blue emission of CQDs originated from carbon topological defects, and the theoretical analysis and experiments indicate that the excitons and doped carriers tend to localized around the disorder structures or the defects. Though a detailed characterization and analysis, a model based on defect-bound trion in the CQDs is proposed to explain the shoulder peaks in the blue emission at high pH, and the calculated results agree well with experimental data concerning the integral intensity ratio of the trion to exciton emissions versus pH. Our experimental and theoretical results demonstrate for the first time emission from trions in CQDs.3. The quaternized CQDs are prepared by bottom-up approach and hydroxyl terminal polyethylene glycol (PEG-OH) is applied to further passivation the quaternized CQDs. In addition, polyethylene glycol (PEG)/CQDs composite solid film is also prepared, which exhibit strong and tunable blue-red emission. The fluorescent quantum yield of the composite film reaches 12.6% which is comparable to that of many liquid CQDs. The results of experiments also indicate that the quaternized CQDs solid film represents weak and untunable blue emission originated from carbon defects and red emission originated from oxygen-related states. Than is to say, amide linkage cannot effectively passivation the CQDs. Only after further passivation with PEG-OH, the solid film presents a strong long wavelength emission caused by quantum confinement effect and weaker blue emission from carbon topological defects. According to the quantum confinement effect mode, the most probable radius carbon quantum dots and their band gap have a good corresponding relationship.