Preparation And Optoelectronic Properties Of Graphene Quantum Dots

As a new type of carbon based material,graphene quantum dots (GQDs) possesses a series of new characteristics.In recent years, graphene related research works have widely aroused people’s attentions, which has become the frontier of condensed matter physics, photoelectric information and material science.In this thesis, we have mainly summarized the preparation,research status, characterization techniques and potential applications of GQDs. Based on liquid phase exfoliation, graphene quantum dots with a certain size distribution were successfully prepared.The morphology, structure, composition, optical absorption,electronic transition, photoluminescence and other optical properties of as-prepared GQDs were investigated by Raman, Transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectra, Photoluminescence (PL) spectra, Photoluminescence excitation (PLE) spectra and Ultraviolet-visible-near infrared absorption (UV-Vis-NIR) spectra, etc. The influence rules of size effect, quantum confinement effect, and the doping effect on graphene level structure were explored. Combined with the infrared detection technology, photoelectric detectors based on GQDs were fabricated, and their photoelectric properties were discussed through current-voltage (I-V), capacitance-voltage (C-V) measurement. The main contents and conclusions are as follows:(1) Preparation and characterization of GQDs through ultrasonic liquid phase exfoliationUltrasonic method has been widely used for the preparation of graphene, which is the simplest way of liquid phase exfoliation, and a commonly used method of GQDs preparation. Non-conventional carbon source (degreasing cotton was carbonized at1000℃into carbon fibers) and N-methyl pyrrolidone (NMP) were used. Under the action of ultrasonic energy, carbon fibers were disintegrated. As a result, as-prepared GQDs with a size distribution of3～10nm. The prepared GQDs was characterized by FTIR and XPS, which confirmed the existence of a lot of oxygen-containing functional groups on their surface. The results explain why GQDs can dissolve in water and many organic solvents and exhibit luminescence excitation dependent and adjustable luminescence properties. Therefore GQDs possess potential applications in light emitting material,organic photovoltaic device and biomedical imaging and other fields.(2) Graphene quantum dots dopingResearches show that doping is an important method for the band and optoelectronic properties modulation for carbon based materials.In this paper, carbon fibers were for the first time treated with HC1for the realization of graphene quantum dot doped Cl (Cl-GQDs). Doped quantum dots showed interlayer spacing increasing and Raman peak redshift (Av=15cm-1), which indicates that chlorine can weaken the Raman scattering, resulting in n-type doping. Cl-doping introduces a new level into the electronic structure, which is optimized the optical performance, and the photovoltaic device performance substantially improved based on Cl-GQDs.(3) Study on the fundamental of GQDs based photoelectric detectorThe first vertical (Cl)-GQDs based photovoltaic detectors have been fabricated via a facile solution process. The Ⅰ-Ⅴ and C-V characteristics under different light and frequency conditions were studied. The results show that an exceptionally big ratio of photocurrent to dark current (Ion/Ioff) as high as105at room temperature using a405nm laser (incident light intensity equals165mW·cm-2) irradiation was achieved. At the same time, the band structures of devices and the mechanism of photodetectors were analyzed. Combine with our devices, the important photovoltaic detectors parameters such as saturation current densities (Jo), ideal factor (η), barrier heights (Φb), and carrier concentrations (N) have been calculated and discussed by studying I-V and C-V characteristics under different illuminations. These results provide some scientific basis for the preparation and development of high-performance graphene based optoelectronic devices.