Synthesis, Characterization, And Photoluminecence Mechanism Of Graphene Quantum Dots

In recent years, Graphene quantum dots(GQDs), as a new kind of carbon nanomaterials, have attraced great interst due to their unique properties, such as excellent solubility, low biotoxicity, steadily bright fluorescence. Abundant synthesis methods of GQDs were continually reported and these GQDs were widely employed in the applications on bioamaging, optoelectric devices, sensors, drug delivery and photocatalyst. However, the photoliumiescence(PL) mechanism of GQDs is still a matter of current debate. The cooperation or competition between intrinsic state emission and defect state emission leads to GQDs prepared by various methods probably exhibits distinct PL mechanisms. Based on the fact, an exploratory study on the systhesis method of well crystallized GQDs with high quantum yield(QY) has been carried out. Moreover, a further investigateion of the PL mechanism in GQDs has been maken. The main achievements are presented as the following:1. The systheis of high quality GQDsThe well crystallized GQDs with 14.3% quantum yield of blue fluorescence were prepared by a hydrothermal method(200 oC and 3 hours) by using glucose, concentrate sulfuric acid and deionized water as precursors. Structural characterization demonstrates that the average size and height of GQDs is 3.35 nm and 1 nm, respectively, and our GQDs have a disk shape with 1~3 layers. Optical characterization indicates that these GQDs can exhibit nearly excitation-independent ultraviolet and blue fluorescences.2. Synthesis mechanism of GQDsBy controlling the quantity of concentrate sulfuric acid, it is found that the as-product is transformed from amorphous carbon dots with low QY and excitation-dependent green fluorescence to well crystallized GQDs with high QY and excitation-independent blue fluorescence. The optimization of reaction conditions, including the reaction temperature, reaction temperature and glucose usage, can obtain GQDs with better quality. Based on the experimental results, a probable mechanism for the synthesis of GQDs is proposed. The carban core is firstly formed through dehydration in glucose. Then with the aid of enough energy provide by the reaction circumstance, the carban core is continually expanded and oxygen-containing functional groups are constantly removed, and finally GQDs become to be bigger size and higher crystallinity.3. PL mechanism of GQDsAccording to the indepth analysis of absorption, PL and PL excitation(PLE) spectra, we have presented that ultraviolet and blue emissions are derived from the recombination of electron-hole pairs localized in the C=C bonds and electron transition of sp2 domains. Temperature-dependent PL measuements further proved the above PL mechanism and illustrate the exsit of strong electron-electron scattering and electron-phonon interactions, similar to temperature behavior of inorganic semiconductor quantum dots.4. The good conductivity of GQDs thin filmGQDs thin film on the polished silicon wafer is fabricated, and its drak and photo current are also measured. The drak current of GQDs thin film is about 9 mS, indicating the strong electronic coupling between GQDs and good conductivity. The ~50% relative improvement of photo to dark conductance implies it is possible to make the photodetector with GQDs.This work is supported by the National Natural Science Foundation of China under Grant Nos. 11304197, 61370042, 11474201 and 61234005.