Graphene And Its Quantum Dots:Doping, Magnetic And Photoluminescent Properties

Graphene is a two-dimensional (2D) network shape materials composed of sp2 hybridized carbon atoms packed into hexagonal structure. Due to its unique structure, graphene features extraordinary mechanical, thermal, optical, and electrical properties. Because of the weak spin orbit interaction, graphene has a long spin diffusion length, which provides favorable conditions for regulating its spin and magnetism. And then, graphene has great potential applications in the fields of spintronic devices. The research shows that the doping of graphene by nitrogen is an effective route to introduce local magnetic moment in the graphene. It is worth noting that graphene quantum dots (GQDs) possess unique physical and chemical properties due to their pronounced quantum confinement and edge effects. Many theoretical researches about orbital and spin magnetism of GQDs have been done in the recent years. Recently, the graphitic quantum dots were prepared by Swain et al., which exhibit ferromagnetic behavior at room temperature. They mentioned that the collective effect of defects and various functional groups may play a contribution to the ferromagnetic behavior of the graphitic quantum dots. However, few experimental researches about magnetic properties of doping graphene with nitrogen have been reported so far. Especially, experimental researches about magnetic properties of GQDs have never been reported. Thus, how to introduce high unpaired spin intensity in graphene by an effective route, and then lead to the existence of the magnetic ordering of graphene, is very important for promoting the research of magnetism of graphene. And then, which will facilitate its application in spintronics. Magnetism and photoluminescence (PL) of NGO and RGOQDs are mainly investigated in the thesis. The main contens are as follows:1. The photochemical route is a fast, clean, low-temperature, simple but effective experimental method. The NGO was prepared by the photochemical route. The results showed that irradiation of GO for only 5 min, the N content of NGO obtained can reach high up to 13.05 at.%. More interesting, N-doping can result in magnetic moment enhancement of for all NGO samples compared to that of GO. Especially, the NGO-10 and NGO-150 exhibited significant saturation magnetization enhancement with a high enhancement ratio of 839.13%, and 1391.30%, respectively. The results show that doping GO by photochemical route is an effective means to enhance the magnetic moment of GO.2. Graphene oxide quantum dots (GOQDs) with a diameter of ca.1～4.5nm were successfully synthesized by hydrothermal method used GO as precursor. The RGOQDs-300、RGOQDs-500, and RGOQDs-700 can be obtained by annealing the GOQDs under Ar atmosphere at 300,500, and 700℃, respectively. The GOQDs exhibited weak ferromagnetism at 2 K. However, the RGOQDs showed obvious ferromagnetism. The results showed that the ferromagnetic saturation magnetization of GOQDs was ca.0.0394 emu/g. However, which increased to 0.081,0.276, and 0.348 emu/g for RGOQDs-300, RGOQDs-500, and RGOQDs-700, respectively. On the other hand, compared to that of GOQDs, the enhancement ratio of the total magnetization reached to 198.97,838.14, and 1209.28% for RGOQDs-300、 RGOQDs-500, and RGOQDs-700, respectively. Note that, the RGOQDs also showed obvious ferromagnetic behavior at room temperature. Especially, the Curie temperature of RGOQDs-500 was about 940.1 K.3. The PL properties of NGO synthesized by photochemical route were systematically investigated. The results showed that compared with GO, all the NGO samples exhibited significant PL enhancement. Specially, the NGO obtained by irradiation of GO for 10 min had a high enhancement ratio of approximately 1501.57%. Interestingly, the PL enhancement ratio presented a same trend to the changing of the [contentN-A-contentN-6] content of the NGO samples except NGO-5. Therefore, it demonstrated further that N-A can effectively enhance the PL of GO. All the results suggested that doping of the GO with N by photochemical ways was an effectively route to enhance the PL intensity of GO. Obviously, the NGO synthesize by photochemical means can be used as an effective fluorescence-enhanced nanomaterial.4. The PL properties of RGOQDs were systematically investigated. The results showed that GOQDs exhibit bright blue PL at ca.436 nm. One can find that all the RGOQDs exhibit significant PL blue-shift, and the maximal blue-shift of RGOQDs-700 can reach up to 76 nm. Note that, after annealing under Ar at high temperature, with the fraction of the sp2 of the RGOQDs increasing signally, the size of the RGOQDs decreased slightly. The combined effect of the enhancement of sp2 and the decrease of size may be responsible for the PL blue-shift of the RGOQDs. On the other hand, the PL of GOQDs can be enhanced or can be quenched by annealing the GOQDs under Ar atmostpere. The results showed that, annealing the GOQDs at low temperature, the oxygen-containing functional groups, such as carboxyl, decreased slightly. And then, the fraction of the sp2 of the RGOQDs increased slightly, which resulted in the enhancement of the PL of the RGOQDs. When the GOQDs were annealed at higher temperature, the content of the graphitic-like carbon increased, which resulted in the quenching of the PL of the RGOQDs. One can find that annealing the GOQDs under Ar atmosphere is an effectively route to regulate the PL peak and the PL intensity of GOQDs.