| Graphene, a two-dimensional (2D) network of sp2 hybridized carbon atoms packed into hexagonal structure, is a basic building block for graphitic materials of all other dimensionalities. Since long-range π-conjugation in graphene yields extraordinary mechanical, thermal, optical, and electrical properties, an enormous effort has been devoted to exploration of its many applications in nanoelectronics, materials science, and condensed-matter physics. However, most electronic applications are limited by the absence of a bandgap in the intrinsic material. In the quest to opening and tuning an energy gap in graphene, various approaches have been developed to improve the semiconducting properties, exemplified by forming confined geometries of quantum dots and doping graphene. One of the most feasible methods to control the semiconducting properties of graphene is by doping (N or F), which is used to tailor the optical and electrical properties of graphene. Recently, magnetic field effects (MFEs) have drawn strong interest toward the development of the spintronics. It is clear that MFEs will not only open an area for nonmagnetic semiconducting materials to be used for spintronics, they also can lead to the development of new multifunctional devices with integrated electronic, optical, and magnetic properties for energy conversion, optical communication, and sensing technologies. However, MFEs of photoluminescence (MFEPL) of doping graphene have never been reported so far. Photoluminescence (PL) and MFEPL of nitrogen-or fluorine-doped graphene are mainly investigated in the thesis. Specific content as follows:1. Nitrogen doped graphene (NG) samples have been prepared by annealing reduced graphene oxide (RGO) in ammonia under various conditions (pressure, temperature). The nitrogen content and the type of nitrogen of the NG samples are analyzed. The PL and MFEPL of RGO and NG have been examined systematically. The results show that RGO exhibits ultraviolet (UV) PL in 367 nm, doping RGO with nitrogen can quench its fluorescence, and the fluorescence quenching of NG obtained in vacuum is more efficient than that prepared in atmosphere. NG samples that prepared in different temperature also show that doping RGO with nitrogen can quench RGO fluorescence, and the quenching efficiency is proportional to the pyridine N content. It may attribute to the effective charge transfer between N and graphene. Thus, by controlling the content of N, it is possible to tune the quenching efficiency of NG samples. In addition, no MFEPL is observed from RGO and NG samples.2. A hydrothermal approach was developed for the synthesis of N-doped graphene quantum dots (N-GQDs) by cutting N-doped graphene. The N-GQDs have a N/C atomic ratio of ca.5.6% and diameter of 1-7 nm. The PL and MFEPL of the N-doped graphene quantum dots were investigated. It was found that N-GQDs possess bright blue PL and excellent upconversion PL properties. Compared to the GQDs, the N-GQDs show a clear red-shifted in the positions of the upconversion PL peaks and exhibit a lower the energy difference (5E) between the σ and π orbitals. We proposed that this may be due to the shift of Fermi level and the increasing of bandgap because of N-doping. Theoretical calculations indicated that the carbon p-α orbitals hybridizes with the nitrogen p-σ orbitals below approximately EF-3.5 eV, while the graphene% orbitals hybridizes with the nitrogen p-z orbitals over a wide range and dominates the states near the Fermi level. The consequences are that (1) the Fermi level is shifted above the Dirac point and (2) because of the shift of the π orbitals arising from N-doping, an extra bandgap around the Dirac point is induced in the system. Thus, the δE of π and σ orbitals of N-GQDs is lower than that of GQDs. Similarly, the N-GQDs show a red-shifted in the positions of the upconversion PL peaks with respect to those of GQDs. In addition, no MFEPL is observed from N-GQDs. As a result, the strong upconverted PL of the N-GQDs is excited by visible light, which indicates that N-GQDs may be used as an effective energy-transfer component in photocatalyst design for environmental and energy.3. Multi-layer fluorographene (FG) was synthesized by a simple liquid-phase exfoliation of graphite fluoride in hexanol solvent, and the PL and MFEPL of the multi-layer FG suspended in hexanol solvent were studied. Results showed that FG has layer spacing of 0.62 nm,7-8 layers of multilayer structure. FG suspended in hexanol solvent exhibit bright UV PL at 394 nm. Most interestingly, we report a new phenomenon:giant MFEPL with a magnitude of 32.5% from multi-layer FG suspended in hexanol solvent, which is observed in inorganic materials for the first time.4. The number of layers, the dielectric constant of host solvent, concentration and annealing temperature having the influence on the MFEPL of FG were systematically investigated and the mechanism were preliminary discussed. Results showed that MFEpL can be tuned by changing the number of layers, the dielectric constant of host solvent, concentration and annealing temperature. In general, inter-layer excited states in multi-layer FG can be formed with both anti-parallel and parallel spin configurations due to electron-spin multiplicities, leading to both singlets and triplets. The singlets and triplets can then experience an intersystem crossing in inter-layer excited states. The intersystem crossing is governed by two competitive processes: spin-conserving process from spin-exchange interaction and spin-random process from hyperfine or spin-orbital interaction. The spin-conserving and spin-random processes can eventually reach an equilibrium in intersystem crossing, leading to certain singlet and triplet ratios in multi-layer FG Perturbing either spin-conserving or spin-random processes can then break the equilibrium in intersystem crossing with the consequence of changing the singlet and triplet ratios. It is noted that that the graphene possesses negligible spin-orbital coupling due to the absence of heavy-elements. This leaves hyperfine interaction responsible for spin-random process. Therefore, the equilibrium in intersystem crossing is essentially determined by spin-exchange interaction and hyperfine interaction. Normally, hyperfine interaction in organic carbon-based materials corresponds to a magnetic field less than 10 mT. Therefore, at the field larger than 10 mT breaking the equilibrium in intersystem crossing depends on whether applied magnetic field can be comparable with spin-exchange interaction. For intramolecular excited states in individual FG, the spin-exchange interaction can be much larger than a field of a few Tesla due to short electron-hole separation distance. This means that a low magnetic field may not be able to compete with the spin-exchange interaction, generating negligible effect on the singlet and triplet ratios in intramolecular excited states in multi-layer FG. However, in inter-layer excited states the spin-exchange interaction can be much weaker due to the large electron-hole separation distance. In this case, a low magnetic field (< 1 Tesla) can be comparable with the spin-exchange interaction, leading to a change on singlet and triplet ratios in inter-layer excited states in multi-layer FG As a consequence, magnetically changing the singlet/triplet ratio in inter-layer excited states can lead to a MFEPL in multi-layer FG, which provides a fundamental possibility to use graphene derivatives for spintronic applications. |
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