Photoluminescent And Photothermal Properties Of Some Carbon Based Nanomaterials

Deep investigation on the fundamental physics of nanomaterials, such as structures, electronic properties, and the interactions between photons are significant. It will not only allow better manipulation of the material’s optoelectronic properties, but also favors in broadening and developing their applications in Optoelectronic Information, Biology-Medicine, and Energy-Environment etc. Photoluminescent (PL) techniques like stable PL, PL excitation (PLE) and time-resolved PL (TRPL) are powerful tools for the research of optoelectronic properties and related physical effects. Recently, carbon-based nanomaterials have gained much attention, because of their abundance, low environmental hazard, low toxicity and various novel optoelectronic properties. In this doctoral dissertation, several optoelectronic properties of graphene oxide (GO), reduced graphene oxide (RGO), carbon nanodots (CNDs) and diphenylalanine (FF) nanotubes are studied by PL spectroscopy combining with other microstructural and spectroscopic characterization tools and first principle calculations. The details are described as follows:1. RGO and CNDs were fabricated through top-down and bottom-up approaches respectively. Both of them exhibit similar luminescence, specifically, as the excitation wavelength increases, the blue emissions are almost fixed whereas the long-wavelength (LW,480-550 nm) emissions red shift remarkably and uniformly. We proposed that there should be two types of radiative recombination centers in this kind of carbon nanomaterials. According to the features of the PL spectra, each asymmetrical PL peak was Gaussian fitted into two subpeaks. The structural variations during the reduction of GO and the formation of CNDs are studied by HRTEM, FTIR, Raman. Combining with the first principle calculations, carbon defects are believed to be responsible for the blue emission, whereas the excitation wavelength-dependent LW emissions are ascribed to the radiative recombination of electron-hole pairs in the sp2 carbon clusters due to the size effect.2. The Mn2+-bonded RGO was synthesized using MnO2-decorated GO as the intermediate products, subsequent hydrothermal treatment reduced the GO to RGO and introduced Mn to RGO. The emissions in the range of 500-600 nm are enhanced remarkably and the PL can be ideally tuned from 430 to 600 nm. These unprecendented PL spectra indicate an effective way to manipulate the PL of RGO, which would be of vital significance for promoting their applications in optoelectronic areas. The spectral analysis and density functional theory (DFT) simulation reveal that the resonant energy transfer between Mn2+ and sp2 clusters of the RGO should be responsible for the enhanced LW emission.3. The so-called upconversion PL ("UCPL") under excitation of a xenon lamp in GQDs was clarified to be artificial by experiments. The "UCPL" is essentially caused by monochromator. Thus to design photocatalyst and solar cell based on the upconversion of GQDs is infeasible and invalid previously. Additionally, the real UCPL was observed under excitation of a strong femtosecond pulsed laser. The quadratic dependence between the laser power and PL intensity implies that the present emission is a two-photon excitation process.4. The degradation of methylene blue (MB) over P25 TiO2-RGO nanocomposites was studied, the irradiation condition was controlled by the filters and the influence of NIR irradiation was separately and carefully checked. The results reveal that the contribution of the photothermal effect of RGO excited by NIR is substantial in addition to trapping and shuttling photogenerated electrons and increasing both light absorption and pollutant adsorptivity. The result reveals that the photothermal character of graphene-based nanocomposite is crucial to photocatalysis. The photothermal effect leads the photogenerated electrons move faster resulting in improved photodegradation rate. It hints that designing graphene-based nanocomposites with improved photothermal performance is a promising strategy to acquire highly efficient photocatalytic activity.5. The origin of the spontaneous polarization (SP) was analysised based on the structure of the diphenylalanine (FF) nanotube. The SP is believed to arise from orientation ordering of hydrogen bonds. And the reversion mechanism of electric dipole moment under the applied antiparallel electric field is also analyzed. An obvious light-induced Stark effect is observed and it leads to an intensity reduction of the SP field. Based on this idea, we measured the P-E curves of FF microtubes under irradiation of a focused Xe lamp. Ferroelectricity of FF microtubes along the tube axis was confirmed by a saturated P-E loop.