Structural Phase Transition And Photoluminescence Properties Of C₆₀ Quasi-one Dimensional Nanomaterials Under High Pressure And High Temperature

C₆₀, as one of the most important members of carbon family, has become the most focused material for their unique chemical and physical properties in the field of condensed matter physics and nano-materials. Up to now, there is only a small number of published works about high pressure studies for nanomaterials with different dimension and morphology.? In this dissertation, we have systematically studied high pressure and high temperature induced polymeric C₆₀ nanorods and their photoluminescence (PL) properties. And we also have studied PL properties of Y₂O₃/Eu³⁺ nanotubes.High pressure and high temperature (HPHT) can change the structure of materials, and thus change their physical properties. It provides us a very powerful tool to study the relations between the structure and physical properties. In this work, we report that C₆₀ nanorods of three polymeric phases have been synthesized under high pressure and high temperature. The C₆₀ nanorods were polymerized under quasi-hydrostatic pressure conditions by using a piston-cylinder device with silicon oil as pressure medium. C₆₀ nanorods were treated at two different conditions in our experiment. One of the samples was first pressed to 0.5 GPa, then heated to 573 K and finally the pressure was increased to 1.5 GPa. The second used a similar procedure with final conditions 2 GPa and 700 K. From our Raman experimental data, the characteristic peak at 1458 cm-1 indicates that the C₆₀ nanorods has been transformed to the orthorhombic (O) polymeric phase after treatment at 1.5 GPa, 573 K. And the pentagonal pinch mode shifts from 1469 cm-1 to 1448 cm-1 for C₆₀ nanorods, indicating that all samples have been transformed to the tetragonal (T) polymer phase after treatment at 2 GPa, 700 K. Otherwise, we treated C₆₀ nanorods at 4.5 GPa, 973 K using a 6×600 ton cubic high-pressure apparatus with an electric current heating device and vaseline as the pressure medium. The pentagonal pinch mode shifts from 1469 cm-1 to 1408 cm-1 for the C₆₀ nanorods, indicating that the samples have been transformed to the rhombohedral (R) polymeric phase after treatment. To verify whether the rod survived after the HPHT treatment, we performed AFM and SEM measurements. It is shown that quasi-hydrostatic pressure treatment can keep the shape of the nanorods. This result indicates that HPHT treatment is a possible method to obtain various new lattice structures in C₆₀ nanorods.We further studied the photoluminescence properties of polymeric C₆₀ nanorods. The shift of the main fluorescence band increases with the number of intermolecular bonds. In three samples of polymeric nanorods, the PL intensity decreases continuously as the degree of polymerization increases. The PL intensity of the polymeric C₆₀ nanorods has been greatly enhanced compared to that of pristine C₆₀. The main fluorescence band shifted from 730nm for the unpolymerized phase to 748 nm for the O phase, and further to near infrared 780 nm for the T phase and 810 nm for the R phase when pressure and temperature were increased. The enhanced fluorescence with adjustable frequency for different polymeric C₆₀ nanorods suggests potential applications in luminescent nanomaterials. From the comparison between three polymeric C₆₀ nanorods, we tentatively assigned some unclear photoluminescence peaks to various polymeric phases, such that a peak at 1.66 eV is from O phase, 1.58 eV is the main peak in the T phase and 1.53 eV is a characteristic feature of the R phase. We also found that the relative intensities of the 1.74 eV peak vary in different regions on the R phase C₆₀ rods. We suggest that the PL peak at 1.74 eV in polymeric sample is assumed to originate from surface defects.To identify any differences between nanorods and bulk samples, one must eliminate effects caused by changes in the treatment condition or differences in treatment times. Therefore, we report a comparative study of pressure-induced polymerization of C₆₀ nanorods and bulk single crystals under various pressures and temperatures in the same experiment. The O, T and R polymeric phases have been obtained. Raman and PL spectra from C₆₀ nanorods and single crystals have been compared in detail, and we find differences between the two types of materials, especially for the tetragonal phase. And the results show that these differences all can be traced back to the very small dimensions of the nanorods compared to the bulk single crystals, and to the resulting differences in domain structures of the final materials. Meantime, C₆₀ nanosheets with polymeric phases have been obtained under various high pressures and high temperatures, including O and T polymeric phases. The structures have been identified and compared with those of nanorods by PL and Raman spectra. The difference of PL and Raman spectra between nanosheets and nanorods samples treated under the same conditions is probably caused by different polymerization degree in these samples because of different shapes. We carried out laser heating experiments of C₆₀ rods for the purpose of direct conversion to diamond at 16 GPa. A light transparent phase has appeared after heating, which is quenchable at ambient conditions. The Raman spectrum shows that the phase is a diamond. We obtain some initial experimental results for synthesis of diamond nanorod from C₆₀ nanorods in a laser-heated diamond anvil cell.We also studied PL of Y₂O₃/Eu³⁺ nanotubes as a function of applied hydrostatic pressure using the diamond anvil cell technique. It is observed that the emission peaks intensity relative to 5D0– 7FJ (J=0,1,2) transitions increased with the pressure before 8.2 GPa and decreased rapidly as the pressure increases above 8.2 GPa under 633 nm laser light. And structure transition of Y₂O₃/Eu³⁺ nanotubes was also investigated by synchrotron angle dispersive X-ray-diffraction. From X-ray-diffraction study, it is observed that a new peak begin to appear in the spectra pressure above 21.9 GPa. The transition pressure of Y₂O₃/Eu³⁺ nanotubes is higher than that of the bulk samples. The Y₂O₃/Eu³⁺ nanotubes can be considered progressively more disordered and eventually near amorphous at pressures beyond 32.6GPa. However, from our X-ray experiment, there is no structural transition at about 8 GPa. The PL intensity of Y₂O₃/Eu³⁺ is closely related to the local environments of Eu³⁺. It is well known that high pressure can compress the crystal lattice and reduce the distance between two ions. This will enhance the crystal field, resulting in influence of the 5D0-7FJ (J=0, 1, 2) emissions because that is sensitive to the crystal field. Before 8 GPa, the Y₂O₃ is cubic phase and the Eu2O3 is cubic and monoclinic coexist phase. As the pressure increase, the Y3+ of cubic phase and Eu³⁺ of cubic and monoclinic coexist phase have different compressibility. Comparison of the difference structure transition pressure for Y₂O₃ and Eu2O3 makes it clear that some lattice distortion of Y₂O₃/Eu³⁺ nanotubes must take place when the pressure increase. The local environments of Eu³⁺ has great influenced by lattice distortion with pressure increase. Thus, local strain fields introduced by lattice distortion can drastically influence electrical and optical properties. Thus, the lattice distortion is believed to enhance the thermally excited Eu³⁺ ions, resulting in the stronger red emission. So the intensity of PL increased with the pressure before 8.2 GPa. The lattice distortion induced by difference structure transition pressure for Y₂O₃ and Eu2O3 is believed to enhance the thermally excited Eu³⁺ ions, resulting in the stronger red emission before about 8 GPa. And the phenomena at about 8 GPa on PL spectra of Y₂O₃/Eu³⁺ nanotubes is possible relative to lattice distortion.