Synthesis Of Fullerene Nanocrystals And Investigation Of Their High Pressure Phase Transition

As typical zero- dimensional materials, due to their unique chemical and physical properties, fullerenes have attracted much attention since their initial discovery. In recent years, one-dimensional and two-dimensional nanocrystals consistent of these zero- dimensional molecules, which have potential application as nanodevices and functional materials, become the focus of research. However, to control fullerene nanocrystals to designed morphologies is still an important and challenging task. On the other hand, for application as in many special fields, single nanocrystals could not meet the requirement, however, present research on the fabrication of fullerene nanomaterials are mainly about single nanocrystals. Efforts have thus been made to assemble individual nanocrystals together, whereas, it is difficult to keep their initial morphologies and structures during the assembling process. To avoid this difficulty, we attempt to fabricate C60 nanocrystal assembly by one-step self-assembling method.Phase transitions usually occur on fullerenes under high pressure, and fullerene could be translated into various polymeric phases under high pressure and high temperature conditions. However, little research has been reported on the high pressure studies of fullerene nanocrystals. Our previous tentative high pressure research on C60 nanorods and nanosheets showed many properties different from that of bulk crystals, such as, higher bulk modulus and higher phase transition pressure. However, it is still unclear how different crystal morphologies of fullerene influence their behaviors under high pressure. It is now an important and challenging task to investigate the phase transitions and polymerization of fullerene nanocrystals with different morphologies.In this paper, we have fabricated C70 nanocrystals with different morphologies by introducing various kinds of alcohols into the saturated solution of C70 in m-xylene. We have investigated the shape controlling and luminescence enhancing effect of different alcohols on C70 nanocrystals. The molecular size and polarity affect greatly on the controlling of C70 nanocrystals morphologies. When the number of carbon atoms is larger than 2, the morphologies of the C70 nanocrystals fabricated using these alcohols are one dimensional nanorods/tubes, and the alcohol molecules were introduced in lattice of C70 nanocrystals to form an orthorhombic solvated structure, and the alcohol molecules were paralleled to the c axis; while there is less than 2 carbon atoms in the alcohols employed, C70 nanocrystals with irregular nanoparticle shapes were obtained, the small alcohols were randomly inserted in the gaps of C70 nanoparticles to form a hexagonal structure similar to that of bulk crystals. The different position of alcohols with different sizes is the key reason for the formation of different crystals morphologies. Furthermore, we have found that the luminescence intensities of C70 nanocrystals were highly enhanced by the introduction of alcohols.Using the solution evaporation method, we have fabricated C60 nanosrystal assembly with different morphologies by controlling temperature. Under the cooperation of toluene and m-xylene, at room temperature, we have fabricated microtube shaped C60 nanocrystal assembly; at the temperature around 0℃, microtube shaped C60 nanocrystal assemblies were obtained; when the temperature were in the range of -10～-20℃, we have obtained C60 flower like nanocrystal assembly; and parallel arrays of C6o nanorods were fabricated under the temperature in the range of -40℃～-50℃. We have investigated the mechanism of the formation of these C60 nanocrystal assemblies with different morphologies, which should be attributed to the initial crystal seed formed under different temperatures.Using synchrotron radiation XRD diffraction, for the first time, we have investigated the pressure induced phase transition of fullerene (C60 and C70) nanotubes with initial face-centered-cubic (fcc) structure. For C60 nanotubes, our investigation indicated that the a phase transition from fcc structure to single cubic (sc) structure occurred under the pressure in the range of 1.46-2.26 GPa, which should be due to an orientation related phase transition. Amorphization occurred under the pressure～20 GPa. Both Raman and PL spectra measured on the C60 nanotubes released from high pressure revealed that the cage structure of C60 in nanotubes persisted to at least 31.1 GPa, and the amorphization became irreversible after the high pressure cycling to 34.3 GPa.For C70 nanotubes, In-situ high pressure Raman spectroscopy and X-ray diffraction have been employed to study the structural stability and phase transitions of the pristine sample. We show that the molecular orientation related phase transition from the fcc structure to a rhombohedral structure occurs at about 1.5 GPa, which is～1 GPa higher than in bulk C70. Also, the C70 molecules themselves are more stable in the nanotubes than in bulk crystals, manifested by a partial amorphization at～20 GPa. The crystal structure of C70 nanotubes could partially return to the initial structure after a pressure cycle above 30.8 GPa, and the C70 molecules were intact up to 43 GPa. The bulk modulus of C70 nanotubes is measured to be～50 GPa, which is twice larger than that of bulk C7o.C60 nanotubes with outer diameters ranging from 400 to 500 nm were polymerized at 1.5 GPa,573 K and 2.0 GPa,700 K, respectively. Raman and photoluminescence spectroscopy were employed to characterize the polymeric phases of the treated samples. Both Raman and photoluminescence spectra showed that the C60 nanotubes transformed into the dimer and orthorhombic phases under the two different conditions, respectively. The photoluminescence peaks were tuned from visible to near infrared range. Comparative studies indicated that C60 nanotubes were more difficult to polymerize than bulk C60 material under the same conditions due to the nanoscale size effect in the C60 nanotubes. For C70 nanotubes with fcc structures, it is impossible to form one dimensional polymeric chain, but could still translated to dimer structures. More importantly, the PL peaks of C70 nanotubes could be tuned from visible range to near infrared area, which suggested that the high pressure and high temperature treatment is an effective way to change the optical properties of C70 nanotubes.Synchrotron radiation in situ XRD diffraction investigation have been recoeded on both the pristine C60 nanotubes with fcc structure and polymerized C60 nanotubes with O structure. The results indicated that the bulk modulus of C60 nanotubes is three times larger than that of bulk crystals; comparing with that of pristine C60 nanotubes the bulk modulus of C60 nanotubes with orthorhmbic polymeric structure increased by-50%. It is proved that synthiesis of C60 nanocrystals is an effective way to improve its mechanical properties.