Synthesis Of Peapod (C₆₀@SWNTs) And Investigation Of Their Structure And High Pressure Induced Phase Transition

Single walled carbon nanotubes (SWNT) and C₆₀, as 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.C₆₀-peapod, a SWNT including C₆₀ molecules inside the tube, is quasi one dimensional nanostructure material, now attracting much interest. The unique structure of peapod and their fascinating physical and chemical properites imply a great potential in nanoscience and in technological applications. The discovery of peapods has opened new and fascinating possibilities to make building blocks for nanoengineering applications.Many experimental and theoretical studies have recently found that The C₆₀ molecules into carbon nanotubes may significantly alter their electronic, conducting, or mechanical properties and thermal conductivity.One of the most widely used methods, the vapor phase reaction method, can produce peapod with a high filled ratio and high purity. Here, we synthesize peapod in high ratio and high purity by this method. The SWNTs were purified and opened and then sealed with C₆₀ powder (purityof 99.9%) in a Y-shaped quartz tube at 1.2*10^-4 Pa, and finally heated in a furnace at 823 K for 72 h to complete the encapsulation process by a vapor phase reaction. It is a still an open question whether the C₆₀ molecules inside a nanotube are separated or whether they are connected by covalent bonds to form dimers, trimers, or longer oligomer chains in the nanotubes at ambient conditions. Therefore studies on the structures of C₆₀ molecules in SWNTs and the possibility to synthesize one-dimensional C₆₀ polymers inside has become a significant and interesting research field.High pressure 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.A resonant Raman spectroscopy study has been carried out under high pressure, using a diamond anvil cell, on carbon nanotube peapods synthesized in high yield in our laboratory. The Raman signal was excited by a near IR laser (830 nm) to avoid photopolymerization of C₆₀ and thus obtain the intrinsic vibrational information on the C₆₀ molecules in the nanotubes. It is found that the surrounding tubes create an effective pressure on the encapsulated C₆₀ due to tube-fullerene interactions, resulting in a shift of the intrinsic Ag(2) vibrational mode to 1474 cm?1 at ambient pressure. High pressure Raman spectroscopy indicates that (C₆₀)2 dimers form near 1 GPa, and that a further polymerization of C₆₀ occurs near 23 GPa, creating linear chains of covalently linked C₆₀ molecules in the tubes. These studies provide helpful information on the structures of peapods both at ambient and high pressure.Polymerization of C₆₀ molecules in SWNTs under high pressure and simultaneous irradiation of UV laser (325 nm) has been carried out for the first time by using diamond anvil cell. Raman spectra for the peapod samples decompressed from high pressure indicated that C₆₀s form one-dimensional orthorhombic polymer in SWNTs with the UV laser irradiation at high pressure of 21.5 GPa, which is lower than that for the polymerized sample only induced by high pressure. The polymerization is an irreversible phase transition in the peapod.Peapods present a model system for studying the properties of dimensionally constrained crystal structures, whose dynamical properties are very important. However, how the C₆₀ molecules rotate inside SWNTs, especially at room temperature, is still an interesting open question. We have recently studied the rotational dynamics of C₆₀ molecules confined inside SWNT by analyzing the IFM (intermediate frequency mode) lattice vibrations using NIR Raman spectroscopy. The rotation of C₆₀ was tuned to a known state by applying high pressure, at which condition C₆₀ first forms covalently bonded dimers at low pressure and then forms a single-chain non-rotating polymer structure at high pressure, in which state the molecules form chains with a two-fold symmetry. We propose that the C₆₀ molecules in carbon nanotubes exhibit an unusual type of ratcheted rotation due to the interaction between C₆₀ and SWNT walls in the"hexagon orientation", and the characteristic vibrations of ratcheted rotation becomes more obvious with decreasing temperature. This study has given new insights and has provided an effective method (combination of NIR Raman and high pressure) to study the one-dimensional confinement effects. Not only do the results have important implications for the future utilization of peapods in new nano-materials, but they have also increased our basic understanding of the dynamics of a unique dimensionally constrained crystal structure, which is important for many applications in various fields, such as superhard and electronic materials, geology and geophysics.Peapod can be taken as a representative qusi-one-dimensional model material. As a fundamental data, thermal stability of polymeric peapod is necessary for its potential application in the nanodevice and nanoengineering field. We thus have investigated the thermal stability and the de-plymerization process for this particular polymeric structure for the first time and compared with that of bulky polymeric structure. We found that polymeric C₆₀ Chain confinement within SWNTs has better stability than the bulky sample. The chemical thermal stability of polymeric C₆₀ molecules inside SWNT was analyzed by heat in Ar atmosphere. The de-polymeric temperature of the polymeric C₆₀ molecules inside nanutube is higher than that of polymeric C₆₀ powder, respectively, about 40⁷0 K for dimer polymeric and about 60⁸0 K for orthorhombic polymer. These results indicated that the thermal stability of polymeric C₆₀ is increased due to the encapsulation, the host SWNT indeed plays an exactly important role to prevent the heated-depolymerizing.We investigated the different diameter transition of host SWNT for both dimer and one-chain polymer of peapod in polymerization and de-polymerization process. The nanotube diameter reduction from the pristine peapod to the dimer and one-chain polymer are observed; however, the reversibility of diameter only occurs in the smaller nanotube. This work can help us to further understand the physical and structure properties of peapod, and provide a new approach to tune the diameter of SWNTs.The investigation of their structural evolution under high pressure is always an important topic, which can provide information on the stability and phase transitions of SWNTs under compression.