High Pressure-induced Structural Transitions And Superhard Phase Of Carbon Nanotubes And Their Confined Systems

Carbon nanotubes have attracted intensive attention in many aspects due to their quasi-one-dimensional topological structure and unique electrical,optical and mechanical properties.The molecules encapsulated in carbon nanotubes exhibit many new structures and properties in nanoconfinent environments,which are different from those of the bulk materials.High pressure can modify efficiently the distances between molecules or atoms,and then affect their interaction and bonding styles,so as to obtain new materials with different structures and properties.Carrying out high pressure experiments on carbon nanotubes and their nanoconfinement systems can not only help us understand deeply their various properties and reveal the mechanism of phase transformation,but also provide an new strategy to construct the novel carbon strutures with desired properties.The studies on the structural transition mechanishm and property change of carbon namotubes and their nanoconfinement systems under pressure are still unclear and researches on constructing new carbon allotropes with desired structure and proterty using starting carbon nanotubes as starting carbon source are very few.Based on these questions,we performed high pressure studies on some typical one-dimentional carbon nanomaterials,including aligned multi-walled carbon nanotube arrays(MWNTAs),C70 peapod and GNRs@SWCNTs,as well as multi-walled carbon nanotube fibers by using diamond anvil cell(DAC)to investigate their structural transitions under high pressure.1.High pressure polarized Raman scattering studies have been carried our on aligned multi-walled carbon nanotube arrays(MWNTAs)up to 40 GPa,suggesting that the polarized Raman technology can be used to identify the intertube interactionchanges between tubes,structural phase transformation,and bonding states of carbon nanotubes under high pressure.Unlike previous literature,we found that the MWNTAs exhibit a polarization dependence similar to that of isolated single-walled carbon nanotubes at ambient conditions.Upon compression,the polarization dependence weakens gradually with increasing pressure up to ~20 GPa.Above 20 GPa,the depolarization effect vanishes.The nanotube arrays released from 22 GPa almost preserved the initial alignment and morphology.Thus we suggest that pressure-induced enhancement of intertube interactions should be the main reason for the gradually weakened depolarization effect.The intertube interaction obviously weakens the depolarization in the perpendicular direction and attributed this phenomenon to the depolarization of the electronic states,in this case,the material behavies like a bulk metal.On the other hand,the disappearance of the polarization dependence can be explained by the formation of interlinked sp3 bonding in the MWNTAs.Our results show that polarized Raman spectroscopy is an efficient method to explore not only intertube interaction but also structural transition changes in MWNTs.This also solves the longrunning polarization dispute on multi-walled carbon nanotubes under pressure and it will provide important way to explore pressure-induced transition in carbon nanotubes.2.A novle carbon allotrope which can be quenched to ambient conditions has been synthesized by cold compression of C70 peapods(C70@SWNTs)up to 80 GPa.Raman spectroscopy,transmission electron microscopy and X-ray diffraction,as well as theoretical simulation explicitly indicate that this is a sp3 carbon allotrope.Indeed,our experimental XRD data show a larger number of distinguishable diffracted peaks compared with the diffraction data from cold compressed graphite and carbon nanotubes.Our predicted a new carbon allotrope,V carbon,which has a lower energy than that of all the previously predicted sp3 carbon allotropes under pressure up to 100 GPa.V carbon'energy is only slightly higher than that of diamond,while its high hardness and bulk modulus are comparable to those of diamond.In addition,good match between the diffracted peaks in experiment and from the simulation gives satisfactory identification of our produced new carbon phase.Note that these experimental peaks can not be explained by any of previously predicted structures in theory.This further supports the “new structure” of our experimentally produced phase and the credibility of our identification as V carbon.This result emphasizes the importance of the initial configuration in the precursor for the creation of new carbon allotropes.In summary,we have developed a new strategy by starting from the peapod structure containing 5-carbon rings as a building block to construct a new sp3 carbon allotrope under pressure.Starting from pure graphite-like materials containing only 6-membered carbon rings it is difficult to quench or identify new sp3 bonded phases,but when such materials are combined with fullerenes or other materials containing odd-numbered rings the transformation becomes easier and the reaction leads to a material.We think our results will motivate further experimental and theoretical studies to design new members for carbon family by using fullerene@nanotubes carbon precursor as building blocks containing odd-numbered rings in the further.3.High pressure studies have been carried out on single wall carbon nanotubes filled with perylene molecules synthesized via a vapor-phase doping method,revealing that the effects of temperacture on the growth of the confined nanoribbons and structural phase transiton behavior under high pressure.It is found that the perylene molecules formed short-chain nanoribbons inside the tubes by polymerization.The polymerization of perylene molecules is found to be dependenton the annealing temperature and thus the length of the formed nanoribbons.High pressure transformation of the formed hybrid nanostructure has been studied by means of Raman spectroscopy combined with theoretical simulation.It is found that the encapsulated nanoribbons can act as a probe for identifying the structural transitions of the host nanotubes.In comparison to empty carbon nanotube,filling with nanoribbon into the nanotubes obviously decreases the collapse pressure of the nanotubes,which can be explained in terms of their inhomogeneous interactions between the fillers and carbon nanotubes.Our theoretical calculation gives further insight into the experimental observations.4.High temperature and high pressure studies have been performed on multiwalled carbon nanotubes fibers up to 15 GPa and 2000 K using laser heated diamond anvil cell.Our results suggested that one form,the nanocrystalline diamonds(NCDs)with size of about 50–100 nm was synthesized.Another possible crystalline form,“n-diamond” with a lattice parameter of 0.4312 nm was found in the obtained products.