Investigations On Structural Transitions Of Typical Carbon Nanomaterials Under High Pressure

Since their unique chemical and physical properties and great potentialapplications in the fields of nanoelectronics and multi-functional materials, carbonnanomaterials, such as single-wall carbon nanotubes （SWNTs） and graphene havebeen the subject of intense investigations. High pressure can effectively change thedistances between molecules and atoms to affect the electron distribution and energyband structure of materials, which will greatly influence or even change the propertiesof materials. High pressure is a powerful method to synthesize new materials, to findnew laws and theories of physics. Carrying out high pressure experiments on carbonnanomaterials can help us deeply understand their various properties and furtherprovide an important way to obain new carbon strutures and new carbon materialssuch as the superhard carbon materials, due to the flexibility of carbon to form sp, sp2,and sp3bonds. These arouse great value in the field of high pressure research oncarbon nanomaterials.In this thesis, we carried out high pressure research on some typical carbonnanomaterials, such as single-wall carbon nanotubes （SWNTs）, C₆₀peapods(C₆₀@SWNTs), and graphene nanoplates by using diamond anvil cell （DAC） to makedetailed research on their structural transitions under pressure.Extensive high pressure studies on SWNTs have shown that the cross-section ofnanotubes will change in shape under pressure. However, what structural transitionswill happen in a higher pressure region after the pressure-induced collapse hasoccurred in SWNTs? This issue is still not clear up to now. In our work, we studiedthe high-pressure Raman spectra of high quality single-walled carbon nanotubes（arc-SWNTs） with a narrow diameter distribution of1.3nm by using different lasers.Through detailed analysis of the Raman signals of our SWNTs sample, we obtained the whole physical picture of the structural transitions in SWNTs under pressure. Thepressure-induced changes in Raman signals at around2GPa and5GPa can beattributed to the nanotubes’ cross-section changes from circle to ellipse and then to aflattened shape, respectively. At around15-17GPa both the Raman wavenumber andthe linewidth of the G-band as a function as pressure exhibit anomalies. We suggestthat the interlinked configuration with sp3bonds is formed among nanotubes or evenbetween the two opposite walls of a collapsed nanotube for this anomaly, which couldbe corresponding to the anomalies in G-band wavenumber and width. Our HRTEMobservations and Raman measurements on the decompressed samples show that theSWNTs are almost recovered even from31GPa. It is proposed that the formation ofinterlinking sp3C-C bonds in SWNTs stabilize the nanotube structure and thusenhance significantly the high pressure stability of SWNTs.Raman spectra of HiPco-SWNTs with diameters of0.6–1.3nm was furtherstudied under high pressure. It is found that the pressure dependence of the radialR-band frequency, dω/dp, is diameter and experimental condition dependent.Compared with the theoretical calculation, we believe that besides thepressure-induced hardening of the C-C bond, the effect from PTM contributessignificantly to the pressure-induced upshift of RBMs in the experiments with PTMs,and the interaction between PTM and nanotubes becomes stronger in nanotubes withdiameters smaller than⁰.8nm. In the experiments without a PTM, the intertubeinteractions dominate the upshift of RBMs, i.e., the smaller the nanotube diameter, thestronger the intertube interactions. The results thus indicate that the smaller diameternanotubes should have stronger coupling to the PTM in the case with a PTM, andinstead stronger intertube interactions between each other in the case without a PTM.For the HiPco SWNTs upon decompression, it is found that the pressure for thecollapse of a nanotube is diameter dependent in the experiments without a PTM butshows little diameter dependence in the case with a PTM. In contrast, the arc-SWNTsshow high stability even after31GPa compression with or without a PTM. Theseresults suggest that the reversibility of the collapse for a nanotube depends on thenanotube itself and also on other experimental conditions.The encapsulation of C₆₀molecules into SWNTs can result in a self-assemblednanotube-C₆₀hybrid structure, the so-called peapods C₆₀@SWNTs, a new carboncomposite nanomaterial. Nowadays the high pressure studies on C₆₀peapods mainly focus on the behaviors of confined C₆₀in a relative low pressure region. The structuraltransitions of C₆₀peapods under the higher pressure region are still absent both in theexperimental and theoretical studies. By using Raman spectroscopy, HRTEM andXRD technique, we investigated the structural phase transitions of decompressed C₆₀peapod samples from different pressures of31GPa,37GPa,52GPa,81GPa. It isfound that the initial structure of C₆₀peapod is absolutely destroyed when it isdecompressed from37GPa and the above pressure value. The X-ray diffractionpattern and HRTEM observations on the52GPa and81GPa decompressed samplesindicate that a new quenchable suprhard carbon phase is formed, which is highlyconsistent with the Cco-C8structure proposed by a theoretical work. The ring crackindentations on the diamond anvils following the original boundary of sample in thegasket are found, indicating the exceptional hardness of this new phase. Compared tothe carbon nanotubes, the pressure value to get the quenchable suprhard carbon phaseis obviously reduced. This indicates that the insertion of C₆₀into SWNTs could highlylower the pressure condition to obtain the quenchable suprhard carbon phase. Ourexperimental findings have great guiding significance for the synthesis of newsuperhard carbon materials and the materials with unique properties.Graphene nanoplate is a quasi two-dimensional carbon nanomaterial with thethickness in nano-scale. High pressure Raman study on graphene nanoplates has beencarried out in a diamond anvil cell. A phase transformation has been observed ingraphene nanoplates at15GPa, which can be explained by the interlayer couplingwith sp3bonds formed in the material. For graphite and micro-graphite, the transitionpressure is19GPa, which is obvious higher than that of graphene nanoplates. Thedifferent thickness of graphene nanoplates/graphite could be responsible for thedifferent phase transition pressure in our experiments. And the lower phase pressurein graphene nanoplates is explained by their specical limited-number layer structureand nucleation process in phase transition. Our TEM observations and Ramanmeasurements on the decompressed samples of graphene nanoplates further support the proposed transformation picture.