The Irradiation Effects Of HOPG And Graphene Induced By Energetic Heavy Ions

Graphite materials have many advantages, such as high melting and boiling points,good thermal and electrical conductivity, stable chemical property, corrosion resistance,thermal shock resistance and good plasticity. What’s more important is it can slowdown the fast neutrons. Thus graphite is the ideal neutron moderation materials whichcould be used in nuclear reactors. In high-temperature gas-cooled reactor, graphite isthe only choice for structure and reflector materials. Heat neutron irradiation inducesatom displacement, results in defects and disturbance which would further lead to thephysical and chemical properties and even the macroscopic dimensions changes of thegraphite. Therefore, the study of irradiation effects in graphite is always a veryimportant research topic all over the world. Graphene is single layer graphite, whichcould be viewed as the building block for various carbon allotropes. In addition,graphene has excellent electrical, thermal and optical properties. The investigation ofthe irradiation effects on graphene paves a way to understand the property of carbonallotropes. Moreover, it could provide valuable experimental data for the applicationof graphene.In this thesis, monolayer graphene samples were successfully fabricated bymicromechanical cleavage technique. Both of Swift Heavy Ions (SHI) and HighlyCharged Ions (HCI) were used to irradiate Highly Oriented Pyrolytic Graphite(HOPG), HOPG films with thickness of several nanometers and monolayer graphene.SHI were provided by the accelerator HIRFL of IMP and the linear acceleratorUNILAC of GSI. HCI were offered by320kV high-voltage platform of IMP. Afterirradiation, the samples were investigated by Raman spectroscopy, X-ray photoelectron spectroscopy, scanning tunneling microscopy, transmission electronmicroscopy and atomic force microscopy. The detailed analysis and discussion of theexperimental results are divided into three parts as follows.The results of irradiation effects caused by SHI indicate that:(1) Nanoscalehillock latent tracks and sp3component were detected on the irradiated HOPG surface.The amount of hybridization (Isp3/Isp2) strongly depends on the electronic energy lossand the fluence of incident ions in the samples. The area ratio of Raman D and D′peak to G peak (ID/IG) increases with the increasing ion fluence, this is correspondingto the T-K relation.(2) The irradiation damage of HOPG films depends on thethickness of the samples. It’s much easier to induce defects into thinner films thanthicker ones. The results showed that the monolayer graphene has the weakest anti–irradiation properties. The possible defect types in samples with different thicknesswere also discussed via different intensity ratio ID/ID′.(3) Nanoscale holes wereobserved in monolayer graphene by TEM. Raman test shows that the electronic energyloss (dE/dx)eplays an important role in the degree of damage in graphene. Lucchese’stheoretical model was improved in this work to study the evolution of ID/IGwith latenttrack spacing (Ld) of graphene irradiated by SHI. In the improved model, the directrelationships of the damage in graphene to the impacting ions parameters includingelectronic energy loss dE/dx and impact ion energy ε were concluded. With this model,the irradiation effects of graphene caused by SHI could be predicted well.(4) Bycomparing the irradiation effects of HOPG and graphene, we found that monolayergraphene is much easier to be damaged than bulk graphite. A turning point wasdetected in the evolution progress of ID/IGwith the fluence obtained from graphene,while such point was never found in HOPG in the fluence range of this work. Thevelocity effect was measured in HOPG However, it was not observed in graphene inthis experiment.The results of the HCI irradiation experiment were concluded as following:(1)Hillock latent tracks were detected on the irradiated HOPG surface.Compressed HOPG lattice structure was imaged on the top of some latent tracks.(2)The Lucchese’s phenomenological model was improved to give full line fitting of theexperiment data of HOPG and graphene. According to the improved model, the energetic ions may cause both structurally disordered and activated regions ingraphene. The competing mechanism of these two regions results in three variationregions of the ID/IGof graphene. In HOPG, however, only activated region is inducedby energetic ions, then two variation regions of ID/IGobtained from HOPG has bedetected.The different irradiation results of the HOPG and graphene caused by SHI andHCI were discussed. The main results show that:(1) Under the same irradiation ionfluence, the greater intensity ratios of the D peak to G peak (ID/IG) were found inHOPG impacted by HCI than that by SHI. While no obvious differences of ID/IGweredetected in graphene irradiated by those two different kinds of ions.(2) Comparing thegraphene samples impacted by SHI and HCI ions, the larger radius of the activatedregion rAin HCI irradiated graphene was observed.