Irradiation-induced amorphization of graphite

Defect formation in multiwalled carbon nanotubes under low-energy He and Ne ion irradiation

The mechanical, structural, electronic and magnetic properties of carbon nanotubes can be modified by electron or ion irradiation. In this work we used 25 keV He+ and Ne+ ion irradiation to study the influence of fluence and sample thickness on the irradiation-induced damage of multiwalled carbon nanotubes (MWCNTs). The irradiated areas have been characterised by correlative Raman spectroscopy and TEM imaging. In order to preclude the Raman contribution coming from the amorphous carbon support of typical TEM grids, a new methodology involving Raman inactive Au TEM grids was developed. The experimental results have been compared to SDTRIMSP simulations. Due to the small thickness of the MWCNTs, sputtering has been observed for the top and bottom side of the samples. Depending on thickness and ion species, the sputter yield is significantly higher for the bottom than the top side. For He+ and Ne+ irradiation, damage formation evolves differently, with a change in the trend of the ratio of D to G peak in the Raman spectra being observed for He+ but not for Ne+. This can be attributed to differences in stopping power and sputter behaviour.

Using defects to store energy in materials - a computational study

Energy storage occurs in a variety of physical and chemical processes. In particular, defects in materials can be regarded as energy storage units since they are long-lived and require energy to be formed. Here, we investigate energy storage in non-equilibrium populations of materials defects, such as those generated by bombardment or irradiation. We first estimate upper limits and trends for energy storage using defects. First-principles calculations are then employed to compute the stored energy in the most promising elemental materials, including tungsten, silicon, graphite, diamond and graphene, for point defects such as vacancies, interstitials and Frenkel pairs. We find that defect concentrations achievable experimentally (~0.1–1 at.%) can store large energies per volume and weight, up to ~5 MJ/L and 1.5 MJ/kg for covalent materials. Engineering challenges and proof-of-concept devices for storing and releasing energy with defects are discussed. Our work demonstrates the potential of storing energy using defects in materials.

The electronic states of a double carbon vacancy defect in pyrene: a model study for graphene

The unpaired density changes from the polyradical to closed shell character upon geometry relaxation.

The contribution made by lattice vacancies to the Wigner effect in radiation-damaged graphite

Models for radiation damage in graphite are reviewed and compared, leading to a re-examination of the contribution made by vacancies to annealing processes. A method based on density functional theory, using large supercells with orthorhombic and hexagonal symmetry, is employed to calculate the properties and behaviour of lattice vacancies and displacement defects. It is concluded that annihilation of intimate Frenkel defects marks the onset of recovery in electrical resistivity, which occurs when the temperature exceeds about 160 K. The migration of isolated monovacancies is estimated to have an activation energy of E(a) ≈ 1.1 eV. Coalescence into divacancy defects occurs in several stages, with different barriers at each stage, depending on the path. The formation of pairs ultimately yields up to 8.9 eV energy, which is nearly 1.0 eV more than the formation energy for an isolated monovacancy. Processes resulting in vacancy coalescence and annihilation appear to be responsible for the main Wigner energy release peak in radiation-damaged graphite, occurring at about 475 K.

Degradation and healing mechanisms of carbon fibers during the catalytic growth of carbon nanotubes on their surfaces

This study reports on the main cause of the reduced tensile strength of carbon fibers (CFs) by investigating the microstructural changes in the CFs that are undergoing mainly two processes: catalyst nanoparticle formation and chemical vapor deposition (CVD). Interestingly, the two processes oppositely influenced the tensile strength of the CFs: the former negatively and the latter positively. The catalysts coating and nanoparticle formation degraded the CF surface by inducing amorphous carbons and severing graphitic layers, while those defects were healed by both the injected carbons and interfaced CNTs during the CVD process. The revealed degradation and healing mechanisms can serve as a fundamental engineering basis for exploring optimized processes in the manufacturing of hierarchical reinforcements without sacrificing the tensile strength of the substrate CFs.

Raman Spectroscopy for Quantitative Analysis of Point Defects and Defect Clusters in Irradiated Graphite

We report the development of Raman spectroscopy as a powerful tool for quantitative analysis of point defect and defect clusters in irradiated graphite. Highly oriented pyrolytic graphite (HOPG) was irradiated by 25 keV He+ and 20 keV D+ ions. Raman spectroscopy and transmission electron microscopy revealed a transformation of irradiated graphite into amorphous state. Annealing experiment indicated a close relation between Raman intensity ratio and vacancy concentration. The change of Raman spectra under irradiation was empirically analyzed by “disordered-region model,” which assumes the transformation from vacancy-contained region to disordered region. The model well explains the change of Raman spectra and predicts the critical dose of amorphization, but the nature of the disordered region is unclear. Then, we advanced the model into “dislocation accumulation model,” assigning the disordered region to dislocation dipole. Dislocation accumulation model can simulate the irradiation time dependencies of Raman intensity ratio and the c-axis expansion under irradiation, giving a relation between the absolute concentration of vacancy and Raman intensity ratio, suggesting an existence of the barrier on the mutual annihilation of vacancy and interstitial.

The influence of Ga(+) irradiation on the transport properties of mesoscopic conducting thin films

We studied the influence of 30 keV Ga(+)-ions-commonly used in focused-ion-beam (FIB) devices-on the transport properties of thin crystalline graphite flakes, and La(0.7)Ca(0.3)MnO(3) and Co thin films. The changes in electrical resistance were measured in situ during irradiation and also the temperature and magnetic field dependence before and after irradiation. Our results show that the transport properties of these materials strongly change at Ga(+) fluences much below those used for patterning and ion-beam-induced deposition (IBID), seriously limiting the use of FIB when the intrinsic properties of the materials of interest are of importance. We present a method that can be used to protect the sample as well as to produce selectively irradiation-induced changes.

Pathway for the transformation from highly oriented pyrolytic graphite into amorphous diamond

We report the discovery of a novel pathway for the transformation from highly oriented pyrolytic graphite foils into amorphous diamond platelets. This pathway consists of three stages of neutron irradiation, shock compression, and rapid quenching. We obtained transparent platelets which show photoluminescence but no diamond Raman peak, similar to the case of amorphous diamond synthesized from C60 fullerene. Wigner defects formed by irradiation are considered to make a high density of diamond nucleation sites under shock compression, of which growth is suppressed by rapid quenching.

Multiscale imaging and tip-scratch studies reveal insight into the plasma oxidation of graphite

The plasma oxidation process of highly oriented pyrolytic graphite (HOPG) has been investigated through a combination of multiscale (micrometric to atomic) imaging by atomic force and scanning tunneling microscopies (AFM/STM) and STM tip-scratching of the HOPG substrate. Complementary information was obtained by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Repetitive imaging of the same HOPG location following a series of consecutive plasma treatments allowed an accurate determination of the plasma etch rates along both the a and c crystallographic directions of the graphite lattice. The etch rates were typically in the range of a few nm per min along the a axis, and the equivalent of 1-6 graphene layers per min along the c axis. The results pointed to the existence of two main plasma etching regimes, related to short (<20-30 min) and long (> or =30 min) treatment times. This was inferred not only from the measured plasma etch rates but also from the observation of fundamental differences in the atomic-scale surface structure of the plasma-treated HOPG samples, and from the general mechanical behavior of the materials under the action of the AFM tip. In particular, atomic-scale STM imaging suggested a change from a defected, but essentially graphitic, surface in the first regime to an amorphous carbon surface in the second regime. Together with AFM and STM, Raman spectroscopy and XPS provided a consistent picture of the surface structure and chemistry of the plasma-modified HOPG in the two regimes. The implications of these results as well as the possible mechanism that drives the plasma etching process in the two regimes are discussed.

A method to evaluate the tensile strength and stress-strain relationship of carbon nanofibers, carbon nanotubes, and C-chains

A method is introduced to assess the tensile strength of carbon nanofibers, carbon nanotubes (CNTs), and linear chains of carbon atoms (C-chains) obtained from thin amorphous carbon films by electron irradiation. Transmission electron microscopy images show that the nanofibers undergo a radiation-induced necking process, characterized by CNT formation and often followed by the formation of a C-chain. Simulations of the necking process are carried out to determine the tensile stress supported by the nanofiber and CNT neck.

Formation and transformation of carbon nanoparticles under electron irradiation

This article reviews the phenomena occurring during irradiation of graphitic nanoparticles with high-energy electrons. A brief introduction to the physics of the interaction between energetic electrons and solids is given with particular emphasis on graphitic materials. Irradiation effects are discussed, starting from microscopic mechanisms that lead to structural alterations of the graphite lattice. It is shown how random displacements of the atoms and their subsequent rearrangements eventually lead to topological changes of the nanoparticles. Examples are the formation of carbon onions, morphological changes of carbon nanotubes, or the coalescence of fullerenes or nanotubes under electron irradiation. Irradiation-induced phase transformations in nanoparticles are discussed, e.g. the transformation of graphite to diamond, novel metal-carbon phases in nanocomposite materials or modified phase equilibria in metal crystals encapsulated in graphitic shells.