B.S. Elman

Ion channeling studies of regrowth kinetics of disordered surface layers on graphite

The crystallization of disordered surface layers on highly oriented pyrolytic graphite (HOPG) has been studied by Rutherford backscattering spectrometry and channeling techniques. Disordered layers (∼1000–3000 A thick) are produced on the surface of HOPG by the implantation of various ions. The disordered layers are regrown by thermal annealing of the samples in an inert environment. Isochronal anneals reveal two distinct regrowth processes: one, a rapid process of low activation energy (Ea∼0.15 eV) which is observed primarily in regions where the disorder is sufficient to prevent the channeling of the ions, but insufficient to totally destroy the graphitic structure. This low activation energy may indicate annealing of the damage by migration of interstitials where the interstitials are the knock‐on carbon atoms produced by the primary ions. A regrowth process with higher activation energy (Ea∼1.2 eV) occurs primarily in regions where the disorder is close to the saturation disorder produced by ion impla...

Channeling studies in graphite

The channeling characteristics of helium ions in the polycrystalline semimetal graphite have been studied using Rutherford Backscattering Spectrometry in the 1.2–2.4 MeV energy range. Axial channeling is investigated in the geometry where the ion beam is parallel to the highly preferred c‐axis direction of a sample of highly‐oriented pyrolytic graphite (HOPG). Assuming a Gaussian distribution of crystallite orientations and measuring the energy and angular dependencies of the backscattering yield, it was possible to extract the minimum yield and the critical angle for channeling in single‐crystal graphite, and the standard deviation for the spread in c‐axis crystallite orientations. An unusual increase of the measured angular width for channeling with depth was observed, and is attributed to the polycrystalline nature of HOPG.

Observation of superlattice-induced Raman modes in graphite-potassium-amalgam compounds

Abstract The first definitive observation is reported of zone-folded Raman modes associated with in-plane superlattice ordering for the stage 1 graphite-potassium-amalgam intercalation compounds. Raman spectra for stages 2 and 3 are also reported and show only staging effects, consistent with spectra previously reported for donor compounds.

Retention of impurities in ion implanted graphite

The implantation and annealing conditions required to introduce and retain impurities in the graphite lattice have been studied using the backscattering-channeling technique. When highly oriented pyrolytic graphite (HOPG) is implanted at room temperature, the damage caused by the implantation can be completely annealed by heating the sample to temperatures Ta ≳ 2500°C. As the temperature is raised, the first recrystallization (graphitization) step involving two-dimensional or in-plane ordering occurs for annealing temperatures in the range 1500°C ≲ Ta ≲ 2300°C. In this range, the impurities diffuse parallel to the basal planes and out of the sample. This ejection of the implanted species appears to be correlated with a decrease in the interplanar spacing of the graphite structure during the graphitization process. A second step of graphitization occurs at temperatures higher than 2300°C where three-dimensional or c-axis ordering takes place. However at these high temperatures all the impurities have already diffused out of the substrate. Retention of the impurities and simultaneous annealing of radiation defects can be achieved when the sample is implanted at an elevated temperature (200 < Ti < 800°C) and subsequently annealed at ∼ 2300°C. However this behavior depends on the implanted species; e.g., As ions are retained by the graphite lattice after post-hot-implantation annealing while no trace of Si implants can be found after the same implantation and annealing procedure.

High Temperature Implantation in Graphite

In previous studies it was found that when highly oriented pyrolytic graphite (HOPG) is implanted at room temperature, the damage caused by the implantation could be completely annealed by heating the sample to temperatures higher than ∼ 2500°C. However at these high temperatures, the implanted species was found to diffuse out of the sample, as evidenced by the disappearance of the impurity peak in the Rutherford backscattering (RBS) spectrum. If, on the other hand, the HOPG crystal was held at a high temperature (≥ 600°C) during the implantation, partial annealing could be observed. The present work further shows that it is possible to anneal the radiation damage and simultaneously to retain the implants in the graphite lattice by means of high temperature implantation (T i ≥ 450°C) followed by annealing at 2300°C.

Ion-Implantation Studies of Graphite

Ion implantation of highly oriented pyrolytic graphite (HOPG) is studied using various characterization techniques, including Raman spectroscopy and Secondary Ion Mass Spectroscopy (SIMS). Particular attention is given to the annealing of the implantation-induced lattice damage using both hot substrate implantation (200 1

Raman Spectra of Ion Implanted Graphite

Raman spectroscopy is used in a variety of ways to monitor different aspects of the lattice damage caused by ion implantation into graphite. Particular attention is given to the use of Raman spectroscopy to monitor the restoration of lattice order by the annealing process, which depends critically on the annealing temperature and on the extent of the original lattice damage. At low fluences the highly disordered region is localized in the implanted region and relatively low annealing temperatures are required, compared with the implantation at high fluences where the highly disordered region extends all the way to the surface. At high fluences, annealing temperatures comparable to those required for the graphitization of carbons are necessary to fully restore lattice order.

Nanosecond Time Resolved Reflectivity Measurements At The Surface of Pulsed Laser Irradiated Graphite

Time resolved reflectivity measurements at the surface of pulsed laser irradiated graphite are found to be complicated by the evolution of carbon atoms from the surface which attenuates the probe beam. The evolution of the species occurs concomitant with the observation of the melt on a time scale of 5 cm/s. The experimental results suggest the formation of large clusters at the molten surface though more experiments are required to clearly distinguish the effect of clusters from those due to the formation of a randomly reflecting plasma.

Stoichiometric determination of SbCl5‐graphite intercalation compounds using Rutherford backscattering spectrometry

Rutherford backscattering spectrometry (RBS) is used for the first time to characterize the stoichiometry of a graphite intercalation compound (GIC). This technique provides direct information about the depth and lateral distribution of intercalant, which is of particular importance for this class of materials. Such information is difficult to obtain by other techniques. In this work, explicit application is made to the SbCl5‐GIC system. Results for the stoichiometry of Cξn SbClm are reported in terms of values of ξ and m for cleaved and as grown (uncleaved) samples. Because of the depth sensitivity of the RBS technique, we are able to show that the stoichiometry at the surface of as‐prepared intercalated graphite samples may differ significantly from the Cl:Sb ratio of 5:1 and need not be similar to that of the bulk. However, within experimental error, the RBS results on the cleaved samples are consistent with those obtained by analysis of (00l) x‐ray diffraction spectra.

MAGNETOREFLECTION IN ION-IMPLANTED GRAPHITE.

The use of a hot stage (T ∼ 600°C) for ion implantation into graphite permits the introduction of foreign species into the host material while eliminating most of the lattice damage associated with ion implantation at room temperature. This permits the use of the magnetoreflection technique for examination of changes in the electronic band structure induced by implantation Samples of graphite implanted with 31 P and 11 B at various energies and fluences are examined, and the in-plane and c-axis disorder are characterized using Raman spectroscopy and Rutherford Backscattering Spectrometer (RBS) techniques. Implantation-induced changes in the electronic band structure are interpreted in terms of the Slonczewski-Weiss- McClure band model. Small changes are found relative to the band parameters that describe pristine graphite.