Abstract

The interactions of fully stripped Argon-40 heavy ion beams with 140 MeV/nucleon with a series of increasingly polygonal carbon onions are investigated by high-resolution transmission electron microscopy and thermogravimetric analysis. The experimentally observed graphene layer linking is compared with expected results from the displacement and dislocation migration models. The results suggest that dislocation-driven mechanisms may play a significant role in graphene layer linking induced by heavy ion interactions with carbon onions.

Introduction

Carbon nanostructures and their composites are under investigation as structural elements 1 and/or lubricants 2 in environments that involve heavy ion exposure, including space and particle accelerator environments. In the present work, we report the results of an investigation of carbon onion interactions with Argon-40 heavy ion primary beams with an initial kinetic energy of 140 MeV/nucleon. This is comparable to the energy of heavy ions in the solar wind. The investigation was performed at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University, which enabled well-calibrated energy depositions to be achieved. Irradiation times were based on experimental masses and volumes and selected to result in a 10,000 Gray (Gy: Joule/kg) cumulative total dose for each sample.

One observed experimental response was the linking of graphene layers. The actual results were compared expected layer linking results from two different models. The best-studied model for radiation interactions with graphene is interaction through displacement “knock-on” collisions, which produce dangling bonds and interstitial carbon atoms 3, 4, 5, 6, 7 . The dangling bonds and loose carbon atoms then rearrange into energy-lowering configurations. A new model for graphene layer rearrangement by “zipping” driven by dislocation migration mechanisms that are only available in multi-layer radial situations has been proposed recently 8 . Our results suggest that the newly identified dislocation-driven mechanisms may also play a significant role in graphene layer linking induced by heavy ion interactions with carbon onions.

Experiment

An Argon-40 18+ (fully stripped) primary beam with energy of 140 MeV/nucleon (MeV/u) was focused to uniformly irradiate a circular 300 mm 2 area measured from known dimensional markings on a beam-viewing scintillator plate prior to the experiments. After passing through a 0.075 mm zirconium (Zr) foil exit window (ΔE = -1.45 MeV/u), a 493.8 mm air gap (ΔE = -2.64 MeV/u), and a 0.2 mm mica coverslip that restrained the samples in concavity wells (ΔE = -1.91 MeV/u), the beam-on-target energy determined to be 134.0 MeV/u, calculated by SRIM 9 . Before and after the experiment, the beam current was measured at the exit point using an electronsuppressed Faraday cup, and the upstream current was measured continuously (10 msec intervals) in the cyclotron extraction channel using a non-invasive beam probe. By scaling the probe current to the Faraday cup readout, continuous monitoring of the beam current was achieved.

Three sets of carbon onion samples were prepared at Tokyo Institute of Technology from crystalline diamond nanoparticles having an average diameter of 5 nm as previously described 10 . The diamond nanoparticles were slowly heated in argon gas flow in an infrared gold image furnace to 1700°C, 2000°C and 2300°C, respectively, which produced a known 11 spherical to polygonal transition of carbon onion morphology as a function of synthesis temperature.

The mass of each sample was determined prior to irradiation using a Denver Instruments M-220D scale with 0.01 mg sensitivity. The approximate area and thickness of each sample was measured with Mitutoyo CD-6″CS electronic calipers with 0.001 mm accuracy. The experimentally measured dimensions and densities were used in the SRIM calculations to determine the required exposure times to achieve a cumulative 10,000 Gy total dose.

High-resolution transmission electron microscopy (HRTEM) investigation of pre and post radiation carbon onions was performed in a JEOL 2200FS operated at 200 kV. Samples were suspended in ethyl alcohol and dispersed onto carbon lacey film 200 mesh copper grids (SPI). Care was taken to acquire images from samples that were well suspended over holes and not the carbon lacey film. Thermogravimetric analysis (TGA) was performed using a Rigaku TG8120 in air ambient.

Transmission Electron Microscopy Analysis

Radiation effects appeared to depend on the polygonal character of the carbon onion sample. The effect Argon-40 heavy ions on the spherical carbon onions synthesized at 1700 o C, shown in Figure 1a was to induce defects into the individual graphene layers but without serious disruption to the onion structure. This is shown by the ‘wavy’ appearance of the irradiated onion layers in Fig. 1b . Fusion of three or more onions into structures with linked outer layers was also typically observed. Carbon onions synthesized at 2000 o C carbon onions already had some polygonal character, as shown in Figure 2a . These samples become highly polygonal when irradiated, as shown in Figure 2b , and resembled carbon onions synthesized at 2300 o C, shown for comparison in Figure 2c . Figure 2 . Interaction of Argon-40 heavy ions with (a) polygonal carbons produced (b) highly polygonal onions that resembled those (c) synthesized at a higher temperature.