Control of Carbon Nanotube Nucleation Rate with a Hydrogen Beam PlasmaPaolo Santos1, Dorothée Alsentzer 3, Thomas B. Clegg2,3, Sergio Lemaitre 2,3, and Brian R. Stoner 3

1-University of North Carolina at Pembroke, 2-Triangle Universities Nuclear Laboratory, 3-University of North Carolina at Chapel Hill

Proposed Model for Nucleation and Growth of Carbon Nanotubes

A B C D E F

A: Thin Fe film deposited on diamond surface

B: Fe beads forms nucleation sites after heating

C: Carbon atoms coalesces on bead surface

D: “Liftoff” occurs to reduce lattice strain

E: Nucleation of successive tubular structures

F: Growth of multi-walled nanotubes

Hydrogen Plasma Jet source developed at TUNL for production of intense spin-polarized H or D beams.

Sample holder shown without heat shield. Four samples are mounted on the four faces

of the holder and rotated into beam.

Heat shield and sample holder mounted on extension rod of

sample manipulator.

Abstract

Carbon nanotubes can form when hemispherical beads of iron of 1 to 10 nm in diameter are placed on a diamond surface, and held near ~750 C in the presence of hydrogen. However, the actual nucleation of these nanotubes is not well understood. It is known that at this temperature, carbon from the diamond is soluble in iron. Thus, it is believed carbon atoms move within the iron and coalesce on the bead’s surface. Any hydrogen present then preferentially etches non-graphitic surface carbon and leads to one or more graphite-like layers covering the hemispherical bead. It is postulated then, that as more carbon migrates to the surface, strain within these layers causes their ‘liftoff’ from the bead, nucleating the growth of single- or multiwalled tubular structure(s) above it. In an effort to verify this process, an experiment is underway to control the rate of this nanotube growth. By varying the diamond substrate temperature, to control carbon mobility, and the H+ ion flux at the bead’s surface, to control the rate of graphite formation, measurements will investigate whether rates of nucleation, ‘liftoff’, and nanotube growth can be sufficiently throttled to reveal clearly this nucleation process. Initial experimental results will be reported.

Abstract

Description of the Experiment Optical and Scanning Electron Microscope Images

Work supported in part by the US Dept. of Energy under Grant DE-FG05-88ER40442 and by the National Science Foundation

Optical microscope image of Sample #1 at 1000x magnifica-tion showing tiny black spots

characteristic of graphitic carbon agglomeration.

SEM image of Sample #4 at low resolution showing that Fe islands have formed which are larger than

ideal to provide easy nucleation sites for carbon nanotubes.

Higher resolution SEM image of these highly irregular Fe islands.

Experimental hypothesis - Atoms in a thin film of iron, when deposited on a diamond substrate and raised to temperatures above ~ 650C, begin to migrate and agglomerate, to form separate tiny islands which are believed to have initial radial dimensions of 10 to 50 nm. We sought to prepare samples with such structures, and then expose them after further heating to a hydrogen plasma jet. The hydrogen etches away carbon structures with weaker bonds, preferentially leaving graphitic structures. We expected that some of these structures would initiate nucleation of carbon nanotubes. Experimental plan - Because we did not know optimal experimental conditions for producing such nanotube growth, we planned to expose our samples for various lengths of time, and over a range of temperatures, to a very-low-energy H+ beam. Several temperatures were chosen, selected to promote successively higher carbon atom solubility and mobility within iron. At each temperature, exposure times were varied to bracket conditions believed to be best for nanotube nucleation. Experimental method - Samples were first prepared of poly-crystalline diamond film grown by plasma-enhanced chemical vapor deposition on 0.5mm thick silicon substrates. These were cut into 5mmx5mm squares. Then, a ~15 nm thick iron film was grown atop the diamond. Twelve such samples, in three groups of four, were then heated to temperatures of 745C, 810C, and 860C, respectively, and exposed to the hydro-gen plasma beam. In each group, a numbered sample was exposed for 36 mins, 6 mins, 1 min, or 10 sec. Before and after each group of samples was irradiated, and between the longest exposures within any sample set, the incident H-flux was monitored downstream by removing the sample from the beam and measuring the pressure rise when the plasma entered a previously calibrated chamber. Measurements implied that the H+ intensity incident on the samples was 0.6 +/- 0.3 mA/mm2. Prior measurements of the plasma had shown that H2

+ ions represented < 5% of the flux. Estimated mean H-ion temperature was ~ 1 eV. Sample analysis and conclusions - After irradiation, several samples were thoroughly scanned with an optical microscope. Regions of likely graphitic growth were few, but were most apparent in Samples #1-4, which were exposed at the lowest temperature. Most interesting and promising structures are shown at the right. These and other sample regions were then investigated with higher resolution using a scanning electron microscope. A possible region of hemispherical carbon growth is exhibited in Sample #1. Other SEM images of Sample #4 demonstrate that iron often coalesced substantially, into regions far too large to support the formation of carbon nanotubes. This indicates that iron mobility at the temperature used was higher than optimal. Thus, thinner Fe film and lower H flux is likely indicated for future experiments.

SEM image of different region of Sample #1 indicating

hemispherical surface growth ~200nm diameter.

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