Control of Carbon Nanotube Nucleation Rate with a Hydrogen Beam Plasma

1
User TUNL

Control of Carbon Nanotube Nucleation Rate with a
Hydrogen Beam Plasma
Paolo 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
Abstract
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 beads 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 beads 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.
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.
Proposed Model for Nucleation and Growth of
Carbon Nanotubes
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
C
D
E
F
A
B
Optical and Scanning Electron Microscope Images
Description of the Experiment
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 lt 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.
Optical microscope image of Sample 1 at 1000x
magnifica-tion showing tiny black spots
characteristic of graphitic carbon agglomeration.
SEM image of different region of Sample 1
indicating hemispherical surface growth 200nm
diameter.
Higher resolution SEM image of these highly
irregular Fe islands.
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.
Work supported in part by the US Dept. of Energy
under Grant DE-FG05-88ER40442 and by the National
Science Foundation