Transcript No Slide Title

User TUNL:
Control of Carbon Nanotube Nucleation Rate with a Hydrogen Beam Plasma
Paolo
1
Santos ,
Dorothée Alsentzer
3,
Thomas B.
2,3
Clegg ,
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 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.
Hydrogen Plasma Jet source
developed at TUNL for production of
intense spin-polarized H or D beams.
Heat shield and sample holder
mounted on extension rod of
sample manipulator.
Proposed Model for Nucleation and Growth of Carbon Nanotubes
Sample holder shown without heat shield.
Four samples are mounted on the four faces
of the holder and rotated into beam.
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
A
B
C
D
E
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 plasmaenhanced 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 hydrogen 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.
Work supported in part by the US Dept. of Energy under Grant DE-FG05-88ER40442 and by the National Science Foundation
F
F: Growth of multi-walled nanotubes
Optical and Scanning Electron Microscope Images
Optical microscope image of
Sample #1 at 1000x magnification 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.
SEM image of different region of
Sample #1 indicating
hemispherical surface growth
~200nm diameter.
Higher resolution SEM image of
these highly irregular Fe islands.