Abstract
Carbon nanotubes (CNTs) possess extraordinary thermal and mechanical properties on an
individual nanotube level. Gaining these benefits on a macro-scale has been elusive due to the
poor nanotube linkages (tube-to-tube as well as wall-to-wall for multiwall CNTs). Hence, in
this study, ‘defect decoration’ at various nanotube locations using boron ion-implantation was
followed by high-pressure (HP) treatments in a diamond anvil cell (DAC). This was done to
promote intra nanotube wall-to-wall and inter tube-to-tube interlinking in double wall carbon
nanotubes (DWCNTs). The present study has shown, for the first time, the close-interval
monitoring of the D band evolution with pressure up to 25 GPa, for DWCNTs, in spite of
intense obscuring signals from the diamond window. Further, to the best of our knowledge,
no other study on combined ion implantation pre-processing and subsequent pressurization to
high pressures of DWCNTs has been reported.
DWCNTs were pressurized up to 25 GPa at room temperature. Due to sample
inhomogeneity it was necessary to use an appropriate large area scanning mode to obtain an
average from 64 spectra per pressure. Vis-Raman spectroscopy (532 nm laser excitation)
measurements were done at intervals of 2-3 GPa. A separate DAC containing boron ionimplanted
DWCNT sample from the same batch as the reference (starting, non-implanted),
was prepared. 11B+ ions of 150 keV per ion at a fluence of 5×1012 per cm2 were used in the
implantation. The same probe and measurement protocols as for the reference sample were
repeated in the HP Raman measurements on the implanted sample. The Raman probing depth
is ~30 nm. Appropriate sample preparation, DAC loading and implantation procedures have
been developed to ensure an unambiguous probing of the shallow implanted surface. In
addition, UV-Raman (244 nm) measurements were done on recovered samples decompressed
from 20 GPa as well as starting DWCNT material in search of the characteristic signature for
sp3 bonds.
The D to G+ (axial) band intensity ratio for implanted DWCNTs evolves with pressure
following a trajectory consistent with an empirically determined trend for proliferation of sp3
bonds in carbon materials [A.C. Ferrari and J. Robertson, Phys. Rev. B 61, 14095 (2000)]. A
maximum of ~10% volume fraction of sp3 bonds was determined. The pressure dependences
of the G+ band position for implanted DWCNTs delineated three pressure regimes
0 ≤ P ≤ 3 GPa, 3 < P ≤ 11 GPa and P >11 GPa. The behaviour of the G+ band position with
pressure indicates two competing mechanisms namely the proliferation of sp3 bonds and
expected phonon mode hardening. Notable changes in the pressure dependences of the Raman
spectral parameters of the D and G bands occur at low and higher critical pressure. The
lower critical pressure for both implanted and unimplanted DWCNTs is 2-3 GPa. Due to
introduction of nucleation sites as well as enhanced nanotube interaction in the implanted
DWCNTs, the lower critical pressure for implanted DWCNTs corresponds to the onset in
proliferation of sp3 bonding in the DWCNT structures followed by a subsequent saturation of
sp3 formation beyond 9 GPa. The lower critical pressure for unimplanted DWCNTs is...
M.Sc. (Physics)