5 research outputs found
Modeling Pressure Induced Structural Modification of Armchair Single-Wall Nanotubes
Based on the helical and rotational symmetries and Tersoff-Brenner potential
with couple of modified parameters, we investigate the variation of bond
length/lengths in equilibrium structure due to tube length as well as due to
applied hydrostatic pressure for a series of high symmetry armchair (n,n)
single-wall nanotubes having different radii. Assuming that two different bond
lengths dictate the tube geometry, these are monitored as a function of radius.
It turns out that one of these bond lengths is greater than bond length of
graphite whereas other one was less than it. These deviations from graphite
value appear to be related to the curvature-induced rehybridization of the
carbon orbitals. Lengths are found to have very important effect on the values
of both bond lengths. The results under hydrostatic pressure indicate many
linear behaviors having different slopes in the values of bond lengths with
increasing pressure leading to a pressure induced-phase transition. This
behaviour is strongly dependent on the tube radius. We also calculate the bulk
moduls for this structure which reflects clearly this behavior of armchair
nanotubes and thus predicts mechanical resilience of nanotubes.Comment: 15 pages, 22 references, 6 figures. Submitted to Phys. Rev.
Modeling and characterizing single-walled carbon nanotubes by pressure probe
We compare the behavior of bond lengths, cross sectional shape and bulk
modulus in equilibrium structure at ambient conditions and under hydrostatic
pressure of all the three kinds of uncapped single walled carbon nanotubes.
Results of our numerical calculations show that two bond lengths completely
describe the structure of achiral SWNT whereas only one bond length is required
to determine structure of chiral SWNT. In armchair tubes, one bond length is
found to be larger than that of graphitic value while in zigzag tubes one bond
length has a constant value. These bond lengths are very sensitive to tube
radius. In chiral tubes, the value of bond length is found to depend on the
chirality and slightly on the tube radius. Different responses of these bond
lengths are found on application of pressure. At some critical pressure, both
bond lengths become equal to each other in achiral tubes. An analysis regarding
the cross sectional shape of the nanotubes and its pressure dependence has also
been done. The shape transition, from circular to oval shape takes place. At
this transition, the behavior of bond lengths is found different and dependent
on the chirality of the tubes. Chiral tubes with chiral angle which is mid way
between zigzag and armchair tubes are found to have most prominent effects of
chirality. Thus we demonstrate that pressure is a useful probe to characterize
various kinds of carbon nanotubes.Comment: pages 8, references 19, Figures
Structure of chiral single-walled carbon nanotubes under hydrostatic pressure
We investigate the structural parameters, i.e. bond lengths and bond angles
of chiral tubes of various chiralities. The procedure used is based on helical
and rotational symmetries and Tersoff potential. The results indicate that at
ambient condition, there are equal bond lengths and three unequal bond angles
in the structure of chiral tubes. The bond length depends much more on the
chirality and very slightly on the tube radius. Length of the tubes does not
play very significant role on bond length and bond angles. These C-C bonds were
recalculated under hydrostatic pressure. The bond length compresses with
pressure while the bond angles remain practically unchanged. We also carry out
analysis regarding the cross sectional shape of chiral tubes and its pressure
dependence. It is found that at some pressures, transition from circular to
oval cross section takes place. The transition pressure is found to strongly
depend on the radius and chirality of tube. At this transition, corresponding
to given elliptical cross section, the bond length for all chiral tubes is
identical. This behavior of bond length is different from achiral tubes.Comment: 16 pages, 9 figures, 32 reference
Bond Lengths of Single-Walled Carbon Nanotubes
Results of the bond lengths for various chiralities of single-wall carbon
carbon nanotubes (SWNTs) (armchair, zigzag and chiral) are obtained. We use
modified helical and rotational symmetries to describe the structure of SWNTs
and Tersoff potential to minimize the energy of these tubes. It emerges that in
general, two bond lengths are required for obtaining minimum energy structure,
in contrast to one bond length commonly used. The difference in bond lengths
depends on chirality and radius of achiral tubes. Significantly, even a small
deviation from zigzag or armchair character leads to interesting behavior of
bond lengths. A reduction in diameter is responsible for difference in the bond
lengths of achiral nanotubes. We also calculate the bond lengths under
hydrostatic pressure. The behavior of bond lengths for armchair single-wall
nanotubes when calculated under pressure shows that the larger bond length
decreases faster with pressure in comparison to the shorter bond length. At
some critical pressure (depending upon the radius of the tube), the two bond
lengths become equal to each other, reversing their difference above this
critical pressure. We suggest that this behavior can be exploited to
experimentally determine the chirality and radius of the carbon nanotubes, for
example by observing the presence and disappearance of modes typical of two
different bond lengths. This change occurs only within a few GPa of pressure.Comment: 11 pages, 7 figures, 16 references, 1 table. Submitted to PR
Pressure effects on bond lengths and shape of zigzag single-walled carbon nanotubes
We investigate structural parameters, i.e., bond lengths and bond angles of
isolated uncapped zigzag single-wall nanotubes in detail. The bond lengths and
bond angles are determined for several radii tubes by using a theoretical
procedure based on the helical and rotational symmetry for atom coordinates
generation, coupled with Tersoff potential for interaction energy calculations.
Results show that the structure of zigzag tubes is governed by two bond
lengths. One bond length is found to have a value equal to that of graphite,
while the other one is larger. Furthermore, the tube length is found to have
significant effect only on larger bond length in zigzag tubes. With the
application of the pressure, only the larger bond length compresses, the other
one remaining practically constant. At some critical pressure, this bond length
becomes equal to constant bond length. This behavior of bond lengths is
different from those of armchair tubes. An analysis regarding the cross
sectional shape has also been done. At some higher pressure, transition from
circular to oval cross section takes place. This transition pressure is found
to be equal 2.06GPa for (20,0) tube. Some comparison with chiral tubes has also
been made and important differences on bond length behavior have been observed.Comment: pages 21, figures 7 and references 34. In press in Computational
Material Science (2008