arXiv subjects

V. K. Jindal

Publications and source records attributed to V. K. Jindal.

At least 37 records · Page 2Linked to original sources

arXiv2008

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.

cond-mat.mtrl-sci↗

arXiv2008

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.

cond-mat.mtrl-sci↗

arXiv2007

Behaviour of a Bucky-ball under Internal and External Pressures

We study the behaviour of the C60 molecule under very high internal or external pressure using Tersoff as well as Brenner potentials. As a result, we estimate the critical internal and external pressures that lead to its instability. We also calculate stretching force constant and bulk modulus of this molecule at several pressures under which the molecule remains stable. The values of these estimated here at zero pressure agree closely with those obtained in earlier calculations. We also observe that at high pressures, a finite value of parameter of Tersoff potential gives physically acceptable results in contrast to its value zero, which is usually taken for the carbon systems.

cond-mat.mtrl-sci↗

arXiv2007

Dimerization and Fusion of two C60 Molecules

We investigate the dimerization and fusion of C60 molecules to form various C60 dimers when pushed against each other at several inter-molecular distances. We study the stability of this dimerized C60 molecule based on its binding strength provided by intramolecular interactions. Tersoff potential, which is considered to represent intramolecular interactions well, has been used to calculate potential energy at these distances of separation and for all possible orientations of the molecules. We observe that several minimum energy configurations exist at various distances between the C60 molecules. Our calculation shows that apart from the dumbbell structures, many interesting composite phases also result, such as fused, peanut and carbon nanotubes of geometry (5,5) and (10,0) of certain specific length.

cond-mat.mtrl-sci↗

arXiv2006

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.

cond-mat.mtrl-sci↗

arXiv2006

Stability of different phases of (C60)2 Structures

We investigate the possible binding configurations of pairs of C60 molecules when pushed against each other. Tersoff potential, which represents intramolecular interactions well, has been used to calculate potential energies. We begin relaxation of atomic coordinates at various distances of separation and for all possible mutual orientations of the two molecules. As a result, we have been able to show that several minimum energy configurations exist. Some of these configurations have not been reported earlier. Only two types of dimer structures, involving interlinkage through a single bond, or through so called 2+2 cycloaddition, have been commonly referred in the literature. Our calculation shows that apart from these configurations, many interesting composite phases also result, such as fused and peanut structures and (5,5) and (10,0) nanotubes. A link with experiment to find these structures can be established by application of suitable critical applied pressure in the solid phase, accompanied by high temperature corresponding to orientational melting so that suitable mutual orientations are available. High energy molecular beams of C60 incident upon C60 layers could also achieve the same.

cond-mat.mtrl-sci↗

arXiv2006

Behavior of Bucky Ball under extreme Internal and External Pressures

We study the behavior of the C60 molecule under very high internal and external pressure using Tersoff potential. As a result, we calculate the critical internal and external pressures leading to its instability. We also calculate stretching force constant, breathing mode frequency and bulk modulus of this molecule. The data estimated here at zero pressure agrees closely to that obtained in earlier calculations. If subjected to extreme pressures the molecule can withstand upto 58.23% of compression and 174.89% of dilation in terms of its volume. We also observe that above some critical external pressure the coordination number of the carbon atoms of C60 molecule suddenly increases resulting in an abrupt change in the bulk modulus of the molecule.

cond-mat.mtrl-sci↗

arXiv2006

Lattice dynamics of MC60 compounds in FCC phase

Phonon dynamics of alkali metal atom, M doped in C60, forming MC60 solids in fcc phase has been studied. The calculations take into account Van-der-Waals and Coulomb interactions for M-C60 and C60-C60 and show fairly good agreement with reported neutron scattering results for RbC60. The calculations have also been done following Rigid Shell Model (RSM). We also perform calculations for specific heat, Gruneisen parameter, thermal expansion and thermal expansion coefficient.

cond-mat.mtrl-sci↗

arXiv2006

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.

cond-mat.mtrl-sci↗

arXiv2005

Strains in Axial and Lateral Directions in Carbon Nanotubes

Analytic expressions for strains in carbon nanotubes of all chiralities in lateral and axial directions have been obtained. These expressions have been related to bond angle and bond length flexibilities. This finally results in obtaining very generalized results for Poisson ratio of all chirality and all diameter carbon nanotubes. It turns out that the Poisson ratio is always independent of carbon nanotube diameter, but depends on the their chirality. On the assumption of fixed bond lengths, (only flexible bond angle), the Poisson ratio of all chirality tubes is obtained to be 1.When realistic flexibility of bond lengths and bond angles is assumed, the Poisson ratio is between ~0 and 0.3, depending upon chirality. The present calculation of the Poisson ratio does not require any detail of molecular interactions under the assumption of either bond angle or bond length flexibility alone, but does depend on the interaction potential if both are assumed flexible. Further, it has been found that the Poisson ratio is sensitive to applied initial stress, varying significantly when obtained under different initial stress values. It is observed that Poisson ratio can be used to justify the interaction potential by finding out the bond length flexibility in comparison to bond angle flexibility.

cond-mat.mtrl-sci↗

arXiv2004

Model for Pressure Induced Deformations in Carbon Nanotube Materials

We report the results of a model calculation for studying the effects of hydrostatic pressure on a bunch of carbon nanotubes. At pressures that we work with, the deformation in axial direction comes out to be negligibly small. We find that hydrostatic pressure is an ideal probe to study the radial deformations of the nanotubes. The nanotubes are considered to be flexible, identified by a flattening of cylinders under pressure through a parameter f. We use the 6-exponential and Brenner potentials to account for inter and intra-tube interactions respectively. We calculate the total energy of the deformed tubes in bunches. The free energy thus calculated enables us to calculate phase changes at various pressures. From our calculations, we find the phase transformation to occur at about 5GPa.

cond-mat.mtrl-sci↗

arXiv2002

Swift Heavy Ion Induced Modification Studies of C60 Thin Films

Modification induced by 110 MeV Ni ion irradiated thin film samples of C60 on Si and quartz substrates were studied at various fluences. The pristine and irradiated samples were investigated using Raman spectroscopy, electrical conductivity and optical absorption spectroscopy. The Raman data and band gap measurements indicate that swift ions at low fluences result in formations that involve multiple molecular units like dimer or polymer. High fluence irradiation resulted in sub-molecular formations and amorphous semiconducting carbon, indicating overall damage of the fullerene molecules. These sub-molecular units have been identified with nanocrystalline diamond and nanocrystalline graphite like formations.

cond-mat.mtrl-sci↗

arXiv2001

Model for High Temperature Phase of C70 Solid

Depending on the temperature, the C70 solid crystallizes in several structures. At high temperature (T > 340K), the ellipsoidal C70 molecule rotates freely in all directions and may be treated as a uniform thick spherical shell with inner and outer radii as the minimum and the maximum distance of C-atom from the center of the molecule. At lower temperatures the free rotations of molecules freeze out. We have calculated the lattice parameters, energies and bulk modulus at the minimum energy configuration of fcc and hcp phase of pure C70 solid at high temperature using a simple model based on atom-atom potential.

cond-mat.mtrl-sci↗

arXiv2000

Bulk and Lattice Properties for Rigid Carbon Nanotubes Materials

We use an atom-atom potential between carbon atoms to obtain an interaction potential between nanotubes (assumed rigid), thereby calculating the cohesive energy of a bunch of nanotubes in hexagonal two dimensional packing. The model proposed is quite similar to our earlier work on fullerenes and organic molecular crystals. The results for inter-nanotube distances, energy per unit length, bulk modulus and phonons for inter-nanotube vibrations are obtained and compared with available data from measurements and other available calculations. We also model formation of multi-wall nanotubes. We find the results for various calculated quantities agreeing very well with measured structural parameters and other calculations. The reversible energy stored on compression of the bunch of nanotubes on application of pressure up to 30 Kbar calculated in this rigid molecule model is overestimated by about 30% when compared with measured results, signifying the appreciable flexibility of tubes at high pressures. The model is considered very suitable for incorporating flexible nanotubes in bunches of single and multi-wall nanotube materials of various types.

cond-mat.mtrl-sci↗

arXiv2000

Orientational Ordering and Binding in Alkali doped C60 solids

The binding energy of A3C60, a conductor, is described well by an ionic solid type calculation. This succeeds because there is little overlap between molecular wave functions on neighbouring sites, so that electrons are practically localized on-shell. This leads one to believe that even in A4C60 and A6C60 systems such calculation may suffice. However, for large charge on the anion, there is a possibility for some electrons to delocalize and go into the s-band. We calculate binding energy, keeping these delocalised electrons x, as a parameter and minimize the energy w.r.t. it. We take the intermolecular interaction to be arising out of a C-C potential of 6-exp form and a screened Coulomb interaction between the anions and cations and among themselves. Model calculations are presented for K1C60, K3C60, K4C60 and K6C60 for which the minimum energy state shows no delocalisation. Cohesive Energy dependence on Lattice constant is used to calculate Bulk Modulus for all systems. We have got a reasonably good resemblance with experimental values. Further, we observe that the cohesive energy shows poor resemblance with experimental values. Further, delocalisation of a fraction of electron at the centre of double bond show considerable increase in cohesive energy.

cond-mat.mtrl-sci↗

arXiv2000

Anisotropic Compression in Dimer C60 Fullerene Solid

Dimer C60 solids are monoclinic crystals with molecular longer axis aligned along the c-axis. A simple model based on atom-atom potential used to calculate various bulk, structural and thermodynamic properties of this fullerite, has also been used to investigate the pressure and temperature dependent properties such as bulk modulus, lattice and orientational structure, phonon dispersion relations, Gruneisen parameters, heat capacity and entropy. An important feature of this solid evidently concerns its anisotropic pressure compression. We calculate these directional compressibilities and find the anisotropy for pressure compression very significant. It is expected that the properties of dimer solid C60 calculated here will motivate further experimentation on this solid.

cond-mat.mtrl-sci↗

arXiv2000

Dynamics of a Bunch of Carbon Nano-tubes

Nano-diameter tubules made of folded graphitic sheets have drawn the attention of physicists ever since the advent of fullerene research. These are interesting because of their conductivity (depends upon chirality) as well as their mechanical properties. Bunches of such tubes, single-walled and each of same diameter (~14A), have been produced in the laboratory. In this work we explore the mechanical properties of the bunch (hexagonal packing), having assumed, for simplicity, uniform continuous distribution of carbon atoms on each tube. The atom-atom potential (6-exp) is of Kitaigorodsky type where only inter-tube interactions are considered. Intra-tube interactions are not considered (implying a rigid tube model). This approximation works very well for pure C60 solid. We find that the range of external modes is not well separated from the radial mode frequency of the single tube as indicated by measured data. Therefore interpretation of experimentally observed frequencies needs to be done in terms of total modes of the bunch, including internal as well as external modes. We have also studied pressure dependent deformations of such a bunch which is found to be very interesting and is likely to provide information about their mechanical strength.

cond-mat.mtrl-sci↗

arXiv2000

Theoretical Model Upper Limit on The Lattice Parameter of Doped C60 Solid

The results on the lattice instability, invoked by applying a negative pressure on pure C60 solid, are described. The results of such a calculation are used to correlate the experimentally achieved results on the superconducting transition temperature Tc by doping C60 with alkali metals. A simple model had been used already to interpret the bulk, structure and thermodynamic properties Of C60 solids. As a result of this calculation an upper limit on the lattice parameter results which is around 14.5A and is a suggestion of an upper limit of Tc as well in doped C60.

cond-mat.mtrl-sci↗