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Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
| ABSTRACT - Internet Electron. J. Mol. Des. May 2005, Volume 4, Number 5, 316-328 |
Computational Studies of 1T and 2H TaS2 in Crystalline
and Nanotubular Forms: Structural and Electronic Properties
Andrew N. Enyashin, Igor R. Shein, Nadezhda I. Medvedeva, and Alexander L. Ivanovskii
Internet Electron. J. Mol. Des. 2005, 4, 316-328
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Abstract:
Since the discovery of carbon nanotubes in 1991, inorganic
nanotubes (NTs) have received much attention. The layered
chalcogenides, having structures analogous to graphite, are known to
be unstable toward bending and show high propensity to form
curved structures. Recently, apart from carbon nanotubes, nanotubes
of the layered d-metal disulfides (MS2, M = Mo, W, Nb, Ta, Ti, Zr)
have been synthesized. However the information on the structural
and electronic properties of curved nanostructures based on tantalum
disulfide is lacking to date. On the other hand, the study of TaS2 bulk
materials has been actively pursued during the last years owing their
interesting properties and potential applications. One of the most
important and intriguing problem that attracts the attention of
researchers is the modification of properties of inorganic systems
going from bulk to nanoscale state. In the present paper the
electronic band structure of 1T and 2H polytypes of layered tantalum
disulfide in crystalline and nanotubular forms has been studied by
means of band structure approaches. For the first time we discuss the
changes in electronic properties of TaS2 nanotubes depending from
the atomic arrangement in tube walls (octahedral or trigonal
prismatic) and from the tube diameters and geometry (zigzag or
armchair-like). The electronic band structures, densities of states,
electron density maps, bonding indices - crystal orbital overlap
populations and total band energies of 1T and 2H polytypes of TaS2
bulk and nanotubes have been obtained using the ab initio full-potential
linear muffin tin orbitals (FPLMTO) and the semi-empirical tight-binding
EHT band structure approaches. The energy
bands for 1T and 2H polytypes of crystalline TaS2 are obtained;
using of the total energy calculations, we found that there is a gain in
energy of 0.231 eV/formula unit for 2H structure, reflecting the fact
that the ideal 1T phase is unstable at low temperatures. The DOS at
the Fermi energy N(EF) is controlled by the overlap between the S p
and Ta d states. This overlap results in a metallic state for TaS2. The
DOS curves of 2H and 1T polytypes obtained by the semi-empirical
EHT calculations are in reasonable agreement with ab initio FLMTO
data. The atomic models of 1T and 2H TaS2 nanotubes have been
constructed. The electronic structure and bond indices of nanotubes
have been calculated and analyzed as a function of the tubes
diameters (D) in the armchair- and zigzag-like forms. Within the
range of diameters examined, the zigzag configurations of 1T and 2H
like tubes appear to be the most stable than armchair configurations.
All NTs are metallic-like and their electronic spectra are similar to
the DOS of the corresponding planar layers. The Fermi level is
located in the region of the non-bonding Ta d band. For the
armchair-like 2H TaS2 NTs there is a pronounced DOS peak at the
Fermi level. Since the this sharp DOS peak for such NTs is half-filled
with high N(EF), one would expect that the some 2H TaS2 NTs
may be superconducting, as it has been obtained recently for NbSe2
nanotubes (2003). The electronic properties and chemical bonding of
2H and 1T polytypes of layered tantalum disulfide in crystalline and
nanotubular forms have been studied with the ab initio FPLMTO
and the semi-empirical tight-binding EHT approaches. We show that
both zigzag- and armchair-like nanotubes are metallic-like, and the
tube stability trends to vanish for very small NT diameters. Zigzag-like
nanotubes were found to be more stable. It was established that
Ta-S covalent bonds are the strongest interactions in TaS2 bulk and
NTs, whereas Ta-Ta bonds are much weaker.
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