Title:
Response of Transition Metal nanotubes and their Janus variants to mechanical deformations: an ab initio study
Response of Transition Metal nanotubes and their Janus variants to mechanical deformations: an ab initio study
Author(s)
Bhardwaj, Arpit
Advisor(s)
Suryanarayana, Phanish
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Abstract
In the past three decades, the importance of nanotubes has significantly increased since
the synthesis of carbon nanotubes. Among them, transition metal nanotubes, such as transition
metal dichalcogenide (TMD) nanotubes, have gained attention due to their unique
properties, including high tensile strength and mechanically tunable electronic properties,
which make them ideal candidates for various applications such as reinforcement in
nanocomposites, mechanical sensors, nanoelectromechanical (NEMS) devices, and biosensors.
However, despite their potential, TMD nanotubes have not been thoroughly investigated
for their elastic properties and electromechanical response, particularly concerning
torsional deformations, using first-principles calculations. This is primarily due to the limitations
imposed by standard periodic conditions, which require a large number of atoms.
TMD nanotubes are generally multi-walled with large diameters because of the relatively
high energies required to bend their 2D material analogs. To address this issue,
we introduce asymmetry in TMD nanotubes and form Janus TMD nanotubes, which are
expected to exhibit unique and fascinating properties typically associated with quantum
confinement effects. Moreover, Janus TMD nanotubes can form small single-walled nanotubes,
thereby providing additional opportunities for their potential applications. Another
promising class of transition metal nanotubes is transition metal dihalides (TMH), which
have not yet been synthesized. However, due to the fascinating usage of their 2D analogs
in piezoelectric-ferromagnetic, and ferrovalley materials, it is anticipated that TMH nanotubes
will exhibit advantageous features similar to those of their 2D counterparts.
In this thesis, we employ symmetry-adapted DFT simulations to calculate the elastic
properties of TMD and Janus TMD nanotubes, including Young’s modulus, Poisson’s ratio,
and torsional modulus. Additionally, we investigate the electromechanical response
of TMD nanotubes to torsional deformations and explore the behavior of Janus TMD and
TMH nanotubes under axial and torsional deformations. Furthermore, we investigate the effect of spin-orbit coupling on mechanically deformed TMD and Janus TMD nanotubes
and observe Zeeman and Rashba spin-splitting, which are highly relevant for spintronics
applications. Overall, our research provides valuable insights into the mechanical and electronic
properties of these nanotubes, which could lead to their potential applications in a
wide range of fields, such as electronics, spintronics, and sensors.
Our calculations reveal that the Young’s and torsional moduli of TMD nanotubes follow
the trend MS2 > MSe2 > MTe2, while for Janus TMD nanotubes, the trend is MSSe >
MSTe > MSeTe. Furthermore, TMD nanotubes are isotropic, while Janus TMD nanotubes
are anisotropic, with the ordering being MSTe > MSeTe > MSSe. We also observe that
strain engineering has little to no effect on metallic nanotubes, while it generally reduces
the bandgap of semiconducting nanotubes, leading to semiconductor-to-metal transitions.
This reduction in bandgap is typically observed to be linear with axial strain and quadratic
with shear strain. Moreover, it results in a decrease in the effective mass of holes and
an increase in the effective mass of electrons, leading to transitions from n-type to p-type
semiconductors.
The TMD and Janus TMD nanotubes exhibit inversion symmetry, which leads to the
absence of Rashba spin-splitting without any mechanical deformations. However, the introduction
of twist in these nanotubes breaks the symmetry and induces Rashba spin-splitting,
with relatively high values of the Rashba coefficient. We also investigate the Zeeman spinsplitting
in these nanotubes under axial and shear strain. Our results reveal that the splitting
values at the VBM (Valence Band Maximum) and CBM (Conduction Band Minimum) levels
decrease monotonically, and in most cases of VBM with axial strain, it reaches 0. This
is a crucial finding as the maximum splitting value at VBM is significant, reaching 0.46 eV
in the WSe2 nanotube before becoming zero with axial strain.
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Date Issued
2023-04-25
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Dissertation