Response of Transition Metal nanotubes and their Janus variants to mechanical deformations: an ab initio study

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Bhardwaj, Arpit
Suryanarayana, Phanish
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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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