Flexoelectric radial polarization of single-walled nanotubes from first-principles

Abstract

Flexoelectricity is the polar response of an insulator to strain gradients such as bending. While the size dependence of this effect makes it weak in bulk systems in comparison to piezoelectricity, it suggests that it plays a bigger role in nanoscale systems such as thin films and nanotubes (NTs). In this paper we demonstrate using first-principles calculations that the walls of carbon nanotubes (CNTs) and transition metal dichalcogenide nanotubes (TMD NTs) are polarized in the radial direction, the strength of the polarization increasing as the size of the NT decreases. For CNTs and TMD NTs with chiral indices $(n,m)$, the radial polarization of the walls $P_R$ starts to diverge below $C(n,m)/a=\sqrt{n^2+nm+m^2}\sim 10$, where $C(n,m)$ is the circumference of the NT and $a$ is the lattice constant of the 2D monolayer. For CNTs, $P_R$ drops to zero above this value but for TMD NTs there is a non-zero polarization above this value, which is an ionic rather than electronic effect. The size dependence of $P_R$ in the TMD NTs is interesting: it increases gradually and reaches a maximum of $P_R\sim 100{\text{C/cm}}^2$ at $C(n,m)/a\sim 15$, then decreases until $C(n,m)/a\sim 10$ where it starts to diverge. Measurements of the radial strain on the chalcogen atoms with respect to the 2D monolayers shows that this polarization is the result of a significantly larger strain on the outer bonds than the inner bonds, but did not offer an explanation for the peculiar size dependence. These results suggest that while the walls of smaller CNTs and TMD NTs are polarized, the walls of larger TMD NTs are also polarized due to a difference in strain on the inner and outer atoms in the walls. This result may prove useful for the application of NTs for screening in liquid or biological systems.

7 pages, 5 figures

References in corpus (9)

• Computational 2D Materials Database: Electronic Structure of Transition-Metal Dichalcogenides and Oxides
• Strain gradient induced polarization in SrTiO3 single crystals
• Quantum Flexoelectricity in Low Dimensional Systems
• Towards a bulk theory of flexoelectricity
• Intrinsic bending flexoelectric constants in two-dimensional materials
• Nanoscale Smoothing and the Analysis of Interfacial Charge and Dipolar Densities
• From Layers to Nanotubes: Transition Metal Disulfides TMS2
• First-principles theory of spatial dispersion: Dynamical quadrupoles and flexoelectricity
• High flexoelectric constants in Janus transition-metal dichalcogenides

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• Curvature-Controlled Band Alignment Transitions in 1D van der Waals Heterostructures
• Theory of polarization textures in crystal supercells
• Electrically tunable stacking domains and ferroelectricity in moiré superlattices