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arXiv 2005.02257cond-mat.mes-hallcond-mat.mtrl-sci
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单壁纳米管挠曲电径向极化的第一性原理研究

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

Daniel Bennett

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7 pages, 5 figures
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摘要：

挠曲电性是绝缘体对应变梯度（例如弯曲）的极性响应。虽然这种效应的尺寸依赖性使得它在体系统中比压电性弱，但它表明它在纳米级系统如薄膜和纳米管（NT）中起着更大的作用。在本文中，我们使用第一性原理计算证明了碳纳米管（CNT）和过渡金属二硫属元素纳米管（TMD NT）的壁在径向方向上被极化，极化强度随着NT尺寸的减小而增加。对于具有手性指数$(n,m)$的CNT和TMD NT，壁$P_R$的径向极化在低于$ C(n,m)/a = \sqrt{n^2 +nm + m^2} \sim 10$时开始发散，其中$C(n,m)$是NT的周长，$a$是2D单层的晶格常数。对于CNT，$P_R$在该值以上下降到零，但是对于TMD NT，在该值以上存在非零极化，这是离子效应而不是电子效应。TMD NT中$P_R$的大小依赖性是有趣的：它逐渐增加，并在$C(n,m)/a \sim 15$处达到最大值$P_R \sim 100 \ \text{C/cm}^2$，然后降低，直到$C(n,m)/a \sim 10$，在那里它开始发散。相对于2D单层的硫属原子上的径向应变的测量表明，这种极化是一个显着更大的应变的结果，比内部债券的外部债券，但没有提供一个解释的特殊的大小依赖性。这些结果表明，虽然较小的CNT和TMD NT的壁被极化，但较大的TMD NT的壁也被极化，这是由于壁中的内部和外部原子上的应变差异。这一结果可能被证明是有用的应用NTs筛选在液体或生物系统。

英文摘要：

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.

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