Strong Variation of Electronic Properties of MoS₂ and WS₂ Nanotubes in the Presence of External Electric Fields

Transition-metal dichalcogenides attracted a huge international research focus from the point of two-dimensional materials. These materials exist also as nanotubes; however, they have been mostly studied for their lubricant properties. Despite their interesting electronic properties, quite similar to their 2D counterparts, nanotubes remain much less explored. Like in 2D materials, electronic properties of nanotubes can be strongly modulated by external means such as strain or electric field. Here, we report on the effect of external electric fields on the electronic properties of MoS ₂ and WS ₂ nanotubes using density functional theory. We show that the electric field induces a strong polarization in these nanotubes, which results in a nearly linear decrease of the band gaps with the field strength and eventually in a semiconductor-metal transition. In particular for large tube diameters, this transition can occur for field strengths between 1-2 V·nm ^-1 . This is an order of magnitude weaker than fields required to close the band gaps in the corresponding 2D mono- and bilayers of transition-metal dichalcogenides. We also observe splittings of the degenerate valence and conduction band states due to the Stark effect. Furthermore, we show that the work-function of these nanotubes can be modulated under the applied external electric field, which also depends on the chirality and the tube diameter. Finally, we examine the effect of external electric field on dielectric properties of these materials. The field strengths applied here for the calculation of RPA dielectric function do not significantly affect the optical absorption spectra. However, the armchair nanotubes are found to undergo a red-shift in absorption toward the visible range with the tube diameter and noticeable plasmon excitations compared to the zigzag ones. Accordingly, these findings provide a physical basis for potential applications of transition-metal dichalcogenide nanotubes in nano- and optoelectronics. As such, they could be used as logical switches, even at moderate field strengths that can be achieved experimentally, for example by applying a gate voltage.