Indirect-to-direct band gap crossover of single walled MoS$_2$ nanotubes

TL;DR

This study uses density functional theory to show that single-walled MoS₂ nanotubes transition from indirect to direct band gaps as their diameter increases, affecting their optical properties.

Contribution

It reveals the diameter-dependent electronic structure crossover in MoS₂ nanotubes, linking nanotube geometry to band gap nature and potential photoluminescence behavior.

Findings

Small diameter NTs are indirect gap semiconductors.

Large diameter NTs become direct gap semiconductors.

Band gap crossover is due to strain-induced shifts in the valence band.

Abstract

Using density functional theory, the electronic structures of single walled molybdenum disulfide nanotubes (MoS$_2$ NTs) were investigated as a function of diameter. Our calculations show that the electronic structure near the band gap is sensitive to the NT diameter: armchair MoS$_2$ NTs act as indirect gap semiconductors for diameters up to approximately 5.0 nm, while armchair MoS$_2$ NTs with larger diameters act as direct gap semiconductors with band edges located in the vicinity of $k = 2\pi/3$. This finding implies that MoS$_2$ NTs with large diameters should exhibit similar photoluminescence to 2D monolayer MoS$_2$ sheets. This indirect-to-direct band gap crossover is ascribed to the upward shift of the valence band peak at the $\Gamma$ point in small diameter NTs, which is caused by the tensile strain resulting from their tubular structures.