Photoluminescence from Single-Walled MoS2 Nanotubes Coaxially Grown on Boron Nitride Nanotubes
Abstract
Single- and multi-walled molybdenum disulfide (MoS2) nanotubes have been coaxially grown on small diameter boron nitride nanotubes (BNNTs) which were synthesized from heteronanotubes by removing single-walled carbon nanotubes (SWCNTs), and systematically investigated by optical spectroscopy. The strong photoluminescence (PL) from single-walled MoS2 nanotubes supported by core BNNTs is observed in this work, which evidences a direct band gap structure for single-walled MoS2 nanotubes with around 6 - 7 nm in diameter. The observation is consistent with our DFT results that the single-walled MoS2 nanotube changes from an indirect-gap to a direct-gap semiconductor when the diameter of a nanotube is more than around 5 nm. On the other hand, when there are SWCNTs inside the heteronanotubes of BNNTs and MoS2 nanotubes, the PL signal is considerably quenched. The charge transfer and energy transfer between SWCNTs and single-walled MoS2 nanotubes were examined through characterizations by PL, XPS, and Raman spectroscopy. Unlike the single-walled MoS2 nanotubes, multi-walled MoS2 nanotubes do not emit light. Single- and multi-walled MoS2 nanotubes exhibit different Raman features in both resonant and non-resonant Raman spectra. The method of assembling heteronanotubes using BNNTs as templates provides an efficient approach for exploring the electronic and optical properties of other transition metal dichalcogenide nanotubes.