Atomically thin MoS2: A new direct-gap semiconductor

TL;DR

This paper investigates ultrathin MoS2 crystals, revealing a transition from indirect to direct band gap as the material becomes monolayer, resulting in significantly enhanced photoluminescence.

Contribution

It demonstrates that monolayer MoS2 is a direct-gap semiconductor with strong light emission, a novel finding for this material.

Findings

Monolayer MoS2 exhibits a direct band gap.

Luminescence quantum efficiency increases over 1000 times.

Quantum confinement shifts the band gap by over 0.6 eV.

Abstract

The electronic properties of ultrathin crystals of molybdenum disulfide consisting of N = 1, 2, ... 6 S-Mo-S monolayers have been investigated by optical spectroscopy. Through characterization by absorption, photoluminescence, and photoconductivity spectroscopy, we trace the effect of quantum confinement on the material's electronic structure. With decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by more than 0.6 eV. This leads to a crossover to a direct-gap material in the limit of the single monolayer. Unlike the bulk material, the MoS2 monolayer emits light strongly. The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 1000 compared with the bulk material.