Layer thickness dependent band gap of MBE grown single- to few-layer MoS$_{2}$

TL;DR

This study investigates how the band gap of MoS₂ varies with layer thickness, using experimental and theoretical methods, revealing a significant decrease in band gap with increasing layers and effects of growth conditions.

Contribution

It provides new experimental data on layer-dependent band gaps of MoS₂ grown by MBE and compares it with advanced theoretical calculations to understand screening effects.

Findings

Band gap decreases significantly with increasing layer number.

Pinning of the conduction band vanishes above 4 layers.

Additional screening effects are introduced by growth conditions.

Abstract

In light of the rise of transition metal dichalcogenides as 2D semiconductors for device applications, band engineering becomes very important from an application point of view. In many of these materials, such as the canonical example of MoS$_{2}$, the semiconductor band gap depends on the layer number. It changes from indirect to direct as it evolves from a bulk semiconductor to a monolayer. Interestingly, it was predicted and experimentally confirmed that, by thinning the material from bulk to a bilayer, the indirect transition shows a strong blue-shift. Here, we present the results of scanning tunnelling spectroscopy measurements on MoS$_{2}$ that has been grown \textit{in situ} via molecular beam epitaxy on graphene on Ir(111) at thicknesses ranging from 1 to 5 layers. We find a drastic decrease of the band gap with increasing layer number, to values even below the band gap in bulk. We also observe that the pinning of the conduction band vanishes above 4 layers. Comparing our experimental data with density functional theory and \textit{GW} calculations indicates that an additional screening is introduced by the sample growth conditions.