Direct Imaging of Band Profile in Single Layer MoS₂ on Graphite: Quasiparticle Energy Gap, Metallic Edge States, and Edge Band Bending

Abstract

Abstract:Using Scanning Tunneling Microscopy and Spectroscopy, we probe the electronic structures of single layer MoS 2 on graphite. We show that the quasiparticle energy gap of single layer MoS 2 is 2.15 ± 0.06 eV at 77 K. Combining this with temperature dependent photoluminescence studies, we deduce an exciton binding energy of 0.22 ± 0.1 eV, a value that is lower than current theoretical predictions. Consistent with theoretical predictions, we directly observe the metallic edge states of single layer MoS 2 . In the bulk region of MoS 2 , the Fermi level is located at 1.8 eV above the valence band maximum, possibly due to the formation of a graphite/MoS 2 heterojunction. At the edge, however, we observe an upward band bending of 0.6 eV within a short depletion length of about 5 nm, analogous to the phenomena of Fermi level pinning of a 3D semiconductor by metallic surface states.

Edge and confinement effects allow in situ measurement of size and thickness of liquid-exfoliated nanosheets

“…This value is larger than would be expected for effects solely due to electronic edge states, suggesting more complex phenomena to be at work. However, it is worth noting that scanning tunnelling microscopy measurements on MoS 2 on graphite have shown a distinct edge region at least 5 nm thick 48 . Nevertheless, the effects described here may not be universal but perhaps specific to nanosheets surrounded by a weakly interacting environment (for example, surfactant micelle or solvent solvation shell).…”

Spectroscopic Signatures for Interlayer Coupling in MoS2–WSe2 van der Waals Stacking

“…In this context, it has been demonstrated by scanning tunneling microscopy (STM) that the majority of the adsorbates and residuals can be removed after thermal annealing at elevated temperatures. 28 Therefore, as-prepared MoS2/WSe2 samples were then annealed in an hydrogen/Ar environment (atmosphere pressure; H2:Ar = 1:4) at 300 o C for 4hr. The detailed fabrication process is described in the Experimental Methods.…”

“…According to our previous work on STS 28 and-XPS 48 , we know that the energy band gap for MoS2 and WSe2 are 2.34 eV and 2.46 eV, respectively, and the valence band offset is 0.42 eV. For consistency, we measure the PL spectrum at 77K to set the energy band alignment.…”

Probing Critical Point Energies of Transition Metal Dichalcogenides: Surprising Indirect Gap of Single Layer WSe2

“…Using angle resolved photoemission (ARPES), it is difficult to probe the conduction band structures 6,7 . In principle, scanning tunneling spectroscopy (STS) would be an ideal probe to determine both the valence and conduction band structures.However, the reported results have been controversial thus far, even for the determination of the quasi-particle band gaps [8][9][10] . As we will show, this is due primarily to the intriguing influence of the lateral momentum in the tunneling process, making certain critical points difficult to access in the conventional scanning tunneling spectroscopy acquired at a constant tip-to-sampledistance (Z).…”