Understanding quasiparticle band structures of transition metal dichalcogenides (TMDs) is critical for technological advances of these materials for atomic layer electronics and photonics. Although theoretical calculations to date have shown qualitat
ively similar features, there exist subtle differences which can lead to important consequences in the device characteristics. For example, most calculations have shown that all single layer (SL) TMDs have direct band gaps, while some have shown that $SL-WSe_2$ have an indirect gap. Moreover, there are large variations in the reported quasiparticle gaps, corresponding to large variations in exciton binding energies. By using a comprehensive form of scanning tunneling spectroscopy, we have revealed detailed quasiparticle electronic structures in TMDs, including the quasi-particle gaps, critical point energy locations and their origins in the Brillouin Zones (BZs). We show that $SL-WSe_2$ actually has an indirect quasi-particle gap with the conduction band minimum located at the Q point (instead of K), albeit the two states are nearly degenerate. Its implications on optical properties are discussed. We have further observed rich quasi-particle electronic structures of TMDs as a function of atomic structures and spin-orbital couplings.
The emergence of transition metal dichalcogenides (TMDs) as 2D electronic materials has stimulated proposals of novel electronic and photonic devices based on TMD heterostructures. Here we report the determination of band offsets in TMD heterostructu
res by using microbeam X-ray photoelectron spectroscopy ({mu}-XPS) and scanning tunneling microscopy/spectroscopy (STM/S). We determine a type-II alignment between $textrm{MoS}_2$ and $textrm{WSe}_2$ with a valence band offset (VBO) value of 0.83 eV and a conduction band offset (CBO) of 0.76 eV. First-principles calculations show that in this heterostructure with dissimilar chalcogen atoms, the electronic structures of $textrm{WSe}_2$ and $textrm{MoS}_2$ are well retained in their respective layers due to a weak interlayer coupling. Moreover, a VBO of 0.94 eV is obtained from density functional theory (DFT), consistent with the experimental determination.
Using Scanning Tunneling Microscopy and Spectroscopy, we probe the electronic structures of single layer ${small MoS_2}$ on graphite. We show that the quasiparticle energy gap of single layer ${small MoS_2}$ is 2.15 $pm$ 0.07 eV at 77 K. Combining wi
th temperature dependent photoluminescence studies, we deduce an exciton binding energy of 0.22 $pm$ 0.1 eV, a value that is much lower than current theoretical predictions. Consistent with theoretical predictions we directly observed metallic edge states of single layer ${small MoS_2}$. In the bulk region of ${small MoS_2}$, the Fermi level is located at 1.8 eV above the valence band maximum, possibly due to the formation of a graphite/${small MoS_2}$ 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.
