Electronic Structure of a Quasi-Freestanding MoS₂ Monolayer

Abstract

Several transition-metal dichalcogenides exhibit a striking crossover from indirect to direct band gap semiconductors as they are thinned down to a single monolayer. Here, we demonstrate how an electronic structure characteristic of the isolated monolayer can be created at the surface of a bulk MoS2 crystal. This is achieved by intercalating potassium in the interlayer van der Waals gap, expanding its size while simultaneously doping electrons into the conduction band. Our angle-resolved photoemission measurements reveal resulting electron pockets centered at the K̅ and K' points of the Brillouin zone, providing the first momentum-resolved measurements of how the conduction band dispersions evolve to yield an approximately direct band gap of ∼1.8 eV in quasi-freestanding monolayer MoS2. As well as validating previous theoretical proposals, this establishes a novel methodology for manipulating electronic structure in transition-metal dichalcogenides, opening a new route for the generation of large-area quasi-freestanding monolayers for future fundamental study and use in practical applications.

Spin-resolved photoemission study of epitaxially grown MoSe2and WSe2thin films

“…The size of these splitting is much larger in WSe 2 ∼ 470 meV compared to that in MoSe 2 ∼ 180 meV, due to the stronger SOC induced by heavier element W. Third, the band maximum at the Γ-point locates at the higher binding energy than that at the K-point, leaving the valence band maximum at the K-point. Combining this with the conduction band minimum being at the K-point in the unoccupied state [20,21,26], the 1ML MoSe 2 and WSe 2 are direct band gap semiconductors.…”

“…There have been a number of ARPES studies either on bulk MX 2 samples [26,27,28] or a few layer samples [20,21,29,30] prepared by various methods such as exfoliation, chemical vapor deposition (CVD), and epitaxial growth. They all observe sizable splitting in VB at the K-point, with size ∼ 170 meV for MoS 2 and MoSe 2 and ∼ 450 meV for WS 2 and WSe 2 as W induces larger SOC.…”

Graphene versus MoS2: A short review

“…Electrons in SLMoS 2 are normal fermions with parabolic energy dispersion, and SLMoS 2 is a semiconductor with a direct band gap above 1.8 eV [174][175][176][177]. This finite band gap endorses SLMoS 2 for transistor applications [178,179].…”

“…On the other hand, SLMoS 2 has a direct band gap of about 1.8 eV [174,175]. This optical-range band gap leads to a high absorption coefficient for incident light, so SLMoS 2 has very high sensitivity in photon detection [182].…”

Phonon and Raman scattering of two-dimensional transition metal dichalcogenides from monolayer, multilayer to bulk material

“…8,9,26 This can be probed by a variety of spectroscopic tools. 8,9,[26][27][28][29][30][31] For example, as an indirect-gap material, bandgap PL in bulk 2H-MoS 2 is very weak because it is a phonon-assisted process and known to have negligible quantum yield. Appreciable PL is observed in FL-MoS 2 and surprisingly bright PL is detected in 1L-MoS 2 , which is indicative of it being a direct-gap semiconductor.…”