Electronic band gaps and exciton binding energies in monolayerMoxW1−xS2transition metal dichalc

Abstract

Using scanning tunneling spectroscopy (STS) and optical reflectance contrast measurements, we examine band-gap properties of single layers of transition metal dichalcogenide (TMDC) alloys: MoS 2 , Mo 0.5 W 0.5 S 2 , Mo 0.25 W 0.75 S 2 , Mo 0.1 W 0.9 S 2 , and WS 2 . The quasi-particle band gap, spin-orbit separation of the excitonic transitions at the K/K' point in the Brillouin zone, and binding energies of the A exciton are extracted from STS and optical data.The exciton binding energies change roughly linearly with tungsten concentration. For our samples on an insulating substrate, we report quasi-particle band gaps from 2.17 ± 0.04 eV (MoS 2 ) to 2.38 ± 0.06 eV (WS 2 ), with A exciton binding energies ranging from 310 to 420 meV.

Colloquium : Excitons in atomically thin transition metal dichalcogenides

“…3a). The reported values, as summarized in the Table II, range from 0.22 eV for MoS 2 (Zhang et al, 2014a) to 0.55 eV for MoSe 2 (Ugeda et al, 2014); further reports include (Bradley et al, 2015;Chiu et al, 2015;Rigosi et al, 2016;Zhang et al, 2015a). The differences can be related to (i) the overall precision in extracting the onsets of the tunneling current and (ii) to the use of different conducting substrates required for the STS, i.e., the influence of different dielectric environments and related proximity effects.…”

Excitonic structure of the optical conductivity in MoS2 monolayers

“…The optical conductivity, absorption, and reflectance spectra of few and single-layer MoS 2 has been extensively studied in recent experiments 12,[23][24][25][26][27][28][29][30][31][32][33] .…”

“…It is well established that the onset of optical absorption in clean, undoped monolayers occurs at 1.8 ± 0.1 eV, as measured spectroscopically by reflection and photoluminescence 12,23,24,27,31,32 , absorption [26][27][28]33 , photoemission 29 , and second-harmonic analysis 35,38,39 .…”

Comprehensive tunneling spectroscopy of quasifreestanding MoS2 on graphene on Ir(111)

“…This leads to the predicted freestanding bandgap of E g ≈ 2.8 eV [13][14][15][16] being considerably reduced. For example the bandgap measured by constant height STS is E g = 1.74 eV on an Au substrate [17], 2.01 eV on graphene/SiC [18], 2.17 eV on quartz [19], 2.20 eV on graphene/Au [20], and variously 1.9 eV [21], * murray@ph2.uni-koeln.de 2.15 eV [22] or 2.40 eV [23] on graphite. In addition to simply reducing the bandgap size, substrate coupling will affect each band differently -due to the differing planar nature of the Mo and S orbitals, the band structure is distorted inhomogeneously across the MoS 2 Brillouin zone (BZ) [17].…”