Nevertheless, the measured bandgap sizes are significantly smaller than the accurate GW bandgap of 2.8 eV. As such, the measured electronic bandgap sizes have been in a wide range (1.39–2.16 eV), and the bandgap changes have been speculated to be introduced by the EDS 10, 15, 17 or carrier-induced bandgap renormalization 19 effect. In a ML MoS 2 phototransistor that has the Al 2O 3/MoS 2/SiO 2 stack structure, the electronic transport bandgap of the ML MoS 2 has been measured to be 1.8 eV, in which the optically excited excitons are separated to generate electron and hole carriers by applying the source and drain bias voltages 18. 17), which are larger than other measured values, but still significantly lower than the predicted GW value of 2.8 eV (refs 8, 9, 10, 11, 12). 16) and that on a bilayer graphene to be 2.16 eV (ref. In scanning tunneling spectroscopy (STS) measurements, the bandgap of a ML MoS 2 on graphite substrate has been measured to be 2.15 eV (ref. For a chemical vapor deposition (CVD) grown ML MoS 2 on a Au(111) substrate, the ARPES bandgap of about 1.39 eV has been measured, which is very small 15. With intercalated potassium (K) in a bulk MoS 2, a quasi ML MoS 2 has been fabricated from a bulk MoS 2, and a direct bandgap of 1.86 eV at the K valley has been measured using angle-resolved photo-emission spectroscopy (ARPES) 14. Eliminating the excitonic effect, the measurements of the electronic bandgap have given diverse results. Since the exciton binding energy is large, the measured optical bandgaps are not accurate representation of semiconductor bandgap, determined by the energy difference between valence and conduction band edges. Due to the strong exciton binding (~1 eV) 8, 9, 10, 11, 12, the optical bandgap has been obtained to be about 1.8 eV from the photoluminescence (PL) and optical absorption experiments 13, which agree with the theoretical Bethe-Salpeter-Equation (BSE) calculations 9, 10, 11, 12. Within the GW approximation, the electronic bandgap of a freestanding ML MoS 2 has been predicted to be about 2.8 eV (refs 8, 9, 10, 11, 12). freestanding 2D semiconductors), the strong unscreened Coulomb interaction (through the space outside of the 2D materials) makes the quasiparticle (QP) renormalization of electrons huge. Furthermore, in a spatially isolated low dimensional system (e.g. It is well established that the bandgap size of MoS 2 layers has a strong dependence on the number of layers. Nonetheless, an accurate evaluation of the bandgap in low dimensional semiconductors has not been as simple as in conventional bulk semiconductors. Furthermore, the exciton binding energies have also been reported to be affected by the EDS effect strongly in 2D semiconductors 6, 7.Ī moderate bandgap size is a determining characteristic property of a semiconductor. The EDS effect has also been reported to change the defect level with the band gap and induce deep- to shallow-level transition of dopants, enhancing the carrier concentrations significantly and the electrical conductivities 5. Without the top gate high-k dielectric, large reduction of the electron mobility has been reported 1, 2, 3, and it has been believed to be due to the environmental dielectric screening (EDS) effect suppressing the Coulomb scattering of carriers with charged impurities in the 2D semiconductors 2, 4. However, the superior properties have been achieved only with a supporting substrate and a gate dielectric in a top gate FET structure, such as the HfO 2/MoS 2/SiO 2 stack 1, 2. Monolayer (ML) molybdenum disulfide (MoS 2) has shown high electron mobility of about 217 cm 2 V −1 s −1 and an excessively high current on/off ratio of an order of 10 8 in a field effect transistor (FET) 1, 2. Atomically thin two-dimensional (2D) semiconductors have attracted a great deal of attention for their superior properties in electronic devices.
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