Measurement of direct and indirect bandgaps in synthetic ultrathin MoS2 and WS2 films from photoconductivity spectra

Abstract

Exploring the thickness-dependent electronic properties of ultrathin transition metal dichalcogenides is crucial for novel optoelectronic devices. Particularly important is experimental information regarding the bandgap width. This information is scarce and often inconsistent among the several measurement techniques that were employed for this task, such as optical absorption, scanning tunneling spectroscopy and photoconductivity. Here, we present photoconductivity measurements in large-area synthetic MoS 2 and WS 2 films (1-5 monolayers and the bulk crystal) grown on insulating layers (SiO 2 , Al 2 O 3 or HfO 2 ). The excitonic peaks of MoS 2 and WS 2 were detected in both the photocapacitor and traditional in-plane geometries.Their contribution to the photoconductivity is explained by the electric field-assisted dissociation mechanism. We have separated the excitonic and free carrier components in the photocurrent spectra, and extracted the direct and indirect bandgaps using the Tauc plot, revealing their dependencies on the number of monolayers.

Controlled growth of 3D assemblies of edge enriched multilayer MoS2 nanosheets for dually selective NH3 and NO2 gas sensors

“…It is an n-type semiconductor with a band gap ranging from 1.2 eV for the bulk MoS 2 (indirect band gap) to 1.83 eV for the monolayer MoS 2 (direct and wider band gap). 17 Indeed, the bandgap of TMD materials depends on the number of their atomic layers, which provides different possibilities for their application in various fields such as optoelectronics, 18 solar cells, 19 photocatalysis and gas sensors. 20 It was reported that a high quality monolayer fit well with FET devices, whereas few layer TMDs are suitable for gas sensing applications.…”

Top-Gate Stack Engineering Featuring a High-κ Gadolinium Aluminate Interfacial Layer for Field-Effect Transistors Based on Two-Dimensional Transition-Metal Dichalcogenides

“…The reported band gaps of MoS 2 depend on the number of layers, deposition method, characterization techniques, etc. 10,55 We use a typical value (1.8 eV) of the band gap for 1L MoS 2 . 56,57 The conduction-band edge can be calculated knowing the band gap and valence-band edge.…”

“…The 10-nm-thick dielectrics used here is thick enough to screen the contribution from the underlying substrates. The reported band gaps of MoS 2 depend on the number of layers, deposition method, characterization techniques, etc. , We use a typical value (1.8 eV) of the band gap for 1L MoS 2 . , The conduction-band edge can be calculated knowing the band gap and valence-band edge. Figure b shows the band alignment extracted by XPS with core-level calibration to include the effect of the interface dipole , (Figure S5).…”

“…1 2D TMDs can preserve a decent mobility at a monolayer channel thickness of ∼0.7 nm, 2−6 while the mobility of the bulk semiconductors such as Si severely degrades below 5 nm due to the thickness fluctuation scattering. 7,8 In addition, the large band gaps 9,10 and good ambient stability 11−13 compared to other types of 2D materials make 2D TMDs promising candidates as future channel materials.…”

“…Reducing the semiconductor channel thickness in metal–oxide semiconductor field-effect transistors (MOSFETs) is critical to mitigate the short-channel effects for further device scaling . 2D TMDs can preserve a decent mobility at a monolayer channel thickness of ∼0.7 nm, − while the mobility of the bulk semiconductors such as Si severely degrades below 5 nm due to the thickness fluctuation scattering. , In addition, the large band gaps , and good ambient stability − compared to other types of 2D materials make 2D TMDs promising candidates as future channel materials.…”