Indirect Band Gap Emission by Hot Electron Injection in Metal/MoS₂ and Metal/WSe₂ Heterojunctions

Abstract

Transition metal dichalcogenides (TMDCs), such as MoS2 and WSe2, are free of dangling bonds and therefore make more "ideal" Schottky junctions than bulk semiconductors, which produce Fermi energy pinning and recombination centers at the interface with bulk metals, inhibiting charge transfer. Here, we observe a more than 10× enhancement in the indirect band gap photoluminescence of transition metal dichalcogenides (TMDCs) deposited on various metals (e.g., Cu, Au, Ag), while the direct band gap emission remains unchanged. We believe the main mechanism of light emission arises from photoexcited hot electrons in the metal that are injected into the conduction band of MoS2 and WSe2 and subsequently recombine radiatively with minority holes in the TMDC. Since the conduction band at the K-point is 0.5 eV higher than at the Σ-point, a lower Schottky barrier exists for the Σ-point band, making electron injection more favorable. Also, the Σ band consists of the sulfur pz orbital, which overlaps more significantly with the electron wave functions in the metal. This enhancement in the indirect emission only occurs for thick flakes of MoS2 and WSe2 (≥100 nm) and is completely absent in monolayer and few-layer (∼10 nm) flakes. Here, the flake thickness must exceed the depletion width of the Schottky junction, in order for efficient radiative recombination to occur in the TMDC. The intensity of this indirect peak decreases at low temperatures, which is consistent with the hot electron injection model.

Pt‐Nanostrip‐Enabled Plasmonically Enhanced Broad Spectral Photodetection in Bilayer MoS2

“…The indirect bandgap of MoS 2 indicates that its Σ‐point is at a lower Schottky barrier height compared to the K‐point. Moreover, in the MoS 2 metal junctions sulfur atoms (S 3pz orbitals) can directly interact with the metal surface which favors the electron conduction into the Σ‐point and therefore the possibility arises of decaying K‐valley electrons to the Σ‐point . As the deposited Pt nanostrip can interact with the top Mo‐S layer there is a possibility of destabilization of the MoS 2 lattice.…”

Exciton physics and device application of two-dimensional transition metal dichalcogenide semiconductors

“…It has further been shown that the emission is tunable between neutral excitons and trions [38,113]. Although monolayers are usually preferred for electroluminescent devices because of their direct band gap, EL has also been observed in TMD multilayers by carrier redistribution from the indirect to the direct valleys in a high electric field [118], by indirect valley filling [119], or by the injection of hot carriers from a metal/TMD junction [120].…”

Optoelectronic Devices Based on Atomically Thin Transition Metal Dichalcogenides

“…Monolayer TMDs are usually used for EL-devices because of their direct band gap. Light emission from multi-layer TMDs was reported by carrier redistribution from the indirect to the direct valleys in a high electric field [64], by filling the indirect valleys in the conduction and valence bands in an electric-double-layer transistor [65], or by the injection of hot electrons from metal/TMD junction [66].…”