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

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 <i>p</i>_<i>z</i> 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

Layer Control of WSe₂ <i>via</i> Selective Surface Layer Oxidation

We report Raman and photoluminescence spectra of mono- and few-layer WSe₂ and MoSe₂ taken before and after exposure to a remote oxygen plasma. For bilayer and trilayer WSe₂, we observe an increase in the photoluminescence intensity and a blue shift of the photoluminescence peak positions after oxygen plasma treatment. The photoluminescence spectra of trilayer WSe₂ exhibit features of a bilayer after oxygen plasma treatment. Bilayer WSe₂ exhibits features of a monolayer, and the photoluminescence of monolayer WSe₂ is completely absent after the oxygen plasma treatment. These changes are observed consistently in more than 20 flakes. The mechanism of the changes observed in the photoluminescence spectra of WSe₂ is due to the selective oxidation of the topmost layer. As a result, <i>N</i>-layer WSe₂ is reduced to <i>N</i>–1 layers. Raman spectra and AFM images taken from the WSe₂ flakes before and after the oxygen treatment corroborate these findings. Because of the low kinetic energy of the oxygen radicals in the remote oxygen plasma, the oxidation is self-limiting. By varying the process duration from 1 to 10 min, we confirmed that the oxidation will only affect the topmost layer of the WSe₂ flakes. X-ray photoelectron spectroscopy shows that the surface layer WO_<i>x</i> of the sample can be removed by a quick dip in KOH solution. Therefore, this technique provides a promising way of controlling the thickness of WSe₂ layer by layer