Layer Control of WSe₂ via Selective Surface Layer Oxidation

Abstract

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, N-layer WSe2 is reduced to N-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 WOx 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.

Effects of energetic ion irradiation on WSe2/SiC heterostructures

“…The ionization is responsible for the charge transfer at the WSe 2 /SiC interface, which causes the XPS core-level peak shift. Although a small positive shift of W 4 f peak to the order of ~0.15 eV can be induced by extensive oxidation 37, 38 and there could be oxidation below the detection limit of XPS, the relatively large W 4 f peak shift (~0.6 eV for a fluence of 10 16 protons/cm 2 and ~0.8 eV for a fluence of 10 17 protons/cm 2 ) observed in this work should be mostly attributed to the charge transfer at the WSe 2 interface. Based on SRIM/TRIM simulations, the electronic energy loss within the WSe 2 and SiC at the sample surface is approximately 56 eV/nm and 39 eV/nm, respectively.…”

Impact of Post‐Lithography Polymer Residue on the Electrical Characteristics of MoS2 and WSe2 Field Effect Transistors

“…Plasma cleaning can also be challenging. For example, O 2 plasma is found to oxidize edges/defects on graphene and the top layer of WSe 2 , and is used in etching and p‐type doping of TMDs . H 2 plasma can lift graphene off SiO 2 , and Cl 2 plasma can cause a heavy p‐doping to graphene .…”

Spectroscopic investigation of defects in two-dimensional materials

“…By defect engineering, the bandgap of graphene can be opened to allow switching of graphene-based transistors with a high on/off ratio [13]. The properties of TMDs can also be tailored by introduction of defects; for example, line defects can act as one-dimensional metallic stripes [28]; laser and ion irradiation can be utilized to thin and dope TMDs [29,30]; GBs influence the electroluminescence (EL) behavior of WS 2 [31]; structural defects or active edge sites can be applied in electrocatalysis [32]; and a strong photoluminescence (PL) enhancement of monolayer MoS 2 can be realized through defect engineering and oxygen bonding [33]. Hence, the investigation of defects is a crucial step for 2D materials research.…”