Oxidation of Monolayer WS₂ in Ambient Is a Photoinduced Process

Abstract

We have studied the ambient air oxidation of chemical vapor deposition (CVD) grown monolayers of the semiconducting transition metal dichalcogenide (S-TMD) WS2 using optical microscopy, laser scanning confocal microscopy (LSCM), photoluminescence (PL) spectroscopy, and atomic force microscopy (AFM). Monolayer WS2 exposed to ambient conditions in the presence of light (typical laboratory ambient light for weeks or typical PL spectroscopy map) exhibits damage due to oxidation which can be detected with the LSCM and AFM, though may not be evident in conventional optical microscopy due to poorer contrast and resolution. Additionally, this oxidation was not random and was correlated with “high-symmetry” high intensity edges and red-shifted areas in the PL spectroscopy map, areas thought to contain a higher concentration of sulfur vacancies. In contrast, samples kept in ambient and darkness showed no signs of oxidation for up to 10 months. Low-irradiance/fluence experiments showed that samples subjected to excitation energies at or above the trion excitation energy (532 nm/2.33 eV and 660 nm/1.88 eV) oxidized in as little as 7 days, even for irradiances and fluences 8 and 4 orders of magnitude lower (respectively) than previously reported. No significant oxidation was observed for 760 nm/1.63 eV light exposure, which lies below the trion excitation energy in WS2. The strong wavelength dependence and apparent lack of irradiance dependence suggests that ambient oxidation of WS2 is initiated by photon-mediated electronic band transitions, that is, photo-oxidation. These findings have important implications for prior, present, and future studies concerning S-TMDs measured, stored, or manipulated in ambient conditions.

Emerging Opportunities for 2D Semiconductor/Ferroelectric Transistor‐Structure Devices

“…[172,173] The 2D semiconductor layer itself could also attract environmental adatoms through its reaction with oxygen and water. [174][175][176] Consequently, the FeFET performance can be strongly deteriorated by the collective environment chemistry effects on 2D channel and ferroelectric gate. Hence, critical for future application of practical devices and further improvement of device performance and stability is to minimize the influence of device operating environment on 2D FeFETs.…”

WS2/MoS2 Heterostructures through Thermal Treatment of MoS2 Layers Electrostatically Functionalized with W3S4 Molecular Clusters

“…Curiously, the commercial 2H‐WS 2 used as a reference contains approximately the same oxide percentage. The explanation of this fact could be that WS 2 edges are spontaneously oxidized to a certain extent if manipulated in ambient conditions . The S 2p spectrum (Figure b) displays two well‐defined peaks at approximately 162.3 and 163.5 eV, which correspond to S 2− 2p 3/2 and 2p 1/2 components, respectively.…”

Controllable Water Vapor Assisted Chemical Vapor Transport Synthesis of WS2–MoS2 Heterostructure

“…[62][63][64][65] The enhanced PL on the edge of MoS2 might be related to the oxygen binding to defects on the edge after the sample is exposed to ambient environment. [66][67][68][69] Note the boundaries between WS2 and MoS2 also showed brighter response in both PL and Raman. This could be related to potential charge transfer at the heterojunction of monolayer WS2 and MoS2, 70 which needs to be further invesigated.…”