Effect of Adventitious Carbon on Pit Formation of Monolayer MoS₂

Abstract

Forming pits on molybdenum disulfide (MoS2) monolayers is desirable for (opto)electrical, catalytic, and biological applications. Thermal oxidation is a potentially scalable method to generate pits on monolayer MoS2, and pits are assumed to preferentially form around undercoordinated sites, such as sulfur vacancies. However, studies on thermal oxidation of MoS2 monolayers have not considered the effect of adventitious carbon (C) that is ubiquitous and interacts with oxygen at elevated temperatures. Herein, the effect of adventitious C on the pit formation on MoS2 monolayers during thermal oxidation is studied. The in situ environmental transmission electron microscopy measurements herein show that pit formation is preferentially initiated at the interface between adventitious C nanoparticles and MoS2, rather than only sulfur vacancies. Density functional theory (DFT) calculations reveal that the C/MoS2 interface favors the sequential adsorption of oxygen atoms with facile kinetics. These results illustrate the important role of adventitious C on pit formation on monolayer MoS2.

Direct Identification of Surface Bound MoO3 on Single MoS2 Flakes Heated in Dry and Humid Air

“…It has been already observed that adventitious carbon cannot be avoided for the MoS 2 samples handled and manipulated in air. [ 53 ] To illustrate the carbon interferences further, the S/Mo ratio drops to 1.2 on the oxidized portion of the flake, where indeed substantial amount of the MoO x is present, but at the same time adventitious carbon content is ≈65%. An expected S/Mo ratio is 2, but since Auger technique is particularly sensitive for lighter elements, such a heavy carbon content effectively masks the presence of the MoS 2 surface buried underneath the MoO x layer and the carbon layer.…”

Operando Study of Thermal Oxidation of Monolayer MoS2

“…Past studies on the thermal oxidation of MoS 2 reported that the oxidation process is fast when the temperature is above ≈250 °C (e.g., 250 °C in the air for 1 h, [ 12 ] 360 °C in the air for 5 min, [ 13 ] and 380 °C in the air for 10 min; Table S1, Supporting Information). [ 10 ] These findings were supported by ex situ observations of pits and crack formation, and identification of molybdenum oxides via atomic force microscopy (AFM), [ 12–16 ] scanning electron microscopy (SEM), [ 13,15,17 ] transmission electron microscopy (TEM), [ 17 ] X‐ray photoelectron spectroscopy (XPS), [ 12,16,17 ] nanomechanical means, [ 16 ] and Raman spectroscopy. [ 14 ] Such ex situ characterization methods, although very informative, may not detect subtle morphological and chemical compositional changes brought on by MoS 2 oxidation.…”

Strategies to improve electrocatalytic performance of MoS2-based catalysts for hydrogen evolution reactions

“…Forming pits on MoS 2 monolayers has been found to be desirable for promoting HER performance. Park et al 170 investigated the effect of adventitious C on the pit formation in the MoS 2 layers. They used an aperture type in situ ETEM to study the reaction between MoS 2 monolayers and O 2 .…”