On-stack two-dimensional conversion of MoS ₂ into MoO ₃

Abstract

Chemical transformation of existing two-dimensional (2D) materials can be crucial in further expanding the 2D crystal palette required to realize various functional heterostructures. In this work, we demonstrate a 2D 'on-stack' chemical conversion of single-layer crystalline MoS 2 into MoO 3 with a precise layer control that enables truly 2D MoO 3 and MoO 3 /MoS 2 heterostructures. To minimize perturbation of the 2D morphology, a nonthermal oxidation using O 2 plasma was employed. The early stage of the reaction was characterized by a defect-induced Raman peak, drastic quenching of photoluminescence (PL) signals and sub-nm protrusions in atomic force microscopy images. As the reaction proceeded from the uppermost layer to the buried layers, PL and optical second harmonic generation signals showed characteristic modulations revealing a layer-by-layer conversion. The plasma-generated 2D oxides, confirmed as MoO 3 by x-ray photoelectron spectroscopy, were found to be amorphous but extremely flat with a surface roughness of 0.18 nm, comparable to that of 1L MoS 2 . The rate of oxidation quantified by Raman spectroscopy decreased very rapidly for buried sulfide layers due to protection by the surface 2D oxides, exhibiting a pseudo-self-limiting behavior. As exemplified in this work, various on-stack chemical transformations can be applied to other 2D materials in forming otherwise unobtainable materials and complex heterostructures, thus expanding the palette of 2D material building blocks.

Is Single Layer MoS2 Stable in the Air?

“…This is the reason for the strong buckling of the Mo−O group at the edge (Figure ). Experimentally, it seems that the misfit between MoS 2 and MoO 3 makes the MoO 3 layer amorphous, and may cause the experimentally observed cracking …”

“…However, the local order at the oxidised edge is probably similar to the fully oxidised structure depicted in Figure . X‐ray photoemission spectroscopy experiments on oxidised MoS 2 revealed a shift of the Mo 3p and Mo 3d states towards higher binding energies, suggesting that the samples contained MoO 3 . To support this hypothesis, we used the Slater–Janak (half‐core hole) approach to calculate the core level shifts for Mo atoms in the fully oxidised nanostripe (Figure ).…”

“…Ko et al. studied the chemical transformation of crystalline MoS 2 into MoO 3 using an O 2 plasma, and observed that oxidation started at MoS 2 edge sites and then progressed layer‐by‐layer, but was much slower in inner layers protected by outer oxides. This passivation by surface oxides may stabilise bulk MoS 2 against further oxidation and explain the generally accepted view that MoS 2 is air‐stable.…”

“…Nevertheless, the experiments reported by Ko et al. indicate that plasma‐generated reactive oxygen species create sub‐oxide defects (MoS x O 1− x ) on the basal plane of MoS 2 prior to full oxidative conversion into MoO 3 , indicating that substitution is the primary oxidative process. Fleischauer and Lince examined the effect of oxidation on sputtered MoS 2 films, and found that the majority of the oxygen in the samples was not removed by annealing, indicating that most of it was tightly bonded within the film.…”

Structural and Optical Properties of Single- and Few-Layer Magnetic Semiconductor CrPS4

“…The home-built setup for the micro-Raman and PL measurements has been described elsewhere. 63,64 Briefly, the output of visible lasers (two solid state lasers for 457.9 and 514.3 nm;…”