Interface-Confined Doubly Anisotropic Oxidation of Two-Dimensional MoS₂

Abstract

Despite their importance, chemical reactions confined in a low dimensional space are elusive and experimentally intractable. In this work, we report doubly anisotropic, in-plane and out-of-plane, oxidation reactions of two-dimensional crystals, by resolving interface-confined thermal oxidation of a single and multilayer MoS supported on silica substrates from their conventional surface reaction. Using optical second-harmonic generation spectroscopy of artificially stacked multilayers, we directly proved that crystallographically oriented triangular oxides (TOs) were formed in the bottommost layer while triangular etch pits (TEs) were generated in the topmost layer and that both structures were terminated with zigzag edges. The formation of the Mo oxide layer at the interface demonstrates that O diffuses efficiently through the van der Waals (vdW) gap but not MoO, which would otherwise sublime. The fact that TOs are several times larger than TEs indicates that oxidation is greatly enhanced when MoS is in direct contact with silica substrates, which suggests a catalytic effect. This study indicates that the vdW-bonded interfaces are essentially open to mass transport and can serve as a model system for investigating chemistry in low dimensional spaces.

Crossover between Photochemical and Photothermal Oxidations of Atomically Thin Magnetic Semiconductor CrPS4

“…4d), which is consistent with the negligible reaction in Fig. 4d considering that the threshold temperature for thermal oxidation is above 300 o C. [36][37][38] When the average power was raised to 11 mW for which i σ reached its maximum (Fig. 4c), opt T reached 320 o C where thermal oxidation readily occurs.…”

Direct measurements of interfacial adhesion in 2D materials and van der Waals heterostructures in ambient air

“…This is consistent with the low-temperature surface oxidation of MoS 2 , which is initiated at ~100 °C and significantly increases at 300 °C 26 , resulting in the negative impact of oxygen adsorption on the mobility and homogeneity of MoS 2 /G heterostructure devices after annealing above 150 °C 27 . We hypothesize two possible interfacial oxidation mechanisms responsible for the weaker interaction of MoS 2 /hBN(G) at 300 °C: (1) replacement of sulfur atoms with oxygen atoms results in a lower surface energy in the oxidized MoS 2 (MoO 3 ) than unreacted MoS 2 ; (2) partial protrusions (0.36 ± 0.25 nm 28 ) at the interface due to formation of interfacial MoO 3 patches along with the presence of gaseous reaction products (e.g., SO 2 , which cannot diffuse out of interface owing to very high vdW pressure on the trapped molecular layers 29 ) can give rise to local interlayer decoupling of unreacted MoS 2 crystal from underlying hBN and G substrates.…”

Surface-Bound and Volatile Mo Oxides Produced During Oxidation of Single MoS2 Crystals in Air and High Relative Humidity

“…To investigate the presence of Mo oxides on the MoS 2 crystals, we prepared single 2H MoS 2 crystals on silicon/silica substrates, as explained in the Materials and Methods. We have studied flakes thicker than 10 nm, i.e., of more than 15 MoS 2 monolayers due to known dependencies of several physico-chemical properties of the MoS 2 flakes on their thickness, particularly for the flakes thinner than 10 monolayers [ 27 ]. First, the MoS 2 flakes were localized using light microscopy and AFM; see Figure 1 .…”