Precursor Concentration Ratio: The Key to Controllable Lateral-to-Standing MoO2 Flake Transition

Transition-metal dichalcogenide (TMD) materials with a standing orientation on the substrate have attracted much attention due to enhanced electronic and optical properties. Although growing standing metal oxide flakes is a well-adopted first step in growing standing TMD materials, the orientation control of the metal oxide and, most importantly, the mechanism for orientation transition are far from well understood. In this paper, we present a method of tuning the orientation of chemical vapor deposition-grown MoO2 crystals by controlling the concentration ratio between S and Mo precursors (S/Mo). Through delicately tuning S/Mo, we find that a low S/Mo favors lateral MoO2 flakes, whereas a high S/Mo favors standing MoO2 flakes. Results also consistently show that such a S/Mo-mediated orientation transition is widely applicable to different substrates and across a range of growth temperatures. We further present a model of the S/Mo-mediated orientation transition based on the change of the chemical reaction mechanism, namely, heterogeneous (on the substrate) and homogeneous (in vapor) reactions. Modulating the reaction mechanism provides an effective and highly realizable means for the orientation engineering of low-dimensional materials.

Automated summary

This paper presents a method for tuning the orientation of MoO2 crystals grown via chemical vapor deposition by controlling the concentration ratio of S and Mo precursors (S/Mo). It was found that a low S/Mo ratio favors the growth of lateral MoO2 flakes, while a high S/Mo ratio favors standing MoO2 flakes. The results show that this S/Mo-mediated orientation transition is widely applicable to different substrates and growth temperatures.