Bottom-up synthesis of vertically oriented two-dimensional materials

Understanding nucleation and growth of two-dimensional (2D) and layered materials is a challenging topic due to the complex van der Waals interactions between layers and substrate. The morphology of 2D materials is known vary depending on experimental conditions. For the case of MoS₂, the morphology has been shown to vary from rounded (molybdenum rich) domains to equilateral triangular (sulfur rich) domains. These different morphologies can result in drastically different properties, which can be exploited for applications in catalytic reactions, digital electronics, optoelectronics, and energy storage. Powder vaporization (PV) synthesis of molybdenum disulfide (MoS₂) can yield vertical domains, however, these domains are often ignored when the morphology evolution of MoS₂ is discussed, thereby completely omitting a major part of the impact of the Mo:S ratio to the growth mode of MoS₂ during PV. Combining experimental and numerical simulation methods, we reveal a vertical-to-horizontal growth mode transition for MoS₂ that occurs in the presence of a molybdenum oxide partial pressure gradient. Transmission electron microscopy reveals that the growth of vertical MoS₂ results from initial seeding of single crystalline molybdenum dioxide, followed by sulfurization from the substrate upward to form vertically oriented MoS₂ domains.