Synthesizing a material with the desired polymorphic phase in a chemical vapor deposition (CVD) process requires a delicate balance among various thermodynamic variables. Here, we present a methodology to synthesize rhombohedral (3R)-phase MoS2 in a well-defined sword-like geometry having lengths up to 120 μm, uniform width of 2-3 μm and thickness of 3-7 nm by controlling the carrier gas flow dynamics from continuous mode to pulsed mode during the CVD growth process. Characteristic signatures such as high degree of circular dichroism (∼58% at 100 K), distinct evolution of low-frequency Raman peaks and increasing intensity of second harmonic signals with increasing number of layers conclusively establish the 3R-phase of the material. A high value (∼844 pm/V) of second-order susceptibility for few-layer-thick MoS2 swords signifies the potential of MoS2 to serve as an atomically thin nonlinear medium. A field effect mobility of 40 cm2/V-s and Ion/Ioff ratio of ∼106 further confirm the electronic-grade standard of this 3R-phase MoS2. These findings are significant for the development of emerging quantum electronic devices utilizing valley-based physics and nonlinear optical phenomena in layered materials.
Keywords: 2D materials; 3R-phase; Low-frequency Raman; MoS2 swords; Pulsed-CVD; Second harmonic generation; Valley polarization.