Monolayers of molybdenum disulfide are of vital importance in the fabrication of optical and nanoelectronic devices. The development of thin and low-cost devices has increased the demand for synthesis processes. Usually, the synthesis of molybdenum disulfide monolayers requires temperatures of approximately 800 °C, which is a drawback for the applications mentioned above. Here, we propose a route using the atmospheric pressure chemical vapor deposition technique to grow monolayers of MoS2 at 550 °C mediated by using sodium as a catalyst. We produced single crystals and polycrystalline films by controlling the NaNO3/MoO3 catalyst/precursor ratio and the growth time. Using first-principles calculations, we determined that sodium was the nucleation site of the growth process. The precursor’s ratio is crucial to decrease the formation energy and the synthesis temperature. First-principles calculations and experiments showed that the ideal precursor ratio was 0.3 and that the synthesis temperature should be decreased by 250 °C. We investigated the monolayers with optical microscopy, high-resolution scanning transmission electron microscopy, X-ray photoelectron spectroscopy, atomic force microscopy, Raman spectroscopy, photoluminescence spectroscopy, and transport experiments. The optical and electrical performances were comparable to those of monolayers grown at higher temperatures. We believe that a low-temperature synthesis recipe is essential to drive the fabrication of nanoscale optoelectronic devices.

The stacking of few layers of transition metal dichalcogenides (TMDs) and their heterostructures allows us to create new structures, observe new physical phenomena, and envision new applications. Moreover, the twist angle in few-layer TMDs can significantly impact their electrical and optical properties. Therefore, controlling the TMD material and obtaining different stacking orientations when synthesizing TMDs via chemical vapor deposition (CVD) is a powerful tool, which can add functionality to TMD-based optoelectronic devices. Here, we report on the synthesis of few-layer MoS[Formula: see text] and WS[Formula: see text] crystals, as well as their heterobilayer structures with [Formula: see text] and [Formula: see text] twist angles between layers via CVD. Raman and photoluminescence spectroscopies demonstrate the quality, crystallinity, and layer count of our grown samples, while second harmonic generation shows that adjacent layers grow with 0[Formula: see text] or 60[Formula: see text] twist angles, corresponding to two different crystal phases. Our study based on TMDs with different and multiple stacking configurations provides an alternative route for the development of future optoelectronic and nonlinear optical devices.

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The transfer of two-dimensional materials from grown substrates onto target substrates is critical for device applications and postgrown analysis. The traditional transfer method for as-grown two-dimensional materials requires a wet chemical etching process that damages the crystals and the substrates. These issues deteriorate the electrical and optical performances of two-dimensional-material-based devices fabricated afterward. Herein, we developed an etching-free method and nanoimaging for transferring and analyzing monolayers of MoS2 onto arbitrary substrates using polyurethane as a sacrifice polymer. The polymer layer and the MoS2 crystals can be peeled off from the substrate by tweezers and transferred to any substrate. We analyzed the transferred samples to a glass coverslip substrate with optical microscopy, atomic force microscopy, tip-enhanced Raman spectroscopy, and tip-enhanced photoluminesce spectroscopy. Also, we transferred monolayers of MoS2 to a transmission electron microscopy grid and acquired scanning electron microscopy together with high-resolution scanning transmission electron microscopy images. The results of nanoimaging indicate that the method preserved the sample’s optical and structural properties and avoid undesirable cracks, wrinkles, and polymer residues. The etching-free transfer method of as-grown two-dimensional materials will improve the quality of transferred samples enhance the devices’ performance.

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