Vertically standing MoS2 nanoflakes are favourable in applications such as energy storage devices, hydrogen evolution reactions, and gas sensors due to their large surface area and high density of exposed edges. In this work, we report the effect of Mo vapor concentration on the morphology of vertical MoS2 nanoflakes prepared by chemical vapor deposition at atmospheric pressure. A series of MoS2 samples were grown under different Mo vapor concentrations by varying the separation distance (x) between the MoO3 source and the substrate. Field emission scanning electron microscopy showed the sample grown at x = 1 cm had a high density of vertical flakes (7 vertical flakes µm−2) with an average flake length of ~770 nm and thickness of ~10 nm. As x increased to 4 cm, the average flake length was reduced to ~150 nm while the flake orientation changed from vertical to lateral. That is, high Mo vapor concentration favours the formation of large and vertical MoS2 nanoflakes. However, oversupply of Mo vapor results in significantly thicker flakes. Raman spectra of all samples showed two main peaks at 380 and 407 cm−1 that correspond to the E12g and A1g vibrational peaks of MoS2. As x decreased from 4 to 1, the peak intensity ratio (E12g/A1g) reduced from 0.58 to 0.42, suggesting greater dominance of vertical flakes at low x. X-ray diffraction data showed a prominent peak at 14.4°, which corresponded to the (002) diffraction peak of 2H MoS2. Transmission electron microscopy verified the flakes consist of eight layers with an interlayer spacing of 0.62 nm. Based on hydrogen evolution reaction measurements, samples with thin flakes have high catalytic activity. This work highlights the importance of optimizing Mo vapor concentration to obtain a high density of thin, large, and vertically standing MoS2 nanoflakes.

Multi-step chemical vapor deposition (CVD) is a synthesis method which is capable of producing a uniform, large area, and high-quality thin films. In this work, we report the effect of post-annealing on the structural and optical properties of few-layers (FL) MoS2 thin films synthesized by multi-step CVD. Based on atomic force microscopic image, the thickness of the MoS2 thin film is ∼3 nm, which is equivalent to five layers. After annealing at 900°C for 17 min, intensity of the A1g and [Formula: see text]full-width-at-half-maximum (FWHM) Raman modes increased by ∼3 times while the reduced from ∼10 cm−1 to ∼7.5 cm−1 for A1g and from ∼13.6 cm−1 to ∼7.5 cm−1 for [Formula: see text]. Both of the as-grown and annealed samples showed X-ray (002) diffraction peak at 14.2° but the intensity was more prominent for the annealed sample. It was found that the annealed sample showed clear and distinct absorbance peaks at 666, 615, 448, 401, and 278 nm which correspond to the A, B, C, D, and E excitons, respectively. The results indicate that annealing significantly improved the optical and structural quality of the MoS2 film. Field-effect transistor based on annealed MoS2 thin film was fabricated and showed electron mobility of 0.21 cm2V−1s−1, on/off ratio of 1.3 × 102 and a threshold voltage of 0.72 V. Our work highlights the importance of high-temperature annealing in multi-step CVD to obtain a uniform and high-quality FL MoS2 thin films.

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The fabrication of high quality graphene has become the main interest in current chemical vapour deposition (CVD)