Effects of Mo vapor concentration on the morphology of vertically standing MoS₂ nanoflakes

Abstract

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.

Formation and properties of mesoporous MoS-=SUB=-2-=/SUB=- films

“…that exhibit the properties typical of graphene [15]. Similar mesoporous TMD (e.g., MoS 2 ) films have been examined earlier as high-porosity catalyst carriers and materials for supercapacitors [16][17][18].…”

Получение и свойства мезопористых пленок MoS-=SUB=-2-=/SUB=-

“…), которые проявляют свойства, присущие графену [15]. Аналогичные мезопористые пленки ДПМ, например MoS 2 , исследовались ранее в качестве высокопористой основы для катализаторов и в конденсаторах высокой емкости [16][17][18].…”

Effects of post-annealing on MoS2 thin films synthesized by multi-step chemical vapor deposition

“…Two-dimensional (2D) transition metal dichalcogenides (TMDs) have received significant interest due to their unique and diverse properties which are useful for fundamental studies and various electronics and electrochemistry applications. [1][2][3][4][5][6][7] Semiconducting 2D TMDs such as MoS 2 , MoSe 2 , WS 2 , and WSe 2 are favorable for realizing ultrasmall transistors due to their finite bandgap which is not found in graphene. [8][9][10] Bulk MoS 2 is an indirect bandgap material (*1.2 eV) but the bandgap energy increases to *1.8 eV and becomes direct when the thickness reduces to monolayer.…”