Ambient Pressure Chemical Vapor Deposition of Flat and Vertically Aligned MoS2 Nanosheets

Abstract

Molybdenum disulfide (MoS₂) got tremendous attention due to its atomically thin body, rich physics, and high carrier mobility. The controlled synthesis of large area and high crystalline monolayer MoS₂ nanosheets on diverse substrates remains a challenge for potential practical applications. Synthesizing different structured MoS₂ nanosheets with horizontal and vertical orientations with respect to the substrate surface would bring a configurational versatility with benefit for numerous applications, including nanoelectronics, optoelectronics, and energy technologies. Among the proposed methods, ambient pressure chemical vapor deposition (AP-CVD) is a promising way for developing large-scale MoS₂ nanosheets because of its high flexibility and facile approach. Here, we show an effective way for synthesizing large-scale horizontally and vertically aligned MoS₂ on different substrates such as flat SiO₂/Si, pre-patterned SiO₂ and conductive substrates (TaN) benefit various direct TMDs production. In particular, we show precise control of CVD optimization for yielding high-quality MoS₂ layers by changing growth zone configuration and the process steps. We demonstrated that the influence of configuration variability by local changes of the S to MoO₃ precursor positions in the growth zones inside the CVD reactor is a key factor that results in differently oriented MoS₂ formation. Finally, we show the layer quality and physical properties of as-grown MoS₂ by means of different characterizations: Raman spectroscopy, scanning electron microscopy (SEM), photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS). These experimental findings provide a strong pathway for conformally recasting AP-CVD grown MoS₂ in many different configurations (i.e., substrate variability) or motifs (i.e., vertical or planar alignment) with potential for flexible electronics, optoelectronics, memories to energy storage devices.

Keywords: 2D TMD; AP-CVD; MoS2; growth selectivity; large area growth; pattern substrates.

Figure 1

(a) Schematic diagram of the horizontal two-zone CVD furnace for the synthesis of flat-MoS₂. (b) Modified CVD setup used to synthesize vertical-MoS₂ nanosheets by adjusting the boat position during the growth temperature ramp on different substrates (c) Different steps of temperature profile (left y-axis) adopted for the synthesis of flat-MoS₂ nanosheets with growth ramp at 750 °C (solid line, SiO₂/Si) and 650 °C (dash line, TaN) for 20 min; in the same graph the Ar flux changes during the CVD process are also plotted (blue solid line, right y-axis). (d) Three step temperature profile (left y-axis) used for the synthesis of vertical-MoS₂ nanosheets with growth ramp at 750 °C (solid line, SiO₂/Si) and 625 °C (dash line, TaN) for 20 min; in the same graph the Ar flux changes during the CVD process are also plotted (blue solid line, right y-axis).

Figure 2

Schematic illustration of horizontal and vertical growth of MoS₂ using AP-CVD and SEM image (a) large area monolayer MoS₂ grown on flat SiO₂/Si substrate (b) isolated flake of flat monolayer MoS₂ domain on patterned SiO₂/Si; inset TEM cross sectional image shows conformal monolayer MoS₂ growth on patterned trenches (thin dark line). (c) flat MoS₂ on TaN. (d–f) vertically standing MoS₂ nanosheets (d) on SiO₂/Si (e) on pre-patterned substrate, inset: SEM cross section (f) on TaN.

Figure 3

(a) Raman spectra obtained at different positions on the as-grown horizontal monolayer MoS₂ nanosheets on flat SiO₂/Si; the inset shows the SEM image with the positions of the Raman measures with red open circles (b) Raman spectrum taken on monolayer MoS₂ on pattern substrate; the inset shows the SEM image with the position of the Raman measure with green open circle. (c) Raman spectra of bare TaN substrate (red) and after three layers MoS₂ grown on TaN (black); the inset shows the SEM image with the positions of the Raman measures on TaN substrate with red and MoS₂ grown on TaN black open circles (d) PL spectra of a MoS₂ on flat Si (red), pattern SiO₂/Si (green) and TaN (black) substrates.

Figure 4

(a) Raman spectra obtained on vertically aligned MoS₂ on pre-patterned (red) and equivalent thick flat (black) SiO₂/Si substrate, (b) magnified Raman spectra of panel (a) evidencing E¹_2g (in-plane), A_1g (out of plane) phonon modes of MoS₂ and their frequency difference. (c) Raman spectrum of vertical aligned MoS₂ on TaN substrate and (d) magnification of Raman spectrum in panel (c) in the low Raman shift region (dashed box in (c)), where the presence of co-deposited TaS₂ peak is evidenced by its E¹_2g (in-plane) phonon mode.

Figure 5

(a) XPS spectrum shows Mo(3d) and S(2p) core level regions of as-grown MoS₂ on SiO₂/Si (b) XPS spectra of Ta(4d), Mo(3d) and S(2s) core level regions of as-grown vertical (blue) and flat (magenta) MoS₂ on TaN and bare TaN substrate (red).