Probing Polymorphic Stacking Domains in Mechanically Exfoliated Two-Dimensional Nanosheets Using Atomic Force Microscopy and Ultralow-Frequency Raman Spectroscopy

Abstract

As one of the key features of two-dimensional (2D) layered materials, stacking order has been found to play an important role in modulating the interlayer interactions of 2D materials, potentially affecting their electronic and other properties as a consequence. In this work, ultralow-frequency (ULF) Raman spectroscopy, electrostatic force microscopy (EFM), and high-resolution atomic force microscopy (HR-AFM) were used to systematically study the effect of stacking order on the interlayer interactions as well as electrostatic screening of few-layer polymorphic molybdenum disulfide (MoS₂) and molybdenum diselenide (MoSe₂) nanosheets. The stacking order difference was first confirmed by measuring the ULF Raman spectrum of the nanosheets with polymorphic stacking domains. The atomic lattice arrangement revealed using HR-AFM also clearly showed a stacking order difference. In addition, EFM phase imaging clearly presented the distribution of the stacking domains in the mechanically exfoliated nanosheets, which could have arisen from electrostatic screening. The results indicate that EFM in combination with ULF Raman spectroscopy could be a simple, fast, and high-resolution method for probing the distribution of polymorphic stacking domains in 2D transition metal dichalcogenide materials. Our work might be promising for correlating the interlayer interactions of TMDC nanosheets with stacking order, a topic of great interest with regard to modulating their optoelectronic properties.

Keywords: atomic force microscopy; electrostatic screening; stacking order; transition metal dichalcogenides; ultralow-frequency Raman spectroscopy.

Scheme 1

Scheme depicting the preparation and characterization of mechanically exfoliated MoS₂ and MoSe₂ with different stacking orders. (a,b) Peeling off of the multilayer nanosheets from the 2H-phase crystal using scotch tape. (c,d) Deposition of the 2H- and non-2H-phase MoS₂ or MoSe₂ nanosheets from the bulk 2H crystal via mechanical exfoliation. (e) EFM characterization of 2H- and non-2H-phase MoS₂ or MoSe₂ nanosheets.

Figure 1

Optical images of the MoS₂ nanosheets consisting of 2H 6L and non-2H 7L layers (a) before and (b) after their transfer onto a PDMS film. (c) The ULF Raman spectra of regions P1, P2, and P3 in the 6L and 7L MoS₂ layers before and after the transfer. The red dashed lines are referred to the breathing-mode peak, the blue and green dashed lines are referred to the shear-mode peaks.

Figure 2

(a) Optical and (c) AFM topography images. (b) ULF Raman spectra of the MoS₂ nanosheet on the 300 nm SiO₂/Si substrate. The red and blue dashed lines are referred to the shear- and breathing-mode peaks of 2H 2L MoS₂, the orange and green dashed lines are referred to the shear- and breathing-mode peaks of 2H 5L MoS₂. The purple arrows are referred to the new shear-mode peaks of non-2H 5L MoS₂. (d) Simulated atomic lattice images of regions P2 and P3 shown in (c) and according to the measured twist angle.

Figure 3

(a) Optical image, (b) AFM topography image, (c) ULF Raman spectra, the red and blue dashed lines are referred to the shear-mode peaks of non-2H 6L MoS₂, (d) Raman mapping in the range of 25.5 to 35.5 cm⁻¹, (e) EFM amplitude image, and (f) EFM phase image of the 6L MoS₂ sample with different stacking orders.

Figure 4

(a) AFM topography image. (b–h) EFM phase images of 2L and 3L MoS₂ nanosheets shown in (a), with the tip bias changed from −2 V to +2 V. (i) EFM phase shift as a function of the applied tip bias.

Figure 5

(a) Optical image, (b) AFM topography image, (c) ULF Raman spectra, the green and blue dashed lines are referred to the shear-mode peaks of non-2H 6L MoSe₂, (d) Raman mapping in the range of 12.4 to 14.4 cm⁻¹ and 17.1 to 20.7 cm⁻¹, (e) EFM amplitude image, and (f) EFM phase image of the 6L MoSe₂ sample with different stacking orders.