Atomically thin materials such as semiconducting transition metal diselenide materials, like molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2), have received intensive interests in recent years due to their unique electronic structure, bandgap engineering, ambipolar behavior, and optical properties and have motivated investigations for the next-generation semiconducting electronic devices. In this work, we show a nondestructive method of characterizing the layer number of two-dimensional (2-D) MoSe2 and WSe2 including single- and few-layer materials by Raman spectroscopy. The related photoluminescence properties are also studied as a reference. Although Raman spectroscopy is a powerful tool for determining the layer number of 2-D materials such as graphene and molybdenum disulfide (MoS2), there have been difficulties in precisely characterizing the layer number for MoSe2 and WSe2 by Raman spectroscopy due to the uncertain shifts during the Raman measurement process and the lack of multiple separated Raman peaks in MoSe2 and WSe2 for referencing. We then compared the normalized Si peak with MoSe2 and WSe2 and successfully identified the layer number of MoSe2 and WSe2. Similar to graphene and MoS2, the sample layer number is found to modify their optical properties up to 4 layers.
Because of their unique structure and exotic physical and mechanical properties, two-dimensional (2-D) materials have drawn tremendous interests since their discovery [
Recently, transition metal diselenides MoSe2 and WSe2 have become the new 2-D stars owing to the creation of moiré excitons in their heterostructures [
In this paper, we characterized the layer number of MoSe2 and WSe2 by Raman spectroscopy with referencing to the substrate silicon’s Raman peaks. Both materials possess increasing Raman peak intensity with increasing layer numbers until reaching 4 layers (L). We also found 1L MoSe2 and WSe2 flakes possess higher PL intensity than the few-layer flakes.
MoSe2 and WSe2 bulk crystals (SPI Supplies) were used for mechanical exfoliation to obtain few-layer materials. The MoSe2 or WSe2 crystal was procured and exfoliated by using a scotch tape. The crystal of size 3 mm × 3 mm was used. After exfoliation on the scotch tape for ∼8 times, the crystal with scotch tape was pressed onto a clean SiO2 (285 nm)/Si substrate, and the end of a sharpie was used to abrade for 3 minutes. After removing the scotch tape, the flakes are left on the SiO2 (285 nm)/Si substrate. 285 nm SiO2 was used because it provides the best contrast for identifying the thin flakes under an optical microscope (Nikon Eclipse 150) (Figure
(a) Optical image and (b) atomic force microscope (AFM) image of 1–4L MoSe2. The scale bar is 2
A micro-Raman spectrometer (RENISHAW InVia Raman Microscope system) was used to measure the Raman spectra (Figures
(a) Raman spectra of different layer numbers of MoSe2 supported by SiO2 (285 nm)/Si substrate by 633 nm laser. (b) Raman peak position of MoSe2’s A1g peak and Si peak for different layer numbers (left vertical axis) and their difference (right vertical axis). (c) Raman peak intensity of MoSe2’s A1g peak and Si peak for different layer numbers (left vertical axis) and their ratio (right vertical axis). (d) Raman peak line width of MoSe2’s A1g peak and Si peak.
(a) Raman spectra of different layer numbers of WSe2 supported by SiO2 (285 nm)/Si substrate by 633 nm laser. (b) Raman peak position of WSe2’s
(a) Photoluminescence (PL) of 1L–4L MoSe2. 1L MoSe2 has highest PL intensity—stronger PL intensity indicates that bandgap transforms from indirect to direct. Bandgap increases with the decrease in thickness. (b) PL of 1L–3L and bulk WSe2, with the same trend as MoSe2.
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Laser used in the Raman measurement was focused on the MoSe2 and WSe2 flakes using the 100x objective lens with a numerical aperture of 0.90 and 1 mW laser power. Laser with a wavelength of 633 nm is used to provide a comprehensive characterization. For MoSe2, A1g Raman peak is the only observable peak and is the most visible and studied peak. Figure
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In this work, two atomically thin transition metal diselenide materials, MoSe2 and WSe2, with layer number from 1L to up to 15L have been studied by Raman spectroscopy, and the Si Raman peak from the substrate has been used as a reference for the precise characterization. By characterizing both the sample and substrate’s Raman spectra, we are able to determine MoSe2 and WSe2’s layer number for up to 4 layers. We believe that the Si-peak analysis remains the most powerful tool for determining the number of layers of MoSe2 and WSe2. Furthermore, this identification method can be exploited for van der Waals heterostructures made of various 2-D materials such as hBN and TMDCs, when substrate-related peaks are found and the relationship with the number of layers is verified. Their PL properties are also studied as additional information. It has been confirmed with the previous study that there is an enhanced PL in single-layer MoSe2 and WSe2 because of the transition from indirect to direct bandgap electronic structures, and the bandgap decreases with an increase in thickness. These results demonstrate more robust measurements of thickness of transition metal diselenide materials and provide potential of new optical and electrical applications of van der Waals semiconducting materials. In addition, the potential of combination of 2-D materials and other emerging materials such as organic materials will attract extensive attention from researchers in the organic, electronic, and nanotechnology communities.
The data used to support the findings of this study are available at
The author declares no conflicts of interest.
This work is supported by the startup funding of the Stevens Institute of Technology.