Probing stacking configurations in a few layered MoS2 by low frequency Raman spectroscopy

Abstract

Novel two-dimensional (2D) layered materials, such as MoS2, have recently gained a significant traction, chiefly due to their tunable electronic and optical properties. A major attribute that affects the tunability is the number of layers in the system. Another important, but often overlooked aspect is the stacking configuration between the layers, which can modify their electro-optic properties through changes in internal symmetries and interlayer interactions. This demands a thorough understanding of interlayer stacking configurations of these materials before they can be used in devices. Here, we investigate the spatial distribution of various stacking configurations and variations in interlayer interactions in few-layered MoS2 flakes probed through the low-frequency Raman spectroscopy, which we establish as a versatile imaging tool for this purpose. Some interesting anomalies in MoS2 layer stacking, which we propose to be caused by defects, wrinkles or twist between the layers, are also reported here. These types of anomalies, which can severely affect the properties of these materials can be detected through low-frequency Raman imaging. Our findings provide useful insights for understanding various structure-dependent properties of 2D materials that could be of great importance for the development of future electro-optic devices, quantum devices and energy harvesting systems.

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

“…4, b). Specifically, the line at 25 cm −1 , which is observed in crystallites 100 nm in size, corresponds to 2−3 layers of MoS 2 of polytype 2H, while the spectra of films with flakes 500 nm in size feature a line at 32 cm −1 that corresponds to 5−6 layers of MoS 2 of polytype 2H [22,23]. These low-frequency lines were not found in the spectra of films with flakes ∼ 1 µm in size.…”

Dilute Rhenium Doping and its Impact on Defects in MoS2

“…34 Notably, no lowfrequency modes were detected in the spectra, indicating the absence of bilayer or thicker regions in the analyzed area. 35 The Re concentration in doped MoS 2 samples is confirmed by XPS quantification of Re 4f 7/2 and Re 4f 5/2 peaks (SI, Figure S3). By modulating H 2 carrier gas flow through the Re 2 (CO) 10 bubbler, Re content in MoS 2 is tuned from alloying (>6 at.…”

“…The average value for E 2 g ′- A 1 g peak distance (Δω) in the mapped area is measured to be 19.51 ± 0.21 cm –1 , which closely matches Δω 1L values in literature . Notably, no low-frequency modes were detected in the spectra, indicating the absence of bilayer or thicker regions in the analyzed area . The Re concentration in doped MoS 2 samples is confirmed by XPS quantification of Re 4 f 7/2 and Re 4 f 5/2 peaks (SI, Figure S3).…”

Polarization Raman Imaging of Organic Monolayer Islands for Crystal Orientation Analysis

“…Additionally, as the Raman spectrum itself contains chemical information, the multimodal analysis would provide more details about organic semiconducting samples by correlating molecular orientation with the information of chemical bonds. Particularly, the low-frequency region of the Raman spectrum contains intermolecular interaction information, − which could give important insights into how densely DPh-DNTT molecules are packed in the herringbone structure. Furthermore, if a higher spatial resolution is required in a crystal orientation image, tip-enhanced Raman spectroscopy could be utilized. ,,− Although the polarization control of a near-field light is still challenging, a technique based on a defocused imaging technique to evaluate and control the polarization of near-field light at a metallic tip apex has recently been demonstrated. , …”