Thickness-Dependent Differential Reflectance Spectra of Monolayer and Few-Layer MoS2, MoSe2, WS2 and WSe2 (1810.04745v1)

Abstract: The research field of two dimensional (2D) materials strongly relies on optical microscopy characterization tools to identify atomically thin materials and to determine their number of layers. Moreover, optical microscopy-based techniques opened the door to study the optical properties of these nanomaterials. We presented a comprehensive study of the differential reflectance spectra of 2D semiconducting transition metal dichalcogenides (TMDCs), MoS2, MoSe2, WS2, and WSe2, with thickness ranging from one layer up to six layers. We analyzed the thickness-dependent energy of the different excitonic features, indicating the change in the band structure of the different TMDC materials with the number of layers. Our work provided a route to employ differential reflectance spectroscopy for determining the number of layers of MoS2, MoSe2, WS2, and WSe2.

An Analytical Evaluation of Thickness-Dependent Differential Reflectance in Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDCs), specifically MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$, represent a critical area of paper within two-dimensional (2D) materials research, primarily due to their distinctive optical and electronic properties. The paper by Yue Niu et al. offers a thorough investigation into the thickness-dependent differential reflectance spectra of these TMDCs, providing valuable insights into how their excitonic features change with varying layer number.

Key Findings and Methodology

The authors employed mechanical exfoliation techniques to prepare TMDC nanosheets ranging from monolayers to six layers, on polydimethylsiloxane (PDMS) substrates. Employing a combination of home-built micro-reflectance and transmittance spectroscopy setups, along with density functional theory (DFT) and the LDA+GdW approximation, they examined the excitonic features from near-infrared to near-ultraviolet regions (1.4 eV to 3.0 eV). These setups, supported by a solid theoretical framework, allowed Yue Niu et al. to precisely measure and analyze the excitonic features, labeled A, B, C, and D, depending on the material.

The research identified characteristic excitonic peaks corresponding to direct band gap transitions at the K point in the Brillouin zone. Variations in these peaks are reflective of changes in the band structures due to layer thickness, offering deeper insights into the quantum confinement effects influencing excitonic generation.

Significant Numerical Results

The paper provides substantial numerical evidence showcasing the dependence of the excitonic features on layer thickness. For instance, the A exciton demonstrates a redshift with increased thickness due to quantum confinement and interlayer interactions. Quantitative analyses have shown that the separation between A and B excitonic peaks is larger in W-based TMDCs due to heavier W atoms inducing more substantial spin-orbit splitting. For single-layer MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$, the spin-orbit induced splitting between A and B excitonic peaks are experimentally measured to be 124 ± 5 meV, 219 ± 10 meV, 371 ± 5 meV, and 398 ± 10 meV respectively.

Implications and Future Scope

Practically, this work provides a reliable method for determining TMDC flake thickness via differential reflectance spectroscopy, supplementing traditional techniques like Raman spectroscopy, which falter particularly for multilayers. This advances the optical characterization methodologies crucial for integrating TMDCs in optoelectronic devices. Theoretically, the findings enhance understanding of exciton physics in 2D materials, contributing to decision-making processes regarding which TMDCs to exploit for specific applications based on their excitonic landscape.

Future research should focus on expanding the spectroscopic framework to include less-studied excitonic features and more complex 2D heterostructures. Moreover, further developments in optical imaging and spectroscopy techniques could refine the measurement of interlayer interactions, enhancing device fabrications using layered TMDCs. As TMDCs continue to bridge gaps in optoelectronic application potentials, understanding their fundamental excitonic behaviors could propel innovations in photonic and electronic systems, paving the way for applications ranging from quantum computing to flexible electronics.