Lattice dynamics in mono- and few-layer sheets of WS2 and WSe2 (1304.0911v2)

Abstract: Thickness is one of the fundamental parameters that define the electronic, optical, and thermal properties of two-dimensional (2D) crystals. Phonons in molybdenum disulfide (MoS2) were recently found to exhibit unique thickness dependence due to interplay between short and long range interactions. Here we report Raman spectra of atomically thin sheets of WS2 and WSe2 in the mono- to few-layer thickness regime. We show that, similar to the case of MoS2, the characteristic and modes exhibit stiffening and softening with increasing number of layers, respectively, with a small shift of less than 3 cm-1 due to large mass of the atoms. Thickness dependence is also observed in a series of multiphonon bands arising from overtone, combination, and zone edge phonons, whose intensity exhibit significant enhancement in excitonic resonance conditions. Some of these multiphonon peaks are found to be absent only in monolayers. These features provide a unique fingerprint and rapid identification for monolayer flakes.

• The paper demonstrates that the A1g mode stiffens and the E2g mode softens with increasing layer number in WS₂ and WSe₂.
• The study uses Raman spectroscopy to identify distinct multiphonon bands and characteristic shifts, facilitating rapid monolayer identification.
• The findings emphasize excitonic resonance's role in enhancing Raman signals, offering insights essential for nanoelectronic and photonic applications.

Lattice Dynamics in Mono- and Few-Layer Sheets of WS₂ and WSe₂

This paper presents an analytical investigation into the lattice dynamics of mono- and few-layer sheets of WS₂ and WSe₂ through Raman spectroscopy. The research aims to comprehend the phonon behavior in these tungsten-based transition metal dichalcogenides (TMDs), which are isoelectronic compounds of MoS₂. It provides comprehensive insights into the thickness-dependent shifts in phonon modes and observes unique characteristics in their Raman spectra attributable to excitonic resonance conditions.

The authors successfully demonstrate that, consistent with MoS₂, the A1g and E2g modes in WS₂ and WSe₂ exhibit distinguished shifts with layer number increments. Notably, the A1g mode stiffens, while the E2g mode softens. These shifts are less than 3 cm⁻¹ due to the relatively larger atomic masses within these materials. Beyond the A1g and E2g modes, the paper discusses the presence of multiphonon bands, whose intensity significantly enhances under excitonic resonance, particularly at certain excitation resonances, aiding in the rapid identification of monolayers.

Key Findings

• Phonon Behavior: The analysis indicates a significant interplay between short and long-range interactions in determining the behavior of phonons, influencing the stiffening of A1g and softening of E2g modes as the layer number increases. These behaviors remain consistent across the studied materials, albeit with varying shift magnitudes across different compounds in the same group.
• Monolayer Identification: The research highlights the degeneracy of the A1g and E2g modes in monolayer WSe₂, which splits into distinct peaks with increasing thickness. Such fingerprint features are vital for the rapid identification of monolayers, especially in the presence of multiphonon resonance.
• Excitonic Resonance: Detailed multiphonon bands are observed in excitonic resonance, which play a crucial role in enhancing the Raman cross-section and providing additional characterization features for these TMDs.

Implications and Future Directions

1. Material Characterization: The results contribute to a more profound understanding of phonon behavior in layered TMDs, essential for their application in nanoelectronics and photonics, notably in devices that exploit their spintronic and valleytronic properties.
2. Spin and Valley Physics: The observed sizable spin-orbit interactions and phonon characteristics in WS₂ and WSe₂ furnish a foundational platform to further explore and exploit spin and valley degrees of freedom in these materials.
3. Sensor Technologies: The unique Raman fingerprints and thickness-dependence presented offer a specific application avenue in developing sensitive detection platforms and sensor technologies.

Future research may delve into further exploring the intricate behavior of multiphonon modes, specifically in the context of different excitation environments and their potential contributions to TMDs' optoelectronic properties. Emphasis on resolving theoretical-experimental discrepancies in phonon assignments across TMDs also remains crucial. Through continued efforts in this domain, a more comprehensive framework for the practical application of 2D materials can be realized, driving innovations in material science and applied physics.