Theoretical Analysis of the Nanoscale Composition, Tip-Enhanced Raman Spectroscopy, and Electronic Properties of Alloys in 2D MoS₂–WS₂ Heterostructures

Abstract

Recent studies on atomically thin lateral heterostructures have demonstrated the formation of complex interfaces that can be exploited for tailoring the properties of 2D semiconductor systems for optoelectronic applications. In order to understand the compositional disorder and the resulting optical and electronic properties at these interfaces, tip-enhanced Raman scattering (TERS) imaging and spectroscopy have been used to characterize 2D lateral heterostructures at different sites across the interface, showing a continuous evolution of the Raman-active modes when transitioning from one pristine material to the other. Here, we use density functional theory (DFT) for calculating the evolution of vibrational modes, nonresonant Raman spectra, optical absorption, and electronic structure of 1L-MoS2, WS2, and Mo x W1–x S2 alloys. The calculations reproduce the evolution of the Raman modes observed in the TERS measurements, explicitly confirm the extended alloyed nature of the heterostructure interface, and provide a direct mapping of the TERS spectra to local nanoscale alloy compositions. We further elucidate how S vacancies activate a defect mode in these systems and how the mode evolves with composition, providing a second direct comparison to the nonresonant TERS measurements. Leveraging the explicit determination of the composition, we calculate how the realistic interfacial composition affects the band alignment between the two 2D materials. Our study serves as a roadmap for how the same computational approach can predict the compositional-dependent properties of additional lateral heterostructures, providing a valuable resource for quantitatively interpreting state-of-the-art nanoscale characterization measurements such as TERS imaging and spectroscopy.

Thermal properties of single-layer MoS2–WS2 alloys enabled by machine-learned interatomic potentials

“…Consistent with our previous work in ref. 33, we observe a blue-shift of the A 2 ′′ mode (at 477 and 454 cm −1 for pristine 1L-MoS 2 and WS 2 , respectively) and red-shift of the E′′ mode (at 288 and 307 cm −1 for pristine 1L-MoS 2 and WS 2 , respectively) with increased Mo content.…”

Photoluminescence, Reflection Contrast, Raman, and Second Harmonic Generation Spectroscopies Spatially Resolve Strain, Alloying, Defects, and Electronic Characteristics of Lateral MX2 Heterostructures

“…Raman spectroscopy is sensitive to structural changes. Raman-active modes, specifically the E′ in-plane and A 1 ′ out-of-plane modes, are sensitive to the extent of alloying, 19,24,29 defects (such as sulfur vacancies), 30 and lattice strain. 30−33 SHG imaging is sensitive to factors that destroy the inversion symmetry of the lattice.…”

“…Raman spectroscopy is sensitive to structural changes. Raman-active modes, specifically the E ′ in-plane and A 1 ′ out-of-plane modes, are sensitive to the extent of alloying, ,, defects (such as sulfur vacancies), and lattice strain. − SHG imaging is sensitive to factors that destroy the inversion symmetry of the lattice. In this article, we demonstrate how correlations between Raman, PL, reflection contrast, and SHG imaging can resolve and map the importance of alloying, strain, defects, electronic concentrations, and inversion symmetry changes that control the properties of lateral heterostructures.…”

“…An important question for this class of 2D heterostructures is the importance of Mo x W 1– x X 2 (0 < x < 1) phases since these grow under similar growth conditions for pure MX 2 monolayers . Recent near-field spatial mapping spectroscopies have provided enhanced details of defects and alloying near the heterojunction, demonstrating that synthetic variations could strongly influence optical behaviors near the interface. ,− These PL variations in past works not only underscore the sensitive relationship between photophysical behaviors and synthetic procedures but also imply that impurities and defects could play an important role in the electronics of reported lateral heterostructures. To effectively utilize these heterostructures, it is important to understand the heterogeneity and defects within the individual components and the junctions within these heterostructures.…”

Nanophotonics for Chemical Imaging and Spectroscopy

“…The quest to characterize transition metal dichalcogenides (TMDs) on the nanoscale led to the wide adoption of TERS and tip-enhanced photoluminescence (TEPL) by active practitioners in this exciting field of low-dimensional quantum materials. , In this VSI, TERS and TEPL were used to better understand the optoelectronic properties of grain boundaries in single layer WSe 2 , to characterize the interplay between 2D materials and plasmonic nanoparticles, and to rationalize the excitation wavelength dependence of nano-Raman spectral images of TMDs . An interesting theoretical analysis of the optical and electronic properties of TMD heterostructures is one of several fundamental TERS studies in this issue. The influence of an imperfect AFM probe on ambient TERS spectra and images as well as the information content in STM-TERS spectral images of a three-dimensional double-decker molecule are also reported as part of this VSI.…”