Nano-optics of transition metal dichalcogenides and their van der Waals heterostructures with electron spectroscopies

Abstract

The outstanding properties of transition metal dichalcogenide (TMD) monolayers and their van der Waals (vdW) heterostructures, arising from their structure and the modified electron-hole Coulomb interaction in two-dimension, make them promising candidates for potential electro-optical devices. However, the production of reproducible devices remains challenging, partly due to variability at the nanometer to atomic scales. Thus, access to chemical, structural, and optical characterization at these lengthscales is essential. While electron microscopy and spectroscopy can provide chemical and structural data, accessing the optical response at the nanoscale through electron spectroscopies has been hindered until recently. This review focuses on the application of two electron spectroscopies in scanning (transmission) electron microscopes, namely cathodoluminescence and electron energy-loss spectroscopy, to study the nano-optics of TMD atomic layers and their vdW heterostructures. How technological advancements that can improve these spectroscopies, many of which are already underway, will make them ideal for studying the physics of vdW heterostructures at the nanoscale will also be discussed.

Figures (8)

• Figure 1: a Sketch of an SEM which includes an electron source, electron focusing optics (mostly purely magnetic), scanning optics (sequence of dipoles), a sample, and a light injection/collection system (CL). b Sketch of a STEM which, in addition to the elements in an SEM, contains electron beam de-scanning optics (EELS, diffraction), possibly a projection system, and an EELS spectrometer. The typical electron kinetic energy for SEMs is in the range of 1--30 keV, while for STEMs it is in the range of 30--300 keV. Samples in both setups can be cooled down to liquid-helium temperature, but this option is not standard for STEMs due to large mechanical vibrations and the short operation time of currently available helium-cooled sample holders.
• Figure 2: Sketch of inelastic scattering on a target. A primary electron can exchange energy with a target through its electromagnetic field at a finite distance, an effect usually referred to as delocalization Muller1995. The energy lost creates excitations in the target, which can propagate. These excitations eventually decay, leading to the emission of phonons in the lattice and/or free photons and electrons. Spectroscopy of the energy lost (EELS) and the emitted photons (CL) helps us understand the optical properties of the target.
• Figure 3: a Five spectra integrated at different positions across the interface a MoS$_2$-MoSe$_2$ chemically diffuse interface showing the excitonic A and B transitions. b Comparison of the fitting coefficient profiles with the chemical profiles measured from core-loss EELS of the S L and Se M edges on the same interface. c Temperature-dependent absorption spectra for MoS$_2$, where a shift towards lower energy in the onset (noted by the light-blue linear fitting curve) is observed with increasing temperature. d Comparison of EELS and optical absorption spectra of h-BN encapsulated WSe$_2$. Panels a,b adapted with permission from ref. Tizei2015, panel c adapted with permission from ref. Tizei2016, panel d adapted with permission from ref. Woo2024.
• Figure 4: Twist angle-dependent EELS spectra of MoS$_2$/WSe$_2$ van der Waals heterostructures and twisted bilayer WSe$_2$. a Comparison of the EELS spectra of monolayer MoS$_2$ and WSe$_2$ with that of aligned (anti-aligned) and misaligned van der Waals heterostructures. b,c STEM-HAADF images of 50$^\circ$ MoS$_2$/WSe$_2$ heterostructure and its inset image fast Fourier transfer (b), and WSe$_2$ bilayer with 3.4$^\circ$ relative twist angle (c). d EELS spectra from twisted bilayer WSe$_2$ with various twist angles compared to monolayer (1L) WSe$_2$ and AA$^\prime$ stacked bilayer. The dotted line highlights the change in the C exciton energy positions. Panels a,b adapted with permission from ref. Gogoi2019, panels c,d adapted with permission from ref. Woo2023.
• Figure 5: a CL spectra at positions 1 and 2 in (b) are shown together with a PL spectrum acquired at position 1 in (b). b Monochromatic CL map of the h-BN/WSe$_2$/h-BN heterostructure filtered at 1.66 eV, the WSe$_2$ neutral exciton emission energy (color coded in red). The yellow outlines the WSe$_2$ monolayer, and the blue outlines the top h-BN layer. c Schematic showing the process of the generation, diffusion, and recombination of electron-hole (e-h) pairs. The minor number of e-h pairs generated in the WSe$_2$ layer is ignored in this model. Most electron inelastic scattering occurs in the h-BN layers Bonnet2021, as shown in panel d of a low-loss EELS spectrum of h-BN encapsulated WS$_2$ monolayer. Panels a--c reproduced with permission from ref. Zheng2017, panel d adapted with permission from ref. Bonnet2021.