Illuminating the Electronic Properties of WS₂ Polytypism with Electron Microscopy

Abstract

Tailoring the specific stacking sequence (polytypes) of layered materials represents a powerful strategy to identify and design novel physical properties. While nanostructures built upon transition‐metal dichalcogenides (TMDs) with either the 2H or 3R crystalline phases have been routinely studied, knowledge of TMD nanomaterials based on mixed 2H/3R polytypes is far more limited. In this work, mixed 2H/3R free‐standing WS2 nanostructures displaying a flower‐like configuration are fingerprinted by means of state‐of‐the‐art transmission electron microscopy. Their rich variety of shape‐morphology configurations is correlated with relevant local electronic properties such as edge, surface, and bulk plasmons. Machine learning is deployed to establish that the 2H/3R polytype displays an indirect band gap of EnormalBG=1.6−0.2+0.3eV. Further, high resolution electron energy‐loss spectroscopy reveals energy‐gain peaks exhibiting a gain‐to‐loss ratio greater than unity, a property that can be exploited for cooling strategies of atomically‐thin TMD nanostructures and devices built upon them. The findings of this work represent a stepping stone towards an improved understanding of TMD nanomaterials based on mixed crystalline phases.

Morphology-induced spectral modification of self-assembled WS2 pyramids

“…This microchip is composed of a silicon frame with nine windows over which a continuous silicon nitride (Si 3 N 4 ) film is spanned. Preceding the CVD growth procedure, 16 tungsten trioxide (WO 3 ) is deposited onto the microchip. This deposition is achieved by dispersing 50 mg of WO 3 in 1 mL of isopropanol, followed by a deposition by means of a pipette.…”

“…This deposition is achieved by dispersing 50 mg of WO 3 in 1 mL of isopropanol, followed by a deposition by means of a pipette. The subsequent sulfurization process 16 is carried out in a gradient tube furnace from Carbolite Gero. The microchip is placed in the middle zone and a crucible holding 400 mg of sulfur is placed upstream from it.…”

“…[10][11][12][13][14][15] While CVD can reproduce horizontal layers as found in naturally occurring TMDs, adjusting the growth conditions enables the growth of nanostructures with exciting properties and applications in nanotechnology, for instance vertical walls, owers and pyramids. 16 Under certain CVD growth parameters TMDs materials form pyramid-like structures, 17,18 having many active adsorption site useful for applications in hydrogen sensors 19 and water disinfection, 20 at the same time possessing interesting electronic properties like ferromagnetism 21 and high mobility. 22,23 Moreover, pyramid-like TMDs structures have applications in non-linear optics, 22,24,25 as they exhibit higher non-linear optical conversion efficiency than monolayers due to the thickness increase, while demonstrating a much larger nonlinear optical response than multilayer TMDs.…”

Spatially Resolved Band Gap and Dielectric Function in Two-Dimensional Materials from Electron Energy Loss Spectroscopy

“…As a proof of concept we apply our strategy to the determination of the band gap and the complex dielectric function in two representative van der Waals materials, InSe flakes and polytypic WS 2 nanoflowers. 27 Both electronic properties are evaluated across the whole specimen and can be correlated among them, e.g., to assess the interplay between the band gap energy or the location of plasmonic resonances with the local thickness. Our approach is amenable to generalization to other families of nanostructured materials, is suitable for application to higher-dimensional data sets such as momentum-resolved EELS, and is made available as a new release of the EELSfitter open-source framework.…”

Spatially-Resolved Band Gap and Dielectric Function in 2D Materials from Electron Energy Loss Spectroscopy

“…As a proof-of-concept we apply our strategy to the determination of the bandgap and the complex dielectric function in two representative van der Waals materials, InSe flakes and polytypic WS 2 nanoflowers. 27 Both electronic properties are evaluated across the whole specimen and can be correlated among them, e.g. to assess the interplay between bandgap energy or the location of plasmonic resonances with the local thickness.…”