First-Principles Calculation of Optoelectronic Properties in 2D Materials: The Polytypic WS₂ Case

Abstract

The phenomenon of polytypism, namely unconventional crystal phases displaying a mixture of stacking sequences, represents a powerful handle to design and engineer novel physical properties in two-dimensional (2D) materials. In this work, we characterize from first-principles the optoelectronic properties associated with the 2H/3R polytypism occurring in WS2 nanomaterials by means of density functional theory (DFT) calculations. We evaluate the band gap, optical response, and energy-loss function associated with 2H/3R WS2 nanomaterials and compare our predictions with experimental measurements of electron energy-loss spectroscopy (EELS) carried out in nanostructures exhibiting the same polytypism. Our results provide further input to the ongoing efforts toward the integration of polytypic 2D materials into functional devices.

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

“…The map of Figure a is consistent with the findings of ref , which obtained a value of the band gap of 2H/3R polytypic WS 2 of E bg = (1.6 ± 0.3) eV with an exponent of b = 1.3 –0.7 +0.3 from a single spectrum. These results also agree within uncertainties with first-principles calculations based on density functional theory for the band structure of 2H/3R polytypic WS 2 . Furthermore, the correlation between the thickness and band gap maps points to a possible dependence of the value of E bg on the specimen thickness, though this trend is not statistically significant.…”

“…These results also agree within uncertainties with first-principles calculations based on density functional theory for the band structure of 2H/3R polytypic WS 2 . 32 Furthermore, the correlation between the thickness and band gap maps points to a possible dependence of the value of E bg on the specimen thickness, though this trend is not statistically significant. Further details about the band gap analysis of the WS 2 nanoflowers are provided in section S6 .…”

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

“…These results also agree within uncertainties with first-principles calculations based on Density Functional Theory for the band structure of 2H/3R polytypic WS 2 . 32 Furthermore, the correlation between the thickness and bandgap maps points to a possible dependence of the value of E bg on the specimen thickness, though this trend is not statistically significant. Further details about the bandgap analysis of the WS 2 nanoflowers are provided in Supporting Information Sect.…”

“…Furthermore, this spatially-resolved determination of E bg is in agreement within uncertainties with first-principles calculations based on Density Functional Theory (DFT) of the band structure of 2H/3R polytypic WS 2 . 32 These DFT calculations, which also account for spin-orbit coupling effects, find values of E bg in the range between 1.40 eV and 1.48 eV depending on the settings of the calculation. The DFT predictions are hence consistent with the 68% CL interval for the E bg for a wide region of the specimen, as indicated by Fig.…”

“…( 67) were carried out by fixing the value of the exponent b to b = 0.5 for InSe specimen, motivated by the PL analysis, and b = 1.5 for the WS 2 nanoflower, justified by previous work 26,27 as well as by independent first-principles DFT calculations. 32 Here we present results of fits were both the bandgap energy E bg and type b are simultaneously determined from the ZLP-subtracted EELS spectra in the case of the InSe specimen.…”