The Key Role of Non-Local Screening in the Environment-Insensitive Exciton Fine Structures of Transition-Metal Dichalcogenide Monolayers

Abstract

In this work, we present a comprehensive theoretical and computational investigation of exciton fine structures of WSe2-monolayers, one of the best-known two-dimensional (2D) transition-metal dichalcogenides (TMDs), in various dielectric-layered environments by solving the first-principles-based Bethe–Salpeter equation. While the physical and electronic properties of atomically thin nanomaterials are normally sensitive to the variation of the surrounding environment, our studies reveal that the influence of the dielectric environment on the exciton fine structures of TMD-MLs is surprisingly limited. We point out that the non-locality of Coulomb screening plays a key role in suppressing the dielectric environment factor and drastically shrinking the fine structure splittings between bright exciton (BX) states and various dark-exciton (DX) states of TMD-MLs. The intriguing non-locality of screening in 2D materials can be manifested by the measurable non-linear correlation between the BX-DX splittings and exciton-binding energies by varying the surrounding dielectric environments. The revealed environment-insensitive exciton fine structures of TMD-ML suggest the robustness of prospective dark-exciton-based optoelectronics against the inevitable variation of the inhomogeneous dielectric environment.

Enhanced Photo-excitation and Angular-Momentum Imprint of Gray Excitons in WSe2Monolayers by Spin–Orbit-Coupled Vector Vortex Beams

“…The exciton state of a 2D material is expressed as | S , bold-italicQ false⟩ = 1 normalΩ ∑ v c bold-italick Λ S , bold-italicQ ( v c k ) ĉ c bold-italick + bold-italicQ † ĥ v − bold-italick † | G S false⟩ , where Ω is the area of the 2D material, ĉ c bold-italick † false( ĥ v − bold-italick † false) is defined as the particle operator creating the electron (hole) of wavevector k (− k ) in conduction band c (valence band v ), | GS ⟩ denotes the ground state of the material, Λ S , Q ( vc k ) is the amplitude of the electron–hole configuration ĉ c bold-italick + bold-italicQ † ĥ v − bold-italick † false| G S false⟩ and corresponds to the solution of the BSE for the exciton in momentum space, S is the band index of the exciton state, and Q is the center-of-mass momentum of exciton. For a WSe 2 -ML, the DX states as the exciton ground states are significantly lower than the bright ones by ∼48.8 meV, as shown in Figures b,c and a. , The calculated BX-DX fine structure is in excellent agreement with the experimental observation . Carefully examining the lowest DX states, one notes a small splitting between the DX doublet resulting.…”

“…For the studies of excitons, we employ the theoretical methodology developed by refs , to solve the Bethe–Salpeter equation (BSE) established in first-principles for the exciton fine structure spectra of encapsulated 2D materials (see the Supporting Information for details). The exciton state of a 2D material is expressed as | S , bold-italicQ false⟩ = 1 normalΩ ∑ v c bold-italick Λ S , bold-italicQ ( v c k ) ĉ c bold-italick + bold-italicQ † ĥ v − bold-italick † | G S false⟩ , where Ω is the area of the 2D material, ĉ c bold-italick † false( ĥ v − bold-italick † false) is defined as the particle operator creating the electron (hole) of wavevector k (− k ) in conduction band c (valence band v ), | GS ⟩ denotes the ground state of the material, Λ S , Q ( vc k ) is the amplitude of the electron–hole configuration ĉ c bold-italick + bold-italicQ † ĥ v − bold-italick † false| G S false⟩ and corresponds to the solution of the BSE for the exciton in momentum space, S is the band index of the exciton state, and Q is the center-of-mass momentum of exciton.…”

“… 93 , 94 The calculated BX-DX fine structure is in excellent agreement with the experimental observation. 63 Carefully examining the lowest DX states, one notes a small splitting between the DX doublet resulting. Combined with the SOI of quasi-particle, the intervalley exchange interaction splits the lowest DX doublet and turns one of them, referred to as GX, to be slightly bright.…”

“…Atomically thin transition-metal dichalcogenide monolayer (TMD-ML) is one of the most promising optoelectronic 2D materials with superior light–matter interactions that are dictated by excitons. − In TMD-MLs, excitons are strongly bound by the enhanced Coulomb interaction, leading to the atypical band dispersion and exciton fine structures associated with the diverse degrees of freedom inherent in excitons, including spin and valley properties as well as the center-of-mass motion of exciton. − The pronounced exciton fine structure of a TMD-ML enables the unambiguous spectral resolution of diverse exciton complexes, such as the bright exciton (BX) and various dark exciton (DX) states, each possessing distinct degrees of freedom. In darkish W-based TMD-MLs, e.g.…”

Excitons and Phonons in Two-Dimensional Materials: From Fundamental to Applications

“…The purpose of this Special Issue entitled "Excitons and Phonons in Two-Dimensional Materials: From Fundamental to Applications" is to provide a unique international forum and to cover the entire range of fundamental and applied research associated with excitonic complexes and phonon modes in two-dimensional layered materials. This Special Issue is composed of nine published papers [6][7][8][9][10][11][12][13][14] devoted to investigations of different 2D materials, such as S-TMDs, perovskites, and the multilayered structure of thin films, with theoretical [7,9,11,13] and experimental [10] approaches, as well as their combination [6,8,12,14].…”