Distinctive Signatures of the Spin- and Momentum-Forbidden Dark Exciton States in the Photoluminescence of Strained WSe₂ Monolayers under Thermalization

Abstract

With both spin and valley degrees of freedom, the low-lying excitonic spectra of photo-excited transition-metal dichalcogenide monolayers (TMDC-MLs) are featured by rich fine structures, comprising the intra-valley bright exciton states as well as various intra-and inter-valley dark ones. The latter states can be classified as those of the spin-and momentum-forbidden dark excitons according to the violated optical selection rules. Because of their optical invisibility, these two types of the dark states are in principle hardly observed and even distinguished in conventional spectroscopies although their impacts on the optical and dynamical properties of TMDC-MLs have 1 arXiv:1811.06728v3 [cond-mat.mes-hall] 9 Apr 2019 been well noticed. In this Letter, we present a theoretical and computational investigation of the exciton fine structures and the temperature-dependent photo-luminescence spectra of strained tungsten diselenide monolayers (WSe 2 -MLs) where the intra-valley spin-forbidden dark exciton lies in the lowest exciton states and other momentumforbidden states are in the higher energies that are tunable by external stress. The numerical computations are carried out by solving the Bethe-Salpeter equation for an exciton in a WSe 2 -ML under the stress-control in the tight-binding scheme established from the first principle computation in the density functional theory. According to the numerical computation and supportive model analysis, we reveal the distinctive signatures of the spin-and momentum-forbidden exciton states of strained WSe 2 -MLs in the temperature-dependent photo-luminescences and present the guiding principle to infer the relative energetic locations of the two types of dark excitons.

Strain Engineering on the Electronic and Optical Properties of WSSe Bilayer

“…Besides, strain engineering will alter the structural symmetry, which may give rise to the polarized characteristics of 2D materials and endow them with great prospects in future applications. As has been reported, the strained WSe 2 monolayers show obvious variation in electronic band structure [18][19][20][21][22] and demonstrate unique advantages in the applications of photoactive devices [23], valleytronics [18,24], photodetectors [25], and anode material for Li-ion battery [26]. Nevertheless, strain engineering on the electronic and optical properties, such as band evolution and optical anisotropy of 2D Janus WSSe bilayer has not yet been reported so far.…”

Measurement of the spin-forbidden dark excitons in MoS2 and MoSe2 monolayers

“…Numerous ab-initio calculations have been proposed to predict the exciton bright-dark splitting but the results are highly dispersed with values in the range 10-40 meV and more importantly with different signs: depending on the methods applied, dark excitons lie above or below the dark ones [16][17][18][19][20] . It is therefore crucial to have a clear experimental determination.…”

Non-equilibrium diffusion of dark excitons in atomically thin semiconductors

“…First, we present theoretical calculations with the goal to microscopically describe spatially and temporally resolved dynamics of the phonon-assisted emission from dark excitons. The energy and wavefunction of bright and dark exciton states 7,13,16,17,22 are accessed via numerical solution of the Wannier equation 8,47,48 including material-specific parameters for the electronic bandstructure 49 . Due to the resonant optical excitation and considerable binding energies, we focus our study on the three energetically lowest 1s states, which in our calculations are KK excitons followed by KΛ and bright KK ones.…”

“…A plethora of Coulomb-bound states including bright, spin-and momentum-dark excitons [1][2][3][4][5][6][7][8][9] as well as spatially-separated interlayer excitons in van der Waals heterostructures [10][11][12] dominate optical response and ultrafast dynamics of these technologically promising materials. Particularly, rich excitonic manifolds are observed in tungsten-based TMDs 7,8,[13][14][15][16][17][18][19][20] that exhibit several sharp resonances in their photoluminescence (PL) spectra. Most of these resonances stem from dark exciton states, which can not directly interact with in-plane polarized light due to either spinor momentum-conservation.…”