Diffusion quantum Monte Carlo study of excitonic complexes in two-dimensional transition-metal dichalcogenides

Abstract

Excitonic effects play a particularly important role in the optoelectronic behavior of twodimensional semiconductors. To facilitate the interpretation of experimental photoabsorption and photoluminescence spectra we provide (i) statistically exact diffusion quantum Monte Carlo bindingenergy data for a Mott-Wannier model of (donor/acceptor-bound) excitons, trions, and biexcitons in two-dimensional semiconductors in which charges interact via the Keldysh potential, (ii) contact pair-distribution functions to allow a perturbative description of contact interactions between charge carriers, and (iii) an analysis and classification of the different types of bright trion and biexciton that can be seen in single-layer molybdenum and tungsten dichalcogenides. We investigate the stability of biexcitons in which two charge carriers are indistinguishable, finding that they are only bound when the indistinguishable particles are several times heavier than the distinguishable ones. Donor/acceptor-bound biexcitons have similar binding energies to the experimentally measured biexciton binding energies. We predict the relative positions of all stable free and bound excitonic complexes of distinguishable charge carriers in the photoluminescence spectra of WSe2 and MoSe2.

Variational and diffusion quantum Monte Carlo calculations with the CASINO code

“…Charge-carrier complexes play a particularly important role in the optoelec-tronic properties of layered and 2D materials. In the field of 2D materials, the DMC-calculated binding energies of trions [165][166][167] and the binding energies of larger complexes such as biexcitons and quintons (charged biexcitons) 154,168 have been reported, as have the DMC-calculated binding energies and VMC-calculated recombination rates of ion-bound charge carrier complexes in heterobilayers of transition-metal dichalcogenides. 169,170 2.…”

Quantum Monte Carlo calculations of energy gaps from first principles

“…The characteristic size of an exciton is usually the exciton Bohr radius a * B = /µ, where µ = m * e m * h /(m * e + m * h ) is the electron-hole reduced mass, is the permittivity, and m * e and m * h are the electron and hole effective masses, respectively. (Note that the size of an exciton is different in 2D materials where the screened interaction is of Keldysh form; 76 in that case the size of the exciton is r 0 = r * /(2µ).) If the simulation supercells used are of linear size much less than the characteristic exciton size then the exciton is artificially confined and the kinetic energy dominates the Coulomb interaction.…”

“…Using the exciton fitting function developed in Ref. 76 and phosphorene parameters available in the literature, 166,168 we show the dependence of the exciton binding energy on the dielectric medium surrounding the monolayer in Fig. 11.…”

“…(Color online) Exciton binding energy in phosphorene within the effective-mass approximation with the Keldysh interaction between charges as a function of the permittivity of the surrounding medium . Results were obtained using the fitting formula from Ref 76…”

Anisotropic Keldysh interaction

“…The important role played by the dielectric screening in determining excitonic properties of various two-dimensional semiconductor heterostructures has been the subject of numerous investigations over the last several decades (for recent studies see, for example, [1][2][3][4][5][6][7][8][9][10][11][12][13]). A particularly interesting direction of current experimental research involves perovskite chalcogenide films whose in-plane dielectric anisotropy gives rise to some rather unusual optical behavior [14,15].…”