Fluence dependent dynamics of excitons in monolayer MoSi₂Z₄ (Z = pnictogen)

Abstract

Reduced dielectric screening in two-dimensional materials enables bound excitons, which modifies their optical absorption and optoelectronic response. Here, we demonstrate the existence of excitons in the bandgap of the monolayer family of the newly discovered synthetic MoSi2Z4 (Z = N, P, and As) series of materials. All three monolayers support several bright and strongly bound excitons with binding energies varying from 1 eV to 1.35 eV for the lowest energy exciton resonances. We show that on increasing the pump fluence or photo-excited carrier density, the lowest energy exciton first undergoes a redshift followed by a blueshift, due to the renormalized exciton binding energies. The exciton binding energy varies as a Lennard-Jones-like potential as a function of the inter-exciton spacing. This establishes an atom-like attractive and repulsive interaction between excitons depending on the inter-exciton separation. Our study shows that the MoSi2Z4 series of monolayers offer an exciting test-bed for exploring the physics of strongly bound excitons and their non-equilibrium dynamics.

Emerging Versatile Two‐Dimensional MoSi2N4 Family

“…coupling in α 1 MoSi 2 N 4 , [35][36][37] spin polarization and plasmon properties in α 1 MoSi 2 N 4 , [38,39] superconductivity in α 1 TaSi 2 N 4 and NbSi 2 N 4 , [40] topological insulating property in β 2 SrGa 2 Se 4 and SrGa 2 Te 4 , [29] ferromagnetic nature in δ 4 VSi 2 P 4 , [29] valley halfsemiconducting property in α 1 VSi 2 N 4 , [41] Mott transition in XSi 2 N 4 , [42] etc. These intriguing properties could enable prom ising applications of MA 2 Z 4 family in nanoelectronic devices such as magnetic tunnel junction, field effect transistors, highly sensitive and reusable gas sensors, etc.…”

“…The experimentally synthesized and the most widely studied structure is α 1 ‐MA 2 Z 4 . Due to the rich compositions and diverse structures, MA 2 Z 4 family has exhibited intriguing physical and chemical characteristics by using the Vienna ab initio simulation package (VASP) based on DFT, [ 30,31 ] such as non‐linear optics and second harmonic response, [ 32,33 ] magnetooptical properties, [ 34 ] quantum behavior of strong exciton‐phonon coupling in α 1 ‐MoSi 2 N 4 , [ 35–37 ] spin polarization and plasmon properties in α 1 ‐MoSi 2 N 4 , [ 38,39 ] superconductivity in α 1 ‐TaSi 2 N 4 and NbSi 2 N 4 , [ 40 ] topological insulating property in β 2 ‐SrGa 2 Se 4 and SrGa 2 Te 4 , [ 29 ] ferromagnetic nature in δ 4 ‐VSi 2 P 4 , [ 29 ] valley‐half‐semiconducting property in α 1 ‐VSi 2 N 4 , [ 41 ] Mott transition in XSi 2 N 4 , [ 42 ] etc. These intriguing properties could enable promising applications of MA 2 Z 4 family in nanoelectronic devices such as magnetic tunnel junction, field effect transistors, highly sensitive and reusable gas sensors, etc.…”

Electronic and Excitonic Properties of MSi2Z4 Monolayers

“…The recent calculations within GW approximation and Bethe-Salpeter equation (GW + BSE) highlighted the band gap renormalization of more than 1.0 eV and exciton binding energy of several hundreds of meV. [44][45][46][47][48][49][50] The convergence of GW band gap and exciton binding energy with respect to calculation parameters was carefully checked only in refs. [47,48,50], yielding indirect and direct gap of 2.79-2.86 and 2.96-3.13 eV, respectively, and exciton A binding energy of 0.63-0.65 eV in 1L MoSi 2 N 4 .…”

Room temperature electron–hole liquid phase in monolayer MoSi2Z4 (Z = pinctogen)

“…Here, we predict that monolayer MoSi 2 Z 4 series (Z = N, P, or As) can host the EHL phase for temperatures above room temperature. This is facilitated by the strongly bound excitons in the MoSi 2 Z 4 series having binding energies of up to 1 eV [34][35][36][37][38]. The binding energy (E ex ) of the free exciton is a significant factor in determining the critical temperature for the EHL phase, which follows the empirical relation, T c ∼ 0.1 E ex .…”

“…Therefore, the monolayers of MoSi 2 Z 4 are promising platform for observing the EHL phase at higher temperatures and experimentally accessible exciton density. The MoSi 2 N 4 class of materials have emerged as a new series of synthetic 2D material with excellent physical properties [34][35][36][37][38][39][40][41][42][43][44]. We calculate the ground state energy and the phase diagram of the EHL phase in the MoSi 2 Z 4 series, taking into account the kinetic, exchange, and correlation energy of the electron-hole pairs.…”