Exciton States in Monolayer MoSe2 and MoTe2 Probed by Upconversion Spectroscopy

Abstract

Transitions metal dichalcogenides (TMDs) are direct semiconductors in the atomic monolayer (ML) limit with fascinating optical and spin-valley properties. The strong optical absorption of up to 20 % for a single ML is governed by excitons, electron-hole pairs bound by Coulomb attraction. Excited exciton states in MoSe2 and MoTe2 monolayers have so far been elusive due to their low oscillator strength and strong inhomogeneous broadening. Here we show that encapsulation in hexagonal boron nitride results in emission line width of the A:1s exciton below 1.5 meV and 3 meV in our MoSe2 and MoTe2 monolayer samples, respectively. This allows us to investigate the excited exciton states by photoluminescence upconversion spectroscopy for both monolayer materials. The excitation laser is tuned into resonance with the A:1s transition and we observe emission of excited exciton states up to 200 meV above the laser energy. We demonstrate bias control of the efficiency of this non-linear optical process. At the origin of upconversion our model calculations suggest an exciton-exciton (Auger) scattering mechanism specific to TMD MLs involving an excited conduction band thus generating high energy excitons with small wave-vectors. The optical transitions are further investigated by white light reflectivity, photoluminescence excitation and resonant Raman scattering confirming their origin as excited excitonic states in monolayer thin semiconductors.

Coulomb interaction in monolayer transition-metal dichalcogenides

“…Our calculations are focused on ML-MoSe 2 and ML-WSe 2 for which there are well-established results for the binding energies of trions in various ML configurations 41,43,58,[64][65][66][67][68][69][70] . When benchmarking the calculated values against empirical results, we focus on the trion binding energies and the energy difference between the 1s and 2s neutral-exciton states, ∆ 12 .…”

“…The former is directly measured from the energy difference between the spectral lines of the neutral and charged excitons in photoluminescence or reflectivity experiments. Similarly, ∆ 12 is directly extracted from the energy difference between the spectral lines of the 1s and 2s neutral-exciton states in reflectivity experiments 41,66,67,70 . Table II includes compiled empirical results of WSe 2 and MoSe 2 in three common ML configurations: suspended in air, supported by SiO 2 , and encapsulated in hBN.…”

Electro‐Optic Upconversion in van der Waals Heterostructures via Nonequilibrium Photocarrier Tunneling

“…Considering the sub‐bandgap CW excitation, the relatively large increase in photon energy (370 meV), and bias‐dependent intensity, the observed upconversion cannot be explained by parametric nonlinear processes, [ 30 ] two‐photon absorption, [ 31,32 ] defect‐ [ 33 ] or phonon‐assisted processes, [ 34,35 ] or biexciton annihilation processes. [ 36–38 ] In order to gain insight into this electro‐optic upconversion mechanism, we measured photocurrent ( I ph ) under NIR laser excitation condition. The UPL intensity ( Figure a) and I ph (Figure 3b) were both found to increase linearly with excitation power suggesting that photo‐induced interlayer charge transfer is responsible for the upconversion mechanism (Figure 3a).…”

Phonon wind and drag of excitons in monolayer semiconductors

“…We assume that initially the exciton density is high enough so that the Auger-like exciton-exciton annihilation process takes place as well. It results in the non-radiative "bi-molecular" recombination where one exciton recombines with its energy and arXiv:1905.01621v2 [cond-mat.mes-hall] 16 Jul 2019 momentum being transferred to another exciton which ends up in a highly-excited state [10,12,15,16,[34][35][36]. It starts losing energy emitting more phonons.…”

“…In recent work [10] different regimes of exciton transport have been revealed in monolayer WS 2 : At low exciton densities the classical diffusion of excitons has been observed. An increase in the density of excitons enables the Auger process [7,[11][12][13][14][15][16] giving rise to an efficient non-radiative exciton-exciton annihilation. It changes the shape of the exciton distribution profile in the real space and gives rise to an apparent increase of the observed diffusion coefficient.…”