Direct Determination of Band Gap Renormalization in Photo-Excited Monolayer Mos2

Abstract

A key feature of monolayer semiconductors, such as transition-metal dichalcogenides, is the poorly screened Coulomb potential, which leads to large exciton binding energy (Eb) and strong renormalization of the quasiparticle bandgap (Eg) by carriers. The latter has been difficult to determine due to cancellation in changes of Eb and Eg, resulting in little change in optical transition energy at different carrier densities. Here we quantify bandgap renormalization in macroscopic single crystal MoS2 monolayers on SiO2 using time and angle resolved photoemission spectroscopy (TR-ARPES). At excitation density above the Mott threshold, Eg decreases by as much as 360 meV. We compare the carrier density dependent Eg with previous theoretical calculations and show the necessity of knowing both doping and excitation densities in quantifying the bandgap. Atomically thin transition-metal dichalcogenide (TMDC) monolayers and heterojunctions are being broadly explored as model systems for a wide range of electronic, optoelectronic, and quantum processes. The commonly studied TMDC monolayers possess direct bandgaps in the visible to near-IR region [1-3]. Because of the strong many-body Coulomb interactions in monolayer TMDCs, both exciton binding energy (Eb) and bandgap renormalization energy are large [3]. The former lowers the optical transition energy by hundreds meV from Eg, while the latter decreases Eg by similar amounts in the presence of charge carriers or excitons. The bandgap renormalization energy (DEg) and decrease in exciton binding energy (DEb) tend to be of similar magnitudes but counteract each other, leading to comparatively modest changes in optical transition energies [4,5]. Since the quasiparticle bandgap Eg is the most fundamental quantity and

The Schottky–Mott Rule Expanded for Two-Dimensional Semiconductors: Influence of Substrate Dielectric Screening

“…In fact, strong indication for excess electrons in experiments with ML-MoS 2 on SiO 2 as supporting substrate has been reported by the observations of enhanced trion photoluminescence, , in line with our suggestion that electron transfer occurs from the supporting substrate to ML-MoS 2 in the E F -pinning region. This is also in line with a decrease in IE due to increased carrier screening. − While an increase in EA would be concomitantly expected due to band gap renormalization by doping, it is apparently not observed here. This is at odds with the simple picture of electron transfer to gap states, but could possibly be due to partial filling and electron correlation induced splitting of the conduction band by transferred electrons, so that the measured EA does no longer correspond to the conduction band minimum.…”

“…The origin of the E g renormalization due to screening is the Coulomb interaction between confined holes/electrons inside the 2D monolayer and the surrounding dielectric medium, , and we can safely hypothesize that the change in E g of ML-MoS 2 monolayer is thus governed by the Coulomb potential that is proportional to 1/ε r . Based on this, E g of ML-MoS 2 as a function of ε r of the substrate can be empirically expressed as where E g,∞ is the single-particle band gap of ML-MoS 2 on a substrate with infinite ε r , and α is an empirical constant (of the dimension energy) that includes information about the polarizability of the combined system.…”

“…Our present data are thus consistent with previous theoretical predictions by firstprinciples calculations using the GW approximation and further experimental data from scanning tunneling microscopy measurements. 36,65 The origin of the E g renormalization due to screening is the Coulomb interaction between confined holes/electrons inside the 2D monolayer and the surrounding dielectric medium, 54,66 and we can safely hypothesize that the change in E g of ML-MoS 2 monolayer is thus governed by the Coulomb potential that is proportional to 1/ε r . Based on this, E g of ML-MoS 2 as a function of ε r of the substrate can be empirically expressed as…”

Optical generation of high carrier densities in 2D semiconductor heterobilayers

“…We point out that the spectroscopy measurements probe the combined responses of the electron and hole plasmas across the heterobilayer interface. Resolving the individual response of the electron or hole plasma is challenging but possible with time and angle-resolved photoemission spectroscopy, which is underway in our laboratory ( 39 ). The combined PL and transient reflectance measurements also reveal the participation of intervalley scattering and dark exciton/carrier reservoirs in radiative recombination dynamics.…”

Defect-Enhanced Exciton–Exciton Annihilation in Monolayer Transition Metal Dichalcogenides at High Exciton Densities

“…19,35 This is primarily attributed to the competition of various physical processes of exciton relaxation, namely, first-order recombination (such as exciton radiation recombination and defect-assisted nonradiation recombination) and bimolecular recombinations. 14,[19][20][21][22][24][25][26]35,36 The reduced dimensional systems can enhance the many-body interaction of excitons, such that at higher carrier densities, the physical process of exciton− exciton annihilation (EEA) becomes extremely effective and dominant in the exciton dynamics and significantly reduces the PLQY. 22,37−39 The main reason is that EEA is a scattering mechanism in which one exciton nonradiation recombination transfers its energy and momentum to another exciton excited to a higher energy state, and the former exciton subsequently relaxes to a lower energy state and loses the originally obtained energy through electron−phonon interaction.…”