Rydberg Excitons and Trions in Monolayer MoTe₂

Abstract

Monolayer transition metal dichalcogenide (TMDC) semiconductors exhibit strong excitonic optical resonances, which serve as a microscopic, noninvasive probe into their fundamental properties. Like the hydrogen atom, such excitons can exhibit an entire Rydberg series of resonances. Excitons have been extensively studied in most TMDCs (MoS2, MoSe2, WS2, and WSe2), but detailed exploration of excitonic phenomena has been lacking in the important TMDC material molybdenum ditelluride (MoTe2). Here, we report an experimental investigation of excitonic luminescence properties of monolayer MoTe2 to understand the excitonic Rydberg series, up to 3s. We report a significant modification of emission energies with temperature (4 to 300 K), thereby quantifying the exciton–phonon coupling. Furthermore, we observe a strongly gate-tunable exciton–trion interplay for all the Rydberg states governed mainly by free-carrier screening, Pauli blocking, and band gap renormalization in agreement with the results of first-principles GW plus Bethe–Salpeter equation approach calculations. Our results help bring monolayer MoTe2 closer to its potential applications in near-infrared optoelectronics and photonic devices.

Quasiparticle and Optical Properties of Carrier-Doped Monolayer MoTe2 from First Principles

“…The reduction in the exciton binding energy almost exactly compensates for the renormalization of the QP gap, resulting in an almost constant exciton energy with doping density. This is shown in Figure c, which also validates the efficiency of our PPM, as the theoretical results are in good qualitative and quantitative agreement with photoluminescence data reported in ref , in the experimentally accessible doping region. The corresponding computed absorption spectra can be found in Figure S7.…”

“…Transitions associated with the 2s and 3s states of the A excitons are included under this second peak and fall at an energy of 1.31 and 1.43 eV, respectively. Relevant results for the exciton states are reported in Table and are in very good agreement with energies obtained from our photoluminescence measurements, which are detailed elsewhere (ref ). The slight remaining discrepancy is likely due to the dielectric environment, i.e., hBN substrate, present in the experimental setup and not considered in the calculations.…”

“…Here, we detail the development of a general, accurate, and computationally efficient approach for predicting excitonic effects in carrier-doped materials using the ab initio GW-BSE method. We illustrate this approach for doped MoTe 2 monolayer, a less-studied member of the monolayer TMD class, comparing to detailed spectroscopy measurements . Central to our approach is a modified plasmon-pole model (PPM) that accurately captures two poles in the frequency-dependent dielectric function, i.e., the high-frequency optical intrinsic plasmon and the low-frequency acoustic carrier plasmon (Figure ).…”

“…For monolayer MoTe 2 , doping dependence of (a) the QP band gap obtained with our PPM (eq ) with the carrier plasmon screening included in the head matrix element only (δε 00 –1 ( q ,ω) in the right-hand side of eq ) or in the full dielectric matrix ( δ ε boldG boldG false′ − 1 ( q , ω ) in eq ), compared to a full-frequency calculation (black star markers), (b) the exciton binding energy obtained self-consistently from BSE calculations with the effective dielectric function accounting for dynamical effects in the head matrix element δ ε̃ 00 − 1 ( q , E b S ) only or in the full matrix δ ε̃ boldG boldG false′ − 1 ( q , E b S ) (eqs & ), and (c) the computed exciton energy with the PPM (cyan curve) obtained as ΔΩ = Δ E g – Δ E b , compared to experimental data (gray markers) from ref .…”

“…(a) Evolution of the oscillator strength with the doping density for monolayer MoTe 2 , obtained from a self-consistent BSE calculation using the effective dielectric function defined in eqs & , with carrier plasmon screening accounted for in all matrix elements, δ ε̃ boldG boldG false′ − 1 ( q , E b S ) , or in only the head matrix element, δ ε̃ 00 − 1 ( q , E b S ) . Experimental data (gray markers) from ref are shown for comparison. (b–m) Representation of the lowest-energy exciton envelope wave function in k -space (normalized such that ∑ k | A ( k )| 2 = 1), centered around K(0,0), for the intrinsic MoTe 2 monolayer, and doped monolayer MoTe 2 , with doping density of 2.3 × 10 11 , 1.6 × 10 12 , 3.0 × 10 12 , 4.5 × 10 12 , and 5.9 × 10 12 cm –2 .…”