[Paper Review] Broadband pump-probe study of biexcitons in chemically exfoliated layered WS$_{2}$

Rup Kumar Chowdhury, Snehasish Nandy+6 more|arXiv (Cornell University)|Dec 4, 2017

2D Materials and Applications59 references16 citations

TL;DR

This study uses broadband femtosecond pump-probe spectroscopy to investigate ultrafast excitonic dynamics in chemically exfoliated monolayer to quadlayer WS₂ at room temperature. It experimentally confirms the existence of biexcitons with binding energies of ~69 meV (AA) and ~66 meV (BB), in excellent agreement with theory, and reveals distinct non-radiative and radiative lifetimes for excitons and biexcitons, demonstrating room-temperature stability crucial for optoelectronic devices.

ABSTRACT

Strong light-matter interactions in layered transition metal dichalcogenides (TMDs) open up vivid possibilities for novel exciton-based devices. The optical properties of TMDs are dominated mostly by the tightly bound excitons and more complex quasiparticles, the biexcitons. Instead of physically exfoliated monolayers, the solvent-mediated chemical exfoliation of these 2D crystals is a cost-effective, large-scale production method suitable for real device applications. We explore the ultrafast excitonic processes in WS$_{2}$ dispersion using broadband femtosecond pump-probe spectroscopy at room temperature. We detect the biexcitons experimentally and calculate their binding energies, in excellent agreement with earlier theoretical predictions. Using many-body physics, we show that the excitons act like Weiner-Mott excitons and explain the origin of excitons via first-principles calculations. Our detailed time-resolved investigation provides ultrafast radiative and non-radiative lifetimes of excitons and biexcitons in WS$_{2}$. Indeed, our results demonstrate the potential for excitonic quasiparticle-controlled TMDs-based devices operating at room temperature.

Motivation & Objective

• To investigate ultrafast excitonic and biexcitonic dynamics in chemically exfoliated WS₂ at room temperature.
• To experimentally confirm the presence of biexcitons and measure their binding energies in monolayer to quadlayer WS₂.
• To determine the radiative and non-radiative decay lifetimes of excitons and biexcitons using time-resolved spectroscopy.
• To establish the role of many-body interactions and short-range Coulomb forces in shaping excitonic behavior in 2D WS₂.
• To validate theoretical predictions of biexciton binding energies and exciton formation mechanisms via first-principles calculations.

Proposed method

• Employed broadband femtosecond pump-probe spectroscopy with a 405 nm pump and a 350–750 nm white-light probe to measure transient absorption (TAS) dynamics.
• Used a 200 fs instrument response function (IRF) to deconvolve time-resolved TAS signals and extract decay components.
• Fitted the TAS data with a three-exponential decay model to separate non-radiative (T₁, T₂) and radiative (T₃) recombination processes.
• Performed density functional theory (DFT) calculations to analyze band degeneracy, spin-orbit coupling effects, and the origin of A, B, and C excitons.
• Correlated experimental peak shifts with many-body interactions, including exciton cooling and pump-power-dependent red shifts.
• Calculated biexciton binding energies from peak energy shifts and compared them with theoretical predictions.

Experimental results

Research questions

• RQ1 Do biexcitons exist in chemically exfoliated monolayer to quadlayer WS₂ at room temperature?
• RQ2 What are the binding energies of AA and BB biexcitons in chemically exfoliated WS₂, and how do they compare to theoretical predictions?
• RQ3 How do the radiative and non-radiative lifetimes of excitons and biexcitons differ in WS₂, and what causes the anomaly in the B-exciton lifetime?
• RQ4 What is the role of many-body interactions, such as Coulomb screening and Pauli blocking, in shaping the ultrafast dynamics of excitonic quasiparticles?
• RQ5 How does the presence of excited-state absorption (ESA) and biexcitonic transitions affect the radiative recombination dynamics in WS₂?

Key findings

• Biexcitons in chemically exfoliated WS₂ were experimentally confirmed with binding energies of ~69 meV (AA) and ~66 meV (BB), in excellent agreement with theoretical predictions.
• The radiative lifetime of A and C excitons was measured to be on the order of nanoseconds, while the B-exciton radiative lifetime was significantly reduced to ~300 ps due to strong coupling with ESA and BB biexciton states.
• Non-radiative decay components (T₁ and T₂) were found to be similar across all excitonic and biexcitonic states, indicating a common relaxation pathway via defect states.
• The pump-power-dependent red shift of ~90 meV for AA and ~80 meV for BB biexcitons arises from many-body interactions among quasiparticles.
• Exciton cooling induced a blue shift of ~90 meV in the BB biexciton peak, confirming the role of thermal relaxation in quasiparticle evolution.
• First-principles DFT calculations confirmed that spin-orbit coupling and band degeneracy at K and K′ points are preserved from monolayer to quadlayer, supporting the formation of A, B, and C excitons via short-range Coulomb interactions akin to Weiner-Mott excitons.