Vibronically Coherent Exciton Trapping in Monolayer WS2

Abstract

Defect engineering in transition metal dichalcogenide (TMD) monolayers enables applications in single-photon emission, sensing, and photocatalysis. These functionalities critically depend on defect type, density, spatial distribution, relative energy, and the dynamics of exciton trapping at the defect sites. The latter are mediated by coupling to optical phonons through mechanisms not yet fully understood. Traditionally, exciton or carrier trapping at defects in inorganic crystals has been described by incoherent multiphonon emission within the Born-Oppenheimer approximation─an approach that underpins the widely used Shockley-Read-Hall framework for nonradiative recombination. Here, we use impulsive vibrational spectroscopy to investigate exciton trapping in defect-modified monolayers of WS₂ grown through metal-organic chemical vapor deposition. We find that the phonon coherences of the Raman-active A' and E' modes persist throughout the ultrafast (∼100 fs) exciton trapping process, indicating a continuous evolution of the excitonic wave function. This observation is consistent with a conical intersection-mediated trapping process, in which a potential energy surface crossing between the free and trapped excitonic states acts as a funnel to drive this nonadiabatic transition. Such a molecular-like, vibronically coherent mechanism lies beyond the Born-Oppenheimer approximation, in stark contrast to classical, incoherent trapping models in solids. Moreover, the faster dephasing of the E' mode in the trapped exciton state compared to the free exciton suggests it acts as a vibrational coordinate that promotes the trapping process. These findings provide mechanistic insights into exciton-phonon interactions at defects in TMD monolayers and inform strategies for engineering quantum and energy functionalities.

Keywords: conical intersections; defect engineering; defect photophysics; exciton trapping; exciton−phonon coupling; transition metal dichalcogenide defects; vibronic coherence.

1

n-BuLi treatment of monolayer WS₂induces a bright lower-energy emission. (a) Schematic of n-BuLi treatment. The proposed mechanism involves a positively charged lithium ion extracting a sulfur atom, leaving a sulfur vacancy. The interaction of vibrations with the defects are studied with ultrafast impulsive vibrational spectroscopy (IVS) in this work. (b) Photoluminescence spectra of pristine and treated WS₂. The assigned emission peaks of the treated film are indicated in the inset. CB: conduction band. VB: valence band. (c) Steady-state Raman spectra of pristine and treated WS₂. Optical measurements are taken at room temperature.

2

Ultrafast exciton trapping. (a) Femtosecond temporal evolution of the transient absorption spectra for a pristine WS₂ film. (b) Transient absorption spectra for a n-BuLi treated WS₂ film. (c) Difference spectra (after normalization of the full TA map) for both films at 75 fs. (d) Difference spectra at 700 fs. (e) Zoomed-in view of (d). The spectra in (c) and (d) are aligned to the exciton A bleach of the pristine film to account for the treatment-induced 5 nm blueshift. From the difference spectra the treatment induces a faster exciton A decay (600–625 nm), as well as a stronger lower-energy bleach (670–720 nm).

3

Exciton trapping modulates phonon coherences. (a) Transient absorption spectra of n-BuLi treated WS₂ monolayer. (b) Kinetics integrated between 600–625 nm (free exciton in orange) and 670–720 nm (trapped exciton in red). Gray transparent box indicates coherent artifact. (c) Electronic-decay free oscillation and its corresponding damped sinusoid function fit for the free exciton. (d) Electronic-decay free oscillation and its corresponding damped sinusoid function fit for the trapped exciton. (e) Early time scale electronic-decay free oscillations. (f) Fast Fourier transform (FFT) over the complete time range. (g) Schematic 1D illustration of the coupling/promoting role of the E’ mode to drive exciton trapping.

4

Configuration coordinate diagram for ultrafast coherent exciton trapping. In the case of traditional thermally activated trapping, an activation barrier (E_A) precedes the surface crossing leading to an incoherent process. For a barrierless trapping the coherence generated in the free excitonic state may be transferred to the trapped excitonic state. Coupling modes experience stronger dephasing than tuning or spectator modes as they nonadiabatically couple the free and trapped excitonic states via a conical intersection (indicated with a blue circle). Conventionally, modes that couple trapped to free states are classified as promoter modes, whereas modes absorbing the excess energy as a result of trapping are called accepting modes.