Tip-Enhanced Dark Exciton Nanoimaging and Local Strain Control in Monolayer WSe₂

Abstract

Dark excitons in transition-metal dichalcogenides, with their long lifetimes and strong binding energies, provide potential platforms from photonic and optoelectronic applications to quantum information science even at room temperature. However, their spatial heterogeneity and sensitivity to strain is not yet understood. Here, we combine tip-enhanced photoluminescence spectroscopy with atomic force induced strain control to nanoimage dark excitons in WSe2 and their response to local strain. Dark exciton emission is facilitated by out-of-plane picocavity Purcell enhancement giving rise to spatially highly localized emission, providing for higher spatial resolution compared to bright exciton nanoimaging. Further, tip–antenna-induced dark exciton emission is enhanced in areas of higher strain associated with bubbles. In addition, active force control shows dark exciton emission to be more sensitive to strain with both compressive and tensile lattice deformation facilitating emission. This interplay between localized strain and Purcell effects provides novel pathways for nanomechanical exciton emission control.

Spatial Imaging and Control of Dark Excitons in Monolayer Transition Metal Dichalcogenides

“…To obtain the definite spatial information on the dark excitons, we assume that the system is observed on the order of 10 K. 19 The term involving coefficient Θ in eq 1 describes the coupling between the bright and dark excitons. It can be induced by several factors, such as the in-plane magnetic field, 16−19 phonon scattering, 28−30 strain, 31,32 and spin−orbit coupling. 33 Here we will mainly focus on the effect of the inplane magnetic field and ignore other scattering mechanisms at low temperatures, in accordance with the previous experiments.…”

Near-Field Coupling with a Nanoimprinted Probe for Dark Exciton Nanoimaging in Monolayer WSe2

“…Since the first demonstration by Betzig et al of single-molecule detection using an aperture-type near-field probe, advanced near-field probes (configurations) with improved throughput have been proposed to study new luminescent nanomaterials. Among them, two types of near-field probes, including the “Campanile” probe with in-plane near-field enhancement and probe–substrate-based gap mode plasmonic with sensitivity in the out-of-plane direction have been successfully employed to optically map the subdiffraction limit localized excitonic states in two-dimensional (2D) transition-metal dichalcogenides (TMDs) , across local heterogeneities such as point defects, , twin boundaries, and localized strain. − …”

“…Gelly et al have shown that dynamic control of strain can facilitate the funneling of X D and increase the emission. Moreover, Hasz et al reported an increase in the X D emission with increasing strain using a scattering-type near-field probe at the nanobubble sites. These findings further underscore the significance of our technique in investigating dark excitons.…”

“…This confined excitation spot is critical for probing the dark state emission, as it minimizes the amount of bright excitons that are excited, which can act as background noise and obscure the signal from the dark states in the room-temperature spectra. In contrast, the scattering-type near-field technique typically has a far-field spot size coupled from the side objective lens typically on the order of micrometers, leading to a larger amount of bright excitons being excited. Moreover, the addition of a gold coating to the pyramid probe not only forms the gap mode with the gold substrate but also helps to reduce the relative intensity of the sidebands beyond the apex of the probe.…”

“…The plasmonic coupling also limits the collection volume of the dark state emission, resulting in extremely high imaging resolution. In contrast to scattering-type near-field techniques, , this novel near-field configuration offers a reduced interaction between the excitation laser and the probe, which minimizes potential instability factors such as the probe wear, electrostatic potential, and thermal drift . The heating effect generated by the excitation laser can be significantly reduced, by up to 5 orders of magnitude, through the combination of reduced near-field enhancement, as discussed above, 2 orders of magnitude larger spatial localization of excitation laser at the probing region.…”