Imaging Charged Exciton Localization in van der Waals WSe₂/MoSe₂ Heterobilayers

Abstract

Exciton localization in transition-metal dichalcogenide monolayers is behind a variety of interesting phenomena and applications, including broad-spectrum solar cells and single-photon emissions. Strain fields at the periphery of topographically distinct features such as nanoscopic bubbles were recently associated with localized charge-neutral excitons. Here, we use tip-enhanced photoluminescence (PL) to visualize excitons in WSe2/MoSe2 heterobilayers (HBL). We find strong optical emission from charged excitons, particularly positively charged trions, in HBL supported by interlayer charge transfer. Our results reveal strong trion confinement, with a localization length scale comparable to the trion size, at the apex region inside individual nanoscopic bubbles. Nano-PL mapping also shows sub-10-nm spatial variations in the localized trion emission spectra, which stem from atomic-scale potential energy fluctuations. These findings demonstrate the possibility of confining charged exciton complexes that are electrically tunable, opening up further opportunities to probe many-body exciton physics and to explore additional possible sites for strong exciton localization that can lead to quantum emission.

Ultralocalized Optoelectronic Properties of Nanobubbles in 2D Semiconductors

“…Two-dimensional (2D) transition-metal dichalcogenides (TMDs) have been investigated as a possible route toward producing well-controlled optical emitters, critical components in quantum technologies and photonics. − The band gap of monolayer TMDs is uniquely sensitive to doping, − dielectric environment, and mechanical strain, ,,− thereby providing an ideal platform to create quantum confinement by local mechanical and electrical fields. Indeed, previous research has investigated moiré patterns, − wrinkles, ,, bubbles, , and lithographically fabricated dielectric arrays as effective methods to create localized optical emitters. In spite of these notable achievements, a basic band diagram picture across such localized structures remains elusive.…”

“…1−12 The band gap of monolayer TMDs is uniquely sensitive to doping, 13−17 dielectric environment, 18 and mechanical strain, 8,11,19−21 thereby providing an ideal platform to create quantum confinement by local mechanical and electrical fields. Indeed, previous research has investigated moirépatterns, 22−26 wrinkles, 2,4,8 bubbles, 1,27 and lithographically fabricated dielectric arrays as effective methods 28 to create localized optical emitters. In spite of these notable achievements, a basic band diagram picture across such localized structures remains elusive.…”

Multimodal (Non)Linear Optical Nanoimaging and Nanospectroscopy

“…11,23 Under ambient conditions, spatial resolution on the order of 1 to a few nanometers was repeatedly demonstrated. 22,24−27 Note that this is the case not only for Raman, 24,26,27 photoluminescence, 25,28 and extinction (vide infra) but also for nonlinear nano-optical processes, such as tip-enhanced four-wave mixing. 22 Model systems that are often used to access the few-nanometer length scale in optical spectroscopy include bare and chemically functionalized plasmonic silver and gold nanoparticles.…”

“…Useful Model Systems . Even though this Perspective is focused on ambient TOM, one cannot but mention the seminal UHV nano-Raman measurements that allowed the real space visualization of molecules and their vibrational modes with subnanometer spatial resolution. , Under ambient conditions, spatial resolution on the order of 1 to a few nanometers was repeatedly demonstrated. ,− Note that this is the case not only for Raman, ,, photoluminescence, , and extinction ( vide infra ) but also for nonlinear nano-optical processes, such as tip-enhanced four-wave mixing . Model systems that are often used to access the few-nanometer length scale in optical spectroscopy include bare and chemically functionalized plasmonic silver and gold nanoparticles .…”

Nano-optical Visualization of Interlayer Interactions in WSe2/WS2 Heterostructures

“…Increasing the spatial resolution in such measurements would allow us to better understand the interplay between excitons and local nanostructural motifs that are well-known to significantly affect the properties of TMDs. This is bolstered through tip-enhanced Raman spectroscopy (TERS) and tip-enhanced photoluminescence (TEPL) measurements of TMD monolayers and heterostructures, where nanoscale structure–function relationships become possible. − …”