Drift-dominant exciton funneling and trion conversion in 2D semiconductors on the nanogap

Abstract

Understanding and controlling the nanoscale transport of excitonic quasiparticles in atomically thin two-dimensional (2D) semiconductors are crucial to produce highly efficient nano-excitonic devices. Here, we present a nanogap device to selectively confine excitons or trions of 2D transition metal dichalcogenides at the nanoscale, facilitated by the drift-dominant exciton funneling into the strain-induced local spot. We investigate the spatiospectral characteristics of the funneled excitons in a WSe 2 monolayer (ML) and converted trions in a MoS 2 ML using hyperspectral tip-enhanced photoluminescence imaging with <15-nm spatial resolution. In addition, we dynamically control the exciton funneling and trion conversion rate by the gigapascal-scale tip pressure engineering. Through a drift-diffusion model, we confirm an exciton funneling efficiency of ∼25% with a significantly low strain threshold (∼0.1%), which sufficiently exceeds the efficiency of ∼3% in previous studies. This work provides a previously unexplored strategy to facilitate efficient exciton transport and trion conversion of 2D semiconductor devices.

Ultralocalized Optoelectronic Properties of Nanobubbles in 2D Semiconductors

“…This alignment we believe is critical for near-field observation of inter-layer exciton luminescence, which has substantially weaker oscillator strength. We also expect that strain-localized PL signal is enhanced by funneling effects, where excitons are preferentially shuttled towards the lower energy states [34][35][36] Figure 3c shows point spectra at various locations across the nanobubble edges, identified by the colored markers in Fig. 3a and 3d.…”

High-Density, Localized Quantum Emitters in Strained 2D Semiconductors

“…Moreover, as these mechanical strains can strongly control band structure, it is possible to use mechanical strain to tune electronic and photonic performance. , For this reason, the identification of quantum emitters (QEs) in TMDCs has generated considerable excitement in the field of 2D nanophotonics − and quantum information science and engineering . Despite these intriguing properties, the fundamental origin of quantum emission in TMDCs is not completely clear, and thus far, it is believed that emissions are formed by excitons bound to defects, − impurities, or 3D transformations (nanostructures, nanoindents) induced by the strain gradients. − ,− In addition to this fundamental question, there is a need to create a high spatial and number density of quantum emitters in 2D semiconductors. The high number density and high brightness of individual quantum emitters are needed to create dense integration of quantum light sources with other photonic elements in scalable applications.…”

Charge Separation in Monolayer WSe2 by Strain Engineering: Implications for Strain-Induced Diode Action

“…Spatially varying strain in 2D TMDCs bends the band structure and generates a spatial bandgap gradient, thus offering exciton funneling and trion drift . Work utilizing a strain gradient includes both static straining devices, e.g., micro/nano pillars, wrinkled substrate, and dynamic straining devices, e.g., piezoelectric actuators, , atomic force microscopy tips, , nanogap, and MEMS devices . Moreover, exciton funneling in TMDCs may lead to critical densities for droplet formation and strong PL enhancement (e.g., MoS 2 , WS 2 , and WSe 2 ).…”

“…Hence, local strain can physically move excitons and trions toward or away from a strain apex, depending on the type of band bending that localized strain causes. If the band “pinches” together, it will collect quasi-particles and charge carriers, which is interesting for studying high-density phenomena such as crossover between an exciton gas and electron–hole liquid. , If on the other hand the bandgap decreases while both bands bend in the same direction (“type-II”), it may cause charge separation.…”

“…22 Moreover, exciton funneling in TMDCs may lead to critical densities for droplet formation and strong PL enhancement (e.g., MoS 2 , 23 WS 2 , 24 and WSe 2 25 ). Hence, local strain can physically move excitons and trions toward or away from a strain apex, 21 depending on the type of band bending that localized strain causes. If the band "pinches" together, it will collect quasi-particles and charge carriers, which is interesting for studying high-density phenomena such as crossover between an exciton gas and electron−hole liquid.…”