Dark exciton anti-funneling in atomically thin semiconductors

Abstract

Transport of charge carriers is at the heart of current nanoelectronics. In conventional materials, electronic transport can be controlled by applying electric fields. Atomically thin semiconductors, however, are governed by excitons, which are neutral electron-hole pairs and as such cannot be controlled by electrical fields. Recently, strain engineering has been introduced to manipulate exciton propagation. Strain-induced energy gradients give rise to exciton funneling up to a micrometer range. Here, we combine spatiotemporal photoluminescence measurements with microscopic theory to track the way of excitons in time, space and energy. We find that excitons surprisingly move away from high-strain regions. This anti-funneling behavior can be ascribed to dark excitons which possess an opposite strain-induced energy variation compared to bright excitons. Our findings open new possibilities to control transport in exciton-dominated materials. Overall, our work represents a major advance in understanding exciton transport that is crucial for technological applications of atomically thin materials.

Dark-Exciton Driven Energy Funneling into Dielectric Inhomogeneities in Two-Dimensional Semiconductors

“…This feature may arise from regions of compressive strain between closely spaced nanobubbles, which carries an opposite effect to tensile strain (raised KK and lowered KQ exciton energies) ,, and could lead to dark-state trapping that would explain the absence of this feature in the PL image. Similarly, recent work suggests that momentum-dark excitons experience antifunneling (i.e., move away) from tensile strain regions, which may explain our observation of trapped momentum-indirect excitons between closely spaced strain nanobubbles.…”

“…Similarly, recent work suggests that momentum-dark excitons experience antifunneling (i.e. move away) from tensile strain regions, 69 which may explain our observation of trapped momentum-indirect excitons between closely-spaced strain nanobubbles.…”

Microscopic origin of anomalous interlayer exciton transport in van der Waals heterostructures

“…The spatiotemporal dynamics of excitons can be accessed through the temporal evolution of the exciton Wigner function [36][37][38], which is directly extracted from the Heisenberg equation of motion for the off-diagonal density matrix X † Q X Q . Extending the approach introduced by Hess and Kuhn [39,40] to excitons, we can quantitatively describe the spatiotemporal evolution of the interlayer exciton density n(r, t) through the following drift-diffusion equation: ṅ(r, t) = ∇ • (D(n(r, t))∇n(r, t)) with…”

Theoretical Analysis of the Nanoscale Composition, Tip-Enhanced Raman Spectroscopy, and Electronic Properties of Alloys in 2D MoS2–WS2 Heterostructures

“…Two-dimensional (2D) single-layer transition metal dichalcogenide (1L-TMD) semiconductors host intense light–matter interactions that can be tailored on the nanoscale for optoelectronic and quantum technologies. − Top-down perturbations such as strain or modified dielectric screening can effectuate localized states to generate quantum states of light, − control the 2D diffusion of excited states, , enhance light–matter interactions, , alter charge separation dynamics, and engineer model many-body physical systems . From the bottom-up, material stoichiometry can be tailored through the introduction of defects, ,, interfaces in lateral heterostructures, − or alloying, , providing additional opportunities to tune electronic structure and optoelectronic phenomena.…”