Two dimensional semiconductors: optical and electronic properties

TL;DR

This paper reviews the optoelectronic and electronic properties of two-dimensional semiconductors, especially transition-metal dichalcogenides, focusing on excitons, their controllability, and recent advances in understanding their complex behaviors.

Contribution

It provides a comprehensive overview of the physical phenomena, experimental techniques, and open questions related to excitons and energy landscapes in 2D TMD materials and heterostructures.

Findings

Insights into exciton control via strain and fields

Observation of non-linear exciton effects like halo formation

Discussion of exciton diffusion and anti-funneling phenomena

Abstract

In the last decade atomically thin 2D materials have emerged as a perfect platform for studying and tuning light-matter interaction and electronic properties in nanostructures. The optoelectronic properties in layered materials such as transition-metal-dichalcogenides (TMDs) are governed by excitons, Coulomb bound electron-hole pairs, even at room temperature. The energy, wave function extension, spin and valley properties of optically excited conduction electrons and valence holes are controllable via multiple experimentally accessible knobs, such as lattice strain, varying atomic registries, dielectric engineering as well as electric and magnetic fields. This results in a multitude of fascinating physical phenomena in optics and transport linked to excitons with very specific properties, such as bright and dark excitons, interlayer and charge transfer excitons as well as hybrid and moir\'e excitons. In this book chapter we introduce general optoelectronic properties of 2D materials and energy landscapes in TMD monolayers as well as their vertical and lateral heterostructures, including twisted TMD hetero- and homobilayer bilayers with moir\'e excitons and lattice recombination effects. We review the recently gained insights and open questions on exciton diffusion, strain- and field-induced exciton drift. We discuss intriguing non-linear many-particle effects, such as exciton halo formation, negative and anomalous diffusion, the surprising anti-funneling of dark excitons.