Local Strain Engineering of Two-Dimensional Transition Metal Dichalcogenides Towards Quantum Emitters

Abstract

Two-dimensional transition metal dichalcogenides (2D TMDCs) have received considerable attention in local strain engineering due to their extraordinary mechanical flexibility, electronic structure, and optical properties. The strain-induced out-of-plane deformations in 2D TMDCs lead to diverse excitonic behaviors and versatile modulations in optical properties, paving the way for the development of advanced quantum technologies, flexible optoelectronic materials, and straintronic devices. Research on local strain engineering on 2D TMDCs has been delved into fabrication techniques, electronic state variations, and quantum optical applications. This review begins by summarizing the state-of-the-art methods for introducing local strain into 2D TMDCs, followed by an exploration of the impact of local strain engineering on optical properties. The intriguing phenomena resulting from local strain, such as exciton funnelling and anti-funnelling, are also discussed. We then shift the focus to the application of locally strained 2D TMDCs as quantum emitters, with various strategies outlined for modulating the properties of TMDC-based quantum emitters. Finally, we discuss the remaining questions in this field and provide an outlook on the future of local strain engineering on 2D TMDCs.

1. Introduction

Two-dimensional transition metal dichalcogenides (2D TMDCs) have garnered significant attention in recent years due to their exceptional electronic [1, 2], optical [3, 4], and mechanical properties [5]. The atomically thin nature of 2D TMDCs gives rise to a strong quantum confinement effect, which confines photons and electrons within the planar dimensions. This confinement enables a diverse range of distinct phenomena, such as direct-to-indirect bandgap transitions [6, 7], rich excitonic complexes [7, 8], and valley pseudospin [9]. These unique properties of 2D TMDCs make them promising candidates for a wide range of applications, including high-performance electronics [10], optoelectronics [8], and energy storage [11].

The extraordinary flexibility and mechanical properties of 2D TMDCs enable them to tolerate significant structural curvature and accommodate large deformations in both the in-plane and out-of-plane directions. Specifically, 2D TMDCs can withstand up to 10% strain, which is an order of magnitude higher than their bulk counterparts [12–15]. As morphology deformation occurs, lattice distance and symmetry are altered, leading to the tuning in electronic band structures. This in turn modulates the optical properties and excitonic behaviors of 2D TMDCs [16, 17]. For example, a direct-to-indirect bandgap transition has been detected in monolayer MoS2 under tensile strain in uniaxial or biaxial directions [18, 19]. Out-of-plane deformation can introduce a non-uniform strain gradient in 2D TMDCs, enabling novel phenomena such as exciton funnelling [20, 21] and exciton-exciton interactions [22, 23]. The formed local excitons exhibit non-classical quantum emission, whose positions and emission characteristics are deterministically controlled by local strain fields [24–27]. The energies of emitted photons can be easily adjusted by strain field [17]. These characteristics make 2D TMDCs competitive candidates for the construction of single-photon light sources [28, 29].