Heterostructuring in Mechanically Deformed Van Der Waals Crystals

Heterostructures (HSs) of van der Waals (vdW) crystals are created by stacking atomically thin layers with different compositions and relative angular alignment thus prompting an endless number of combinations and novel physical phenomena. Another key feature of vdW HSs, and of their constituent layers, is their highly sensitive response to mechanical deformations stemming from their virtually two-dimensional (2D) structure. Strain indeed has been demonstrated to be a most powerful tool to modify controllably the electronic properties of 2D materials [1].<br/>Here, we report two exemplary cases, where the interplay between heterostructuring and strain add new functionalities to and improve the optoelectronic characteristics of 2D crystals and their HSs.<br/>In the first instance, we consider HSs formed by strained transition metal dichalcogenide (TMD) monolayers (MLs) and hexagonal boron nitride (h-BN). Strain is achieved by proton irradiation of TMD bulk flakes, in which protons lead to a local blistering of the crystal just beneath the topmost plane and hence to the formation of highly strained ML micro/nano-domes filled with H₂ [2]. The capping with h-BN promotes an elastic energy transfer from the domes to h-BN that eventually prevents the dome deflation at the liquefaction temperature of H₂ (~33 K). In turn, this preserves the dome strain field and enables the fabrication of spatially controlled quantum emitters [3].<br/>In the second case, InSe/TMD dome HSs are presented. InSe features excellent transport properties but poor emission efficiency in the few layer limit. We exploit the strain-induced transition of the valence band maximum from K to Γ in MS₂ (M=Mo,W) ML domes [4] to create a type-I HS with InSe. Therein, a direct transfer of holes from the Γ point of the ML dome to that of the InSe occurs, while electrons undergo a defect-assisted tunnelling from the conduction band of the strained ML to that of InSe. As a result of these processes an increase by up to two orders of magnitude in the emission efficiency of InSe is observed.<br/><br/>[1] E. Blundo <i>et al</i>., Appl. Phys. Rev. <b>8</b>, 021318 (2021)<br/>[2] E. Blundo <i>et al</i>., Phys. Rev. Lett. <b>127</b>, 046101 (2021)<br/>[3] S. Cianci <i>et al</i>., Adv. Opt. Mater. DOI:10.1002/adom.202202953<br/>[4] E. Blundo <i>et al</i>., Phys. Rev. Res. <b>2</b>, 012024 (2020)