Bubbles as a platform for investigating work function tunability of 2D materials under biaxial strains

Strain engineering has been adopted as a convenient way to tune the band structure of two-dimensional (2D) materials. While uniaxial strain can be easily applied, achieving nonvolatile biaxial strain is still challenging, hindering the investigation of biaxial strain engineering. Taking ReS₂ as a demo, we showed that bubbles formed simultaneously during mechanical exfoliation can serve as a convenient platform for investigating work function tunability of 2D materials under biaxial strains. Equi-biaxial tensile strain at the bubble vertex, varying with the bubble sizes, was revealed in situ by lateral force microscopy, as can be predicted by an interfacial strong-shear model. It is found that the local work function of ReS₂ increases with the biaxial tensile strain, leading to the decrease of detected surface potential by kelvin probe force microscopy (KPFM). First-principles calculations, which show that a biaxial strain of 0.2 % can increase the work function of tri-layer ReS₂ by 0.35 %, revealed that the increased work function under biaxial tensile strain stemmed from the strain-tunable band energies. These findings pave a convenient way to investigate the biaxial strain engineering of ReS₂ and tune its local working function for potentially constructing electronic devices of high performance, which could be easily extended to other 2D materials.