Conventionally, tuning materials’ properties can be done
through strategies such as alloying, doping, defect engineering, and
phase engineering, while in fact mechanical straining can be another
effective approach. In particular, elastic strain engineering (ESE),
unlike conventional strain engineering mainly based on epitaxial growth,
allows for continuous and reversible modulation of material properties
by mechanical loading/unloading. The exceptional intrinsic mechanical
properties (including elasticity and strength) of two-dimensional
(2D) materials make them naturally attractive candidates for potential
ESE applications. Here, we demonstrated that using the strain effect
to modulate the physical and chemical properties toward novel functional
device applications, which could be a general strategy for various
2D materials and their heterostructures. We then show how ultralarge,
uniform elastic strain in free-standing 2D monolayers can permit deep
elastic strain engineering (DESE), which can result in fundamentally
changed electronic and optoelectronic properties for unconventional
device applications. In addition to monolayers and van der Waals (vdW)
heterostructures, we propose that DESE can be also applied to twisted
bilayer graphene and other emerging twisted vdW structures, allowing
for unprecedented functional 2D material applications.