  Symmetry breaking in two-dimensional layered materials plays a significant
role in their macroscopic electrical, optical, magnetic and topological
properties, including but not limited to spin-polarization effects,
valley-contrasting physics, nonlinear Hall effects, nematic order,
ferroelectricity, Bose-Einstein condensation and unconventional
superconductivity. Engineering symmetry breaking of two-dimensional layered
materials not only offers extraordinary opportunities to tune their physical
properties, but also provides unprecedented possibilities to introduce
completely new physics and technological innovations in electronics, photonics
and optoelectronics. Indeed, over the past 15 years, a wide variety of
physical, structural and chemical approaches have been developed to engineer
symmetry breaking of two-dimensional layered materials. In this Review, we
focus on the recent progresses on engineering the breaking of inversion,
rotational, time reversal and spontaneous gauge symmetries in two-dimensional
layered materials, and illustrate our perspectives on how these may lead to
potential new physics and applications.
