Engineering interface polarization
Many properties can emerge at the interface of van der Waals materials created by rotating the layers of a single material or by creating heterointerfaces between different materials. Akamatsu et al. formed an interface that intentionally broke in-plane inversion symmetry by combining crystals of tungsten diselenide with threefold rotational symmetry and black phosphorus with twofold rotational symmetry. This interface creates in-plane electronic polarization that results in a spontaneous photovoltaic effect only along the polarization direction. This effect was explained in terms of a shift current mechanism.
Science, this issue p. 68
Van der Waals interfaces can be formed by layer stacking without regard to lattice constants or symmetries of individual building blocks. We engineered the symmetry of a van der Waals interface of tungsten selenide and black phosphorus and realized in-plane electronic polarization that led to the emergence of a spontaneous photovoltaic effect. Spontaneous photocurrent was observed along the polar direction and was absent in the direction perpendicular to it. The observed spontaneous photocurrent was explained by a quantum-mechanical shift current that reflects the geometrical and topological electronic nature of this emergent interface. The present results offer a simple guideline for symmetry engineering that is applicable to a variety of van der Waals interfaces.