Symmetry Engineering in Nanostructures of Transition Metal Dichalcogenides

Symmetry often plays crucial roles in the properties and functions of materials. In bulk materials, symmetry is basically determined by the space group of single crystals. In sharp contrast, in nanomaterials, symmetry can be controlled as designed. For instance, graphene and bilayer graphene have totally different symmetry, and needless to say, rolling them into tublar structures makes their symmetry reduced to chiral.<br/>In this presentation, we discuss one of the symmetry sensitive properties, bulk photovoltaic effect, in nanotubes [1], van der Waals (vdW) heterostructures [2], and strained [3] transition metal dicalcogenides (TMD). Bulk photovoltaic effect is the photovoltaic effect of uniform materials without p-n junctions, which have been known as a characteristic property of ferroelectric or polar bulk materials. Monolayer TMD has a trigonal structure, which is a noncentrosymmetric but nonpolar structure. Therefore no bulk photovoltaic effect for random light polarization is expected. However, TMD can be changed to polar structures by making tubular structures, vdW heterostructures with twofold rotational symmetry, or strained 3R structure, and thus bulk photovoltaic effect emerges. The light intensity dependence of photocurrent exhibits a crossover from linear to route mean square dependence, in agreement with the quantum mechanical shift current mechanism. Importantly, the observed photocurrent density is rather large comparing to those in bulk polar materials. The present result may indicate a novel route to create new functionalities based on nanostructures through symmetry engineering.<br/><br/>[1] Y. J. Zhang et al., Nature 570, 349 (2019).<br/>[2] T. Akamatsu et al., Science 372, 68 (2021).<br/>[3] Y. Dong et al., submitted for publication.