Enhanced intrinsic photovoltaic effect in tungsten disulphide nanotubes

Transition metal dichalcogenides (TMDs) are representative 2D materials. Group VI-B TMDs in 2H-type crystal structure have a semiconducting band structure. One of their unique features is the controllability of the crystal symmetry. The inversion symmetry inherent in bulk TMDs can be broken by isolating a monolayer. Various opto-electronic functionalities have been demonstrated by breaking this inversion symmetry. A further reduction of crystal symmetry can be achieved by rolling up 2D sheets to form a tubular structure. In such TMD nanotubes, not only the inversion symmetry but also mirror and rotation symmetries can be broken simultaneously, leading to a polar structure. We have investigated the photovoltaic effect in intrinsic WS₂ with different crystal symmetries and observed a sizable photovoltaic effect from TMD nanotubes without p-n junction. Such a phenomenon is called bulk photovoltaic effect (BPVE). In our study BPVE was observed in WS₂ nanotubes only, indicating a importance of the polar structure to enhance BPVE.