Chemically grown multi-walled WS2 nanotubes are dispersed on SiO2/Si++ substrates. Isolated nanotubes were selected under an optical microscope. single crystals of WS2 were grown by chemical vapor transport. Bilayers and monolayers were mechanically peeled using tape.

The short-circuit current (I_sc) under laser illumination is an important parameter for the evaluation of photovoltaic effects. We measured this current for WS2 bilayer device with different crystal symmetries. For each device, we scanned the laser spot from one electrode to the other to distinguish the BPVE from the Schottky barrier photovoltaic effect as well as the photothermal effect near the contact.

The bulk photovoltaic effect (BPVE) in WS2 nanotube devices is quite stable in terms of quality and quantity. The large decrease in the short-circuit current (Isc) with decreasing temperature cannot be explained simply by a decrease in the absorption coefficient, because the band gap is blue-shifted with decreasing temperature. Light with a wavelength of 632.8 nm (1.96 eV) almost resonates with the A-exciton of WS2 (a specific bonded state of an electron and a hole) and therefore produces the strongest signal.

The bulk photovoltaic effect (BPVE) found in tungsten disulfide devices could further enhance energy conversion rates. The BPVE in conventional p-n junctions - where p-type materials (with excess holes) are adjacent to n-type materials (with excess electrons) - generates current through the light-induced generation and separation of electron-hole pairs. This BPVE is particularly important in energy applications, and its efficiency is now approaching its theoretical limit.