PHOTOACTIVE TUNGSTEN DISULFIDE (WS_2) NANOSHEETS, PREPARED BY RAPID CRYSTALLIZATION FROM LIQUID SULFIDES

摘要

We have shown, that photoactive, highly (001)-textured tungsten disulfide (WS_2) films can be prepared both on insulating as well as metallic substrates. The WS_2 films are crystallized from X-ray-amorphous WS_(3+x) films by a rapid annealing process in an H_2S atmosphere with the assistance of metal-sulfide droplets, which are formed, when the eutectic temperature in the system metal-sulfur is exceeded. As promoters Ni, Co and Pd can be used, where the eutectic temperatures with sulfur are in the range from 620 to 680°C. From ex situ- and in situ-analysis a modified model for the formation of the WS_2 nanosheets was derived, which resembles the well-known VLS (vapor-liquid-solid)-model for the formation of nanorods (or whiskers), except that in our systems the precursor is also an (amorphous) solid, and instead of 1-dimensional rods, 2-dimensional nanosheets are grown. Especially, the preparation on metallic layers (W, TiN), which was not successful before, offers now the opportunity, to prepare thin film solar cells from these WS_2 films. First experiments to prepare thin film solar cells show, that short circuits occur, most probably at grain boundaries, which have to be avoided or passivated.