Semiconducting ZnO＆WS2 heterojunction composite films: Fabrication, characterization and ultrafast nonlinear properties

Abstract

In this work, a series of WS2, ZnO base films and ZnO&WS2 (ZnO@WS2 and WS2@ZnO) heterojunction composite films were synthesized by the single/two-step Radio Frequency Magnetron Sputtering. Interestingly, it is found that structure, morphology and growth mechanism of the samples depends on the regulation different experimental parameters. The result of morphology characterization demonstrated that the standing WS2 with different morphology depend on growth time and the multi-directional growth of the ZnO based film. And using the WS2 base films promoted the growth of ZnO sheet-like morphology. The result of X-ray diffraction characterization showed that the location and the shape of the characteristic peaks existed in the ZnO&WS2 heterojunction composite films are analogous to ZnO (002) and WS2 (002)/(101) alone. Furthermore, the ultrafast nonlinear optical behavior and dynamics of the samples were studied. The “W” and “M” type image of open-aperture Z-scan curve were analyzed. From the results, the samples showed multiphoton absorption induced the ground state or excited state saturated absorption or reverse saturated absorption behavior. And the two-photon absorption and three-photon absorption coefficients of ZnO@WS2 heterojunction composite films reached 1.93 × 10−9 m/W, −2.22 × 10−23 m3/W2, respectively. The bi-exponential decay of the positive signal assigned to the pump-induced deep-level or excited state absorption. Moreover, these findings indicate that the excitation wavelength increases and the detection wave band widen. The study shows that the carrier lifetime of ZnO@WS2 heterojunction composite films is significantly higher than that of the WS2 base films. ZnO&WS2 heterojunction composite films will be promising candidates in nonlinear optical material.