Effect of layer variation on the electronic structure of stacked MoS2(1-x) Se2x alloy

Abstract

Two-dimensional transition metal dichalcogenides (TMDCs) have the extensive application prospect in multifunctional electronics and photonics due to their unique electro-optical properties. In order to further expand their application scope in micro-nano optoelectronic devices and improve the performance of devices, the band-gap and defective engineering have been studied to tune the band-gap, morphology and structure of two-dimensional semiconductor materials. The tunning of the bandgap of MoS_{2(1-<i>x</i>)} Se_2<i>x</i> alloy has been typically achieved by controlling the Se concentration. Theoretical calculations revealed that layered stacked two-dimensional alloy materials with a larger aspect ratio, exposed edges and obvious edge dangling bonds show enhanced HER activity as compared with TMDCs. In this paper, the properties of stacked MoS_{2(1-<i>x</i>)} Se_2<i>x</i> alloy grown by the chemical vapor deposition method in a quartz tube furnace are investigated by using optical microscopy (OM), atomic force microscopy (AFM), scanning tunneling microscopy (SEM), Raman, photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS). The OM and SEM images of the as-synthesized stacked MoS_{2(1-<i>x</i>)}Se_2<i>x</i> alloy show apparent interface between layers and their thickness is further acquired by AFM. Unlike most of single-layer or few-layer MoS_{2(1-<i>x</i>)}Se_2<i>x</i> alloys, stack-grown stepped MoS_{2(1-<i>x</i>)} Se_2<i>x</i> alloy materials all present the strong luminescence properties despite the thickness increasing from 2.2 nm (~3 layers) to 5.6 nm (~7 layers). And even till 100 nm, the emission spectrum with two luminescence peaks can still be observed. The two exciton luminescence peaks A and B are derived from the valence band splitting caused by the spin-orbit coupling, respectively. As the thickness increases, the two luminescence peaks are red-shifted and exhibit a band-bending effect that is only present when the alloy doping concentration is changed. As the sample thickness is 5.6 nm, a C-peak at 650 nm at the high energy end of the PL spectrum is observed, which may be attributed to the transition luminescence from the defect energy level introduced by Se (S) substitution, interstice or cluster. When the number of layers is small, the number of defects is small, so that the luminescence is not observed. As the number of layers increases, the defects increase to form a defect energy level. However, when the material thickness continuously increases until the bulk material is formed, the luminescence disappears in the PL spectrum because the band gap is reduced and the band gap is made smaller than the defect energy level. Raman spectroscopy gives two sets of vibration modes:like-MoS₂ and like-MoSe₂. The Raman peak is almost unchanged as the thickness increases, but the two vibration modes E_{2g (Mo-Se)} and E_{2 g (Mo-S)} in the plane gradually appear and increase. At the same time, the intensity ratio and line width of Mo-Se related vibration mode E_2g/A_1g increase with thickness increasing, which indicates the enhancement of the Mo-Se in-plane vibration mode and the incorporation of randomness of Se into the lattice. Obviously, the defects and stress are the main factors affecting the electronic structure of stacked MoS_{2(1-<i>x</i>)} Se_2<i>x</i> alloy, which provides a meaningful reference for preparing the special functional devices and studying the controllable defect engineering.