层数变化对堆叠生长的MoS_(2(1-x))Se_(2x)电子结构的影响

二维过渡金属硫化物因其独特的光电特性在多功能光电器件方面具有广泛的应用前景.为了进一步拓展其在微纳光电子器件方面的应用范围,并提高器件性能,人们开展了通过合金手段改变端组分材料配比实现对二维半导体材料带隙调控的带隙工程以及调控生长条件改变材料形貌和结构的缺陷工程研究.本文利用光学、原子力和扫描电子显微镜等设备以及拉曼和光致发光光谱等手段对由化学气相沉积法生长出来的堆叠状MoS_(2(1-x))Se_(2x)合金的性质进行了研究.不同于大多数单层或少层MoS_(2(1-x))Se_(2x)合金的情况,堆叠生长的阶梯状MoS_(2(1-x))Se_(2x)合金材料在厚度从2.2 nm (约3层)一直增加到5.6 nm (约7层)时都显出了较强的发光特性,甚至在100 nm厚时,样品的发光谱线仍具有两个发光峰.两个激子发光峰分别来源于自旋轨道耦合造成的价带劈裂.随着厚度的增加,两个峰都逐渐红移,显示了合金掺杂时的能带弯曲效应.拉曼光谱给出了类MoS_2和类MoSe_2两套振动模.随着厚度的增加,拉曼峰位几乎不移动,但面内的两个振动模E_(2g(Mo-Se))和E_(2g(Mo-s))逐渐显现并增强.显然缺陷和应力是影响堆叠生长MoS_(2(1-x))Se_(2x)合金样品电子结构的主要因素,这为特殊功能器件的制备和可控缺陷工程的研究提供了有益的参考.

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

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-x) Se_2x 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-x) Se_2x 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-x)Se_2x 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-x)Se_2x alloys, stack-grown stepped MoS_2(1-x) Se_2x 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-x) Se_2x alloy, which provides a meaningful reference for preparing the special functional devices and studying the controllable defect engineering.