非层状二维CdSe的制备及厚度对带隙的影响

二维半导体材料的天然带隙有望弥补石墨烯的零带隙缺陷，打破后者在场效应晶体管、开关器件、逻辑电路等领域的应用瓶颈.相较于层状半导体材料，非层状半导体材料多以较强的离子键/共价键结合，且各向同性，因此要获取其二维结构存在一定挑战.本论文通过化学气相沉积法实现了非层状Cd Se在云母衬底上的二维各向异性生长.详细表征了二维Cd Se的微观形貌、晶体结构和光学特性等.结果表明，样品具有显著的光致发光(PL)效应，说明厚度减薄至纳米级时不会破坏Cd Se的直接带隙属性.此外，随着厚度减小，样品的PL峰逐渐蓝移.为了进一步解释该现象，采用基于密度泛函理论的第一性原理计算方法，研究了不同厚度的Cd Se的能带结构，结果显示均为直接带隙，且随厚度减小，带隙值增大，与实验现象吻合.由此可知，通过生长参数调控二维Cd Se的厚度，即可实现对其带隙的有效调控，这对相关光电器件的性能提升具有指导意义.

Preparation of nonlayered two-dimensional CdSe and effect of thickness on bandgap

Two-dimensional semiconductor materials with natural bandgap are expected to make up for the zero bandgap defect of graphene and break the bottleneck of its application in fields such as field effect transistors, switching devices, logic circuits. Compared with layered semiconductor materials, non-layered semiconductor materials are usually bound by strong ionic/covalent bonds and are isotropic, so it is a challenge to obtain their two-dimensional structures. In this paper, two-dimensional anisotropic growth of nonlayered CdSe on mica substrate was achieved by chemical vapor deposition. The microstructure, crystalline structure and optical properties of two-dimensional CdSe were characterized in detail. The results show that the sample has a significant photoluminescence (PL) effect, indicating that the thickness thinning to nanometer level does not destroy the direct bandgap property of CdSe. In addition, with the decrease of thickness, PL peaks of the sample are gradually blue-shifted. In order to further explain this phenomenon, the energy band structures of CdSe with different thickness were studied using the first-principles caculation based on density functional theory. The results show that they all have direct bandgap, and the bandgap increases with the decreasing thickness, which is consistent with the experimental phenomenon. Therefore, the effective control of the bandgap can be achieved by adjusting the thickness of two-dimensional CdSe through the growth parameters, which has guiding significance for the performance improvement of related optoelectronic devices.