悬空单层金属及金属氧化物二维晶体的原位制备及稳定性研究

Monolayer two-dimensional (2D) nanomaterials will present many novel physical phenomena compared to their bulk counterpart due to the quantum confinement effect. Most of the research about 2D materials focus on the graphene-like systems, such as TMDs, which their bulk counterparts have the layered structures. The non-layered systems attract less attention due to the hard preparation of the monolayer structures. The monolayer preparation methods for layered materials are not suitable for that of non-layered materials. However, the non-layered monolayer systems might also have many novel physical phenomena as their structures might be modulated with the thickness decreasing. .Here I propose a novel monolayer preparation method for the non-layered materials based on Cs-corrected (scanning) transmission electron microscopy. Several metal and metal oxide nanoparticles are chosen. The high energy electron beam is employed to bomb the nanoparticles. Some surface atoms will diffuse along graphene and be trapped by graphene nanopore. With the increase of trapped atoms, one free-standing metal or metal oxide monolayer will be formed suspended in graphene nanopore. In particular, we will focus on the formation process in the atomic scale, its lattice structure, its thickness and its component. Meanwhile, we will analyze its stability upon temperature and air. Finally, we will setup the atomic model according our experimental results to calculate the formation energy, phonon dispersion, electronic structure, and its spin information etc. Our findings will enrich the novel physics about 2D materials and increase the possible research fields. In addition, our finds will provide experimental and theoretic supports on the applications of 2D materials.

随着材料厚度降低到单原子层厚，量子限域效应将极大影响材料的性质，单层二维晶体材料将呈现出许多体材料未有的新奇物理现象。关于二维材料的研究主要集中在石墨烯和过渡金属硫族化物等少数几类具有层状结构的材料上，更多潜在的非层状二维晶体材料尚未被发掘。非层状材料一般具有强的金属键、离子键或共价键，随着厚度的降低，表面悬挂键比例增大，材料的结构可能发生重构，同时可能存在全新的物理现象。.本项目将基于球差校正的（扫描）透射电子显微术，选择非层状金属和金属氧化物纳米颗粒为研究对象，利用高能电子束的轰击剥离效应，在石墨烯纳米孔中形成金属和金属氧化物的悬空二维结构，实时观察其形成过程，分析其结构和成分信息；利用原位加热系统，研究其热稳定性和空气条件下的稳定性；结合理论计算，分析其电子结构和光电性质。项目研究成果将丰富二维材料新奇性质，拓展二维材料研究领域，为开发新型器件，实现二维材料的实际应用提供依据。

单层二维晶体材料可具有许多体材料未有的新奇物理性质，将非层状块体材料转变为单层二维晶体材料并分析其形成机理是一个重要的科学问题。本项目以石墨烯为辅助衬底，利用高能电子束轰击非层状块体材料将其转变为单层二维材料，主要研究电子束剂量、温度、气氛、颗粒尺寸等对二维材料形成过程的影响规律，分析单层二维材料的形成机理。研究了Cu、Ni、Au、Ag、Pt、Li、Na、Si、CuO、SnO2、Co3O4、CaO、CeO2等纳米颗粒制备悬空单原子层厚二维晶体的可行性，成功制备了多种单原子层厚二维晶体材料，获得了数种二维晶体材料实时生长的原子级高分辨图像；由于边缘原子存在悬挂键，Si、CuO等原子/分子会首先被石墨烯边缘原子捕获，接着其他原子/分子会向石墨烯孔内部迁移直至将孔洞填满以降低系统能量；一定的束流密度可以促进单原子层厚二维晶体材料的形成，过高的束流密度会使形成的单层二维晶体重新塌缩成纳米颗粒；形成单原子层厚二维晶体所需的加速电压集中在60-80 kV；形成的单层二维晶体与石墨烯存在一定的取向关系，同时通过异质结构界面处原子位置的偏移实现不同晶格的匹配，通过GPA分析证明界面处存在应力；形成的单层二维晶体表现出不同于体材料的物理性质，如单层的CuO表现出D型反铁磁有序性质，同时有3.37eV的间接带隙。本项目研究结果表明，非层状块体材料是可以通过一定的方法制备出单原子层厚二维晶体的，这将大大拓展二维晶体材料的种类，为二维晶体材料库提供更多不同性质的候选材料，为加快二维晶体材料研发和实际应用提供实验和理论依据。