基于二维磷烯异质结的外延生长制备及其电子结构研究

The research on two-dimensional (2D) layered materials with diverse electronic properties has rapidly developed. Recently, van der Waals heterostructures, which are vertical stacks of multiple 2D materials layer by layer, have created a new paradigm in materials science. The layers in the van der Waals heterostructures are held together by weak van der Waals forces and the combination of different 2D materials offers a platform for exploring new physics and devices with new architectures. However, up to now, most of the van der Waals heterostructures are realized by mechanical transfer using scotch tape. Although this method can produce high quality heterostructures suitable for lab research, the lack of scalability, the poor output, the low controllability of the alignment between different layers, as well as the presence of possible contaminations during the complicated transfer process limit its applications. .For the transition from the realm of fundamental research to practical applications, a facial and scalable synthesis approach for the construction of 2D heterostructures need to be developed. An alternative way to fabricate the vertical 2D heterostructures with clean and atomically sharp interfaces as well as good scalability is the direct growth of one kind of 2D material onto another. Despite the considerable efforts and some successful examples to date, the realization of the bottom up fabricated vertical heterostructures are mainly limited to stacks between two transition metal dichalcogenides (TMDs), or BN/Gr. The direct growth of 2D vertical stacks based on other 2D members are rarely reported, e.g., phosphorus based 2D vertical heterostructures. Due to its intriguing physical properties, hybridization of phosphorene with other 2D members could provide opportunities of novel electronic and optoelectronic devices, e.g. stacking the p-type phosphorene and n-type TMDs to construct vertical p-n junctions, inserting BP between two graphene layers to investigate its performance as a tunnel barrier, or studying the photocurrent generation process on phosphorene based heterostructures, etc. .Although conceptually simple, the growth of phosphorene based vertical heterostructures or even the phosphorene itself is a considerable challenge. To date, only the growth of single layer blue phosphorus (BlueP) has been realized on Au(111). The challenges for the growth of phosphorene lies in that (1) the formation of phosphorene is very sensitive to the substrate, strong interaction with phosphorus will break up the phosphorus flakes into nanoclusters or even single atoms and (2) atomically thin phosphorus are too delicate, water and oxygen may induce the oxidation of few phosphorus layers in ambient environment. To grow phosphorene as well as phosphorene based heterostructures, it is therefore essential to select substrates with relatively weak interactions with phosphorus and perform the experiment in the ultrahigh vacuum (UHV) conditions to avoid the undesired contaminations such as water and oxygen. Moreover, special attention should be paid to select the substrates with appropriate symmetry and better matched lattice constant. .This project will be carried out through three main interlinked sub-projects: (1) Structural characterization of the as-grown BlueP based vertical heterostructures by low-temperature scanning tunnelling microscopy (LT-STM), in-situ X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy (XPS/UPS) and angle-resolved photoemission spectroscopy (ARPES); (2) Development of transfer technology and the fabrication of FETs devices to evaluate the charge transport properties and optoelectronic properties of the as-grown BlueP based vertical structures; (3) Use STS as a probe to understand the electrical and electronic properties of the vertical heterostructures.

二维磷应用的一个巨大挑战是难以实现薄层磷尤其是单层磷膜的自下而上的高质量大面积制备。实验研究表明，薄层磷在大气中由于水氧的存在会发生氧化。而磷生长对衬底也具有很强的依赖性。太弱的磷-衬底作用不足以稳定磷吸附体系，而太强的磷-衬底相互作用又会打破磷-磷键，不利于二维磷的生长。选择二维材料作为衬底既避免了磷-衬底之间成键对二维磷生长的抑制，还可以完成二维磷和二维磷范德华异质结的一步生长。本项目拟通过超高真空分子束外延生长的方法在不同性质的二维衬底上：绝缘体（比如h-BN），半导体（比如MoS2)）以及半金属（比如石墨烯）制备二维磷异质结，并通过结合低温扫描隧道显微镜，利用基于同步辐射光源的光电子能谱，软/硬X-射线近边/拓展边吸收光谱技术，在原子/分子尺度研究二维磷异质结的微观形貌，电子结构和电学性能。为制备大面积高质量的基于二维磷的异质结提供指导和依据，探索适用于不同功能器件的异质结体系。

二维层状磷（包括黑磷，蓝磷）是新兴的二维材料，最近成为电子学和电子器件研究的前沿热点。本课题组利用超高真空扫描隧道显微镜（STM）、光电子能谱（XPS/UPS）并结合先进的同步辐射技术如角分辨光电子能谱（ARPES）和近边吸收精细结构（NEXAFS）等技术，通过分子束外延生长的方法，在超高真空环境中在二维材料衬底上原位外延制备二维磷,并对不同阶段不同衬底表面磷的微观形貌和电子结构进行精确的表征，深入理解衬底及制备环境对二维磷生长的影响。从原子尺度上理解其生长机理从而选择合适的二维磷生长衬底和条件,为基于二维磷异质结的大面积生长提供理论基础和技术手段。这些工作具有一定的原始创新性，其中，我们首次报道采用硅插层的办法，实现了分子束外延生长大面积高质量的蓝磷单层薄膜, 在原子尺度上探索了蓝磷薄膜的氧化机制，并发展了基于二维黑磷薄膜的光电器件的应用及器件性能探索研究。这些工作总结在标注了本项目号的40 篇SCI论文中。同时也基于本课题组在二维层状磷的工作，获得了2021年Nano Research Young Innovators (NR45) Award。通过本项目的研究，初步获得了在原子尺度上理解二维层状磷生长机理，并为选择合适的二维磷生长衬底和条件提供了坚实的实验和理论基础。在项目结题后，我们还将继续相关的研究工作，细化一些研究要点，争取取得更多的成绩。

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